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.