Ball-milled sulfide iron-copper bimetals based composite permeable materials for Cr (VI) removal: Effects of preparation parameters and kinetics study.

Zero-valent iron (ZVI) and modified ZVI have been investigated extensively for groundwater remediation. However, ZVI based powder was difficult to be applied directly as permeable reactive barrier (PRB) materials due to their low water permeability and usage rate. In this study, sulfide iron-copper bimetal was prepared by ball milling, which is environment-friendly without second contamination. The optimal preparation parameters of sulfide iron-copper bimetal for Cr(VI) removal were determined (Cu/Fe ratio (w/w), 0.018; FeS/Fe ratio (w/w), 0.1213; ball milling speed, 450 rpm; ball milling time, 5 h). A composite permeable material was prepared by sintering a mixture of sulfide iron-copper bimetal, sludge, and kaolin. The parameters for composite permeable material preparation including sludge content and particle size, and sintering time were optimized, which were 60%, 60-75 mesh, and 4 h, respectively. The optimal composite permeable material was characterized by SEM-EDS, XRD, and FTIR. The results demonstrated preparation parameters can affect the hydraulic conductivity and hardness of composite permeable material. High sludge content, small particles size, and moderate sintering time resulted in high permeability of composite permeable material and were beneficial for Cr(VI) removal. The dominant Cr(VI) removal mechanism was reduction, and the reaction followed pseudo-first order kinetics. Conversely, low sludge content and large particle size, and long sintering time lead to low permeability of composite permeable material. Chromate removal was mainly by chemisorption following pseudo-second order kinetics. The hydraulic conductivity and hardness of the optimal composite permeable material achieved 1.732 cm/s and 50, respectively. The results of column experiments indicated that its Cr(VI) removal capacity was 0.54 mg/g, 0.39 mg/g and 0.29 mg/g at pH 5, 7 and 9, respectively. The ratio of Cr(VI) to Cr(III) on composite permeable material surface was similar under acidic and alkaline conditions. This study will provide an effective reactive material of PRB for field application.