Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters

An estimated 30-50 million people in Bangladesh consume groundwater with ar senic contents far above accepted limits. A better understanding of arsenic redox kinetics and simple water treatment procedures are urgently needed. We have studied thermal and photochemical As(III) oxidation in the laborato ry, on a time scale of hours, in water containing 500 mug/L As(III), 0.06-5 mg/L Fe(II,III), and 4-6 mM bicarbonate at pH 6.5-8.0. As(V) was measured colorimetrically, and As(III) and As(tot) were measured by As(III)/As(tot)- specific hydride-generation AAS. Dissolved oxygen and micromolar hydrogenpe roxide did not oxidize As(III) on a time scale of hours. As(III) was partly oxidized in the dark by addition of Fe(II) to aerated water, presumably by reactive intermediates formed in the reduction of oxygen by Fe(ll). In sol utions containing 0.06-5 mg/L Fe(II,III), over 90% of As(III) could be oxid ized photochemically within 2-3 h by illumination with 90 W/m(2) UV-A light . Citrate, by forming Fe(III) citrate complexes that are photolyzed With hi gh quantum yields, strongly accelerated As(lll) oxidation. The photoproduct of citrate (3-oxoglutaric acid) induced rapid flocculation and precipitati on of Fe(III). in laboratory/tests, 80-90% of total arsenic was removed aft er addition of 50 muM citrate or 100-200 muL (4-8 drops) of lemon juice/L, illumination for 2-3 h, and precipitation. The same procedure was able to r emove 45-78% of total arsenic in first field trials in Bangladesh.