Evaluation of arsenic immobilization in red mud by CO2 or waste acid acidification combined ferrous (Fe2+) treatment

► Acidification and ferrous treatment were combined for arsenic immobilization in the RM. ► Effects of CO2 and waste acid on the RM acidification were evaluated. ► Comparison of effects on arsenic immobilization by CO2-ferrous treatment and waste acid-ferrous treatment was evaluated. ► Costs of arsenic immobilization in the RM by acidification combined ferrous treatment were evaluated.