Advanced oxidation processes for 2,4-dichlorophenoxyacetic acid in aqueous media: evidence gaps, mechanistic insights and emerging directions

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) remains a relevant aquatic contaminant because parent-compound disappearance does not imply mineralization, detoxification or treatment readiness. This systematic review synthesizes recent evidence on advanced oxidation processes (AOPs) and closely related chemically coupled routes for aqueous 2,4-D treatment. Records from Scopus, Web of Science and Google Scholar within a documented 2020–2025 review window were screened using explicit eligibility criteria and assessed with an evidence-completeness rubric covering key reporting domains for cross-study comparison. The final corpus comprised 204 studies across photocatalytic/photo-assisted, electrochemical/electro-Fenton, Fenton-type, sulfate-radical, ozone-based, sonochemical, plasma-assisted and hybrid systems. Parent 2,4-D removal or analytical confirmation was reported in 202 studies, while carbon-based mineralization or organic-load evidence was reported in 116, transformation products in 95, residual toxicity in 27, real-water validation in 67 and reactor-scale or transferability information in 37. These patterns show stronger evidence for parent-compound disappearance than for treatment completeness, residual hazard and transferability. No AOP family was universally superior across removal, mineralization, toxicity reduction, matrix robustness, energy demand, durability and scale readiness. Future studies should standardize reporting of mineralization, transformation products, toxicity, energy boundaries, leaching, durability and reactor descriptors.