Synergistic enhanced passivation of phosphorus and cadmium in sediment by Ca/Al co-modified biochar

Currently, lakes are facing increasingly severe combined pollution from eutrophication and heavy metals. A novel material was prepared by modification peanut shell biochar with calcium (Ca)-aluminum (Al) (CA@BC) in this study. Its maximum adsorption capacity (Qmax) reached 97.9 and 88.7 mg/g for phosphorus (P) and cadmium (Cd), respectively, and it showed synergistic enhancement during synchronous adsorption. Not entirely consistent with the adsorption mechanisms for P or Cd alone, the phosphate precipitation and inner sphere complexes (the coordination bonds formation between carbon and metal with phosphate, C/Al/Cd-O-P) were accounted for P adsorption by CA@BC during synchronous adsorption. And the Cd removal was caused by the formation of phosphate and hydroxide precipitates, as well as a small amount of Cd-O-P and Cd-O-C inner sphere complexes. The sediment results of individual P and simultaneous P and Cd indicated that the CA@BC capping had a good immobilization effect on sediment P and Cd, especially in alleviating their increased release intensity caused by anoxic conditions, and helped to maintain consistently low P and Cd concentrations in the overlying water. Both active P and Cd in surface sediments could be converted to their stable states by the capping of CA@BC, in which iron bound P (Fe-P) and the weak acid extractable Cd and reducible Cd were primarily transformed to Ca bound P (Ca-P) and residual Cd, respectively. In addition, CA@BC has a stronger transformation effect on the active Fe-P in surface sediments during simultaneous passivation. Therefore, CA@BC showed a great potential in the passivation of sediment P and Cd, and provided a good solution for the remediation of eutrophic heavy metal-contaminated sediments.

Automated summary

This study prepared a new material CA@BC by modifying peanut shell biochar, which showed great potential in the remediation of eutrophic heavy metal-contaminated sediments. CA@BC exhibited high adsorption capacity for phosphorus and cadmium, and showed synergistic enhancement during synchronous adsorption. The adsorption mechanisms for phosphorus and cadmium during synchronous adsorption involved phosphate precipitation and inner sphere complexes. The results demonstrated that CA@BC could effectively immobilize active phosphorus and cadmium in sediments and maintain low concentrations of phosphorus and cadmium in the overlying water.