Phytic acid-modulated iron phosphide/biochar catalyst activates persulfate for rapid sulfamethoxazole removal: Synergy between iron phosphides and biochar

Abstract

The efficient activation of persulfate (PS) is essential for rapidly degrading refractory organic pollutants in water. Traditional metal-based catalysts are limited by defects such as the metal ion redox cycle and secondary pollution, resulting in insufficient catalytic performances. In this research, highly active micron-sized iron phosphides (FeP and Fe2P) are in-situ synthesized on biochar, and the primary crystal phase can be switched by adjusting the phytic acid (PA) amount. Under the conditions of 0.15 g/L catalyst, 0.5 mM PS, and pH 7, sulfamethoxazole (SMX) at concentrations of 0.25, 0.50, 1.00, and 10.00 mg/L can be completely oxidized within 3, 5, 15, and 180 min, respectively. The leached ferric ion amount is below 0.020 mg/L. Meanwhile, ideal SMX abatements are achieved in sodium humate solution and surface water matrices, demonstrating the potential of this catalytic system for drinking water and wastewater treatment. Moreover, SO4-•, •OH, and 1O2 are crucial reactive oxygen species for SMX removal. Based on density functional theory (DFT) calculations, it is found that the partially charged Fe ion (≡Feδ+) is the pivotal site for PS activation and ≡Fe2+ regeneration. FeP and Fe2P are typically semi-metallic. Biochar can enhance the catalytic efficacies of FeP and Fe2P by shifting d-band centers (spin up and down) towards fermi level and reducing band gaps (spin down). Besides, biochar may donate electrons to iron phosphides when activating PS. The enhancement of FeP by biochar is more significant, which is why FeP/biochar is superior to Fe2P/biochar in PS activation.