Photocatalytic degradation of microcystin-LR in aqueous solutions

A Perspective on Removal of Cyanotoxins from Water Through Advanced Oxidation Processes

This perspective discusses the challenges associated with the removal of cyanotoxins from raw water sources for drinking water treatment and the emergence of sulfate radical-based advanced oxidation processes (SR-AOPs) as an effective treatment technique. The advantage of SR-AOPs is that they can be activated using a variety of methods, including heat, UV radiation, and transition metal catalysts, allowing for greater flexibility in treatment design and optimization. In addition, the byproducts of SR-AOPs are less harmful than those generated by •OH-AOPs, which reduces the risk of secondary contamination. SR-AOPs generate sulfate radicals (SO4 •-) that are highly selective to certain organic contaminants and have lower reactivity to background water constituents, resulting in higher efficiency and selectivity of the process. The presence of natural organic matter and transition metals in the natural water body increases the degradation efficiency of SR-AOPs for the cyanotoxins. The bromate formation is also suppressed when the water contaminated with cyanotoxins is treated with SR-AOPs.

Exploring the photocatalytic inactivation mechanism of Microcystis aeruginosa under visible light using Ag3PO4/g-C3N4

In this work, a series of Ag₃PO₄/g-C₃N₄ (AG) photocatalysts were synthesized. After characterizing the properties, the effects of mass ratio, light intensity, and material dosages on photodegradation were investigated. The material with a 1/2 mass ratio of Ag₃PO₄/g-C₃N₄ showed the highest photocatalytic activity under visible light, and the removal efficiency reached 90.22% for an initial suspended algae concentration of 2.7 × 10⁶ cells/mL, 0.1 g of AG, and 3 h of irradiation. These results showed that the conductivity was increased while the total protein and COD contents of the algae suspension were declined rapidly. In contrast, the variations in the malondialdehyde (MDA) level suggested that the algae cell wall was severely damaged and that selective permeability of the membrane was significantly affected. A possible photocatalytic mechanism was proposed and •O₂^- was shown to be the major reactive oxygen species in the photocatalysis. In summary, during the visible light photocatalytic process, the cell structure was destroyed, which caused the leakage of electrolyte, the inactivation of protein, and the inhibition of photosynthesis; finally, the cells died. This study provides a reference for photodegradation of algae pollution in water bodies.

Effect of ultraviolet radiation (type B) and titanium dioxide nanoparticles on the interspecific interaction between Microcystis flos-aquae and Pseudokirchneriella subcapitata

Anthropogenic activities have led to the depletion of the ultraviolet radiation screening ozone layer, exposing aquatic biota to its harmful effects. Also, the rising applications of nanotechnology are resulting in the release and contamination of aquatic ecosystems with engineered nanometals like titanium dioxide nanoparticles (nTiO₂). The rise in ultraviolet radiation interacts with nanometals, increasing their bioactivities to susceptible aquatic organisms such as algae and cyanobacteria. The effect of ultraviolet radiation B (UVB) and nTiO₂ on Microcystis flos-aquae and Pseudokirchneriella subcapitata during inter-specific interaction was investigated. The specific growth rate (d^-1) of M. flos-aquae exposed to nTiO₂ increased significantly under monoculture conditions but was suppressed during co-culture with P. subcapitata. Contrarily, UVB stimulated the growth of the cyanobacterium regardless of the presence or absence of the green microalgae. However, there was a general decline in the growth of P. subcapitata following cultivation with M. flos-aquae and exposure to UVB and nTiO₂. The chlorophyll-a and total chlorophyll content of the monocultures of M. flos-aquae exposed to nTiO₂ increased while other co-culture treatments significantly decreased these parameters. The experimental treatments, UVB, nTiO₂, and UVB + nTiO₂ had differential effects on the pigment content of P. subcapitata. The total protein content, intracellular H₂O₂, peroxidase (POD), and glutathione S-transferase (GST) activity of both M. flos-aquae and P. subcapitata increased at varying degrees as a function of the treatment condition. Microcystin content was highest in co-cultures exposed to UVB. The results of this study suggest that increasing levels of nTiO₂ and UVB significantly alter the growth and cellular metabolic activity of M. flos-aquae and P. subcapitata, but the cyanobacterium will probably be favored by increasing UVB levels and its interaction with nanometals like nTiO₂ in aquatic ecosystems.

Study of cyanotoxin degradation and evaluation of their transformation products in surface waters by LC-QTOF MS

In the present work, the degradation of three cyanotoxins from the hepatotoxins group was investigated under laboratory-controlled experiments in water samples. Surface waters spiked with microcystin-LR (MC-LR), nodularin (NOD) and cylindrospermopsin (CYN) were subjected to hydrolysis, chlorination and photo-degradation, under both sunlight (SL) and ultraviolet (UV) radiation. A total of 12 transformation products (TPs) were detected and tentatively identified by liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (LC-QTOF MS). These comprised: 6 chlorination TPs (3 from CYN and 3 from MC-LR, 2 isomers); 4 UV TPs (all from CYN); and 2 sunlight TPs (one isomer from MC-LR and another from NOD). No TPs were observed under hydrolysis conditions. The chemical structures for all TPs were tentatively proposed based on the accurate-mass QTOF MS full-spectra. Analysis of real-world samples collected from the Peñol reservoir (Antioquia, Colombia) revealed the presence of MC-LR and CYN as well as a sunlight TP identified in the laboratory experiments. Data presented in this article will assist further research on TPs potentially formed in future tertiary degradation processes applied for the removal of organic micro-pollutants in water; as well as improving available knowledge on the toxic implications of cyanobacterial toxins TPs in surface waters.

Microcystin Content in Phytoplankton and in Small Fish from Eutrophic Nyanza Gulf, Lake Victoria, Kenya

The human health risks posed by exposure to cyanobacterial toxins such as microcystin (MC) through water and fish consumption remain poorly described. During the last two decades, coastal regions of Lake Victoria such as Nyanza Gulf (Kisumu Bay) have shown severe signs of eutrophication with blooms formed by Microcystis producing MC. In this study, the spatial variability in MC concentration in Kisumu Bay was investigated which was mostly caused by Microcystis buoyancy and wind drifting. Small fish (<6 cm) mainly composed of Rastrineobola argentea were examined for MC content by means of biological methods such as ELISA and protein phosphatase inhibition assay (PPIA) and partly by chemical-analytical methods such as LC-MS/MS. Overall, the MC content in small fish was related to the MC content observed in the seston. When comparing the MC content in the seston in relation to dry weight with the MC content in small fish the latter was found three orders of magnitude decreased. On average, the ELISA-determined MC contents exceeded the PPIA-determined MC contents by a factor of 8.2 ± 0.5 (SE) while the MC contents as determined by LC-MS/MS were close to the detection limit. Using PPIA, the MC content varied from 25–109 (mean 62 ± 7) ng/g fish dry weight in Kisumu Bay vs. 14 ± 0.8 ng MC/g in the more open water of L. Victoria at Rusinga channel. Drying the fish under the sun showed little effect on MC content, although increased humidity might indirectly favor photocatalyzed MC degradation.

Detection of free microcystins in the liver and muscle of freshwater fish by liquid chromatography-tandem mass spectrometry

MC analysis of biological tissue is considered to be very difficult due to the lack of validated methods. This is the primary limiting factor for monitoring potential risks in both the flesh of aquatic organisms and the aquatic ecosystem. In this study, an effective method to determine free MCs (MC-LR and MC-RR) in the muscle and liver tissues of freshwater cultured fish was developed using solid-phase extraction (SPE) and liquid chromatography-tandem mass spectrometry (LC/MS-MS). The extraction solvent, time of extraction, eluent and purification of the extract were optimized. Various SPE cartridges were also investigated. In this optimized analytical procedure, an 85% methanol/water solution (v/v) was selected as the extraction solvent, after which the extracts were purified by removing fats and proteins; a HLB cartridge was chosen for MCs enrichment; and 90% methanol containing 0.02% formic acid/water solution (v/v) was used as the eluent. Under the optimized pretreatment conditions and instrument parameters, good recoveries of MC-LR and MC-RR were obtained at three concentrations (0.5, 1.0 and 2.0 µg g^-1 dry weight (DW)), with values ranging from 92.5 to 98.3% and 92.1 to 98.6%, respectively. The method detection limit (MDL) for muscle samples was 0.5 µg kg^-1 and 0.4 µg kg^-1 (DW) for MC-LR and MC-RR, respectively. The MDL for the liver samples was 0.8 µg kg^-1 (DW) for both MC-LR and MC-RR. The developed procedure was successfully applied to analyze MCs in the muscle and liver of fish samples collected from a Chinese freshwater aquaculture pond during bloom seasons. The MC-LR concentrations ranged from below the MDL to 4.17 µg kg^-1 and the MC-RR concentrations ranged from below the MDL to 2.64 µg kg^-1.

Degradation mechanisms of Microcystin-LR during UV-B photolysis and UV/H2O2 processes: Byproducts and pathways

The removal and degradation pathways of microcystin-LR (MC-LR, [M+H]⁺ = 995.6) in UV-B photolysis and UV-B/H₂O₂ processes were examined using liquid chromatography-tandem mass spectrometry. The UV/H₂O₂ process was more efficient than UV-B photolysis for MC-LR removal. Eight by-products were newly identified in the UV-B photolysis ([M+H]⁺ = 414.3, 417.3, 709.6, 428.9, 608.6, 847.5, 807.4, and 823.6), and eleven by-products were identified in the UV-B/H₂O₂ process ([M+H]⁺ = 707.4, 414.7, 429.3, 445.3, 608.6, 1052.0, 313.4, 823.6, 357.3, 245.2, and 805.7). Most of the MC-LR by-products had lower [M+H]⁺ values than the MC-LR itself during both processes, except for the [M+H]⁺ value of 1052.0 during UV-B photolysis. Based on identified by-products and peak area patterns, we proposed potential degradation pathways during the two processes. Bond cleavage and intramolecular electron rearrangement by electron pair in the nitrogen atom were the major reactions during UV-B photolysis and UV-B/H₂O₂ processes, and hydroxylation by OH radical and the adduct formation reaction between the produced by-products were identified as additional pathways during the UV-B/H₂O₂ process. Meanwhile, the degradation by-products identified from MC-LR during UV-B/H₂O₂ process can be further degraded by increasing H₂O₂ dose.

UV-B Exposure Affects the Biosynthesis of Microcystin in Toxic Microcystis aeruginosa Cells and Its Degradation in the Extracellular Space

Microcystins (MCs) are cyclic hepatotoxic heptapeptides produced by cyanobacteria that can be toxic to aquatic and terrestrial organisms. MC synthesis and degradation are thought to be influenced by several different physical and environmental parameters. In this study, the effects of different intensities of UV-B radiation on MC biosynthesis in Microcystis cells and on its extracellular degradation were investigated by mRNA analysis and degradation experiments. Exposure to UV-B at intensities of 1.02 and 1.45 W/m² not only remarkably inhibited the growth of Microcystis, but also led to a decrease in the MC concentration. In addition, mcyD transcription was decreased under the same UV-B intensities. These results demonstrated that the effects of UV-B exposure on the biosynthesis of MCs in Microcystis cells could be attributed to the regulation of mcy gene transcription. Moreover, the MC concentration was decreased significantly after exposure to different intensities of UV-B radiation. Of the three MC variants (MC-LR, -RR and -YR, L, R and Y are abbreviations of leucine, arginine and tyrosine), MC-LR and MC-YR were sensitive to UV-B radiation, whereas MC-RR was not. In summary, our results showed that UV-B radiation had a negative effect on MC production in Microcystis cells and MC persistence in the extracellular space.

Bioreactor study employing bacteria with enhanced activity toward cyanobacterial toxins microcystins

An important aim of white (grey) biotechnology is bioremediation, where microbes are employed to remove unwanted chemicals. Microcystins (MCs) and other cyanobacterial toxins are not industrial or agricultural pollutants; however, their occurrence as a consequence of human activity and water reservoir eutrophication is regarded as anthropogenic. Microbial degradation of microcystins is suggested as an alternative to chemical and physical methods of their elimination. This paper describes a possible technique of the practical application of the biodegradation process. The idea relies on the utilization of bacteria with a significantly enhanced MC-degradation ability (in comparison with wild strains). The cells of an Escherichia coli laboratory strain expressing microcystinase (MlrA) responsible for the detoxification of MCs were immobilized in alginate beads. The degradation potency of the tested bioreactors was monitored by HPLC detection of linear microcystin LR (MC-LR) as the MlrA degradation product. An open system based on a column filled with alginate-entrapped cells was shown to operate more efficiently than a closed system (alginate beads shaken in a glass container). The maximal degradation rate calculated per one liter of carrier was 219.9 µg h-1 of degraded MC-LR. A comparison of the efficiency of the described system with other biological and chemo-physical proposals suggests that this new idea presents several advantages and is worth investigating in future studies.

A novel photocatalytic material for removing microcystin-LR under visible light irradiation: degradation characteristics and mechanisms

BACKGROUND AND PURPOSE

Microcystin-LR (MC-LR), a common toxic species in contaminated aquatic systems, persists for long periods because of its cyclic structure. Ag3PO4 is an environment-friendly photocatalyst with relatively good degradation capacity for hazardous organic pollutants. This study aimed to investigate the degradation capacity of Ag3PO4 for MC-LR under visible light.

METHODS

An Ag3PO4 photocatalyst was synthesized by the ion-exchange method and characterized by X-ray diffraction, field-emission scanning electron microscope, and UV-Vis spectrophotometer. MC-LR was quantified in each sample through high-performance liquid chromatograph. The degradation efficiency of MC-LR was affected by initial pH, initial Ag3PO4 concentration, initial MC-LR concentration, and recycle experiments. The degradation intermediates of MC-LR were examined by liquid chromatography-mass spectrometry (LC/MS).

RESULTS

The degradation process can be well fitted with the pseudo-first-order kinetic model. The maximum MC-LR degradation rate of 99.98% can be obtained within 5 h under the following optimum conditions: pH of 5.01, Ag3PO4 concentration of 26.67 g/L, and MC-LR concentration of 9.06 mg/L. Nine intermediates were detected and analyzed by LC/MS. Three main degradation pathways were proposed based on the molecular weight of the intermediates and the reaction mechanism: (1) hydroxylation on the aromatic ring of Adda, (2) hydroxylation on the diene bonds of Adda, and (3) internal interactions on the cyclic structure of MC-LR.

CONCLUSION

Ag3PO4 is a highly efficient catalyst for MC-LR degradation in aqueous solutions.

Preparation and characterization of TiO2–Graphene@Fe3O4 magnetic composite and its application in the removal of trace amounts of microcystin-LR

A facile and feasible approach is presented to prepare a novel magnetic photocatalyst TiO₂–graphene@Fe₃O₄ composite exhibiting a superior performance in the removal of trace microcystin-LR in water samples.