Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron

Removing arsenic from contaminated groundwater in Bangladesh is challenging due to high concentrations of As(III), phosphate and silicate. Application of zero-valent iron as a promising removal method was investigated in detail with synthetic groundwater containing 500 microg/L As(III), 2-3mg/L P, 20mg/L Si, 8.2mM HCO3-, 2.5mM Ca2+, 1.6mM Mg2+ and pH 7.0. In a series of experiments, 1L was repeatedly passed through a mixture of 1.5 g iron filings and 3-4 g quartz sand in a vertical glass column (10mm diameter), allowing the water to re-aerate between each filtration. At a flow rate of 1L/h, up to 8 mg/L dissolved Fe(II) was released. During the subsequent oxidation of Fe(II) by dissolved oxygen, As(III) was partially oxidized and As(V) sorbed on the forming hydrous ferric oxides (HFO). HFO was retained in the next filtration step and was removed by shaking of the sand-iron mixture with water. Rapid phosphate removal provided optimal conditions for the sorption of As(V). Four filtrations lead to almost complete As(III) oxidation and removal of As(tot) to below 50 microg/L. In a prototype treatment with a succession of four filters, each containing 1.5 g iron and 60 g sand, 36 L could be treated to below 50 microg/L in one continuous filtration, without an added oxidant.