Photocatalytic Degradation of Microcystins by TiO2 Using UV-LED Controlled Periodic Illumination (original) (raw)
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Water Research, 2002
The increasing incidence of algal blooms in fresh water supplies and the consequent possibility of cyanobacterial microcystin contamination of potable water is a cause of recent concern. Heterogeneous photocatalytic oxidation forms part of a family of advanced water treatment technologies comprising the generation of reactive oxidizing species in water media and results in the complete oxidative degradation (mineralization) of organic pollutants to yield carbon dioxide, water and inorganic ions. A new experimental laboratory-scale 'falling film' reactor has been developed to study the photocatalytic degradation of microcystins in aqueous solution. The reactor consisted of a fiberglass sheet impregnated with immobilized titanium dioxide (TiO 2) catalyst over which the microcystin solution was pumped (as a falling film) while being irradiated from UV-C germicidal lamps. The design of the system obviated the necessity to separate suspended catalyst from treated water as required in slurry reactors. The photocatalytic degradation was characterized by pseudo-first order reaction kinetics. Rapid degradation of microcystins LR, YR and RR was observed in natural lake water with half lives less than 10 min, while even faster rates were achieved in laboratory distilled water. Although low pH (pH 3) marginally improved reaction rates, the presence of radical scavengers such as sulfate ions was detrimental to the photocatalytic oxidation process.
Applied Catalysis B: Environmental, 2014
The effect of using only visible light to induce nitrogen-and fluorine-doped titanium dioxide (NF-TiO 2) photocatalysis of degradation products of microcystin-LR (MC-LR), the most common problematic cyanotoxin, was explored by looking at the intermediate degradation products. Although the degradation mechanisms and products of conventional UV-based TiO 2 photocatalysis of MC-LR have been well elucidated, the same is not true for visible light-based TiO 2 photocatalysis. The results of LC/MS 2 (and in one case LC/MS 3) indicated that the intermediates are not drastically altered in comparison to traditional TiO 2 photocatalysis using UV light. The data hint that the degradation is driven by hydroxyl radicals, as is UV-based TiO 2 photocatalysis, although the mechanism for producing hydroxyl radicals is unclear since studies indicate drastically slower kinetics for visible light-based photocatalysis of MC-LR. Notably, the data indicate that visible light-induced NF-TiO 2 photocatalysis degraded the portion of MC-LR that is responsible for biological toxicity. As a result of this, it was concluded that doping TiO 2 with nitrogen and fluorine is an effective method for increasing utilization of visible light while degrading MC-LR in water, although it should still be noted that degradation kinetics are still slower than UV-based TiO 2 photocatalysis.
Applied Catalysis B: Environmental, 2009
This study investigated the use of thin transparent TiO 2 photocatalytic films, prepared with novel sol-gel methods containing surfactants as templating materials, for the degradation of the cyanotoxin, microcystin-LR (MC-LR). MC-LR is an emerging contaminant from the Contaminant Candidate Lists (CCLs 1-3) of the USEPA. The effects of UV-A radiation, solution pH, initial toxin concentration, coated surface area of the TiO 2 films and their structural properties (porosity, crystallinity and thickness) on the degradation rate of MC-LR were investigated. Photolysis did not occur with UV-A radiation. Acidic pH was more efficient for the degradation of MC-LR due to toxin interaction with the catalyst surface and increased adsorption into the porous films. The degradation profiles of the toxin at different initial concentrations were fitted with pseudo-first order kinetics. Films prepared with three coatings (0.3 mm thickness) had the best performance at acidic and neutral pH, while the exclusion of surfactant from the preparation method resulted in non-porous films with decreased performance. The parameter that mostly affected the degradation rate was the solution pH. The toxicity of the treated samples, evaluated by an in-house protein phosphatase 1 assay, indicated that treatment with the TiO 2 photocatalytic films indeed resulted in complete removal of MC-LR's toxicity. ß
Photocatalytic degradation of microcystin-LR in aqueous solutions
Chemosphere, 2013
In order to investigate the catalytic performance of anodic TiO 2 nanotubes and their practical application in the treatment of refractory microcystins (MCs) in natural-water samples, TiO 2 nanotubes of diameter of 50-80 nm were fabricated by anodization in C 2 H 2 O 4 ·2H 2 O containing NH 4 F. Under irradiation with natural sunlight, MC-LR was totally degraded after 1 d using the anodic TiO 2 nanotubes. In contrast, the removal efficiency without TiO 2 nanotubes was as low as 47.7% within 20 d. In addition, a mixture of anatase and rutile TiO 2 gave higher photocatalytic activity than the single phase did. The pH also influenced the adsorption capacity of the TiO 2 nanotubes. The order of MC-LR degradation efficiencies at different pH values was 3.5 > 8.0 > 10.0. After five repeated experiments on the degradation of MC-LR for 7 h, the degradation efficiency was still stable.
Materials
Microcystin-LR (MC-LR), a potent hepatotoxin produced by the cyanobacteria, is of increasing concern worldwide because of severe and persistent impacts on humans and animals by inhalation and consumption of contaminated waters and food. In this work, MC-LR was removed completely from aqueous solution using visible-light-active C/N-co-modified mesoporous anatase/brookite TiO2 photocatalyst. The co-modified TiO2 nanoparticles were synthesized by a one-pot hydrothermal process, and then calcined at different temperatures (300, 400, and 500 °C). All the obtained TiO2 powders were analyzed by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscope (TEM), specific surface area (SSA) measurements, ultraviolet-visible diffuse reflectance spectra (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) analysis. It was found that all samples contained mixed-phase TiO2 (anatase and brookite), and the...
Photocatalytic Degradation of Microcystin-LR: Conceptual Model and Pilot Scale Studies
Zeitschrift für Physikalische Chemie, 1999
The applicability of heterogeneous photocatalytic degradation of low concentrations of the cyanobacterial toxin, microcystin-LR, in a natural organic-aqueous matrix is examined using titanium dioxide as the photocatalyst. The initial rate of toxin degradation is strongly pH dependent in a manner mirrored by the pH dependence of toxin adsorption to Ti02. Rapid degradation of toxin occurs in the acidic pH range in the presence of light and Ti02 with a maximum initial rate of degradation occurring at pH 3.5 while at higher concentrations and pH, a distinct lag is observed prior to commencement of toxin degradation. A proposed conceptual model for toxin degradation is developed and the veracity of the proposed model is tested by determining species concentrations for assumed kinetic constants. Very similar dependencies and trends to those observed in the laboratory studies were obtained suggesting that adsorption and sensitization effects are critical. In particular, under conditions where the contaminant adsorbs strongly to semiconductor surface sites, the primary degradation step appears to involve reaction between surface-located long-lived organic radicals and adsorbed trace contaminant. Significant presence of superoxide at low pH appears to enhance trace contaminant degradation via solution phase formation of highly oxidizing organic peroxy radicals from "bulk" background organic. Under conditions where the trace contaminant shows no observable adsorption, surface degradation adequately predicts removal due to surface concentration effects. Initial pilot scale studies using a solar fixed-bed photocatalytic reactor have been successful in achieving 96% removal of a 100 L microcystin-LR spiked drinking water solution. The model successfully predicts the much slower degradation achieved where a trace contaminant is a very small fraction of a natural organic-aqueous matrix and number of sites is significantly reduced.
Environmental Science & Technology, 2007
The presence of the harmful cyanobacterial toxins in water resources worldwide drives the development of an innovative and practical water treatment technology with great urgency. This study deals with two important aspects: the fabrication of mesoporous nitrogen-doped TiO 2 (N-TiO 2 ) photocatalysts and their environmental application for the destruction of microcystin-LR (MC-LR) under visible light. In a nanotechnological sol-gel synthesis method, a nitrogen-containing surfactant (dodecylammonium chloride) was introduced as a pore templating material for tailor-designing the structural properties of TiO 2 and as a nitrogen dopant for its visible light response. The resulting N-TiO 2 exhibited significantly enhanced structural properties including 2-8 nm mesoporous structure (porosity 44%) and high surface area of 150 m 2 /g. Red shift in light absorbance up to 468 nm, 0.9 eV lower binding energy of electrons in Ti 2p state, and reduced interplanar distance of crystal lattices proved nitrogen doping in the TiO 2 lattice. Due to its narrow band gap at 2.65 eV, N-TiO 2 efficiently degraded MC-LR under visible spectrum above 420 nm. Acidic condition (pH 3.5) was more favorable for the adsorption and photocatalytic degradation of MC-LR on N-TiO 2 due to electrostatic attraction forces between negatively charged MC-LR and +6.5 mV charged N-TiO 2 . Even under UV light, MC-LR was decomposed 3-4 times faster using N-TiO 2 than control TiO 2 . The degradation pathways and reaction intermediates of MC-LR were not directly related to the energy source for TiO 2 activation (UV and visible) and nature of TiO 2 (neat and nitrogen-doped). This study implies a strong possibility for the in situ photocatalytic remediation of contaminated water with cyanobacterial toxins and other toxic compounds using solar light, a sustainable source of energy.
Visible-light-driven photocatalytic degradation of microcystin-LR by Bi-doped TiO 2
Research on Chemical Intermediates, 2011
Bi-doped nano-crystalline TiO2 (Bi–TiO2) has been synthesized by sonocrystallization at low temperature. The Bi–TiO2 materials have narrower bandgaps than pristine TiO2, which endow them with significant visible light absorption. Accordingly, these materials had enhanced photocatalytic activity in the degradation of organic dye pollutants and the cyanotoxin microcystin-LR (MC-LR) under visible irradiation. It was found that degradation of MC-LR is rather