Electrochemical Generation Of H₂ For Catalytic Hydrodechlorination Of Chlorinated Organic Compounds In Groundwater

Chlorinated organic compounds (COCs) have aroused intensive concern due to their high refractory nature, bio-accumulation and toxicity. The toxicity of COCs is strongly related to the extent of chlorination. Generally, more chlorination is associated with higher toxicity and difficult oxidative degradation. The release of chlorine leads to the decrease in toxicity and the increase in oxidative degradability. Therefore, reductive dechlorination is considered as an effective method for the remediation of COCs contaminated groundwater. Zero-valent iron (ZVI) and catalytic hydrodechlorination are the two main processes for COCs dechlorination in groundwater. However, the continuous corrosion of iron during decades of field running makes the surface of iron covered with iron hydrates and oxides, which results in the decrease in long-term performance. As to catalytic hydrodechlorination, the practical application of this process is inconvenient because the in situ supply of H2 in the subsurface is difficult and dangerous. In present study, a novel electrolytic system, which is based on sequential catalytic hydrodechlorination and anodic oxidation, was developed to reach in-situ remediation of COCs contaminated groundwater. The Pd/C catalyst was firstly synthesized, and then the catalytic hydrodechlorination using external H2 was carried out, which was used to investigate the influence of the factors which are involved in groundwater remediation by electrochemical process, on the hydrodechlorination of COCs. Secondly, in a divided electrolytic system, the catalytic hydrodechlorination of COCs in cathode compartment by H2 generated at cathode was conducted to verify the feasibility of hydrodechlorination by the H2 generated at cathode and test the influence of current intensity. Finally, a dynamic column system, in which groundwater flew through cathode, Pd/BC catalyst and anode in turn, was employed to evaluate the feasibility of in-situ sequential catalytic hydrodechlorination of 2,4-DCP by the H2 generated at cathode and direct anodic oxidation of Phenol produced. Main conclusions are summarized as follows:(1) Pd was successfully loaded on the bamboo charcoal. The Pd content was measured as 10.11%(w/w), and the Pd particles of FCC structure were evenly scattered on BC surface and the particle size was about 20 nm. The dominant component of the Pd3d binding energy is confirmed as Pd(0). After long-term reaction, the Pd particles on BC was gathered, and the size was slightly increased from 20 nm to 23 nm. The weight fraction of Pd3d decreased from 10.96% to only 3.69%.(2) The catalytic hydrodechlorination using external H2 showed that within 20 min the hydrodechlorination of 2,4-DCP was 100%,95.6%, and 21.4% at the pH of 3.5,5.5 and 11 respectively. Phenol was the main products and it could be further reduced to nontoxic cyclohexanone. During the reaction progress, intermediate products of 2-chlorophenol or 4-chlorophenol emerged. The concentration Na2SO4 and humic acid also had affect on catalytic hydrodechlorination of 2,4-DCP, which lower concentration corresponded to high hydrodechlorination efficiency.The catalytic hydrodechlorination of 2,4-DCP under the Na2SO4 concentration of 10,70 and 140 mmol/L reached 84%,89% and 23% at 60 min, respectively. At the Na2SO4 concentration of 70mmol/L, the hydrodechlorination efficiency of 2,4-DCP were 49% and 39% corresponding to the humic acid concentration of 20 and 50 mg/L in the reaction aquid, respectively. The synthesized Pd/BC catalyst was efficient for 2,4-DCP hydrodechlorination and the performance can be sustained for a long time.(3) In a divided electrolytic system,2,4-DCP could be reduced by the H2 generated at cathode in the attendence of Pd/C, and higher current resulted in faster reduction. For 60 min treatment, reduction at 50 mA led to 100% removal of 2,4-DCP, while at 20 mA the time of complete dechlorination would extended to 120 min at which the hydrodechlorination efficiency of 2,4-DCP was only 41% when 10 mA. Likewise, Phenol was the main dechlorination product, and 2-CP rather than 4-CP was identified as the minor monochlorophenol intermediate. 2,4-DCP reductive dechlorination products of Phenol could be oxidized at the anode degradation, and oxidation products were benzoquinone, catechol, hydroquinone and other fatty acids.(4) In the dynamic flow-through electrolytic system, The final effluent of 2,4-DCP was decreased by 60% than influent concentration at pH of 5.5, and the degradation of Phenol was completely. When the influent pH of 2, the final effluent concentration of 2,4-DCP and Phenol were below the detection limit, which showed that 2,4-DCP was first complete hydrodechlorination and then subsequent thorough oxidation. Low pH is conducive to the removal of pollutants. In the reducing end of dynamic system, the reduction of 2,4-DCP and Phenol formation changes could detect. Dynamic system is not only verify the effective operation of electrochemical generation of H2 for catalytic hydrodechlorination of chlorinated organic compounds in groundwater, but also confirmed that the techniques combined with electrochemical and hydrodechlorination was feasibility and efficiency for the treatment of chlorinated organic compounds in groundwater.