Improvement Of Zero Valent Iron PRB Media Efficiency By Bimetallic System And Multi-metallic System

Permeable reactive barrier（PRB） is a new technology for in-situ remediation of contaminated groundwater. Zero valent iron（Fe⁰） as a medium is used widely in the laboratory research and engineering application. However, the Fe⁰-PRB technique faced critical issues for in-situ applications which included low efficiency, pore plugging, significant loss of porosity and permeability of porous media. They limited its large-scale application and development.In order to overcome the above-mentioned shortcomings, a small amount of copper or nickel was added directly into the Fe⁰ system to form Fe/Cu, Fe/Ni bimetallic system and Fe/Cu/Ni multi-metallic system. Reagent iron powder of 0.074 mm and industrial scrap iron of 0.150⁰.270 mm were used as reaction media. The mechanism of Cr（Ⅵ） reduction and its removal efficiency by Fe⁰, Fe/Cu, Fe /Ni bimetallic system and Fe/Cu/Ni multi-metallic system were investigated in batch experiments. Then the removal effect of Cr（Ⅵ） with different PRB reaction media and the long-term stability of PRB were studied in PRB column experiments. So it was useful to provide theoretical guidance and technical support for improving the efficiency of Fe⁰-PRB reaction media.Results of batch experiments indicated that Cr（Ⅵ） removal and Fe⁰ efficiency were improved significantly by bimetallic system and multi-metallic system. When the reaction was balanced, Cr（Ⅵ） removal of reagent Fe⁰ and industrial Fe⁰ were 45% and 20%, respectively. And Cr（Ⅵ） removal of Fe/Cu bimetallic system and Fe/Cu/Ni multi-metallic system were more than 90%. While Cr（Ⅵ） removal of reagent Fe/Ni and industrial Fe/Ni were 76.6% and 44.0%, respectively. Low mass concentration of souble Cr（Ⅲ） was observed during the process of Cr（Ⅵ） reduction. The pH increased from 5.0 to 10.0 before and after the reaction. The total iron mass concentration was more than 0.30mg/L in the initial stage of the reaction, but the other met the standard. Cu²⁺ mass concentrations was less than 0.40mg/L, while Ni²⁺ mass concentrations was lower than the minimum detection limit.Results of PRB column experiments indicated that Fe（Ⅲ）-Cr（Ⅲ） hydroxide or Fe（Ⅲ）-Cr（Ⅲ） oxide hydroxides co-precipitation could be attached in the reaction media when media and contaminated Cr（Ⅵ） groundwater reacted. When Cr（Ⅵ） concentration in the effluent was 20.00 mg/L, the effluent volume of different PRB columns from high to low as follows: reagent Fe/Cu/Ni column > reagent Fe/Cu column > industrial Fe/Cu column > reagent Fe/Ni column > industrial Fe/Cu/Ni column > reagent Fe column > industrial Fe/Ni column > industrial Fe column. Low mass concentration of souble Cr（Ⅲ） was observed in the effluent. The total iron mass concentration was less than 0.30mg/L. The p H increased from 6.5 in the influent to 10.0 in the effluent. The conductivity first increased rapidly and then decreased, and finally there was a small increase in amplitude.Cr（Ⅵ） removal and Fe⁰ efficiency were improved significantly by bimetallic system and multi-metallic system. And the effluent volume of PRB column were also improved effectively. So it was significant to prolong the life of the PRB column. Therefore, to a certain extent, bimetallic system and multi-metallic system could improve efficiency Fe⁰-PRB media efficiency.