Role of Major Groundwater Ions on Reductive Cr(VI) Immobilization in Subsurface Systems with Pyrite
2016
Bulbul, Muhammet Samet | Kantar, Cetin | Keskin, Selda
Laboratory batch and column experiments were performed to better understand the effects of Ca²⁺, Mg²⁺, and HCO₃ ⁻ on Cr(VI) removal from aqueous systems with pyrite. Batch results show that increasing HCO₃ ⁻ concentration led to an increase in Cr(VI) removal by pyrite due to pH buffering capacity of HCO₃ ⁻. However, while Ca²⁺ and Mg²⁺ individually had no effect on Cr(VI) removal at pH 4, the addition of Ca²⁺ or Mg²⁺ to systems containing HCO₃ ⁻ resulted in a significant decrease in Cr(VI) removal at pH 8 relative to the systems containing HCO₃ ⁻ alone. The XPS data proved that while Ca²⁺ precipitated as CaCO₃₍S₎ onto pyrite surface, Mg²⁺ sorbed and/or accumulated as Mg(OH)₂₍S₎ onto oxidized pyrite surface. The formation of surface precipitates (e.g., CaCO₃) inhibited further Cr(VI) reduction by blocking electron transfer between Cr(VI) and pyritic surface sites. While the precipitation of Ca²⁺ as CaCO₃ led to a significant decrease in effluent pH, the decrease in effluent pH was very low in systems containing Mg²⁺, most probably due to much higher solubility of Mg²⁺ at pH 8. Zeta potential measurements provided further evidence that while Ca²⁺ or Mg²⁺ had no effect on zeta potential of pyrite particles under acidic conditions (e.g., pH < 7), the addition of Ca²⁺ or Mg²⁺ to systems containing Cr(VI) reversed the pyrite surface potential from negative to positive under alkaline pH conditions (e.g., pH > 8) relative to system containing only Cr(VI), suggesting the sorption and/or accumulation of surface precipitates on pyrite surface.
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