Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Water Disinfection in Rural Areas Demands Unconventional Solar Technologies Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Chiheng Chu, Eric C. Ryberg, Stephanie K. Loeb, Min-Jeong Suh, and Jae-Hong Kim* Department of Chemical and Environmental Engineering and Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06511, United States Acc. Chem. Res. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/03/19. For personal use only.
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CONSPECTUS: Providing access to safe drinking water is a prerequisite for protecting public health. Vast improvements in drinking water quality have been witnessed during the last century, particularly in urban areas, thanks to the successful implementation of large, centralized water treatment plants and the distribution of treated water via underground networks of pipes. Nevertheless, infection by waterborne pathogens through the consumption of biologically unsafe drinking water remains one of the most significant causes of morbidity and mortality in developing rural areas. In these areas, the construction of centralized water treatment and distribution systems is impractical due to high capital costs and lack of existing infrastructure. Improving drinking water quality in developing rural areas demands a paradigm shift to unconventional, innovative water disinfection strategies that are low cost and simple to implement and maintain, while also requiring minimal infrastructure. The implementation of point-of-use (POU) disinfection techniques at the household- or community-scale is the most promising intervention strategy for producing immediate health benefits in the most vulnerable rural populations. Among POU techniques, solar-driven processes are considered particularly instrumental to this strategy, as developing rural areas that lack safe drinking water typically receive higher than average surface sunlight irradiation. Materials that can efficiently harvest sunlight to produce disinfecting agents are pivotal for surpassing the disinfection performance of conventional POU techniques. In this account, we highlight recent advances in materials and processes that can harness sunlight to disinfect water. We describe the physicochemical properties and molecular disinfection mechanisms for four categories of disinfectants that can be generated by harvesting sunlight: heat, germicidal UV radiation, strong oxidants, and mild oxidants. Our recent work in developing materials-based solar disinfection technologies is discussed in detail, with particular focus on the materials’ mechanistic functions and their modes of action for inactivation of three common types of waterborne pathogens (i.e., bacteria, virus, and protozoa). We conclude that different solar disinfection technologies should be applied depending on the source water quality and target pathogen due to significant variations on susceptibility of microbial components to disparate disinfectants. In addition, we expect that ample research opportunities exist on reactor design and process engineering for scale-up and improved performance of these solar materials, while accounting for the infrastructure demand and capital input. Although the practical implementation of new treatment techniques will face social and economic challenges that cannot be overlooked, novel technologies such as these can play a pivotal role in reducing water borne disease burden in rural communities in the developing world.
1. INTRODUCTION
forty-seven years in 1900 to sixty-three years in 1947; half of this increase among city dwellers is attributed to the public supply of safe drinking water.1 Consequently, urban water infrastructure was identified as the fourth most important engineering feat of the twentieth century, even more so than electronics and the Internet.2 Typical drinking water infra-
1.1. Unsafe Water in Developing Rural Areas
The ongoing battle between humans and pathogenic microorganisms has a legacy dating from the earliest days of human civilization. During the past century, humans have achieved unprecedented victories by drastically reducing waterborne infections through advances in drinking water treatment infrastructure. The impact on human health was immediate and prominent: the average American lifespan increased from © XXXX American Chemical Society
Received: November 16, 2018
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DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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complex, and cost-prohibitive infrastructure needed for centralized water treatment and distribution. Immediate consumption of treated water also obviates the risk of pathogen regrowth,6 a challenge in regions that lack the means to safely store treated water. Several POU technologies have been developed, some derived from ancient approaches, and others that miniaturize and simplify more complex treatment processes used at large scale. Their actual implementation, despite sporadic successes, has faced challenges unique to each technology. Boiling water, which has been practiced for over 6000 years,2 is still the most widely used POU disinfection method today; however, heating water is energy-inefficient (high heat capacity of water), labor-intensive (for collection of fuel such as wood), and destructive to environment (i.e., burning an average of 1.7 pounds of wood per day for a family of four9). Solar disinfection (SODIS), the practice of exposing water contained in clear plastic bottles to sunlight, requires little capital investment and no supply of energy or chemicals since it uses solar irradiation as a disinfecting agent. Yet its widespread implementation is hampered by its low disinfection efficiency, requiring excessively long treatment times (>6 h on sunny days10) to ensure disinfection. Chlorination (e.g., adding chlorine tablets) has proven efficient at inactivating most pathogens (e.g., provided water is clear, free chlorine can inactivate 99.99% of enteric bacteria and viruses at doses of a few mg/L for ∼30 min treatment time).7 However, the disinfection efficiency of chlorination dramatically decreases with increasing turbidity and natural organic matter (NOM) content in the source water, while formation of toxic chlorinated byproducts is a problem that cannot be easily resolved.9 Additionally, residual chlorine can leave an unpleasant odor and taste that reduces user acceptance.7 Filtration using porous ceramic pot filters or sand filters has a higher rate of user acceptance, although filtration exhibits low removal rates for small pathogens (e.g., viruses) and has a high risk of recontamination if the filters are not well maintained.7 In pursuit of future POU technologies that can overcome these challenges, we recognize that developing rural areas with limited access to basic drinking water services largely overlap with areas of high surface sunlight intensity throughout the entire year (Figure 2a). Among the 130 countries with data available for both surface solar irradiation and rural access to basic drinking water services, 64 countries with less than 80% rural access to basic drinking water services have significantly greater solar irradiation (5.59 kWh/m2/day) than the global average (4.70 kWh/m2/day; Figure 2b). This correlation highlights an untapped opportunity to exploit solar irradiation as a major source of energy for water disinfection. Innovative materials and processes that can effectively harness sunlight and inactivate pathogens in water are key to surpassing the limitations of the conventional SODIS approach. In this article, we discuss select material-based POU strategies that have emerged over the past decade with the goal of better equipping rural populations in the fight against waterborne pathogens by using the free and endless supply of sunlight.
structure consists of centralized water treatment plants and large underground networks of pipes for distributing water. This centralized water treatment scheme has been particularly cost-effective in densely populated urban areas. Unfortunately, as of 2015, over 800 million people have not yet experienced this victory and still lack access to basic drinking water services.5 Approximately 81% of those who do not have access to basic drinking water service live in rural areas.5 This disparity exists worldwide but is more pronounced in certain regions such as sub-Saharan Africa, where 57% of the rural population lacks a basic drinking water service, as compared to 17% among the urban population (Figure 1). In
Figure 1. Worldwide access to basic drinking water service (i.e., improved sources of drinking water that required no more than 30 min per trip to collect water) and infrastructure (indicated as access to electricity) in urban and rural areas. Data for access to electricity and access to basic drinking water service were adopted from World Bank65 and WHO,5 respectively (Table S1).
these rural communities, waterborne diseases (e.g., gastroenteritis, typhoid, encephalitis, meningitis, and hepatitis) remain among the most significant causes of morbidity and mortality, with waterborne gastroenteritis alone responsible for over 480 000 deaths each year.3,6 This failure to protect the public health of rural populations appears to be related to a lack of basic infrastructure such as an electrical grid (Figure 1). Such infrastructure is essential for the construction and operation of the centralized water treatment and distribution systems that have been a prominent strategy in the battle against infectious disease. The problem, however, demands a more urgent tactical adjustment than merely hoping for general economic growth; innovative strategies that are low cost, simple to implement and maintain, and require minimal infrastructure are central to meeting the demand for improved drinking water in developing rural communities.
2. ADVANCES IN SOLAR TECHNOLOGIES FOR WATER DISINFECTION
1.2. Toward Water Treatment at Point of Use
Household- or community-scale water treatment at the pointof-use (POU) is one of the most promising intervention strategies that can immediately benefit the most vulnerable population in rural areas.6−8 It does not require the large,
2.1. Solar Photothermal Disinfection
Relatively effective against all major pathogens of concern (Figure 3),11,12 heat treatment induces the irreversible B
DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. (a) Coincidence of solar irradiation with deceased access to basic drinking water services. Global horizontal irradiance (GHI, kWh/m2/ day, color gradient scale) of sunlight demonstrates regions where solar-based POU interventions have the greatest potential. The patterns of percentage access to basic drinking water services among rural populations highlights regions where POU interventions are needed. The juxtaposition of these two parameters underlines the potential of using sunlight as a resource for increasing drinking water access for rural populations. GHI data at 30 arcsecond resolution between 60°N and 45°S were taken from the Global Solar Atlas. Latitudes outside this range (continents with white background) are not available due to satellite imagery accuracy limitations. The map was created using Esri ArcMap software. Map images are the intellectual property of Esri and used herein with permission. Copyright 2019 Esri and its licensors. All rights reserved. (b) Worldwide access to basic drinking water service and solar irradiation (GHI) in urban and rural areas. Data on access to basic drinking water services and solar irradiation were taken from the WHO3 and World Bank,4 respectively.
that induce multiple scattering events, increasing the probability of photon absorption and concentrating light to a small spatial domain. These materials can enable intense localized heating without heating the entire water volume(Figure 4a).14 The highly absorptive properties required for efficient sunlight-to-heat conversion can be achieved in metallic nanoparticles through surface plasmon resonance (SPR). High absorption cross-sections are achieved through a resonant amplification of the charge-density oscillations at
denaturation, coagulation, and breakdown of proteins and genomes, causing structural damage and interfering with vital metabolic functions,12 and in the case of viruses, disrupting their ability to recognize and bind to host cells.13 Heating bulk water by simply absorbing sunlight to the required temperature for efficient disinfection (>55 °C for many bacterial pathogens11) is nearly impossible due to poor absorption of the solar spectrum by water as well as its high heat capacity. Recently, this challenge has been addressed through the use of photothermal nanomaterials: strongly light-absorbing particles C
DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. Schematic illustration of the structure and components of the three classes of pathogens that are primarily associated with drinking waterrelated diseases. The disinfectants that each microbial components is vulnerable to during the inactivation processes are listed below each microbial component.
Figure 4. Solar-driven POU disinfection techniques that generate various disinfectant species.
has recently demonstrated for the first time that localized heating using nanoparticles can be exploited for the direct photothermal inactivation of bacteria and viruses in water.19 Current research is at an embryonic stage: the inactivation kinetics remains slow, the heterogeneous temperature gradients are poorly defined, and more research is needed to optimize materials and demonstrate effectiveness in larger flow-through scale systems. Notable broadband absorbing materials such as carbon black are cost-effective and can be sourced through the pyrolysis of biomass, while in comparison, prototypical SPR materials such as Au nanoparticles are cost-
the nanoparticle surface, yielding local electric-field enhancement.15 Alternatively, blackbody-like materials, including some semiconductors and carbon materials, can be used to absorb a large number of photons across all solar wavelengths.16 In an early demonstration of the localized heating effect, steam was generated in seconds when suspensions of SiO2-coated Au nanoparticles or carbon black were irradiated with sunlight, leaving the bulk water temperature relatively unchanged.17 Nanoparticle-enhanced rapid steam generation has since been employed in the fabrication of solar photothermal devices such as solar evaporator for clean water production.16,18 Our group D
DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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hindered by present high costs and low outputs (i.e., low external quantum efficiency). For instance, several chips must be used in conjunction to achieve sufficient light intensity, which is vital for both timely disinfection and suppression of microbial DNA repair and regrowth. Additionally, radiation patterns emitted by LEDs are less spatially uniform compared to mercury lamps, creating reactor design and engineering challenges that must be overcome to optimize LED-based POU water disinfection devices.
prohibitive and far from ideal for applications in developing rural regions. Recently developed plasmonic Al nanoparticles18 show promise as an appealing cost-effective substitution. 2.2. Germicidal UV Irradiation
Disinfection of drinking water using germicidal UV (UV−C) light became popular in the 1990s because it produces virtually no disinfection byproducts (e.g., chlorinated organic compounds) and is effective against protozoan cysts that are not easily inactivated by conventional disinfectants such as chlorine. UV irradiation specifically targets the genome of bacteria,20 viruses,21 and protozoa (Figure 3).12 Induced lesions, which are mainly formed by the photochemical formation of cis−syn cyclobutane pyrimidine dimers, inhibit the replication of genomes,12 cause DNA intrastrand and nucleic acid−protein cross-links,12 and, in the case of viruses, inhibit genome injection through modification of related proteins.21 However, UV-based disinfection is frequently complicated by various engineering challenges (e.g., influence of water quality parameters such as turbidity on UV delivery) as well as microbial regrowth after the treatment due to DNA repair in the case of insufficient UV dosage).12 The terrestrial sunlight spectrum does not contain UV−C wavelengths that overlap with the absorption maximum for genomic materials, typically peaking at 250−270 nm. For the majority of solar wavelengths to achieve radiation based disinfection, low-energy, nongermicidal photons must be converted to high-energy, germicidal photons (i.e., photon upconversion22). In a novel approach enabling UV disinfection using only visible light photons, we realized direct conversion of visible light to shorter wavelengths using lanthanide-doped inorganic upconversion materials (e.g., Y2SiO5:Pr,Gd,Li and Lu7O5F9:Pr3+),23−25 which absorb two or more photons of lower energy and spontaneously emit a photon at higher energy (Figure 4b). Viability for practical application, however, remains in question as the quantum yield of current materials is prohibitively low.24,26 Higher efficiency materials are needed to achieve discernible levels of inactivation in water with concentrated sunlight and surpass the kinetics of spontaneous, light-activated DNA repair.27 Converting sunlight to electricity using photovoltaic cells, and producing germicidal UV using electricity-powered lamps, are presently more practical approaches than the aforementioned photoluminescent upconversion approach. Gas-discharge lamps (e.g., lamps filled with low-pressure mercury vapor) have proven efficient for inactivating all categories of pathogens and are the current industry standard.28 Nevertheless, challenges for their application in developing rural regions include the required energy inputs and maintenance needs. For POU applications, solid-state light emitting diodes (LEDs) present emerging opportunities for safer, more durable, and more energy-efficient UV disinfection (Figure 4b).29,30 An LED chip fabricated using the In−Al−Ga-N materials system produces monochromatic radiation corresponding to the material bandgap, which can be readily tailored by adjusting the Al fraction.31 The wavelength tunability of LEDs is particularly beneficial, compared to the fixed emission spectrum of mercury-filled lamps, because overlap between the lamp emission spectrum and the absorption spectrum of DNA/RNA (or the germicidal action spectra, if known) can be maximized through the selection and array combination of UV-LEDs with different peak emission wavelengths.32 The application of UV-LEDs, however, is
2.3. Photogenerated Strong Oxidants
Strong, nonselective chemical oxidants such as hydroxyl radicals (•OH) can lead to indiscriminate oxidative destruction of microbial surface components. For bacteria and protozoa, the dominant cause of microbial death by •OH is known to be cell membrane damage through lipid oxidation,33−35 accompanied by the loss of inner proteins, DNA, polysaccharides, and ions (Figure 3).12,20 Lipid peroxidation, involving propagation of delocalized peroxide radicals from •OH attack on lipids, occurs readily in eukaryotes as their membranes contain polyunsaturated lipids, and protozoa likely follow similar mechanisms.34 This process is less probable for bacteria, as bacterial lipids mostly contain saturated and monounsaturated fatty acids, which form more localized radical intermediates that are unable to propagate the chain reaction.34,36 Instead, cell membrane damage is more likely to occur through direct lipid oxidation by •OH, disrupting vital cellular activities including compounds transportation, enzymatic reactions, and cell signaling.11 The strong reactivity of • OH with cell membranes also results in strong •OH quenching, which in turn limits •OH transport to the cytoplasm and subsequent damage of intracellular components. However, an influx of mild oxidants (e.g., lipid radicals or H2O2) may occur through the •OH-induced damaged surface sites and lead to intracellular oxidation. Viruses are less susceptible to oxidative inactivation compared with bacteria due to fewer target oxidation sites (e.g., enzymes).33 Inactivation of viruses appears to occur primarily through the denaturation of capsid proteins, which are mostly responsible for virus structural integrity and attachment to host cells.12,37 Compared to nonenveloped viruses, enveloped viruses are more susceptible to chemical disinfectants, likely due to damage of envelope lipids and proteins. Since •OH is short-lived, it is produced on site in typical advanced oxidation processes (AOPs) by UV-activation of H2O2 or O3.20,38 Translating these technologies established in large scale plants to POU application in developing rural regions is challenging given the high energy and chemical demands. In marked contrast, photocatalytic processes utilizing sunlight can achieve essentially the same goal without the continuous dosing of chemicals.38 In photocatalysis, semiconductors absorb photons with energy higher than their band gaps to generate photoexcited electrons and holes, which subsequently produce •OH from water and oxygen (Figure 4c). Our group has adopted TiO2 as a benchmark photocatalyst for its low cost, robustness, and high •OH yield, and demonstrated its effectiveness for inactivating multiple pathogens.37,39 Nevertheless, TiO2 is a poor absorber in the visible light range due to its relatively large bandgap and therefore not ideal for solar applications. Efforts to develop visible light photocatalysts have increased substantially over the past decade, but photocatalysts other than TiO2 have not yet seen widespread implementation in water treatment because E
DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research their incremental enhancements in solar-to-•OH conversion efficiency cannot offset the low cost and physiochemical robustness of TiO2.38,40 Another critical hurdle for •OH-based disinfection is the scavenging of •OH by NOM present in source waters, which may greatly diminish the effectiveness of photocatalytic disinfection in complex water matrices.41 Alternatively, solar energy can be harvested to photocatalytically synthesize H2O2, a precursor that can be stored and later activated to produce •OH. This approach overcomes a default limitation of solar-based water treatment technologies, operating only during hours of sunlight. Catalytic selectivity (e.g., reduction of O2 to H2O2 instead of H2O) has been a key challenge in meeting this goal. In our recent publications,42−44 anthraquinone molecules were anchored to the surface of semiconductors such as C3N4 to exploit anthraquinone’s exceptional selectivity toward photocatalytic H2O2 production (i.e., selective reductive hydrogenation of O2, the foundation of current benchmark high-pressure industrial processes).42 Stored H2O2 can be readily activated to generate •OH without electricity by using heterogeneous Fenton catalysts that enable the Fe2+/Fe3+ redox cycle on the catalyst surface.45 Both steps can be enabled in small modular reactors tailored for off-grid POU treatment, but these reactors have yet to be evaluated beyond lab-scale under realistic conditions.
fusion with the host cell.52 In contrast, the disinfection mechanism of nonenveloped viruses by 1O2 is reliant on genomic and capsid damage. Nonenveloped viruses are generally less vulnerable to 1O2 due to substantial speciesdependent variation in genomic and protein composition that exhibit disparate 1O2 susceptibility,53 influencing the impairment of viral functions, including genome replication, host binding, and genome injection.21,54 For instance, RNA is more susceptible to 1O2 than DNA, and single-stranded (ss) nucleic acids are more vulnerable than double-stranded (ds) nucleic acids, leading to a general phage-susceptibility trend in genomic material: ssRNA > ssDNA > dsRNA > dsDNA. To seize the advantageous properties of 1O2, our group has been developing sunlight-driven photosensitization materials for POU disinfection applications (Figure 4d). Synthetic photosensitizers such as fullerene derivatives attracted our initial attention due to their resilience to photobleaching (loss of pigmentation upon oxidation of photosensitizer), high 1O2 yield, and large extinction coefficients in the visible spectrum.55,56 Their disinfection kinetics rival that of TiO2 for Escherichia coli57 and significantly outperform TiO2 for bacteriophage MS2 inactivation under sunlight.58 However, these photosensitizers must be removed from treated water prior to human consumption due to health and cost concerns, leading to the need for substrate-immobilized photosensitizers at the sacrifice of performance.59−61 This challenge has been addressed by employing photosensitizers that are edible. We have recently demonstrated the effectiveness of riboflavin (Vitamin B2) and erythrosine (an FDA-approved food dye) for rapid inactivation of viruses in a complex and turbid water matrix, at a cost comparable to current POU interventions.49 While these edible photosensitizers photobleach, the accompanying distinct color change (e.g., from erythrosine red to transparent) occurs at a rate comparable to the disinfection, providing a safety indication that disinfection is completed, a much-needed function lacking in other POU technologies.49 Ongoing studies are attempting to establish disinfection solutions derived from locally available resources (e.g., natural photosensitizers in plants and fruits) to further improve the accessibility of photosensitization materials in the rural developing world.
2.4. Photogenerated Mild Oxidants
In contrast to strong oxidants, mild oxidants, predominantly singlet oxygen (1O2), exhibit lower but more selective reactivity, especially toward biomolecules including purine and pyrimidine bases, amino acids containing aromatic or sulfur functionalities, and unsaturated fatty acids and steroids.46 This makes DNA/RNA, proteins, and lipid membranes targets of 1O2-oxidation. The selective nature of 1 O2 means it is less hindered by constituents in complex water matrices compared to nonselective ROS such as •OH,41,47 as reflected by its relatively long lifetime (∼4 μs) and its ability to oxidize targets distant from the site of generation in aqueous systems.48 This property is particularly beneficial when disinfecting water with poor initial quality (e.g., turbidity, NOM), where POU technologies based on strong oxidants lose effectiveness.41,49 1O2 can be photochemically generated through a photosensitization process: the photosensitizer in the singlet ground-state is first promoted to the singlet excitedstate upon light absorption, then it undergoes intersystem crossing to create the triplet excited-state, and then finally interacting with triplet ground-state oxygen through a Dexter energy transfer to yield 1O2 (Figure 4d).50 Bacterial inactivation by exogenous 1O2 has been related to the oxidation of the inner cellular membrane and subsequent disruption of cell integrity (Figure 3).46 In Gram-positive bacteria, the large pores in the peptidoglycan layer likely allow passage of photosensitizers permitting localized photosensitization near the inner membrane.46 In contrast, the outer membrane of Gram-negative bacteria is highly selective and complex, providing protection from exogenous photosensitization.46,50 Consequently, electrostatically attracting cationic photosensitizers to the negatively charged surface and generating 1O2 in close proximity to the cellular membrane has been found to be particularly effective in inactivating Gram-negative bacteria.51 1O2 is a broadly potent disinfectant against enveloped viruses, where 1O2 reacts with the lipid envelope, decreasing membrane fluidity and increasing the energy required for host binding, thus inhibiting membrane
3. CONCLUSION AND OUTLOOK Significant opportunities exist in developing breakthrough POU disinfection techniques for developing rural regions that lack sufficient water treatment and distribution infrastructure. The research presented here focuses on sunlight-assisted POU techniques since these regions often receive high surface solar irradiation. It should be noted that different solar POU disinfection approaches come with not only uniquely advantageous features, but also with distinctive challenges that must be resolved through further research. We do not envision a single most effective solution that can solve all present challenges, especially considering the nature of water and types of pathogens in the water vary both spatially and temporally. It is difficult to compare the effectiveness of different approaches because their performance is highly dependent on the types of water to be treated.26 For instance, the performance of a disinfection technique using localized heat (photothermal) or mild oxidants (1O2) is likely less affected by source water quality variations than that of a disinfection technique relying on UV or strong oxidants ( • OH). In addition, the susceptibility of various pathogen species to disparate F
DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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capital input (Figure 5), where the solar-powered techniques could be integrated for energy-efficient disinfection. Rapid advances in materials and processes specifically designed for solar-powered water disinfection are bringing these much-needed POU technologies closer to fruition. Ideal solar POU disinfection technologies will likely be based on unconventional approaches because the obstacles in rural developing regions are often different from the challenges that have been well-addressed with centralized water treatment in populous urban areas. These technologies also need to demonstrate characteristics that receive less attention in other application scenarios such as cost-effectiveness, low maintenance-requirements, and ease of distribution. Once successfully developed and implemented, these solar POU technologies will help humankind claim final victory in the ancient battle against pathogens in drinking water. We note that victory is not yet assured since the threat on human life by waterborne pathogens is still very real for millions of people around the world.
disinfectants (i.e., heat, UV, and oxidants) differs significantly. The local economy, disinfectant-availability logistics, existing infrastructure, and public acceptance are additional compounding factors that diverge depending on location and culture. We expect that, for real-world implementation, these POU disinfection technologies will most likely be integrated with other POU techniques, such as media filters, that provide some level of pretreatment. More than one disinfection technique could be employed, like the multibarrier approach widely adopted in urban water treatment (e.g., strong disinfectant treatment followed by milder residual disinfectant treatment).20,62 This highlights the need to develop multiple technologies for inclusion into the POU disinfection toolbox. These techniques must also be scaled up with appropriate reactor design and process engineering considerations (Figure 5). The simplest POU technique can be implemented at the
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00578.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chiheng Chu: 0000-0001-9493-9120 Eric C. Ryberg: 0000-0001-9390-951X Jae-Hong Kim: 0000-0003-2224-3516 Figure 5. Advancing solar-powered disinfection techniques from a single water bottle scale to small community scale. Facilities with different scales of infrastructure demands and water treatment capacities can be deployed, contingent on the economic situation.
Notes
The authors declare no competing financial interest. Biographies Chiheng Chu is currently postdoctoral associate at Yale University. He received his B.S. degree from Peking University, M.E. from University of Tokyo, and Ph.D. from ETH Zürich. His research interests focus on photochemical and electrochemical processes in natural and engineered water systems.
scale of a single water bottle, for example, the currently practiced SODIS technique augmented with edible photosensitizers.49 However, meeting the daily household water demand of 20 L by using tens of bottles, treated in small batches, is a cumbersome task.8,63 More advanced devices, such as parabolic solar collectors, can treat larger amounts of water to supply a household in a continuous fashion. Upscaling to a flow-through roof-top system with longer sunlight exposure time can potentially produce a sufficient amount of disinfected water for a small community. In these systems, photothermal or photocatalytic nanoparticles can significantly enhance solar disinfection performance. These nanoparticles must be immobilized onto the reactor inner surface or transparent packing media, as recently demonstrated in labscale photothermal disinfection, so that they do not leach out into the product water and cause adverse impacts on human health.64 Alternatively, edible photosensitizers can be employed to provide additional safety. In the long term, with continued economic growth in developing rural areas, we expect a transition from POU techniques to community-scale water treatment and distribution that requires relatively higher
Eric C. Ryberg is currently a Ph.D. candidate at Yale University. He pursued his B.S. in Chemistry and Earth and Planetary Sciences at the Johns Hopkins University. Focusing on edible photosensitizers for water treatment, his research interests span environmental photochemistry and public health impacts in the developing world. Stephanie K. Loeb is currently a Ph.D. candidate at Yale University. She received her B.S. in physics jointly from the University of Toronto and the National University of Singapore, as well as a M.ASc. in Environmental Engineering from the University of Toronto. Her research interests focus on photonic and fluorescent materials for environmental remediation and sensing. Min-Jeong Suh is a Ph.D. candidate in the Department of Chemical and Environmental Engineering at Yale University researching adsorption-photocatalysis composite nanomaterials for water treatment. She received her M.Chem. in Chemistry from the University of Oxford. G
DOI: 10.1021/acs.accounts.8b00578 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Jae-Hong Kim is currently Henry P. Becton Sr. Professor of Engineering and Department Chair of Chemical and Environmental Engineering at Yale University. Kim received his B.S. and M.S. degrees in Chemical and Biological Engineering from Seoul National University in Korea in 1995 and 1997, respectively, and a Ph.D. degree in Environmental Engineering from the University of Illinois at Urbana−Champaign in 2002.
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ACKNOWLEDGMENTS This work was partially supported by National Science Foundation Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC-1449500).
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