Reactivity Characteristics of Hexavalent Chromium Removed by PRB in Simulated Ground Water

The zero-valent iron (Fe⁰) with particle size of 0.15-0.42 mm and the activated carbon (AC) with particle size of 0.15 mm were used as PRB reaction mediums. By continuous flow simulation experiments, the reactivity characteristics and removal effect of hexavalent chromium (Cr(Ⅵ)) contaminated ground water by Fe⁰/AC-PRB were investigated. The removal effect of Cr(Ⅵ) by AC, the reducing action of Cr(Ⅵ) by Fe⁰ and their redox products of chromium deoxidized, and collaborative removal effect of Fe⁰/AC were analyzed, and the speciation of chromium remained in aquifer was investigated. The results showed that Cr(Ⅵ) was removed to some extend mainly by adsorption under the conditions of influent pH 6.9-7.1. Meanwhile, the chromium could be deoxidized by Fe⁰ more effectively, with the efficiency up to 96% while continuously inputting simulated groundwater containing 10 mg/L hexavalent chromium. During the process, the redox products, Fe³⁺ and Cr³⁺, were precipitated on the reaction medium; therefore they did not transfer into downstream water. Micro-electrolysis improved the reduction capacity and efficiency of Fe⁰, and compared with the effect of sole Fe⁰, the ratio of chromium to iron increased by more than 1 time, the pH increased from 7.0 of influent to 8.0 of effluent, and the total iron concentration of effluent was below 0.20 mg/L. Chromium was easily transported and extracted by bioavailable weak acid when the aquifer was flushed with deionized water. Chromium was distributed mainly in organic/sulphide fractions and residue fraction, which indicated that a low ecological risk of chromium remained in aquifer to environment.