Adsorption of Ni(II), Pb(II) and Zn(II) on Ca(NO3)2-Neutralised Red Mud

This study aimed to investigate a novel method 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 alkalinity of red mud and calcium from calcium nitrate to form calcite (CaCO3). The maximum adsorption capacity 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 adsorption 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 adsorption capacity losses of Ni(II), Pb(II) and Zn(II).