Manganese-modified biochar for highly efficient sorption of cadmium

In this study, corn stalk was modified by manganese (Mn) before (MBC₁) and after (MBC₂) pyrolysis at different temperatures (400~600 °C) under anaerobic conditions for Cd sorption in both water and soil. Batch experiments in aqueous solution were conducted to evaluate the optimum sorption capability by biochar with and without manganese-modified. Both types of manganese modification can improve the sorption capacity of Cd(II) on biochar, which is superior to the corresponding pristine biochar without modification, especially, pyrolyzed at 500 °C with 5:1 modification ratio. Under the optimal preparation conditions, the sorption percentage on MBC₂ was 11.01% higher than that of MBC₁. The maximum sorption capacity of MBC₂ was 191.94 mg g^-1 calculated by isotherm model. The performance of MBC₂ was also verified in soil stabilization experiments in Cd-contaminated soil. We can conclude from the results of BCR extraction that all the application rates of MBC₂ (1%, 2%, and 3%) can reduce the mild acid-soluble fraction Cd. The reducible, oxidizable, and residual fraction Cd showed an upward trend, thus controlling the migration, transformation, and enrichment of Cd in soil. The characteristic analysis showed biochar has more irregular fold and more particle-aggregated surface after modification. The main components of these aggregated particles are manganese oxides (MnO_x) with high sorption capacity, such as the MnO_x crystal structure loaded on MBC₂ is a mixed structure of δ-MnO₂ and MnO. However, these particles may block the biochar pores, or some of the pores may collapse at high temperatures during the modification process. The specific surface area was reduced, even if the sorption effect of MBC was strongly enhanced. Meanwhile, under the action of the secondary pyrolysis of MBC₂ modification process, the MBC₂ has a higher degree of aromatization with more potential active sorption sites for Cd. The study concluded that the MBC₂ could be a promising amendment for Cd in both water and soil real field applications.