Sulfate Removal from Acid Mine Drainage using adsorption onto acid- activated neutralized red mud: Effect of various parameters, isotherm and kinetic studies

Abstract

Abstract Red mud (RM) as an alternate adsorbent was explored for the removal of sulfate from the aqueous solution by the batch equilibration technique. The results revealed that both seawater (BRM) and acid treatment RM (RMH) can modify high rates of pH and improve the adsorption capacity of raw RM. The study also focused on effect of various factors on sulfate removal, including acid dosage, pH, adsorbent dose, adsorbate concentration and contact time. The most sorption capacity and sulfate removal for RM, BRM, and RMH was equal to 12.7, 15.9 and 23.9 mg/g and 25.5, 31.9 and 41.3 % at pH rates of 5.5, 6.5 and 4.5, respectively. Sulfate removal reached equilibrium in 30, 60 and 60 min contact time for RM, BRM, and RMH. Based on kinetic studies, pseudo-second-order is the best model for sulfate adsorption onto RMH and BRM, and chemisorption interaction is controlling step. Isotherm studies demonstrated RM surface is heterogeneous and adsorption occurs through particle diffusion and multilayer and Freundlich model was the best isotherm for description sulfate adsorption on RMH and BRM. FTIR results illustrated a sharp intensity (1100 cm-1) related to SO2-4 in RM samples after sulfate adsorption that is sharper for HRM than BRM and RM. EDX studies showed increasing Ca peak in BRM and Fe and Ti peaks in RMH results. It seems that mononuclear exchange with OH- groups and binuclear ligand exchange are the main mechanisms of sulfate adsorption on RM.