Adsorption of Ni(II), Pb(II) and Zn(II) on Ca(NO3)2-Neutralised Red Mud Beatriz Cestaro Pichinelli & Mariana Scicia Gabriel da Silva & Fabiano Tomazini da Conceição & Amauri Antonio Menegário & Maria Lucia Pereira Antunes & Guillermo Rafael Beltran Navarro & Rodrigo Braga Moruzzi Received: 21 July 2016 /Accepted: 6 December 2016 /Published online: 14 December 2016 # Springer International Publishing Switzerland 2016 Abstract This study aimed to investigate a novel meth- od of red mud neutralisation by Ca(NO3)2 (NRM), keeping its adsorption capacity in relation to natural red mud (RM) for Ni(II), Pb(II) and Zn(II). Results pointed out that the neutralisation process decreases the pH and electrical conductivity values on NRM due to reaction between the carbonate and bicarbonate alka- linity of red mud and calcium from calcium nitrate to form calcite (CaCO3). The maximum adsorption capac- ity values of RM and NRM, respectively, were 1.78 and 1.79 mmol g−1 for Ni(II), 2.13 and 2.23 mmol g−1 for Pb(II) and 1.14 and 1.06 mmol g−1 for Zn(II). Pseudo- second-order model is the main responsible for the adsorption of these metals on RM and NRM. The ad- sorption reaction is endothermic and these metals have affinity to RM and NRM. Thus, it is possible to neutralise the red mud with Ca(NO3)2 without adsorp- tion capacity losses of Ni(II), Pb(II) and Zn(II). Keywords Ca(NO3)2-neutralised redmud . Trace metals . Adsorption . Environmental management 1 Introduction Brazil is the third-largest producer of aluminium, mak- ing Brazil a contributor to global aluminium production (Brasil 2014). The world bauxite reserves totalled 25.6 billion tonnes in 2013, with the mineable Brazilian bauxite reserves of 714 million tonnes, being produced in 2013 approximately 33 million tonnes, behind Aus- tralia and China (Santana 2014). About 95% of the world bauxite production is used in production of alu- mina (Al2O3), which is subjected to an electrolytic re- duction, removing oxygen and producing aluminium metal (Mártires 2012). During the process of alumina extraction from baux- ite by Bayer process, it is generated an insoluble waste called red mud (Hind et al. 1999). The amount of generated red mud can reach twice the amount of alu- mina produced, requiring a large area for disposal. Bra- zilian red mud presents high alkaline content and is composed primarily of fine particles of SiO2, Al, Fe, Ca and Ti oxides and hydroxides (Antunes et al. 2012). Disposal methods result in high costs and environmental risks for the aluminium industry. Improper disposal of red mud may lead to problems such as contamination of soil, groundwater and surface water by NaOH, iron, Water Air Soil Pollut (2017) 228: 24 DOI 10.1007/s11270-016-3208-1 B. C. Pichinelli :M. S. G. da Silva UNESP—Universidade Estadual Paulista, Faculdade de Engenharia de Bauru, Rio Claro, Brazil F. T. da Conceição (*) :G. R. B. Navarro :R. B. Moruzzi UNESP—Universidade Estadual Paulista, Instituto de Geociências e Ciências Exatas, Avenida 24-A, n° 1515, C. P. 178, CEP 13506-900, Bela Vista, Rio Claro, São Paulo, Brazil e-mail: ftomazini@rc.unesp.br A. A. Menegário UNESP—Universidade Estadual Paulista, Centro de Estudos Ambientais, Rio Claro, Brazil M. L. P. Antunes UNESP—Universidade Estadual Paulista, Instituto de Ciência e Tecnologia, Rio Claro, Brazil http://crossmark.crossref.org/dialog/?doi=10.1007/s11270-016-3208-1&domain=pdf aluminium or others chemical agents and damage to flora and fauna in the region around the disposal (Hind et al. 1999; Silva Filho et al. 2007). Neutralisation is a method to minimise the potential environmental impacts caused by red mud disposal or make the red mud reutilisation possible. Neutralisation treatments include water washing (Apak et al. 1998a, b; Cengeloglu et al. 2006; Smiciklas et al. 2014), boiling with acids (Apak et al. 1998a, b; Cengeloglu et al. 2006), washing with acids (Santona et al. 2006; Nadaroglu et al. 2010; Grudić et al. 2013; Liang et al. 2014), and treatment by CO2 (Sahu et al. 2011) and seawater (McConchie et al. 2000; Palmer et al. 2010; Grudić et al. 2013; Souza et al. 2013a). Calcium nitrate (Ca(NO3)2) is water soluble and frequently used to extract the exchangeable phase in the desorption of Cd, Pb and Zn from red mud (Santona et al. 2006; Costa et al. 2009). Surprisingly, the use of Ca(NO3)2 to neutralise red mud has not been previously proposed. Studies indicated that red mud has substantial adsorp- tive properties, especially when it receives some type of thermal or chemical treatment (Wang et al. 2008). Be- sides, red mud is considered a low-cost adsorbent, which can be used in water and wastewater treatment for removal of toxic trace metals (Nadaroglu et al. 2010; Smiljanic et al. 2010; Sahu et al. 2011; Geyikçi et al. 2012; Pulford et al. 2012; Grudić et al. 2013; Smiciklas et al. 2014; Conceição et al. 2016), arsenic (Akin et al. 2012), fluoride (Liang et al. 2014), dyes (Fu et al. 2010; Ratnamala et al. 2012; Souza et al. 2013a, b) and bac- teria (Rai et al. 2012). Trace metals such as lead (Pb), nickel (Ni) and zinc (Zn) are used in various industrial activities such as electroplating and production of alloys, batteries, paints, pigments, dyes and electronic components. These metals are pollutants not compatible with biological treatments. Various technologies have been used to re- move trace metals during the water and industrial waste- water treatment, i.e. ion exchange, reverse osmosis, chemical precipitation and adsorption (Tchobanoglous et al. 2003). Thus, the aims of this study were to investigate a novel method of red mud neutralisation by Ca(NO3)2 and evaluate its capacity to adsorb Ni(II), Pb(II) and Zn(II), comparing with adsorption capacity of natural red mud. Natural red mud samples were treated with Ca(NO3)2 and then both materials, RM and Ca(NO3)2- neutralised red mud (NRM), were characterised in terms of mineralogy, pH, specific surface area, electrical conductivity, specific surface area, chemical and miner- alogical composition. The pH influence, adsorption iso- therms, kinetics and thermodynamics parameters were studied to better understand Ni(II), Pb(II) and Zn(II) adsorption characteristics in aqueous solutions using natural red mud and Ca(NO3)2-neutralised red mud. 2 Theoretical Background 2.1 Adsorption Isotherms and Kinetic Studies Equations used to describe the adsorption isotherms were developed by Freundlich (Eq. 1) and Langmuir (Eq. 2) (Tchobanoglous et al. 2003; Di Bernardo 2005). qe ¼ K FC1=n e ð1Þ qe ¼ qmaxKLCe 1þ KLCe ð2Þ where K F = F r e u n d l i c h c a p a c i t y f a c t o r ((mmol g−1)(L mmol−1)1/n); 1/n = Freundlich intensity parameter; qmax = maximum amount of adsorption cor- responding to complete monolayer coverage on the surface (mmol g−1); KL = Langmuir constant related to the energy of adsorption (25 mL mmol−1). Models of pseudo-first order Lagergren (Eq. 3), pseudo-second order (Eq. 4), Elovich (Eq. 5) and intraparticle diffusion (Eq. 6) were applied to explain the reaction kinetics and analyse the control mechanism of the adsorption process (Ho andMckay 1998; Cheung et al. 2000; Önal 2006). dqt dt ¼ k1 qe−qtð Þ ð3Þ dqt dt ¼ k2 qe−qtð Þ2 ð4Þ dqt dt ¼ α e−βqt ð5Þ qt ¼ kint t0;5 þ C ð6Þ where 24 Page 2 of 13 Water Air Soil Pollut (2017) 228: 24 qt and qe = amount adsorbed at equilibrium (mmol g−1) and at any time t (min); k1 and k2 = rate constant of pseudo-first-order rate (min−1) and pseudo-second- order adsorption (g. mmol−1 min−1); α = initial sorption rate (mmol g−1 min−1); β = extent of surface coverage and activation energy for chemisorption (g mmol−1); kint = constant intraparticle diffusion (mmol g−0,5 min- 0,5); C= intercept (mmol .g−1). 2.2 Thermodynamic Studies The thermodynamic study is able to verify the nature of the adsorption reactions, endothermic or exothermic. The variation of standard free energy or Gibbs energy (ΔG°) in J mol−1 was calculated from Eq. 7 (Önal 2006). The kc value was obtained by Eq. 8. ΔG� ¼ −R:T :lnKc ð7Þ kc ¼ CAe Ce ð8Þ where: R = gas constant (J mol−1 K−1); kc = equilibrium con- stant; T = temperature (K); CAe and Ce: equilibrium con- centration of adsorbate on the adsorbent and in the solution (mmol mL−1), respectively. The enthalpy change (ΔH°) in J mol−1 and the en- tropy (ΔS°) in J mol−1 K−1 adsorption were obtained from van’t Hoff equation (Eq. 9) (Önal 2006). ln kc ¼ − ΔH� R : T þ ΔS� R ð9Þ where -ΔH°/R = slope of the van’t Hoff chart; ΔS°/R = in- tercept of the van’t Hoff chart. 2.3 Average Relative Error The average relative error (ARE), together with R2, was chosen to analyse the best-fitting in the isotherm models and kinetic studies to the experimental data, after non- linear adjustment. The ARE values were calculated by Eq. 10, adapted from Behnamfard et al. (2014). These values can be positives or negatives, what indicates they were underestimated or overestimated, respectively, compared to the experimental data. Smallest ARE absolute values indicate in average more accurate esti- mation of qe values, because the calculated value is closer to the experimental value. Selecting the smallest ARE absolute values, it was possible to decide which model best represented the adsorption and kinetics of the metals by RM and NRM. ARE ¼ 100 N XN i¼1 qe exp−qe cal qe exp ! i ð10Þ where: qe exp = experimental amount of metal adsorbed onto RM and NRM (mmol g−1); qe cal = calculated amount of metal adsorbed onto RM and NRM (mmol g−1); N = number of measurements made. 3 Materials and Methods 3.1 Sampling, Activation Procedures and Characterisation The red mud used in this study was collected in an aluminium plant in São Paulo State. Natural red mud samples (RM) were defragmented in porcelain cruci- bles, dried for 12 h at 60 °C and sieved to <150 μm. Samples of neutralised red mud were prepared by wash- ing with an aqueous solution of Ca(NO3)2 0.1 N for 2 h at the ratio 1:25 (wt/wt) of red mud/Ca(NO3)2 solution. After treatment, the Ca(NO3)2-neutralised red mud (NRM) was washed with distilled water and dried for 12 h at 60 °C. The pH and electrical conductivity (EC) values for RM andNRMwere determined in 1:25 ratio of red mud/ distilled water, using a YSI 556 meter. The combined electrode was calibrated using the following high purity standards at pH 4.00 (4.00 ± 0.01 at 25 °C ± 0.2 °C) and 7.00 (7.00 ± 0.01 at 25 °C ± 0.2 °C). The conductivity metre was calibrated using a 1.0mmol L−1 KCl solution, which corresponds to 147 μS cm−1 at 25 °C. The deter- mination of cation exchange capacity (CEC), in mmol(+) kg−1, was performed according to the procedure of Embrapa (1997). The specific surface area (SBET) was characterised by BET/N2 adsorption methods using a Micromeritics ASAP 2020 instrument. The RM and NRM samples were analysed for SiO2, Al2O3, Fe2O3t, TiO2, K2O, Na2O, MgO, MnO, CaO, P2O5 and LOI (loss on ignition at 1000 °C) by X-ray fluorescence (XRF—Phillips PW 2510). The identification of Water Air Soil Pollut (2017) 228: 24 Page 3 of 13 24 minerals was performed by X-ray diffractometry (XRD—Siemens D5000) on powdered samples, using a wide angle X-ray diffractometer, operating at 40 kV and 40 mA, with CuKα radiation. In addition, the morphology of RM and NRM samples was observed under a JEOL JSM-6010LA Scanning Electron Micro- scope (SEM), attached with an energy dispersive X-ray spectrometer (EDS). 3.2 Adsorption Studies The Ni(II), Pb(II) and Zn(II) adsorption experiments were conducted using RM and NRM as adsorbents at Environmental Geochemistry Laboratory—IGCE, UNESP, Rio Claro. The solutions of these trace metals have been obtained from their analytical grade nitrate salts : Ni(NO3)2.6H2O, Pb(NO3)2 and Zn(NO3)2.6H2O. The influence of pH on the adsorption experiments have been studied at pH values of 2.0, 4.0, 7.0, 10.0 and 12.0. For the experiments with Ni(II), Pb(II) and Zn(II), 1.00 g (±0.01 g) of RM and NRM was mixed to the metal solution at concentration of 1 mmol 25 mL−1. The samples were stirred for 5 h at 145 rpm and the pH control was made with HCl and NaOH, 0.1 M. Isothermal studies were promoted by contact of 1.00 g (±0.01 g) of adsorbent and 25 mL of Ni(II), Pb(II) or Zn(II) solutions with initial concentrations ranging from 0.5 mmol 25 mL−1 to 4.0 mmol 25 mL−1 at pH 5.0–5.5. The system was stirred at 145 rpm, for 24 h, at temperature of 25 °C. The study of the kinetics of adsorption was carried out by the addition of 1.0 g (±0.01 g) of RM or NRM in 1 mmol 25 mL−1 trace metal solution, at pH 5.0–5.5. The samples were stirred at 145 rpm and removed after 15, 30, 60, 120, 420, 660 and 1440 min. The influence of temperature on the adsorption was studied by adsorption tests at different temperatures, i.e. 303 K (30 °C), 313 K (40 °C) and 323 K (50 °C), using 1.00 g (±0.01 g) of RM or NRM in 1 mmol 25 mL−1 trace metal solution, at pH 5.0–5.5, with the samples stirred at 145 rpm, for 7 h. After each experiment, an aliquot was collected from the supernatant and was centrifuged for 25 min at 3000 rpm for analysis of the remaining metal concentra- tion. The trace metals concentrations were determined by inductively coupled plasma optical emission spectrome- try (ICP OES), iCAP 6000 SERIES machine Thermo Scientific, with the detection limits of 0.002, 0.006 and 0.001mg L−1 for Ni(II), Pb(II) and Zn(II), respectively, at Center for Environmental Studies—UNESP, Rio Claro. Deionised water was used as a blank sample. All exper- iments were performed in triplicate. The amount of metal adsorbed onto the RM and NRM, qe, in mmol g−1, was calculated using Eq. 11. The Eq. 12 was used to obtain the adsorption percentage (%A) of metals. qe ¼ C0−Ce m : V ð11Þ %A ¼ C0−Ce Ce :100 ð12Þ where C0 and Ce = initial and equilibrium concentration of metal, respectively, (mmol 25 mL−1); m=mass of ad- sorbent sample (1 g); V = volume of solution (25 mL). 4 Results and Discussion 4.1 Characterisation of RM and NRM Properties of RM and NRM are provided in Table 1. Natural red mud possesses alkaline pH and presents the EC value of 3800 μS cm−1. After the neutralisation process by Ca(NO3)2, pH and EC values decrease. CEC values found were practically identical to RM and NRM. Specific surface area (SBET) obtained for NRM is higher than value found for RM. Pore sizes of RM and NRM ranging from 3.0 to 4.5, indicating the dimensions of mesopores. RM and NRM are composed of particles of different size, shape and texture (Fig. 1), since red mud is a heterogeneous material with particles ranging from <1 to >10 μm of diameter. The mineral morphology was not changed after the Ca(NO3)2 activation. Red mud composition is extremely dependent on the bauxite ore origin and the applied technological process, Table 1 Characterisation of RM and NRM Parameter RM NRM pH 10.4 7.8 EC (μS cm−1) 3800 122 CEC (mmol(+) kg −1) 108 110 SBET (m 2 g−1) 31.25 40.07 Pores size (nm) 3 to 4 3.5 to 4.5 24 Page 4 of 13 Water Air Soil Pollut (2017) 228: 24 and may have different ratios of oxides in other locations (Antunes et al. 2012). Chemical composition of RM and NRM are shown in Table 2. Principal components in both materials are iron, aluminium and silicon oxides. However, the results indicated that the NRM exhibits more calcium oxide than RM. X-Ray diffraction (XRD) patterns of RM and NRM are given in Fig. 2. RM is composed of gibbsi te (Al(OH)3) , kaol in i te (Al2Si2O5(OH)4), quartz (SiO2), goethite (FeO(OH)), he- matite (Fe2O3) and sodalite (Na8Al6Si6O24Cl2). NRM possesses the same mineral phases and calcite (CaCO3), which appearing in several peaks in XRD patterns. The characterisation results of RM and NRM clearly indicate that red mud pH was neutralised, besides EC has decreased (Table 1). EC value reduction occurred due to solubilisation of cations adsorbed in RM during Ca(NO3)2-neutralisation, because Ca(NO3)2 has extract- ed the exchangeable phases of red mud (Santona et al. 2006; Costa et al. 2009), especially those cations that are adsorbed in sodalite mineral phase, such as aluminium, silicon and sodium, amongst others. Several studies have evaluated the neutralisation of red mud with sea- water, which concluded that reduction of pH values is associated with reactions between the carbonate and bicarbonate alkalinity of red mud and calcium to form calcite (CaCO3) (McConchie et al. 2000; Hanahan et al. 2004). These studies were carried out to evaluate the adsorption capacity of cadmium (Grudić et al. 2013), arsenate, vanadate and molybdate (Palmer et al. 2010). Recently, Souza et al. (2013a) explained the influence of the thermal treatment of seawater neutralised red mud on the adsorption of Reactive Blue 19 dye. During the activation, the Ca(NO3)2 is dissolved in water solution (Ca(NO3)2→Ca2+ + (NO3)2 2−) and the Ca2+ reacts with alkalinity from red mud to precipitate calcite, according to Eqs. 13 and 14. XRD and MEV- EDS analysis confirmed the presence of calcite in NRM (Figs. 1 and 2). Besides, the chemical composition of NRM also indicates an increase in Ca2+ content in relation to RM due to calcite formation during Ca(NO3)2-neutralisation (Table 2). The increase in SBET value can also be explained by the formation of calcite in the surface of red mud grains (Fig. 1b). OH− aqð Þ þ CO2 aqð Þ→ HCO3 − aqð Þ ð13Þ 2HCO3 − aqð Þ þ Ca2þ aqð Þ→CaCO3 sð Þ þ H2CO3 aqð Þ ð14Þ Fig. 1 SEM-EDS of RM (a) and NRM (b) Table 2 Chemical com- position (wt%) of RM and NRM Oxide RM NRM SiO2 16.29 14.85 TiO2 8.66 8.72 Al2O3 17.50 16.49 Fe2O3 35.77 35.06 MnO 0.24 0.27 MgO 0.10 0.12 CaO 3.55 8.19 Na2O 4.45 3.31 K2O 0.37 0.29 P2O5 0.37 0.39 LOI 12.60 12.32 Total 99.90 100.01 Water Air Soil Pollut (2017) 228: 24 Page 5 of 13 24 4.2 Influence of pH in Adsorption Experiments The removal of Ni(II), Pb(II) and Zn(II) by RM and NRM at different values of pH is given in Fig. 3. In the experiments at pH 10 and 12, values of C0 e Ce were lower than metal limit detection of ICP OES device, propably due to precipitation of these metals, as indicated in the literature (Bhattacharyya and Gupta 2008; Zhou and Haynes 2011). This precipitation was confirmed using the software CHEAQS (CHemical Equilibria in Aquatic Sys- tems) (Verweij 2014), which function is to calculate the chemical equilibrium in aqueous systems and find the chemical speciation of chemical com- pounds. Thus, the results obtained in experiments performed at pH 10 and 12 were not considered, because this study had interest to investigate ad- sorption of dissolved metals in aqueous solution. For all metals, the removal efficiency increases at pH 7. The removal efficiency was practically equal for the RM and NRM in all pH values considering the associated standard deviation. Thus, the Ca(NO3)2 neutralisation process does not affect the removal efficiency of Ni(II), Pb(II) and Zn(II) in aqueous solution. For the concentra- tion tested, at pH 7 the highest values of Fig. 2 XRD patterns of RM and NRM. Sodalite Sdl,GibbsiteGbs, Goethite Gt, Kaolinite Kln, Hematite Hem, Quartz Qtz and Calcite Cal Fig. 3 Effect of pH on the adsorption of Ni(II), Pb(II) and Zn(II) by RM (a) and NRM (b) in different pH conditions. Bars indicate standard deviation 24 Page 6 of 13 Water Air Soil Pollut (2017) 228: 24 percentages removal were 53% for Ni(II) in RM, 97% for Pb(II) in NRM and 99% for Zn(II) in NRM. In highly acidified solution, at pH 2 and 4, the highest removal efficiency was ca. 30%, indi- cating a weak adsorption process in relation to solution at pH 7. Although the value of 7 has been found as the opti- mum pH, the initial pH of RM and NRM samples with metals in aqueous solutions varied between 5.0 and 5.5, which is a value widely used in adsorption studies (Santona et al. 2006; Nadaroglu et al. 2010; Smiljanic et al. 2010; Smiciklas et al. 2014). This value is consid- ered ideal because at acid pH, metals precipitation does not occur, and pH is not so low, which could adversely affect the adsorption. In addition, when carrying up the other adsorption experiments at natural pH of the medi- um, studies are more practical and economical because it is not necessary pH adjustment and use of acidic and basic reagents. 4.3 Adsorption Isotherms The total amount of Ni(II), Pb(II) and Zn(II) in the aqueous solutions was higher than the cationic ex- change capacity of RM and NRM. The removal of these metals by RM and NRM was higher than their respective CEC. Figure 4 shows adsorption isotherms adjusted by Freundlich and Langmuir models for the Ni(II), Pb(II) and Zn(II) onto RM and NRM. Freundlich and Langmuir isotherm’s parameters are shown in Table 3. For all samples, the relative standard deviation was lower than 4%. Metal adsorption on RM and NRM was practi- cally identical and increased in the following order: Zn(II) ≤Ni(II) < Pb(II) (mmol g−1) (Fig. 4). Ni(II), Pb(II) and Zn(II) adsorption was higher than 68% at lower concentrations (0.5 to 1.0 mmol 25 mL−1). When the Ni(II), Pb(II) and Zn(II) concentrations increase, the adsorption in RM and NRM decreases, indicating that the efficiency of adsorption is lower for the highest concentrations. The highest values of R2 were obtained for Freundlich model for both materials, as well as this model tends to better represent the average of experimental, considering the ARE values (Table 3). Thus, for this applica- tion, the Freundlich model represents the phenom- enon of adsorption of Ni(II), Pb(II) and Zn(II) on RM and NRM. The maximum adsorption capacity values (qmax) of RM and NRM were 1.78 and 1.79 mmol g−1 for Ni(II), 2.13 and 2.23 mmol g−1 for Pb(II) and 1.14 and 1.06 mmol g−1 for Zn(II), at pH 5.0–5.5, re- spectively. The values of RM and NRM adsorption capacity of Ni(II) and Pb(II) (in mmol g−1) were higher than the values obtained in the literature to natural or activated red mud (Table 4) (Apak et al. 1998b; Gupta et al. 2001; Santona et al. 2006; Hannachi et al. 2010; Smiljanic et al. 2010; Fig. 4 Adsorption isotherms of Ni(II), Pb(II) and Zn(II) by RM and NRM (a and b, respectively) using Langmuir (continuous line) and Freundlich (dashed line) adsorption models. Bars indicate standard deviation Water Air Soil Pollut (2017) 228: 24 Page 7 of 13 24 Pulford et al. 2012; Smiciklas et al. 2014). For Zn(II), adsorption capacity was lower than the nat- ural red mud (Vaclavikova et al. 2005; Santona et al. 2006) and treated by HCl (Santona et al. 2006) and higher than the red mud neutralised by CO2 (Sahu et al. 2011), treated by CaSO4 (Lopez et al. 1998) and thermally treated (Gupta and Sharma 2002) (Table 4). Sodalite (calcium and sodium tectosilicate) possesses open porous structure, being found in RM and NRM. Table 4 Comparison of RM and NRM adsorption capacity (mmol g−1) with other studies Metal Treatment Adsorption capacity Reference Ni(II) RM—natural 1.78 Present study NRM—Ca(NO3)2 1.79 Present study Natural red mud 0.23 Hannachi et al. (2010) Red mud—HCl 0.19 Smiciklas et al. (2014) Red mud—thermal 0.37 Smiljanic et al. (2010) Pb(II) RM—natural 2.13 Present study NRM—Ca(NO3)2 2.23 Present study Natural red mud 1.88 Santona et al. (2006) Red mud—carbonised 0.45 Pulford et al. (2012) Red mud—HCl 0.84 Apak et al. (1998b) Red mud—HCl 0.77 Santona et al. (2006) Red mud—thermal and H2O2 0.35 Gupta et al. (2001) Zn(II) RM—natural 1.14 Present study NRM—Ca(NO3)2 0.96 Present study Natural red mud 2.05 Vaclavikova et al. (2005) Natural red mud 2.47 Santona et al. (2006) Red mud—CaSO4 0.19 Lopez et al. (1998) Red mud—CO2 0.23 Sahu et al. (2011) Red mud—HCl 1.59 Santona et al. (2006) Red mud—thermal 0.18-0.22 Gupta and Sharma (2002) Table 3 Parameter of adsorption using Freundlich and Langmuir models for Ni(II), Pb(II) and Zn(II) adsorption by RM and NRM, at pH 5–5.5 Freundlich parameters Langmuir parameters KF ((mmol g−1) (25 mL mmol−1)1/n) 1/n R2 ARE qmax (mmol g−1) KL (25 mL mmol −1) R2 ARE RM Ni(II) 1.36 0.21 0.99 0.69 1.78 4.56 0.97 8.11 Pb(II) 1.68 0.20 0.99 0.58 2.13 11.37 0.99 1.37 Zn(II) 1.12 0.12 0.99 −0.59 1.14 −46.76 0.96 11.72 NRM Ni(II) 1.29 0.23 0.96 −0.27 1.79 3.57 0.97 6.59 Pb(II) 1.88 0.13 0.99 0.34 2.23 23.9 0.99 10.59 Zn(II) 0.86 0.16 0.99 −0.65 1.06 10.34 0.92 −2.69 24 Page 8 of 13 Water Air Soil Pollut (2017) 228: 24 This mineral is considered as zeolite-type mineral and exhibits permanently negatively-charged surface (Smiciklas et al. 2014), which is responsible for the adsorption capacity of RM and NRM. Besides, other minerals (Al and Fe oxides) also allows the adsorption of Ni(II), Pb(II) and Zn(II) (Santona et al. 2006), which were found in RM and NRM (Fig. 2). 4.4 Kinetic Studies The adsorption kinetics of Ni(II), Pb(II) and Zn(II) by RM and NRM, at pH 5.0–5.5 are shown in Figure 5. It can be seen that the stabilisation of adsorption has started from 420 min for all metals. This time of 7 h was considered as minimum time for adsorption tests. Fig. 5 qt versus adsorption time of Ni(II), Pb(II) and Zn(II) using RM and NRM adjusted by pseudo-first-order Lagergren—PFO, pseudo-second-order—PSO, Elovich—ELO and intraparticle diffusion—INT models (a, b, c, d, e and f, respectively). Bars indicate standard deviation Water Air Soil Pollut (2017) 228: 24 Page 9 of 13 24 The parameters shown in Table 5 were obtained from the adjustment of the kinetic models to the adsorption experimental data, at pH 5.0–5.5, using pseudo-first order Lagergren (Fig. 5a, b), pseudo-second order (Fig. 5a, b), Elovich (Fig. 5c, d) and intraparticle diffu- sion (Fig. 5e, f) models. Comparing the parameters of the pseudo-first-order Lagergren and pseudo-second-order models, and their adjustment to the experimental data based on the ARE values and R2 (Table 5), it is clear that ARE values are lower for the model of pseudo-second order, for all metals, for RM and NRM. The R2 values obtained to pseudo-second order were higher than values found for pseudo-first Lagergren model (except for Ni(II) adsorp- tion on NRM). For the pseudo-second-order model, qe values are closer to the values of qe(exp), except for the Ni(II) adsorption on NRM. Besides, according to the ARE and R2 values, the pseudo-second-order model indicated the best fit to the experimental data for all metals by RM and NRM. The absolute values of ARE for Elovich and intraparticle diffusion models can be considered low, except for the Ni(II) and Zn(II) adsorption on NRM. The R2 varied from 0.48 to 0.95 for Elovich model and from 0.73 to 0.99 for intraparticle diffusion model. The intercept values presents in Table 5 suggest that the surface adsorption and intraparticle-diffusion were only concurrently occurring during the adsorption of Ni(II), Pb(II) and Zn(II) on RM and NRM. 4.5 Thermodynamic Studies The percentage of adsorption of Ni(II), Pb(II) and Zn(II) by RM and NRM at different temperatures (303, 313 and 323 K), at pH 5.0–5.5, is given in Fig. 6a, b, re- spectively. The percentage adsorption of these metals increases with increasing temperature. The constants ΔH° and ΔS° were obtained from the slope and inter- cept of plot ln Kc versus T−1103 (van’t Hoff chart) (Fig. 6c, d), respectively. Table 6 presents the thermo- dynamic parameters obtained, ΔH° (kJ mol−1), ΔS° (J (mol K)−1) and ΔG° (kJ mol−1). Positive values for ΔH° indicated the occurrence of endothermic reactions and positive values ofΔS° indicated affinity of the RM Table 5 Kinetic parameters ob- tained by pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion models for Ni(II), Pb(II) and Zn(II) adsorp- tion by RM and NRM, at pH 5– 5.5 qe(exp) (mmol g−1) RM NRM Ni(II) Pb(II) Zn(II) Ni(II) Pb(II) Zn(II) 0.77 1.28 0.90 0.58 0.71 0.61 Pseudo-first order qe (mmol g−1) 0.24 0.39 0.26 0.57 0.47 0.35 k1 × 10 3 (min −1) 1.15 3.70 7.13 1.38 0.46 0.69 R2 0.97 0.98 0.92 0.99 0.33 0.51 ARE 88.54 0.82 79.94 22.11 0.81 80.81 Pseudo-second order qe (mmol g−1) 0.78 1.20 0.90 0.81 0.68 0.63 k2 × 10 3 (g (mmol min)−1) 25.42 33.81 88.16 1.75 6.01 7.67 R2 0.99 0.99 0.99 0.87 0.71 0.90 ARE 12.73 0.06 1.87 1.66 0.13 11.32 Elovich α (mmol (g min)−1) 59.87 95.25 76.75 0.0046 0.03 0.12 β (g mmol−1) 19.96 11.09 15.41 8.67 11.52 17.01 R2 0.86 0.95 0.94 0.84 0.61 0.48 ARE −0.16 0.001 −0.14 22.78 0.05 −8.29 Intraparticle diffusion kint (mmol (g min½)−1) 0.0071 0.01 0.0078 0.0165 0.013 0.0096 C 0.49 0.89 0.66 −0.06 0.12 0.18 R2 0.98 0.88 0.78 0.99 0.78 0.73 ARE 0.15 0.002 −0.42 9.74 0.03 −5.24 24 Page 10 of 13 Water Air Soil Pollut (2017) 228: 24 and NRM for Ni(II), Pb(II) and Zn(II) (Chowdhury et al. 2011). For ΔG°, negative and positive values were observed. Negative values confirmed the natural spon- taneity of the adsorption process, which is the case at 303 K for Pb(II) and Zn(II) adsorption on RM, at 313 K for Pb(II) adsorption on RM and Zn(II) adsorption on RM and NRM and at 323 K for all cases, except for Ni(II) adsorption on NRM. The growth of the absolute value of ΔG° with increasing temperature suggests an increase in adsorption affinity of Ni(II), Pb(II) and Zn(II ) for RM and NRM with temperature (Chowdhury et al. 2011). Table 6 Thermodynamic parameters obtained for Ni(II), Pb(II) and Zn(II) adsorption by RM and NRM, at pH 5–5.5 ΔH° (kJ mol−1) ΔS° (J (mol K)−1) ΔG° (kJ mol−1) RM 303 K 313 K 323 K Ni(II) 27.69 87.27 1.30 1.13 −0.44 Pb(II) 23.54 80.46 −0.83 −1.66 −2.44 Zn(II) 26.17 89.24 −0.69 −2.15 −2.45 NRM Ni(II) 36.86 110.54 3.63 1.69 1.46 Pb(II) 68.95 219.46 2.29 0.62 −2.12 Zn(II) 52.58 168.40 1.69 −0.42 −1.66 Fig. 6 Adsorption of Ni(II), Pb(II) and Zn(II) by RM and NRM at different temperatures (303, 313 and 323 K) and van’t Hoff chart (lnKc versus T−1103) for Ni(II), Pb(II) and Zn(II) by RM and NRM (a, b, c and d, respectively). Bars indicate standard deviation Water Air Soil Pollut (2017) 228: 24 Page 11 of 13 24 5 Conclusion The chemical treatment with Ca(NO3)2 has caused pH neutralisation of the red mud, which is a very important factor for the reuse of this waste of bauxite refining. This occurs due to reactions between the carbonate and bicar- bonate alkalinity of red mud and calcium from calcium nitrate to form calcite (CaCO3), fact confirmed with the chemical andmineralogical results of RM andNRM. The adsorption of Ni(II), Pb(II) and Zn(II) on RM and NRM was dependent on the pH of the solution, with the highest percentage of adsorption occurs at pH 7. The adsorption isotherm model that best represented the adsorption of thesemetals on RM andNRMwas the Freundlichmodel. The qmax values of RM and NRM, respectively, were 1.78 and 1.79 mmol g−1 for Ni(II), 2.13 and 2.23 mmol g−1 for Pb(II), and 1.14 and 1.06 mmol g−1 for Zn(II), respectively. Sodalite is responsible for the adsorption capacity of RM and NRM. The pseudo- second kinetic order model presented the best fit to the experimental data for these metals. 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A comparison of inorganic solid wastes as adsorbents of heavy metal cations in aqueous solution and their capacity for desorption and regeneration. Water, Air, and Soil Pollution, 218, 457–470. Water Air Soil Pollut (2017) 228: 24 Page 13 of 13 24 Adsorption of Ni(II), Pb(II) and Zn(II) on Ca(NO3)2-Neutralised Red Mud Abstract Introduction Theoretical Background Adsorption Isotherms and Kinetic Studies Thermodynamic Studies Average Relative Error Materials and Methods Sampling, Activation Procedures and Characterisation Adsorption Studies Results and Discussion Characterisation of RM and NRM Influence of pH in Adsorption Experiments Adsorption Isotherms Kinetic Studies Thermodynamic Studies Conclusion References