Review of the potential for selenium remobilization in semi-passive treatment systems of mine impacted waters

Biological semi-passive mine water treatment technologies are used in the mining industry as an alternative to or in conjunction with active treatment systems to remediate mine impacted water (MIW) containing nitrate and selenium oxyanions such as selenate and selenite. In semi-passive biological treatment systems, MIW is pumped through a saturated, porous media (either a gravel bed or waste rock) which provides ample surface area for biofilm growth and the creation of anoxic, subaqueous environments. Additional nutrients and carbon sources are pumped into the system to encourage the growth of microbes that biochemically reduce selenate and selenite to insoluble reduced Se⁰ species such as selenium nanoparticles (SeNP) by respiring selenate and selenite. One such semi-passive treatment process, the saturated rock fill (SRF), injects MIW into backfilled open pits. Currently, it is contemplated that the SRF technology will treat over 170,000 m³/day of MIW in Southwest British Columbia in Canada and operate for decades if not longer. Following closure, the SRFs will retain significant quantities of selenium removed from water in perpetuity. Despite advancements in physical design and understanding operational performance of SRFs, there has been little publicly available information on the long-term fate and stability of the reduced Se⁰ retained in the system following closure (i.e., 100 or more years into the future). Here, we review available and relevant scientific literature to highlight the significance of this knowledge gap. Based on an extensive literature review and analyses using known, published chemical reactions, we discuss conditions in which immobilized Se⁰ formed from selenate and selenite bioreduction in the SRFs can remobilize and propose actionable steps to better understand the future environmental implications of implementing the SRFs.