The effect of biogeochemical redox oscillations on arsenic release from legacy mine tailings.

Exposed and un-remediated metal(loid)-bearing mine tailings are susceptible to wind and water erosion that disperses toxic elements into the surrounding environment. Compost-assisted phytostabilization has been successfully applied to legacy tailings as an inexpensive, eco-friendly, and sustainable landscape rehabilitation that provides vegetative cover and subsurface scaffolding to inhibit offsite transport of contaminant laden particles. The possibility of augmented metal(loid) mobility from subsurface redox reactions driven by irrigation and organic amendments is known and arsenic (As) is of particular concern because of its high affinity for adsorption to reducible ferric (oxyhydr)oxide surface sites. However, the biogeochemical transformation of As in mine tailings during multiple redox oscillations has not yet been addressed. In the present study, a redox-stat reactor was used to control oscillations between 7 d oxic and 7 d anoxic half-cycles over a three-month period in mine tailings with and without amendment of compost-derived organic matter (OM) solution. Aqueous and solid phase analyses during and after redox oscillations by mass spectrometry and synchrotron X-ray absorption spectroscopy revealed that soluble OM addition stimulated pyrite oxidation, which resulted in accelerated acidification and increased aqueous sulfate activity. Soluble OM in the reactor solution significantly increased mobilization of As under anoxic half-cycles primarily through reductive dissolution of ferrihydrite. Microbially-mediated As reduction was also observed in compost treatments, which increased partitioning to the aqueous phase due to the lower affinity of As(III) for complexation on ferric surface sites, e.g. ferrihydrite. Oxic half-cycles showed As repartitioned to the solid phase concurrent with precipitation of ferrihydrite and jarosite. Multiple redox oscillations increased the crystallinity of Fe minerals in the Treatment reactors with compost solution due to the reductive dissolution of ferrihydrite and precipitation of jarosite. The release of As from tailings gradually decreased after repeated redox oscillations. The high sulfate, ferrous iron, and hydronium activity promoted the precipitation of jarosite, which sequestered arsenic. Our results indicated that redox oscillations under compost-assisted phytostabilization can promote As release that diminishes over time, which should inform remediation assessment and environmental risk assessment of mine site compost-assisted phytostabilization.