Recent advances toward chromium oxidation and reduction reaction mechanisms during thermal treatment of solid waste: A critical review

Chromium is widely presented in industrial solid wastes like tannery sludge, electroplating sludge and metallurgical slag. These industrial solid wastes usually undergo thermal treatment process to reduce volume and toxicity. However, a significant amount of low-toxicity and low-mobility Cr(III) is determined to be oxidized to highly-toxic and highly-mobile Cr(VI) at high temperature, posing a greater threat to humans and the ecological environment. This paper summarizes the forms of Cr in solid wastes containing Cr, redox reactions mechanisms for different Cr forms, and methods to inhibit Cr(VI) formation during thermal treatment process. The Cr(III) compounds in solid waste containing Cr mainly include Cr(III) hydrates, Cr(III) oxides, Cr(III) hosting spinels and Organic-Cr(III). Cr(III) hydrates are usually oxidized at temperatures above 100 °C, even without the induction of alkali and alkaline earth metals. Compared to the direct reaction of Cr(III) oxides and spinels with O2, Cr(III) can be induced to oxidize at lower temperatures by alkali and alkaline earth metals. A large amount of Cr(III) is oxidized usually at 600–900 °C. Organic-Cr is generally pyrolyzed to CrO3(g), CrO2Cl(g) and Cr2O3(s) at high temperature. CrO2Cl(g) can be released directly into the atmosphere with CrO3(g), or captured by CaO to form CaCrO4. The reduction of Cr(VI) at high temperatures includes the decomposition of unstable Cr(VI) compounds driven solely by temperature, as well as reduction facilitated by acidic oxides. The reduction of Cr(VI) at high temperatures involves the decomposition of unstable Cr(VI) compounds, driven solely by temperature, as well as reduction facilitated by acidic oxides. Typical unstable Cr(VI) compounds include CrO3 and CaCrO4, which begin to decompose at temperatures above 270 °C and 1000 °C, respectively. Cr(III) oxidation and Cr(VI) reduction at high temperature are strongly dependent on the system basicity and the temperature. Subsequently, reducing oxygen content in atmosphere and the system basicity by adding common acidic oxides such as silicon dioxide, phosphate and sulfates exhibited a significant effect on inhibiting Cr(VI) formation during heating solid waste containing Cr. However, Cr oxidation and reduction mechanisms at molecular level have not yet been explored, and more effective measures to inhibit Cr(III) oxidation during thermal treatment of solid waste also should be developed in further works.