ABIOTIC AND BIOTIC SEQUESTRATION OF SELENIUM IN ANOXIC COAL WASTE ROCK

Abstract

Waste rock dumps at coal mines in the Elk Valley are the primary source of selenium (Se) in the Elk River. The mobile Se oxyanions, Se(IV) and Se(VI), can be adsorbed to mineral phases or reduced biotically or abiotically to Se solid phases, such as Se(0). Microbial analyses and geochemical testing were conducted to determine if Se can be sequestered in anoxic coal waste rock by these mechanisms. Testing was conducted on a waste rock sample collected from the West Line Creek waste rock dump in the Elk Valley and a water sample collected from the rock drain flowing beneath the dump. Microbial analyses included culturing with plate and broth media and 16S rRNA gene sequencing. The key finding from these analyses was that bacteria capable of reducing Se(VI) to Se(0) were present in the waste rock sample and bacteria capable of reducing Se(IV) to Se(0) were present in the rock drain water sample. Geochemical tests included abiotic batch tests on waste rock and pure minerals, non-sterile batch tests on waste rock, and desorption batch tests on waste rock, as well as several solids analysis techniques, such as X-ray Absorption Spectroscopy and Energy Dispersive X-ray Spectroscopy. Batch tests were conducted at circumneutral pH in an anoxic glove box to simulate an anoxic zone of a waste rock dump. Batch testing on pure minerals present in the waste rock sample determined that siderite can reduce Se(VI) to Se(IV) and that siderite, pyrite, and sphalerite can adsorb Se(IV) and reduce Se(IV) to Se(0). Under sterile conditions, waste rock did not reduce Se(VI) but it did adsorb Se(IV) and reduce Se(IV) to Se(0). Under non-sterile conditions, Se(VI) was reduced to Se(IV) much faster than the sterile tests with siderite and Se(IV) was removed from solution by a combination of adsorption, biotic reduction, and abiotic reduction. Desorption batch testing showed that a fraction of sequestered Se was susceptible to desorption but that the fraction decreased over time. These findings suggest that sequestration of Se in anoxic coal waste rock is a viable method to decrease the amount of Se discharging from Elk Valley waste rock dumps.