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

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 SiO 2 on formation and reduction of Cr(VI) were investigated during heating of Cr-bearing simulated sludge (Cr-SS) and Cr 2 O 3 , 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 Cr 2 O 3 samples. Above 300℃, oxidation rates of Cr(III) in different samples of Cr-SS and Cr 2 O 3 exhibit the same trend with temperature increasing. Above 1000℃, new Cr-containing crystalline phases were observed, like CaCr 2 O 4 and Ca 3 (CrO 4 ) 2 . The oxidation rate of Cr(III) is positively connected with the adding amount of CaO. Besides, doping SiO 2 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 Ca 5 Cr 2 SiO 12 and CaCr 2 O 4 . 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 inhibit Cr(III) oxidation and promote Cr(VI) reduction. Introducing SiO 2 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. • Oxidation rate of Cr(III) is positively correlated with the adding amount of CaO. • Content of Cr(VI) in CaO and Cr-SS system reaches peak value at 300℃ and 900℃ respectively. • Multiple chromium species were detected, like CaCrO 4 , Ca 3 (CrO 4 ), CaCr 2 O 4 and Cr 2 O 3 . • Reducing the number of valence band holes of Cr 2 O 3 crystals can inhibit Cr(III) oxidation. • Introducing SiO 2 and controlling temperature are the effective strategies to inhibit Cr(VI) generation.