An Improved Method of Arsenic (III) Removal By Reverse Osmosis Membrane

Arsenic is a Group 1 carcinogen as there is abundant research to support that ingestion of arsenic in drinking water and food can lead to liver, lung, kidney, or bladder cancer in humans. The recommend World Health Organization (WHO) arsenic standard in drinking water is 10 µg/L, while the Environmental Protection Agency's (EPA) Maximum Contaminant Level (MCL) of arsenic in drinking water is 10 µg/L. Globally, at least 40 million people face more than 10 µg/L arsenic contamination in their drinking water. As(III) (trivalent state, such as arsenite), and As(V) (pentavalent state, such as arsenates) are the dominant arsenic forms in water. Because the pKas of arsenite and arsenate are 9.2 and 2.2, respectively, the dominant form of arsenite in natural water is as a neutral molecule, while arsenate is present in ionic form. According to previous research, removal of As(III) by RO treatment is 50 to 80%, but more than 98% As(V) is removed as the dissociated arsenate ions are amenable to removal by RO. Therefore, oxidizing As(III) to As(V) can improve arsenic removal by RO membranes. In this study, electrolytic oxidation was used to oxidize arsenite in the feed water, and then arsenic removal using RO membrane was measured. The use of electrochemical pretreatment ahead of RO membranes has not been explored deeply, and no existing studies address the feasibility of electrolytic oxidation to improve arsenic removal by RO membranes. The results of this study showed that electrolytic oxidation using Ti/IrO2 electrodes under 30 mA DC current in 500 mg/L NaCl solution can completely oxidize 360 µg/L As(III) to As(V) in 10 seconds. The subsequent arsenic removal by RO membranes increased from 54.2% (without oxidation) to 98.2%. The main oxidant generated in the electrolytic oxidation process was free chlorine. Using combined electrolysis-RO, the arsenic removal significantly increased beyond RO alone, even in the presence of ferrous and alkalinity and natural organic matter. The presence of sulfide impacted the As(III) form in water, producing ionic species, which increased the As(III) removal to close 90% without electrolytic oxidation.