Temperature dependent reduction of Cr(VI) to Cr(V) aroused by CaO during thermal treatment of solid waste containing Cr(VI)

Cr(VI) compounds at high temperature usually tend to decompose and reduce into Cr(III) due to thermodynamically instability for Cr(VI). This study found Cr(VI) could be reduced into Cr(V) instead of Cr(III) in the presence of CaO during heating solid waste containing Cr(VI). CaCrO 4 is prepared and mixed with CaO as simulated solid waste containing Cr(VI). It was found that CaCrO 4 reacted with CaO and formed a new product Ca-5(CrO4)(3)O-0.5 at temperature range of 800 and 1000 degrees C. The valence state of Cr inCa(5)(CrO4)(3)O-0.5 is determined to be +5 b y XPS analysis, and the color for new formed Cr(V) is observed in green, similar to Cr(III) compounds. The temperature and CaO are two keys to arouse the reduction reaction of Cr(VI) into Cr(V). In particular, the reduction of Cr(VI) into Cr(V) is strongly depended on temperature (800-1000 degrees C), this reaction can be balanced within 10 min, while prolonging sintering time has little help for promoting the reduction of Cr(VI) to Cr(V). Additionally, it was found Cr(V) can keep stable and not be re-oxidized into Cr(VI) at 800-1000 degrees C. Above results offers some new understanding and knowledge about the formation of Cr(V) in presence of much CaO or CaCO3 during heating solid waste containing Cr(VI). (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study found that at high temperatures, Cr(VI) can be reduced to Cr(V) instead of Cr(III) in the presence of CaO in heating solid waste containing Cr(VI). The reduction reaction of Cr(VI) into Cr(V) is strongly dependent on temperature (800-1000 degrees C) and the presence of CaO, with little impact from prolonging sintering time. Additionally, Cr(V) remains stable and does not re-oxidize into Cr(VI) at 800-1000 degrees C.