Electrocatalytic dechlorination of chloropicolinic acid mixtures by using palladium-modified metal cathodes in aqueous solutions

Abstract

In China, chloropicolinic acid (ClPA) mixtures comprising 3,5,6-trichloropicolinic acid, 3,6-dichloropicolinic acid (3,6-D), 3-ClPA, and 6-ClPA are discharged as organic wastes at a rate of approximately 300 tons per year. In this work, we developed an aqueous phase electrocatalytic hydrogenation (ECH) system based on Pd catalyst to dechlorinate the ClPA mixtures into picolinic acid (PA) at room temperature. Firstly, we evaluated the influence of cathode support and Pd loading on the catalytic performance of cathodes, as well as the effects of operating parameters on the intermediate product selectivity and dechlorination efficiency of the ECH process with 3,6-D as the target compound. Secondly, we analyzed the ECH dechlorination mechanism of 3,6-D with regard to the surface condition of cathode and catholyte pH, and the rate-limiting step of the dechlorination process was also discussed. Finally, we assessed the practicability of the ECH system to dechlorinate the ClPA mixtures into PA by using a plate-and-frame cell. Results demonstrated that Pd/Ni foam cathodes with Pd loading of 2.25–3.6mgcm−2 exhibited the optimum ECH dechlorination performance, and the basic aqueous solution and high 3,6-D concentration favored the ECH process. The ClPA mixtures with 47gL−1 concentration (the total concentration of ClPAs was approximately 250mM) can be selectively dechlorinated into PA with 99% yield, 76.3% current efficiency, and 2.47kWhkg−1 PA specific electric energy consumption at a current density of 208Am−2 in a 1.25M NaOH aqueous solution.