Detoxification/immobilization of hexavalent chromium using metakaolin-based geopolymer coupled with ferrous chloride

The detoxification and immobilization of Cr(VI) using metakaolin-based geopolymer coupled with ferrous chloride was solved successfully. Adding potassium dichromate and ferrous chloride in the base colloid formed by mixing metakaolin and the alkaline activator, Cr(VI)-contaminated metakaolin-based geopolymer was prepared, whose compressive strength was more than 30 MPa. The optimized capacity of chromium in the geopolymer was 0.8 wt.% with an optimized Fe2+/Cr(VI) mole ratio of 4:1, which ensured a low leached chromium concentration below 5 mg/L. This product was mainly amorphous except by-product sodium chloride. According to XRD, SEM and XPS, the mechanism of Cr detoxification/immobilization was speculated and explained. In the colloid, the negatively charged [AlO4] was first formed because of the geopolymerization, and at the same time, Cr(VI) was reduced to Cr3+ under the participation of Fe2+. Then, Cr3+ was attracted by [AlO4] due to the electrostatic attraction and stabilized in geopolymerical matrices. Therefore, the immobilization of Cr(VI) in geopolymer contained a redox and a fixation process. This method is expected to be used for the disposal of Cr(VI)-contaminated solid wastes.