Continuous-flow skeletal editing for front-end remediation of 1,4-dioxane at environmentally relevant concentrations

1,4-Dioxane is a probable human carcinogen, a highly mobile groundwater contaminant, and notably resistant to conventional remediation. We report an oxidant-free continuous-flow photochemical platform that mitigates this challenge via selective molecular transformation rather than mineralization. At environmentally relevant influent levels (∼1 g L −1 , 11.4 mM), blue LED irradiation and aryl diazoacetates enable controlled skeletal expansion of 1,4-dioxane into 1,4-dioxepane derivatives through C–O bond insertion. The resulting dioxepanes are structurally stable, oxygen-rich cyclic ethers with established value as medicinal and functional scaffolds and exhibit markedly lower water miscibility, enabling efficient phase separation and downstream removal. A segmented biphasic millifluidic reactor, optimized using Design of Experiments and generalized linear modeling, achieved >93% conversion and up to 95% effective removal under spiral geometries without uncontrolled by-products. Although toxicological profiles require further study, reduced mobility and separability support dioxepanes as transient intermediates in front-end load reduction. This work positions skeletal editing as a scalable, energy-efficient complement to mineralization-based water treatment.