Effect of Modified Biochars on Soil Cadmium Stabilization in Paddy Soil Suffered from Original or Exogenous Contamination

Abstract

To investigate the passivation of different modified biochars on the speciation and availability of cadmium contaminated soil, the modified biochars were treated by different approaches (acid/base treatment, impregnation with manganese oxides, magnetic modification) and biochars(BC) were used as soil passivating agents for soil culture experiments. The result indicated that the content of available cadmium decreased significantly by BC and modified biochars in originally contaminated soil. Compared with CK, the percentage of available cadmium in originally contaminated soil was reduced by more than 50% using impregnable biochars by KMnO4(BC-KMnO4) and basic biochars by NaOH(BC-NaOH). And the content of available cadmium decreased significantly by three modified biochars which were BC-KMnO4, BC-NaOH and FeCl3 magnetization biochars(BC-FeCl3) in exogenously contaminated soil. Particularly, the best performance was observed with BC-KMnO4 that reduced 30% available cadmium in exogenously contaminated soil. However, the passivation of BC was not significant, and the content of available cadmium slightly increased (3.8%-24.5%) by BC-HNO3 in exogenously contaminated soil. Furthermore, the content of exchangeable cadmium was increased by 20.2% with 2.5% BC-HNO3 in exogenously contaminated soil, while significantly decreased by other modified biochars and BC, and the 10% BC-KMnO4 reduced 65.1% exchangeable cadmium in originally contaminated soil. Meanwhile, soil pH was increased significantly by BC, BC-KMnO4 and BC-NaOH, while was reduced by BC-HNO3. The contents of organic carbon and exchangeable base cations in soil were improved by all the treatments. The results of regression analysis showed that the content of available cadmium in originally contaminated soil was significantly negatively correlated with soil pH, soil exchangeable Na+, while the content of available cadmium in exogenously contaminated soil was significantly negatively correlated with soil pH, soil organic carbon, soil exchangeable Mg2+, Na+, K+. Accordingly, the lower available cadmium in contaminated soil may correlate with the increasing content of organic carbon, exchangeable base cations and pH. In summary, the materials of BC-KMnO4 could be used as a superior passivating agent for in situ remediation of cadmium pollution, while the materials of BC-HNO3 could slightly activate cadmium in the soil, leading to some risk in in situ remediation.