Cadmium, Lead, Arsenate, and Phosphate Sorption On Red Mud and Gypsum Mixtures.

Red mud (RM) and gypsum (G) are mining by-products that have potential to be used as soil amendments in contaminated areas. They can be used in natura or be subjected to a pretreatment in order to change their properties aiming for a better sorption performance. This work evaluated cation (Cd and Pb) and anion (As and P) adsorption on RM, comparing it with G and a mixture of both by-products at different proportions: 100% G, 75% G + 25% RM, 50% G + 50% RM, 25% G + 75% RM and 100% RM. After the adsorbent preparation, adsorption of Cd, Pb, As, and P was carried out using nitrate salts for cations and sodium as the accompanying cation for anions (ionic strength of 30 mmol L^-1). Following adsorption, samples were reacted with 30 mL of Ca(NO₃)₂ and NaCl solutions to promote desorption of previously adsorbed elements. After 72h of reaction, samples were centrifuged and the supernatant collected for analyses. The adsorbed concentrations of Cd and As increase with increasing RM proportion in the mix with G, with the highest Cd adsorption occurring for the mixture 25% G + 75% RM. Decreasing the proportion of G mixed with RM decreased Pb adsorption significantly, but did not affect P adsorption. Lead showed the highest Kd value followed by P, As, and Cd for RM containing G; for RM without G the decreasing order was P<Pb<As=Cd. Lead and Cd had the highest and As and P the lowest desorbed amount upon increasing the proportion of RM in the mixture with G. Effectively adsorbed percentages of P and Pb were the highest followed by As and Cd. Sponsored by CNPq, CAPES, and FAPEMIG.