Effects of Reduced Sulfur Compounds on Pd-Catalytic Hydrodechlorination of Trichloroethylene in Groundwater by Cathodic H₂under Electrochemically Induced Oxidizing Conditions

Abstract

Reduced sulfur compounds (RSCs) poison Pd catalysts for catalytic hydrodechlorination of contaminants in anoxic groundwater. This study investigates the effects of RSCs on Pd-catalytic hydrodechlorination of trichloroethylene (TCE) in oxic groundwater. Water electrolysis in an undivided electrolytic cell is used to produce H2 for TCE hydrodechlorination under oxidizing conditions. TCE is efficiently hydrodechlorinated to ethane, with significant accumulation of H2O2 under acidic conditions. Presence of sulfide at concentrations less than 93.8 μM moderately inhibits TCE hydrodechlorination and H2O2 production. Presence of sulfite at low concentrations (≤ 1 mM) significantly enhances TCE decay, while at high concentration (3 mM) inhibits initially and enhances afterwards when sulfite concentration declines to less than 1 mM. Using radical scavenging experiments and electron spin resonance assay, SO3•− which is generated from sulfite under oxidizing conditions is validated as the new reactive species contributing to the enhancement. This study reveals a distinct mechanism of effect of sulfite on TCE hydrodechlorination by Pd and H2 in oxic groundwater and presents an alternative approach to increasing resistance of Pd to RSCs poisoning.

Effect of operating parameters on trichloroethylene degradation by extended release of nanoscale zero-valent iron

“…Moreover, chloride ions are a major byproduct of TCE degradation by PV3A-nZVI. According to previous studies [42,43] and our findings, the proposed pathways for the reductive dechlorination of TCE are displayed in Fig. 10.…”

“…Only a few reports have evaluated the degradation products of TCE using the photochemical method [42] and a Pd-catalytic approach [43]. Jung et al [42] investigated the photochemical degradation of TCE through sulfite-mediated UV irradiation and found that degradation byproducts included 1,1-dichloroethylene (1,1-DCE), cis-DCE, trans-DCE, vinyl chloride (VC), chloroacetylene, ethylene, and ethane.…”

“…Jung et al [42] investigated the photochemical degradation of TCE through sulfite-mediated UV irradiation and found that degradation byproducts included 1,1-dichloroethylene (1,1-DCE), cis-DCE, trans-DCE, vinyl chloride (VC), chloroacetylene, ethylene, and ethane. Yuan et al [43] indicated that the most abundant byproduct is ethane. In this study, when TCE and PV3A-nZVI concentrations were both 10 mg/L, and the solution pH 0 was 6.8, the degradation products of TCE were analyzed using GC-MS (Fig.…”

Pd-catalytic hydrodechlorination of chlorinated hydrocarbons in groundwater using H2 produced by a dual-anode system

“…Besides, addition of FeSO 4 to a TCE solution resulted in a negligible TCE removal . Direct oxidation of TCE on the MMO anode was also not favored because of the low overpotential for O 2 evolution (Scialdone et al, 2008), as evidenced in our previous investigation (Yuan et al, 2013a). In contrast, it has been reported that TCE can be reduced by the MMO cathode, showing dependence on the applied potential (Petersen et al, 2007).…”

“…Without current applied to the iron anode (20/0), TCE was only reduced by 25.3% within 4 h; when 5 mA current was applied to the iron anode (15/5), the removal increased to 66.0%; and when the applied current further increased, the removal efficiency stabilized at about 79.5%. Previous studies showed that TCE degradation shifted from reductive dechlorination in the absence of Fe(II) to OH oxidation in the presence of Fe(II) under acidic conditions, but OH oxidation was negligible at the pH above 6 because the generation of H 2 O 2 was limited and most Fe(II) was precipitated (Liao et al, 2013;Yuan et al, 2013aYuan et al, , 2012. Therefore, hydrodechlorination mechanism predominated for TCE degradation under the neutral circumstance in this work.…”