Sulfate Adsorption Process from Acid Mine Drainage by Seawater and Acid-Activated Neutralized Red Mud

Acid mine drainage (AMD) is an environmental concern which contributed to mining activities and its discharge into surrounding environment can lead to acidification of water bodies and soil. In this research, red mud (RM) with alkaline pH as aluminum industry and alternate adsorbent was studied for the removal of sulfate which can be available in AMD by the batch equilibration technique. Seawater washing and acid treatment were employed as RM modification methods. The results reveal that both methods can decrease high rates of pH and improve the sorption capacity of raw RM. The study also focuses on the effect of various factors on sulfate removal, including acid dosage, pH, adsorbent dose, adsorbate concentration, and contact time. Among these factors, pH is the most important and effective factor in the adsorption of sulfate ions and in strongly acid medium, sulfate adsorption increases due to the high concentration of hydrogen ions and more positive charge on the adsorbent surface. The most sorption capacity and sulfate removal for RM, RMS (RM/Seawater), and RMH (RM/HCl) is 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 reaches equilibrium in 30, 60 and 60 min contact time for RM, RMS, and RMH, respectively. Based on kinetic studies, pseudo-second order is the best model for sulfate adsorption onto RMS and RMH, and the chemisorption interaction controls step. Isotherm studies demonstrated adsorption is heterogeneous and occurs through particle diffusion and multilayer and Freundlich model is the best isotherm for description sulfate adsorption on RMH and RMS. FTIR results illustrate a sharp intensity (1100 cm⁻¹) related to SO₄²⁻ in RM samples after sulfate adsorption that is sharper for RMH than RMS and RM. EDX studies show increasing Ca peak in RMS due to calcite and aragonite precipitation during seawater treatment and result of acid washing illustrates increasing Fe and Ti peaks in RMH and calcite minerals are decreased. It seems that mononuclear exchange with OH⁻ groups and electrostatic interaction at lower pH, and binuclear ligand exchange with bridge bond formation are the main mechanisms of sulfate adsorption on RM.