Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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The Prospect of Electrochemical Technologies Advancing Worldwide Water Treatment Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Brian P. Chaplin* Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, Illinois 60607, United States Downloaded via WEBSTER UNIV on February 15, 2019 at 23:24:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
CONSPECTUS: Growing worldwide population, climate change, and decaying water infrastructure have all contributed to a need for a better water treatment and conveyance model. Distributed water treatment is one possible solution, which relies on the local treatment of water from various sources to a degree dependent on its intended use and, finally, distribution to local consumers. This distributed, fit-for-purpose water treatment strategy requires the development of new modular point-of-use and point-of-entry technologies to bring this idea to fruition. Electrochemical technologies have the potential to contribute to this vision, as they have several advantages over established water treatment technologies. Electrochemical technologies have the ability to simultaneously treat multiple classes of contaminants through the in situ production of chemicals at the electrode surfaces with low power and energy demands, thereby allowing the construction of compact, modular water treatment technologies that require little maintenance and can be easily automated or remotely controlled. In addition, these technologies offer the opportunity for energy recovery through production of fuels at the cathode, which can further reduce their energy footprint. In spite of these advantages, there are several challenges that need to be overcome before widespread adoption of electrochemical water treatment technologies is possible. This Account will focus primarily on destructive electrolytic technologies that allow for removal of water contaminants without the need for residual treatment or management. Most important to the development of destructive electrochemical technologies is a need to fabricate nontoxic, inexpensive, highsurface-area electrodes that have a long operational life and can operate without the production of unwanted toxic byproducts. Overcoming these barriers will decrease the capital costs of water treatment and allow the development of the point-of-use and point-of-entry technologies that are necessary to promote more sustainable water treatment solutions. However, to accomplish this goal, a reprioritization of research is needed. Current research is primarily focused on investigating individual contaminant transformation pathways and mechanisms. While this research is important for understanding these technologies, additional work is needed in developing inexpensive, high-surface-area, stable electrode materials, minimizing toxic byproduct formation, and determining the life cycle and technoeconomic analyses necessary for commercialization. Better understanding of these critical research areas will allow for strategic deployment of electrochemical water treatment technologies to promote a more sustainable future.
1.0. INTRODUCTION
reduce the costs and energy usage associated with water transport.2 In developing countries, this approach removes the need for a complex distribution infrastructure, which is expensive to build and maintain. The development of reliable and affordable point-of-use and point-of-entry treatment technologies would allow for household-level to communitylevel water security in both the developed and developing world. Such an accomplishment would help to reduce the nearly 1.0 billion people who lack access to clean potable water
The gap between water demand and supply is increasing in both developed and developing countries and has resulted in the need to utilize nontraditional water supplies. As a result, traditional treatment technologies and infrastructure may be an ineffective means to treat and convey water. For this reason, a decentralized fit-for-purpose approach has been suggested as a new paradigm for water treatment, which focuses on local water treatment and distribution, and the extent of treatment is dictated by its intended use.1 The shift toward distributed water treatment in urban areas can increase water supply and resiliency, reduce pollution, and © XXXX American Chemical Society
Received: November 30, 2018
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DOI: 10.1021/acs.accounts.8b00611 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 1. Treatment capabilities of electrochemical technologies. Examples include (a) electrochemical oxidation of phenol (C6H5OH); (b) electrochemical reduction of nitrate (NO3−) to N2; (c) electro-deionization of NaCl; (d) micro-organism inactivation by electrochemically produced Cl2 and OH•; (e) electrodeposition of lead; (f) electrocoagulation of metals; and (g) electrosorption of arsenate.
with these technologies, and provide a vision for future research. This Account differs from past reviews on the subject, by providing a more thorough critique of the disadvantages and a more visionary discussion of the potential of electrochemical technologies to transform the future of water treatment.
and 1.8 million children who die each year from diarrhea, which is primarily caused by water contamination.3 Electrochemical technologies can contribute to decentralized water treatment by providing a flexible technology to treat and reuse water from various sources, as either a standalone technology or part of a treatment train approach. Electrochemical cells can electrochemically oxidize organic contaminants,4 electrochemically reduce toxic oxyanions,5 remove toxic heavy metals,6 and disinfect the water.7 The great promise of these technologies is hampered by toxic halogenated byproduct formation and challenges associated with large-scale applications because of expensive, low-surface area electrodes, and in developing countries, the promise is hampered by the inability to power these systems. The primary objectives of this Account are to underscore the advantages of destructive electrochemical technologies for distributed water treatment, discuss the challenges associated
2.0. ADVANTAGES OF ELECTROCHEMICAL TECHNOLOGIES Electrochemical technologies have several advantages over established water treatment technologies. These advantages include (1) the ability to treat multiple classes of contaminants; (2) in situ production of chemicals; (3) modular cell construction; and (4) the possibility for energy recovery. These areas of research are actively being B
DOI: 10.1021/acs.accounts.8b00611 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
H+, OH−, reactive oxygen species (ROS) (i.e., OH•, H2O2, O3), Cl2, adsorbed hydrogen (Had), and oxidizing and reducing electrons. Detailed studies comparing the economic and life cycle costs of on-site generation of chemicals with commercial purchase do not exist. However, recent work has investigated electrochemical NaOH production from desalination RO brines at the laboratory scale, where the NaOH was generated for utilization within the desalination process.23,24 On-site energy estimates for this small-scale process in divided electrochemical cells were 3.25.9 kWh/kg of NaOH24 and are comparable to the on-site energy usage of the industrialscale chlor-alkali process (2.1 to 2.3 kWh/kg of NaOH),23 indicating that on-site chemical generation could be a feasible approach.
investigated at the laboratory scale, and key results are discussed below. 2.1. Treatment of Multiple Contaminant Classes
Treatment capabilities of electrochemical cells (summarized in Figure 1) include electrochemical oxidation of organic contaminants,4,8 electrochemical reduction of oxyanions and halogenated organic compounds,5,9 electro-deionization of salts,10 bacteria and virus inactivation,7 and the removal of toxic metals through electrodeposition, electrocoagulation, and electrosorption processes.6,11,12 The wide-ranging capabilities of electrochemical technologies have created opportunities for these technologies to replace multiple traditional technologies. Key examples are the use of electrochemical advanced oxidation processes (EAOPs) instead of traditional advanced oxidation processes (AOPs) and electrochemical disinfection in place of Cl2 disinfection. While electrochemical technologies have been utilized for a single purpose (e.g., electrochemical disinfection), there are opportunities to use a single electrochemical cell to accomplish various treatment goals. For example, simultaneous metal removal and organic contaminant oxidation,13 oxidation and reduction of contaminants,14 electrochemical disinfection and electrocoagulation,15 and electrochemical disinfection and contaminant oxidation16 are but a few examples. Recent work has also transformed traditional separation membranes into reactive surfaces, known as reactive electrochemical membranes (REMs), which further expand the application of electrochemical technologies to simultaneous filtration and electrochemical oxidation/reduction of contaminants.17,18 Destructive technologies are desired, because they transform contaminants to innocuous or less toxic products, and they do not suffer from residuals management issues. Therefore, this Account will focus primarily on destructive electrochemical treatment technologies, which include electrochemical oxidation and reduction. These processes will be discussed in more detail in the subsequent sections.
2.3. Scalable Modular Technologies
Modular water treatment strategies have been proposed as a means to achieve flexibility when treating variable water quality and quantity.2,25,26 This strategy relies on approximately linear scalable treatment technologies, where modules can be added or removed to accommodate variable input water flow or composition. Electrochemical technologies have been proposed as viable candidates for this module treatment approach.2,26 In addition, the control of potentials and currents, which determines the system performance of electrochemical technologies, is suitable for automation and remote system control. Dimensional similarity is often used for reactor scale-up.27 Scale-up of electrochemical reactors relies on the use of geometric, kinematic, thermal, and current/potential similarities between reactors.27 Analysis indicates that optimal scaleup of electrochemical reactors is achieved by using multicell electrode stacks in both parallel and series.28 This modular scale-up approach allows for optimized results that are obtained at the bench scale to easily be extrapolated to large-scale applications by simply adding more electrode stacks. Therefore, it is important to consider reactor design with regards to current and potential distributions and hydrodynamics when conducting bench-scale electrochemical experiments. Doing so allows bench-scale results to be used to determine the feasibility of process scale-up. However, this approach is not a substitute for pilot-scale testing at representative module size, which is needed to access technoeconomic feasibility.
2.2. In Situ Chemical Production
Oxidation and reduction technologies typically require the addition of chemicals. For example, chlorine disinfection, H2O2-based AOPs, and catalytic hydrogenation all require the addition of chemical oxidants/reductants.19,20 Chemical addition can result in high salt concentrations, chemical residuals, and substantial expenditures of raw materials and fossil fuels for manufacture, transport, and storage. These expenditures increase total treatment costs and can cause environmental pollution. Although railcar transport is the most efficient transport method, it uses approximately 0.005 gallons of fuel per ton-mile and emits approximately 0.46, 0.64, and 1.83 lbs/ton-mile of hydrocarbons, CO, and NOx, respectively.21 Additionally, the total CO2 emitted is approximately 22.2 lbs/gallon of fuel used,22 an equivalent of 0.11 lbs/tonmile. The total cost of transport was estimated at $2.53/tonmile.21 There also is a need for on-site storage of potentially dangerous chemicals (e.g., Cl2), which decreases safety and increases the treatment cost and footprint. The in situ electrochemical generation of chemicals for water treatment would eliminate many of these costs and the negative impacts associated with pollution and worker/user safety. Several chemicals can be generated from the electrochemical oxidation and reduction of water and reduction of oxygen. These chemicals can be generated on-site from available water sources, without the addition of electrolyte salts, and include
2.4. Energy Recovery
The most researched application of environmental electrochemical technologies is electrochemical oxidation, where reactions at the cathode are often ignored. However, in the near future, it may be possible to utilize cathodic reactions to recover energy through the production and utilization of fuels. The reduction of CO2 to hydrocarbon fuels (e.g, CO, HCOOH, CH3OH, and CH4) is possible, and active research is addressing the challenges associated with high overpotentials, low reaction rates, and expensive catalysts.29 Thermodynamics can be used to estimate the maximum energy recovery for a given cathodic reaction, i.e., change in enthalpy of the combustion reaction of the fuel (ΔHcomb) divided by the electrochemical work done by the cell to produce the fuel (ΔGrxn = −nFErxn), where n is the number of electrons transferred, and F is Faraday’s constant. This simplified analysis assumes that cell reactions are under standard state conditions, overpotentials associated with activation energies, ohmic resistances, and mass transfer are C
DOI: 10.1021/acs.accounts.8b00611 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Electrochemical reactor operation in (a) flow-by mode operation and (b) flow-through mode operation.
treatment technologies be developed? (3) Are power and energy demands too high? (4) Will electrochemical technologies result in harmful consequences?
negligible, and the combustion process is 100% efficient. For example, if OH• is produced on the anode (E° = 2.5 V/SHE), and the cathodic reaction is the reduction of CO2 to CO, HCOOH, CH3OH, or CH4, energy recoveries of 98, 98, 51, or 50% are calculated, respectively. This analysis indicates that energy recovery from electrochemical cells in the form of fuels is possible. However, if more realistic systems are considered where significant activation energies, ohmic resistances, and mass transport limitations exist, cell potentials would increase significantly, thereby reducing the actual energy recoveries. In addition, collecting, separating, or concentrating the produced fuel all require energy and financial expenditures that may not justify the energy recovery process. Recently, an electrochemical cell was tested for simultaneous oxidation of an azo dye (model wastewater) and reduction of carbon dioxide to formic acid.30 The overall energy consumption was calculated to be approximately 20% higher for the combined process compared to that of the oxidation process alone because of the need for a membrane separator, but the energy consumption is still lower than the sum of the two processes (roughly double). Further work is needed to optimize the combined process by finding inexpensive, robust, high-surface-area electrode materials and eliminating the need for a membrane separator.
3.1. Are the Capital Costs Too High?
Electrode materials are a primary contributor to the capital costs of electrochemical cells. Therefore, the development of affordable, long-life electrodes is necessary. Because of the need for reverse polarity operation of electrochemical cells to remove mineral scale (e.g., CaCO3(s)) on the cathode, electrodes must be cathodically and anodically stable. Cathodic stability is met by many materials, but anodic stability is not. Active electrode materials are generally used for water treatment, because they produce OH•.4 By contrast, inactive electrode materials do not generate OH•, and many are not anodically stable (e.g., carbon), making them ineffective for long-term treatment of recalcitrant contaminants. Common active electrodes for electrochemical oxidation include borondoped diamond (BDD), doped SnO2, and substoichiometric and doped TiO2.4 BDD film electrodes have been the most promising electrodes for water treatment because of their high anodic stability, high OH• yield, and wide potential window.4 However, BDD electrodes are typically synthesized using chemical vapor deposition methods, which are slow and expensive and have low production rates. These factors translate to expensive electrodes (∼$7125 per m2). It is also difficult to produce high-surface-area BDD electrodes, which results in the use of numerous expensive electrodes to achieve a given treatment objective. Therefore, it is critical to find affordable alternatives to BDD to reduce capital costs associated with electrochemical
3.0. CHALLENGES FOR ELECTROCHEMICAL TECHNOLOGIES Although electrochemical technologies hold promise for promoting modular water treatment, there are several challenges that must be addressed. Important questions include: (1) Are the capital costs too high? (2) Can compact D
DOI: 10.1021/acs.accounts.8b00611 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
power is not available. Demonstration studies have shown that solar cells can power electrochemical water treatment.39,40 Energy consumption of electrochemical technologies must be competitive with current technologies to make them viable economical options. The most useful figure-of-merit for energy consumption is the electrical energy per order (EEO), which measures the kWh per m3 of water treated to degrade a contaminant by 1 order of magnitude. Competitive energy consumptions for electrochemical cells include the oxidation of atrazine in a flow-through Bi−SnO2/Ti4O7 REM (EEO < 0.42 kWh/m3) and electrochemical reduction of NDMA in a flowthrough carbon−Ti4O7 composite REM (EEO = 0.086 to 0.43 kWh/m3).41 The energy for pumping was low and estimated at 0.03 kWh/m3 for a flow rate of 600 L/m2/h.38 The EEO values for electrochemical oxidation of atrazine were comparable to those for a UV/H2O2 AOP (0.2 to 0.98 kwh m−3).42 However, the REM showed complete mineralization of atrazine,38 whereas the UV/H2O2 AOP formed several oxidation products with similar or higher toxicity than that of atrazine.43,44 The EEO values for NDMA were between 0.5 and 0.9 kWh/m3 for ozonation, 0.3 to 1.62 kWh/m3 for UV/H2O2 oxidation,42 and ∼2.0 kWh/m3 for RO.45 These results indicate that electrochemical technologies can be competitive with many mature treatment technologies. The overall energy consumption for electrolysis reactions can be lowered using a current modulation technique, which continuously adjusts the applied current to that of the limiting current during electrochemical treatment.46 Such a strategy has shown the ability to reduce the energy consumption by approximately 5-fold when operating in a batch mode. To take advantage of this technique in flow-through mode, reactors in series should be operated at progressively lower current densities.
treatment. Although doped SnO2 electrodes have shown acceptable performance and cost, they have short service lives.4 Recent work comparing various electrodes (i.e., BDD, Ti4O7, Pt, SnO2) for organic compound oxidation showed that Ti4O7 electrodes were comparable to BDD electrodes for efficiency of organic compound oxidation,31 and nonoptimized, lab-scale (e.g., cm 2 ) Ti4 O7 electrodes can be synthesized at much lower costs (∼ $0.36 per m2). The significantly lower costs relative to BDD are due to the much higher specific surface area of porous Ti4O7 electrodes (a = 2.9 × 106 m−1)32 relative to BDD electrodes (a = 1.0 × 10−3 m−1) as well as lower fabrication costs (see Supporting Information). In recent years, Ti4O7 electrodes have shown the ability to oxidize and reduce several different contaminants33,34 and are electrochemically stable if continuous reverse polarity treatments are employed.35,36 These results suggest that Ti4O7 may be an appropriate electrode material from both a technical and economic standpoint. 3.2. Can Compact Treatment Technologies Be Developed?
Traditional electrochemical cells utilize flat plate electrodes that are operated in flow-by mode. This operational mode, as shown in Figure 2a, results in the formation of a thick diffusional boundary layer (∼100 μm). Therefore, reaction rates of contaminants become diffusion-limited at low current densities (e.g., < 5 mA/cm2).37 Utilization of high-specificsurface-area electrodes in flow-by mode results in only modest gains in reaction rates, because the length scale of the features of electrode roughness is much smaller than that of the diffusional boundary layer, and therefore, the features get averaged into the diffusional field. By contrast, the use of high-specific-surface-area electrodes in flow-through mode, where the water passes directly through the electrode pores, significantly increases reaction rates (Figure 2b). This increase is attributed to the convective transport of water through the small pores of the electrode, which decreases the diffusional boundary layer thickness to a distance comparable to the pore radius, and features of electrode roughness are now on the same length scale as the diffusional field, and thus, a large fraction of the surface area is electroactive. These combined effects have resulted in order of magnitude increases in the observed reaction rate constants of microporous electrodes when operated in flow-through mode relative to flow-by or batch mode.34 A more detailed comparison of these two flow modes has been discussed by Trellu et al.17 The high mass transport rate constants and large surface areas of porous electrodes have resulted in significant removal of contaminants in a single pass through the electrode when operated in flow-through mode.18,38 For example, a Pd−Cu/ Ti4O7 REM reduced NO3− from 1 mM to below the EPA’s regulatory maximum contaminant level (700 μM) (residence time ≈ 2 s)18 and a Bi-doped SnO2/Ti4O7 REM achieved oxidation of atrazine and clothianidin from 10 μM to below the detection limit of 10 nM (residence time ≈ 3.6 s).38
3.4. Does Electrochemical Water Treatment Result in Harmful Consequences?
Recent studies have shown that electrochemical oxidation can lead to the formation of toxic organic and inorganic byproducts (Figure 3). Electrochemical oxidation in the presence of halide ions can result in the production of halogenated organic
3.3. Are Power and Energy Demands Too High?
Optimized electrochemical cells treating moderately concentrated waste streams typically operate at
