Adsorption Mechanism of Modified Biochar on Typical Alkyl and Chlorinated Organophosphorus Flame Retardants

In recent years, the widespread occurrence of alkyl and chlorinated organophosphate flame retardants (OPFR) pollution in various environmental media has attracted increasing attention. Tris(2-butoxy) ethyl phosphate (TBOEP), tripropyl phosphate (TPrP) and tris(1, 3-dichloroisopropyl) phosphate (TDCPP) are representative compounds of alkyl- and chlorinated- OPFRs, respectively. In this study, using discarded peanut shells as biochar precursor and KOH as a modifying agent, four types of modified peanut shell-derived biochars were prepared at different pyrolysis temperatures via a simple pyrolysis process. These biochars were subsequently applied for adsorption removal of the three selected OPFR pollutants. Characterization techniques including scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, Fourier-transform infrared spectroscopy (FT-IR), x-ray photoelectron spectroscopy (XPS), zero potential measurement, and water contact angle analysis reveal that the biochar pyrolyzed at 600 ℃ (PBC600) exhibited a high specific surface area (SSA), well-developed pore structure, and abundant surface functional groups such as —OH, —NH and —COOH. Under the experimental conditions of an initial OPFR concentration of 5 mg·L^-1 and pH 5, the addition of 10 mg of PBC600 resulted in removal efficiencies of 73%, 80% and 89% for TBOEP, TPrP and TDCPP, respectively, after 3-h of adsorption. Common coexisting ions in environmental water systems, including K⁺, Na⁺, Ca²⁺, Mg^2+, Al³⁺, Cl^- and SO₄^2-, exhibited negligible interference with the adsorption process. Comparative analysis of SEM-EDS and FTIR spectra before and after adsorption, combined with adsorption experiments, model fitting, and density functional theory (DFT) calculations, indicates that the adsorption of TDCPP by PBC600 followed a pseudo second-order kinetics model, suggesting a chemical adsorption. In contrast, TBOEP adsorption was better described by the pseudo first-order kinetics model, indicating a physisorption mechanism. The theoretical maximum adsorption capacity (Q_m) (110.62 mg·g^-1) calculated from the model was consistent with the experimentally observed value (108.2 mg·g^-1). The adsorption capacities of PBC600 for the three OPFRs followed the order: TDCPP > TPrP > TBOEP, which was attributed to differences in their molecular electrostatic potentials. The adsorption isotherms of all three OPFRs on PBC600 were better fitted by the Langmuir model; however, discrepancies between theoretical and experimental adsorption capacities suggested the occurrence of non-uniform multilayer adsorption. The primary adsorption mechanism involved pore-filling, hydrogen bonding, and hydrophobic interactions, with TDCPP also exhibiting weak electrostatic interactions. In real-world waters, the removal efficiency of OPFRs slightly decreased, and the adsorption capacity diminished after five regeneration cycles. These findings are expected to provide a theoretical basis for the development of eco-friendly adsorbent materials for the remediation of OPFR-contaminated environments.