Multi-contaminant removal from synthetic mine-impacted water by permeable reactive barriers under cold conditions

Permeable reactive barriers (PRB) effectively attenuate multiple groundwater contaminants in temperate climates, but their efficacy remains uncertain in sub(Arctic) climates where cold temperatures inhibit kinetic biogeochemical reactions governing contaminant removal. This study applies bench-scale columns mimicking PRBs, containing varying proportions of zero-valent iron (ZVI), gravel, and wood chips, to treat synthetic mine-impacted water containing nitrate, arsenic, and uranium at low temperatures (5 °C) over 36 weeks. Columns were amended with sodium acetate during weeks 20-33 to stimulate microbial activity. Speciation was investigated by combining X-ray Absorption Spectroscopy and geochemical speciation modeling using PHREEQC. Arsenic removal efficiency exceeded 95 % over the experimental duration in all ZVI-bearing columns and was mostly driven by adsorption and coprecipitation with ZVI oxidation products. Nitrate removal was limited in the absence of acetate amendments but improved to ∼50 % during the amendment. Denitrification to N₂ gas was incomplete, likely due to kinetic limitations on the various nitrogen reduction reaction steps. Uranium removal was >95 % in ZVI-bearing columns before the acetate amendment and was predominantly explained by U(VI) adsorption onto Fe-(oxyhydr)oxides. However, U was remobilized during the amendment, likely due to increased aqueous complexation of U by calcium and carbonate that drove the desorption of U from Fe-(oxyhydr)oxides. These experiments show that PRB technology holds promise for multi-contaminant removal under cold conditions, while exposing ongoing challenges associated with concurrent removal of contaminants exhibiting contrasting geochemical behavior.