Conditional remediation performance of wheat straw biochar on three typical Cd-contaminated soils.

Undoubtedly biochar has excellent remediation performance for Cd-contaminated soil. Nevertheless, the remediation performance may be not invariable considering highly variable soil conditions including soil properties and environmental conditions. This work investigated the fate of Cd in three typical Cd-contaminated soils (acidic, neutral and saline-alkali soils) treated with wheat straw biochar and its driving mechanisms under specific soil conditions through aging and remediation experiment, Cd availability experiment and leaching column experiment. The results indicated that biochar addition facilitated Cd immobilization and reduced the uptake of Cd by green vegetables in acidic, neutral and saline-alkali soils under wetting-drying conditions. In contrast to neutral and saline-alkali soils, the release of exchangeable aluminum from biochar-treated acidic soil under flooding-drying cycles lowered the pH of leachate, thus promoting the leaching of Cd from leaching column, especially at 7 and 14 days, when the leaching of Cd increased by 25.3 and 32.6 times, respectively. This result was further supported by the increase in the exchangeable fraction and total leaching amounts of Cd in the topsoil layer (0-20 cm) of biochar-treated acidic soil of leaching column. Additionally, the leaching of Cd was positively correlated with DOC contents of leachate in biochar-treated neutral and saline-alkali soils. In summary, the remediation performance of biochar for Cd-contaminated soils is conditional, and its remediation effect is better in neutral and saline-alkali soils. Notably, the inherent conditions of soil must be fully considered when applying biochar for Cd remediation, especially in acidic Cd-contaminated paddy soils in South China.