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Photocatalytic Disinfection of Agriculturally Impacted Waters: Characterization of Solution Chemistry on Bacterial Adhesion and Disinfection

Investigators
Kinsinger, Nichola
Institutions
University of California - Riverside
Start date
2013
End date
2015
Objective
My goal as a USDA Fellow and post-doctoral researcher is to continue my education and development of necessary skills to be on the cutting edge of environmental and water quality research and continued development of my mentoring skills to be an effective leader in the scientific community. I aim to use my background in engineering and material science to conduct research and develop new technologies for environmental remediation and protection of public health. Dr. Walker's expertise in bacterial adhesion and water quality will allow me to take my expertise in new directions, focusing on the application of TiO2 photocatalytic disinfection for water and food safety. The proposed work seeks to evaluate the effectiveness of TiO2, in the degradation of a model bacteria and pesticide, and with the mechanistic understanding of these processes, optimize a reactor for water treatment. This training, and my research interests, provides me with the multidisciplinary skills that make me ideally suited to complete the proposed post-doctoral study merging material science and environmental engineering. Rationale and Significance With increasing awareness of water conservation and protection, photocatalytic technology has the potential to be applied to a wide variety of water treatment applications such alternatives for agriculture runoff treatment, dairy water recycling, produce rinse water disinfection to protect the food supply from potential bacterial outbreaks while continuing to protect our water supply. Outbreaks observed in produce are becoming increasing common possibly due to contaminated irrigation waters or contaminated waters used during processing. These waters may become contaminated animal manure from runoff or leached into surface and ground waters supplies during rain events 1. This becomes specifically problematic is more rural farming communities that may rely solely on well water which may become contaminated and is of key concern in developing countries. Typical TiO2 photocatalytic systems are optimized under ideal slurry reactor conditions based on photocatalyst material properties such as particle size, surface area, phase, and surface charge. However, not only the photocatalyst material properties are important, since adhesion to the catalyst surface significantly effects disinfection due to the short lifespan of the reactive radical species. There are limitations to overcome before such technology is deployable for water treatment applications. The photocatalytic activity also has the versatility for antimicrobial surface applications such as self-cleaning coatings for use as food preparation surfaces that will also reduce outbreaks and spread of bacteria such as E. coli. This research plan responds to the USDA Program Area Priority #2, Food Safety, Nutrition, and Health; by ensuring that water onsite is free from microbial contamination this will be meeting the USDA's goal of ensuring food safety and security. Research Objectives Quantification of bacterial adhesion under flow to immobilized food grade TiO2 nanoparticles surfaces under simulated conditions of agriculturally impacted surface waters. Quantification of batch photocatalytic generation of reactive oxygen species by immobilized food grade TiO2 nanoparticles surfaces under simulated conditions of agriculturally impacted waters. Quantification of batch photocatalytic degradation of model organic pollutants (i.e. pesticides) using immobilized food grade TiO2 nanoparticles surfaces under simulated conditions of agriculturally impacted waters. Evaluate different photocatalytic disinfection reactor designs to optimize cell adhesion, organic degradation, and ROS formation (based upon mechanistic understanding gained from objectives 1-3)
More information

NON-TECHNICAL SUMMARY:
My aim as a post-doctoral researcher is to use my background in engineering and science to conduct research and develop new technologies for environmental remediation and protection of public health. Dr. Walker's expertise in bacterial adhesion and water quality will allow me to take my work in new directions, focusing on the application of TiO2 photocatalytic treatment for water and food safety. This USDA Fellowship Grant will enable me to build the skills necessary for a career in environmental research and college-level teaching. This goal is a product of a longstanding enthusiasm for academic pursuits combined with the desire to protect our natural resources. My doctoral training and research interests provide me with the multidisciplinary skills that make me ideally suited to complete the proposed post-doctoral study, which merges material science and environmental engineering. Water is an essential resource for life, yet each year Escherichia coli infects 75,000 people and caused 61 deaths within the United States from contaminated food and drinking water Drinking water sources are frequently contaminated by agricultural run-off or soil penetration from animal waste water lagoons. Many pathogenic viruses and bacteria are known to be resistant to chlorine disinfection. While increasing chlorine concentration will increase its efficacy, it will also lead to an increase in carcinogenic trihalomethanes (THMs) causing increased concern for public health and safety. Disinfection via oxidation of organics by hydroxyl radicals (OH•) is an alternative and supplemental technology for water treatment that does not require significant capital investment compared with processes such as reverse osmosis. It also eliminates the concern of producing secondary byproducts such as THMs due to complete mineralization. Various methods are used to produce OH• including degradation of H2O2, O3, and H2O2/O3, using ultra-violet radiation (UV). Adding a photocatalytic material can significantly accelerate the production of these radicals that degrade organics non-selectively. Photocatalysts work by absorbing sunlight and, in the presence of water, generates OH• radicals. TiO2 is an inexpensive example of a photocatalytic material that can be easily integrated into an existing treatment system and isolated from the effluent liquid stream. Typical TiO2 photocatalytic systems are optimized under ideal slurry reactor conditions based on photocatalyst material properties such as particle size, surface area, phase, and surface charge. However not only the photocatalyst material properties are important as it has been shown that reduced interaction distances between the target (organic or bacteria) and photocatalyst surface enhance the efficiency activity since to the short lifespan of the reactive oxygen species. Photocatalytic technology has the potential to be applied a wide variety of water treatment applications such alternative agriculture runoff treatment, dairy water recycling, and produce rinse water disinfection. However there are limitations to overcome before such technology is deployable for water treatment applications. The proposed work seeks to evaluate the effectiveness of TiO2, in the degradation of a model bacteria and pesticide, and with the mechanistic understanding of these processes, optimize a reactor for water treatment. The specific objectives of this work are as follows: Research Objectives Quantification of bacterial adhesion under flow to immobilized food grade TiO2 nanoparticles surfaces under simulated conditions of agriculturally impacted surface waters. Quantification of batch photocatalytic generation of reactive oxygen species by immobilized food grade TiO2 nanoparticles surfaces under simulated conditions of agriculturally impacted waters. Quantification of batch photocatalytic degradation of model organic pollutants (i.e. pesticides) using immobilized food grade TiO2 nanoparticles surfaces under simulated conditions of agriculturally impacted waters. Evaluate different photocatalytic disinfection reactor designs to optimize cell adhesion, organic degradation, and ROS formation (based upon mechanistic understanding gained from objectives 1-3).

APPROACH:
Objective 1: Ideal and Simulated Bacterial Adhesion Operating parameters such as pH have been reported to greatly affect heterogeneous photocatalytic performance due to particle aggregation which will reduce accessible surface active sites and block excitation events within the catalyst. The system pH also affects the ionization state of catalyst surface and the organic compounds which has been shown to affect the sorbate-surface interactions which impact performance since OH• radicals are easily scavenged. Commercial food grade TiO2 (E171) materials will be selected to vary surface area, particle size, and phase (anatase/brookite/rutile) which are all know to affect photocatalyst performance. These TiO2 materials will be immobilized on a glass annular photoreactor and glass surfaces (to be used in a parallel plate (PP) flow chamber) utilizing a dip-coating procedure outlined by van Grieken et al. The clean glass reactor will be dip-coated using a 150g/L suspension of TiO2 in deionized water adjusted to pH1.5 using nitric acid. The reactor will be dried at 110°C for 24 hours following each coating cycle and then calcined at 500°C for 2 hours heating at a rate of 5°C per minute. Catalyst Material Characterization: TiO2 materials immobilized on the glass surfaceswill be characterized using methods Scanning Electron Microscopy, Bright-field Transmission Electron Microscopy, and X-Ray diffraction will be used to investigate the size, morphology, phase, and thickness of the TiO2 films both prior to and following calcination. The Brunauer, Emmett, and Teller method will be used to determine surface area and pore size distributions. The zeta potential will be determined for each sample surface by measuring the streaming potential. A streaming potential analyzer (EKA, Anton Paar, Graz, Austria) with an asymmetric clamping cell will be used to determine the electrokinetic properties of the sample surfaces. Roughness will be measured by atomic force microscopy to provide a measure of the surface roughness and uniformity of the surface. Bacterial Adhesion Characterization: Escherichia coli O157:H7 will be used as a model pathogen, to characterize effect of TiO2 properties on adhesion under various simulated conditions of agriculturally impacted waters (artificial surface water spiked with model pesticides). Individual solution chemistry and environmental parameters including pH, ionic strength, natural organic matter, water temperature, and the effects of photocatalyst material and morphology and morphology will be investigated. Adhesion studies will be conducted by recycling a stock suspension of E. coli (approximate concentration of 108 cells mL?1) for 6 cycles at various flow rates to account for the effect of shear. Following adhesion the influent solution will be switch to a bacteria-free suspension at the same chemical composition. The effluent will be measured using a UV-visible spectrophotometer at a fixed wavelength of 546nm to characterize the bacteria breakthrough curves. Parallel Plate Bacterial Adhesion Characterization: The mass transfer of bacteria onto the solid-water interface under range of solution chemistry will be measured in a parallel plate (PP) flow chamber system available at UCR. This flow chamber is installed on the stage of an inverted fluorescent microscope (BX-52 Olympus) allowing for the quantification of bacterial adhesion under flowing conditions. The bottom of the flow cell is a microscope slide, which is typically used as the test surface in PP studies. However, the flow cell has been modified to insert a test surface on the upper surface (installed into a grove in the upper acrylic flow deck) such that deposition of cells onto a non-transparent test surface - in this case a immobilized TiO2 film on glass) - will be investigated. Bacterial samples will be suspended in solutions (described above) and injected into a parallel flow chamber system at flow rates within the PP system will be tested across a range of diffusion dominated to turbulent conditions, mimicking the type of flows experienced in various stages within the photocatalytic reactor. We will apply different shear rates in close proximity to the test membrane surface (where shear rate is maximum) in the PP channel that are between 1 to 103 sec-1. Objective 2: Photocatalytic Generation of Reactive Oxygen Species (ROS) Measurement of steady-state hydroxyl radical (OH•) concentrations will be performed using a batch reactor illuminated by a 450W O3-free xenon arc lamp to produce a collimated beam with a water filter to remove infrared radiation. A 305nm long pass filter will be used to produce light in the UV and visible range. Steady-state concentrations of OH• will be determined using phenol as a probe. Prepared sample of immobilized TiO2 on the glass substrate will be immersed in know starting concentrations of phenol (100?M). These reactors will be exposed to the light over several hours will 1mL aliquots taken every hour and analyzed using High Performance Liquid Chromotagraphy. Objective 3: Degradation of Model Organic Pollutants Photocatalytic degradation of organic pollutants (such as herbicide: Atraezine, or insecticide: Diazinon) will be conducted to characterize the effect of immobilized TiO2 properties on degradation rates of such pollutants. During the photocatalytic degradation studies system parameters investigated in Objectives 1-2 will be adjusted individually to identify the effect of individual constituents within agriculture impacted waters such as ionic strength and natural organic material. Subsequently photocatalytic degradation studies using agriculturally impacted water with the cocktail of constituents will be performed. Objective 4: Photocatalytic Disinfection Based on results from Objectives 1-3, studies will be conducted to characterize the effect of bacterial adhesion to the TiO2 photoreactor on disinfection rates. E. coli will be used again as the model pathogen and the same annular photoreactor with immobilized TiO2 (prepared as described above) will be used to conduct the disinfection studies. The disinfection rates will be measured by quantifying the concentration of viable bacteria at predetermined times via serial dilution procedures. During the photocatalytic disinfection studies system parameters investigated in Objective 1 will be adjusted individually to identify the effect of individual constituents within agriculturally impact waters on disinfection. Subsequently photocatalytic disinfection studies using simulated agriculturally impacted water with the combination of constituents will be performed. These detailed studies will develop understanding necessary for real applications of this technology and will guide photocatalytic reactor and system designs to account for natural fluctuations within the influent from various sources and seasonal variations. There are some limitations and issues that might be encountered during this project. Such issues may include low reactive oxygen generation by certain nanoparticle/surface combinations resulting in immeasurable disinfection rates or pesticide degradation, due to material properties or insufficient UV exposure of TiO2 in the reactor geometry. In these cases, different reactor geometries, photocatalyst design (nanoparticle type and coating), and enhanced retention time in reactors will be considered. Annual Milestones: Year 1 Completion of the preparation and characterization of the photocatalytic reactors to be used for bacterial adhesion Completion of bacterial adhesion studies Conference attendance and presentation on bacterial adhesions studies Year 2 Completion of photocatalytic ROS generation studies Completion of pesticide photocatalytic degradation and disinfection studies. Conference attendance and presentation on correlation of adhesion to disinfection rate Manuscript submitted for publication on resulting studies.

Funding Source
Nat'l. Inst. of Food and Agriculture
Project source
View this project
Project number
CALW-2013-03306
Accession number
1001293
Categories
Natural Toxins
Escherichia coli
Bacterial Pathogens
Food Defense and Integrity
Sanitation and Quality Standards
Chemical Contaminants