Optimizing arsenic removal during iron removal: Theoretical and practical considerations

New health effects research prompted the United States Environmental Protection Agency (USEPA) to reduce the drinking water standard for arsenic from 0.05 to 0.010 mg l⁻¹ (10 μg l⁻¹), and as a result many drinking water systems (particularly smaller ones) throughout the country will no longer be in compliance. In waters that contain natural iron, arsenic removal can be achieved during iron removal, but the effectiveness of iron to remove arsenic depends on many variables. The objective of this study was to identify the operational and water quality factors that impact arsenic removal during iron removal. Bench-scale (‘batch’ and standard jar) tests were used to evaluate the effects of pH, phosphate, other water chemistry variables, and the oxidant used to oxidize Fe(II) on the removal of arsenic. Treatment operation considerations, including sequence of oxidant addition and contact time, were also considered. Results showed that (1) arsenic removal improves with increasing iron concentration and particle surface area; (2) freshly precipitated iron particles had a much greater capacity to remove arsenic than preformed particles that were formed by oxidation of ferrous iron with either oxygen or chlorine; (3) chlorination, or application of a stronger oxidant, may be necessary to improve arsenic removal at many drinking water treatment plants; (4) the point of strong oxidant addition in the treatment train is important; and (5) the pH and other competing water quality variables such as phosphate play significant roles in the amount of arsenic removed.