Objectives The environmental damage of heavy metals mainly depends on their speciation, especially the content and proportion of bioavailable ones. Biochar had been attempted as artificial measures for reducing the content of bioavailable Cd in Cd contaminated soils. The paper examined the transformation of Cd forms in red soils and cinnamon soils when a certain amount of biochar were added, which would deepen the understanding of the remediation mechanism of biochar to Cd pollution.

Methods Natural air-dried red soil (pH 5.21) and cinnamon soil (pH 7.75) of less than two 2 mm in size were selected as tested soils for a incubation experiment. 40 kg of the tested soil were loaded into a 20 L-plastic box, Cd (NO₃)₂ solution was added to make the exogenous Cd content of the soil to 5 mg/kg, and kept stood for two weeks at (25 ± 2)℃, keeping the maximum soil water content of 70% of filed capacity. Then, biochar were evenly mixed with the soil in ratio of 0, 5, 10 and 20 g/kg soil, and cultured at room temperature for 50 days. Soil samples were collected at 1, 4, 7, 14, 21, 35, and 49 days of culture. The soil pH and organic carbon content were determined, and the soil Cd was divided into five forms by Tessier classification method, and the contents were analyzed.

Results The red soil pH was significantly increased with the increase of biochar addition, and after 14 days of cultivation at the biochar addition rate of 20 g/kg, the soil pH changed from acidic to weak alkaline. While the pH of cinnamon soil did not significantly changed with biochar addition. The organic carbon content increased with the increasing amount of biochar in both red soil and drab soil. In red soil after 49 days of cultivation, the content of exchangeable Cd decreased by 0.31 to 0.82 mg/kg. At the biochar addition rate of 2%, the exchangeable Cd content was the lowest (1.24 mg/kg) , the carbonate bound Cd was the highest (1.06 mg/kg). The proportion of carbon-bound Cd were increased by 3.14%−14.21%, and that of Fe and Mn oxide-bound Cd increased by 8.20%−23.96%. In cinnamon soil after 49 days of incubation, the carbon-bound Cd and Fe and Mn oxides bound Cd were increased by 0.94%−2.61% and 0.80%−7.90% with the increase of biochar addition rate. The soil organic carbon content and biochar addition rate presented significant and negative correlation with the exchangeable Cd, yet had a great positive correlation with carbonate bound Cd, Fe, Mn oxide-bound Cd and organic-bound Cd. The pH, soil organic carbon content and biochar application rate presented significant and negative correlation with the exchangeable Cd in red soil, yet had a significant and positive correlation with other four forms of Cd. However, the correlation coefficient in red soil was greater than that in cinnamon soil.

Conclusions Comprehensively analyzing the form of Cd in the two types of soil, the remediation effect of biochar in red soil is due to the transform of bioavailable Cd to unavailable form, which is driven by the increased soil pH. Therefore, biochar can be referred as a kind of conditioner in acidic Cd-contaminated soil.