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

Selective production of singlet oxygen for harmful cyanobacteria inactivation and cyanotoxins degradation: Efficiency and mechanisms

Knowledge about the impact of singlet oxygen (¹O₂) on the characteristics and inactivation of harmful cyanobacterial organic matter is limited. In this study, the feasibility of using an improved single-iron doped graphite-like phase carbon nitride catalyst (FeCN) to activate peroxymonosulfate (PMS) catalytic production of ¹O₂ to inactivate four harmful cyanobacteria was investigated. The inactivation efficiencies at 30 min were 92.77%, 66.84%, 91.06%, and 93.45% for Microcystis aeruginosa (M. aeruginosa), Nodularia harveyana, Oscillatoria sp., and Nostoc sp., respectively. This was associated with adjusting experimental parameters, such as the FeCN and PMS doses and initial pH, to obtain the maximum ¹O₂ yield. The quenching experiment results and electron paramagnetic resonance spectra showed that ¹O₂ generated via the non-radical pathway might play a dominant role in inactivating harmful cyanobacteria and degrading harmful algal toxins (Microcystin-LR and Nodularin). In addition, the FeCN-PMS system not only effectively destroyed the integrity of harmful cyanobacterial cells but also effectively degraded cyanobacterial toxins, thereby preventing severe secondary contamination by cell rupture. A possible removal mechanism was proposed. This reveals the potential of ¹O₂ to simultaneously inactivate harmful cyanobacteria and degrade harmful cyanobacterial toxins.

Degradation mechanisms of cyanobacteria neurotoxin β-N-methylamino-l-alanine (BMAA) during UV254/H2O2 process: Kinetics and pathways

In this work, the UV₂₅₄/H₂O₂ process was utilized to remove β-N-methylamino-l-alanine (BMAA), a kind of cyanobacteria neurotoxin, and the influence of reaction parameters and environmental factors on the degradation of BMAA has been systematically investigated. The results showed that BMAA could be effectively removed in the UV₂₅₄/H₂O₂ system compared to UV or H₂O₂ alone and OH was confirmed as the main ROS to degrade BMAA. The degradation rate of BMAA increased first and then decreased with the increase of pH and the maximum k_obs was 0.1545 min^-1 obtained at pH 9. The removal of BMAA in the UV₂₅₄/H₂O₂ system was inhibited in actual water, while the degradation rate of BMAA in actual water could still exceed 90% by appropriately extending the reaction time. The decrease in the degradation efficiency of BMAA in actual water was primarily due to the ultraviolet light absorption and competition effects of NOM, and anions (Cl^- and HCO₃^-) would also inhibit the degradation of BMAA. Five by-products ([M - H]^- = 118, 103, 88, 87 and 59) were identified in this study and the degradation pathways of BMAA were proposed. The production of by-products was attributed to the fracture of the C-N bond and hydroxylation reaction. This study is worthwhile to deepen the understanding of the degradation mechanism of BMAA in the UV₂₅₄/H₂O₂ system.

Comparison of UV-A photolytic and UV/TiO2 photocatalytic effects on Microcystis aeruginosa PCC7813 and four microcystin analogues: A pilot scale study

To date, the high cost of supplying UV irradiation has prevented the widespread application of UV photolysis and titanium dioxide based photocatalysis in removing undesirable organics in the water treatment sector. To overcome this problem, the use of UV-LEDs (365 nm) for photolysis and heterogeneous photocatalysis applying TiO₂ coated glass beads under UV-LED illumination (365 nm) in a pilot scale reactor for the elimination of Microcystis aeruginosa PCC7813 and four microcystin analogues (MC-LR, -LY, -LW, -LF) with a view to deployment in drinking water reservoirs was investigated. UV-A (365 nm) photolysis was shown to be more effective than the UV/TiO₂ photocatalytic system for the removal of Microcystis aeruginosa cells and microcystins. During photolysis, cell density significantly decreased over 5 days from an initial concentration of 5.8 × 10⁶ cells mL^-1 until few cells were left. Both intra- and extracellular microcystin concentrations were significantly reduced by 100 and 92 %, respectively, by day 5 of the UV treatment for all microcystin analogues. During UV/TiO₂ treatment, there was great variability between replicates, making prediction of the effect on cyanobacterial cell and toxin behavior difficult.

NiMoO4 nanorods photocatalytic activity comparison under UV and visible light

Waste water remediation is the ongoing hot research topic that can reduce the water scarcity all over the world. By reducing the pollutants in the waste water drawn from industries and other sources will be more useful for domestic purposes. To reduce the rate of pollutants in water may also help in improving the aquatic environment and decreases other side effects. Efficient and cost effective catalysts were in search for both dye degradation and water remediation treatment applications. NiMoO₄ nanorods were prepared by employing co-precipitation method with different stirrer time (2 h, 4 h and 6 h). The formation of NiMoO₄ was substantiated employing X-ray diffractometer analysis (XRD). Vibrational and rotational property of the samples was analyzed by FT-IR spectra and Raman spectra. The optical property was further confirmed by UV-vis spectral studies. Morphological analysis studies revealed growth of nanorods with 6 h stirrer time. The photocatalytic behavior of the obtained product was carried out under both UV light (364 nm) and visible light irradiation. The samples subjected to visible light environment showed better efficiency on degrading the methylene blue (MB) dye. The efficiency obtained under UV irradiation were 20%, 31%, 33%, 41% and efficiency obtained in visible light irradiation were 27%, 42%, 46%, 55% with respect to bare methylene blue (MB), MB with NiMoO₄ (2 h), MB with NiMoO₄ (4 h), MB with NiMoO₄ (6 h) catalyst added. NiMoO₄ sample with 6 h stirrer time and fine nanorods growth will be the good candidate for future use.

Kinetics of Microcystin-LR Removal in a Real Lake Water by UV/H2O2 Treatment and Analysis of Specific Energy Consumption

The hepatotoxin microcystin-LR (MC-LR) represents one of the most toxic cyanotoxins for human health. Considering its harmful effect, the World Health Organization recommended a limit in drinking water (DW) of 1 µg L⁻¹. Due to the ineffectiveness of conventional treatments present in DW treatment plants against MC-LR, advanced oxidation processes (AOPs) are gaining interest due to the high redox potential of the OH^• radicals. In this work UV/H₂O₂ was applied to a real lake water to remove MC-LR. The kinetics of the UV/H₂O₂ were compared with those of UV and H₂O₂ showing the following result: UV/H₂O₂ > UV > H₂O₂. Within the range of H₂O₂ tested (0–0.9 mM), the results showed that H₂O₂ concentration and the removal kinetics followed an increasing quadratic relation. By increasing the initial concentration of H₂O₂, the consumption of oxidant also increased but, in terms of MC-LR degraded for H₂O₂ dosed, the removal efficiency decreased. As the initial MC-LR initial concentration increased, the removal kinetics increased up to a limit concentration (80 µg L⁻¹) in which the presence of high amounts of the toxin slowed down the process. Operating with UV fluence lower than 950 mJ cm⁻², UV alone minimized the specific energy consumption required. UV/H₂O₂ (0.3 mM) and UV/H₂O₂ (0.9 mM) were the most advantageous combination when operating with UV fluence of 950–1400 mJ cm⁻² and higher than 1400 mJ cm⁻², respectively.

Effects of electron beam irradiation on proteins and exopolysaccharide production and changes in Microcystis aeruginosa

Purpose: Microcystis aeruginosa often threaten human health and safety for the toxin and unpleasant odor and removal difficulties during water treatment process. In order to remove it, a novel method high energy was studied in this research.Materials and methods: The electron beam generated by an accelerator was applied to irradiate M. aeruginosa at various doses of 1, 2, 3, 4 and 5 kGy. The effects of irradiation on M. aeruginosa characteristics and mechanism have been researched through surveying the changes in pH and conductivity, changes of algae cell community structure and respiration rate, and changes of protein and exopolysaccharides production were also detected.Result: The data showed that exposure to 2-5 kGy radiation could make pH decrease. Microcystis aeruginosa increased through its own photosynthesis and physiological regulation. The increasing damage to algal cells led to the exosmosis of the contents, which increased the electrical conductivity of algae liquid and decreased the area of algae cells colony. 2-5 kGy irradiation decreased protein content and destroy the antioxidant system and thus reduced the secretion of extracellular polysaccharidesConclusions: 2-5 kGy radiation could control the algae growth and produced obvious effect. The respiration rate decreased obviously that made M. aeruginosa lose activity in a short time. The results proved that irradiation could change the algae growth and affect its life characteristic efficiently in a short time.

Degradation of cyanotoxin microcystin-LR in synthetic and natural waters by chemical-free UV/VUV radiation

This study investigated the capability of ultraviolet radiation at 254 nm and 185 nm (UV/VUV) to degrade cyanotoxin microcystin-LR (MC-LR). Results showed 70% toxin reduction solely by 254 nm direct photolysis (ε₂₅₄ = 13,225 ± 814 M^-1cm^-1; Φ₂₅₄ = 0.29 ± 0.03 mol/Einstein). The addition of 185 nm increased MC-LR degradation through advanced oxidation by ^•OH (k_•OH,MC-LR = 2.25 ± 0.39 × 10¹⁰ M^-1s^-1). Alkalinity and organics (DOC) reduced MC-LR degradation by scavenging ^•OH (k_obs,MilliQ = 0.117 cm²/mJ; k_{obs,50ppmAlk.} = 0.0497 cm²/mJ; k_obs,6ppmDOC = 0.019 cm²/mJ). Chloride absorbed 185 nm, impacting ^•OH formation and generating Cl^•, while also scavenging ^•OH. However, Cl^• is reactive and ^•OH scavenging is reversible, resulting in relatively low impact on MC-LR degradation (k_obs,50ppmCl = 0.0939 cm²/mJ). In natural water, MC-LR could be degraded from a typical concentration (˜15 μg/L) to below detection (<0.5 μg/L) with a UV₂₅₄ fluence of 200 mJ/cm² using UV/VUV. The presence of cyanobacterial cells impeded MC-LR degradation; however, 90% MC-LR degradation could still be achieved. UV/VUV is a promising chemical-free technology capable of MC-LR degradation in a variety of water conditions, and a potentially suitable treatment option for small, remote communities.

Elimination of trichloroanisoles by UV/H2O2: Kinetics, degradation mechanism, water matrix effects and toxicity assessment

The elimination of 2,3,6-trichloroanisole (2,3,6-TCA), which produces a musty-earthy off-odor in water, by an ultraviolet (UV)/H₂O₂ process was assessed. The removal of 88.1% of 2,3,6-TCA in ultrapure water (UPW) was achieved using an initial 2,3,6-TCA concentration of 1 μg L^-1 (4.73 nM), a H₂O₂ concentration of 20 mg L^-1 (0.588 mM), a UV intensity of 1.44 mW cm^-2 and a pH of 8.2. The reaction was found to be pseudo first order with a rate constant (k_obs) of 0.0340 min^-1. Both the removal efficiency and k_obs increased significantly upon increasing the H₂O₂ concentration from 10 to 50 mg L^-1. The second order rate constant (k_{HO·,2,3,6-TCA}) in competition kinetic trials was determined to be 8.17 × 10⁷ M^-1s^-1. Degradation products generated during both the UV photolysis and UV/H₂O₂ treatment of 2,3,6-TCA solutions were analyzed using ultrahigh resolution gas chromatography/mass spectrometry, and the degradation mechanism was proposed. The toxicities of water solutions during both processes were assessed via a luminescence method in conjunction with Vibrio fischeri. The pH and Cl^-, HCO₃^- and natural organic matter concentrations of the aqueous medium were all found to significantly affect the removal of 2,3,6-TCA. The degradation rates of trichloroanisoles (TCAs) in real-world water samples demonstrated that UV/H₂O₂ has significant potential with regard to controlling TCAs as pollutants in water.

•OH Inactivation of Cyanobacterial Blooms and Degradation of Toxins in Drinking Water Treatment System

Cyanobacterial blooms continue to serve as one of the most serious global issues threatening water supply and human health. During cyanobacterial bloom season, a large •OH-yield equipment was developed and installed after coagulation settling in a 12000 ton/day drinking water treatment system in Xiamen, China. An •OH concentration of 7.76∼57.8 μmol/L was formed by using the oxygen activated species generated by strong ionisation discharge combining with the effect of water jet cavitation. •OH pre-treatment at a dose of 1.0 mg/L inactivated cyanobacterial blooms in the process of conveying bloom water within only 20s, which were then removed by sand filtration. Under SEM observation, dominant Microcystis sp. colonies connected by mucilage were dispersed into individuals that still retained the cell integrity, indicating no release of intracellular organic matter (IOM). According to a flow cytometry analysis, the main cause of •OH inactivation was the breakage of DNA strands. Meanwhile, the •OH-mineralized microcystin-LR was by breaking the C=C conjugated diene bond and crucial opening the persistent benzene ring to carboxylic acid m/z 158.0. During •OH pre-treatment of 1.0 mg/L and NaClO disinfection of 0.5 mg/L, all water quality indexes and disinfection by-product (DBP) contents complied with the Chinese Sanitary Standards for Drinking Water. Therefore, the •OH based on the strong ionisation discharge showed great prospect for large-scale drinking water treatment in the removal of cyanobacterial blooms while retaining cell integrity as well as the degradation of toxins.

Insight into elemental mercury (Hg0) removal from flue gas using UV/H2O2 advanced oxidation processes

Elemental mercury (Hg⁰) emitted from coal-fired power plants and municipal solid waste (MSW) incinerators has caused great harm to the environment and human beings. The strong oxidized ^•OH radicals produced by UV/H₂O₂ advanced oxidation processes were studied to investigate the performance of Hg⁰ removal from simulated flue gases. The results showed that when H₂O₂ concentration was 1.0 mol/L and the solution pH value was 4.1, the UV/H₂O₂ system had the highest Hg⁰ removal efficiency. The optimal reaction temperature was approximately 50 °C and Hg⁰ removal was inhibited when the temperature was higher or lower. The yield of ^•OH radicals during UV/H₂O₂ reaction was studied by electron paramagnetic resonance (EPR) analysis. UV radiation was the determining factor to remove Hg⁰ in UV/H₂O₂ system due to ^•OH generation during H₂O₂ decomposition. SO₂ had little influence on Hg⁰ removal whereas NO had an inhibitory effect on Hg⁰ removal. The detailed findings for Hg⁰ removal reactions over UV/H₂O₂ make it an attractive method for mercury control from flue gases.