Effects of Straw and Cow Dung Biochars on the Cadmium Passivation in the Vegetable Fields around the Mining Area and the Growth of Chinese Cabbage

The remediation of heavy metal contamination for large areas of paddy soils in non-ferrous metal mining areas has received much attention. However, the safe use of soil in scattered vegetable fields near mining areas has not yet been adequately studied, which will seriously threaten the health of local residents. In this study, five types of biochar remediation materials (rice husk biochar, corn straw biochar, sesame straw biochar, cow dung biochar and potassium ferrate modified rice husk biochar) were prepared by pyrolysis method. Taking the vegetable soil around a mining area in Tongling that suffered from cadmium contamination as the object of remediation, potting remediation experimental study of four-season Chinese cabbage (Brassica chinensis), which is widely planted in the local area, was carried out in order to reveal the effects of different biochar materials on the passivation mechanism of cadmium-contaminated soils, remediation effect, and the growth of Chinese cabbage. It was found that the application of different biochar materials at the rate of 1.5% increased the pH and cation exchange of different potting soils by different degrees, and the content of available Cd in soil was also reduced by 2.87% (rice husk biochar), 7.03% (corn stover biochar), 14.38% (sesame straw biochar), 32.19% (cow dung biochar) and 24.66% (potassium ferrate modified rice husk biochar), respectively. Compared to the control group, the application of different biochar materials resulted in different reductions in aboveground Cd content and increases in biomass (i. e., plant height and fresh weight) of Chinese cabbage. Among five types of biochar, the passivation effect of sesame straw biochar, cow dung biochar and potassium ferrate modified rice husk biochar were more obvious, which not only significantly reduced the cadmium content in Chinese cabbage, but also promoted its growth and development. Such results might be related to the fact that these materials have rich surface functional groups as well as high ash content. In conclusion, this study can provide important reference for the safe use of heavy metal contaminated vegetable land soil in mining areas and for food safety.