Galvanic sulphide oxidation as a metal-leaching mechanism and its environmental implications

Abstract

Following a brief review of the theoretical aspects of galvanic sulphide oxidation, the significance of galvanic interaction in the oxidation of natural mixed sulphide assemblages is demonstrated using test materials prepared from sulphide-containing rocks in a series of chemical and microbial weathering experiments. The test results indicate that: (1) metal leaching can effectively proceed even in a near-neutral environment; and (2) the occurrence of acid mine drainage can be delayed due to cathodic protection of an acid-generating sulphide such as pyrite from oxidative dissolution. While microbial mediation may enhance the weathering of sulphides with a high electrode potential, the competing galvanic processes diminish the dominating role of the microbes in effecting the oxidation of sulphides with a low electrode potential when the sulphides occur in a mixed assemblage. In-situ potential measurements on sulphide surfaces with micro-electrodes demonstrated the occurrence of a significant potential difference between a contacting sulphide pair sufficient to sustain galvanic interaction for the duration of the experiments. It is also shown that galvanic sulphide oxidation and hence metal leaching can occur even under an oxygenated water cover. Therefore, subaqueous disposal may not be the best management option for all reactive mine wastes, especially those containing metals or trace elements that remain mobile under near-neutral pH conditions.