Biomass inherent metal interfere carbothermal reduction modification of biochar for Cd immobilization

Metal salt laden are frequently used to enhance the heavy metal adsorption capacity of biochar. The present study indicates that CaS loading biochar can be modified from the carbothermal reduction reaction between CaSO₃ (modification agent) and carbon matrix. The CaS transformation ratio as indicated by XPS spectra was significantly improved by the CaSO₃ loading content. The coprecipitation reaction induced by the CaS in biochar can significantly enhance the adsorption capacity of heavy metals (Cd). And, the Cd adsorption capacity can be enhanced up to >100 mg/g and increases with increasing CaS ratio in the biochar. In addition, the adsorption process was rapid and could be balanced within several minutes (~ 5 min). Furthermore, the interaction reaction between the modification agent and the inherent metal in the biomass was examined in the biochar pyrolysis preparation process. Interestingly, MgCl₂ inherent metal salt can combine with the original CaSO₃ to produce a new mineral, resulting in a decrease in CaS. However, KCl, a more thermally stable biomass-derived metal salt, exhibited a weak combination ability with the modification agent. Accordingly, this type of secondary reaction reduces the Cd adsorption capacity owing to the decrease in the number of adsorption sites (CaS).