Abstract:

As a result of mining activity, Hg-bearing waste can be produced (e.g., gold production), which should be managed with strict regulations to prevent Hg release into nature. However, the chemical stability of such waste is currently being assessed only by short-term leaching tests (e.g., TCLP tests). Thus, we conducted air oxidation tests (168 hours) of Hg-adsorbed pyrite, as a common Hg-bearing waste, under different DO (8.0 and 0.2 mg/L) and pH (2, 6, and 10) conditions to examine its long-term chemical stability. The results showed that a larger amount of Hg was desorbed under high DO conditions, along with the progress of pyrite oxidation. This trend was supposed to be more obvious at alkaline pH, in which pyrite oxidation is theoretically enhanced. However, the amount of Hg desorbed at pH 2 was 150 times higher than that at pH 10. Solid analysis with XPS and XAFS found that the formation of Fe (oxy)hydroxide at alkaline pH, as a result of pyrite oxidation, contributed to the re-immobilization of once-desorbed Hg. We also developed the kinetic model, considering the (i) pyrite oxidation, (ii) Fe precipitation, (iii) Hg desorption, and (iv) Hg re-immobilization, and confirmed that the model successfully reproduced reactions that occurred in the experimental system. At last, we attempted to give some suggestions on the safe disposal and stable management of Hg-bearing wastes.

Keywords: mercury (Hg), Hg-bearing waste, pyrite (FeS2), long-term chemical stability, kinetic modeling

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Dr. Keishi Oyama