Coupled extraction and spontaneous room-temperature degradation of haloacetic acids in a hydrophobic deep eutectic solvent

Haloacetic acids (HAAs) are persistent, highly mobile disinfection byproducts in drinking water distribution systems and disinfected wastewater. Their small size and hydrophilic nature make the removal and degradation in water difficult. Here, we examine a hydrophobic deep eutectic solvent (HDES) as an integrated extraction and degradation phase for representative HAAs. The HDES efficiently partitioned mono-, di-, and trihalogenated HAAs from clean water and wastewater effluent into a distinct nonaqueous phase. Whereas degradation of PFAS in the same HDES system required heating, the enriched HAAs underwent extensive room-temperature C-halogen and C-C bond cleavage in basic HDES without external energy input, a reactivity inaccessible in water. Product distributions revealed halogenation-dependent pathway selection: mono- and dihalogenated substrates followed C2-preserving hydrolytic dehalogenation, whereas trihalogenated substrates shifted toward decarboxylation-derived C1 fragmentation. Comparative density functional theory calculations showed that HDES lowers activation barriers for competing dehalogenation and decarboxylation initiation steps relative to water and revealed an α-halogenation-dependent crossover in pathway accessibility. Five sequential extraction/destruction cycles in wastewater effluent retained both separation and reaction functionality. By uniting extraction and room-temperature destruction in a single engineered phase, this work advances solvent-phase engineering as a strategy for contaminants that are difficult to both remove and degrade in water.