[Effect of Water Regimes on Pb and Cd Immobilization by Biochar in Contaminated Paddy Soil]

An incubation experiment was conducted to explore the influence of 30% water holding capacity (WHC), flooding, and alternate dry-wet conditions on changes in heavy metal fractions with 1% rice straw biochar in Pb and Cd co-contaminated paddy soils, to provide a scientific basis for a water regime of biochar remediation on heavy metal contaminated paddy soil. Results showed that flooding and alternating wet-dry conditions could significantly increase soil pH, the contents of dissolved organic carbon (DOC), and amorphous iron oxide (Fe_o) after adding biochar. Compared with a 30% WHC treatment, when the soil is flooded and alternating wet-dry conditions, the content of the TCLP extractable Pb decreased by 31.87% and 20.33%, respectively, and the content of the TCLP extraction Cd decreased by 25.29% and 16.07%, respectively. Under flooding, the acid soluble Pb and Cd content decreased by 24.78% and 20.14%, respectively, and the acid soluble Cd content decreased over time. The decreasing order of available Pb and Cd content was flooding > alternating dry-wet > 30% WHC. Correlation analysis results showed that soil pH and Fe_o have significant negative correlation with available heavy metals, which means flooding with biochar could effectively immobilize Pb and Cd by increasing soil pH and Fe_o content. Flooding and biochar have a synergistic interaction on promoting the transformation of Pb and Cd to more stable fractions in acidic co-contaminated heavy metal polluted paddy soil.