Impact of SiO2 and CaO on Cr(VI) formation during heat treatment of Cr(III)-bearing sludge

Abstract

There is a key issue about formation of highly toxic and easily dissoluble Cr(VI) during heat treatment of Cr-bearing solid waste, resulting in a prominent increase of environmental risk of incineration product. In this study, the effects of CaO and SiO₂ on formation and reduction of Cr(VI) were investigated during heating of Cr-bearing simulated sludge (Cr-SS) and Cr₂O₃, exploring Cr(III) oxidation mechanism. The findings indicates that doping CaO significantly promotes Cr(III) oxidation to Cr(VI), and chromium speciation vary with temperature. At 300℃, leaching amount of Cr(VI) in Cr-SS samples increases sharply to 9.3 % of total Cr. In contrast, no Cr(VI) was detected in Cr₂O₃ samples. Above 300℃, oxidation rates of Cr(III) in different samples of Cr-SS and Cr₂O₃ exhibit the same trend with temperature increasing. Above 1000℃, new Cr-containing crystalline phases were observed, like CaCr₂O₄ and Ca₃(CrO₄)₂. The oxidation rate of Cr(III) is positively connected with the adding amount of CaO. Besides, doping SiO₂ to sludge prominently inhibits Cr(III) oxidation or facilitate Cr(VI) reduction at 600℃-1200℃, which is also confirmed by thermodynamic calculation, attributing to formation of Ca₅Cr₂SiO₁₂ and CaCr₂O₄. The number of holes in valence band will be changed through substitution of Cr(III) ions by cations of different valence states, thus facilitate or inhibit Cr(III) oxidation. The ions (<III) can promote Cr(III) oxidation, while ions (>III) can inhibit Cr(III) oxidation and promote Cr(VI) reduction. Introducing SiO₂ or Si-enriched materials to chromium-bearing wastes may be a promising alternative to solve the problem of highly toxic Cr(VI) formation during heat treatment.