Adsorption–desorption properties and control mechanism of aromatic organophosphate esters on biochar–mineral complexes

With the widespread application of biochar, biochar-mineral complexes are formed in large quantities in the environment. However, there are few reports on their adsorption and desorption mechanisms for organophosphate esters (OPEs). In this study, biochar-mineral complexes (MBC@FH and MBC@F) were prepared through co-ball-milling with biochar and minerals (α-FeO(OH) and Fe₂O₃). The properties of pristine biochar and MBC@FH/F were compared, and the adsorption-desorption mechanisms of OPEs on all biochar materials were analyzed. The results showed that MBC@FH/F complexes have a larger specific surface area, more oxygen-containing functional groups, stronger aromaticity, and larger pore structures, but less persistent free radicals than pristine biochar. MBC@FH/F complexes exhibited lower secondary kinetic adsorption rate constant (k₂), higher intraparticle diffusion rate constant (k_1p) and adsorption capacity than pristine biochar. Hydrophobic interactions controlled the adsorption of TPhP and TPPO on MBC@FH/F complexes, and π-π interactions, pore filling, hydrogen bonding also participated in the adsorption process. MBC@F had higher aromaticity than MBC@FH, thus having stronger adsorption capacity than MBC@FH. The large spatial volume of TPPO inhibited its diffusion on biochar materials, demonstrating a smaller adsorption amounts than TPhP. The desorption was only observed for TPhP, while no desorption occurred for TPPO. The retention amount of TPhP on MBC@FH/F was represented by subtracting the desorption amount from their adsorption amount, which increased by 1.22-6.98 times, reaching 4.48-41.07 mg/g. In addition, the desorption hysteresis decreases with increasing pore diameter of MBC@FH/F. This study illustrated that the formation of biochar-mineral complexes may be more conducive to the adsorption of OPEs.