Electrochemically active carbon nanotube (CNT) filters have been developed as a highly efficient technology for water and wastewater treatment during the last few years. CNT filters have been widely used to adsorb chemical and biological contaminants due to their high stability, great flexibility, and large specific surface area. Electrochemically active CNT filters provide additional electrooxidation of the adsorbed contaminants and have been proven to be a highly effective treatment technology in a few recent lab-scale studies. The working principles, impacting factors, and some of the latest development of electrochemically active CNT filters are reviewed in this paper. The existing challenges and future perspectives are also discussed.
Carbon nanotube (CNT) filters have been extensively studied for water and wastewater treatment during recent years. Owing to the unique electronic, mechanical, and chemical properties, carbon nanotubes (CNTs) received extensive attention since 1990  and are useful for a variety of applications in energy studies and biomedical devices. The high surface area (30–500 m2 g-1) and electrochemical degradation (104~106 S m-1) of CNT filters result in a high adsorption capability for water and wastewater treatment [2–4]. The application of CNTs in water and wastewater treatment has been extensively reviewed previously [5, 6].
The combination of electrochemistry and CNT filters has further expanded the application of CNT filters for water and wastewater treatment [2, 7–18]. Electrochemistry has been widely applied in separation, corrosion control, electroanalytical sensors, electroplating, and fuel cells, as well as in environmental applications, such as wastewater treatment, environmental sensing, and batteries . The environmental application of electrochemistry had gained more and more popularity [7, 20] and has been extensively reviewed in previous studies [21, 22]. Previous studies have demonstrated that CNT filters were effective to remove aqueous organic pollutants, such as salts , proteins , viruses , azo dyes , pharmaceuticals , perfluorinated chemicals , and phenol . In addition, the application of electrochemistry could reduce filter fouling rate by in situ foulant destruction and biological inactivation [17, 23, 27] and reduce maintenance efforts for optimal membrane permeability . A CNT-polyvinylidene fluoride porous non-Faradaic cathode was shown to significantly reduce organic fouling in ultrafiltration (UF) membranes and has potential to reduce energy requirements by up to twofold compared to the unmodified UF system . The electrochemical applications of CNT filters are related with their good electrocatalytic properties for electrochemical reactions. For example, a CNT-based class electrode showed a very low overpotential and higher peak current values than those observed in bare glass electrodes . As a result, CNT-based electrodes are good candidates for environmental applications, such as wastewater treatment and micropollutant sensors .
Although the application of CNT filters for adsorption has been extensively studied and reviewed, a detailed review on the working principles and electrochemical processes of electrochemically active CNT filters is not yet available. The working principles and impacting factors of electrochemically active CNT filters are reviewed in this paper, and a few recently developed CNT-based electrochemical treatment techniques are discussed. The existing challenges and future perspectives are also discussed. A comprehensive review of existing literature on electrochemically active CNT filters will help advance our understanding of the advantages and limitations of electrochemically active CNT filters and help develop an efficient and cost-effective technique for water and wastewater treatment.
2 Working principles of electrochemical CNT filters for water and wastewater treatment
CNT filters are composed of tens of hundreds of individual tubes that are entangled with each other through van der Waals forces of attraction , which results in a high surface area (30–500 m2 g-1) that can be used to adsorb chemical and biological contaminants. The configuration of a representative electrochemical CNT filter is shown in Figure 1 . Vecitis et al. modified a commercial polycarbonate filtration casing and inserted stainless steel or titanium cathode and a titanium anode ring so a multiwalled nanotube (MWNT) anode can be used for electrooxidation.
The overall electrochemical filtration includes three processes: (1) hydrodynamically enhanced mass transfer, (2) temperature-dependent physical adsorption/desorption, and (3) voltage-dependent direct electron transfer . Electrochemical oxidation for organic contaminants takes much less time than that of conventional biological wastewater treatment systems . Furthermore, recalcitrant compounds that are often difficult to be treated can be electrochemically oxidized . CNT filters have been widely used for adsorption of various chemical contaminants [30, 31], but previous studies also indicated that breakthrough of CNT filters happened in few hours in fix bed columns  or <30 min in a dead end filter  after CNT active sites were occupied with adsorbed contaminants. Electrochemical CNT filters can further break down adsorbed contaminants and, therefore, significantly improve their performance in wastewater treatment. Electrochemical CNT filters have been successfully applied to treat various contaminants, and the results of CNT-based electrochemical treatment systems are summarized in Table 1. The results indicated that CNT-based electrochemical treatment systems were highly efficient to treat different chemical and biological contaminants with a typical removal efficiency >90%.
|Contaminants||Anode||Cathode||Removal efficiency||System configuration||References|
|C.I. basic yellow 2||Fe||CNT-PTFE||96%||Batch|||
|Methyl orange||Sb-doped SnO2 CNT||Stainless steel||95%||Filtration|||
|C.I. basic blue 3||Pt||CNT-PTFE||94%||Batch|||
|E. coli||CNT||Stainless steel||75%||Filtration|||
|C.I. direct red 23||Pt||CNT-PTFE||94%||Batch|||
3 Impacting factors of electrochemical CNT filters
The treatment efficiency of electrochemical CNT filters may be affected by a number of factors and the main impacting factors are discussed as follows.
3.1 Anode potential
Anode potential is one of the most important impacting factors for electrooxidation with CNT filters. Anode potential needs to be higher than the redox potential for a chemical to be oxidized. However, water splitting may also happen at a high anode potential, and there is a direct competition between organic oxidation and oxygen evolution (Eq. 1).
The potentials of oxygen evolution of different anodes, such as Pt, IrO2, PbO2, and SnO2, have been reviewed previously . Anode materials with low oxygen evolution potentials are not efficient for electrochemical treatment of pollutants because competition with oxygen evolution results in low current efficiencies. High anode potential may also result in CNT degradation  and limits the applications of CNTs for electrochemical treatment of water and wastewater. To minimize the competition and potential CNT degradation, anode potentials should not be too high, and some high oxygen-overvoltage anode materials, such as a boron-doped diamond (BDD) electrode, could be used to minimize the competition between anoxic oxidation and oxygen evolution [45–47], although these materials are generally more expensive than CNTs.
Surface coating is one technique to improve the overpotential of oxygen evolution for electrochemical applications. In a recent study, Liu et al. fabricated a binder-free, porous, and conductive 3D CNT network uniformly coated with bismuth-doped tin oxide (BTO) nanoparticles via a simple electrosorption-hydrothermal method . The BTO layer is composed of 3.9±1.5-nm-diameter nanoparticles, and further testing of BTO-coated CNT filters showed that anode stability and efficiency for flow-through organic electrooxidation were improved in BTO-coated CNT filters . Around 98% of oxalate was oxidized in BTO-coated CNT filters with relatively high current efficiencies in the range of 32% to >99%. The overpotential of oxygen evolution in the BTO-CNT nanocomposite was determined to be 1.71 V (vs. Ag/AgCl), which is 0.44 V higher than that of an uncoated CNT anode, and therefore, anodic oxidation of organic contaminants on BTO-CNT surface can take place with less competition of oxygen evolution, and anodic stability of CNT filters was also improved . Additional experiments on the electrooxidative filtration of ethanol, methanol, formaldehyde, and formate showed that BTO-CNT anode displayed two to eight times greater TOC removal efficiencies, 1.5–3.5 times greater mineralization current efficiencies, and four to five times less energy consumption than those of the uncoated CNT anode .
The effect of applied potentials on anoxic oxidation has been reported in previous studies. Under a total applied potential of 3 volts (V), MS2 virus with an influent concentration of 106 ml-1 was removed below detection limit in the effluent, and viability of Escherichia coli was lost by 99% after electrochemical CNT filtration . Although anode potentials were not reported in these studies, the results suggest that higher anode potentials result in higher anoxic oxidation when total applied potential is below 3 V. Further electrooxidation experiments on ferroxyanide oxidation confirmed this trend, and anoxic oxidation rate increased under higher anode potentials [15, 42]. Current densities were observed to increase with anode potentials [13, 39].
Anode potentials are also related with CNT reactive sites. The sp2-conjugated sidewall CNT sites are dominant when anode potentials are low (≤0.2 V), and CNT tips were also electroactive when anode potentials are high (≥0.3 V). A quantitative 2D electrooxidative CNT filter model has been developed to describe the correlation among target molecule, mass transport, adsorption, and electron transfer and product desorption, and this model has been used to accurately predict effluent concentrations over a much larger range of conditions .
3.2 Flow rate
Flow rate is another factor that may affect the electrooxidation rate of CNT filters, and it is related with reaction kinetics. A positive correlation between ferroxyanide electrooxidation and flow rate up to 4 ml min-1 was observed when mass transfer dominated the electrooxidation process at an anode potential of 0.3 V . When the anode potential was below 0.3 V, the electrochemical CNT filtration was under mixed mass transfer and reaction control . Under the typical configuration of CNT filters, electrooxidation rate is mass transfer limited by the convective flow rate through the electrode and subsequent replenishment of the target molecule, but not by the diffusional mass transfer of ferroxyanide from the bulk to the electrode surface . Current efficiency was found to increase slightly with increasing flow rate from J=0.5–4.0 ml min-1 and reached a plateau at the higher flow rates, which was likely due to increased mass transfer to the anode surface with increased flow rates .
3.3 Ionic strength
The electrooxidation rate may be affected by ionic strength in the solution. A comparison of three different concentrations (1, 10, and 100 mm) of electrolyte (NaSO4) in the electrochemical CNT filtration system indicated that the Na2SO4 concentration had minimal effect on the ferrocyanide oxidation . Such a result was related with the electrooxidation mechanism of ferrocyanide oxidation, which is a direct electron transfer [8, 10] instead of reactions with other oxidizing species that may be directly affected by ionic strength . For the electrochemical systems where indirect oxidation of the target species in the bulk solution was the primary mechanism, the primary chemical oxidant is a Cl-radical species . Reactor configuration may also affect the effects of ionic strength on electrooxidation in CNT filters. The constant convective renewal of the solution near the CNT surface in the flow-through configuration resulted in faster charge neutralization than the batch system , and the hydrodynamic reductions of charge transfer resistances enabled flow-through electrochemical CNT filtration to be a relative robust treatment technology that is less sensitive to the change of ionic strength.
3.4 Cathode material
Many materials have been used in electrochemical systems, such as graphite felt , carbon felt , and MWNT , and the performance of an electrochemical system is directly related with cathode materials because different cathodes resulted in significantly different extents of oxidation under the electron transfer-limited regime. Only 50% of influent ferrocyanide was oxidized at 0.3-V anode potential with Ti cathode, while over 80% of the influent ferrocyanide was oxidized under the same conditions with the CNT cathode . Coating electrodes with other materials may improve the performance of electrochemical CNT filtration system  or electrocatalytic performance in other treatment systems .
4 Latest development on electrochemical CNT filters
A few recently developed CNT-based electrochemical treatment techniques are also discussed to improve the overall performance of electrochemical CNT filters.
4.1 Surface modification and functionalization of CNT filters
The surface of CNT filters can be modified to improve their functions and performance through covalent attachment of functional groups or the non-covalent adsorption of functional molecules onto the surface of CNTs . Although there are many previous studies on the surface modification and functionalization of CNT filters, most of them focus on electroanalytical applications , adsorption , sensors [55–57], capacitor devices [58, 59], thermal stability , solubility , and only limited studies have been done to functionalize CNT filters for electrochemical treatment. Mishra et al. fabricated a new type of flexible carbon fabric-supported MWNT (Fe3O4-MWNTs) nanocomposite-based supercapacitor using a chemical vapor deposition (CVD) technique and applied it for arsenic removal and seawater desalination . The Fe3O4-MWNTs nanocomposite-based electrodes were used to remove both types of arsenic ions (arsenate and arsenite) and salts from seawater and achieved high desalination efficiency (removal of sodium, magnesium, and calcium) of seawater and high repeatability for the simultaneous removal of arsenic (both arsenate and arsenite) and sodium . In addition to the surface coating of CNTs with BTO nanoparticles , Liu et al. also coated CNT filters with titanium dioxide (TiO2) via a simple filtration-steam hydrolysis method and evaluated its performance for arsenic removal . The TiO2-coated CNT filters significantly enhanced arsenic sorption kinetics and capacity, which is possibly due to increased mass transport, improved sorption site accessibility, and reduced TiO2 negative surface charge. The TiO2-CNT arsenic sorption kinetics also increased with both increasing flow rate and cell potential. The spent TiO2 filter was successfully regenerated by 5 mm of NaOH .
4.2 Anode oxidation coupled with in situ generated H2O2 in the CNT cathode
Within a typical electrochemical CNT filtration system, organic pollutants were only adsorbed and oxidized via a direct/indirect oxidation process on the anodic CNT filters , and a Ti ring or CNT filters were used as a counter cathode to provide the required potential [15, 16]. However, the role of a cathode in electrochemical filters beyond a counter electrode has been largely neglected. A typical cathode provides electrons and only supports reduction instead of oxidation, and therefore, it cannot be directly used to oxidize organic pollutants in wastewater, which are mostly reduced hydrocarbons. A recent study utilized the cathode in a CNT filtration system to produce hydrogen peroxide (H2O2) for further oxidation of chemical contaminants . As CNT filers can work as an oxygen reduction reaction catalyst [64–66], a CNT cathode could be used to reduce oxygen to generate H2O2 (Eq. 2) with the counter electrode serving as a functional cathode [67–70].
As a strong oxidant (E0=1.763 V vs. SHE), H2O2 can oxidize various organic pollutants [67, 71, 72] and produce oxygen and water as by-products after oxidation [73–76]. A schematic of an electrochemical CNT filter coupled with in situ generated H2O2 is shown in Figure 2.
Under optimized conditions (C-CNT-HCl, applied cathode potential=-0.4 V vs. Ag/AgCl, influent DO flux=1.95 mol l-1 m-2, and [Na2SO4]=10 mmol l-1, flow rate=1.5 ml min-1), H2O2 flux increased linearly with increased flow rates, indicating that the filtration system was under mixed mass transfer and oxygen reduction reaction control. Contrarily, at medium flow rate conditions (1.5–4.0 ml min-1), the system became mass transfer-limited, i.e. the electroreduction rate of O2 was limited by the flow rate of the influent Na2SO4 solution throughout the cathode and subsequent replenishment of O2 to produce H2O2. This was further confirmed by the stable effluent DO of 6.9±0.1 mg l-1 and the stable DO efficiency of 58±2.6%. High flow rates above 4 ml min-1 may be detrimental to the reduction kinetics due to greatly increased pressure within the current filtration casing, which needs delicate engineering design to prevent such a negative effect .
H2O2 flux increased with higher initial DO flux, and the maximum H2O2 flux was obtained at an applied cathode potential of -0.4 V (vs. Ag/AgCl) and an O2 flux of 1.95 mol l-1 m-2, which was 1.7-, 2.8-, and 46.8-fold higher than those at the DO flux of 1.33, 0.66, and 0 mol l-1 m-2, respectively. However, the maximum current efficiency for H2O2 generation was only 52.9±1.2%, suggesting the existence of H2O2 decomposition processes or other electron-consuming reactions. For example, H2O2 could undergo chemical decomposition to O2 either on the anode (heterogeneous process) or in the medium (homogeneous process). Reduction from H2O2 to OH- on the CNT cathodes could also consume H2O2 and electrons [77, 78]. Additionally, H2O2 could be consumed indirectly by other reactive species, such as OH˙ to produce HO2˙ at the anode  and eventually decomposed into O2 [67, 77]. The electrochemical filter coupled with in situ generated H2O2 was further tested with three additional organic compounds: tetracycline (a typical PPCP), methyl orange (a typical azo-dye), and geosmin (a typical off-flavor compound), and all three organic compounds were efficiently removed with removal efficiencies of 90.3%, 96.0%, and 87.2% for 0.1 mmol l-1 tetracycline, 0.1 mmol l-1 methyl orange, and 0.55 nmol l-1 geosmin, respectively .
A CNT-based electro-Fenton system was developed to further utilize H2O2 to produce a Fenton agent to improve the treatment efficiency of CNT filtration systems . The system is composed of a CNT cathode, a CNT-COOFen+ cathode, a PTFE separator, and a CNT anode. The schematic of the CNT-based electro-Fenton system is shown in Figure 3.
In this system, O2 is reduced on the CNT cathode to produce H2O2 first, and H2O2 further reacts with Fe2+ to produce hydroxyl radical (OH˙), which is a strong oxidant and can be used to oxidize contaminants (Eq. 3). The produced Fe3+ will be regenerated in situ in the cathode (Eq. 4), and Fe2+ is reused in the following reactions. The remaining intermediates or metabolites are further oxidized in the CNT anode. As H2O2 is generated on site, the cost and risk associated with H2O2 storage and transportation can be reduced. The system is operated at neutral pH, and therefore, the acid/base required to keep Fe dissolved during the reaction is not needed. Fe is bound to the electrode, and an in situ electroregeneration of Fe2+ reduced the need for the addition of Fe and the disposal of iron sludge.
The results showed that the sequential electro-Fenton oxidation was a highly efficient electrochemical system for contaminant removal. The removal rate for oxalate was 206.8±6.3 mgC m-2 h-1, which is fourfold greater than the sum of the individual electrochemistry (16.4±3.2 mgC m-2 h-1) and Fenton (33.3±1.3 mgC m-2 h-1) reaction fluxes. The removal rates for refractory trifluoroacetic acid (TFA) and trichloroacetic acid (TCA) also reached 11.3±1.2 and 21.8±1.9 mmol m-2 h-1, respectively .
5 Energy consumption of electrochemical CNT filters
Energy consumption is one of the most important factors for building and operating water and wastewater treatment systems. The energy consumption of a typical electrochemical CNT filtration system was calculated as 4 kW h kg-1 COD at a total cell potential of 2 V and 15 kW h kg-1 COD at a total cell potential of 3 V for (C-CNT-HNO3 coated with Sb-doped SnO2 particles) . The energy consumption for electrochemical filtration system coupled with in situ generated H2O2 was calculated as 3.75 kW h kg-1 COD at a total cell potential of 1.85 V (corresponding to an optimized cathode potential of -0.4 V vs. Ag/AgCl) and a current of 9.05 mA . Additionally, the pumping energy was considered to pump the liquid solution through the filter. At a common back pressure of 15 kPa , a flow rate of 1.5 ml min-1, and a pump efficiency of 75%, the total energy cost for pumping was calculated as 1.35 J , which is only 2.2% of the energy used for electrochemical H2O2 production . The energy consumption for the flow-through sequential regenerative electro-Fenton system was calculated as 45.8 kW h kg-1 TOC . These values are lower than those in state-of-the-art electrochemical oxidation processes whose energy consumptions are typically in the range of 5–100 kW h kg-1 COD . Generally speaking, the energy consumption of electrochemical carbon nanotube filters is low. Although there was additional electrical energy input in the filtration system, the benefits of low applied potentials, high oxidation kinetics, short residence time, and long service time can compensate the additional energy input. The low potential could be provided by a solar panel so electrochemical CNT filters may be widely used as a cost-effective point-of-use treatment system in rural areas of developing countries.
6 Existing challenges
Although electrochemical CNT filters have been proven to be an efficient treatment technology, there still remain a few challenges before such a technology can be widely used in large-scale water and wastewater treatment plants. Although the cost of CNTs has decreased significantly during the last few years, the cost of CNTs are still relatively high than those of other commonly used water treatment materials, such as activate carbon. Porous CNT sheets in larger sizes are also expensive to fabricate. Another issue is bubble formation when the applied voltage is higher than 2 V, which is caused by the continuous production of hydrogen in the cathode and oxygen in the anode. A sample picture is shown in Figure 4. Bubble formation may block the active sites of CNT filters, increase back pressure, reduce mass transfer efficiency, and eventually reduce the efficiency of electrochemical CNT filters.
The CNT surface could be changed after filtration, and a sample SEM image of the CNT surface is shown in Figure 5. After electrochemical filtration for 3 h, the anodic CNT filter showed some extent of organic, possibly polymer, and inorganic, possibly sodium persulfate, buildup on the CNT surface that may increase resistance to electron transfer and hinder electrochemistry .
The existing configuration of dead-end CNT filtration system may limit the overall water flux to be treated, but pilot-scale and full-scale cross-flow CNT filtration systems have not been reported in the literature yet. CNT filters are relatively fragile and can be easily torn apart and fractured. CNT agglomeration is another issue after an extended period of operation and is more obvious under applied potentials higher than 2.5 V. The last but not the least, metabolites can be produced after partial oxidation, and the toxicity of these metabolites are often difficult to characterize, and therefore, their chemical characteristics and toxicity are not well understood.
7 Conclusions and perspectives
Electrochemically active CNT filters have been developed as a highly effective treatment technology for water and wastewater treatment and can be used to treat various chemical and biological contaminants due to its high stability, great flexibility, and large specific surface area. With the continuous reduced costs of CNTs, electrochemical CNT filters could be widely adopted for water purification. Surface modification and functionalization of CNT filters will further improve their applications and treatment efficiency. Fouling control in these electrochemical CNT filters is another area with great potentials, and minimization of membrane fouling will greatly help the development of cost-effective water treatment technologies. Cross-flow configuration provides higher fluxes than that of dead-end filtration, and the development of cross-flow electrochemical CNT systems will build a foundation for scaled-up application of this techniques in pilot-scale and full-scale water and wastewater treatment plants. Because of the simplicity of the reactor configuration, this technology can be easily adopted by developing countries for point-of-use applications as no expensive treatment facilities are needed. The potentials of using these techniques to treat both chemical and biological contaminants can be further explored to provide flexibility to deal with different contamination in water sources. Coupled with solar panels to provide the low applied potentials, electrochemical CNT filters could be one of the most cost-effective treatment technologies to meet ever-increasing needs for adequate clean water.
This work is financially supported by Purdue University.
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