Effect of groundwater ions (Ca2+, Na+, and HCO3-) on removal of hexavalent chromium by Fe(II)-phosphate mineral.

A systematic study was conducted to investigate the effect of major groundwater ions (i.e., Ca2+, Na+, and HCO3-) on removal of hexavalent chromium (Cr(VI)) by an Fe(II)-phosphate mineral (i.e., vivianite). The batch experiments revealed that the second-order rate constant for Cr(VI) removal by vivianite with Ca2+ + CO32- (0.076-1.90 mM) and Na+ + HCO3- (0.26-6.50 mM) was 1.5-5.2 times lower than that without these ions. The removal kinetics of Cr(VI) by vivianite was abruptly slowed down with the increased ion concentration, which showed their inhibitory effect on the reaction. The results of the geochemical modeling and density functional theory calculations showed that the presence of Ca2+ + HCO3- and Na+ + HCO3- can form less favorable Cr(VI) species (i.e., CaCrO4(aq) and NaCrO4-) on the Fe-B site of vivianite surface, leading to the inhibitory effect observed in this study. Finally, the X-ray absorption spectroscopy results showed that reductive immobilization of Cr(VI) to Cr(III) occurred by structural Fe(II) oxidation of vivianite to amorphous mixed-valence Fe-phosphate via an inner-sphere complexation. The results suggest that the presence of Ca2+, Na+, and HCO3- in phosphorous-enriched iron-reducing environments may lower the remedial efficiency of Cr(VI) removal.