Arsenic Removal from Groundwater by Fe-Mn Oxidation and Microfiltration

Although arsenic has been classified as human carcinogen and its acute toxicity has long been known, the long term health effects of trace arsenic levels have only recently been realized. The maximum contaminant level (MCL) for arsenic in drinking water was consequently lowered from 50 µg/L to 10 µg/L in 2002, resulting in many water utilities needing upgraded to achieve compliance. In groundwater treatment, Fe-Mn oxidation and microfiltration has been recognized as a cost-effective technology since arsenic removal is facilitated during the removal of iron and manganese.This study investigated the efficiency of arsenic removal using Fe-Mn oxidation/microfiltration under various process conditions including iron to arsenic ratio, pH, potassium permanganate dose, contact time, oxidation state of iron, and stirring in dead-end filtration cell. Arsenite removal was relatively insensitive to variations in pH in the range of 6.5-8.0 when only aeration was applied, but the impact of pH was important in the potassium permanganate oxidation. At neutral pH, iron to arsenic ratio of 60 was sufficient to reduce the arsenic concentrations ranging from 25-250 ppb to below 10 ppb (primary MCL for arsenic) with aeration and microfiltration. The oxidation state of iron did not considerably affect the arsenic removal. Oxidation with potassium permanganate facilitated additional arsenite removal compared to aeration alone. Although higher arsenic removals were observed at pH 7.0 than at pH 8.0, identical residual arsenic concentrations were obtained with 0.5 mg/L permanganate dose at both pH values.In experiments with various iron levels, concentrations of iron in the permeate remained around 0.01 mg/L, which is far below the secondary MCL for iron (0.3 mg/L), while the manganese standard (0.05 mg/L) was only met when initial concentration of manganese was 0.2 mg/L. Iron and manganese particles were the primary reason for membrane fouling. The results of this study are important for water treatment systems using groundwater with elevated arsenic and iron concentrations as a raw water source. The proposed technology is much simpler to operate than a conventional coagulation process and chemical requirements are minimized by utilizing existing iron concentration in the raw water