SULFIDE-INDUCED MERCURY STABILIZATION

The stabilization of a mercury waste surrogate was performed using sodium sulfide under several different pH and sulfide dosage conditions. Kinetics of the Hg-sulfide reaction was investigated to obtain the optimum treatment time and to determine immobilization pathways. The experimental results indicate that the Hg-sulfide reaction is a relatively slow process that may take more than 100 hours to reach equilibrium. Primary stabilization parameters such as stabilization pH and sulfide/mercury (S/Hg) molar ratio were tested for their influence on stabilization effectiveness. Stabilization efficiency results showed that sulfide is a very effective reagent for stabilizing mercury in the surrogate. The leachability of mercury decreased at low pH values (pH 2–6) compared to that at a higher pH range (pH 7–10). The effects of sulfide dosage on Hg stabilization are much more complicated. It was found that, in the very low pH range (pH 2 and 4), the leachate Hg concentration decreased with an increase of sulfide dosage; while at the higher pH range (pH 5 – 10), the opposite results were observed. Toxicity Characteristics Leaching Procedure (TCLP) tests were conducted to evaluate Hg stabilization effectiveness and to determine optimized process parameters. TCLP Hg results passed the TCLP limit for most of the sulfide-stabilized surrogates. Compared to the TCLP Hg results for untreated surrogates, the stabilization efficiency reached as high as 99% for most stabilization scenarios. From the stabilization scenarios investigated so far, it is concluded that the most effective stabilization occurs at pH 6 combined with a sulfide/mercury molar ratio of 1.