STABILIZATION OF MERCURY-CONTAINING WASTES USING SULFIDE

Stabilization of mercury-containing wastes has received considerable attention recently, due to concerns about air emissions from typically used thermal treatment technologies. Because of the extremely low solubility of mercuric sulfide, sulfide-induced stabilization is considered to be an effective way to immobilize mercury while minimizing mercury emissions. However, little is known of the mechanisms involved. In addition, the process of sulfide-induced stabilization of mercury-containing wastes has not been sufficiently developed; therefore, further research is needed to optimize the process-controlling parameters. In this study, the stabilization of mercury-containing wastes was performed using sodium sulfide. Primary stabilization variables such as stabilization pH, sulfide/mercury (S/Hg) molar ratio, and stabilization time were investigated. Mercury stabilization effectiveness was evaluated using the Toxicity Characteristic Leaching Procedure (TCLP) and constant pH leaching tests. The effectiveness of mercury immobilization by sulfide was tested in the presence of various concentrations of interfering ions. The results demonstrate that stabilization pH and sulfide dosage have significant effects on the stabilization efficacy. It was found that the most effective mercury stabilization occurs at pH 6 combined with a sulfide/mercury molar ratio of 1. The mercury stabilization efficiency reached 99%, even in the presence of interferents. The constant pH leaching results indicate that sulfide-treated mercury wastes produce significantly higher mercury concentrations in high pH (pH >10) leachants relative to others. Nevertheless, the mercury stabilization efficiency was still as high as 99%, even with exposure of the wastes to high pH leachants. Therefore, it is concluded that sulfide-induced stabilization is an effective way to stabilize mercury-containing wastes. The treatment optimization study indicates that the combined use of increased dosage of sulfide and ferrous ions (S/Hg = 2 and Fe/Hg = 3 at pH = 6) can significantly reduce the interferences by chloride and/or phosphate during sulfide-induced mercury immobilization. Visual MINTEQ simulation results indicate that the precipitation of cinnabar is the main mechanism that contributes to the mercury stabilization by sulfide. However, the formation of soluble mercury sulfide species at excess sulfide dosage due to the common ion effect can cause mercury remobilization from sulfide sludge under conditions that can exist in the landfills.