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.

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.

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.

"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.

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 transformation of thichloroethylene in groundwater by Ni-containing cathodes.

In this study, we evaluate the use of different stainless steel (SS) materials as cost-effective cathode materials for electrochemical transformation of trichloroethylene (TCE) in contaminated groundwater. Ni, which is present in certain SS, has low hydrogen overpotential that promotes fast formation of atomic hydrogen and, therefore, its content can enhance hydrodechlorination (HDC). We a flow-through electrochemical reactor with a SS cathode followed by an anode. The performance of Ni containing foam cathodes (Fe/Ni and Ni foam) was also evaluated for electrochemical transformation of TCE in groundwater. SS type 316 (12% Ni) removed 61.7% of TCE compared to 52.6% removed by SS 304 (9.25% Ni) and 37.5% removed by SS 430 (0.75% Ni). Ni foam cathode produced the highest TCE removal rate (68.4%) compared with other cathodes. The slightly lower performance of SS type 316 mesh is balanced by the reduction in treatment costs for larger-scale systems. The results prove that Ni content in SS highly influences TCE removal rate.

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.

Degradation of anthraquinone dye reactive blue 4 in pyrite ash catalyzed Fenton reaction.

Pyrite ash (PA) is created by burning pyrite in the chemical production of sulphuric acid. The high concentration of iron oxide, mostly hematite, present in pyrite ash, gives the basis for its application as a source of catalytic iron in a modified Fenton process for anthraquinone dye reactive blue 4 (RB4) degradation. The effect of various operating variables such as catalyst and oxidant concentration, initial pH and RB4 concentration on the abatement of total organic carbon, and dye has been assessed in this study. Here we show that degradation of RB4 in the modified Fenton reaction was efficient under the following conditions: pH=2.5; [PA]0=0.2 g L(-1); [H2O2]0=5 mM and initial RB4 concentration up to 100 mg L(-1). The pyrite ash Fenton reaction can overcome limitations observed from the classic Fenton reaction, such as the early termination of the Fenton reaction. Metal (Pb, Zn, and Cu) content of the solution after the process suggests that an additional treatment step is necessary to remove the remaining metals from the water. These results provide basic knowledge to better understand the modified, heterogeneous Fenton process and apply the PA Fenton reaction for the treatment of wastewaters which contains anthraquinone dyes.

Simulation of hexavalent chromium removal by electrocoagulation using iron anode in flow-through reactor.

An electrocoagulation (EC) model is developed for hexavalent chromium reduction and precipitation, using iron electrodes. Parallel removal mechanisms such as adsorption of chromium on ferrihydrite and direct reduction at the cathode is assumed negligible due to low concentration of Cr(VI). The reaction model presented for batch system represents species complexation, precipitation/dissolution, acid/base, and oxidation-reduction reactions. Batch reactor simulation is verified using experimental data obtained by Sarahney et al. (2012), where the effect of initial chromium concentration, pH, volumetric current density, and ionic strength is considered (Sarahney et al., 2012). The model couples multicomponent ionic transport in MATLAB with chemical reaction model in PHREEQC, as a widely used computational programming tool and a geochemical reaction simulator with comprehensive geochemistry databases. The suggested current density is 0.05-0.3mA/cm2 and the surface to volume ratio in batch reactor is considered 0.017 1/cm. Design parameters are presented for operation of a flow-through hexavalent chromium removal using electrocoagulation by iron electrode to treat Cr(VI) in range of 10-50 mg/L. The operational parameters for a flow-through EC reactor for Cr(VI) removal is suggested to follow [Formula: see text] .

Electrokinetic Treatment of Cr-, Cu-, and Zn-Contaminated Sediment: Cathode Modification.

Enhanced electrokinetic (EK) removal of Cr, Cu, and Zn from sediment by using original and modified integrated ion exchange (IIX™) cathodes was investigated. IIX cathode design and EK device process modifications were made to improve performance: separation of IIX cathode components (IIXS), combination of modified IIX cathode with pulsed electric field (IIXSP), and separation of IIX cathode components with addition of an anion exchange resin compartment (IIXA). After using the IIXSP, overall Cr, Cu, and Zn removal efficacies were significantly improved compared with the other treatments investigated. No improvements in overall Cr, Cu, and Zn removal efficacies were achieved by utilization of IIXA. Nevertheless, significant removal efficacies occurred at the anode region since distribution of the alkaline front was prevented. However, metal accumulation in the cathode region occurred. This was a consequence of metal cation complexation with Cl- released from the anion exchange resin that changed the direction of metal migration. Enhancing EK remediation of Cr-, Cu-, and Zn-contaminated sediment can be achieved by using a modified IIX cathode.

Correlation of different pollution criteria in the assessment of metal sediment pollution.

In this article an assessment of the sediment metal pollution (cadmium, copper, chromium, lead, nickel, zinc) in the Veliki Backi canal (Serbia) was carried out using pseudo-total metal content, contamination factor (CF), pollution load index (PLI) and enrichment factor (EF). The study also encompassed pore-water metal concentrations and an assessment of sediment pollution based on the analysis of simultaneously extracted metals (SEM), acid volatile sulphides (AVS) and the sequential extraction procedure. The concentrations of metals are likely to result in harmful effects based on the comparison with sediment quality guidelines (Dutch, Canadian, US EPA - United States Environmental Protection Agency). The ratio of simultaneously extracted metals and volatile acid sulphides was found to be greater than 1 in only one location, which is already recognized as a place of high risk based on the criteria applied. Other samples had Σ[SEM]/[AVS] < 1, despite their high risk classification based on the applied criteria. According to the sequential extraction procedure, zinc and nickel exhibit high risk in most samples, whereas other metals show low and medium risk. The CF values for Cr, Cu and Zn were > 6 in most samples, which denotes very high contamination by these metals. The PLI values indicated moderate and high pollution. The EF values for all metals studied except for Cd in some cases were >1.5, suggesting anthropogenic impact. The obtained results will be invaluable for future activities regarding sediment monitoring and will facilitate the selection of appropriate criteria when evaluating sediment quality.

Spectroscopic monitoring of photocatalytic degradation of the insecticide acetamiprid and its degradation product 6-chloronicotinic acid on TiO2 catalyst.

Two spectroscopic methods, (1)H NMR and FTIR, were developed for the monitoring of the photocatalytic degradation of acetamiprid, a widely used pyridine-based neonicotinoid insecticide, in UV-irradiated aqueous suspensions of O(2)/TiO(2). The (1)H NMR method allowed also the identification of the intermediates such as 6-chloronicotinic and formic acids, as well as separate monitoring of the kinetics of degradation of acyclic and aromatic moieties based on the different chemical shifts of the protons belonging to the methyl group of the acyclic and selected proton of the heterocyclic aromatic moiety. The FTIR procedure enabled the monitoring of the kinetics of degradation of the cyano group of the compound. The obtained results are in good agreement with the comparative HPLC-DAD and HPLC-MS/MS measurements, which also enabled the identification of certain intermediates. To get a deeper insight into the complex photocatalytic process, the photocatalytic degradation of 6-chloronicotinic acid, a stable degradation intermediate of acetamiprid, was also investigated by (1)H NMR and HPLC-DAD methods. Based on the obtained data, a tentative reaction mechanism was proposed for the photocatalytic degradation of acetamiprid.

Correlation between the results of sequential extraction and effectiveness of immobilization treatment of lead- and cadmium-contaminated sediment.

The assessment of the quality of sediment from the Great Backi Canal (Serbia), based on the pseudo-total lead (Pb) and cadmium (Cd) content according to the corresponding Dutch standards and Canadian guidelines, showed its severe contamination with these two metals. A microwave-assisted BCR (Community Bureau of Reference of the Commission of the European Union) sequential extraction procedure was employed to assess their potential mobility and risk to the aquatic environment. Comparison of the results of sequential extraction and different criteria for sediment quality assessment has led to somewhat contradictory conclusions. Namely, while the results of sequential extraction showed that Cd comes under the high-risk category, Pb shows low risk to the environment, despite its high pseudo-total content. The contaminated sediment, irrespective of the different speciation of Pb and Cd, was subjected to the same immobilization, stabilization/solidification (S/S) treatments using kaolinite, montmorillonite, kaolinite-quicklime, montmorillonite-quicklime, fly ash, zeolite, or zeolite-fly ash combination. Semi-dynamic leaching tests were conducted for Pb- and Cd-contaminated sediment in order to assess the long-term leaching behavior of these metals. In order to simulate "worst case" leaching conditions, the semi-dynamic leaching test was modified using 0.014 M acetic acid (pH = 3.25) and humic acid solutions (20 mg TOC l-1) as leachants instead of deionized water. The effectiveness of S/S treatment was evaluated by determining diffusion coefficients (De) and leachability indices (LX). The standard toxicity characteristic leaching procedure (TCLP) was applied to evaluate the extraction potential of Pb and Cd. A diffusion-based model was used to elucidate the controlling leaching mechanisms. Generally, the test results indicated that all applied S/S treatments were effective in immobilizing Pb and Cd, and the treated sediments may be considered acceptable for "controlled utilization" based on LX values, irrespective of their different availability in the untreated samples. In the majority of samples, the controlling leaching mechanism appeared to be diffusion, which indicates that a slow leaching of Cd and Pb could be expected when the above S/S agents were applied. The TCLP results showed that all S/S samples were nonhazardous.

Behavior of zinc, nickel, copper and cadmium during the electrokinetic remediation of sediment from the Great Backa Canal (Serbia).

This paper describes a bench-scale study dealing with the removal of heavy metals by electrokinetic (EK) remediation from sediment of the Great Backa Canal (Vojvodina, Republic of Serbia), with an emphasis on the dependence of removal efficacies on the physicochemical states of the heavy metals and sediment chemistry. Sediment samples were spiked with the following heavy metals (mg kg(-1)): Zn 4400, Ni 900, Cu 1140 and Cd 57. In addition to determining the pseudo-total metal content in the contaminated sediment before and after EK treatment, BCR sequential extraction was also performed to examine the distribution of the contaminants in the sediment. Conventional EK remediation (EXP I) was ineffective in removing the heavy metals investigated, so two enhanced processes were developed. In both these processes, the mass of treated sediment was reduced to avoid the presence of inactive electric field areas in the sediment and increase current density. The first enhanced experiment (EXP II) used acetic acid (HAc) solution (pH 2.9) as an anolyte. Combined with the smaller sediment mass, this resulted in an increase in overall removal efficacies (9% for Zn, 15% for Ni, 10% for Cu and 15% for Cd). The second enhanced experiment (EXP III), as well as using HAc solution as an anolyte, made use of a cation exchange membrane in the cathodic chamber to minimize pH changes in the region adjacent to the cathode, which negatively influenced the removal of some heavy metals. However, no improvement in removal efficacy was achieved in EXP III. Since the redox potential of the sediment drops during the EK process, metals removal is limited by the formation of their sulfides. In conclusion, the removal of heavy metals by EK remediation is governed by a complex interplay of the complexation, precipitation and reduction processes, and the difficulties encountered in their optimization can explain the unsatisfactory effectiveness achieved by the described remediation procedure. Improved understanding of the behavior of metal ions during EK treatment can be useful in predicting and enhancing the efficacy of the process.

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.

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.