Immobilization of hexavalent chromium in aqueous solution through the formation of 3CaO·(Al,Fe)2O3·Ca(OH) 2·xH2O phase, ettringite and C-S-H gel

Abstract

The immobilization of hexavalent chromium in aqueous solution is found to occur through the formation of Cr6+-bearing AFm phase, ettringite and C-S-H gel, which are formed by calcium aluminate or ferrite, a mixture of lime and Al2(SO4)3 or Fe2(SO 4)3, and calcium silicate, respectively. In the case of AFm phase, which is most effective one to the immobilization of Cr6+, the Cr6+ concentration in aqueous solution decreases from initial 50ppm to below 0.05 ppm. The Cr6+-containing AFm phases identified are 3CaO· Al2O3· Ca[(OH)2, CrO4] ·18H2O, 3CaO·(Al,Fe)2O 3· Ca[(OH)2,CrO4]·18H 2O, 3CaO·Fe2O3·Ca(CO 3,CrO4)·12H2O and 3CaO·Al 2O3·CaCrO4-12H2O. The immobilization occurs through the OH- or CO32- site substitution with CrO42-. No significant amount of Cr6+ is identified in either ettringite phase or C-S-H gel, indicating that these two phases do not play an important role for the immobilization of Cr6+ in cement-based solidification/stabilization process. The immobilization takes place by the substitution of SO 42- site in the case of ettringite, while in the case of C-S-H gel the sorption mechanism is more possible. © 2002 ISIJ.