Immobilized palladium-catalyzed electro-Fenton's degradation of chlorobenzene in groundwater.

This study investigates the effect of palladium (Pd) form on the electrochemical degradation of chlorobenzene in groundwater by palladium-catalyzed electro-Fenton (EF) reaction. In batch and flow-through column reactors, EF was initiated via in-situ electrochemical formation of hydrogen peroxide (H2O2) supported by Pd on alumina powder or by palladized polyacrylic acid (PAA) in a polyvinylidene fluoride (PVDF) membrane (Pd-PVDF/PAA). In a mixed batch reactor containing 10 mg L-1 Fe2+, 2 g L-1 of catalyst in powder form (1% Pd, 20 mg L-1 of Pd) and an initial pH of 3, chlorobenzene was degraded under 120 mA current following a first-order decay rate showing 96% removal within 60 min. Under the same conditions, a rotating Pd-PVDF/PAA disk produced 88% of chlorobenzene degradation. In the column experiment with automatic pH adjustment, 71% of chlorobenzene was removed within 120 min with 10 mg L-1 Fe2+, and 2 g L-1 catalyst in pellet form (0.5% Pd, 10 mg L-1 of Pd) under 60 mA. The EF reaction can be achieved under flow, without external pH adjustment and H2O2 addition, and can be applied for in-situ groundwater treatment. Furthermore, the rotating PVDF-PAA membrane with immobilized Pd-catalyst showed an effective and low maintenance option for employing Pd catalyst for water treatment.

Efficient H2O2 electrogeneration at graphite felt modified via electrode polarity reversal: Utilization for organic pollutants degradation.

Electrochemical synthesis of H2O2 offers a great potential for water treatment. However, a significant challenge is the development of efficient cathode materials for the process. Herein, we implement a practical electrochemical cathode modification to support efficient H2O2 electrogeneration via the reduction of dissolved anodic O2. Graphite felt (GF) is in situ anodically modified by electrode polarity reversal technique in an acid-free, low-conductivity electrolyte. The modified GF exhibits a significantly higher activity towards O2 reduction. Up to 183.3% higher H2O2 yield is obtained by the anodized GF due to the increased concentrations of oxygen-containing groups and the hydrophilicity of the surface, which facilitates electron and mass transfer between GF and the electrolyte. Another significant finding is the ability to produce H2O2 at a high yield under neutral pH and low current intensity by the modified GF (35% of the charge need to produce the same amount by unmodified GF). Long-term stability testing of the modified GF showed a decay in the electrode's activity for H2O2 production after 30 consecutive applications. However, the electrode regained its optimal activity for H2O2 production after a secondary modification by electrode polarity reversal. Finally, in situ electrochemically modified GF is more effective for removal of reactive blue 19 (RB19, 20 mg/L) and ibuprofen (IBP, 10 mg/L) by the electro-Fenton process. The modified GF removed 62.7% of RB19 compared to only 28.1% by the unmodified GF in batch reactors after 50 min. Similarly, 75.3% IBP is removed by the modified GF compared to 57.6% by the unmodified GF in a flow-through reactor after 100 min.

Degradation of 4-Chlorophenol in Aqueous Solution by Sono-Electro-Fenton Process.

Electro-Fenton (EF) and ultrasound radiation (US) have been of interest for the removal of chlorinated compounds from water. This study evaluates the effects of different parameters on sono-electro-Fenton (SEF) for degradation of 4-chlorophenol (4-CP) in an aqueous solution. This study uses pulsing US waves along with Pd-catalyzed EF to degrade contaminants in water while maintaining temperature. The usage of pulsing US waves along with Pd catalyzed EF to remove contaminants while maintaining temperature has not been reported previously. SEF ability to degrade 4-CP was compared with the performance of each process (EF and sonolysis) alone. Initial pH, current density, background electrolyte, Fe2+ concentration, Pd/Al2O3 catalyst concentration, US waves, and sonifier amplitude were optimized in a two electrode (Ti/mixed metal oxide or Ti/MMO) batch system. The degradation of 4-CP increased from 1.85% by US to 83% by EF to nearly >99.9% by coupled SEF. With US radiation under 70% amplitude and 1:10 ON/OFF ratio, the removal rate of 4-CP increased to 98% compared to 62% under EF alone within the first 120 min in the presence of 80 mg L-1 Fe2+, 16.94 mA cm-2 of current density, 1 g L-1 Pd/Al2O3 catalyst (10 mg Pd), and initial pH of 3. However, the degradation rate decreased after 120 min of treatment, and complete 4-CP removal was observed after 300 minutes. The sonolysis impacted the 4-CP removal under coupled SEF, mostly due to the contribution of mass transfer (micromixing), while radical formation was found to be absent under the conditions tested (20kHz). The pulsed US was found to increase the temperature by only 8.7°C, which was found not to impact the 4-CP volatilization or degradation. These results imply that low-level US frequency through pulses is a practical and efficient approach to support electro-Fenton reaction, improving reaction rates without the need for electrolyte cooling.

Review-Physicochemical hydrodynamics of gas bubbles in two phase electrochemical systems.

Electrochemical systems suffer from poor management of evolving gas bubbles. Improved understanding of bubbles behavior helps to reduce overpotential, save energy and enhance the mass transfer during chemical reactions. This work investigates and reviews the gas bubbles hydrodynamics, behavior, and management in electrochemical cells. Although the rate of bubble growth over the electrode surface is well understood, there is no reliable prediction of bubbles break-off diameter from the electrode surface because of the complexity of bubbles motion near the electrode surface. Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) are the most common experimental techniques to measure bubble dynamics. Although the PIV is faster than LDA, both techniques are considered expensive and time-consuming. This encourages adapting Computational Fluid Dynamics (CFD) methods as an alternative to study bubbles behavior. However, further development of CFD methods is required to include coalescence and break-up of bubbles for better understanding and accuracy. The disadvantages of CFD methods can be overcome by using hybrid methods. The behavior of bubbles in electrochemical systems is still a complex challenging topic which requires a better understanding of the gas bubbles hydrodynamics and their interactions with the electrode surface and bulk liquid, as well as between the bubbles itself.

Electrochemical degradation of trichloroethylene in aqueous solution by bipolar graphite electrodes.

In this study, we tested the use of the bipolar electrodes to enhance electrochemical degradation of trichloroethylene (TCE) in an undivided, flow-through electrochemical reactor. The bipolar electrode forms when an electrically conductive material polarizes between feeder electrodes that are connected to a direct current source and, therefore, creates an additional anode/cathode pair in the system. We hypothesize that bipolar electrodes will generate additional oxidation/reduction zones to enhance TCE degradation. The graphite cathode followed by graphite anode sequence were operated without a bipolar electrode as well as with one and two bipolar graphite electrodes. The system without bipolar electrodes degraded 29% of TCE while the system with one and two bipolar electrodes degraded 38% and 66% of TCE, respectively. It was found that the removal mechanism for TCE in bipolar mode includes hydrodechlorination at the feeder cathode, and oxidation through reaction with peroxide. The results show that the bipolar electrodes presence enhance TCE removal efficiency and rate and imply that they can be used to improve electrochemical treatment of contaminated groundwater.

Influence of humic substances on electrochemical degradation of trichloroethylene in limestone aquifers.

In this study we investigate the influence of humic substances (HS) on electrochemical transformation of trichloroethylene (TCE) in groundwater from limestone aquifers. A laboratory flow-through column with an electrochemical reactor that consists of a palladized iron foam cathode followed by a MMO anode was used to induce TCE electro-reduction in groundwater. Up to 82.9% TCE removal was achieved in the absence of HS. Presence of 1, 2, 5, and 10 mgTOC L-1 reduced TCE removal to 70.9%, 61.4%, 51.8% and 19.5%, respectively. The inverse correlation between HS content and TCE removal was linear. Total organic carbon (TOC), dissolved organic carbon (DOC) and absorption properties (A=254 nm, 365 nm and 436 nm) normalized to DOC, were monitored during treatment to understand the behavior and impacts of HS under electrochemical processes. Changes in all parameters occurred mainly after contact with the cathode, which implies that the HS are reacting either directly with electrons from the cathode or with H2 formed at the cathode surface. Since hydrodechlorination is the primary TCE reduction mechanism in this setup, reactions of the HS with the cathode limit transformation of TCE. The presence of limestone gravel reduced the impact of HS on TCE removal. The study concludes that presence of humic substances adversely affects TCE removal from contaminated groundwater by electrochemical reduction using palladized cathodes.