Removal of arsenic using reverse osmosis

Abstract

Arsenic is an element that naturally occurs in rocks and sediments. In some areas, arsenic is released to ground water and can contaminate drinking water sources. Since arsenic is toxic in high doses and a carcinogen, the World Health Organisation (WHO) recommends arsenic levels lower than 10 μg/L in drinking water. This thesis describes the removal of arsenic in a reverse osmosis system. Arsenic is usually present in water in two oxidation states, as arsenite (As(III)) or arsenate (As(V)). Arsenate is usually easier to remove than arsenite, and the experiments have emphasized the removal of arsenite in double filtration process and compared to the removal of arsenate. Previous studies at UMB have found reverse osmosis as an efficient way to remove arsenite when the water is filtrated twice, almost as efficient as arsenate removal in one filtration. One hypothesis that was tested was that arsenite was oxidized during the removal process and was removed easily as arsenate in the second filtration. The verification of arsenate removal gave a removal efficiency of 97-99 % of As(V) after one filtration. Double filtration of arsenite gave a total arsenic removal between 91.8-94.3 %. Oxidation of arsenite to arsenate was tested and showed a small oxidation rate of between 5 and 10 %. During the filtration processes, about 5 % of arsenite was oxidized. However, an effective membrane and RO unit rather than oxidation caused the great removal efficiency of arsenite.