Fabrication Of Activated Silver And Palladized Silver Electrodes And Their Properties For Reductive Dechlorination

Chlorinated organic compounds （COCs）, including chloroacetic acid and chlorophenoxy acids, are widespread environmental contaminants found both in groundwater and soil, many of which are toxic, persistent and strongly resistant to chemical, physical or biological degradation. They can accumulate gradually in the environment and cause hazardous influence on living organisms including human beings. Therefore, developing efficient and safe technology for the dechlorination of COCs is highly desirable nowadays.In this study, the roughened Ag electrodes were obtained by oxidation-reduction cycle process （ORC）, and palladized silver electrodes were fabricated by a convenient metallic replacement reaction. Trichloroacetic acid （TCAA） and 2,4-Dichlorophenoxyacetic acid （2,4-D） were selected as model of COCs. The dechlorination degradation of them has been investigated by the means of electroreduction. In addition, the dechlorination pathway and mechanism were analysed.The surface morphology and composition of the electrodes were analyzed by AFM and SEM assembled with energy dispersive spectrometer. The electrochemical reductive dechlorination of chlorinated compounds （TCAA, and 2,4-D） has been investigated at Ag（r） and Pd/Ag（r） electrodes in aqueous solutions, by means of cyclic voltammetry （CV） and of controlled-current electrolysis.Firstly, Ag is an excellent electrocatalytic material for direct electrodechlorination of chloroacetic acid and the complete dechlorination of chloroacetic acids to safe acetic acid （AA） with excellent current efficiency （near 95%） can be successfully achieved by using this peculiar metal. The reduction of TCAA in a stepwise dechlorination fashion was suggested and trichloroacetic acid （TCAA） => dichloroacetic acid （DCAA） => monochloroacetic acid （MCAA） => acetic acid （AA） probably is the main reductive reaction route independent of solutions pH. The reduction activities of some substances, usually co-existing in chlorinated acetate-contaminated drinking water, probably follow the decreasing order: TCAA > DCAA > H+ > MCAA > H2O, by using activated silver electrodes. In addition, it is prognosticated that all of haloacetic acids （except for fluorine-containing haloacetic acids） can be completely dehalogenated to safe AA with high efficiency by using this catalytic system according to the stability sequence of haloacetic acids.Secondly, Pd/Ag（r） exhibited powerful electrocatalytic activity for dechlorination of 2,4-D. The initial concentration of 2,4-D, Pd loading, electrode activity, and current density affected the dechlorination efficiency and the current efficiency. The increase of 2,4-D concentration led to the increase of the current efficiency but the decrease of dechlorination efficiency. It can make an improvement of the electrode activity and achieve the excellent dechlorination efficiency using the approach of activating Pd/Ag（r） electrode by ORC process at regular intervals. The optimal Pd loading and current density for 2,4-D dechlorination was 0.91 mg cm^-2 and 0.69 mA cm^-2 （25 mA）, respectively.Using the obtained optimized conditions, 25 mM of 2,4-D could be wholly dechlorinated to PAA with 85.3 % yield at 298 K by constant current 25 mA after 6 h electrolysis, which gave current efficiencies in range of 66-79.3 %. In addition, a new dechlorination mechanism of 2,4-D was proposed, in which the formation of adsorbed 2,4-D on Ag is a key step.