Effects of Al2O3, Fe2O3, and SiO2 on Cr(VI) formation during heating of solid waste containing Cr(III)

In this study, the influences of Al2O3, Fe2O3, and SiO2 on the Cr(VI) formation during thermal treatment of solid waste containing Cr(III) were studied. Al2O3, Fe2O3, and SiO2 suppressed Cr(III) oxidation by incorporating alkali and alkaline earth metals in the products (i.e. silicates, aluminates, and ferrites). However, the products still have the potential to trigger Cr(III) oxidation, but was less than the original ones. The decrease could be attributed to the interactions of the original alkali and alkaline earth metal salts with Al2O3, Fe2O3 and SiO2, which reducing the basicity of resulting products. Because the potential of alkali and alkaline earth metal salts to trigger Cr(III) oxidation is dominated by their basicity. Additionally, Al2O3, Fe2O3and SiO2 could also trigger Cr(VI) to be reduced, and this ability is related to their acidity. SiO2 is more acidic than Al2O3 and Fe2O3, and can cause Na2CrO4 and K2CrO4 to be reduced at temperatures above 1000 °C, but Al2O3 and Fe2O3 cannot. These results suggested that acidic oxides (e.g. SiO2 and P2O5) played an important role in inhibiting Cr(III) oxidation, and that increasing the fraction of acidic oxide was beneficial to suppress the Cr(III) oxidation during thermal treatment of solid waste containing Cr(III).