PHOSPHATE – INDUCED MERCURY STABILIZATION: A PRETREATMENT TECHNOLOGY

The toxic nature of mercury and its compounds has been known for centuries. The most well established approaches for stabilizing mercury in wastes is precipitation, such as sulfide precipitation. Given the very low solubility of mercury phosphates, a novel approach, phosphate-induced mercury stabilization, has been investigated in this study. In the preliminary stage of the study, soluble phosphate (Na2HPO4) was proved to successfully stabilize mercury both in pure solution and in surrogates. The effects of phosphate: mercury molar ratio, stabilization pH and stabilization time were investigated. P/Hg molar ratios of 3–5 were found to be effective for Hg stabilization. The optimal pH range for the phosphate process was found to be pH 2–5. After 24 hours tumbling, reactions reached equilibrium. Mercury concentrations in the solution were lowered from 400 mg/L down to lower than 2 mg/L at pH 4. The stabilization efficiency was higher than 99%. At higher pH, less mercury was precipitated. The stabilization efficiency was about 80%. For Hg-doped sand, the mercury concentration after phosphate treatment was lower than 10 mg/L at pH 4, compared with above 100 mg/L without phosphate addition. Bentonite was found to improve mercury stabilization in Hg-doped sand. However, the phosphate process alone was unable to stabilize mercury-containing surrogate well enough to pass TCLP test.