Photogenerated-electron fate, interfacial transfer, and evidence for microbial utilization in intimately coupled...

Intimately coupled photocatalysis and biodegradation (ICPB) systems integrate photocatalysts, carriers, and biofilms for recalcitrant-contaminant removal and nitrogen transformation. As research has shifted toward interfacial mechanisms, photogenerated-electron fate and attribution have become central to interpreting photocatalytic-microbial interactions. However, observations of improved removal, enhanced nitrogen conversion, altered photoelectrochemical signals, or upregulation of extracellular electron transfer (EET)-related components do not by themselves establish microbial utilization of photogenerated electrons. This review critically evaluates thermodynamic feasibility, competing electron sinks, photocatalyst-carrier-biofilm configurations, microbial EET-related components, and engineering constraints. Photogenerated electrons may recombine with holes, reduce O₂ to reactive oxygen species, reduce organic or inorganic acceptors, or participate in microbial interfacial exchange, while alternative reducing equivalents are provided by co-substrates, photocatalytic transformation products, and endogenous metabolism. A five-tier claim-based evidence hierarchy distinguishes associative evidence, interfacial transfer/accessibility, causal biological electron acquisition, source-resolved microbial utilization, and bounded quantitative attribution. Robust attribution requires source-resolved evidence linking causal biological electron acquisition to a defined terminal function; specific molecular EET claims require pathway-specific evidence, whereas quantitative attribution requires electron-equivalent accounting within an explicit system boundary. Engineering translation ultimately depends on whether these relationships remain valid in real wastewater matrices, nonuniform illumination, continuous-flow operation, interfacial aging, and process-level energy and cost constraints.