Chromate removal from aqueous wastes by reduction with ferrous ion

Experimental measurements of reaction stoichiometries indicate that optimum conditions for chromate removal from oxygenated, 25/sup 0/C aqueous wastes by reduction with ferrous ion occur at pH less than 10.0 in solutions containing the following anions: ClO/sub 4//sup -/, Cl/sup -/, F/sup -/, NO/sub 3//sup -/, and PO/sub 4//sup 3 -/. At pH greater than 10.0 and at lower pH in solutions that contain greater than 10/sup -4/ M total phosphate, chromate is less efficiently reduced by ferrous ions as evidenced by nonstoichiometric reduction. Nonstoichiometric reduction of the chromate by the ferrous ion is caused by the competitive oxidation of the ferrous ions by dissolved oxygen. Depending on pH and solution composition, the reduction of chromate by ferrous ions causes rapid precipitation of (Cr/sub x/Fe/sub 1-x/)(OH)/sub 3/(s). The solubility of this solid limits Cr(III) concentrations, between pH 5.0 and pH 11.0, to less than the drinking water limit of 10/sup -6/ M.