Unlocking the kinetic processes of cathodic electrochemical hydrodechlorination with an integrated process-resolved kinetic model.

Electrochemical hydrodechlorination (EHDC) is a promising approach for removing chlorinated organic contaminants from water, but its kinetics are often described using apparent rate constants that cannot resolve the underlying controlling processes. Here, a process-resolved kinetic model was developed for EHDC of 2,4-dichlorophenoxyacetic acid (2,4-D) on Pd/Ni cathodes by integrating substrate transport, interfacial adsorption, sequential dechlorination, active-hydrogen turnover, and competition from the hydrogen evolution reaction. Time-resolved concentration profiles of 2,4-D and its dechlorination products, including o-chlorophenoxyacetic acid, p-chlorophenoxyacetic acid, and phenoxyacetic acid, were used for parameter optimization under different cathode potentials and initial 2,4-D concentrations. The model accurately described both 2,4-D depletion and dechlorination product formation. The fitted parameters, supported by local sensitivity and profile weighted residual sum of squares diagnostics, were physicochemically reasonable and indicated that EHDC was mainly controlled by interfacial retention, pathway-specific dechlorination, and active-hydrogen utilization rather than bulk diffusion alone. Model predictions under extended operating conditions, including higher 2,4-D loading and stronger cathodic polarization, were consistent with the experimental trends. In addition, the model described the inhibitory effects of representative water-matrix ions, HCO3- and Ca2+, through matrix-induced active-hydrogen loss and reduced effective reactive-site utilization. This work provides a quantitative framework for interpreting and predicting EHDC performance. Application to other pollutants, cathode materials, or reactor configurations requires system-specific recalibration and validation.