Detoxification of solid waste containing Cr(VI) with phosphate by thermal treatment

The remediation of solid waste containing Cr(VI) has always been challenging due to the toxic and carcinogenic properties of Cr(VI). This study proposed a novel method to detoxify solid waste containing Cr(VI) with phosphate by thermal treatment. Thermodynamic calculation proved that thermally reduction of Cr(VI) with phosphate was thermodynamic feasible. When the heating temperature increased to 1000°C, Cr(VI) was effectively reduced to Cr(III) and more than 97% of chromates could be reduced with the addition of (NH4)2HPO4 at n(P)/n(Cr) ratio of 3. Moreover, CaCrO4 was relatively easier to be reduced comparing with Na2CrO4 and K2CrO4. Additionally, phosphate exhibited greater potential to trigger Cr(VI) reduction in comparison to SiO2, because the potential of oxides to trigger the reduction of Cr(VI) was dominated by their acidity. Chromite ore processing residue was mixed with (NH4)2HPO4 at n(P)/n(Cr) rate of 3 and heated at 1000°C. Toxicity characteristic leaching procedure (TCLP) showed that the concentration of Cr(VI) in leachate was below 5mg/L and met the regulatory limit, it indicated that chromite ore processing residue (COPR) could be successfully remedied with phosphate by thermal treatment. These results suggested the thermal treatment of solid waste containing Cr(VI) with phosphate was a promising and reliable strategy for the detoxification of Cr(VI).