Treatment Of Hexavalent Chromium Using Nanoscale Zero-Valent Iron

Laboratory experiments and theoretical modeling studies were performed to investigate the mechanisms of Cr (Ⅵ) removal from deoxygenated simulated groundwater using nanoscale zero-valent iron, and to evaluate influencing factors and kinetics based on zeta potential, redox potential, ferrous concentrations, and the pe-pH diagram of Fe-Cr-H2O system.Experimental results demonstrate that the removal efficiency of Cr (Ⅵ) decreases with the increasing Cr (VI)/Fe mass ratio. When the Cr (Ⅵ)/nZVI mass ratio is 0.025, 0.050,0.075, and 0.100, the corresponding Cr (Ⅵ) removal rate is 100.0%,85.6%, 72.7% and 39.6%, respectively. The Cr (Ⅵ) removal is favorable at acidic pH with fixed Cr(Ⅵ)/Fe mass ratio of 0.100. When pH is 3.0,5.0,7.0,9.0 and 11.0, the Cr (Ⅵ) removal rate is 73.4%,57.6%,39.6%,44.1%, and 41.2%, accordingly. The Cr(Ⅵ) removal follows the pseudo second-order kinetics. When pH is 7.0 and Cr (Ⅵ)/nZVI mass ratio is 0.025, the rate of Cr (Ⅵ) removal is the highest with the rate constant at 9.76×10-3g·(mg·min)-1.The following conclusion could be inferred by analyzing pe-pH diagram of Fe-Cr-H2O system, redox potential and the concentration of ferrous. The conversion from Cr2O72- to Cr3+ should be instantaneous when Cr2O72- is absorbed on the surface of Fe. The Cr (Ⅵ) was reduced to Cr (Ⅲ), which was subsequently incorporated into the FeOOH shell and formed a Cr-Fe film. The film once formed could further inhibit the electron transfer between Cr2O72- and Fe. Then Cr (Ⅵ) removal was primary controlled by the adsorption process.Deoxygenated COPR extract was treated with nZVI at the mass ratio of Cr (Ⅵ)/nZVI=0.017, and 100% Cr (Ⅵ) was removed; The ZVI could reduce the Cr(Ⅵ) content in COPR from 16 g·kg-1 to less than 0.01 g·kg-1 within 1 d of treatment at nZVI/COPR mass ratio of 4%; the Cr(Ⅵ) in COPR cannot be reduced efficiently with the S/S treatment.