Removal of Mercury from Low-Concentration Aqueous Streams Using Chemical Reduction and Air Stripping

Field, laboratory, and engineering data confirmed the efficacy of chemical reduction and air stripping as a low concentration mercury treatment concept for water containing Hg(II). The process consists of dosing the water with low levels of stannous chloride [Sn(II)] to convert the mercury to elemental mercury (Hg^0). Hg^0 can easily be removed from the water by air stripping or sparging. We studied this concept for groundwater containing initial mercury concentrations of approximately 138 ng/L (0.00069 (mu)mol/L). In undosed samples, sparging removed 0% of the initial mercury. Removal in the treated samples varied by reagent dose. Low reagent doses, with Sn:Hg stoichiometric ratios <1, showed little removal. High reagent doses, with Sn:Hg stoichiometric ratios greater than about 5 to 25, showed relatively complete removal (>94%) and yielded final mercury concentrations <10 ng/L (<0.00005 (mu)mol/L). At intermediate doses, mercury removal was a function of the dose. A kinetic study indicated that addition of the Sn(II) reagent resulted in rapid reduction of Hg(II) to Hg0. When combined with standard supporting engineering techniques (e.g., treating the purge air) as needed, a simple system of chemical reduction and stripping may be useful and cost effective.