Stannous Chloride Reductive Treatment and Kinetics Using Hexavalent Chromium in Water Supplies

Hexavalent chromium [Cr(VI)] occurs in groundwater and poses potential health concern when present above 10 μg/L in drinking water. Cr(VI) redox conversion to Cr(III), a less toxic and soluble chromium species at groundwater pH, is potentially an effective strategy to control Cr(VI). Stannous chloride (SnCl2), which is already approved for use in drinking water treatment, was investigated in model and real Arizona groundwater for Cr(VI) reduction to Cr(III) by using a jar testing apparatus. With <1 min of mixing, SnCl2 chemically reduced Cr(VI), producing both soluble and particulate forms of tin and chromium. The rate and extent of this reaction varied as a function of Sn(II):Cr(VI) dosing ratios and pH. An empirical rate equation was parameterized using experimental data: d[Cr(VI)]/dt = -k[H+]-0.25[Sn2+]0.85[CrO42-]3 with k = 0.033 μM-2.6s-1. Experimental results and equilibrium modeling showed that apparent competition occurs in real groundwater where coexisting oxyanions (e.g., arsenate) likely react with SnCl2. While freshly prepared commercial-grade and reagent-grade SnCl2 solutions behaved similarly, exposure to the air for 1 to 2 weeks resulted in >50% reduction in Cr(VI) removal capability and raised concern over on-site storage or intermittent use as a wellhead treatment strategy. Results suggest that SnCl2 reductive treatment is a simple, rapid, and highly effective method to treat Cr(VI) in groundwater.