Non-radical-dominated photocatalytic degradation of PFOA on NiO/Bi₅O₇I S-scheme heterojunctions: Charge dynamics, Ni³⁺/Ni²⁺ cycling, and DFT-guided reaction pathways

Effective photocatalytic degradation of perfluorooctanoic acid (PFOA) remains challenging due to inefficient charge separation and limited visible-light absorption of conventional catalysts. Pristine Bi₅O₇I suffers from low specific surface area and rapid charge recombination, and NiO is limited by a wide band gap and low Ni³⁺ content. Here, a NiO/Bi₅O₇I S-scheme heterojunction (NB5) was synthesized via one-pot hydrothermal synthesis followed by calcination. In-situ growth of NiO nanoparticles increased surface area, and Bi₅O₇I incorporation increased the Ni³⁺/Ni²⁺ ratio, collectively strengthening the internal electric field and accelerating charge transfer. As a result, NB5 achieved 87.5% PFOA removal with an apparent rate constant of 0.439 h⁻¹ under simulated sunlight. Mechanistic studies revealed a non-radical-dominated synergistic oxidation-reduction pathway involving direct oxidation by h⁺, Ni³⁺-mediated electron transfer, and conduction-band e⁻, facilitated by reversible Ni³⁺/Ni²⁺ redox cycling. Density functional theory (DFT) calculations reveal dual thermodynamically favorable oxidation pathways for PFOA anions, involving direct h⁺ oxidation (ΔG = −9.58 eV) and Ni³⁺-mediated oxidation (ΔG = −8.13 eV). The toxicity and ecological risks of PFOA and its degradation products were predicted based on structure–activity relationships using established in silico approaches. This work provides a facile strategy for S-scheme heterojunction construction and mechanistic guidance for photocatalytic degradation of PFOA.