Synergistic Effects of N-Containing Heterocyclic and Ca Ligand Structures on the Phosphorus Adsorption of N and Ca Co-Doped Biochar

Removal of phosphorus from water using modified biochar is often considered a green and feasible approach to complement traditional methods. Especially metal and heteroatom co-doped biochar shows more active sites and higher affinity for pollutants compared to single-doped biochar by metal or heteroatoms. In this study, N and Ca single-doped and co-doped biochar were synthesized by both direct and active doping, and their possible structural changes and adsorption mechanism were discussed. The results demonstrate that the structural changes of biochar caused by heteroatom doping (represented by N) and metal doping (represented by Ca) are essentially a heterocyclic restructuring of the carbon skeleton. During this restructuring, metal ions are combined with the carbon skeleton through oxidation coordination. Additionally, phosphorus removal is enhanced by the coupling of N-containing heterocycles with Ca ligand structures on the surface of biochar, resulting in synergistic (1+1>2) adsorption effects. The mechanisms of efficient adsorption mainly include pore filling, hydrogen bonding, anion-π interaction, ligand exchange, and chemical precipitation. The aforementioned results offer a thorough comprehension of the structural regulatory direction needed for effective phosphorus adsorption using biochar.