In situ FTIR studies on the electrochemical hydrodechlorination of 3,4,5,6-tetrachloropicolinic acid on Ag cathode

Electrocatalytic hydro-dehalogenation of halogenated organic pollutants from wastewater: A critical review

Halogenated organic pollutants are often found in wastewater effluent although it has been usually treated by advanced oxidation processes. Atomic hydrogen (H*)-mediated electrocatalytic dehalogenation, with an outperformed performance for breaking the strong carbon-halogen bonds, is of increasing significance for the efficient removal of halogenated organic compounds from water and wastewater. This review consolidates the recent advances in the electrocatalytic hydro-dehalogenation of toxic halogenated organic pollutants from contaminated water. The effect of the molecular structure (e.g., the number and type of halogens, electron-donating or electron-withdrawing groups) on dehalogenation reactivity is firstly predicted, revealing the nucleophilic properties of the existing halogenated organic pollutants. The specific contribution of the direct electron transfer and atomic hydrogen (H*)-mediated indirect electron transfer to dehalogenation efficiency has been established, aiming to better understand the dehalogenation mechanisms. The analyses of entropy and enthalpy illustrate that low pH has a lower energy barrier than that of high pH, facilitating the transformation from proton to H*. Furthermore, the quantitative relationship between dehalogenation efficiency and energy consumption shows an exponential increase of energy consumption for dehalogenation efficiency increasing from 90% to 100%. Lastly, challenges and perspectives are discussed for efficient dehalogenation and practical applications.

Atomic Hydrogen in Electrocatalytic Systems: Generation, Identification, and Environmental Applications

Electrochemical reduction has emerged as a viable technology for the removal of a variety of organic contaminants from water. Atomic hydrogen (H*) is the primary species generated in electrochemical reduction processes. In this work, identification and quantification for H* are reviewed with a focus on methods used to generate H* at different positions. Additionally, we present recently developed proposals for the surface chemistry mechanisms of H* on the most commonly used cathodes as well as the use of H* in standard electrochemical reactors. The proposed reaction pathways in different H* systems for environmental applications are also discussed in detail. As shown in this review, the key hurdles facing H* reduction technologies are related to i) the establishment of systematic and practical synthetic methods; ii) the development of effective identification approaches with high specificity; and, iii) an in-depth exploration of the H* reaction mechanism to better understand the reaction process of H*.

2,4-Dichlorophenol removal from water using an electrochemical method improved by a composite molecularly imprinted membrane/bipolar membrane

Low efficiency is often a problem in electrochemical reductive hydrodechlorination (ERHD) to remove chlorinated compounds such as 2,4-dichlorophenol (24DCP) from water. In this study, a composite molecularly imprinted membrane (MIM)/bipolar membrane (BPM) was introduced onto a palladium-coated titanium mesh electrode (BPM/MIM@Pd/Ti) to increase the concentration of 24DCP on the surface of electrode and ERHD efficiency. The efficiency of ERHD of 24DCP increased from 70 to 88% by introduction of the two membranes, from 71 to 89% by increasing current density from 5.0 to 30 mA/cm², and from 80 to 94% by increasing the electrolyte concentration from 0.25 to 1.00 mol/L. Treatment with Fenton's reagent after ERHD achieved 100% 24DCP removal, with chemical oxygen demand and total organic carbon reductions of 91 and 87%, respectively. Notably, these reductions were greater than obtained from the direct oxidation of the 24DCP solution by Fenton's reagent alone (i.e., 98, 84, and 72%, respectively). No products were detected in solution by GC-MS after treatment with the proposed combination technology. The mechanism of 24DCP removal and degradation involved adsorption, electrochemical hydrodechlorination via H_ads, and Fenton oxidation. Results show the process has high potential for removing 24DCP from aqueous solution.

Complete dechlorination of 2,4-dichlorophenol in aqueous solution on palladium/polymeric pyrrole-cetyl trimethyl ammonium bromide/foam-nickel composite electrode

The electrochemically reductive dechlorination of 2,4-dichlorophenol (2,4-DCP) in aqueous solution on palladium/polymeric pyrrole-cetyl trimethyl ammonium bromide/foam-nickel electrode (Pd/PPy-CTAB/foam-Ni electrode) was investigated in this paper. Pd/PPy-CTAB/foam-Ni electrode was prepared and characterized by cyclic voltammetry (CV), scanning electron microscope (SEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) adsorption and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The influences of some experimental factors such as the dechlorination current, dechlorination time and the initial pH on the removal efficiency and the current efficiency of 2,4-DCP dechlorination on Pd/PPy-CTAB/foam-Ni electrode were studied. Complete removal of 2,4-DCP was achieved and the current efficiency of 47.4% could be obtained under the conditions of the initial pH of 2.2, the dechlorination current of 5 mA and the dechlorination time of 50 min when the initial 2,4-DCP concentration was 100 mg L(-1). The analysis of high performance liquid chromatography (HPLC) identified that the intermediate products were 2-chlorophenol (2-CP) and 4-chlorophenol (4-CP). The final products were mainly phenol. Its further reduction product cyclohexanone was also detected. The electrocatalytic dechlorination pathways of 2,4-DCP on Pd/PPy-CTAB/foam-Ni electrode were discussed. The stability of the electrode was favorable that it could keep dechlorination efficiency at 100% after having been reused 10 times. Results revealed that the stable prepared Pd/PPy-CTAB/foam-Ni electrode presented a good application prospect in dechlorination process with high effectiveness and low cost.