Sorption of Cadmium, Lead, Arsenate, and Phosphate on Red Mud Combined with Phosphogypsum

Abstract

The combination of red mud (RM) and phosphogypsum (PG) is an interesting alternative for reusing these by-products as either adsorbent or soil amendment. In this context, this study aimed to evaluate cation (cadmium and lead) and anion (arsenate and phosphate) adsorption and desorption on RM, PG, and their blendings at different proportions (w/w): 100% PG, 75% PG + 25% RM, 50% PG + 50% RM, 25% PG + 75% RM, and 100% RM. Cadmium, lead, arsenate, and phosphate adsorption and desorption tests were carried out using 0.01 mol L−1 Ca(NO3)2 for cations and 0.03 mol L−1 NaCl for anions. The initial concentrations of cations and anions were 0.33 and 0.66 mmol L−1, respectively, and the equilibrium pH was 5.5 ± 0.2 (adsorbent:solution ratio of 1:100). RM adsorbed 99% of phosphate, 92% of lead, 87% of arsenate, and 26% of cadmium. The blending containing 75% of RM and 25% of PG adsorbed 95% of phosphate, 97% of lead, 76% of arsenate, and 32% of cadmium. The amount of cadmium and arsenate adsorbed increased with increasing RM proportion. Cadmium (16%) and arsenate (6.9%) desorption percentages were higher than lead (0.4%) and phosphate (1.3%). Effectively adsorbed percentages followed the decreasing order: phosphate (98%) > lead (91%) > arsenate (83%) > cadmium (19%) for RM and lead (97%) > phosphate (94%) > arsenate (70%) > cadmium (26%) for the mixture containing 75% of RM and 25% of PG.