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Review pubs.acs.org/journal/estlcu
Beyond the Pipeline: Assessing the Efficiency Limits of Advanced Technologies for Solar Water Disinfection Stephanie Loeb,† Ron Hofmann,‡ and Jae-Hong Kim*,† †
Department of Chemical and Environmental Engineering and Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06511, United States ‡ Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, Ontario, Canada M5S 1A4 S Supporting Information *
ABSTRACT: This critical review analyzes and compares the efficiency of select technologies that harness solar energy for point-of-use water disinfection, including photocatalysts, photosensitizing chromophores, UVC light-emitting diodes, and visible-to-UVC upconversion phosphors. The volume rate of water that each material can treat to achieve 99% inactivation of model microorganisms, given the same sunlight exposure, was estimated on the basis of literature data and theoretical predictions, in the context of both currently reported efficiencies and theoretical thermodynamic maximum efficiencies. Each material is further critiqued in terms of the spectral match with sunlight, quantum efficiency, and the relative strength of the resulting disinfecting agent such as hydroxyl radicals, singlet oxygen, and UVC radiation. This review emphasizes critical needs for disinfection strategies that can efficiently inactivate more than one type of microorganism. In addition, the approach described herein can guide future research in efforts to identify more efficient materials and technologies for capturing sunlight for water disinfection.
1. INTRODUCTION The World Health Organization (WHO) declared 2005−2015 the decade of water, with the goal of establishing a framework for providing access to safe drinking water for all people. Despite this, more than 660 million people remain unserved, with 530 million of them living in rural areas.1 The UN Millennium Development Goal (MDG) to halve the portion of the population without access to an improved water source was achieved in 2015.1 However, many sources categorized as improved were found to remain contaminated, indicating that sources that are “simply improved” without proper treatment, and most importantly disinfection, are not sufficient.2,3 Disinfected water is commonly achieved through a centralized treatment plant and piped distribution system, but this type of treatment system is often ineffective for rural or isolated communities, as demonstrated by persistent water quality problems in Sub-Saharan Africa4 and Native Reservations in North America.5,6 Just as we are seeing cellular technologies bridge the urban−rural gap for truly globalized communications networks without the need for connecting wires, we can focus on looking beyond the pipeline to see how advanced materialbased technologies for point-of-use (POU) water treatment can provide safe water in less developed rural communities. Energy is needed to operate most water disinfection processes, but regions that lack access to clean water often © 2016 American Chemical Society
lack energy infrastructure. Decentralized disinfection systems can be powered through photovoltaics (PV) that convert solar energy into electricity. Economical PV technology is now mature, with research-phase PVs surpassing the Shockely−Queisser limit,7,8 reaching efficiencies near 40%.9,10 Alternatively, solar disinfection (SODIS) works by simply exposing water to sunlight, directly exploiting the biocidal action of higher-energy solar wavelengths.11 Under typical weather conditions, however, biocidal radiation makes up only a small portion of the solar spectrum. In an effort to better harness solar energy, materials that photochemically produce reactive oxygen species (ROS), without the need for intermediate production of electricity through PVs, have been explored for disinfection.12 What are the existing and emerging material-based technologies that harness solar energy to disinfect water? How do the treatment capacities of these technologies compare to each other? Is there a significant advantage gained when the solar-toelectricity photovoltaic conversion step is obviated? To answer these questions, we review select solar-driven disinfection technologies that employ hydroxyl radical (•OH)-producing Received: Revised: Accepted: Published: 73
January 21, 2016 February 20, 2016 February 22, 2016 February 25, 2016 DOI: 10.1021/acs.estlett.6b00023 Environ. Sci. Technol. Lett. 2016, 3, 73−80
Review
Environmental Science & Technology Letters
Figure 1. Model concept flow diagram. Incoming solar radiation is derived from AM1.5 spectra, with a total integrated energy flux of 1003 W/m2 and a total photon flux of 3.9 × 1021 photons m−2 s−1.13 By assuming the absorber is a blackbody and that the reactor has no mass transfer limitations, one can determine the number of usable photons in the solar spectrum for each disinfection method. These photons are then propagated through the appropriate series of energy conversions to predict the rate at which water can be treated per square meter of sunlight-exposed surface (liters per square meter per second). The empirical constant used for evaluating disinfection by ROS is the contact time multiplied by the disinfectant concentration (CT; milligrams per minute per liter) for the dominant species; for UVC inactivation, the empirical constant is the UV dose (millijoules per square centimeter).
photocatalysts, singlet-oxygen (1O2)-producing photosensitizers, PV-powered UVC light-emitting diodes (LEDs), and visible-toUVC upconversion phosphors. We herein estimate and compare the efficiencies of current materials, as well as their theoretical limits, based on fundamentals of photon absorption/emission, charge separation, and energy transfer using the empirical inactivation kinetics for three representative microorganisms.
energy greater than the absorption threshold (band gap of semiconductors or HOMO−LUMO gap of chromophores) can be absorbed. Accordingly, the absorption profile of a photocatalyst or solar cell (manufactured from semiconductors) is a step function, rising from zero to unity at wavelengths more energetic than the band gap. Relative to those of semiconductors, chromophores and phosphors have more complex absorption profiles with characteristic peaks due to their molecular electronic structure, meaning some photons with energy above the threshold also may fail to be absorbed. The closeness of a material’s absorption profile to the solar spectrum defines its spectral match. The number of usable photons available in the solar spectrum for a given photoactive material (Np) is determined by summing the photon flux Φp(λ) across the absorption profile of the material A(λ) for photons with energy equal to or greater than the energetic limit (EL) required to form the resulting disinfection agent:
2. METHOD OVERVIEW A schematic overview of the model developed for comparative study is presented in Figure 1. In brief, the model calculates the production rate of water (liters per second) that can be attained by a material-based treatment system that captures sunlight over 1 m2 of area at the earth’s surface to achieve 99% inactivation of three representative microorganisms. By this process, knowledge of the absorbance function and energy transfer efficiency allows for the prediction of a normalized solar water treatment capacity (liters per square meter per second). Calculations following the scheme depicted in Figure 1 were performed for photoactive materials available at the current mass-produced scale of industry and state-of-the-art researchphase materials. Four types of materials-based treatment systems were considered: photocatalysts, photosensitizers, PV-powered UVC LEDs, and upconversion phosphors. An evaluation of conventional SODIS is included as a baseline. All materials considered are detailed in Tables S1−S6 of the Supporting Information. Furthermore, by defining a hypothetical ideal photoactive material for each treatment system, the model can predict a theoretical maximum treatment capacity. This analysis focuses on the effect of the photoactive material on water treatment performance, disregarding some practical and socioeconomic issues. When these techniques are translated into real practice, the importance of process design and operation engineering cannot be ignored. The model’s primary input is wavelength-dependent photon flux from sunlight onto a 1 m2 control surface at sea level using midlatitude yearly average 1.5 atmospheric mass (AM1.5) spectral data from ASTM (Figure S1).13 A photoactive material spanning this control surface is assumed to be exposed to all incoming radiation without shadowing; i.e., arbitrary reactor geometry allows the material to access all energy radiated in 1 m2 of planar solar flux. However, not all accessible photons can be used to drive disinfection, because only photons with
0
Np =
∫EL Φp(λ) × A(λ) dλ
(1)
For example, a hypothetical ideal material absorbs 100% of incident photons (blackbody) in the range defined by its absorption profile, causing the amplitudes to align with the solar spectrum for these wavelengths when expressed in terms of photon flux. However, when energy flux is considered, the number of photons captured is multiplied by the energy of the species immediately produced (ROS, electrical current, or UVC photon) and excess energy is lost. This method is an adaptation from ref 7, the first to produce such a spectrum for silicon PVs. We then converted the number of usable photons into the amount of disinfecting agent produced in each treatment system, •OH, 1O2, or UVC radiation, by following a procedure unique to each material (detailed in the following sections). Inactivation of model microorganisms was estimated on the basis of empirical data for the product of contact time with disinfectant concentration (CT; milligrams per minute per liter) or UV dose (fluence; millijoules per square centimeter) obtained from the literature. We selected three microorganisms, Cryptosporidium parvum oocysts, Escherichia coli bacteria, and MS2 bacteriophage, on the basis of the WHO guidelines for evaluating household water treatment options to serve as indicators for the diversity of organisms responsible for waterborne diseases,14−17 and on the basis of the availability of 74
DOI: 10.1021/acs.estlett.6b00023 Environ. Sci. Technol. Lett. 2016, 3, 73−80
Review
Environmental Science & Technology Letters inactivation kinetic data in the literature for the disinfectants being evaluated. Water is assumed to be ideally mixed such that disinfection reactions have no mass transfer limitations. Considering that natural organic matter (NOM) is the major contributor to UV attenuation and ROS scavenging, the model assumes a NOM concentration of 10−5 M, which is in the range of clear surface water, or tap water,18 and assumes an arbitrary but typical UV-254 nm extinction coefficient and ROS reactivity (Table S7) based on the literature.18−21
Our approach to relate sunlight exposure to production of OH, a well-established disinfecting agent,48,50,51 involves first summing the total solar photon flux from the highest energy available at the Earth’s surface (taken to be 280 nm in this study) to the wavelength corresponding to the specific photocatalyst’s band gap. Once the total photon flux between 280 nm and the band gap is determined, a quantum efficiency (QE, ranging from 0 to 1.0) is applied to account for absorbed photons that will not yield a •OH radical due to futile internal recombination. Therefore, the photon flux, together with the QE, allows the net rate of •OH production to be determined. The steadystate concentration of •OH available for disinfection is then revised assuming that NOM is the major radical scavenger (eqs S6 and S7).47,52,53 The steady-state •OH concentration and its corresponding CT value48,54−56 are used in eqs S11 and 12 to determine the disinfection capacity of a given photocatalyst, followed by the rate of water treatment that can be achieved per square meter of sunlight exposure. The rate of water treatment by photocatalysis is calculated for three circumstances assuming values for band gaps and QEs that are representative of current industry standards, state-of-the-art research, and an estimated theoretical maximum. We assumed (1) industry performance is represented by combined rutile-anatase TiO2 with a band gap of 400 nm and a QE of 3%,57 (2) research performance is represented by modified black-TiO2 with a band gap of 530 nm and a QE of 1%,58,59 and (3) the theoretical maximum performance is a hypothetical semiconductor with a band gap of 539 nm (Figure 2a) and a QE of 100%. •
3. SOLAR WATER DISINFECTION TECHNOLOGIES Established Technologies. Many technologies for capturing sunlight for water treatment exist. PV-powered technologies that mimic compact versions of full scale processes, such as electrochemical chlorine generation22 or UVC disinfection using mercury-filled lamps,23−25 have limitations: chlorine cannot be produced without salt,22 and UVC lamps are expensive and fragile.23−25 Particularly conducive to remote, infrastructuredeficient areas, SODIS involves exposing water to direct sunlight for a minimum of 6 h. Microbial inactivation results from exposure to UVA radiation (320−400 nm), which comprises ∼8% of AM1.5 radiation on a clear day, with a minor, often negligible, contribution from heating.11,26,27 Inactivation doses for SODIS in PET bottles are widely reported in the literature for E. coli28−31 and less extensively for C. parvum oocysts32 and MS2.30,33 Particularly for MS2, reported doses vary widely and are known to depend on organic matter and dissolved oxygen concentrations, which encourage the production of ROS under UVA radiation. Although lauded for its economy and simplicity, SODIS is compromised by unpredictable radiation intensity, and long required exposure times.27 Both the amount and the proportion of UV radiation in the spectrum depend on latitude and cloud coverage due to the preferential scattering of shorter wavelength light. For more detailed discussions comparing SODIS along with other currently employed POU treatment systems, the reader is referred to refs 34−36. Throughout this review, SODIS is used as a representative conventional technique for comparison. It is assumed that SODIS is conducted in 2 L PET bottles (∼10 cm depth), and the effect of synergistic heating is neglected. Performance is calculated on the basis of cumulative UVA radiation (Figure S2) and the response of microorganisms to UVA reported in the literature (Table S1) by employing eqs S3 and S4. Selecting the smallest reported dose, under clear AM1.5 conditions, SODIS is predicted to treat up to 0.057 L m−2 s−1 for 2-log inactivation of E. coli, 0.0016 L m−2 s−1 for C. parvum, and 0.022 L m−2 s−1 for MS2. Innovative augmentations to SODIS would improve the performance beyond what is reported here. Current research continues to improve our understanding of solar-induced mechanisms of inactivation,33,37−40 to develop containers with improved UVB transmissivity,29,30,41 and to develop low-cost, locally sourced chemical additives to improve inactivation rates.30,42−45 Photocatalysis. TiO2 and other semiconductors with band gaps in the UVA and visible range can absorb solar photons with energy equal to or greater than the band gap to photochemically produce conduction band electron (e−) and valence band hole (h+) pairs. In materials with appropriate band edge potentials, • OH is formed as the dominant ROS through oxidation of OH− by h+ or reduction of O2 by e−, as shown in Scheme S1.46−48 The latter reductive pathway is known to have a minimal contribution to gross •OH generation.46 The band gap energy must be equal to or greater than the OH−/O2 redox potential of E° = 2.3 eV (at pH 7) (i.e., up to 539 nm) to generate •OH.49,50
Figure 2. Total energy available at each wavelength for the (a) photocatalytic production of •OH using an ideal material with an Eg of 2.3 eV, (b) photosensitizer production of 1O2 using an ideal material with an infinite bandwidth, (c) photovoltaic production of electricity using a silicon solar cell with an Eg of 1.1 eV, and (d) inorganic visible to UVC upconversion using an ideal material with a 100 nm bandwidth shown in color. Solar AM1.5 is shown in gray for reference. Available energy calculated using eqs S5, S8, and S17.
The predicted water treatment rates for these three photocatalysts are shown in Figure 3. Under ideal conditions, photocatalysts can be predicted to be able to treat (for 2-log inactivation) up to 30 and 10 L of water m−2 s−1 for E. coli and MS2 bacteriophage, respectively, but only 3 L m−2 s−1 of the more resistant Cryptosporidium. While this is an excellent theoretical maximum treatment rate, in current practice, actual rates are
