Stability of selenium in post-closure saturated coal mine rock fills

Selenium release to the environment as a result of industrial activities such as coal mining can impact receiving water. Saturated rock fill (SRF) technology is a robust and sustainable approach to reduce selenate to sorbed selenite and/or insoluble elemental selenium using native bacteria. Following SRF closure, the native microbial community and water chemistry is anticipated to return to conditions representing pre-SRF baseline chemistry in equilibrium with the backfilled rock, maintaining neutral pH, low oxygen, and reduced conditions that support stability of the accreted selenium. Mineralogical analyses of selenium in biomineralized samples recovered from the full-scale Elkview Operations SRF trial identified selenite strongly sorbed to ferrihydrite and reduced Se_nS_n-8 minerals consistent with previous analyses of column residues. Batch tests showed sequestered selenium was not remobilized from active SRF treatment zone samples when nutrient amendment was stopped. Column tests run to accumulate selenium were subsequently shut down and allowed to remain static for 5 years with intermittent flow to mimic post-closure SRF conditions. Selenium was not remobilized from these columns under post-closure conditions. A series of comprehensive selenium adsorption-desorption experiments indicated selenite is strongly sorbed to ferrihydrite in coal mine rock with minimal desorption. Zero valent selenium remained stable when stored sub-aqueously and when air dried and exposed to atmospheric oxygen for over a year in the lab. These results suggest that selenium in SRF treatment reactors will likely be very stable and unlikely to negatively affect surface water or groundwater post closure.