Immobilization of Chromium Contaminated Soil by Co-pyrolysis with Rice Straw

Long-term disposal of tannery waste and effluent without rigorous treatment may result in severe chromium (Cr) contamination in soils. In order to reduce the toxicity of Cr in soil, rice straw was co-pyrolyzed with Cr-contaminated soil under 0.5 MPa pressure. The influences of co-pyrolysis temperature (100–300 °C) and rice straw/soil mixing ratio (0–20 wt%) on Cr immobilization were investigated. The synthetic precipitation leaching procedure (SPLP), total Cr(VI) measurement, and simple bioaccessibility extraction test (SBET) were conducted, and results showed that the leachable Cr, total Cr(VI), and bioavailable Cr in co-pyrolyzed soils were significantly decreased (up to 95%, 86%, 70%, respectively) compared with untreated soil. BCR sequential extraction also indicated that mobile fractions (F1, F2) of Cr were transformed to more stable fractions after pressurized co-pyrolysis treatment. Time-varying desorption tests showed that the pseudo-second order kinetic model fitted better (R2 > 0.9) in all samples and co-pyrolysis could effectively reduce Cr desorption when extracted by citric acid, nitric acid, and CaCl2 solution. X-ray diffraction (XRD) revealed that unstable forms of Cr transformed to stable Cr species and Cr(III) was the dominant species in the soil. In conclusion, pressurized co-pyrolysis of Cr-contaminated soil with 20% of rice straw at 300 °C effectively immobilized Cr and reduced Cr(VI), making it a promising method to minimize the potential bioavailable risk of Cr in soil.