The critical roles of surface-bound radicals in the nickel phosphide/biochar-persulfate catalytic oxidation system for tetracycline removal: The synergistic catalysis between nickel phosphides and biochar

Exploring efficient and cost-effective heterogeneous persulfate (PS) oxidation systems is meaningful for addressing recalcitrant organic pollutants in wastewater. Herein, nickel phosphide/biochar (NixP/biochar) composite, with Ni12P5 as the predominant crystalline phase, is firstly synthesized utilizing a biomass phosphorus source, phytic acid (PA). NixP/biochar demonstrates outstanding catalytic capacity in activating PS for removing tetracycline (TC). At 1 mM PS, 0.15 g/L catalyst, and pH 7.0, 94.55 % of TC (20 mg/L) can be removed within 180 min, markedly superior to Fe-, Cu-, and Co-based phosphide/biochar composites. The NixP/biochar-PS system exhibits superior catalytic degradation efficiency (>94.00 %) against varying concentrations (1–20 mg/L) of TC and robustly resists interferences from complex water matrices, including sodium humate solution, surface water, anions (>90.00 %). The ideal catalytic efficiency is ascribed to surface-bound radicals (including SO4-• and •OH), with •O2– and 1O2 assisting in. Density functional theory (DFT) calculations indicate that due to the electron donations from Ni, P, and C, S2O82- may undergo homolysis to produce the surface-bound SO4-•, which may induce the generation of other ROSs through radical chain reactions. Besides, the electronic structures and work functions of NixP in NixP/biochar composites can be modulated by the biochar, enhancing the catalytic selectivity of NiP, Ni2P, and Ni12P5 with Ni as the electron-donating center, while diminishing that of P-centered Ni3P. These findings may serve as a reference for the theoretical study of Ni-based catalysts and the practical application of NixP in AOPs.