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Abstract: Arsenic was detected in a red mud (RM) produced during alumina production from bauxite known as the Bayer process. The transporting RM was a mixture of RM solid phase (RMsf) and RM liquid phase (RMlf). The mass content of RMsf in RM is about 30–40%. The alkalinities concentrations in the RMlf were in a range of 37.2×103 mg/l to 51.5×103 mg/l. Acidification by CO2 or waste acid (WA) combined with ferrous (Fe2+) treatment was evaluated for arsenic immobilization in the RM. The aqueous arsenic concentration in the RMlf decreased from 6.1mg/l to 0.5mg/l and 0.06mg/l with the addition of CO2 and WA, respectively. Ferrous was then added to decrease the aqueous arsenic concentration to be lower than 0.05mg/l. The cost-effective dosages of CO2 or WA were 80.1g/l or 26.7g/l, and the corresponding dosages of ferrous were both 6g/l. A 23 full factorial design was employed to evaluate the importance of chemical components of the RM in the cost of arsenic immobilization. High concentrations of arsenic and alkalinities in the RM will increase the cost while the effects of alumina contents varied during the different acidifications. Dissolvable arsenic in the RMsf was 8.2% and 9.5% after the CO2 and WA combined ferrous treatments, respectively. [Copyright &y& Elsevier]