Reduction of Cr(VI) by Nanoscale Zero Valent Iron (nZVI): The Reaction Kinetics

Increasing evidence suggests that nanoscale zero valent iron (nZVI) is one of best materials for the treatment of heavy metal ions in water. In this work, the reduction of Cr(VI) by nZVI was investigated. Laboratory experiments with varied Cr(VI) concentrations and nZVI doses were performed. Chromium removal efficiency decreases as the ratio [Cr(VI)]/nZVI increases due to the limited reaction sites on nZVI surface and passivation by the formation and precipitation of Cr(OH)3 at the surface. The reaction rates were fitted to pseudo-first-order kinetics, the observed rate constants (kobs) and the surface-area normalized rate coefficient (kSA) were calculated. At a fixed Cr(VI) concentration of 35.1 mg/L, a sudden increase in both kobs and KAS were observed when the nZVI loading is greater than 2.5g/L, suggesting that only 3.3% of the iron in nZVI is present at active surface sites. The results are consitent with the core-shell model of nZVI, suggesting that nZVI is extremely effective for in situ immobilization of chromium-contaminated soil and groundwater due to the high removal capacity and fast kinetics.