Evaluation of chemical immobilization treatments for reducing arsenic transport in red mud

Red mud (RM) was produced during alumina production from bauxite known as the Bayer process. Arsenic was detected in the solid phase of RM (RMsf) which was disposed in the disposal area. This study investigates the effectiveness of using Zero-valent iron (ZVI), ferrihydrite, ferrous sulfate (FeSO4), waste acid (WA) or CO2 for immobilization of arsenic in the RMsf. To test the effect of the amendments on the arsenic leachability, the RMsf samples were amended with the iron-based materials or acidifiers at various w/w (weight/ weight) ratios (1–10 %) for 30 days. The leachability of arsenic in the RMsf was evaluated by a 4-step water elusion process. After 30-day treatment of the RMsf, the leachability of As decreased from an initial (12.7 %) to (7.0 %) with a w/w ratio of 5 % ZVI (0 %) with 5 % FeSO4 7H2O, (3.4 %) with 5 % ferryhydrite, (2.0 %) with 6 % WA and (11.8 %) with 6 % CO2. FeSO4 7H2O and WA showed more effectively than other amendments for immobilizing arsenic. Arsenic fractionation with a sequential extraction procedure was used to evaluate the arsenic migration potential in the RMsf. FeSO4 and WA were effective in increasing the hydrous oxide combined arsenic in the RMsf. The leachable Cl and SO4 2in the RMsf increased from 2.9 to 14.1 mg/g and 19.9–44.4 mg/g with 6 % WA and 5 % FeSO4 7H2O added, respectively. The estimated cost of the FeSO4 and WA treatment was 0.47 and 0.49 USD per ton, respectively.