Atomically dispersed Fe boosting elimination performance of g-C3N4 towards refractory sulfonic azo compounds via catalyst-contaminant interaction

Abstract

Herein, an oxygen-doped porous g-C₃N₄ photocatalyst modified with atomically dispersed Fe (Fe₁/OPCN) is successfully prepared and exhibits significant superiority in removing refractory sulfonic azo contaminants from water via catalyst-contaminant interaction. The elimination performance of Fe₁/OPCN towards acid red 9, acid red 13 and amaranth containing similar azonaphthalene structure and increasing sulfonic acid groups increases gradually. The amaranth degradation rate of Fe₁/OPCN is 17.7 and 6.1 times as that of homogeneous Fenton and OPCN, respectively. In addition, Fe₁/OPCN also has more outstanding removal activities towards other contaminants with sulfonic acid and azo groups alone. The considerable enhancement for removing sulfonic azo contaminants of Fe₁/OPCN is mainly ascribed to the following aspects: (1) The modified Fe could enhance the adsorption towards sulfonic azo compounds to accelerate the mass transfer, act as e⁻ acceptor to promote interfacial charge separation, and trigger the self-Fenton reaction to convert in-situ generated H₂O₂ into •OH. (2) Fe(Ⅲ) could coordinate with —N=N— to form d-π conjugation, which could attract e⁻ transfer to attack —N=N— bond. Meanwhile, the inhibited charge recombination could release more free h⁺ to oxidize sulfonic acid groups into SO₄⁻•. (3) Under the cooperation of abundant multiple active species (•O₂⁻, h⁺, e⁻, •OH, SO₄⁻•) formed during the degradation reaction, sulfonic azo compounds could be completely mineralized into harmless small molecules (CO₂, H₂O, etc.) by means of —N=N— cleavage, hydroxyl substitution, and aromatic ring opening. This work offers a novel approach for effectively eliminating refractory sulfonic azo compounds from wastewater.