O-doped Graphitic Granular Biochar Enables Pollutants Removal via Simultaneous H2O2 Generation and Activation in Neutral Fe-free Electro-Fenton Process.

H2O2 generation by 2-electron oxygen electroreduction reaction (2eORR) has attracted great attention as an alternative to the industry-dominant anthraquinone process. Electro-Fenton (EF) process, which relies on the H2O2 electrogeneration, is regarded as an important environmental application of H2O2 generation by 2eORR. However, its application is hindered by the relatively expensive electrode materials. Proposing cathode materials with low cost and facile synthetic procedures are the priority to advance the EF process. In this work, a composite cathode structure that uses graphitic granular bamboo-based biochar (GB) and stainless steel (SS) mesh (GBSS) is proposed, where SS mesh functions as current distributor and GB supports synergistic H2O2 electrogeneration and activation. The graphitic carbon makes GB conductive and the oxygen-containing groups serve as active sites for H2O2 production. 11.3 mg/L H2O2 was produced from 2.0 g GB at 50 mA after 50 min under neutral pH without external O2/air supply. The O-doped biochar further increased the H2O2 yield to 18.3 mg/L under same conditions. The GBSS electrode is also effective for H2O2 activation to generate ·OH, especially under neutral pH. Ultimately, a neutral Fe-free EF process enabled by GBSS cathode is effective for removal of various model organic pollutants (reactive blue 19, orange II, 4-nitrophenol) within 120 min, and for their partial mineralization (48.4% to 63.5%). Long-term stability of the GBSS electrode for H2O2 electrogeneration, H2O2 activation, and pollutants degradation were also examined and analyzed. This work offers a promising application for biomass waste for removals of organic pollutants in neutral Fe-free EF process.

"Self-cleaning" electrochemical regeneration of dye-loaded activated carbon.

A low maintenance, "self-cleaning" electrochemical approach is evaluated for regeneration of dye-loaded granular activated carbon (GAC). To do so, batch experiments were conducted using a low-cost granular activated carbon/stainless steel mesh (GACSS) composite cathode and a stable Ti/mixed metal oxides (Ti/MMO) anode without the addition of oxidants or iron catalysts. The GACSS cathode supports simultaneous H2O2 electrogeneration via the in situ supplied O2 from Ti/MMO anode and the subsequent H2O2 activation for ·OH generation, thus enabling the cracking of dye molecules adsorbed on GAC and regenerating the GAC's sorption capacity. Results show that a prolonged electrochemical processing for 12h will achieve up to 88.7% regeneration efficiency (RE). While RE decreases with multi-cycle application, up to 52.3% could still be achieved after 10 adsorption-regeneration cycles. To identify the mechanism, experiments were conducted to measure H2O2 electrogeneration, H2O2 activation, and ·OH generation by various GAC samples. The dye-loaded GAC and GAC treated after 10 adsorption-regeneration cycles were still capable of ·OH generation, which contributes to effective "self-cleaning" and regeneration over multi-cycles.

Activated carbon as effective cathode material in iron-free Electro-Fenton process: Integrated H2O2 electrogeneration, activation, and pollutants adsorption.

Major challenges for effective implementation of the Electro-Fenton (EF) water treatment process are that conventional efficient cathodes are relatively expensive, and H2O2 activation by Fe2+ may cause secondary pollution. Herein, we propose a low-cost activated carbon/stainless steel mesh (ACSS) composite cathode, where the SS mesh distributes the current and the AC simultaneously supports H2O2 electrogeneration, H2O2 activation, and organic compounds (OCs) adsorption. The oxygen-containing groups on the AC function as oxygen reduction reaction (ORR) sites for H2O2 electrogeneration; while the porous configuration supply sufficient reactive surface area for ORR. 8.9 mg/L H2O2 was obtained with 1.5 g AC at 100 mA under neutral pH without external O2 supply. The ACSS electrode is also effective for H2O2 activation to generate ‧OH, especially under neutral pH. Adsorption shows limited influence on both H2O2 electrogeneration and activation. The iron-free EF process enabled by the ACSS cathode is effective for reactive blue 19 (RB19) degradation. 61.5% RB19 was removed after 90 min and 74.3% TOC was removed after 720 min. Moreover, long-term stability test proved its relatively stable performance. Thus, the ACSS electrode configuration is promising for practical and cost-effective EF process for transformation of OCs in water.

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.

Assessment of groundwater quality and remediation in karst aquifers: A review.

Karst aquifers, capable of storing and transmitting large amount of water, are the main source of drinking water in many regions worldwide. Their excessive permeability leads to an enhanced vulnerability to retain and spread the contamination accordingly. From sustainability perspective, the environmental, economic and social impacts of karst contamination on water resources management are gaining more attention. In this study, an overview of hydrogeological processes and concepts regarding groundwater flow and contaminant transport in karstic systems is presented, followed by a short discussion on surface water and groundwater interaction. Due to the complexity of karstic systems, different approaches have been developed by researchers for investigating and understanding hydrogeological processes and groundwater behavior in karst which are reviewed herein. Additionally, groundwater contamination issues and the most common and effective remediation techniques in karstic terrains are discussed. Lastly, modeling techniques and remote sensing methods, as beneficial and powerful tools for assessing groundwater flow and contaminant transport in karst terrains, are reviewed and evaluated. In each section, relevant research works conducted for Puerto Rico are discussed and some recommendations are presented to complement the ongoing hydrogeological investigations on this island.

"Floating" cathode for efficient H2O2 electrogeneration applied to degradation of ibuprofen as a model pollutant.

The performance of the Electro-Fenton (EF) process for contaminant degradation depends on the rate of H2O2 production at the cathode via 2-electron dissolved O2 reduction. However, the low solubility of O2 (≈1×10-3 mol dm-3) limits H2O2 production. Herein, a novel and practical strategy that enables the synergistic utilization of O2 from the bulk electrolyte and ambient air for efficient H2O2 production is proposed. Compared with a conventional "submerged" cathode, the H2O2 concentration obtained using the "floating" cathode is 4.3 and 1.5 times higher using porous graphite felt (GF) and reticulated vitreous carbon (RVC) foam electrodes, respectively. This surprising enhancement results from the formation of a three-phase interface inside the porous cathode, where the O2 from ambient air is also utilized for H2O2 production. The contribution of O2 from ambient air varies depending on the cathode material and is calculated to be 76.7% for the GF cathode and 35.6% for the RVC foam cathode. The effects of pH, current, and mixing on H2O2 production are evaluated. Finally, the EF process enhanced by the "floating" cathode degraded 78.3% of the anti-inflammatory drug ibuprofen after 120 min compared to only 25.4% using a conventional "submerged" electrode, without any increase in the cost.

Electrolytic control of hydrogen peroxide release from calcium peroxide in aqueous solution.

The in situ generation of hydrogen peroxide (H2O2) for water treatment is more practical than the use of liquid H2O2, which is costly to store and transport. Calcium peroxide (CaO2), a solid carrier of H2O2, can release H2O2 on dissolution in water. However, the constant H2O2 release rate of CaO2 has been a bottleneck constraining its wider application. In this study, a practical electrochemical method using a divided cell is developed to control the rate of release of H2O2 from CaO2. The results show that the rate of H2O2 release from CaO2 is enhanced in the anolyte. The increase in H2O2 release is positively correlated with the current. Under a current of 100 mA, the H2O2 concentration was 2.5 times higher after 30 min of electrolysis than in the control experiment in which no current was applied. Water electrolysis in the anodic compartment generates protons that not only: (i) en-hance dissolution of CaO2 and release of H2O2, but also (ii) neutralize the alkaline pH resulting from CaO2 dissolution, thus providing new advantages for the use of CaO2. This effective technique may be suitable for the sophisticated control of H2O2 release in environmental applications.

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.

Transient reactive transport model for physico-chemical transformation by electrochemical reactive barriers.

A comprehensive model that integrates coupled effects of chemical, physical, and electrochemical processes, is necessary for design, analysis, and implementation of the electro-remediation of groundwater under flow conditions. A coupled system of equations to solve for transport and multiple reactions in an electrochemical reactor is numerically intensive due to highly stiff nature of reaction model formulation. In this study, the focus is to develop an efficient model for reactions associated with the transport and physico-chemical transformation in an electrochemical reactor. The model incorporates effects of transport mechanisms as well as chemical and electrochemical reactions. Model verification is provided for pH profiles under different electrolyte compositions in two sets of reactors; a batch and a flow-through reactor. The model is able to predict the concentration of species during the electrochemical remediation process with a close correlation to experimental data (R2 = 0.99 for batch and R2 = 0.78 for flow-through reactor.) Imposing polarity reversal to the system will cause fluctuation of pH, however, the trend stays the same as if no polarity were applied. Ultimately, volumetric charge flow is introduced as a unique parameter characterizing the electroremediation reactor for operating purposes.

BULK CHEMISTRY OF KARST SEDIMENT DEPOSITS.

Sediments are ubiquitous in karst systems and play a critical role in the fate and transport of contaminants. Sorbed contaminants may be stored on immobile sediments or rapidly dispersed on mobile sediments. Sediments may also influence remediation by either enhancing or interfering with the process. To better understand the potential effects of sediments on remediation, we conducted physical and chemical characterizations of 11 sediment samples from 7 cave and spring deposits from karst regions of Tennessee, Virginia, and West Virginia. The samples were analyzed for particle-size distribution using sieves and laser diffraction particle analysis. The sediment size fraction <2 mm (sand, silt, and clay) was analyzed for slurry pH and specific conductivity (SC) using electrodes and for bulk total carbon, organic carbon, nitrogen and sulfur on an ElementarTM Vario MAX Cube CNS. The same <2 mm fraction was subjected to a pseudo-total extraction using aqua regia with subsequent solution analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES). Most of the samples were dominated by the <2 mm size fraction. Their slurry pHs ranged from 6.8 to 8.4 and their SCs ranged from 45 to 206 µS/cm with the exception of two high SC samples (726 and 8500 µS/cm). The fraction of organic carbon (Foc) in the sediments ranged from <0.1 to 2%. The sample from a saltpeter cave historically used for gunpowder production contained the highest concentrations of N and S (~3 g/kg) but lower total C than some of the spring samples. The pseudo-total extractions were analyzed for Al, Ca, Fe, Mg, and Mn. Of those elements, Mg was the most consistent across the locations (2.0-6.1 g/kg), and Ca was the most variable (1.4-52 g/kg). Given the importance of particle size and elemental concentrations in chemical reactions and remediation, more data of this type are needed to predict contaminant fate and transport and to plan successful remediation projects.

LABORATORY TESTING OF THE POTENTIAL FOR THE INFLUENCE OF SUSPENDED SEDIMENTS ON THE ELECTROCHEMICAL REMEDIATION OF KARST GROUNDWATER.

Due to the complicated nature of karst aquifers, many groundwater treatment technologies are difficult to implement successfully. A particular challenge arises because sediments are ubiquitous and mobile in karst systems and may either facilitate contaminant transport or act as long-term substrates for storage via sorption. However, electrochemical remediation is a promising technology to be optimized for karst aquifers due to easy manipulation and control of groundwater chemistry as well as low cost, ability for in situ application, and performance under alternative power sources. This study investigates the effects of suspended karst sediments on the electrochemical remediation of groundwater via electro-Fenton (EF) mechanism. The EF mechanism relies on direct electrolysis (i.e., water electrolysis and ferrous iron release) and indirect, electrochemically-induced processes (i.e., Pd catalyzed H2O2 production). These processes can be optimized for H2O2 generation and support of its activation to hydroxyl radicals - a powerful oxidant capable of degrading and transforming a wide range of contaminants (e.g., chlorinated solvents). In this study, we tested sediments varying in concentrations of Fe, Mn and buffering capacities. When the sediments were introduced into the EF experiments, there were adverse effects on the H2O2 content: at steady state (120 min), Pd catalyzed formation of H2O2 decreased by 60%, 57%, and 75% in the presence of suspended sediment collected from three separate karst locations. Presented results imply that sediments' presence influences EF mechanism in electrochemical systems, but given the flexibility of the technology, it can be optimized in terms of electrode materials, current intensities and current regimes to address these challenges.

Rates of H2O2 Electrogeneration by Reduction of Anodic O2 at RVC Foam Cathodes in Batch and Flow-through Cells.

The Electro-Fenton process for in-situ H2O2 electrogeneration is impacted by low O2 utilization efficiency (<0.1%) and the need of acid for pH adjustment. An electrochemical flow-through cell can develop localized acidic conditions, coupled with simultaneous formation and utilization of O2 to enhance H2O2 formation. Multiple electrode configurations using reticulated vitreous carbon (RVC) foam and Ti/mixed metal oxides (MMO) are proposed to identify the optimum conditions for H2O2 formation in batch and flow-through cells. A pH of 2.75±0.25 is developed locally in the flow-through cell that supports effective O2 reduction. Up to 9.66 mg/L H2O2 is generated in a 180 mL batch cell under 100 mA, at pH 2, and mixing at 350 rpm. In flow-through conditions, both flow rate and current significantly influence H2O2 production. A current of 120 mA produced 2.27 mg/L H2O2 under a flow rate of 3 mL/min in a 3-electrode cell with one RVC foam cathode at 60 min. The low current of 60 mA does not enable effective H2O2 production, while the high current of 250 mA produced less H2O2 due to parasitic reactions competing with O2 reduction. Higher flow rates decrease the retention time, but also increase the O2 mass transfer. Furthermore, 3-electrode flow-through cell with two RVC foam cathodes was not effective for H2O2 production due to the limited O2 supply for the secondary cathode. Finally, a coupled process that uses both O2 and H2 from water electrolysis is proposed to improve the H2O2 yield further.

Drastic Enhancement of H2O2 Electro-generation by Pulsed Current for Ibuprofen Degradation: Strategy Based on Decoupling Study on H2O2 Decomposition Pathways.

Efficient H2O2 electrogeneration from 2-electron oxygen reduction reaction (ORR) represents an important challenge for environmental remediation application. H2O2 production is determined by 2-electron ORR as well as H2O2 decomposition. In this work, a novel strategy based on the systematical investigation on H2O2 decomposition pathways was reported, presenting a drastically improved bulk H2O2 concentration. Results showed that bulk phase disproportion, cathodic reduction, and anodic oxidation all contributed to H2O2 depletion. To decrease the extent of H2O2 cathodic reduction, the pulsed current was applied and proved to be highly effective to lower the extent of H2O2 electroreduction. A systematic study of various pulsed current parameters showed that H2O2 concentration was significantly enhanced by 61.6% under pulsed current of "2s ON + 2s OFF" than constant current. A mechanism was proposed that under pulsed current, less H2O2 molecules were electroreduced when they diffused from the porous cathode to the bulk electrolyte. Further results demonstrated that a proper pulse frequency was necessary to achieve a higher H2O2 production. Finally, this strategy was applied to Electro-Fenton (EF) process with ibuprofen as model pollutant. 75.0% and 34.1% ibuprofen were removed under pulsed and constant current at 10 min, respectively. The result was in consistent with the higher H2O2 and ·OH production in EF under pulsed current. This work poses a potential approach to drastically enhance H2O2 production for improved EF performance on organic pollutants degradation without making any changes to the system except for power mode.

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 dechlorination of trichloroethylene in the presence of natural organic matter, metal ions and nitrates in a simulated karst media.

A small-scale flow-through limestone column was used to evaluate the effect of common coexisting organic and inorganic compounds on the electrochemical dechlorination of trichloroethylene (TCE) in karst media. Iron anode was used to produce ferrous ions and promote reducing conditions in the column. The reduction of TCE under 90 mA current, 1 mL min-1 flow rate, and 1 mg L-1 initial TCE concentration, was inhibited in the presence of humic acids due to competition for direct electron transfer and/or reaction with atomic hydrogen produced at the cathode surface by water electrolysis. Similarly, presence of 10 mg L-1 chromate decreased TCE reduction rate to 53%. The hexavalent chromium was completely reduced to trivalent chromium due to the ferrous species produced from iron anode. Presence of 5 mg L-1 selenate decreased the removal of TCE by 10%. Chromium and selenate complexation with dissolved iron results in formation of aggregates, which cover the electrodes surface and reduce TCE dechlorination rate. Presence of 40 mg L-1 nitrates caused reductive transformation of TCE up to 80%. Therefore, TCE removal is influenced by the presence of other contaminants that are present as a mixture in groundwater in the following order: humic acid, chromate, selenate, and nitrate.

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.

The influence of cathode material on electrochemical degradation of trichloroethylene in aqueous solution.

In this study, different cathode materials were evaluated for electrochemical degradation of aqueous phase trichloroethylene (TCE). A cathode followed by an anode electrode sequence was used to support reduction of TCE at the cathode via hydrodechlorination (HDC). The performance of iron (Fe), copper (Cu), nickel (Ni), aluminum (Al) and carbon (C) foam cathodes was evaluated. We tested commercially available foam materials, which provide large electrode surface area and important properties for field application of the technology. Ni foam cathode produced the highest TCE removal (68.4%) due to its high electrocatalytic activity for hydrogen generation and promotion of HDC. Different performances of the cathode materials originate from differences in the bond strength between atomic hydrogen and the material. With a higher electrocatalytic activity than Ni, Pd catalyst (used as cathode coating) increased TCE removal from 43.5% to 99.8% for Fe, from 56.2% to 79.6% for Cu, from 68.4% to 78.4% for Ni, from 42.0% to 63.6% for Al and from 64.9% to 86.2% for C cathode. The performance of the palladized Fe foam cathode was tested for degradation of TCE in the presence of nitrates, as another commonly found groundwater species. TCE removal decreased from 99% to 41.2% in presence of 100 mg L(-1) of nitrates due to the competition with TCE for HDC at the cathode. The results indicate that the cathode material affects TCE removal rate while the Pd catalyst significantly enhances cathode activity to degrade TCE via HDC.

Hydrodechlorination of TCE in a circulated electrolytic column at high flow rate.

Palladium-catalytic hydrodechlorination of trichloroethylene (TCE) by cathodic H2 produced from water electrolysis has been tested. For a field in-well application, the flow rate is generally high. In this study, the performance of Pd-catalytic hydrodechlorination of TCE using cathodic H2 is evaluated under high flow rate (1 L min(-1)) in a circulated column system, as expected to occur in practice. An iron anode supports reduction conditions and it is used to enhance TCE hydrodechlorination. However, the precipitation occurs and high flow rate was evaluated to minimize its adverse effects on the process (electrode coverage, clogging, etc.). Under the conditions of 1 L min(-1) flow, 500 mA current, and 5 mg L(-1) initial TCE concentration, removal efficacy using iron anodes (96%) is significantly higher than by mixed metal oxide (MMO) anodes (66%). Two types of cathodes (MMO and copper foam) in the presence of Pd/Al2O3 catalyst under various currents (250, 125, and 62 mA) were used to evaluate the effect of cathode materials on TCE removal efficacy. The similar removal efficiencies were achieved for both cathodes, but more precipitation generated with copper foam cathode (based on the experiments done by authors). In addition to the well-known parameters such as current density, electrode materials, and initial TCE concentration, the high velocities of groundwater flow can have important implications, practically in relation to the flush out of precipitates. For potential field application, a cost-effective and sustainable in situ electrochemical process using a solar panel as power supply is being evaluated.

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.