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

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.