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.