Reduction/immobilization processes of hexavalent chromium using metakaolin-based geopolymer

The reduction and immobilization of Cr(VI) were investigated in the metakaolin-based geopolymer using FeSO4·7H2O as reducing agent by two different processes: A. Cr(VI) reduced and immobilized synchronously in the geopolymerization system containing FeSO4·7H2O; B. Cr(VI) reduced firstly in the FeSO4·7H2O solution and then immobilized in the geopolymerization system. The molar ration of Fe(II)/Cr(VI) was set to 3.5 in the both processes. In the process A, the immobilization amount of chromium in the geopolymer was not more than 0.3 wt.% achieving the good immobilization efficiency, with 25 kPa-20.4 MPa of the compressive strength for the final products. In the process B, the immobilization amount of chromium was up to 0.7 wt.%, with 26.3 MPa-9.8 MPa of the compressive strength. The differences were due to the redox reaction mechanism. Process A realized the reduction and immobilization of Cr(VI) synchronously in the geopolymerization system, but process B achieved the better immobilization efficiency and higher immobilization amount.