Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Review Electrocoagulation and advanced electrocoagulation processes: A general review about the fundamentals, emerging applications and its association with other technologies Sergi Garcia-Seguraa, Maria Maesia S.G. Eibanda, Jailson Vieira de Meloa, Carlos Alberto Martínez-Huitlea,b,⁎ a Laboratório de Eletroquímica Ambiental e Aplicada (LEAA), Institute of Chemistry, Federal University of Rio Grande do Norte, Lagoa Nova, CEP 59078-970 Natal, RN, Brazil b Unesp, National Institute for Alternative Technologies of Detection, Toxicological Evaluation and Removal of Micropollutants and Radioactives (INCT-DATREM), Institute of Chemistry, P.O. Box 355, 14800-900 Araraquara, SP, Brazil. A R T I C L E I N F O Keywords: Electrocoagulation Electroflotation Electrochemical reactor Wastewater treatment A B S T R A C T The electrocoagulation (EC) process is an electrochemical means of introducing coagulants and removing sus- pended solids, colloidal material, and metals, as well as other dissolved solids from water and wastewaters. EC process has been successfully employed in removing pollutants, pesticides, and radionuclides. This process also removes harmful microorganisms. More often during EC operation, direct current is applied and electrode plates are sacrificed (dissolved into solution). The electrodissolution causes an increased metal concentration in the solution that finally precipitates as oxides and hydroxides. Due to the process design and low cost material, the EC process is widely accepted over other physicochemical processes. In this frame, this paper presents a general review of efficient EC technologies developed to remove organic and inorganic matter from wastewaters for environmental protection. Fundamentals and main applications of EC as well as progress of emerging EC treatments are reported. The influence of iron or aluminum anode on depollution of synthetic or real effluents is explained. The advantages of EC mechanisms with Al and Fe electrodes are extensively discussed. There are presented the advanced EC processes with in situ generation of hydroxyl radical. The importance of the oper- ating parameters for efficient application of the EC process as well as the combination of this electrochemical technology with electroanalysis techniques and other technologies are commented. 1. Introduction The limitation of hydric sources and the environmental impact to the planet health of polluted wastewater is nowadays an undeniable worldwide concern. Then, water pollution and water recycling are one of the greatest environmental challenges of XXI century [1]. In this context, water treatment technologies emerge as the most direct solu- tion to reduce the pollution impact in water bodies. Centralized water and wastewater treatment plants try to deal with this environmental issue. Among all the water technologies, physico-chemical processes are the most used technologies because these have been known and applied since centuries to make water drinkable for human intake [2]. How- ever, nowadays due to the technological development and the in- dustrial activity the pollutants contained in waters are completely dif- ferent from those of ancient times. Thus, the water treatment technologies have been a hot topic of research to remediate the emer- gent pollution. In this context, EC is an electrochemical technology with wide range of application that can reduce effectively the presence of several pol- lutants in water from heavy metals until persistent organic pollutants. During the last decades this promising technology has been extensively studied to understand its principles, parameters of influence, removal mechanisms and to evidence its applications [3–9]. However, these authoritative reviews summarized and discussed specific cases and no mention about the emerging EC technologies is done (e.g.: photoEC, peroxo EC and coupled EC approaches). For this reason, this review aims to be a reference document that summarizes the fundamentals of EC technologies including, for first time, the advanced EC with in situ generation of oxidant species to improve the pollutants removal effi- cacy as well as the coupling of on-line electroanalytical technologies to http://dx.doi.org/10.1016/j.jelechem.2017.07.047 Received 28 November 2016; Received in revised form 14 July 2017; Accepted 26 July 2017 ⁎ Corresponding author at: Laboratório de Eletroquímica Ambiental e Aplicada (LEAA), Institute of Chemistry, Federal University of Rio Grande do Norte, Lagoa Nova, CEP 59078-970 Natal, RN, Brazil. E-mail address: carlosmh@quimica.ufrn.br (C.A. Martínez-Huitle). Journal of Electroanalytical Chemistry 801 (2017) 267–299 Available online 29 July 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved. MARK http://www.sciencedirect.com/science/journal/15726657 http://www.elsevier.com/locate/jelechem http://dx.doi.org/10.1016/j.jelechem.2017.07.047 http://dx.doi.org/10.1016/j.jelechem.2017.07.047 mailto:carlosmh@quimica.ufrn.br http://dx.doi.org/10.1016/j.jelechem.2017.07.047 http://crossmark.crossref.org/dialog/?doi=10.1016/j.jelechem.2017.07.047&domain=pdf follow the pollutants abatement. Also, the combination of EC with membrane filtration has been introduced. Furthermore, a comprehen- sive and general review about the works reported in the literature has been done in order to become a communication of researchers' ex- perience to stimulate the launch of novel and revolutionary ideas to improve the process performance and future applications. 2. Fundamentals Coagulation is a traditional physico-chemical treatment via phase separation for the decontamination of wastewaters before discharge to the environment [9]. EC is directly related to conventional coagulation process, which has been used as a method (early as 2000 BCE) for water clarification and potabilization [2] and nowadays, it is still extensively used [10]. The process is based on the formation and aggregation of a colloidal system and its further coagulation enhanced by the use of the coagu- lating agents. Metallic and organic pollutants are separated from the aqueous phase by their precipitation with the coagula and subsequently removed from the treated water [11,12]. The aggregates formation is explained by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory where it is assumed that the formation of an aggregate depends on the interaction forces by the sum of Van der Waals and double layer forces [13]. The simplest consideration is the symmetric system (homo-ag- gregation) where the double layer force is repulsive and the Van der Waals forces attractive; then, the attractive force has to overcome the repulsive force in order to form the aggregate. Meanwhile, hetero-ag- gregation systems are from far more complex due to the dual character of the double layer forces which could be attractive, repulsive or both effects simultaneously (while Van der Waals forces are normally at- tractive). Nevertheless, DLVO theory cannot totally explain the coagulation phenomena because in this complex system other interactive forces than electrostatic repulsion (e.g.: hydration, hydrophobic interactions and so on) are involved on the colloids stabilization [14,15]. In coagulation water treatment process, the addition of coagulating agents (such as Fe3+ or Al3+ salts) favors the formation of pollutant aggregates [16], their coagulation and after that, their physical separation from water by precipitation or flotation [17,18], allowing the removal of metal and organic pollutants from water by different coagulation mechanisms, which will be discussed in the subsections below. By adding coagulant agents into water, in general, a decrease on the distance of the electrical double-layer is promoted (due to the counter-ions (coagulants) concentration increase in solution, which reduces the electrostatic repulsion by the pollutants charge shielding [13,19]), diminishing the surface potential and the energy barrier re- quired to form easily the aggregate. Considering the EC approach, similar effects to conventional coa- gulation can be produced [20,21]. This technique uses a current to dissolve Fe, Al or other metals as sacrificial anodes immersed in the polluted water. The electrodisolution promotes an increase on the metal ions in solution or their complexed species with hydroxide ion de- pending on the pH conditions and the sacrificial anode used [22–24]. These species act as coagulants or destabilization agents, helping to separate pollutants from the wastewaters [25]. In general, specific steps take place during an EC treatment [26–28]: (i) Electrodic reactions that produce metal ions from anodes electro- dissolution, and H2 gas evolution at the cathode, (ii) Destabilization of the pollutants, particulate suspension and breaking emulsions, (iii) Formation of aggregates of the destabilized phases and its coagu- lation in the wastewater as flocs, (iv) Removal of coagulated pollutants by sedimentation or by electro- flotation by evolved H2 (electroflotation can be used to disperse the coagulated particles via the bubbles of H2 gas produced at the cathode from water reduction reaction, transporting the solids to the top of the solution), (v) Electrochemical and chemical reactions promoting the cathodic reduction of organic impurities and metal ions onto the cathode surface. Considering the features of EC approach, it presents many ad- vantages to the conventional physico-chemical treatment of coagula- tion. The main advantages that have been reported by several authors [20,29,30] are listed below: Abbreviations AAS Atomic Absorption Spectroscopy AC Alternating current ADE Air diffusion electrode of C-polytetrafluoroethylene ADECNT Air diffusion electrode of carbon nanotubes Alalloy Aluminum alloy BP-S Bipolar electrodes in series connections CMP Chemical Mechanical Polishing COD Chemical oxygen demand (mg of O2·L−1) DC Direct current TOC Total organic carbon (mg of C·L−1) TCO Total cost of operation ($·m−3) DLVO Derjaguin-Landau-Verwey-Overbeek theory DSA Dimensionally stable anode EC Electrocoagulation EEC Electrical energy consumption (kWh·m−3) Gr C graphite i.e. Inner electrode MP-P Monopolar electrodes in parallel connections MP-S Monopolar electrodes in series connections NOM Natural organic matter o.e. Outer electrode PZD Predominance-zone diagrams SEEC Specific electrical energy consumption (kWh·kg−1 of electrode dissolved) SEECP specific electrical energy consumption per pollutant mass (kWh·kg−1 of pollutant) SS Stainless steel St Steel Stwool Steel wool UV Ultraviolet Y Percentage of pollutant removed Symbols E Electrical potential (V) F Faraday constant (96,487C mol−1) I Current (A) j Current density (mA cm−2) Mpol Molecular weight of pollutant (g mol−1) Mw Molecular weight (g mol−1) n Number of electrons R Electrical resistance (Ω) tEC Time of electrocoagulation treatment (h) VS Volume treated (m3) Δmexp Experimental electrodic mass loss Δmtheo Theoretical electrodic mass loss φ Efficiency of anodes dissolution S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 268 (i) More effective and rapid organic matter separation than in coa- gulation, (ii) pH control is not necessary, except for extreme values, (iii) Coagulants are directly electrogenerated, thus chloride or sulfate ions are not added to the solution and consequently, eliminating competitive anions; allowing a maximum adsorptive removal, (iv) The highly-pure electrogenerated coagulant improves the pollu- tants removal, then, a smaller amount of chemicals is required, (v) A direct consequence of (iv) is the lower amount of sludge pro- duced, (vi) The operating costs are much lower than conventional technolo- gies. However, this method presents some major disadvantages [31,32] related to: (i) The possible anode passivation or/and sludge deposition on the electrodes that can inhibit the electrolytic process in continuous operation mode, (ii) Even though lower amount of sludge is produced in comparison with coagulation, the treated effluents still present high con- centrations of iron and aluminum ions in the effluent that avoid their direct release to the environment. Thus, a post-treatment to reduce the metallic ions concentration after the electrochemical process is required in order to attend the environmental legisla- tions, (iii) The sacrificial anodes are consumed and must be replaced peri- odically. (iv) Deposition of hydroxides of calcium, magnesium, etc., onto the cathode, avoiding the release of H2 and the pass of current, when using actual wastewaters. This can solved using alternate current with same anode and cathode materials. 2.1. Sacrificial anode materials for electrocoagulation In this subsection will be presented the main materials used as sa- crificial anodes in EC. The anodic dissolution of the anodes releases in the water the coagulants responsibles of the pollutants removal. 2.1.1. Iron and steel anodes When an iron, steel (St) or stainless steel (SS) anode is used in EC, Fe2+ is dissolved in the treated wastewater by Fe oxidation at the anode, as follows [33,34]: → ++ −Fe Fe 2e2 (1) Meanwhile, hydroxide ion and H2 gas are generated at the cathode from the water reduction reaction: + → +− −2 H O 2e 2 OH H (g)2 2 (2) Significant OH− production from reaction (2) causes an increase in pH during electrolysis leading to the formation of different iron hy- droxocomplexes in solution. Fig. 1 presents the predominance-zone diagrams (PZD) of stability of iron(II) and iron(III) and their hydro- complexes as a function of pH [18,35], which is a control parameter of coagulation in EC. As can be deduced from Fig. 1a, insoluble Fe(OH)2 precipitates at pH > 5.5 and remains in equilibrium with Fe2+ up to pH 9.5 or with other monomeric species such as Fe(OH)+ from pH 9.5 up to 11.4 and Fe(OH)3− from 11.8 to 14.0. The formation of insoluble Fe(OH)2, which favors the coagula precipitation, can be written as [20]: + →+ −Fe 2 OH Fe(OH) (s)2 2 (3) and the overall reaction for the electrolytic process from the sequence of reactions (1)–(3) is: + → +Fe 2 H O Fe(OH) (s) H (g)2 2 2 (4) Even though iron(II) species can generate coagulates, the iron(III) species are those that present higher charge density favoring even more the coagulation-flocculation process. This performance is related to an efficient decrease on the electrical double-layer by major charge va- lence of the metal ions used as coagulant. Thus, the higher charge va- lence the coagulant ion carries, the less the dosage required to obtain the same results [13,14,36]. In the case of iron(III), this species could be directly electro- generated from the sacrificial anode depending on the voltage applied by direct charge transfer (5) involving the anode electrodissolution. Besides, Fe(II) could be easily oxidized by reaction (6) to insoluble Fe (OH)3 in the presence of O2, which is commonly dissolved in water [9,20,37]: → ++ −Fe Fe 3e3 (5) + + → ++ +4 Fe 10 H O O (g) 4 Fe(OH) (s) 8H2 2 2 3 (6) where protons can be neutralized with the OH− produced in reaction (2) or directly reduced to H2 gas at the cathode by means of reaction (7): + →+ −2H 2e 2 H (g)2 (7) The PZD of iron(III) shown in Fig. 1b evidences that Fe(OH)3 coa- gulates since pH > 1.0. Then, this predominant precipitated species is in equilibrium with different soluble monomeric species as a function of the pH range [35]. Thus, Fe(OH)3 is in equilibrium with Fe3+ up to pH 2.0, Fe(OH)2+ from 2.0 up to 3.8, Fe(OH)2+ from 3.8 up to 6.2 and Fe(OH)4− from 9.6 and so on. It is important to indicate that in the diagram, Fe(OH)3 is the unique species present in solution in the range of pH between 6.2 and 9.6. Besides, the complexes have a significant tendency to polymerize as Fe2(OH)24+ and Fe2(OH)42+ complexes between pH 3.5–7.0 [38] depending on the applied current density and -2 0 2 4 6 8 10 pF e( II ) Fe2+ Fe(OH)+ Fe(OH) 2(s) Fe(OH) 3 Fe(OH) 2 − -2 0 2 4 6 8 pF e( II I) Fe3+ Fe(OH)2+ Fe(OH) 3(s) Fe(OH) 4 Fe(OH) 2 − Fe(OH) 3+ 0 2 4 6 8 10 12 14 pH Fig. 1. Predominance-zone diagrams for iron species in aqueous solution in function of pH. Iron species: (a) Fe(II), (b) Fe (III). S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 269 the electrolysis time. All these species with different protecting charge and electrostatic attraction favor the coagulum formation/precipita- tion, in major or less extent, depending on the pollutant characteristics. However, among all the iron(III) species, Fe(OH)3 is considered to be the preferred coagulant agent and the main responsible of pollutants removal. 2.1.2. Aluminum anode In the case of EC with Al, the anodic reaction (8) leads soluble Al3+ [39,40] while the cathodic reaction produces hydroxide ion and H2 gas by reaction (2). → ++ −Al Al 3e3 (8) Aluminum ions in the aqueous medium present a complex equili- brium with different monomeric species such as Al(OH)2+, Al(OH)2+, Al(OH)3 and Al(OH)4− depending on the pH conditions [41], as it is shown in the PZD of Fig. 2 [42]. Several authors have reported the polymerization of the former monomeric species as Al2(OH)24+, Al6(OH)153+, Al7(OH)174+, Al8(OH)204+, Al13O4(OH)247+ and Al13(OH)345+ [40,42,43]. However, the main responsible of the floc- cules and aggregates formation is Al(OH)3, which is formed by complex precipitation mechanisms from the soluble monomeric and polymeric cations. Being the overall reaction (9) in the bulk: + → +Al 3 H O Al(OH) (s) 3 2 H (g)2 3 2 (9) 2.2. Other anodes Although iron/steel and aluminum anodes are the preferred sacri- ficial anodes used in EC, some works have proposed the use of alter- native anodic materials such Zn and Mg anodes [44–46]. The principles are the same that the stated formerly for Al and Fe anodes, consisting on the anodic dissolution of the anodes by reactions (10) and (11) for Zn and Mg, respectively. The ions generated undergo further the formation of their corresponding hydroxides depending on the pH. Zn(OH)2 and Mg(OH)2 formed by Eqs. (12) and (13) are the main species that cause the pollutants coagulation [24]. → ++ −Zn Zn 2e2 (10) → ++ −Mg Mg 2e2 (11) + → ++ +Zn 2H O Zn(OH) (s) 2 H2 2 2 (12) + → ++ +Mg 2H O Mg(OH) (s) 2 H2 2 2 (13) In the case of aluminum and iron ions, these present a major charge valence than that for zinc and magnesium ions, which favors the coa- gulation process with lower coagulant concentration. However, the evaluation of other coagulants electrochemically generated is related to the residual concentrations of the coagulants that remain in the water after the treatment. For instance, United States Environmental Protection Agency (USEPA) suggests limiting concentrations for alu- minum of 0.2 mg/L to avoid health problems; while for magnesium, 30.0 mg/L is the limit established [45]. 2.3. Pollutants removal mechanisms The mechanisms involved in EC are not clearly understood yet [27], but during the last decade several researchers [47–49] have tried to elucidate the mechanisms involved during the removal of pollutants. This subsection presents a brief overview to give an insight on the most important mechanisms considered during EC for removing pollutants from water, which are summarized in Fig. 3. These are classified into two main groups: 2.3.1. Heavy metals removal Heavy metals are mainly removed by EC by two mechanisms: (i) surface complexation and (ii) electrostatic attraction. But, it is im- portant to consider that, the insoluble flocs of the coagulant metal- hydroxide are produced independently on the removal mechanism [37]. Meanwhile, other mechanisms are feasible, such as (iii) adsorp- tion and (iv) direct precipitation by the formation of the pollutant metal hydroxides. The complexation mechanism considers that the heavy metal can act as a ligand to form a complexation bond to the hydrous moiety of the coagulant floc (mainly Fe(OH)3 or Al(OH)3) yielding a surface complex. Subsequently, the formation of these complexes; superior aggregates are formed and the coagula precipitate, allowing the se- paration of the pollutants from the aqueous phase: + → +Metal (HO)OFe metal–OFe H O(s) (s) 2 (14) + → +Metal (HO)OAl metal–OAl H O(s) (s) 2 (15) The second mechanism considers electrostatic attraction between the heavy metal pollutant and the coagulant floc. On the basis of ex- istence of areas of apparent positive or negative charge in the floc, the negative apparent charge area attracts the heavy metal in solutions allowing their coagulation in the floc, which finally precipitates. Furthermore, the large surface areas of freshly formed amorphous coagulant flocs can also adsorb soluble ions and/or trap colloidal par- ticles, which are separated from the aqueous solution by a third me- chanism [50]. It is necessary to indicate that coagulation is not the only removal mechanism of heavy metals in EC. Electrochemical reduction of these species onto the cathode surface is also feasible, improving the removal efficiencies of these pollutants [51]. 2.3.2. Organics removal The different nature of organic pollutants, depending on their structures and functional groups, has an important influence on the mechanisms, involving their coagulation. The main mechanisms of or- ganic pollutants are: complexation, charge neutralization, entrapment, adsorption and/or the combination of them [47,52]. The complexation mechanism is similar to that described on heavy metals removal, where the organic pollutant acts as a ligand. Thus, the organic pollutant is coordinated to the metallic center by their func- tional groups and precipitates within the coagulant floc. On the other hand, the charge neutralization or destabilization is one of the most common mechanisms with organics. The coagulants act as charge shielding, consequently, the double layer of pollutant is compressing, thus favoring the formation of aggregates and their sub- sequent precipitation. Meanwhile, entrapment mechanism consists on the trapping of or- ganic molecules in the hydroxo-metallic coagula that drag the -12 -10 -8 -6 -4 -2 0 0 2 4 6 8 10 12 14 L og ( [A l x(O H ) y3x -y ]/ m ol d m -3 ) pH Al3+ Al(OH)2+ Al(OH) 4Al(OH) 2 − Al(OH) 3 + Fig. 2. Predominance-zone diagram for aluminum species in aqueous solution in function of pH. S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 270 pollutants with them. Consistently, this mechanism is preferred at high dosages of coagulant in solution. The last mechanism is called adsorption, which presents similarities to entrapment approach but with a slight difference. While in entrap- ment the pollutant is physically dragged by the coagula; the pollutant presents physico-chemical interactions that favor its adsorption onto the coagulant species surface during adsorption approach. The predominance of each mechanism depends on the organic pollutant nature (charge, size, hydrophobicity, etc), the coagulant type and its dosage. However, the pH and other water matrix effects can also influence markedly on the EC performance [13]. It is important to highlight that the general mechanism to remove pollutants by EC is very complex because different ways could coexist simultaneously, enhancing the removal efficiency [48]. 2.4. Faraday law Faraday's law (m= AWIt/zF) is obey when EC is applied [53], where m is the total mass of iron or aluminum (g), AW is the atomic weight of the elemental coagulant precursor (i.e., 55.85 g mol−1 for Fe and 26.98 g mol−1 for Al), I is the electric current (A), t is time (s), z is number of electrons transferred, and F is the Faraday's constant (96,486C/eq) [54–57]. Then, electrodissolved coagulant concentrations increase linearly with the amount of electrical charge passed, as pre- dicted by Faraday's law [22,58–60]. Accurate Faraday's law predictions also arise from vigorously cleaning the anode prior to each experiment [22,61] and it also allows to consider the effects of chemical dissolu- tion, pitting corrosion, and chlorine generation. Electrodissolved iron and aluminum undergo hydrolysis to form various mono, di, and polynuclear complexes, which behave as Brønsted acids, consume buffering capacity and tend to reduce pH [62–65]. Hydroxyl ions re- leased at the cathode tend to neutralize the Brønsted acidity of hy- drolysis products even causing a temporary upward drift in pH for ty- pical initial pH values (depending on the buffering capacity of the feed water, current density, and electrolysis duration [22,55,61], as dis- cussed below). Variations in pH conditions have been reported for high alkalinity waters or when electrolysis is performed for short times [54,61,66]. Electrodissolution of highly soluble Fe(II) has been con- firmed by direct aqueous phase measurements [53,67–69] and it can be problematic since it does not directly induce sweep coagulation. Con- sequently, Fe(III) is preferred over Fe(II) as a coagulant for water pur- ification applications (see below sections). 3. Factors affecting electrocoagulation 3.1. Effect of electrode material Obviously, the choice of electrode material is one of EC control parameters that not only impacts the performance and efficiency of the process, but it is also associated to the cost. In the case of EC efficiency, the anodic dissolution, the percentage of pollutant removed and the coagulant required are significant parameters that play an important role. These are directly associated to the ionic metallic species that are released. In this frame, higher charge valence metal-ionic coagulants are preferred due to their greater electrical double-layer compression that enhances the pollutants coagulation. Typically, aluminum and iron electrodes are used because of the coagulating properties of multivalent ions [36]. Nevertheless, other feature is that aluminum and iron chloride salts are the most used coagulants and the most conventionally accepted in coagulation water-treatment [18]. Besides, these materials are also preferred for their easily availability, their low cost and their high electrodissolution rates. 3.2. Effect of pH The pH of the solution plays an important role in electrochemical and chemical coagulation processes [16,70]. The first effect is related to the coagulant in solution that presents different species in equilibrium depending on the pH: the metal ionic species, the monomeric hydro- xide-complexes and the polymeric hydroxide-complexes. The distribu- tion of these species as a function of the pH are usually presented in the PZD by using the relationship between the negative value of the loga- rithm concentration expression as a function of the pH [35,42], giving valuable information about the distribution of these species in the equilibrium [18]. The type and quantity of these species are so relevant because each one of them present different interactions with pollutants, giving different coagulation performances. For example, the species in high alkaline conditions for aluminum and iron anodes are Al(OH)4− and Fe(OH)4−, respectively. These species present poor coagulative activity [71]; then, typically (excluding some polyaluminum products), the coagulation is performed at slightly acidic conditions (Fe: 4–5 and Al: 5–6). pH conditions significatively vary the physiochemical prop- erties of coagulants, such as: (i) the solubility of metal hydroxides, (ii) the electrical conductivity of metal hydroxydes and (iii) the size of colloidal particles of coagulant complexes [72,73]. Thus, neutral and alkaline media are preferred for coagulation. Metals cathodic electrodeposition Me x+ Adsorption Complexation Cathodic reduction Charge neutralization or destabilization Entrapment Fig. 3. Scheme of the most important mechanisms of pollutants removal by electrocoagulation technologies. S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 271 The second noteworthy effect is related to the changes on the che- mical structure of the pollutants due to the effect of the pH. Protonation/deprotonation of functional groups of the pollutants, de- pending on their pKa, directly affects the pollutants net charges as well as their electrostatic interactions. Hence, modifying the double-layer and consequently affecting the aggregates formation. Due to the dif- ferent physico-chemical character of the pollutants as a function of pH, this parameter has to be optimized according to the target pollutant nature and the effluent conditions [26,74,75]. Nevertheless, other effects can be also related to the pH, such as, the influence of other species present in the actual water matrix. This is the case of different anions that could be affected by pH, affecting their apparent charge and consequently their influence on the double-layer shielding of the coagulants [72,76] or their oxidative character [77]. Thus, this inert species have an effect on the optimum pH condition for EC processes. 3.3. Effect of current density The applied current density (j) controls the electrochemical reac- tions that take place [9,20] in solution (e.g.: electrodissolution rate, gas evolution, electroflotation, water reactions, etc. [17]) as well as their extension and kinetics. Consequently, the j defines (with the applied potential) the energy consumption associated to the operation of the electrochemical process. In general, direct current (DC) is the kind of electric current more extensively used in EC [78]. However, the anodic surface can be iso- lated by the formation of a stable oxide layers due to the oxidation reactions that promote the corrosion phenomena, generating passiva- tion effects. The passivation of the sacrificial anode increases the ohmic resistance (R), and consequently, the cell potential rises, increasing the operational costs, but the passivation decreases considerably the EC efficiency [79]. The use of alternating current (AC) can be considered as an alternative because the continuous changes of polarity avoid or re- duce the formation of passivation layers and enlarge the operational life of the sacrificial anodes [80]. 3.4. Effect of supporting electrolyte In electrochemical processes, the supporting electrolyte is required in solution that avoids migration effects and contributes to increase the solution conductivity, diminishing the ohmic drop and the energy con- sumption [9,20]. Alternatively, the electrolyte has appreciable effects on the electrodissolution kinetics of the sacrificial anodes and it can also influence the double layer shielding by the coagulants to form the flocs [81]. Several authors have studied EC process with different supporting electrolytes [23,77,82–85], where the different influences are usually associated to the anion effects rather the cations [86]. In this section, we will discuss about the influence of the electrolyte cationic and anionic nature on the EC by using Al and Fe anodes. The further discussion will be done in means of the commonly used anions (Cl−, SO4 2−, NO3 −) and cations (Na+, K+, NH4 +), although in complex water matrix, they could coexist between them and with other ionic species. Some authors have reported an appreciable affinity of sulfate spe- cies to form complexes with aluminum [87] passivating the anodic surface. Indeed, more positive potential must be applied to incentive the anodic dissolution avoiding passive action regarding the aluminum oxidation. In fact, in case of sulfate presence, a ratio of [Cl−] / [SO4 2−] > 0.1 is suggested to ensure an efficient release of aluminum cations during EC with Al anodes [86]. Conversely, sulfate has not complexation affinity with iron and it does not inhibit iron anodes oxidation. Meanwhile, nitrates inhibit electrodissolution at both sacri- ficial anodes, being required higher applied potentials to oxidize them [77,81]. On the other hand, chloride medium favors significantly the EC process independently of the anodic material used owing to significant corrosive power of chlorides that promotes the release of coagulant species [88,89]. Thus, voltages required for electrodissolution are ap- preciably lower in the presence of chlorides as supporting electrolyte than those required at sulfate or nitrate-based electrolytes [88]. Moreover, the effect of the electrolyte can be also observed from the modification of the electrode surface during the EC treatment, as re- ported by Hu and co-workers [88]. By using sulfate medium, localized pitting is observed, evidencing uniform corrosion rate on the entire electrode surface. Instead, when nitrate is used, crevice corrosion oc- curs. Conversely, in chloride medium the electrode surface presents numerous pits and holes distributed on the surface of the anode, while other parts remain smooth. This is the typical localized pitting corro- sion induced by halogens [88]. These different corrosion types, during anodic dissolution, are related to the chemical reactions involved in the presence of different electrolytes as well as the pH conditions. Sulfate anions are considered inert electrochemical species; while nitrate and chloride are susceptible to electrochemical reactions. Nitrate anions can be reducted by reactions (16)–(18), producing hydroxyde which basifies the solution and consequeltly difficults the anodic disolution by the formation of oxide insoluble films on the anode surface. The formation of the insoluble films produces the formation of crevices on the anode surface [88]. + + → +− − − −3 NO 3H O 6e 3 NO 6 OH3 2 2 (16) + + → +− − −3 NO 18H O 24e 3 NH 27 OH3 2 3 (17) + + → +− − −6 NO 18 H O 30e 3 N 36 OH3 2 2 (18) Whilst, chloride is susceptible to oxidation reactions (19) that pro- duces chlorine that disproporcionates into hipochlorous acid and chloride by reaction (20). Afterwards hipochlorous acid leads to hipo- chlorite by the acid-base equilibrium (21) with pKa = 7.55 depending on the treated solution pH [9,20]. These active chlorine species are highly oxidants and favors the chemical oxidation of the anodic surface that produces the characteristic pitting corrosion. → +− −2 Cl Cl 2e2(aq) (19) + → + +− +Cl H O HClO Cl H2(aq) 2 (20) ↔ +− +HClO ClO H (21) In fact, it is feasible the formation of oxidizing species, such as ac- tive chlorine, that can oxidize organics during the EC process. It has been demonstrated in the very recent article where the case of bronopol was studied and compared to its treatment by EAOPs [90] as well as the EC. Then, this new advantage of the EC approches opens new alter- natives for the applicability of this tecnology or its combination with other processes. Regarding the cations, not enough information about their influence during EC has been reported. However, interesting assertions have been done in the last years. For example, a neutral role in EC was determined for sodium and potassium cations because no significant enhancements have been achieved when they are used [81]. Nevertheless, ammonium cation present in the solution enhances EC efficiency, especially with Al anodes due to its pH regulation effect [81,86]. This trend is due to the buffering effect of ammonium/ammonia couple [91]. Thus, the hy- droxyl ions electrogenerated at the cathode from water reduction re- action (2) are not only consumed to generate the hydroxo-complexes with the metals but also in means of the neutralization reaction (22) releasing ammonia, which is also in equilibrium (23) with ammonium with a pKa = 9.2: + → ++NH OH NH H O4 – 3 2 (22) + ↔+ +NH H NH3 4 (23) Under these controlled pH conditions (≈9.0), a significant amount of coagulants is formed [25], improving consistently the EC efficiency for removing pollutants. S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 272 3.5. Reactor design parameters Electrochemical reactor design is an indispensable stage to reach the maximum EC efficiency. In electrochemical technologies for waste- water treatment is especially relevant the minimization of the IR-drop between electrodes in order to enhance the electrochemical conversion efficiencies and the energy requirements [92]. Generally speaking, the design of EC reactors takes into account different parameters because the effluent and diverse solid species affect the hydrodynamic condi- tions in the reactor during the electrolytic process. In this frame, dif- ferent design inputs and typical reactors described in the literature will be presented in the following subsections. 3.5.1. Inter-electrode gap distance The space between the electrodes has a direct influence on the IR- drop that is minimized decreasing the distance between anode and cathode. However, lower removal efficiencies of the pollutants from water can be achieved when short distances between the electrodes are used because several phenomena can be affected (e.g.: coagulation, flocculation, precipitation, electroflotation, etc.). These effects impact the flocs formation and their precipitation [93], avoiding the formation of aggregates because the high electrostatic effect hinders the particles collision [94]. In contrast, an excessive distance between electrodes decreases significantly the formation of flocs [95,96]. 3.5.2. Electrode arrangements The connection mode of the electrodes in the EC cell affects not only the removal efficiency but also the energy consumption and the cost [97,98]. The most typical arrangements [7,94,97,99] are monopolar electrodes in parallel connections (MP-P), monopolar electrodes in se- rial connections (MP-S) and bipolar electrodes in serial connections (BP-S). These EC arrangements are schematized in Fig. 4. In monopolar electrodes arrangement, each one of the electrode work as anode or cathode depending on its electrical polarity in the electrochemical cell. The difference between the parallel and the serial connection is illustrated in Fig. 4. As it can be observed, in the MP-P, each sacrificial anode is directly connected with other anode in the cell; using the same condition for cathodes. Meanwhile, in the MP-S con- figuration, each pair anode-cathode is internally connected but they are not connected with the outer electrodes (see Fig. 4). In the case of the bipolar electrodes, each one of the electrodes, excepting the external ones, which are monopolar, present different polarity at each one of the electrode sides depending on the charge of the electrode in front it (see Fig. 4). The connection of bipolar electrodes is always in serial mode. It is noteworthy to mention that higher potential differences are required when a serial arrangement is used, but the same current is distributed between all electrodes (Fig. 5). Conversely, in parallel mode, the electric current is divided between the electrodes Fig. 4. Electrodes arrangements in electro- coagulation cells: (a) monopolar electrodes in parallel connection, (b) monopolar electrodes in serial connection and (c) bipolar electrodes in serial connection. Batch Vertical plate 91 % Other reactors 9 % Filter-press cell 72 % Rotating screw 8 % Cylindrical cocentrical electrodes 12 % Rotating cathode 4 % Electrode rod leads 4 % a b Fig. 5. Diagram of the different electrocoagulation reactors usage in the literature: (a) conventional reactors respect others and (b) the alternative reactors usage. S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 273 interconnected, as function of their resistance, in the electrochemical reactor. However, notorious advantages are achieved when parallel arrangements are used in terms of the energy consumption [27,97]. Several authors have compared the performances of these different electrodes arrangements but the results are not completely conclusive because the relative efficiencies strongly depend on operating para- meters discussed previously as well as the water matrix and the nature of the pollutant [7,95,97,99–101]. Nevertheless, restringing our con- clusions to the existing literature about the electrochemical reactors, the MP-P presents generally lower operational costs, while BP-S re- quires lower installation maintenance during its use and sometimes it favors higher pollutant removals [95,98,102,103]. 3.6. Electrocoagulation reactors The type of EC reactor influences on the process performances but it also affects its operation and scale-up (see, Fig.6). The reactor most extensively used is the open batch cell with plate electrodes (Fig. 6a). The electrodes are submerged in the solution, and the effluent is con- ventionally stirred to be homogenized. A variation of the typical batch cell with plate electrodes reactor consist in a cylindrical reactor with concentrically inner electrodes (Fig. 6b). These electrodes (anode and cathode) present a cylinder shape and are placed one inside the other [104,105]. Another feature is that the inner electrode can be replaced by a metallic rod [106]. As regards the currents applied to the inner electrode (i.e.) and the outer electrode (o.e.), it is important to consider that these electrical conditions can be different. A variation of the cylindrical reactor was reported by Un and co- workers [107], where the anode is a cylindrical electrode but the cathode consists in a rotating impeller with two metallic blades to homogenize mechanically the solution and prevent the particles settling in the reactor during the EC (Fig. 6c). However, other electrochemical reactors have been also applied in EC processes, as showed in Fig. 6. Other electrochemical reactor considerably used for EC is the ty- pical filter press-cell (Fig. 6d). Higher removal efficiencies have been achieved by using this kind of EC reactor for treating solutions con- taining metals, non-metallic inorganic and organics pollutants respect to the conventional open batch cell with plate electrodes (Fig. 6a) [108–110]. Other novel EC systems are the continuous reactors with rotating screw type electrodes (Fig. 6e). These have been used to treat cheese whey wastewater [111] and groundwater [112]. These cells are de- signed with a symmetrical section to favor uniform velocity distribution of the flowing liquid around a sacrificial anode rod with a helical cathode, and both electrodes are placed in the middle of the EC reactor (with or without rotation). Fig. 6. Scheme of the different electrocoagulation reactors reported in the literature. S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 274 4. Advanced electrocoagulation processes The advanced EC processes are emergent technologies that use the simultaneous generation of in situ hydroxyl (%OH) radicals and other chemical oxidants by different mechanisms. These highly oxidant spe- cies improve the pollutants removal due to (i) the acceleration of the anodes dissolution by chemical oxidation and (ii) enhance the organic pollutants abatement via the oxidation action of radical species (mi- neralization). These novel EC technologies are presented below. 4.1. Sono-electrocoagulation (Sono-EC) Sono-EC process is based on the combination of ultrasound irra- diation approach with the EC process. Sound energy agitates the sample and it promotes the mixing, emulsification, homogenization and/or dispersion of particles in solution as well as the cativation effect. However, the simultaneous ultrasonication could produce undesirable side-effects [113] such: (i) The destruction of a part of the obtained colloidal hydroxides by ultrasound-waves, which diminishes the solid phase and the re- moval of pollutants, (ii) The destruction of the formed adsorption layer at the colloid par- ticles surface that favors the redissolution of the adsorbed species, (iii) Disorganize the migration processes reducing the pollutants coa- gulation, and (iv) Difficult the flocs formation. Consistently with the first approach, sonication can dissolve or avoid the formation of flocs during the process which reduce the effi- ciency appreciably. However, the generation of high-energy micro- environments in the bulk by ultrasounds depends upon the insonation power and frequency applied [114]. Thus, it is possible to control the undesirable effects and enhance positive ones for EC [113,115]: (i) Sonication creates free radicals that improve the removal effi- ciency by organics oxidation, chemical polishing of the flocs sur- face and/or anodes dissolution by radicals oxidation, (ii) The frequency and the intensity of the collisions between the coagulant and the pollutant particles are promoted by ultrasonic mixing, producing a significant enhancement of the removal effi- ciency, (iii) Ultrasound waves reduce the thickness of the electrical diffusional layer improving the current efficiency, (iv) Ultrasonication reduces anodic passivation effect, and (v) The electrode surfaces can be activated due to the defects gen- eration in the electrodes crystal lattices. The removal efficiency of the pollutants from solution is appreciably increased in sono-EC in comparison with conventional EC process, de- pending on the water matrix conditions and the insonation power; for example, Raschitor et al. [115] reported an increase on the removal efficiencies from 60% in EC up to 95% by sono-EC. Thus, a significant amount of colloidal hydroxide species is produced when ultrasonication approach is used [115], and consequently, an important improvement for water purification is achieved. On the other hand, an increase on the energy consumption is achieved when both approaches are combined (electrical requirements due to the EC and the sonication process), which can be reduced when an optimization is performed. In summary, the sound generated in the treated solution does not destroy sig- nificantly the hydroxide solid phase and does not disturb the process of ion-molecular adsorption on the surface of the obtained colloidal par- ticles, obtaining a synergic effect when both processes are coupled [101,113]. 4.2. Photo-electrocoagulation (photo-EC) Ultraviolet (UV) irradiation is a well-known and extensively applied technology for water disinfection. The disinfection process is based on the UV radiation penetration of the cell wall affecting the genetic ma- terial (DNA and RNA) of bacterial and protozoa organisms, which ob- literate their reproduction [116]. In this frame, the applicability of EC coupled with UV irradiation was proposed as an alternative. Cotillas et al. [117] presented a novel approach by using the simultaneous implementation of UV irradiation during the EC process, where a sy- nergistic effect on the disinfection removal was observed. In this case, the implementation of UV irradiation affected slightly the turbidity reduction, but a great improvement on E. coli depletion rate was no- ticed. These effects are related to the light irradiation promotion of hydroxyl and chlorine radicals by means of hypochlorite decomposition by reactions (24)–(25) when light irradiation is applied. The results showed that it is necessary to apply 0.0085 and 0.085 kWh m−3 to achieve the maximum percentage of E. coli removal for current densities of 1.44 A m−2 and 7.20 A m−2, respectively. The differences observed in the required energy consumption can be related to the higher electric potential when current density increases and/or to the differences be- tween the initial concentrations of microorganisms (at 1.44 A m−2, E. coli0: 750 CFU 100 mL−1; while at 7.20 A m−2, E. coli0: 7000 CFU 100 mL−1). Nevertheless, the energy consumption necessary to obtain reclaimed water is lower than 0.1 kWh m−3 regardless the current density applied and the initial characteristics of the wastewater. On the other hand, the energy consumption required to achieve the complete disinfection of the effluent with the UV irradiation is much higher (around 1 kWh m−3). This result means that the main energy con- sumption of the combined process is related to the electricity consumed by the UV lamp. This parameter can be optimized, improving the ap- plicability of this emerging technology. It is important to remark that, when chloride is in solution, hypo- chlorite is formed from chloride oxidation according to reactions (19)–(21). It should be remembered that chloride ion is quasi-ubiqui- tous in water effluents. + → +− −hνClO O˙ Cl˙ (24) + → +−O˙ H O OH ˙OH2 – (25) Then, the generation of these oxidants enhances the electrode dis- solution by means of chemical oxidation reactions and favors the bac- teria depletion in water. Thus, the combination of UV radiation and EC by the so-called photo-EC approach enhances pollutants, bacterial and turbidity removal. In fact, the effect of active chlorine species was al- ready confirmed by Brillas e co-workers when EC approach was em- ployed [90], and the UV irradiation to promote the production of chlorine is well-known [20]. 4.3. Peroxi-coagulation and peroxi-EC Peroxi-coagulation or peroxi-electrocoagulation consists in the si- multaneous electrogeneration of hydrogen peroxide by the bielectronic cathodic reduction of oxygen by reaction (26) and the anodic dissolu- tion of iron as sacrificial anode [118–120]. In this case, hydrogen peroxide could be electrogenerated on carbonaceous materials such graphite (Gr), but with greater efficiency using air diffusion electrodes (ADE) with carbon-polytetreafluoroethylene or with carbon nanotubes (ADECNT). Under these conditions the hydrated Fe(III) species are generated as coagulants that remove pollutants by their precipitation. Additionally, homogeneous %OH radicals are generated in solution from Fenton's reaction (27) between Fe2+ species and electrogenerated hy- drogen peroxide leading more Fe3+ that enhances the coagulation process [121–123]. Thus, the radicals electrogenerated favor the sa- crificial anode dissolution by direct chemical oxidation reaction (28–29) and organic pollutants are mineralized to CO2, H2O and S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 275 inorganic ions. Nevertheless, competitive coagulation of by-products with the hydrated Fe(III) oxide can be also attained in concomitance with the organics oxidation [124–126]. + + →+ −O (g) 2H 2e H O2 2 2 (26) + → + ++ +Fe H O Fe ˙OH OH2 2 2 3 – (27) + →Fe 2˙OH Fe(OH)2 (28) + →Fe 3˙OH Fe(OH)3 (29) Barrera-Díaz et al. [127] reported a variation of Fe material in peroxi-coagulation by Cu sacrificial anode. Using this anodic material, the main coagulant species was the Cu(OH)2 and here the %OH was generated by the Fenton's like reaction (30) using Cu+ as catalyst in- stead of Fe2+. Where Cu+ is generated by the reaction of Cu2+ with hydroperoxil radicals by reactions: + → + ++ +Cu H O Cu ˙OH OH2 2 2 – (30) + → + ++ + +Cu HO ˙ Cu H O2 2 2 (31) Although the general mechanism that describes the removal en- hancement of peroxi-coagulation involves Fenton's and/or Fenton's like reactions; a peroxi-coagulation process using Al sacrificial anode in- stead Fe and Cu was initially proposed by Roa-Morales and co-workers [128], where the generation of hydroxyl radical is not related to Fen- ton's reaction (27) or Fenton's like reaction (30), but it is justified by the direct generation on the anode surface (M) following a similar me- chanism to the proposed by Miller and Valentine [129] involving re- actions with oxygen reactive species (32)–(36) and the cathodic gen- eration by reaction (37): + → + ++M H O M ˙OH OH2 2 – (32) + → + ++ +M H O M H HO ˙2 2 2 (33) + → + ++ +M HO ˙ M H O2 2 (34) + + → ++ +M HO ˙ H M H O2 2 2 (35) + → ++M ˙OH M OH– (36) + + → +− +H O e H H O ˙OH2 2 2 (37) The peroxi-coagulation with Al anode shown also better perfor- mances which are related to the synergic effect between the %OH and EC, like in the case of Fe anodes [127]. On the other hand, peroxi-coagulation can be implemented with simultaneous UV irradiation, commonly called as photoperoxi-coagu- lation or peroxi-photoelectrocoagulation [130–132]. The simultaneous irradiation with UV light promotes photochemical reactions that en- hance and accelerate the Fenton's reaction, improving the pollutants mineralization by (i) the photolysis of Fe3+ complexes with some or- ganics and degradation products such as the photodecarboxylation of carboxylic acids by general reaction (38) [133]; (ii) the additional photoreduction of Fe(OH)2+ species by reaction (39) [130]; and the feasible photodecomposition of H2O2 by reactions (40)–(41) and/or hypochlorite generating oxidant species by reaction (24). + → + ++ +hνFe(OOCR) Fe CO R˙2 2 2 (38) + → ++ +hνFe(OH) Fe ˙OH2 2 (39) + →hνH O 2˙OH2 2 (40) + → +hνH O H˙ HO ˙2 2 2 (41) The implementation of these photochemical processes favors the oxidation of organic pollutants thanks to the combination of photo- Fenton processes, opening novel alternatives for removing different organic pollutants in water [130,131]. 4.4. Electrocoagulation and membrane filtration Microfiltration (MF) is widely used in a large variety of filtration processes of aquatic solutions containing natural organic matter (NOM), such as: membrane bio-reactor, pretreatment for seawater and wastewater desalination plants, filtration of drinking water, tertiary treatment of wastewater for agricultural irrigation, and treatment of industrial wastes [53,134–138]. However, severe NOM–colloidal fouling is achieved when MF is used for removing NOM or to filter aquatic-NOM solutions [139]. The fouling intensity is governed by very complex relationships between the NOM properties (size, hydro- philicity, and charge), membrane characteristics (hydrophilicity, sur- face charge, and roughness), and the solution chemistry (e.g., pH, di- valent ions such as Ca) [140]. The severe NOM–colloidal fouling in MF has motivated intensive scientific efforts to develop and research fouling mitigation strategies. One potential MF fouling mitigation method that was suggested in re- cent years is pretreatment by EC [66,141]. The coagulants (iron or aluminum) produced in situ are added to the solution by dissolving the anode in an electrochemical cell. These coagulant ions ultimately lead to aggregation of the original particles in the water, which are later removed by sedimentation or filtration processes [142]. While several research groups have observed significant colloidal fouling mitigation in MF as a result of pretreatment with aluminum-based EC [143], the effects of iron-based EC pretreatment in solutions that contain NOM are still being debated. Bagga et al. observed only marginal fouling miti- gation due to pretreatment of iron-based EC in dead-end MF of river water [54]. This marginal effect on fouling was attributed mostly to the presence of NOM, which is prone to complex with electrochemically dissolved ferric ions, and thus reduces coagulation process efficiency. On the other hand, Adin et al. obtained significant fouling mitigation due to iron-based EC pretreatment in MF of both synthetic silica solu- tion without NOM and secondary effluent that contained organic matter [144,145]. Unlike the effect on fouling, the effects of EC pretreatment on contaminant removal abilities in MF were seldom investigated. Improvement resulting in 4-log or greater virus removal rates was ob- served in synthetic water without NOM, as a result of pretreatment of iron-based EC with MF [70,146]. However, the same authors observed low virus removal rates in hybrid EC + MF in the presence of NOM. This observation was explained by the tendency of NOM to complex with ferric ions and thus to lower flocculation efficiency. Regarding heavy metals, Mavrov et al. [147] observed very high and improved removal (> 98%) of several types of heavy metals in hybrid iron-based EC and MF. Regarding aluminum-based EC, to the best of the authors' knowledge, the effect on MFcontaminant removal rates was not pre- viously reported in the scientific literature. However, to date, there has been no comprehensive research on the effect of EC on the perfor- mances of MF for NOM removal, meaning, both the effects on fouling intensity and on removal rates [53]. Consequently, the overall potential of EC &MF as NOM removal methods is not clear. The impreciseness regarding the performance of hybrid EC &MF is enhanced by the am- biguous conclusions found in the literature about the efficiency of iron- based EC as a fouling mitigation method and its effect on virus removal rates in the presence of NOM. Therefore, Ben-Sasson et al. [148] studied the potential of hybrid EC &MF as a NOM removal method. The fouling mechanisms and NOM removal rates were explored for both aluminum and iron anodes under near neutral conditions (pH 6–8). The perfor- mance of the hybrid EC-MF process was compared to UF in order to evaluate its desirability for NOM removal. They concluded that, pre- treatment with both iron- and aluminum-based EC can improve MF filtration of solutions containing NOM in two ways: (1) it may sig- nificantly mitigate NOM fouling and consequently, reduce the filtration energy consumption, and (2) it dramatically improves the ability of the process to remove NOM. The effect of EC on filtration performance is highly dependent on solution pH, anode material, EC treatment time (coagulant dose), and NOM type and concentration. EC treatment times S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 276 that are too short may lead to deterioration of fouling as compared to MF without EC. A pH value of 6 seems to be the best for achieving lower fouling and high NOM removal for both iron and aluminum electrodes. At pH values of 7 and above, iron-based EC led to stronger fouling mitigation and better NOM removal than aluminum-based EC. The filtration performance and NOM removal ability of the EC-MF hybrid process were superior to those of UF. This emphasizes the potential of using a hybrid EC-MF process as an alternative treatment to UF for removal of NOM [148]. In recent authoritative review by Chellam and Sari [53], they have summarized and discussed the results regarding the integration of aluminum EC and MF for drinking water treatment while including limited information on iron EC/MF. They have indicated that aluminum EC significantly reduces MF fouling by inducing the formation of a cake comprised of particles larger than in the raw water. However, alu- minum flocs can compact or compress and relatively worsen MF fouling at higher pressures. New results are also included showing significant improvements in microfiltered water quality by EC pretreatment. Al (OH)3 flocs sorb NOM and DBP precursors, which are then retained on the MF membrane surface. EC/MF induces a slight shift towards bro- minated THMs and HAAs by increasing the Br−/DOC ratio compared to the raw water. In recent reports they have showed that viruses are ef- fectively sweep coagulated by EC and removed subsequently by MF. A thick cake layer of Al(OH)3 flocs further improves virus removal by acting as a dynamic membrane. In this context, they suggest that EC/ MF systems are promising alternatives for small-scale decentralized facilities because they inherently provide multiple barriers against contaminants of health concern and minimize membrane fouling while requiring limited operator attention [142]. 5. Electrocoagulation process with simultaneous coupled electroanalysis EC process is greatly used for removing different heavy metals as pollutants from water bodies, where many conventional analytical methods are used to quantify the remaining concentration in solution after the treatment. Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma-Atomic Emission Spectrometry are ex- tensively used to this purpose achieving great results, but these methods are expensive. Electroanalytical methods are fast and cheap methods of analysis with limits of quantification at ppb range, because of that are considered as a potential alternative to determine the re- sidual concentration of pollutants. Therefore, associated technologies for heavy metals determination during its elimination by electro- chemical treatments like electrocoagulation are being developed. Escobar et al. [149] reported an interesting work where a synthetic wastewater containing heavy metals was treated by EC with simulte- neous metal concentration determination by means of anodic stripping voltammetry. Nanseu-Njiki et al. [150] applied EC to treat synthetic solutions containing Hg(II) and anodic redissolution in the differential pulse mode as coupled electroanalysis after the EC treatment, which allowed to optimize easily the EC parameters to obtain 99.95% of mercury removal. For instance, Eiband et al. [110] studied the EC of Pb2+ solutions with the electroanalytical Adsorptive Stripping Vol- tammetry technique coupled with a glassy carbon electrode. The con- centrations analyzed showed an average diference of 10% in compar- ison with AAS, indicating that the values determined by electroanalytical approach presented high precision of quantification with good sensitivity. In addition, electroanalytical methods are cheaper and faster than the commonly used spectroscopic ones (which also require the use of more toxic and expensive reagents). Hence, the applicability of coupled electroanalysis to follow the metal pollutants abatement during the EC treatment is feasible. The coupled electroanalyis enables the simultaneous EC process monitoring and allows to evaluate the water quality of the treated water before and the exact point where the treatment could be stopped. 6. Electrocoagulation applications EC has been largely used on the wastewater treatment process to remove different pollutants that have been classified as follows: non- metallic inorganic species, heavy metals, organic pollutants and actual industrial effluents. In the next sections, the most important results about the application of EC to remove different pollutants have been summarized and commented. 6.1. Non-metallic inorganic species Non-metallic inorganic species are widespread in the earth en- vironments and are considered inhert and inoccuous species, but up to certain concentrations. However, the human development and the ex- tensively use of fertilizers and detergents have resulted in a growing accumulation of these species in waters. It is the case of nitrates and phosphate, for example. The excesive release and accumulation of these species in water bodies have con- tributed to the eutrophication of waters, which refers to a dramatic growth of algae in continental and coastal waters afecting aquatic ecosystems. Other inorganic species that presents both beneficial and detrimental effects to human health is fluoride. Fluoride has been added to drinking waters to prevent dental cavities, nevertheless an excess in its concentration leads to varius diseases as fluorosis, arthritis and so on. In this frame, the control of these species under the limits re- commended by the World Health Organization of 1.5 mg/L promoted several technologies to remove excesive contents in waters, and it is the case of EC. Thus, several authors have concentred their efforts to find methodologies to reduce the environmental impact of these pollutants [151–196]. The efficient and promising results of their abatement by EC technologies reported in the literature are presented in Table 1. Al- though EC can be applied to remove nitrate and sulfate; economically speking this is not feasible due to the higher consumption achived. Analysing this Table,> 80% of concentration removal has been achieved for ammonia, boron, cyanide, flouride, nitrite, nitrate, phos- phate, powdered actived carbon, silica particles, sulfide and sulfite when Al, Fe and SS electrodes were used. In the case of ammonia, the configuration of Al-SS electrodes favors the efficient elimination of this inorganic compound from wastewaters [151], while that, Al-Al re- moved 80% [152]. Meanwhile, the use of Fe-Fe electrodes did not achieve significant elimination of ammonia, obtaining up to 15% [153]. Higher removal efficiencies were achieved independent of the electrode used for removing boron from synthetic effluents (see Table 1). However, higher decay in the boron concentration was at- tained at synthetic or real effluents by using Al-Al electrodes with MP-P reactors [83,154–162]. On the other hand, no significant differences were observed when supporting electrolyte was changed [83]. In sev- eral cases, the EC arrangement preferentially used was MP-P reactor (see Fig. 4). Regarding the pH conditions, the efficacy of EC approach was improved when pH about 7.0–8.0 was employed during boron removal [83,154–162]. Even when the nature of electrode was not noteworthy parameter for removing boron, the effective combination of Fe, Al or SS electrodes represents a substantial reduction on the elec- trolysis time, depending on the boron concentrations in the effluent. Other electrode combinations such as Mg-SS [158] and Zn-SS [160] were used, obtaining removal efficiencies between 86.3% to 97.3%. Cyanide is a toxic pollutant for water ecosystems, for this reason, its elimination is important. However, the study reported by Moussavi [163] has been the unique work published until now, showing that the elimination of cyanide (300 mg) is feasible by using Al or Fe electrodes. The configuration of Fe-Fe or Fe-Al electrodes allowed to achieve higher removal efficiencies ranging from 87% to 93% in 20 min of treatment. The use of Al-Al arrangement promotes an efficient elimination of fluoride by using MP-P EC reactor [49,78,95,96,107,164–175] but the configuration BP-S was efficiently employed for removing F- from S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 277 Table 1 Electrocoagulation treatment of non-metallic inorganic species by electrocoagulation technologies. Compound [C0]/mg/L Anode- cathode Arrange. Electrolyte pHi j/mA cm−2 Removal/% Time/min Reference Ammonia (NH4 +) 50 Al-SS MP-P n.d 7.0 16.7 99.0 60 [151] 9.88 Al-Al MP-P n.d. 7.5 4.8 80.0 2 [152] 20 Fe-Fe MP-P 2000 mg/L NaCl 7.0 33 15.0 30 [153] Boron (B) 24 Al-Al MP-P Real geothermal water 8.0 6.0 96.0 30 [83] 2500 Al-Al MP-P 0.1 g/L NaCl 8.0 20.0 90.0 50 [154] 500 Al-Al MP-P 15 mM CaCl2 8.0 3.0 92.5 120 [155] 1000 Al-Al MP-P n.d. 8.0 5.0 A 94.0 – [156] 5000 Mg-SS MP-P n.d. 7.0 2.0 5.0 86.3 97.3 180 [158] 5.0 Fe-SS MP-P n.d. 7.0 2.0 93.1 180 [159] 5.0 Zn-SS MP-P n.d. 7.0 2.0 93.2 180 [160] 15.0 Al-Al MP-P Produced water from Crude Oil Terminal 7.0 20.0 98.0 90 [161] 15.0 Al-Al MP-P Produced water from Crude Oil Terminal 7.0 12.5 98.0 90 [162] Cyanide (CN−) 300 Fe-Fe FeeAl AleAl Al-Fe MP n.d. 11.5 15.0 87.0 93.0 35.0 32.0 20 [163] Fluoride (F-) 42 Al-Al Fe-Fe MP-P 0.025 M Na2SO4 3.0 5.0 87.0 56.7 90 [49] 20 Al-Al MP-P n.d. 7.0 10 AC 10 DC 93.0 91.5 60 [78] 25 Al-Al BP-S None 5 mM Cl− 5 mM NO3 − 5 mM SO4 2− 8.16 o.e (5.56 i.e) 100 87.1 85.0 32.6 9 [88] 10 Al-Al MP-P BP-S n.d. 8.0 25.0 78.7 84.0 45 [96] 5.0 Al-Al Fe-Fe MP-P 0.01 M Na2SO4 6.0 2.0 97.6 83.6 30 [107] 16 Al-Al MP-P NaCl 6.0 1.5 87.5 4 [164] 10 Al-Al MP-P n.d. 6.0 5.0 99.0 50 [165] 15 Al-Al MP-P Drinking water 392 mg/L Cl− 5.0 17.1 98.0 30 [166] 19 Al-Al MP-P none 6.5 0.93 95.2 10 [167] 15 Al-Al MP-P Tap water 392 mg/L Cl− 7.0 17.0 93.0 35 [168] 10 Al-Al MP-P Bore water 7.8 2.0 100 60 [169] 15 Al-Al MP-P n.d. 7.4 17.1 96.4 35 [170] 25 Al-Al MP-P NaCl 7.0 11.1 90.0 25 [171] 10 Al-Al MP-P 0.1 M NaCl 4.0–6.0 1.87 90.0 60 [172] 6.0 Al-Al MP-P groundwater 8.4 2.5 68.0 60 [173] 10 Al-Al MP-P 0.5 g/L Na2SO4, 1.5 g/L ClO− 7.7 5.0 90.0 n.d [174] 30 Al-Al MP-P n.d. 7.0 1.85 94.0 30 [175] 2.4 Al-Al BP-S River water 7.6 0.3 83.3 5 [176] 27.4 Al-Al BP-S 2.0 g/L CaCl2 9.0 1 A 85.4 5 [177] 15 Al-Al BP-S n.d. 6.0 n.d. 95.0 20 [178] 806 Al-Al BP-S 13 mg/L Cl− 7.0 2.0 98.0 20 [179] 25 Al-Al BP-S n.d. 7.0 8.2 98.0 20 [180] 5 Al-Al BP-S Steel industrial water 7.0 8.8 93.0 5 [181] 25 Fe-Fe BP-S n.d. 6.0 12.5 60.0 40 [182] Nitrite (NO2 −) 0.21 Al-Al MP-P n.d. 7.5 4.8 80.0 2 [152] 10 Fe-Fe MP-P 2000 mg/L NaCl 7.0 33 97.0 50 [153] Nitrate (NO3 −) 0.18 Al-Al MP-P n.d. 7.5 4.8 70.0 5 [152] 100 Fe-Fe MP-P n.d. 7.0 2.9 V 98.0 120 [183] 100 300 Al-Al MP-P NaHCO3 9.0 2.5 99.0 83.0 60 90 [184] 300 Fe-Fe MP-P n.d. 7.2 25 V 84.0 480 [185] 150 Al-Al BP-S NaCl 9.0 40 V 89.7 60 [186] 55 203 Al-Al MP-P 60 mg/L Cl− 1.0 g/L SO4 2− 107 mg/L HCO3 − 8.2 15 81.8 75.3 120 [187] 100 Al-Fe FeeFe Al-Gr AleAl Fe-Gr FeeAl Gr-Al Gr-Fe MP-P 100 mg/L NaCl 7.0 25 60.0 52.0 50.0 45.0 35.0 26.0 18.0 16.0 60 [188] (continued on next page) S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 278 synthetic waters [88,95,176–182], but the efficient elimination is strongly dependent on the EC reactor and supporting electrolyte. Under pH conditions below 7.0, higher F- removals were attained. Another feature is that when Fe electrodes were used, a decay on the removal efficiencies was observed [49,107,182]. This trend is related with the particular mechanism of fluoride removal by EC, which occurs mainly by the formation of highly insoluble aluminum fluoride. On the other hand, when different studies have considered similar initial con- centration to be eliminated, the final removal efficiency was also si- milar in all cases, but the electrolysis time varied as a function of the kind of the effluent (synthetic or real) as well as current density by using MP-P reactors [49,78,95,107,164–175]. It is important to in- dicate that shorter electrolysis times were also spent in some cases [164,176,177,181], see Table 1. Few studies have been performed to study the elimination of nitrite. Lin and Wu [153] have studied the decay of nitrite concentration (10 mg/L) with Fe-Fe electrodes by using MP-P configuration cell in presence of 2000 mg/L of Cl− in solution, achieving 97% of removal. Meanwhile, when the similar EC reactor was used but with Al-Al electrodes [152], only 80% of removal was obtained, even when the initial concentration was more that 45-folds minor (0.21 mg/L) than that the study performed by Lin and Wu [153]. In the case of nitrate, it is efficiently eliminated from solutions by EC approach [152,183–187], avoiding the oxidation-reduction effect that difficult its complete removal. Even though, the removal efficiency is dependent on the specific operating conditions as well as on the electro- chemical reactions involved. Later behavior is principally due to the formation of flocs or coagulants electrochemically formed from Fe or Al electrodes. However, when these electrodes are changed or combined with other materials, removal efficiencies decrease significantly [188]. For phosphate, sulfide, sulfite and sulfate; EC treatment is a good alternative to remove higher concentrations of these ions [8,108,109,151,152,189–194]. Lower energy requirements are neces- sary because of lower current densities are applied [8,152,189,195,196]. Special attention is given to an innovative EC alternative which employs renewal energy to supply electrical energy for depuration of river water with a reactor MP-P with Al-Al electrodes to eliminate 170 mg/L of phosphate, obtaining removal efficiencies up to 98% [109]. Meanwhile, phosphate (150 mg/L) was also removed from municipal wastewater [192], reaching efficiencies between 97% and 99% when Fe-Fe electrode arragement was employed. 6.2. Heavy metals Heavy metals are known from decades to be highly toxic, mutagenic and carcinogenic pollutants [197]. The intoxication by heavy metals leads to several psychical and physical diseases for the living beings [198]. For this reason, elimination and control are a continuous pre- occupation in environmental pollution issues. Heavy metal pollution increased with the development of extracting industries (metal plating, mining operation), tanneries, batteries production and others. Un- fortunately, heavy metals are not biodegradable and are bio cumulative affecting the whole trofic chain. Therefore, the removal of heavy metals from aquatic environments has been a constant field of research. In this context, EC appeared as a promising and efficient technology to sepa- rate these pollutants from aqueous phase and even recover them as added value sub-product of the water treatment. Table 2 compiles several results reported in the literature classified as a function of the metallic species removed from the aquatic environment. As an important parameter, the combination of electrodes employed as well as the EC reactor influence significantly the removal efficiency achieved in some cases, such as arsenate, arsenite, cadmium, manganes and silver [28,44,51,63,72,73,76,82,98,104,106,185,199–218] Ar- senite(III) was quase completely removed when Al and Fe electrodes were used for treating synthetic effluents [63,75,106,199–202]. The most used EC reactor for arsenite(III) elimination was MP-P by using soft electrolytic conditions (j or E). In the case of arsenate, higher re- moval efficiencies were achieved when effluents with lower con- centrations were treated [76,98,104,106,185,199,203–208]. In both cases, the efficient elimination depends on the pH, current density and Table 1 (continued) Compound [C0]/mg/L Anode- cathode Arrange. Electrolyte pHi j/mA cm−2 Removal/% Time/min Reference Gr-Gr 13.8 Phosphate(PO4 3−) 30 Al-Fe MP-P 1.0 g/L NaClreal wastewater 5.0 10 96.0 93.0 15 60 [8] 83 Al-Al Fe-Fe MP-P 500 mg/L Na2SO4 9.0 3.0 100 100 30 120 [108] 170 Al-Al MP-P River water and NaCl 7.2 Solar Energy (14.0) 97.8 20 [109] 50 Al-SS MP-P n.d 7.0 16.7 99.0 60 [151] 0.18 Al-Al MP-P n.d. 7.5 4.8 98.0 2 [152] 100 200 Al-Al MP-P NaCl 6.2 10 94.0 70.0 20 [189] 150 Al-Al Fe-Fe MP-P n.d. 3.0 0.5 100 42.0 40 [190] 100 Alalloy-SS Al-SS Fe-SS MP-P n.d. 7.0 2.0 99.0 87.0 85.0 30 [191] 150 Fe-Fe BP-S MP-P Municipal wastewater 9.0 25.0 97.0 98.8 40 [192] 306 Al-Al MP-P n.d. 3.0 10 100 30 [194] Powdered Activated Carbon (C) 20 Al-Al MP-P n.d. 7.5 10.0 95.0 50 [196] Silica particles 70 NTU of turbidity Fe-SS MP-P n.d. 9.5 1.4 95.0 60 [369] Sulfide (S2−) 100 500 Fe-Fe MP-P 30 mg/L Cl− 7.0 32 99.0 65.0 15 [195] Sulfite (SO3 2−) 100 500 Fe-Fe MP-P 30 mg/L Cl− 7.0 62 85.0 46.2 15 [195] Sulfate (SO4 2−) 100 500 Fe-Fe MP-P 30 mg/L Cl− 7.0 62 71.3 30.0 15 [195] S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 279 Table 2 Electrocoagulation treatment of heavy metals by electrocoagulation technologies. Compound [C0]/mg/L Anode-cathode Arrange Electrolyte pHi j/mA cm−2 Removal/% Time/min Reference Antimony [Sb3+] 28.6 Al-Al BP-S Mine water 2.0 22 96.5 60 [371] Arsenite (III) [AsO3 3−]7 13.4 Fe-Fe AleAl Fe-Al MP-P 4 g/L NaCl 2.4 4.0 4.0 30 (AC) 99.6 97.8 99.6 60 [63] 0.10 Fe-Fe MP-P None 1 mg/L PO4 3− 5 mg/L SiO2 7.0 0.14 99.9 60 [76] 0.05 Fe-Fe MP-P Synthetic water 6.5 28 75.4 60 [106] 2.0 Fe-Fe AleAl Ti-Ti MP-P n.d. 7.0 0.65 2.19 2.19 99.0 40.0 60.0 60 [199] 2.24 (As total) Fe-Fe MP-P 1 g/L NaCl 2.9 4.6 99.0 1.5 [200] 2.24 (As total) Fe-Fe MP-P 1 g/L NaCl 2.9 4.6 99.7 1 [201] 6.67 μM Fe-DSA MP-P 20 mM Na2SO 7.0 10 99.9 65 [202] Arsenate (V) [AsO4 3−] 0.10 Fe-Fe MP-P None 1 mg/L PO4 3− 5 mg/L SiO2 7.0 0.14 99.9 15 60 15 [76] 20 mM Al-Al BP-S n.d. 7.0 4.0 99.0 40 [82] 0.15 Fe-Fe FeeFe FeeFe AleAl AleAl Al-Al MP-P MP-S BP-S MP-P MP-S BP-S n.d. 6.5 6.5 6.5 7.0 7.0 7.0 25 98.0 98.0 98.0 98.0 98.0 98.0 12.5 4.5 2.5 8 6 4 [98] 2200 Fe-Fe MP-P wasewater 2.0 10 100 120 [104] 0.05 Fe-Fe MP-P Synthetic water 6.5 0.25 78.0 60 [106] 1 Fe-Fe MP-P n.d. 7.2 25 V 75.0 480 [185] 2.0 Fe MP-P n.d. 7.0 0.65 99.0 60 [199] 100 Fe-Fe MP-P n.d. 1.2 12 (AC) 98.0 180 [203] 100 Fe-Fe BP-S n.d. 1.2 12(AC) 98.0 120 [204] 50 100 150 Fe-Fe MP-P n.d. 7.0 15 85.0 64.7 48.5 40 55 55 [205] 100 Fe-SS Al-SS MP-P n.d. 7.0 5.0 86.0 73.0 50 [206] 0.13 Fe-Fe BP-S Underground water 7.22 3.0 92.3 0.5 [207] 10 Fe-Fe Al-Al MP-P 1000 mg/L NaCl 7.0 3.0 99.9 99.9 60 [208] 0.0038 Fe-Fe Al-Al MP-P Paper mill wastewater 7.7 10 86.8 86.8 50 80 [209] 0.059 Al-Al MP-P Groundwater 8.1 4.0 66.0 4 [210] 0.015 Al-Al BP-S Groundwater 5.0 4.3 91.5 50 [211] Cadmium [Cd2+] 20 Zn-Zn MP-P n.d. 7.0 2.0 AC 2.0 DC 97.8 96.9 120 [44] 20 Alalloy-Alalloy MP-P None 65 mg/L HCO3 − 250 mg/L HCO3 − 50 mg/L PO4 3− 15 mg/L Silicate 5 mg/L H2AsO4 − 7.0 2.0 97.5 69.1 16.0 43.0 19.0 31.0 120 [72] 9.0 Al-Al MP-P n.d. 7.1 64 44.4 22 [212] 100 Al-Al MP-P n.d. 7.0 3.68 100 5 [213] Cesium [Cs+] 5 Mg-Zn AleZn ZneZn Fe-Zn MP-P n.d 6.8 0.8 96.8 92.4 90.6 90.0 80 [24] Chromium (III) [Cr3+] 1700 Fe-Fe MP-P BP-S 1820 mg/L Cl− 3.4 10.84 32.52 81.5 99.9 50 [100] 1000 Al-Al MP-P 1000 mg/L NaCl 3.4 48.78 100 60 [102] 485 Fe-Fe MP-P 1000 mg/L NaCl 2.3 6.5 99.4 1 [201] 1000 Fe-Fe MP-P 1000 mg/L NaCl 3.4 48.78 32.78 100 100 40 60 [219] 200 Al-Al MP-P n.d. 4.23 9.14 V 91.0 10 [220] 44.5 Fe-Al MP-P n.d. 3.0 10 100 20 [221] 45 Fe-Fe AleAl FeeAl Al-Fe MP-P Electroplating effluent 3.0 10 100 99.8 100 95.6 60 [222] 1490 Fe-Fe MP-P Electroplating effluent 4.0 50 100 45 [223] 93.2 Fe-Fe MP-P Metal plating wastewater 9.56 4 100 45 [224] (continued on next page) S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 280 Table 2 (continued) Compound [C0]/mg/L Anode-cathode Arrange Electrolyte pHi j/mA cm−2 Removal/% Time/min Reference 150 Fe-Fe MP-P 1000 mg/L NaCl 8 10 100 20 [237] Chromium (VI) [CrO4 2−] [Cr2O7 2−] 50 Al-Al Fe-Fe MP-P 0.025 M Na2SO4 3.0 5.0 25.2 99.8 90 [49] 50 20 10 Al-Al BP-S 1700 mg/L NaNO3 5.5 3.3 70.0 75.0 70.0 150 120 60 [51] 300 150 75 Al-Al MP-P n.d 4.0 40 100 100 100 80 60 40 [75] 300 Fe-Fe MP-P MP-S 1500 mg/L NaCl 4.5 1 98.2 98.8 20 [105] 1.0 Fe-Fe FeeAl Al-Al MP-P 181 mg/L Cl− 376 mg/L SO4 2− 8.0 7.9 100 100 100 3 3 10 [112] 887 Fe-Fe MP-P Electroplating effluent 4.0 50 100 15 [223] 50 Al-Al MP-P n.d 3.4 4.36 42.0 40 [226] 20 Fe-Fe BP-S 1700 mg/L NaNO3 5.0 1.3 38.9 100 13.4 60 60 [227] 50 Al-Fe FeeFe FeeFe Fe-Fe MP-P 1.0 g/L NaCl 1.0 g/L NaCl 1.0 g/L NaNO3 1.0 g/L Na2SO4 5.0 5.0 15.0 99.0 18.0 14.0 30 [228] 100 Al-Al MP-P n.d 5.0 24 V 90.4 24 [229] 200 Al-Al Fe-Fe MP-P wastewater 7.5 15.0 67.3 100 60 [230] 1470 SS-SS MP-P 33.6 mM NaCl 1.84 31.7 100 70 [231] 100 1000 Fe-Al MP-P 30 mg/L NaCl 30 mg/L KCl 30 mg/L NaNO3 30 mg/L Al2(OH)nCl6-n 5.0 15.3 99.0 99.0 72.0 80.0 25 [232] 5 Alalloy-Zn MP-P n.d. 7.0 2.0 98.2 10 [233] 92 52 10 Fe-Fe MP-P NaCl 2.0 10 100 100 100 100 60 20 [235] Cobalt [Co2+] 400 100 25 Al-Al MP-P NaCl 7.0 6.25 95.5 100 100 60 35 15 [216] Copper [Cu2+] 250 Fe-Fe MP-P n.d. 5.5 25 98.0 40 [28] 250 100 50 Al-Al BP-S 1700 mg/L NaNO3 5.5 3.3 100 100 100 15 7 5 [51] 300 150 75 Al-Al MP-P n.d 4.0 40 100 100 100 50 40 20 [75] 50 Al-Al MP-P Copper production wastewater 5.1 3.0 99.8 15 [214] 200 Al-Al MP-P 302 mg/L SO4 2− 224 mg/L Cl− 3.0 15 100 98.0 15 35 [217] 5.0 Fe-Al MP-P Industrial wastewater 6.0 14 100 90 [218] 45 Fe-Al MP-P n.d. 3.0 10 100 20 [221] 45 Fe-Fe Al-Al Fe-Al Al-Fe MP-P Electroplating effluent 3.0 10 100 100 100 95.6 60 [222] 33.3 Fe-Fe MP-P Metal plating wastewater 9.56 4 99.0 45 [224] 50 Al-Al MP-P KCl 6.0 48.0 100 5 [225] 83 Al-Fe Al-Al Fe-Fe Fe-Al MP-P Mechanical polishing wastewater 6.0 30 V 99.0 99.0 99.0 99.0 30 [236] 12.0 Fe-Fe MP-P 173 mg/L HCO3 −, 43 mg/L SO4 2−, 3.3 mg/L NO3 −, 7.3 mg/L Cl− 7.75 1.4 100 150 [239] 20.0 Zn-Zn MP-P n.d. 7.0 0.5 96.6 35 [240] 15.4 Fe-Al MP-P n.d. 5.0 8.0 95.0 60 [241] Indium [In3+] 20 Fe-Al AleFe FeeFe Al-Al MP-P 100 mg/L NaCl 2.5 20 V 78.3 70.1 31.4 15.8 90 [101] Iron [Fe2+] 25 Al-Al MP-P Tap water 7.5 0.4 99.2 35 [95] 220 Al-Al MP-P n.d. 7.1 64 88.6 20 [212] 25 Alalloy-SS MP-P Tap water 6.5 0.6 98.8 60 [243] Lead [Pb2+] 300 Al-Al MP-P 0.5 mol/L NaNO3 7.0 11.8 7.9 3.9 100 100 100 45 75 90 [110] 1420 Al-Al MP-P n.d. 7.1 64 95 22 [212] 9.0 Fe-Fe MP-S Battery industry wastewater 2.8 6.0 97.4 40 [242] (continued on next page) S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 281 kind of the effluent due to the parallel reactions that are involved during the formation of complex or flocs with coagulant material. Meanwhile, an important feature was determined when cadmium was treated by EC process [44,72,212]. The use of different supporting electrolytes influences on the cadmium removal. For example, Vasu- devan and co-workers [72] examined the elimination of cadmium in absence or in presence of specific anions, such as HCO3 −, PO4 3−, si- licate and AsO4 −. The figures demonstrated that different removal ef- ficiencies were achieved ranging from 16% to 97% in 120 min of treatment depending on the anion. However, it is important to indicate that Al alloy electrodes were used which can be other factor that affects the efficacy of the EC approach. In fact, when Al electrodes were used by Pociecha and Lestan [212] to eliminate cadmium, 44.4% was achieved. Meanwhile, higher removal efficiencies were obtained (> 97%) when Zn electrodes were employed by Vasudevan and co- workers [44]. Chromium species cause severe environmental problems in aquatic ecosystems, for this reason, their removal by EC process has been widely studied [49,51,75,100,102,105,112,201,219–253]. In this case, the use of Al and Fe electrodes is suggested because the elimination of chromium Table 2 (continued) Compound [C0]/mg/L Anode-cathode Arrange Electrolyte pHi j/mA cm−2 Removal/% Time/min Reference SS-SS 8.0 91.4 Manganese [Mn2+] 250 Fe-Fe MP-P n.d. 5.5 25 76.0 40 [28] 2.0 Mg-Zn MP-P None 5 mg/L CO3 2− 5 mg/L PO4 3− 5 mg/L H2AsO4 − 5 mg/L Silicate 7.0 0.5 97.2 72.8 64.7 54.6 82.4 100 [73] 6.0 Al-Al MP-P Copper production wastewater 5.1 3.0 84.0 15 [214] 100 Al-Al MP-P NaCl 7.0 6.25 94.0 60 [215] 400 100 25 Al-Al MP-P NaCl 7.0 6.25 86.6 100 100 120 100 40 [216] 200 Al-Al MP-P 350 mg/L SO4 2− 217 mg/L Cl− 3.0 15 67.6 45.6 35 35 [217] 5.0 Fe-Al MP-P Industrial wastewater 6.0 14 89.0 90 [218] Mercury (II) [Hg2+] 0.4 Fe-Fe Al-Al MP-P NaCl 7.0 25 99.9 15 25 [150] Nickel [Ni2+] 250 Fe-Fe MP-P n.d. 5.5 25 97.6 40 [28] 250 100 50 Al-Al BP-S 1700 mg/L NaNO3 5.5 3.3 100 100 100 20 10 5 [51] 300 150 75 Al-Al MP-P n.d 4.0 40 100 100 100 50 40 20 [75] 394 Fe-Al MP-P n.d. 3.0 10 100 20 [221] 394 Fe-Fe AleAl FeeAl Al-Fe MP-P Electroplating effluent 3.0 10 98.0 96.5 99.9 76.3 60 [222] 57.6 Fe-Fe MP-P Metal plating wastewater 9.56 4 98.0 45 [224] 20 Fe-Fe MP-P 173 mg/L HCO3 −, 43 mg/L SO4 2−, 3.3 mg/L NO3 −, 7.3 mg/L Cl− 7.75 1.4 98.0 100 [239] 1.7 Fe-Al MP-P n.d. 5.0 8.0 95.0 60 [241] Silver [Ag+] 50 20 10 Al-Al BP-S 1700 mg/L NaNO3 5.5 3.3 66.0 60.0 90.0 50 30 30 [51] Strontium [Sr2+] 5 Mg-Zn FeeZn AleZn ZneZn MP-P n.d 6.8 0.8 97.0 95.2 91.4 89.6 80 [24] 10 SS-SS Al-Al MP-P 1 M NaCl 5.0 8.0 93.0 77.0 50 [272] Zinc [Zn2+] 250 Fe-Fe MP-P n.d. 5.5 25 98.0 40 [28] 250 100 50 Al-Al BP-S 1700 mg/L NaNO3 5.5 3.3 100 100 100 20 10 5 [51] 300 150 75 Al-Al MP-P n.d 4.0 40 100 100 100 50 40 20 [75] 260 Al-Al MP-P n.d. 7.1 64 65.4 30 [212] 200 Al-Al MP-P 293 mg/L SO4 2− 217 mg/L Cl− 3.0 15 98.0 70.5 35 35 [217] 10.0 Fe-Al MP-P Industrial wastewater 6.0 14 100 90 [218] 20.4 Fe-Fe MP-P Metal plating wastewater 9.56 4 99.0 45 [224] 50 400 Al-Al MP-P KCl 6.0 48 100 100 5 30 [225] 20.0 Fe-Fe MP-P 173 mg/L HCO3 −, 43 mg/L SO4 2−, 3.3 mg/L NO3 −, 7.3 mg/L Cl− 7.75 1.4 100 150 [239] 11.3 Fe-Al MP-P n.d. 5.0 8.0 95.0 60 [241] 3.2 Fe-Fe SS-SS MP-S Battery industry wastewater 2.8 6.0 96.5 92.6 40 [242] S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 282 Ta bl e 3 El ec tr oc oa gu la ti on tr ea tm en t of or ga ni c po llu ta nt s by el ec tr oc oa gu la ti on te ch no lo gi es . C om po un d [C 0 ]/ m g/ L A no de - ca th od e A rr an ge El ec tr ol yt e pH i j/ m A cm − 2 C om po un d R em ov al /% C O D R em ov al /% D O C R em ov al /% C ol ou r re m ov al /% Ti m e/ m in R ef er en ce A ro m at ic s A ni lin e 10 0 Fe -A D E M P- P 0. 05 M N a 2 SO 4 35 m M H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 30 .0 45 .0 45 .0 75 .0 10 0 10 0 10 0 – – – 74 .0 85 .0 98 .0 80 .0 – – – 30 30 30 10 [1 18 ] A ni lin e 10 00 Fe -A D E M P- P 0. 05 M N a 2 SO 4 35 m M H 2 O 2 (p er ox ic oa gu la ti on ) 3. 0 20 0 95 .0 – 95 .0 – 60 [1 24 ] Be nz oq ui no ne 50 A l-S S M P- P N aN O 3 7. 5 2. 0 90 .0 – – – 20 [2 50 ] D im et hy l ph th al at e 10 0 SS -S S M P- P 15 00 m g/ L N aC l 6. 0 22 .5 10 0 10 0 10 .3 70 .0 10 .0 75 .0 – – 35 18 0 [2 51 ] 4- C hl or op he no l 17 8 Fe -A D E M P- P 0. 05 M N a 2 SO 4 35 m M H 2 O 2 (p er ox ic oa gu la ti on ) 3. 5 10 .0 10 .0 10 0 10 0 – – 55 .0 80 .0 – – 60 12 0 [1 21 ] N ap ht al en e su lf on at e K -a ci d 20 0 SS -S S M P- P 10 00 m g/ L N aC l 7. 0 29 .0 98 .0 66 .0 39 .0 – 15 0 [2 52 ] 4- N it ro ph en ol 20 Fe -S S Fe -S S Fe -S S A l-S S A l-S S SS -S S SS -S S SS -S S M P- P M P- S BP -S M P- P BP -S M P- P M P- S BP -S 30 0 m g/ L N aC l 9. 0 10 10 0 99 .2 10 0 15 .4 96 .1 10 0 99 .6 10 0 65 .0 – – 10 [9 4] Ph en ol 2. 5 Fe -G r M P- P H 2 O 2 (p er ox ic oa gu la ti on ) 2. 0 1. 0 92 .0 – – – 30 [1 23 ] Ph en ol ic m ix tu re (3 ,4 ,5 -t ri m et ho xy be nz oi c, 4- hy dr ox yb en zo ic , ga lli c, pr ot oc at ec hu ic , tr an s‑ ci nn am ic an d ve ra tr ic ac id s) 10 0 ea ch on e A l-A l C ue C u Fe e Fe Pb e Pb Zn -Z n M P- P 1. 5 g/ L N aC l 3. 0 11 .9 – 24 .4 12 .0 40 .8 23 .7 48 .8 – – 80 [2 53 ] D ye s A ci d Bl ac k 1 10 0 Fe -F e Fe -S w oo l M P- P 1 g/ L N aC l 6. 5 10 .7 81 .0 99 .0 30 .0 60 .0 – 81 .0 99 .0 12 [2 54 ] A ci d Bl ac k 52 20 0 A l-A l M P- P 2 g/ L N aC l 5. 0 10 92 .0 – – 92 .0 6 [2 55 ] A ci d Br ow n 14 50 A l-A l M P- P 2 g/ L N aC l 6. 4 6. 3 91 .0 87 .0 – 91 .0 18 [2 56 ] A ci d O ra ng e 7 (O ra ng e II ) 50 Fe -F e M P- P 12 g/ L N aC l 7. 5 3. 5 98 .0 84 .0 – 98 .0 5 [3 3] A ci d O ra ng e 7 (O ra ng e II ) 10 Fe -F e BP -S 4 g/ L N aC l 7. 3 15 .9 98 .5 – – 98 .5 5 [2 57 ] A ci d O ra ng e 7 (O ra ng e II ) 10 A l-A l BP -S 4 g/ L N aC l 7. 3 16 .0 94 .5 – – 94 .5 5 [2 58 ] A ci d O ra ng e 7 (O ra ng e II ) 50 Fe -F e A l-A l M P- P N aC l 7. 0 15 .5 98 .0 98 .0 – – – – 98 .0 98 .0 5 5 [2 59 ] A ci d R ed 14 15 0 Fe -F e M P- S M P- P BP -S n. d. 6. 5 8. 0 99 .0 95 .0 95 .0 85 .0 – – – – – 99 .0 95 .0 95 .0 4 [2 60 ] A ci d R ed 14 50 Fe -S S M P- P n. d. 7. 3 10 .2 91 .0 – – 91 .0 4. 5 [2 61 ] A ci d R ed 13 1 10 A l-A l M P- P 10 0 m g/ L N a 2 SO 4 11 62 .5 98 .0 – – 98 .0 12 0 [2 62 ] A ci d R ed 26 6 n. d. Fe -G r A l-G r M P- P 2 g/ L N aC l 4. 0 18 .2 93 .9 94 .9 – – 93 .9 94 .9 6 [2 63 ] A ci d Y el lo w 23 (T ar tr az in e) 50 Fe -F e M P- P N aC l 6. 0 11 .3 98 .0 69 .0 – 98 .0 5 [4 0] A ci d Y el lo w 23 (T ar tr az in e) 40 Fe -F e Fe e Fe Fe e Fe A le A l M P- P M P- S BP -S M P- S 40 0 m g/ L N aC l 5. 8 12 .0 98 .0 10 0. 0 10 0. 0 70 .0 50 .0 55 .0 58 .0 66 .0 – – – – 10 0. 0 10 0. 0 10 0. 0 10 0. 0 6 [3 9] (c on tin ue d on ne xt pa ge ) S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 283 Ta bl e 3 (c on tin ue d) C om po un d [C 0 ]/ m g/ L A no de - ca th od e A rr an ge El ec tr ol yt e pH i j/ m A cm − 2 C om po un d R em ov al /% C O D R em ov al /% D O C R em ov al /% C ol ou r re m ov al /% Ti m e/ m in R ef er en ce A l/ Fe -F e M P- S 10 0. 0 90 .0 – 10 0. 0 A ci d Y el lo w 23 (T ar tr az in e) 27 8 Fe -S S Fe -S S Fe -S S A l-S S M P- P 0. 05 M N aC l 0. 05 M N a 2 SO 4 0. 05 M N aN O 3 0. 05 M N aC l 6. 3 20 .0 10 0. 0 10 0. 0 65 .0 10 0. 0 – – – – – – – – 10 0. 0 10 0. 0 65 .0 7 30 12 0 30 [7 7] A ci d Y el lo w 36 50 Fe -F e M P- P 8 g/ L N aC l 8. 0 12 .8 83 .0 – – 83 .0 6 [2 64 ] A ci d Y el lo w 22 0 20 0 A l-A l M P- P 2 g/ L N aC l 5. 0 10 99 .9 – – 99 .9 3 [2 55 ] A m id o Bl ac k 10 B 10 0 Fe -F e M P- P N aC l 7. 5 17 .8 99 .0 – – 99 .0 60 [2 65 ] Ba si c Bl ue 3 50 Fe -S S M P- P N aC l 7. 0 8. 0 99 .0 75 .0 – 99 .0 5 [3 7] Ba si c Bl ue 3 20 Fe - A D E C N T M P- P H 2 O 2 0. 05 M N a 2 SO 4 (p er ox ic oa gu la ti on ) 3. 0 10 .0 95 .0 – – 95 .0 10 [1 25 ] Ba si c R ed 43 50 Fe -S S M P- P N aC l 7. 0 6. 0 99 .0 99 .0 – 99 .0 5 [3 7] Ba si c R ed 46 20 Fe - A D E C N T M P- P H 2 O 2 0. 05 M N a 2 SO 4 (p er ox ic oa gu la ti on ) 3. 0 10 .0 98 .0 – – 98 .0 10 [2 66 ] Ba si c Y el lo w 2 20 Fe -A D E M P- P H 2 O 2 0. 05 M N a 2 SO 4 (p er ox ic oa gu la ti on ) 3. 0 10 .0 90 .0 10 0 – – – 81 .0 90 .0 10 0 30 36 0 [1 22 ] Ba si c Y el lo w 2 20 Fe -A D E Fe - A D E C N T Fe -A D E Fe - A D E C N T M P- P H 2 O 2 0. 05 M N a 2 SO 4 (p er ox ic oa gu la ti on ) 3. 0 10 .0 62 .0 96 .0 – – – – – – – – 81 .0 92 .0 62 .0 96 .0 – – 10 10 36 0 36 0 [2 66 ] Ba si c Y el lo w 2 20 Fe - A D E C N T M P- P H 2 O 2 0. 05 M N a 2 SO 4 (p er ox ic oa gu la ti on ) 3. 0 10 .0 96 .0 – – 96 .0 10 [1 25 ] Bo m ap le x R ed C R -L 10 0 A l-A l M P- P 2. 5 m M N aC l 3. 0 0. 5 99 .1 – – 99 .1 30 [2 67 ] C ry st al V io le t 20 0 Fe -F e A le A l M P- P 28 4 m g/ L N a 2 SO 4 5. 8 2. 8 95 .4 97 .9 99 .9 95 .7 – – 95 .4 97 .9 5 30 [2 68 ] D ir ec t R ed 23 50 Fe -F e M P- P n. d. 3. 0 95 .0 – – 95 .0 5 [2 69 ] D ir ec t R ed 81 50 A l-A l M P- P 2 g/ L N aC l 2 g/ L N a 2 SO 4 2 g/ L N aN O 3 2 g/ L N a 2 C O 3 6. 0 1. 9 98 .0 86 .7 85 .0 75 .0 – – 98 .0 86 .7 85 .0 75 .0 60 [8 4] D ir ec t R ed 81 50 A l-A l BP -S N aC l 7. 5 20 90 .2 76 .1 – 90 .2 60 [2 70 ] D is pe rs e Bl ue 10 6 50 0 A l-A l M P- P 1 g/ L N aC l 4. 0 4. 5 99 .0 66 .7 0 – 99 .0 13 [7 1] D is pe rs e dy e (N ap ht oi c ac id -N ap ht ol ) 10 0 A l-A l M P- P 2 g/ L N aC l 7. 0 31 .3 85 80 .0 – 85 14 [2 71 ] D is pe rs e dy e (N ap ht oi c ac id -N ap ht ol ) 10 0 A l-A l M P- P 1 g/ L N aC l 7. 8 20 .0 99 .0 98 .0 68 .0 99 .0 20 [2 72 ] D is pe rs e dy e (N ap ht oi c ac id -N ap ht ol ) 10 0 A l-A l M P- P 1 g/ L N aC l 7. 8 20 .8 95 .0 – – 95 .0 14 [2 73 ] D is pe rs e R ed 1 10 0 A l-A l Fe -F e M P- P N aC l 9. 5 4. 0 99 .0 99 .0 – – 99 .0 99 .0 8 [2 74 ] D is pe rs e Y el lo w 54 50 0 A l-A l M P- P 1 g/ L N aC l 4. 0 4. 5 95 .2 54 .0 – 95 .2 13 [7 1] D is pe rs e Y el lo w 21 8 n. d. Fe -G r A l-G r M P- P 2 g/ L N aC l 4. 0 18 .2 87 .8 71 .4 – – 87 .8 71 .4 6 [2 63 ] D is pe rs e Y el lo w 24 1 10 00 A l-A l M P- P n. d. 4. 5 10 5 (D C ) 10 5 (A C ) 25 .0 86 .0 – – 29 .0 81 .0 25 .0 86 .0 70 [7 9] D ri m ar en e K 2L R C D G 50 A l-A l M P- P N aC l 7. 0 12 91 .8 35 .2 – 91 .8 10 5 [2 76 ] Eo si n Y el lo w 10 0 Fe -F e Fe -S t w oo l M P- P 1 g/ L N aC l 6. 5 10 .7 39 .0 98 .0 35 .7 57 .1 – 39 .0 98 .0 12 [2 54 ] Eo si n Y el lo w 10 0 Fe -F e M P- P N aC l 7. 5 17 .8 81 .0 – – 81 .0 60 [2 65 ] Eo si n Y el lo w is h 20 0 Fe -F e M P- P 40 0 m g/ L N aC l 6. 8 16 ,1 98 .0 78 .0 – 98 .0 30 [2 77 ] In di go C ar m in e 50 Fe -S S M P- P 1. 5 g/ L N aC l 5. 0 1. 1 99 .9 – – 99 .9 18 0 [2 78 ] (c on tin ue d on ne xt pa ge ) S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 284 Ta bl e 3 (c on tin ue d) C om po un d [C 0 ]/ m g/ L A no de - ca th od e A rr an ge El ec tr ol yt e pH i j/ m A cm − 2 C om po un d R em ov al /% C O D R em ov al /% D O C R em ov al /% C ol ou r re m ov al /% Ti m e/ m in R ef er en ce In di go C ar m in e 50 Fe -F e M P- P 1. 5 g/ L N aC l an d 0. 5 g/ L ac ti va te d C 5. 0 0. 27 99 .9 – – 99 .9 12 0 [2 79 ] Le va fi x Bl ue C A 12 00 Fe -F e M P- P 25 g/ L N aC l 11 35 .5 99 .5 93 .9 – 99 .5 12 0 [2 80 ] Le va fi x Br ill ia nt Bl ue 25 0 Fe -F e A l-A l M P- P 5 m M N aC l 5. 5 10 .0 86 .7 98 .3 – – 86 .7 98 .3 30 [2 81 ] Le va fi x O ra ng e 25 0 A l-A l M P- P N aC l 6. 4 10 .0 95 .0 – – 95 .0 12 [4 3] M al ac hi te G re en 20 Fe - A D E C N T M P- P H 2 O 2 0. 05 M N a 2 SO 4 (p er ox ic oa gu la ti on ) 3. 0 10 .0 90 .0 – – 90 .0 10 [1 25 ] M al ac hi te G re en 10 0 Fe -F e M P- P N aC l 7. 5 17 .8 98 .0 – – 98 .0 60 [2 65 ] M et hy l O ra ng e 12 5 A l-A l M P- P N aC l 7. 4 18 5 (A C ) 97 .0 – – 97 .0 14 [8 0] M et hy l V io le t 10 0 Fe -F e M P- P N aC l 7. 5 17 .8 99 .0 – – 99 .0 60 [2 65 ] M et hy le ne Bl ue 20 0 Fe -F e M P- P 40 0 m g/ L N aC l 6. 8 16 .1 99 .0 91 .0 – 99 .0 15 [2 77 ] M et hy le ne Bl ue 50 Fe -F e M P- P N aO H 12 8. 0 92 .2 – – 92 .2 15 [2 82 ] M et hy le ne Bl ue 10 0 Fe -F e M P- P N aC l 7. 5 17 .8 97 .0 – – 97 .0 60 [2 65 ] R ea ct iv e Bl ac k 5 10 0 Fe -F e BP -S 2 g/ L N aC l 5. 0 4. 6 98 .8 – – 98 .8 8 [2 83 ] R ea ct iv e Bl ac k 5 25 Fe -S S M P- P 2 g/ L N aC l 2 g/ L N a 2 SO 4 6. 6 7. 5 92 .3 85 .7 – – 92 .3 85 .7 20 [8 5] R ea ct iv e Bl ue 4 10 0 Fe -F e Fe -S w oo l M P- P 1 g/ L N aC l 6. 5 10 .7 84 .0 99 .0 61 .4 69 .3 – 84 .0 99 .0 12 [2 54 ] R ea ct iv e Bl ue 19 n. d. Fe -G r A l-G r M P- P 2 g/ L N aC l 4. 0 18 .2 99 .9 94 .9 – – 99 .9 94 .9 6 [2 63 ] R ea ct iv e Bl ue 49 50 0 A l-A l M P- P 1 g/ L N aC l 4. 0 4. 5 74 .6 61 .9 – 74 .6 13 [7 1] R ea ct iv e Bl ue 14 0 10 0 A l-A l Fe -F e M P- P N aC l 9. 5 4. 0 97 .0 97 .0 – 90 .0 – 97 .0 8 [2 74 ] R ea ct iv e O ra ng e 84 30 0 Fe -F e SS -S S M P- P N aC l 7. 0 13 .0 11 .0 89 .0 99 .8 80 .4 89 .7 85 .4 91 .2 89 .0 99 .8 30 [2 84 ] R ea ct iv e R ed 43 50 Fe -S S A l-S S M P- P 0. 04 6 M N aC l 0. 02 8 M N aC l 8. 5 4. 1 3. 5 3. 9 99 .0 99 .0 – – 90 .6 98 .4 99 .0 99 .0 23 12 [2 85 ] R ea ct iv e Y el lo w 84 50 0 A l-A l M P- P 1 g/ L N aC l 4. 0 4. 5 88 .9 85 .7 – 88 .9 13 [7 1] R ea ct iv e Y el lo w 13 5 10 00 A l-A l M P- P n. d. 4. 5 10 5 (D C ) 10 5 (A C ) 96 .0 99 .9 – – 81 .0 89 .0 96 .0 99 .9 15 [7 9] R em az ol Bl ue 3R 25 Fe -F e A l-A l M P- P 8. 5 m M N aC l 7. 0 3. 0 10 0 74 .5 – – 10 0 74 .5 7. 5 [3 8] R em az ol Br ill ia nt O ra ng e 3R 25 Fe -F e A l-A l M P- P 8. 5 m M N aC l 7. 0 3. 0 10 0 10 0 – – 10 0 10 0 7. 5 [3 8] R em az ol Br ill ia nt Y el lo w G L 25 Fe -F e A l-A l M P- P 8. 5 m M N aC l 7. 0 3. 0 94 .5 52 .7 – – 94 .5 52 .7 7. 5 [3 8] R em az ol R ed R B 25 Fe -F e A l-A l M P- P 8. 5 m M N aC l 7. 0 3. 0 10 0 10 0 – – 10 0 10 0 7. 5 [3 8] R em az ol R ed R B1 33 25 0 A l-A l M P- P N aC l 6. 0 10 .0 93 .0 – – 93 .0 10 [2 6] R em az ol R ed R B 13 3 25 0 A l-S S M P- P N aC l 6. 0 10 .0 96 .9 – – 96 .9 [2 86 ] R em az ol R ed 3B 50 0 Fe -F e M P- P N aC l 6. 0 15 .0 95 .0 – – 95 .0 10 [7 4] R ho da m in e 6G 10 0 Fe -F e M P- P N aC l 7. 5 17 .8 90 .0 – – 90 .0 60 [2 65 ] R ho da m in e 6G 10 0 Fe -F e A l-A l M P- P 3 g/ L N a 2 SO 4 U lt ra so un d as si st ed 6. 0 25 .0 95 .0 92 .0 – – – – 95 .0 92 .0 18 0 30 0 [1 15 ] N at ur al O rg an ic M at er (N O M ) N O M (H um ic ac id ) 20 A l-A l BP -S n. d. 3. 0 4. 8 97 .8 – – – 30 [3 59 ] N O M (H um ic ac id ) 15 A l-A l M P- P n. d. 3. 0 7. 0 11 3. 3 90 .0 96 .0 51 .0 – – – 30 [3 70 ] N O M 9. 3 m g C / L A l-A l BP -S G ro un dw at er 5. 0 5. 8 77 .0 – 71 .0 – 90 [2 96 ] N O M 18 m g C / L A l-A l M P- P R iv er w at er 4. 1 0. 5 78 .0 – 78 .0 – 12 [2 97 ] (c on tin ue d on ne xt pa ge ) S. Garcia-Segura et al. Journal of Electroanalytical Chemistry 801 (2017) 267–299 285 Ta bl e 3 (c on tin ue d) C om po un d [C 0 ]/ m g/ L A no de - ca th od e A rr an ge El ec tr ol yt e pH i j/ m A cm − 2 C om po un d R em ov al /% C O D R em ov al /% D O C R em ov al /% C ol ou r re m ov al /% Ti m e/ m in R ef er en ce N O M 15 m g C / L A l-S S Fe -S S M P- P n. d. 6. 0 1. 4 79 .0 81 .0 – 79 .0 81 .0 – 12 [1 48 ] N O M 5. 5 m g C / L Fe -P t M P- P N at ur al w at er 6. 0 1. 0 70 .9 – 70 .9 – 15 [2 98 ] N O M 9. 3 m g C / L A l-A l BP -S G ro un dw at er 5. 3 8. 86 (A C ) 89 .0 – 70 .0 – 36 0 [2 11 ] N O M 5. 1 A l-A l Fe e Fe A l/ Fe -F e M P- P N at ur al w at er 7. 3 71 .1 59 .8 68 .6 – 71 .1 59 .8 68 .6 – 25 [2 99 ] Pe st ic id es 4- C hl or op he no xy ac et ic ac id (4 -C PA ) 20 0 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 – – 64 .0 91 .0 – – 40 36 0 [1 19 ] 4- C hl or o- 2- m et hy lp he no xy ac et ic ac id (M C PA ) 19 4 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 – – 66 .0 92 .0 – – 40 36 0 [1 19 ] 4- C hl or o- 2- m et hy lp he no xy ac et ic ac id (M C PA ) 18 6 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 10 0 – – – 38 .0 65 .0 80 .0 – – – 30 60 12 0 [1 31 ] 4- C hl or o- 2- m et hy lp he no xy ac et ic ac id (M C PA ) 18 6 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 + U V A (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 10 0 – – – 43 .0 80 .0 85 .0 – – – 30 60 12 0 [1 31 ] 2, 4- D ic hl or op he no xy ac et oc ac id (2 ,4 -D ) 23 0 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 – – 52 .0 81 .0 – – 40 36 0 [1 19 ] 3, 6- di ch lo ro -2 -m et ho xy be nz oi c ac id (d ic am ba ) 23 0 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 – – 55 .0 95 .0 – – 10 24 0 [1 19 ] 2, 4, 5- tr ic hl or op he no xy ac et ic ac id (2 ,4 ,5 -T ) 26 6 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 .0 10 0 10 0 – – 50 .0 97 .0 – – 30 12 0 [1 30 ] 2, 4, 5- tr ic hl or op he no xy ac et ic ac id (2 ,4 ,5 -T ) 26 9 Fe -A D E M P- P 0. 05 M N a 2 SO 4 H 2 O 2 (p er ox ic oa gu la ti on 3. 0 10 .0 10 0 10 0 – – 51 .0 93 .0 – – 40 36 0 [1 19 ] Ph ar m ac eu ti ca ls A zi th ro m yi n 19 0 Fe -F e M P- P 2. 0 m g/ L H 2 O 2 (p er ox ic oa gu la ti on ) 3. 0 20 .0 95 .6 – – – 60 [1 20 ] D ex am et ha so ne 20 00 A l-A l M P- P 2. 0 g/ L N aC l 7. 0 16 .4 38 .0 – – –