Effect of Humic Substance on Thermal Treatment of Chromium(VI)-Containing Latosol Soil

Abstract

Latosol soils contaminated with chromium(VI) [Cr(VI)], which is hazardous, can be recycled as raw materials for porcelain and construction sectors if a proper thermal stabilization process is implemented. This study investigates how thermal treatment affects Cr behavior during the sintering of latosol and deorganic latosol samples; both samples are artificially contaminated with CrO₃. Ap proaches including X-ray absorption spectroscopy, scanning electron microscopy, N₂-based Brunauer Emmett Teller surface analyzer, thermogravimetric analyzer/differential scanning calorimeter, and the toxicity characteristic leaching procedure promulgated by Taiwan Environmental Protection Administration are used in this study. After drying the Cr(VI)-contaminated latosol (i.e., containing 37,120 mg of Cr/kg sample) at 105 °C, ∼80% of the doped CrO₃ is chemically reduced to Cr(OH)₃ by a humic substance naturally existing in the soil. In contrast, in the organics-free CrO₃-contaminated latosol dried at 105 °C, only 9% of the doped CrO₃ is reduced to Cr(OH)₃. Heating the samples at 500 and 1100 °C transforms hazardous Cr(VI) into Cr(III) that is negligibly toxic; Cr₂O₃, which is insoluble, is detected as the most abundant Cr species. Moreover, formation of Cr₂SiO₅, which is suggested to relate to low Cr leaching, is only detected in the sample heated at 1100 °C. Surface morphology, surface area, and thermogravimetric analyzer/differential scanning calorimeter results demonstrate that thermal treatment at 1100 °C can incur considerable soil sintering/melting if the humic substance in the soil has been heated off previously. Finally, Cr concentrations in the toxicity characteristic leaching procedure leachates collected from the samples thermally treated at 1100 °C for 4 hr are ≤0.21 mg of Cr L⁻¹ that are much less than the Taiwan Environmental Protection Administration regulatory limit (<5 mg of Cr L⁻¹); consequently, these two samples are nonhazardous, and they have the potential for resource recycling. Conversely, Cr concentrations in the leachates from all 500 °C and 105 °C samples are in the 25.6–1279 mg L⁻¹ range.