Advanced oxidation pretreatment for lignocellulosic bioconversion: reactive species chemistry, byproduct formation, and downstream impacts

Lignocellulosic biomass is an abundant renewable organic carbon resource, but its compact cell-wall structure and lignin shielding limit bioconversion efficiency. Advanced oxidation processes (AOPs) have emerged as promising pretreatment approaches; however, the links between reactive species (RS) chemistry, structural attack patterns, byproduct formation, and downstream bioconversion remain fragmented. This review integrates current evidence within an RS–attack sites–byproducts–downstream conversion framework. The formation conditions and reaction characteristics of inorganic radicals, organic radicals, and non-radical pathways are compared, with emphasis on their preferential reactions with lignin linkages, aromatic moieties, and carbohydrate structures. Comparative analysis indicates that RS selectivity interacts with feedstock properties and operating conditions to shape the formation and distribution of major byproduct profiles, including phenolic compounds, furan derivatives, and small organic acids. The review further evaluates how these byproducts influence enzymatic saccharification, methanogenic anaerobic digestion, and chain elongation fermentation in lignocellulosic bioconversion processes. The dominant mechanisms involve interfacial adsorption, enzyme deactivation, redox burden, and microbial stress, which collectively reshape process kinetics and stability. Practical implementation remains constrained by oxidant cost, catalyst stability and recyclability, and residual oxidant or inhibitor carryover. Therefore, engineering scalability of AOP-based pretreatment should integrate RS regulation with catalyst recovery, inhibitor mitigation, and downstream tolerance rather than focusing only on delignification or sugar release. Overall, AOP-based pretreatment should shift from maximizing oxidative intensity or delignification toward controlling RS pathways and byproduct carryover according to downstream tolerance.