| This paper presents the study results for a novel stabilization/solidification (S/S) process for high mercury wastes (Hg > 260 ppm). A relatively low-cost powder reactivated carbon (PAC) was used to stabilize mercury in solid wastes. Then the stabilized wastes were subjected to cement solidification. To improve the mercury adsorption capacity, PAC was impregnated with sulfides to obtain sulfurized PAC (SPAC). It was found that sulfurization of PAC by both CS 2 and Na2S significantly improved the mercury stabilization efficiency. For a Hg(NO3)2 solution with 40 mg/L initial Hg2+, the equilibrium concentration of Hg2+ was lowered to 110 μg/L by SPAC, compared with an equilibrium concentration of 4310 μg/L by PAC. The adsorption efficiency was increased by more than one order of magnitude. The mechanism of sulfurization on mercury adsorption was investigated. It is believed that formation of low solubility mercury-sulfide species was the major cause of this phenomenon. The cement-solidified wastes were subjected to TCLP leach testing and constant pH leach testing. For the constant pH leach testing, the wastes were leached at constant pH values of 2, 4, 6, 8, 10, and 12 for 14 days. From the experimental results, it was found that, once in the solidified waste form, SPAC particles retained most of the adsorbed mercury, even in the presence of high chloride concentration, possibly due to the build-up of a gel-membrane outside the carbon pores as the hydration of cement proceeded. Experimental results from constant pH leaching tests indicated that the stabilized and solidified wastes were quite stable over a wide pH range after 14 days. A model was developed to simulate mercury sorption by reactivated carbon in stirred batch reactors. The model involved the coupling of a pseudo-second order kinetic model, surface equilibrium models, including the Langmuir isotherm and the Freundlich isotherm, and a material balance equation based on batch reactors. The predicted and real carbon dosages match each other very well.; It can be concluded that the S/S process by reactivated carbon and cement is a robust and effective technology for immobilization treatment of high mercury wastes. |
