Examination of multiple sources of selenium release from coal wastes and strategies for remediation

Selenium (Se) has been mobilised by leaching from coal and associated waste rock exposed by mining activities in Western Canada, with deleterious impact on aquatic wildlife. Waste rock characterisation indicates that up to 7% of the Se, as Se(IV), may be associated with organic matter, with ≈9%, as Se(0), associated with euhedral pyrite. Small 1-2 µm mineral particles with average Se concentration of 1.0 ± 0.4 wt% account for the remaining Se with the largest components likely to be associated with Fe oxide/hydroxide/carbonate as Se(0) and framboidal pyrite as Se(IV) and Se(0). No evidence was found for the presence of Se(-I), Se(-II) or Se(VI). In the first 8 weeks of leaching Se release was not correlated to the addition of aqueous silicate, added to aid pyrite passivation, but was reduced by approximately one third when the waste was treated with manure. This suggests the primary initial source of leached Se was not pyrite. Added organic C results in increased microbial numbers, particularly aerobic microbes, and promotes the formation of extensive coating of extracellular polymeric substances resulting in depletion of O₂ at particle surfaces, reducing oxidation of Se(IV) and therefore reducing the leach rate of Se. Subsequent to 8 weeks of leaching the rates of release of Se from the treated wastes were similar regardless of treatment strategy but were reduced as compared to the untreated waste rock, suggestive of partial framboidal pyrite geochemical and microbial passivation. Se leaching was not correlated to S leaching, but the source(s) of the leached S was not known as approximately half of the S within the waste rock was non-sulfidic. These results indicate that utilisation of local organic carbon-containing wastes for coverage of coal waste rock may be a cost-effective strategy to reduce Se leaching to acceptable rates of release regardless of whether the Se is associated with framboidal pyrite or organics.