Kevin E. O'Shea

Solar photocatalysis for water disinfection: Materials and reactor design.

As of 2010, access to clean drinking water is a human right according to UN regulations. Nevertheless, the number of people living in areas without safe drinking water is predicted to increase by three billion by the end of this decade. Several recent cases of E. coli and Cryptosporidium contamination in drinking water are also reported in a number of advanced countries. Therefore ensuring the potability of drinking water is urgent, but highly challenging to both the developing and developed world in the future. A combination of solar disinfection and photocatalysis technology offers real possibilities for removing lethal pathogenic microorganisms from drinking water. The time taken for the conventional SODIS process can be greatly reduced by semiconductor (e.g. TiO2, ZnO, nano-heterojunctions) based photocatalysis. This review addresses the fundamental reaction mechanism, advances in materials synthesis and selection and recent developments in the reactor design for solar energy driven photocatalysis using titanium dioxide. The major advantage of using photo-reactors is that they enhance disinfection by increasing photon flux into the photocatalyst. Other major factors affecting such efficiency of solar-based photocatalysis such as the illuminated volume/total volume ratio, catalyst load and flow rate, are discussed in detail. The significance of using immobilised catalysts over the catalyst powder in slurries is also highlighted. It is noted that, despite encouraging early field studies, the commercialisation and mass production of solar photocatalysis systems remains highly challenging. Recommendations for future directions for addressing issues such as mass transfer, requirement of a standard test method, photo-reactors design and visible light absorption by TiO2 coatings are also discussed.

Efficient removal of endosulfan from aqueous solution by UV-C/peroxides: A comparative study

This study explored the efficiency of UV-C-based advanced oxidation processes (AOPs), i.e., UV/S2O8(2-), UV/HSO5(-), and UV/H2O2 for the degradation of endosulfan, an organochlorine insecticide and an emerging water pollutant. A significant removal, 91%, 86%, and 64%, of endosulfan, at an initial concentration of 2.45 μM and UV fluence of 480 mJ/cm(2), was achieved by UV/S2O8(2-), UV/HSO5(-), and UV/H2O2 processes, respectively, at a [peroxide]0/[endosulfan]0 molar ratio of 20. The efficiency of these processes was, however, inhibited in the presence of radical scavengers, such as alcohols (e.g., tertiary butyl alcohol and isopropyl alcohol) and natural organic matter (NOM). The inhibition was also influenced by common inorganic anions in the order of nitrite > bicarbonate > chloride > nitrate ≈ sulfate. The observed pseudo-first-order rate constant decreased while the degradation rate increased with increasing initial concentration of the target contaminant. The degradation mechanism of endosulfan by the AOPs was evaluated revealing the main by-product as endosulfan ether. Results of this study suggest that UV-C-based AOPs are potential methods for the removal of pesticides, such as endosulfan and its by-products, from contaminated water.

Photocatalytic decomposition of organophosphonates in irradiated TiO2 suspensions

Abstract Kinetic analyses of the TiO 2 -catalyzed photodegradation of dimethyl methylphosphonate (DMMP) and diethyl methylphosphonate (DEMP) in oxygenated aqueous solutions are described. The effects of substrate concentration and solution pH are investigated. A number of kinetic models appear to be applicable in accordance with the initial kinetic parameters. The major products formed from the photocatalytic decomposition of DMMP are methylphosphonic acid, phosphoric acid, formaldehyde and formic acid.

Free radical mechanisms for the treatment of methyl tert-butyl ether (MTBE) via advanced oxidation/reductive processes in aqueous solutions.

Department of Civil and Environmental Engineering, University of California, Irvine, Irvine, California 92697, Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 South College Road, Wilmington, North Carolina 28403-5932, Department of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, Department of Chemistry and Biochemistry, Florida International University Miami, Florida 33199, and Leibniz-Institut für Oberflächenmodifizierung (IOM), Permoserstrasse 15 04318, Leipzig, Germany, and Max-Planck-Institut für Bioanorgansche Chemie, Stiftstrasse 34-26 45413, Mülheim an der Ruhr, Germany

High performance sulfur, nitrogen and carbon doped mesoporous anatase–brookite TiO2 photocatalyst for the removal of microcystin-LR under visible light irradiation

Carbon, nitrogen and sulfur (C, N and S) doped mesoporous anatase-brookite nano-heterojunction titania photocatalysts have been synthesized through a simple sol-gel method in the presence of triblock copolymer Pluronic P123. XRD and Raman spectra revealed the formation of anatase and brookite mixed phases. XPS spectra indicated the presence of C, N and S dopants. The TEM images demonstrated the formation of almost monodisperse titania nanoparticles with particle sizes of approximately 10nm. N2 isotherm measurements confirmed that both doped and undoped titania anatase-brookite materials have mesoporous structure. The photocatalytic degradation of the cyanotoxin microcystin-LR (MC-LR) has been investigated using these novel nanomaterials under visible light illumination. The photocatalytic efficiency of the mesoporous titania anatase-brookite photocatalyst dramatically increased with the addition of the C, N and S non-metal, achieving complete degradation (∼ 100 %) of MC-LR. The results demonstrate the advantages of the synthetic approach and the great potential of the visible light activated C, N, and S doped titania photocatalysts for the treatment of organic micropollutants in contaminated waters under visible light.

Improved charge transport of Nb-doped TiO2 nanorods in methylammonium lead iodide bromide perovskite solar cells

Nb-doped rutile nanorod-based methylammonium lead iodide bromide (MAPbI3−xBrx) perovskite solar cells have been developed by integrating an excellent photon-active perovskite sensitizer with the superior electron transporting rutile nanorods. It is found that there are two distinct stages in the formation of the perovskite materials prepared using non-stoichiometric mixed halide precursors, namely the orange colored bromine-rich transient state formed at 105 °C and the dark brown colored iodine-rich crystallized state formed at 155 °C. Optical, compositional, and crystalline properties of the perovskite samples at the two stages are studied by using UV-vis spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The mixed halide materials undergo a transition from an intermediate cubic phase to a well-crystallized tetragonal perovskite phase through complicated diffusion, evaporation, and intercalation processes. Furthermore, a well-crystallized mixed halide perovskite is integrated with Nb-doped rutile nanorods and undoped rutile nanorods to fabricate perovskite solid state solar cells. Perovskite solar cells with Nb-doped rutile nanorods have significantly improved performance including the increased short circuit current and open circuit voltage compared to perovskite solar cells with undoped rutile nanorods. The overall power conversion efficiency enhancement of the device with Nb-doped rutile nanorods is over 50% compared with an undoped nanorod-based device, which is attributed to the superior charge collection efficiency of the Nb-doped rutile nanorods as evidenced by the electrochemical impedance measurement.

Photodegradation of antibiotics under simulated solar radiation: implications for their environmental fate.

Roxithromycin, erythromycin, ciprofloxacin and sulfamethoxazole are frequently detected antibiotics in environmental waters. Direct and indirect photolysis of these problematic antibiotics were investigated in pure and natural waters (fresh and salt water) under irradiation of different light sources. Fundamental photolysis parameters such as molar absorption coefficient, quantum yield and first order rate constants are reported and discussed. The antibiotics are degraded fastest under ultraviolet 254 nm, followed by 350 nm and simulated solar radiation. The composition of the matrix (pH, dissolved organic content, chloride ion concentration) played a significant role in the observed photodegradation. Under simulated solar radiation, ciprofloxacin and sulfamethoxazole degrade relatively quickly with half-lives of 0.5 and 1.5h, respectively. However, roxithromycin and erythromycin, macrolides are persistent (half-life: 2.4-10 days) under solar simulation. The transformation products (15) of the targeted antibiotics produced under irradiation experiments were identified using high resolution mass spectrometry and degradation pathways were proposed.

Can We Effectively Degrade Microcystins? - Implications on Human Health

Microcystins are cyclic heptapeptide toxins produced by a number of genera of cyanobacteria. They are ubiquitous in bodies of water worldwide and pose significant hazard to human, plant, and animal health. Microcystins are primarily hepatotoxins known to inhibit serine-threonine phosphatases leading to the disruption of cascade of events important in the regulation and control of cellular processes. Covalent binding of microcystins with phosphatases is thought to be responsible for the cytotoxic and genotoxic effects of microcystins. In addition, microcystins can trigger oxidative stress in cells resulting in necrosis or apoptosis. Their cyclic structure and novel amino acids enhance their stability and persistence in the environment. Humans are primarily exposed to microcystins via drinking water consumption and accidental ingestion of recreational water. Recreational exposure by skin contact or inhalation to microcystins is now recognized to cause a wide range of acute illnesses which can be life-threatening. Microcystins are primarily degraded by microorganisms in the environment, while sunlight can cause the isomerization of the double bonds and hydroxylation in the presence of pigments. Attempts to utilize these organisms in sand and membrane filters to treat water contaminated with microcystins showed complete removal and detoxification. Conventional water treatment processes may not fully eliminate microcystins when there are high levels of organic compounds especially during harmful bloom events. Combination of conventional and advanced oxidation technologies can potentially remove 100% of microcystins in water even in turbid conditions. This review covers selected treatment technologies to degrade microcystins in water.

Assessment of the roles of reactive oxygen species in the UV and visible light photocatalytic degradation of cyanotoxins and water taste and odor compounds using C-TiO2.

Visible light (VIS) photocatalysis has large potential as a sustainable water treatment process, however the reaction pathways and degradation processes of organic pollutants are not yet clearly defined. The presence of cyanobacteria cause water quality problems since several genera can produce potent cyanotoxins, harmful to human health. In addition, cyanobacteria produce taste and odor compounds, which pose serious aesthetic problems in drinking water. Although photocatalytic degradation of cyanotoxins and taste and odor compounds have been reported under UV-A light in the presence of TiO2, limited studies have been reported on their degradation pathways by VIS photocatalysis of these problematic compounds. The main objectives of this work were to study the VIS photocatalytic degradation process, define the reactive oxygen species (ROS) involved and elucidate the reaction mechanisms. We report carbon doped TiO2 (C-TiO2) under VIS leads to the slow degradation of cyanotoxins, microcystin-LR (MC-LR) and cylindrospermopsin (CYN), while taste and odor compounds, geosmin and 2-methylisoborneol, were not appreciably degraded. Further studies were carried-out employing several specific radical scavengers (potassium bromide, isopropyl alcohol, sodium azide, superoxide dismutase and catalase) and probes (coumarin) to assess the role of different ROS (hydroxyl radical OH, singlet oxygen (1)O2, superoxide radical anion [Formula: see text] ) in the degradation processes. Reaction pathways of MC-LR and CYN were defined through identification and monitoring of intermediates using liquid chromatography tandem mass spectrometry (LC-MS/MS) for VIS in comparison with UV-A photocatalytic treatment. The effects of scavengers and probes on the degradation process under VIS, as well as the differences in product distributions under VIS and UV-A, suggested that the main species in VIS photocatalysis is [Formula: see text] , with OH and (1)O2 playing minor roles in the degradation.

NF-TiO2 photocatalysis of amitrole and atrazine with addition of oxidants under simulated solar light: Emerging synergies, degradation intermediates, and reusable attributes

In order to investigate sustainable alternatives to current water treatment methods, the effect of NF-titania film thickness and subsequent photocatalysis in combination with oxidants was examined under simulated solar light. Such a combination presents a theoretical possibility for a synergistic interaction between the photocatalyst and the oxidant (activation of the oxidant by the catalyst under conditions under which it may not conventionally be activated). To investigate, peroxymonosulfate (PMS) and persulfate (PS) were used as oxidants, and two pesticides, amitrole and atrazine, were used as target contaminants. In the absence of a film, activation of PMS under simulated solar conditions is demonstrated by removal of atrazine, whereas PS provided minimal removal, suggesting inefficient activation. Combining photocatalytic films with PMS and PS manifested synergies for both oxidants. The effect was most pronounced for PS since PMS already underwent significant activation without the photocatalyst. Amitrole degradation results indicated a lack of removal of amitrole by activated PS alone, suggesting that this sulfate radical-based treatment technology may be ineffective for the removal of amitrole. The NF-TiO₂ films demonstrated reusability under solar light both with and without oxidants. Finally, the degradation intermediates were analyzed, and a new intermediate appeared upon incorporating oxidants into the system.