microcystin-LR Degradation Rate Research Articles

Preparation of porous C3N5 nanosheets by temperature modulation: Visible-light induced degradation characteristics and mechanism of microcystin-LR

Microcystin pollution of the environment is a prevalent problem that has gained considerable attention in sustainable development. Photocatalytic degradation is one of the most efficient ways to address this environmental issue. In this study, a porous C3N5 carbon nitride nanosheet (NCN) with high photocatalytic activity was prepared using 3-amino-1,2,4-triazole as the raw material by a simple and environmentally friendly stepwise pyrolysis technique. The morphology, microstructure, chemical composition, and optoelectronic properties of the materials were analyzed by characterization (SEM, TEM, XRD, FI-IR, XPS, UV–vis, PL, Etc.). The specific surface area and charge mobility of NCN were both increased by the porous structure. The specific surface area of NCN was 4.95 times greater than that of CN-650, and it has a narrower band gap and a broader visible absorption range. The degradation rate of microcystin-LR by NCN, driven by visible light, reached 99.99% at 45 min, and the degradation kinetic constant was 9.09 times that of its precursor. Singlet oxygen (1O2) plays the most crucial role in the photocatalytic degradation of MC-LR by NCN. In addition, possible degradation pathways were sought by studying the intermediates of the MC-LR photodegradation process. The NCN photocatalysts developed in this work provide a wide range of possibilities for photocatalyst design and the mechanistic analysis of photocatalytic degradation of microcystins.

Microcystin-LR exposure enhances toxin-degrading capacity and reduces metabolic diversity of sediment microbial communities

Substantial outbreaks of cyanobacterial harmful algal blooms (cyanoHABs) are accompanied by the prevalent existence of microcystins (MCs) in eutrophic freshwater worldwide, which can compromise the safety of aquatic ecosystems and public health. However, the response of the sediment microbial community to MCs exposure remains unclear. In this study, the sediment microbial communities of Lake Taihu were continuously exposed to environmentally relevant concentration (10 μg/L) of microcystin-LR (MC-LR) in microcosms for three months to mimic MCs contamination during cyanoHABs. Meanwhile, the MC-LR degradation characteristics, metabolic profiles, functional genes, and structures of microbial communities were analyzed periodically. The results showed that the MC-LR degradation capacities of microbial communities significantly increased with prolonged toxin exposure time. The enhanced MC-LR degradation rate was positively correlated with the increase in the abundance of mlr genes, which corresponds to the MC-degrading bacterial community. However, the MC-LR degradation pathway was maintained, and one new biodegradation intermediate (Adda-Glu) was first identified. Moreover, the metabolic profiles of the microbial communities indicated that overall carbon metabolic activity displayed a stimulation-suppression trend, and the exposure led to different carbon utilization patterns. Furthermore, microbial community structure analysis showed that long-term MC-LR exposure enriched microbes associated with MC degradation (Pseudomonas, Stenotrophomonas, Sphingopyxis and Sphingomonas) but inhibited MC-sensitive bacterial genera. This study clearly elucidates that MC-LR exposure alters the function, metabolism, and structure of the sediment microbial community to adapt to polluted environments.

Impacts of algal organic matter and humic substances on microcystin-LR removal and their biotransformation during the biodegradation process

The application of bioaugmentation (i.e., injection of contaminant-degrading microorganisms) has shown its potential to remove harmful cyanotoxins like microcystin-LR (MC-LR) from drinking water sources. However, the natural organic matter (NOM) present in both natural and engineered water systems might affect the bacterial biodegradation of MC-LR. Therefore, for the successful application of bioaugmentation for MC-LR removal in water treatment, it is important to understand NOM effects on MC-LR biodegradation. In this study, the impact of NOM [algal organic matter (AOM) and humic substances (HS)] on MC-LR biodegradation was evaluated in the presence of varying concentrations of NOM by monitoring MC-LR biodegradation kinetics. The changes in NOM composition during MC-LR biodegradation were also characterized by a five-component Parallel factor (PARAFAC) model using 336 excitation-emission matrix (EEM) spectra collected at different sampling points. Our results showed decreases in MC-LR biodegradation rate of 1.6-and 3.4-fold in the presence of AOM and HS, respectively. The expression of the functional mlrA gene exhibited a similar trend to the MC-LR degradation rate at different NOM concentrations. EEM-PARAFAC analyses and NOM molecular size fractionation results indicated a relatively greater production of terrestrial humic-like components (57%) and a decrease of protein-like components. Two-dimensional correlation spectroscopy (2D-COS) analyses further confirmed that low molecular weight protein-like components were initially utilized by bacteria, followed by the formation of higher molecular weight humic-like components, likely due to microbial metabolism.

Microcystin-LR degradation kinetics during chlorination: Role of water quality conditions

Microcystin-LR (MCLR) produced during certain cyanobacteria blooms can contaminate drinking water sources and pose a threat to public health. Previous studies of MCLR degradation by free chlorine may have artifacts from using strong reducing agents to quench chlorination reactions, and they also have not explored the influence of water quality characteristics such as pH, alkalinity, temperature and dissolved organic matter (DOM). Using a novel quencher, 1,3,5-trimethoxybenzene (TMB), the apparent MCLR degradation rate constants were found to be higher than those obtained with thiosulfate (S2O32−), a traditionally used strong reducing quencher. Thiosulfate converted N-chlorinated MCLR degradation products back to the parent MCLR, thereby underestimating MCLR loss over time. The second-order rate constants for HOCl (kHOCl) and OCl− (kOCl-) during chlorination of MCLR were determined to be 72 ± 13 and 28 ± 1.8 M−1s−1, respectively, allowing for determination of the apparent MCLR rate constants (kapp,MCLR) for any known pH condition. The MCLR reaction with free chlorine was strongly affected by temperature and the presence of DOM, while changes in ionic strength and alkalinity had little effect. Free chlorine in the presence of DOM, originating from both terrestrial and microbial sources, exhibited two-stage decay. The initial chlorine demand in the first 15 s of reaction can be determined by the dissolved organic carbon (DOC) concentration (initial chlorine demand = 1.8 × DOC), and the second-order rate constants for the later slower decay correlated well with SUVA254 (kapp,DOM = 0.73 × SUVA254 - 0.41). The results yielded a practical model to predict the decay of MCLR during chlorination of waters with varied water quality characteristics.

Photocatalytic degradation of microcystin-LR and anatoxin-a with presence of natural organic matter using UV-light emitting diodes/TiO2 process

Cyanotoxins are released into water bodies when cyanobacterial blooms occur. Representative cyanotoxins are microcystin-LR (MC-LR), which is one of the most-frequently detected, and anatoxin-a (ANTX), threatening human health through the liver damage and nervous system, respectively. One of the advanced oxidation processes, UV/TiO2 process is effective for MC-LR degradation; however, applying conventional UV lamps is an ongoing issue owing to disadvantages, such as use of mercury and high energy consumption. To resolve these, this study aims to use light emitting diodes (LEDs), developed by the Lumens Co., combined with commercial TiO2 for the removal of two types of cyanotoxins, MC-LR and ANTX. With 0.05 g L−1 of TiO2, over 99.9 % of the MC-LR was degraded in 15 min. Under acidic conditions, MC-LR and TiO2 were converted into MC-LRH− and TiO2+, then, electrostatic attraction was generated between them. Therefore, the degradation rate constant (k) of MC-LR was higher under acidic conditions than neutral or basic conditions. The natural organic matter (NOM) served as a scavenger of OH, reducing the MC-LR degradation rate under UV-LED/TiO2 process. 35%–53.6% degradation of NOM due to the decrease in humic substances and building blocks, and the increase in low molecular weight neutrals and low molecular weight acids. The degradation efficiency of MC-LR was higher than that of ANTX, and both cyanotoxins were completely degraded within 15 min. The k of MC-LR and ANTX were similar but significantly reduced due to the NOM and alkalinity of the water collected from the Han River.

Biodegradation kinetics of microcystins-LR crude extract by Lysinibacillus boronitolerans strain CQ5

As the most common variant of microcystins (MCs), microcystin-LR (MCLR) is a kind of toxins produced by some species of harmful cyanobacteria and more and more attention has been paid to it. Biodegradation has been extensively investigated and recognized to be a cost-efficient and environmentally benign method for MC clean-up. In order to further research the growth characteristics of strain and the biodegradation characteristics of MCLR, it is necessary to use the dynamic mathematical models as powerful and useful tools. In this study, strain CQ5 was screened and identified by morphological observation, physiological and biochemical tests, and 16S rDNA sequence analysis. The kinetic models of cell growth and MCLR degradation were established with the Gompertz model and revised Monod kinetic model. The results showed that strain CQ5 had the closest phylogenetic similarity to Lysinibacillus boronitolerans (T-10a, AB199591) in the phylogenetic tree, with 99% bootstrap support. Strain CQ5 could utilize MCLR as the carbon and nitrogen source for growth. When the initial pH value was 7 and the inoculation amount was 3%, strain CQ5 grew well in MSM, in which the MCLR crude extract was used as the carbon and nitrogen source of strain CQ5. Within 244 h, the MCLR concentration changed from 14.12 to 1.57 μg/L and its degradation rate could reach 88.88%. The growth curve fitted with the Gompertz growth model (Nt = 1.3119 * exp(−0.1237 * exp(−6.6341t)), R2 > 0.99). The process of MCLR degradation agreed with the first-order reaction kinetic equation (lnS = 2.64764 − 0.01537t, R2 > 0.99). The linkage relationship between MCLR concentration, cell density, and MCLR degradation rate was consistent with the revised Monod equation (V = 0.342S, R2 > 0.97) at low substrate concentration, where Vmax/ Ks was 0.342. The dynamic relationship in which strain CQ5 degraded MCLR and used it as the carbon and nitrogen source to promote its own growth could be explained by the equation S = 14.12 e− 0.342 Nt (N = 1.08). The growth of strain CQ5 and MCLR concentration in degradation system could be simulated and predicted by the dynamic mathematical models in this study. And the predicted results were very consistent. These results could provide theoretical reference for studying the mechanism of MCLR biodegradation and promote the engineering application of strain CQ5.

Microcystin-LR Degradation and Gene Regulation of Microcystin-Degrading Novosphingobium sp. THN1 at Different Carbon Concentrations.

The bacterium Novosphingobium sp. THN1 (THN1) is capable of degrading microcystin-LR (MC-LR). To study the ability of THN1 to degrade MC-LR and its possible mechanism(s) of regulation, we analyzed the effect of carbon concentrations on the degradation process. The MC-LR degradation rate peaked early and then declined during MC-LR biodegradation. Decreased levels of carbon in the medium caused the degradation peak to occur earlier. The expression of the functional gene mlrA, encoding a microcystinase, showed a similar trend to the MC-LR degradation rate at various carbon concentrations (r2 = 0.717, p < 0.05), suggesting that regulation of mlrA expression may play an important role in MC-LR degradation by THN1. The total bacterial biomass decreased when the carbon source was limited and did not correlate with the MC-LR degradation rate. Transcriptomic analysis showed that MC-LR degradation differentially regulated 62.16% (2597/4178) of THN1 genes. A considerable number of differentially expressed genes (DEGs) during MC-LR degradation encoded proteins related to carbon-, nitrogen-, and amino acid-related pathways. At 2 h of MC-LR degradation, most DEGs (29/33) involved in carbon and nitrogen metabolism were downregulated. This indicated that MC-LR may regulate carbon and nitrogen pathways of Novosphingobium sp. THN1. KEGG pathway analysis indicated that the upregulated DEGs during MC-LR degradation were mainly related to amino acid degradation and substrate metabolism pathways. Particularly, we detected increased expression of glutathione metabolism-related genes from transcriptomic data at 2 h of MC-LR degradation compared with the gene expression of 0 h, such as GST family protein, glutathione peroxidase, S-(hydroxymethyl) glutathione dehydrogenase, and glutathione-dependent disulfide-bond oxidoreductase that have been reported to be involved in microcystin degradation.

Combining solar irradiation with chlorination enhances the photochemical decomposition of microcystin-LR

Microcystin-LR (MC-LR) generated by cyanobacteria is a potent toxin threatening human health. In this study the kinetics and mechanisms of MC-LR elimination from drinking water under solar irradiation with free chlorine (the solar/chlorine process) was evaluated. The rate of MC-LR degradation was dramatically enhanced in the solar/chlorine process (1.1 × 10−2 s−1) compared with chlorination alone (2.6 × 10−3 s−1) or solar irradiation alone (1.2 × 10−4 s−1) with a free chlorine dose of 42 μM (3.0 mg L−1 as Cl2). The enhancement was due to the presence of hydroxyl radicals, reactive chlorine species (RCS), and ozone during free chlorine photolysis. The second-order rate constants of Cl • and Cl2•- reacting with MC-LR were determined to be (2.25 ± 0.07) × 1010 and (5.58 ± 0.42) × 107 M−1s−1, respectively. Cl • was the major RCS contributing to MC-LR elimination. The highest MC-LR degradation rate was observed at pH 8.0. Free chlorine, HO •, Cl • and O3 together accounted for almost 95% of the MC-LR elimination. Hydroxyl- and chloro-MC-LR were generated in the process, followed by dechlorination, dehydration and cleavage of cyclic structures in MC-LR. Aldehyde- and ketone-MC-LR byproducts were also observed. The destruction of dienes led to a great reduction in MC-LR's toxicity. MC-LR removal in natural water samples under natural sunlight irradiation with free chlorine was demonstrated with limited formation of disinfection byproducts. The solar/chlorine process is an energy-efficient approach for MC-LR control, especially suitable for rural areas or where algal blooming threatens.

Ultraviolet photosensitized transformation mechanism of microcystin-LR by natural organic matter in raw water

Microcystins (MCs), produced by cyanobacterial blooms in eutrophic water, are common toxic metabolites and a potential threat to human health. However, the mechanism of MC photodegradation by photosensitizers in raw water remains unclear. In photodegradation and quenching experiments, this study investigates the photosensitized degradation of microcystin-LR (MC-LR) by fulvic acid (FA, a kind of dissolved organic matter with natural photosensitizing properties) under ultraviolet (UV) light irradiation. The photodegradation mechanisms of FA are also explored. The photodegradation process of MC-LR by FA was consistent with second-order reaction kinetics. The degradation rate of MC-LR in FA decreased from 80% to 55% as the pH increased from 3 to 9, because the binding ability of FA to MC-LR reduces as the pH increases. Given that FA can both inhibit and promote MC-LR degradation depending on its concentration, the optimum initial FA concentration for degrading MC-LR was determined as 9.86 mgC·L−1. The excited triplet state of FA (3FA∗) accounted for 50.12% of the MC-LR loss; the remaining loss (49.88%) was contributed by reactive oxygen species and direct photolysis. This implies that the main pathway of MC-LR degradation is reaction with 3FA∗. The MC-LR degradation rate is 36% higher under UV irradiation than that under simulated sunlight irradiation.

Oxidation of Microcystin-LR via Activation of Peroxymonosulfate Using Ascorbic Acid: Kinetic Modeling and Toxicity Assessment.

Advanced oxidation processes (AOPs) have been widely used for the destruction of organic contaminants in the aqueous phase. In this study, we introduce an AOP on activated peroxymonosulfate (PMS) by using ascorbic acid (H2A) to generate sulfate radicals (SO4•-). Sulfate radicals, hydroxyl radicals (HO•), and ascorbyl radicals (A•-) were found using electron spin resonance (ESR). But we found A•- is negligible in the degradation of microcystin-LR (MCLR) due to its low reactivity. We developed a first-principles kinetic model to simulate the MCLR degradation and predict the radical concentrations. The MCLR degradation rate decreased with increasing pH. The scavenging effect of natural organic matter (NOM) on SO4•- was relatively small compared to that for HO•. Considering both energy consumption and MCLR removal, the optimal H2A and PMS doses for H2A/PMS process were determined at 1.0 × 10-6 M and 1.6 × 10-5 M, respectively. In addition, we determined the toxicity using the protein phosphatase 2A (PP2A) test and the results showed that MCLR was readily detoxified and its oxidation byproducts were not hepatotoxic. Overall, our work provides a new type of AOP and a promising, efficient, and environmental-friendly method for removing microcystins in algae-laden water.

Mechanism of Photochemical Degradation of MC-LR by Pyrite

Pyrite was used as catalyst to degrade Microcystin-LR (MC-LR) at pH 6.8 under visible light irradiation (λ>420 nm). X-ray diffraction (XRD) and scanning electron microscope (SEM) characterization showed that pyrite had the layered structure. The ion state of pyrite before and after the reaction was identified using X-Ray Photoelectron Spectroscopy (XPS), confirming the conversion process of Fe(Ⅱ) to Fe(Ⅲ) on the sulfur defect sites. Electron Spin Resonance (ESR) test showed that pyrite photochemical reaction produced hydroxyl radical (·OH). The results of high performance liquid chromatography (HPLC) and liquid chromatograph-mass spectrometer (LC-MS) showed that visible light irradiation could effectively activate pyrite to degrade MC-LR. The degradation rate of MC-LR reached 100% after 10 hours and the mineralization rate reached 60% after 20 hours. The two reaction pathways of photochemical oxidation of MC-LR by pyrite were discussed.

A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes

This study investigated the electrochemical degradation of microcystin-LR (MC-LR) using boron-doped diamond (BDD) anode and mixed metal oxides (MMO, IrO2Ta2O5/Ti) anode in different medium. In-situ electrogenerated oxidants including hydroxyl radical, active chlorine, and persulfate were confirmed in phosphate, chloride, and sulfate medium, respectively. Different from MMO anode, hydroxyl radical was observed to play a significant role in chlorine generation at BDD anode in chloride medium. Besides, BDD anode could activate sulfate electrochemically due to its high oxygen evolution potential, and MC-LR degradation rate increased with the decrease of solution pH. The effects of natural organic matters (NOM) and algal organic matters (AOM) on MC-LR degradation were evaluated and NOM presented stronger inhibition ability than AOM. Furthermore, the intermediates generated in MC-LR degradation in chloride and sulfate medium were identified by LC/MS/MS and possible degradation pathways were proposed based on the experiments results. Benzene ring and conjugated diene bonds of Adda group and double bonds of Mhda group were found to be the reactive sites of MC-LR. Overall, this study broadens the knowledge of electrochemical oxidation in removing microcystins in algae-laden water.

Photodegradation of microcystin-LR catalyzed by metal phthalocyanines immobilized on TiO2-SiO2 under visible-light irradiation.

Microcystins (MCs) are a group of monocyclic heptapeptide toxins produced by species of cyanobacteria. Since MCs exhibit acute and chronic effects on humans and wildlife by damaging the liver, they are of increasing concern worldwide. In this study, we investigated the ability of the phthalocyanine compound (ZnPc-TiO2-SiO2) to degrade microcystin-LR (MC-LR) in the presence of visible light. X-ray diffraction (XRD) and UV-Visible diffuse reflectance spectra (UV-Vis DRS) were utilized to characterize the crystalline phase and the absorption behavior of this catalyst. According to the results, XRD spectra of ZnPc-TiO2-SiO2 powders taken in the 2θ configuration exhibited the peaks characteristic of the anatase phase. UV-Vis DRS showed that the absorption band wavelength shifted to the visible range when ZnPc was supported on the surface of TiO2-SiO2. Subsequently, several parameters including catalyst dose, MC-LR concentrations and pH were investigated. The MC-LR was quantified in each sample through high-performance liquid chromatography (HPLC). The maximum MC-LR degradation rate of 80.2% can be obtained within 300 minutes under the following conditions: catalyst dose of 7.50 g/L, initial MC-LR concentration of 17.35 mg/L, pH 6.76 and the first cycling run of the photocatalytic reaction. Moreover, the degradation process fitted well with the pseudo-first-order kinetic model.

Degradation of Microcystin-LR by Gas-Liquid Interfacial Discharge Plasma

In this study, we report on the degradation of microcystin-LR (MC-LR) by gas-liquid interfacial discharge plasma. The influences of operation parameters such as average input voltage, electrode distance and gas flow rate are investigated. Experimental results indicate that the input voltage and gas flow rate have positive influences on MC-LR degradation, while the electrode distance has a negative one. After 6 min discharge with 25 kV average input voltage and 60 L/h air aeration, the degradation rate of MC-LR achieves 75.3%. H2O2 and O3 generated by discharge both in distilled water and MC-LR solution are measured. Moreover, an emission spectroscopy is used as an indicator of the processes that take place on the gas-liquid boundary and inside plasma. Varied types of radicals (O, ·OH, CO, O3, etc.) are proved to be present in the gas phase during gas-liquid interfacial discharge.

Degradation of microcystin-LR in water by glow discharge plasma oxidation at the gas–solution interface and its safety evaluation

Microcystin-LR (MC-LR) is one of the most commonly found microcystins (MCs) in fresh water and it poses danger to human health due to its potential hepatotoxicity. In the present study, we employed a novel method by using discharge plasma taking place at the gas–solution interface in gas atmosphere to degrade MC-LR in aqueous solution. The initial degradation rate of MC-LR was fastest under acidic conditions (5.41 ± 0.17 × 10−3 mM min−1 at pH 3.04) and decreased to 2.22 ± 0.11 × 10−3 mM min−1 and 0.912 ± 0.02 × 10−3 mM min−1 at pH 4.99 and 7.02, respectively. The effects of total soluble nitrogen (TN), total soluble phosphorus (TP) and natural organic matter (NOM) on the degradation efficiency were studied. The degradation rate was remarkably affected by TP and TN. Mass spectrometry was applied to identify the products of the reactions. Major degradation pathways are proposed according to the results of liquid chromatography/mass spectrometry (LC/MS) results. It suggests that the degradation of MC-LR is initiated via the attack of hydroxyl radicals on the conjugated carbon double bonds of Adda and on the benzene ring of Adda. Finally, the toxicity of intermediates or end-products from MC-LR degraded by this method was assessed using Caenorhabditis elegans. Our findings demonstrates that discharge plasma oxidation is a promising technology for degradation and removal of MC-LR and it may lead us to a new route to efficient treatment of other cyanotoxins from aqueous solutions.

Synthesis, characterization and photocatalytic evaluation of visible light activated C-doped TiO2 nanoparticles

We have demonstrated heterogeneous photocatalytic degradation of microcystin-LR (MC-LR) by visible light activated carbon doped TiO2 (C–TiO2) nanoparticles, synthesized by a modified sol–gel route based on the self-assembly technique exploiting oleic acid as a pore directing agent and carbon source. The C–TiO2 nanoparticles crystallize in anatase phase despite the low calcination temperature of 350 °C and exhibit a highly porous structure that can be optimized by tuning the concentration of the oleic acid surfactant. The carbon modified nanomaterials exhibited enhanced absorption in the broad visible light region together with an apparent red shift in the optical absorption edge by 0.5 eV (2.69 eV), compared to the 3.18 eV of reference anatase TiO2. Carbon species were identified by x-ray photoelectron spectroscopy analysis through the formation of both Ti–C and C–O bonds, indicative of substitution of carbon for oxygen atoms and the formation of carbonates, respectively. Electron paramagnetic resonance spectroscopy revealed the formation of two carbon related paramagnetic centers in C–TiO2, whose intensity was markedly enhanced under visible light illumination, pointing to the formation of localized states within the anatase band gap, following carbon doping. The photocatalytic activity of C–TiO2 nanomaterials was evaluated for the degradation of MC-LR at pH 3.0 under visible light (λ > 420 nm) irradiation. The doped materials showed a higher MC-LR degradation rate than reference TiO2, behavior that is attributed to the incorporation of carbon into the titania lattice.

Effects of water parameters on the degradation of microcystin-LR under visible light-activated TiO 2 photocatalyst

A study was performed to determine the effect of pH, alkalinity, natural organic matter (NOM) and dissolved oxygen in the performance of nitrogen and fluorine doped TiO 2 (NF-TiO 2) for the degradation of hepatotoxin microcystin-LR (MC-LR) in synthetic and natural water under visible light irradiation. The initial degradation rate of MC-LR was fastest under acidic conditions (3.50 ± 0.02 × 10 −3 μM min −1 at pH 3.0) and decreased to 2.29 ± 0.07 × 10 −3 and 0.54 ± 0.02 × 10 −3 μM min −1 at pH 5.7 and 7.1, respectively. Attractive forces between the opposite charged MC-LR and NF-TiO 2 are likely responsible for the enhancement in the photocatalytic decomposition of MC-LR resulting from increased interfacial adsorption. For carbonate buffered solutions, the photocatalytic activity of NF-TiO 2 was reduced when increasing the carbonate concentration up to 150 mg CaCO 3 L −1. The scavenging of radical species by the bicarbonate ion at pH 7.1 is discussed. In the presence of NOM, the degradation rates decreased as pH and initial concentration of the NOM increased. The inhibition was higher with fulvic acid than humic acid under alkaline conditions. Oxygenated solution yields higher NF-TiO 2 photocatalytic degradation of MC-LR compared to nitrogen sparged solution at pH 5.7. The involvement of specific reactive oxygen species implicated in the photodegradation is proposed. Finally, no significant degradation is observed with various natural waters spiked with MC-LR under visible light ( λ > 420 nm) but high removal was achieved with simulated solar light. This study provides a better understanding of the interactions and photocatalytic processes initiated by NF-TiO 2 under visible and solar light. The results indicate solar photocatalytic oxidation is a promising technology for the treatment of water contaminated with cyanotoxins.

Heterogeneous Fenton Photodegradation of Microcystin-LR with Visible Light Irradiation

Under the condition of visible light irradiation (lambda>450 nm), the degradation mechanism of microcystin-LR was studied using H(2)O(2) and FeY catalyst, which is prepared by means of loading Fe(2+) to the molecular sieve NaY. The results indicate that the degradation rate of microcystin-LR is up to 90% in the wide pH range using Vis/FeY/H(2)O(2) system. Based on the intermediates detected by LC-MS analysis, the possible pathways and three vulnerable oxidation sites are put forward: (1) the conjugated double bonds of the Adda chain, (2) the methoxy group of the Adda chain, (3) unsaturated double bond of the Mdha chain. FeY catalyst is of good stability, having catalytic activity after 5 times repetition, which indicates its potential application to the control of the alga wastewater.

The effect of Poterioochromonas abundance on production of intra- and extracellular microcystin-LR concentration

Due to its capability for producing various microcystins, Microcystis aeruginosa is recognized as one of the most toxic, bloom-forming cyanobacteria. In this study, the fates of intra- and extracellular microcystin-LR (MC-LR) were investigated when the mixotrophic golden alga Poterioochromonas sp. (ZX1) was grazing on M. aeruginosa cells. In the control groups, the total MC-LR concentration increased with the growth of M. aeruginosa with an MC-LR content per cell of 0.5–1.5 × 10−8 μg cell−1. In the treatment with ZX1, the total MC-LR decreased linearly throughout the incubation period. In particular, intracellular MC-LR disappeared with a loss of M. aeruginosa cells in the first few days. Part of the intracellular MC-LR was released to the medium under the grazing stress, resulting in an increase of extracellular MC-LR. The degradation rate of MC-LR was positively related to the initial abundance of ZX1 and negatively related to that of M. aeruginosa. The inhibition ratio of MC-LR production dropped sharply from 98 to 67% when the initial abundance of M. aeruginosa increased from 106 to 107 cells ml−1. However, it increased from 84 to 99% when the initial ZX1 abundance increased from 104 to 105 cells ml−1. The effective removal of both M. aeruginosa cells and MC-LR was observed under lower M. aeruginosa abundance ( 1% of M. aeruginosa abundance). Light had little impact on MC-LR degradation, but MC-LR degradation decreased due to the loss of ZX1 after 10 days of darkness. This study showed that the interactions between M. aeruginosa and ZX1 were strongly influenced by their initial abundances.

Experimental and model comparisons of H2O2 assisted UV photodegradation of Microcystin-LR in simulated drinking water

The degradation of Microcystin-LR (MC-LR) in water by hydrogen peroxide assisted ultraviolet (UV/H2O2) process was investigated in this paper. The UV/H2O2 process appeared to be effective in removal of the MC-LR. MC-LR decomposition was primarily ascribed to production of strong and nonselective oxidant-hydroxyl radicals within the system. The intensity of UV radiation, initial concentration of MC-LR, MC-LR purity, dosages of H2O2, the initial solution pH, and anions present in water, to some extent, influenced the degradation rate of MC-LR. A modified pseudo-first-order kinetic model was developed to predict the removal efficiency under different experimental conditions.