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Photocatalytic Water Treatment: So Where Are We Going with This? Ezra L. Cates* Department of Environmental Engineering and Earth Sciences, Clemson University, Anderson, South Carolina 29625, United States potable reuse sector and associated needs for advanced oxidation processes (AOPs), progress on this front needs only to exceed the cost-effectiveness of existing H2O2 and ozone technologies. I believe this is the most promising direction; however, academics often label photocatalytic AOPs as energy-saving, due to their catalytic nature, which is misleading. The Purifics Photo-Cat,2 for example, is the sole PWT packaged unit that has seen legitimate commercialization for small-scale applications. While it is an impressive design, Benotti et al. found that treatment efficiencies of the system for 1-log reduction of many representative contaminants were in the approximate range of 1−3 kWh/m3 when considering the energy consumption by the UV lamps.2 Even this energy requirement places the process on par with seawater reverse osmosis desalination. Moreover, when pumping and catalyst recovery via crossflow ultrafiltration were factored into the Photo-Cat study, the values for total energy consumption rates increased 4-fold. In fact, efficiency improved greatly if the TiO2 in the unit was simply replaced with H2O2 dosing.2 This result illustrates the breakdown of cost-effectiveness of PWT when translating it from a bench experiment to a useable process. Other ongoing PWT research examines the degradation of uring the Cold War era race to the moon between the specific contaminants for which no “go-to” conventional United States and the USSR, the Soviets are now believed approaches exist. Examples include photocatalytic reduction to have possessed a working lunar orbiter, lunar lander, and of oxyanion contaminants (e.g., NO3−, ClO4−), toxic metals functional moon suits at the time of the Apollo 11 success. oxidation/reduction, and destruction of per/polyfluoroalkyl They lacked, however, a reliable rocket capable of getting this substances. Even if effective material systems are developed, payload to the moon. While the quest for photocatalytic water however, the same implementation hurdles that plague more generic PWT (see ref 3) will be every bit as prohibitive. Until treatment (PWT) is a bit less awe-inspiring, parallels can be reactor design and catalyst recovery aspects are solved, any new drawn between the moon race story and the failures of this embodiments are unfortunately nothing more than laboratory enticing form of water treatment. In a 1996 interview with demonstrations. Chemical & Engineering News, chemist James R. Bolton The majority of remaining studies emphasize the need for commented on the surge of recent studies within this new field: visible light catalysts for sustainable solar PWT technology. In “This may be a strong statement, but I think the interest in an unofficial Google Scholar survey of articles relating to PWT TiO2 [in aqueous systems] is a good example of scientific published in the last two years (which excluded air or surface hype.”1 Twenty years later, it is surprisingly difficult to argue applications), I found that approximately 45% targeted visible that he was wrong. The field has ballooned within academia light activity or sunlight-driven treatment. I assert, however, and spawned a growing number of subfields pursuing new that the seemingly urgent need for visible light catalysts is only applications of PWT, improved catalysts, and reaction invoked by those who study them, and the broader water mechanisms. And yet, the fundamental technology has scarcely treatment community has little interest in solar processes. This demonstrated a capability to survive outside the lab. While I am is primarily due to the exorbitant area footprints that would a hopeful believer in PWT and active participant in the field, result in replacing UV lamp reactors with solar irradiation. How “hype” is perhaps still the best descriptor of its present driving much solar collection area is required to replace one 1000 W forces and academic allure. In fact, among the water treatment low pressure high-output mercury lamp? Assuming a 40% lamp fields, photocatalytic processes arguably show the widest efficiency, and that the hypothetical visible light catalyst is disconnect between research directions and the actual needs activated by wavelengths of 450 nm or shorter, 2.6 m2 of solar of the water industry. exposure (AM 1.5) would be needed to achieve the treatment Several recognizable motifs lead off the introduction sections power of one lamp. For a full-scale process, this area would of typical PWT research articles. The more practical studies are motivated by the prospect of enabling large-scale “chemicalfree” ultraviolet PWT reactors for municipal water and Received: November 29, 2016 wastewater treatment plants. With the growth of the direct
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© XXXX American Chemical Society
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DOI: 10.1021/acs.est.6b06035 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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quickly add up due to the energy input requirements of AOPs. I will use the specifications of one TrojanUVPhox 72AL75 UV/ H2O2 reactor4 as a representative UV-driven AOP to illustrate: the system employs 72 lamps and has a total footprint of 2.2 m2. An equivalent solar process, however, would require at least 187 m2 to deliver the same light energy and would do so for a very narrow portion of the day at optimal latitudes. Given the constraints faced by real-world consultants and plant operators in water treatment process selection, the solar scenario is fantastical. Several European solar PWT pilot projects have largely confirmed these challenges. The Solwater TiO2/solar concentrator project, for example, reported disinfection treatment times of over 4 h for a 20 L reactor.5 Even for potential use in developing countries, where disinfection is the primary concern, simpler and effective filtration technologies offer more practical solutions. So where are we going with this? Unfortunately for PWT at this stage, most high-impact journals heavily favor materials development over process engineering. As a result, countless UV and visible light catalysts have been developed that perform admirably in bench experiments, using approaches such as engineered nanomorphologies and composite-type cocatalysts. Particles that are smaller and more complex have emerged, whereas larger, more robust materials are needed. If the relative success of the Photo-Cat system is any indication, slurry reactors are the most promising form of PWT for high treatment rates, and involve repeated pumping cycles with high turbulence. Nanocomposites that use combinations of semiconductors, noble metal particles, graphene derivatives, etc., likely lack the required durability. Still, materials science can play an important role by focusing on the proper objectives. Catalysts that can sustain repeated use and attrition are needed, along with qualities that enable low-cost recovery from effluent streams. Highly efficient noncomposite materials that perform well even with larger particle diameters could facilitate recovery via less-intensive filtration steps or even gravity separation. Overall, the need for elegant systems design is currently greater than the need for catalysts with higher hydroxyl radical yields. To preserve the appealing qualities of PWT that triggered the hype to begin with, technologies that accommodate its multidimensional aspects without accumulating excessive ancillary sources of energy consumption are necessary. These needs are not completely ignored, as new PWT reactor schemes routinely appear in lower impact journals, though with little fanfare. Proposed ideas continue to miss the mark, however, in terms potential large-scale application, and I suspect these efforts could heavily benefit from greater collaboration with industry. Finally, it is time to stop patting ourselves on the back for laboratory “successes” that clearly turn a blind eye on fatal implementation hurdles. Even when we choose to optimistically focus on the metaphorical lunar landers and moon suits, the intended product must be holistically compatible with some vision of a practical water treatment process.
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REFERENCES
(1) Wilson, E. TiO2 appears inefficient for water treatment. Chem. Eng. News 1996, 74 (27), 29. (2) Benotti, M. J.; Stanford, B. D.; Wert, E. C.; Snyder, S. A. Evaluation of a photocatalytic reactor membrane pilot system for the removal of pharmaceuticals and endocrine disrupting compounds from water. Water Res. 2009, 43 (6), 1513−1522. (3) Chong, M. N.; Jin, B.; Chow, C. W. K.; Saint, C. Recent developments in photocatalytic water treatment technology: A review. Water Res. 2010, 44 (10), 2997−3027. (4) TrojanUV Environmental Contaminant Treatment/Product Details. http://www.trojanuv.com/resources//Products/ TrojanUVPhox/TrojanUV_ECT_Products_Brochure.pdf (accessed 11/21/2016). (5) Gumy, D.; Pulgarin, C. Factors influencing photocatalytic drinking water detoxification and disinfection by suspended and fixed TiO2, Thesis, Ecole Polytechnique Federale de Lausanne, 2007.
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[email protected]. ORCID
Ezra L. Cates: 0000-0003-0793-0246 Notes
The author declares no competing financial interest. B
DOI: 10.1021/acs.est.6b06035 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
