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 (k1p) 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.