The role of temperature on Cr(VI) formation and reduction during heating of chromium-containing sludge in the presence of CaO

Abstract In this study, the temperature dependence of Cr(VI) formation and reduction in the presence of CaO was examined during the thermal treatment of sludge that contains chromium. thermogravimetry–differential scanning calorimetry and X-ray diffractometry were used to characterize the thermal behavior and phase transformation, respectively. Na 2 CO 3 leaching procedure was employed to determine the amount of Cr(VI). The result showed that CaO promoted Cr(III) oxidation, however, its influence is very dependent on heating temperature, with the extent of the effect varying with temperature. From 200–400 °C, the presence of CaO facilitated formation of intermediate product Cr 2 O 3+ x containing Cr(VI) during dehydration of chromium hydrate, while Cr 2 O 3+ x would decompose as temperature over 400 °C, accompanied by part of Cr(VI) being reduced to Cr(III). From 500 to 900 °C, Cr(III) reacted with CaO to form a leachable CaCrO 4 product. This product was stable and a prolonged heating time did not reduce the amount of Cr(VI) significantly. At 1000–1200 °C, part of CaCrO 4 was reduced to Ca(CrO 2 ) 2 in 1 h. While extended heating time above 1 h resulted in the Ca(CrO 2 ) 2 being oxidized reversibly to CaCrO 4 at 1200 °C. Since CaCrO 4 is thermodynamically less stable over 1000 °C, MgO could induce CaCrO 4 to be reduced into MgCr 2 O 4 at around 900 °C, lower than that for the reduction from CaCrO 4 into Ca(CrO 2 ) 2 . It suggested that adding MgO might be a potential approach for inhibiting Cr(VI) formation during heating sludge containing chromium.