Sequential electrolysis and reverse osmosis to improve arsenic removal from water

Abstract

Reverse osmosis (RO) typically removes >98% arsenate (As[V]) but removes arsenite (As[III]) comparatively poorly (approximately 50%–80%). Therefore, oxidizing As(III) to As(V) can improve arsenic removal using RO. In this study, electrolytic oxidation was used to oxidize As(III) in the feed water, and an extreme low-pressure RO membrane was subsequently used to remove the arsenic. Using Ti/IrO₂ electrodes under 30 mA DC current in 500 mg/L NaCl solution primarily generated free chlorine, which completely oxidized 300 μg/L As(III) to As(V). Subsequent arsenic removal by RO increased from 54.2% without oxidation to 98.2% with oxidation. Using electrolysis-RO, arsenic removal significantly increased beyond RO alone, even in the presence of ferrous iron and natural organic matter. When sulfide and As(III) are present in water, they react to produce thioarsenate ions, the formation of which increased As(III) removal to 90% without electrolytic oxidation and electrolytic oxidation did not improve arsenic removal beyond these levels.