electrochemical carbon nanotube filters for water...

Nanotechnol Rev 2016; 5(1): 41–50

*Corresponding author: Zhi Zhou, Division of Environmental and Ecological Engineering and School of Civil Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, USA, e-mail: [email protected] A. Jame: Division of Environmental and Ecological Engineering and School of Civil Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, USA

Review

Sadia A. Jame and Zhi Zhou*

Electrochemical carbon nanotube filters for water and wastewater treatmentDOI 10.1515/ntrev-2015-0056Received October 7, 2015; accepted December 19, 2015; previously published online January 28, 2016

Abstract: Electrochemically active carbon nanotube (CNT) filters have been developed as a highly efficient technol-ogy 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.

Keywords: carbon nanotube filters; electrochemical treat-ment; electrooxidation; wastewater treatment; water treatment.

1 IntroductionCarbon 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 [1] and are useful for a variety of applications in energy studies and biomedical devices. The high surface area (30–500 m2 g-1) and electro-chemical 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 pre-viously [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]. Electrochem-istry 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 bat-teries [19]. The environmental application of electrochem-istry 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 [23], proteins [24], viruses [14], azo dyes [16], pharma-ceuticals [25], perfluorinated chemicals [26], and phenol [8]. 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 [28]. A CNT-polyvinylidene fluoride porous non-Faradaic cathode was shown to significantly reduce organic fouling in ultra-filtration (UF) membranes and has potential to reduce energy requirements by up to twofold compared to the unmodified UF system [18]. The electrochemical appli-cations of CNT filters are related with their good electro-catalytic 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 [13]. As a result, CNT-based electrodes are good candidates for environmental applications, such as wastewater treatment and micropol-lutant sensors [17].

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

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42 S.A. Jame and Z. Zhou: Electrochemical CNT filters for water and wastewater treatment

electrochemical treatment techniques are discussed. The existing challenges and future perspectives are also dis-cussed. 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 waste-water treatment.

2 Working principles of electrochemical CNT filters for water and wastewater treatment

CNT filters are composed of tens of hundreds of individ-ual tubes that are entangled with each other through van der Waals forces of attraction [1], 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 [16]. Vecitis et al. modified a commercial polycar-bonate filtration casing and inserted stainless steel or tita-nium 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 trans-fer, (2) temperature-dependent physical adsorption/desorption, and (3) voltage-dependent direct electron

transfer [16]. Electrochemical oxidation for organic contaminants takes much less time than that of con-ventional biological wastewater treatment systems [29]. Furthermore, recalcitrant compounds that are often dif-ficult to be treated can be electrochemically oxidized [22]. CNT filters have been widely used for adsorption of various chemical contaminants [30, 31], but previ-ous studies also indicated that breakthrough of CNT filters happened in few hours in fix bed columns [32] or < 30 min in a dead end filter [2] after CNT active sites were occupied with adsorbed contaminants. Electro-chemical CNT filters can further break down adsorbed contaminants and, therefore, significantly improve their performance in wastewater treatment. Electrochemi-cal 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 electro-chemical treatment systems were highly efficient to treat different chemical and biological contaminants with a typical removal efficiency > 90%.

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.

Figure 1: A typical electrochemical CNT filter. (A) Design of the modified commercial polycarbonate filtration casing consisting of (1) a perforated stainless steel cathode, (2) an insulating silicone rubber separator and seal, (3) a titanium anodic ring that is pressed into the carbon nanotube anodic filter, and (4) the MWNT anodic filter supported by a PTFE membrane. (B, C) Images of the modified filtration casing. (D, E) Images of the MWNT network before and after electrochemical filtration, respectively [16]. Copyright © 2011 American Chemical Society.


S.A. Jame and Z. Zhou: Electrochemical CNT filters for water and wastewater treatment 43

3.1 Anode potential

Anode potential is one of the most important impact-ing 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).

-2 2 01/ 2H O 1/ 4O H e , E 1.03 V vs. Ag / AgCl+→ + + =+ (1)

The potentials of oxygen evolution of different anodes, such as Pt, IrO2, PbO2, and SnO2, have been reviewed pre-viously [21]. 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 [44] 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 over-potential of oxygen evolution for electrochemical applica-tions. 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 [39]. 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 [39]. 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 evo-lution 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 oxida-tion 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 [39]. Additional experiments on the electrooxidative filtration

Table 1: Removal efficiencies of CNT-based electrochemical treatment systems and adsorption systems.

Contaminants Anode Cathode Removal efficiency System configuration References

Phenol SnO2-Sb2O4-CNT Stainless steel 90% Batch [33]C.I. basic yellow 2 Fe CNT-PTFE 96% Batch [34]Methyl orange Sb-doped SnO2 CNT Stainless steel 95% Filtration [8]C.I. basic blue 3 Pt CNT-PTFE 94% Batch [35]4-Chlorophenol LAS-CNT-PbO2 Stainless steel 99.5% Batch [36]Methylene blue CNT CNT 98% Filtration [16]E. coli CNT Stainless steel 75% Filtration [17]MS2 CNT Stainless steel 99.6% Filtration [17]MS2 CNT Stainless steel > 99.9% Filtration [14]m-Cresol Ti/SnO2-Sb2O5-IrO2 CNT 82% Batch [37]C.I. direct red 23 Pt CNT-PTFE 94% Batch [38]Oxalate BTO-CNT Ti 98% Filtration [39]Ethanol BTO-CNT Ti 85.7% Filtration [39]Methanol BTO-CNT Ti 88.7% Filtration [39]Formaldehyde BTO-CNT Ti 33.5% Filtration [39]Formate BTO-CNT Ti 25% Filtration [39]Ferrocyanide CNT Ti > 90% Filtration [15]p-Nitrophenol CNT-Ce-PbO2 Stainless steel 98.2% Batch [40]Chromium Carbon paper CNT 96% Batch [41]Tetracycline Graphene-CNT Ti 88% Filtration [42]Phenol Graphene-CNT Ti 93% Filtration [42]Oxalate Graphene-CNT Ti 87% Filtration [42]Phenol CNT CNT 87.0% Filtration [43]Tetracycline CNT CNT 90.3% Filtration [43]Methyl orange CNT CNT 96.0% Filtration [43]Geosmin CNT CNT 87.2% Filtration [43]Tetracycline CNT CNT 99% Filtration [2]



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 mineraliza-tion current efficiencies, and four to five times less energy consumption than those of the uncoated CNT anode [39].

The effect of applied potentials on anoxic oxida-tion has been reported in previous studies. Under a total applied potential of 3 volts (V), MS2 virus with an influ-ent concentration of 106 ml-1 was removed below detec-tion limit in the effluent, and viability of Escherichia coli was lost by 99% after electrochemical CNT filtration [17]. Although anode potentials were not reported in these studies, the results suggest that higher anode potentials result in higher anoxic oxidation when total applied poten-tial 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 concentra-tions over a much larger range of conditions [48].

3.2 Flow rate

Flow rate is another factor that may affect the electrooxi-dation rate of CNT filters, and it is related with reaction kinetics. A positive correlation between ferroxyanide elec-trooxidation 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 [15]. When the anode potential was below 0.3 V, the electrochemical CNT filtra-tion was under mixed mass transfer and reaction control [15]. Under the typical configuration of CNT filters, elec-trooxidation rate is mass transfer limited by the convective flow rate through the electrode and subsequent replenish-ment of the target molecule, but not by the diffusional mass transfer of ferroxyanide from the bulk to the elec-trode surface [15]. 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 [15].

3.3 Ionic strength

The electrooxidation rate may be affected by ionic strength in the solution. A comparison of three different concen-trations (1, 10, and 100 mm) of electrolyte (NaSO4) in the electrochemical CNT filtration system indicated that the Na2SO4 concentration had minimal effect on the ferrocya-nide oxidation [15]. Such a result was related with the elec-trooxidation 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 [15]. For the electrochemical systems where indirect oxidation of the target species in the bulk solu-tion was the primary mechanism, the primary chemical oxidant is a Cl-radical species [15]. Reactor configuration may also affect the effects of ionic strength on electrooxi-dation 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 [15], and the hydrodynamic reductions of charge transfer resistances enabled flow-through electro-chemical 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 [49], carbon felt [50], and MWNT [17], and the performance of an electrochemi-cal system is directly related with cathode materials because different cathodes resulted in significantly dif-ferent 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 [15]. Coating electrodes with other materials may improve the performance of electrochemical CNT filtration system [39] or electrocatalytic performance in other treatment systems [51].

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 func-tional molecules onto the surface of CNTs [52]. Although there are many previous studies on the surface modifica-tion and functionalization of CNT filters, most of them focus on electroanalytical applications [53], adsorption [54], sensors [55–57], capacitor devices [58, 59], thermal stabil-ity [60], solubility [61], and only limited studies have been done to functionalize CNT filters for electrochemical treat-ment. 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 [62]. The Fe3O4-MWNTs nanocom-posite-based electrodes were used to remove both types of arsenic ions (arsenate and arsenite) and salts from seawa-ter 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 [62]. In addition to the surface coating of CNTs with BTO nanoparticles [39], Liu et al. also coated CNT filters with titanium dioxide (TiO2) via a simple filtration-steam hydrolysis method and evalu-ated its performance for arsenic removal [63]. 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 success-fully regenerated by 5 mm of NaOH [63].

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 [13], 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

Figure 2: Schematic of an electrochemical CNT filter coupled with in situ generated H2O2 [43]. Copyright © 2015 Royal Society of Chemistry.

utilized the cathode in a CNT filtration system to produce hydrogen peroxide (H2O2) for further oxidation of chemical contaminants [43]. 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].

-2 2 2 01/ 2O H e 1/ 2H O , E 0.45 V vs. Ag / AgCl+ →+ + =+ (2)

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 condi-tions (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 [15].

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 exist-ence of H2O2 decomposition processes or other electron-consuming reactions. For example, H2O2 could undergo chemical decomposition to O2 either on the anode (het-erogeneous process) or in the medium (homogeneous process). Reduction from H2O2 to OH- on the CNT cathodes could also consume H2O2 and electrons [77, 78]. Addition-ally, H2O2 could be consumed indirectly by other reactive species, such as OH˙ to produce HO2˙ at the anode [79] and eventually decomposed into O2 [67, 77]. The electro-chemical 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 [43].

4.3 Electro-Fenton

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 [12]. The system is composed of a CNT cathode, a CNT-COOFen+ cathode, a PTFE separator, and a CNT anode. The sche-matic 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 oxi-dized in the CNT anode. As H2O2 is generated on site, the cost and risk associated with H2O2 storage and transporta-tion 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 elec-trode, and an in situ electroregeneration of Fe2+ reduced the need for the addition of Fe and the disposal of iron sludge.

2 32 2 2H O Fe H Fe H O OH+ + +→+ + + + i (3)

3 - 2Fe e Fe+ +→+ (4)

The results showed that the sequential electro-Fenton oxidation was a highly efficient electrochemi-cal system for contaminant removal. The removal rate

Figure 3: Sandwiched electro-Fenton system based on carbon nanotube membrane stacks. (A) Images of the unfolded sandwich membrane stacks including four layers and (B) schematic of main roles of every layer in membrane stacks, and [P] and [P]m are pollutants and their oxidation intermediates, respectively [12]. Copyright © 2015 American Chemical Society.



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 signifi-cantly 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 fabri-cate. Another issue is bubble formation when the applied voltage is higher than 2 V, which is caused by the continu-ous 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 even-tually 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, possi-bly polymer, and inorganic, possibly sodium persulfate, buildup on the CNT surface that may increase resistance to electron transfer and hinder electrochemistry [2].

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.

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) reac-tion 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 [12].

5 Energy consumption of electrochemical CNT filters

Energy consumption is one of the most important factors for building and operating water and wastewa-ter treatment systems. The energy consumption of a typical electrochemical CNT filtration system was cal-culated 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) [8]. The energy consumption for electrochemical filtra-tion system coupled with in situ generated H2O2 was cal-culated 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 [43]. Addi-tionally, the pumping energy was considered to pump the liquid solution through the filter. At a common back pres-sure of 15 kPa [8], 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 [13], which is only 2.2% of the energy used for electrochemical H2O2 production [43]. The energy consumption for the flow-through sequential regenerative electro-Fenton system was calculated as 45.8 kW h kg-1 TOC [12]. 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 [20]. Generally speaking, the energy consumption of electrochemical carbon nanotube filters is low. Although there was additional electrical energy input in the filtra-tion 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 electro-chemical CNT filters may be widely used as a cost-effective point-of-use treatment system in rural areas of developing countries.

6 Existing challengesAlthough electrochemical CNT filters have been proven to be an efficient treatment technology, there still remain

Figure 4: Bubble formation after electrochemical CNT filtration [42]. Copyright © 2014 Royal Society of Chemistry.



7 Conclusions and perspectivesElectrochemically 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, electrochemi-cal CNT filters could be widely adopted for water purifica-tion. Surface modification and functionalization of CNT filters will further improve their applications and treat-ment efficiency. Fouling control in these electrochemi-cal CNT filters is another area with great potentials, and minimization of membrane fouling will greatly help the development of cost-effective water treatment technolo-gies. 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 coun-tries for point-of-use applications as no expensive treat-ment facilities are needed. The potentials of using these techniques to treat both chemical and biological contami-nants 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.

Figure 5: SEM image of CNT surface after filtration. The white circles and squares showed some extent of inorganic or organic build upon the CNT surface, respectively. Scale bar is 2 μm [2]. Copyright © 2015 American Chemical Society.

Acknowledgments: This work is financially supported by Purdue University.

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