Kinetics of reactions between chlorine and the cyanobacterial toxins microcystins

Individual and combined effects of sodium dichloroisocyanurate and isothiazolinone on the cyanobacteria-Vallisneria natans-microbe aquatic ecosystem

The use of algaecides to control high-density cyanobacterial blooms is often complicated by secondary pollution and the toxicity to non-target organisms. This study investigates the individual and combined effects of sodium dichloroisocyanurate (NaDCC, 5, 50, and 100 mg/L) and isothiazolinone (0.1, 0.5, and 1.5 mg/L) on a cyanobacteria-Vallisneria natans-microbe aquatic ecosystem, focusing on their interactions and ecological impacts. Results indicate that NaDCC could achieve a higher algae removal rate than isothiazolinone within 15 days, but has a greater negative effect on Vallisneria natans. Both algaecides disrupt nutrient and secondary metabolite balances at low and high concentrations, increasing nutrient loads and harmful substances. Optimal results were obtained with low concentrations of NaDCC (5 mg/L) and isothiazolinone (0.1 mg/L), effectively controlling cyanobacteria while minimizing harm to Vallisneria natans and reducing nutrient loads and microcystin accumulation. Algaecide application enhanced microbial diversity in water and leaves, shifting the dominant community from cyanobacteria to organisms adapted to the post-cyanobacterial decay environment. Metabolomic analysis indicated increased secretion of lipids and organic acids by cyanobacteria in response to algaecide stress. High concentrations of NaDCC and isothiazolinone disrupted nitrogen metabolism in cyanobacteria and induced ROS overproduction, affecting unsaturated fatty acid synthesis and other metabolic pathways. These findings highlight the importance of exploring different combinations of algaecides to reduce their concentrations, balance algal control with ecological stability, and offer insights for effective eutrophication management.

Oxidation of Microcystis aeruginosa and Microcystins with Peracetic Acid

Peracetic acid (PAA) shows potential for use in drinking water treatment as an alternative to prechlorination, such as for mussel control and disinfection by-product precursor destruction, though its impact as a preoxidant during cyanobacterial blooms remains underexplored. Here, Microcystis aeruginosa inactivation and microcystin-LR and -RR release and degradation using PAA were explored. The toxin degradation rates were found to be higher in alkaline conditions than in neutral and acidic conditions. However, all rates were significantly smaller than comparable rates when using free chlorine. The inactivation of M. aeruginosa cells using PAA was faster at acidic pH, showing immediate cell damage and subsequent cell death after 15-60 min of exposure to 10 mg/L PAA. In neutral and alkaline conditions, cell death occurred after a longer lag phase (3-6 h). During cell inactivation, microcystin-LR was released slowly, with <35% of the initial intracellular toxins measured in solution after 12 h of exposure to 10 mg/L PAA. Overall, PAA appears impractically slow for M. aeruginosa cell inactivation or microcystin-LR and -RR destruction in drinking water treatment, but this slow reactivity may also allow it to continue to be applied as a preoxidant for other purposes during cyanobacterial blooms without the risk of toxin release.

Physico-chemical treatments for the removal of cyanotoxins from drinking water: Current challenges and future trends

Cyanobacteria are highly prevalent blue-green algae that grow in stagnant and nutrient-rich water bodies. Environmental conditions, such as eutrophication and human activities, increased the cyanobacterial blooms in freshwater resources worldwide. The excessive bloom formation has also resulted in an alarming surge of cyanobacterial toxins. Prolonged exposure to cyanotoxins is a potential threat to natural ecosystems, animal and human health by the spoilage of the quality of bathing and drinking water. Various molecular and analytical methods have been proposed to monitor their occurrence and understand their global distribution. Moreover, different physical, chemical, and biological approaches have been employed to control cyanobacterial blooms and their toxins to mitigate their occurrence. Numerous strategies have been engaged in drinking water treatment plants (DWTPs). However, the degree of treatment varies greatly and is primarily determined by the source, water properties, and operating parameters such as temperature, pH, and cyanotoxin variants and levels. A comprehensive compilation of methods, from traditional approaches to more advanced oxidation processes (AOPs), are presented for the removal of intracellular and extracellular cyanotoxins. This review discusses the effectiveness of various physicochemical operations and their limitations in a DWTP, for the removal of various cyanotoxins. These operations span from simple to advanced treatment levels with varying degrees of effectiveness and differing costs of implementation. Furthermore, mitigation measures applied in other toxin systems have been considered as alternative strategies.

Responses of Vallisneria natans and Pistia stratiotes to Cu2+ and Mn2+ stress: Occurrence of caffeic acid and its degradation kinetics during chlorination

Macrophytes are crucial in maintaining the equilibrium of aquatic ecosystems. However, the pattern of macrophyte-derived caffeic acid (CA) release under heavy metal stress is yet to be fully understood. More importantly, due to its functional groups, CA may be a precursor to the formation of disinfection by-products, posing threats to water ecology and even safety of human drinking water. This study analyzed the responses of CA released by Vallisneria natans (V. natans) and Pistia stratiotes (P. Stratiotes) when exposed to Cu2+ and Mn2+ stress. Additionally, the CA levels in two constructed wetland ponds were detected and the degradation kinetics of CA during chlorination were investigated. Results indicated that CA occurred in two constructed wetland ponds with the concentrations of 44.727 μg/L (planted with V. natans) and 61.607 μg/L (planted with P. Stratiotes). Notably, heavy metal stress could significantly affect CA release from V. natans and P. Stratiotes. In general, under Cu2+ stress, V. natans secreted far more CA than under Mn2+ stress, the level could reach up to 435.303 μg/L. However, compared to V. natans, P. Stratiotes was less affected by Cu2+ and Mn2+ stress, releasing a maximum CA content of 55.582 μg/L under 5 mg/L Mn2+ stress. Aquatic macrophytes secreted more CA in response to heavy metal stresses and protected macrophytes from harmful heavy metals. CA degradation followed the pseudo first-order kinetics model, and the chlorination of CA conformed to a second-order reaction. The reaction rate significantly accelerated as NaClO, pH, temperature and Br- concentration increased. A new pathway for CA degradation and a new DBP 2, 2, 3, 3-tetrachloropropanal were observed. These findings pointed at a new direction into the adverse effect of CA, potentially paving the way for new strategies to solve drinking water safety problems.

Graphene as a rational interface for enhanced adsorption of microcystin-LR from water

Cyanotoxins such as microcystin-LR (MC-LR) represent a global environmental threat to ecosystems and drinking water supplies. The study investigated the direct use of graphene as a rational interface for removal of MC-LR via interactions with the aromatic ring of the ADDA1 chain of MC-LR and the sp2 hybridized carbon network of graphene. Intra-particle diffusion model fit indicated the high mesoporosity of graphene provided significant enhancements to both adsorption capacities and kinetics when benchmarked against microporous granular activated carbon (GAC). Graphene showed superior MC-LR adsorption capacity of 75.4 mg/g (Freundlich model) compared to 0.982 mg/g (Langmuir model) for GAC. Sorption kinetic studies showed graphene adsorbs 99% of MC-LR in 30 min, compared to zero removal for GAC after 24 hr using the same MC-LR concentration. Density functional theory (DFT), calculations showed that postulated π-based interactions align well with the NMR-based experimental work used to probe primary interactions between graphene and MC-LR adduct. This study proved that π-interactions between the aromatic ring on MC-LR and graphene sp2 orbitals are a dominant interaction. With rapid kinetics and adsorption capacities much higher than GAC, it is anticipated that graphene will offer a novel molecular approach for removal of toxins and emerging contaminants with aromatic systems.

Transformation of Algal Toxins during the Oxidation/Disinfection Processes of Drinking Water: From Structure to Toxicity

With the increase of algal blooms worldwide, drinking water resources are threatened by the release of various algal toxins, which can be hepatotoxic, cytotoxic, or neurotoxic. Because of their ubiquitous occurrence in global waters and incomplete removal in conventional drinking water treatment, oxidation/disinfection processes have become promising alternative treatment options to destroy both the structures and toxicity of algal toxins. This Review first summarizes the occurrence and regulation of algal toxins in source water and drinking water. Then, the transformation kinetics, disinfection byproducts (DBPs)/transformation products (TPs), pathways, and toxicity of algal toxins in water oxidation/disinfection processes, including treatment by ozonation, chlorination, chloramination, ultraviolet-based advanced oxidation process, and permanganate, are reviewed. For most algal toxins, hydroxyl radicals (HO•) exhibit the highest oxidation rate, followed by ozone and free chlorine. Under practical applications, ozone and chlorine can degrade most algal toxins to meet water quality standards. However, the transformation of the parent structures of algal toxins by oxidation/disinfection processes does not guarantee a reduction in toxicity, and the formation of toxic TPs should also be considered, especially during chlorination. Notably, the toxicity variation of algal toxins is associated with the chemical moiety responsible for toxicity (e.g., Adda moiety in microcystin-LR and uracil moiety in cylindrospermopsin). Moreover, the formation of known halogenated DBPs after chlorination indicates that toxicity in drinking water may shift from toxicity contributed by algal toxins to toxicity contributed by DBPs. To achieve the simultaneous toxicity reduction of algal toxins and their TPs, optimized oxidation/disinfection processes are warranted in future research, not only for meeting water quality standards but also for effective reduction of toxicity of algal toxins.

Chlorination of amides: Kinetics and mechanisms of formation of N-chloramides and their reactions with phenolic compounds

Amides are common constituents in natural organic matter and synthetic chemicals. In this study, we investigated kinetics and mechanisms of the reactions of chlorine with seven amides, including acetamide, N-methylformamide, N-methylacetamide, benzamide, N-methylbenzamide, N-propylbenzamide, and N-(benzoylglycyl)glycine amide. Apparent second-order rate constants for the reactions of the amides with chlorine at pH 8 are in the range of 5.8 × 10-3 - 1.8 M-1s-1 and activation energies in the range of 62-88 kJ/mol. The second-order rate constants for the reactions of chlorine with different amides decrease with increasing electron donor character of the substituents on the amide-N and N-carbonyl-C in the amide structures. Hypochlorite (‒OCl) dominates the reactions of chlorine with amides yielding N-chloramides with species-specific second-order rate constants in the range of 7.3 × 10-3 - 2.3 M-1s-1. Kinetic model simulations suggest that N-chlorinated primary amides further react with HOCl with second-order rate constants in the order of 10 M-1s-1. The chlorination products of amides, N-chloramides are reactive towards phenolic compounds, forming chlorinated phenols via electrophilic aromatic substitution (phenol and resorcinol) and quinone via electron transfer (hydroquinone). Meanwhile, N-chloramides were recycled to the parent amides. At neutral pH, apparent second-order rate constants for the reactions between phenols and N-chloramides are in the order of 10-4-0.1 M-1s-1, comparable to those with chloramine. The findings of this study improve the understanding of the fate of amides and chlorine during chlorination processes.

Chlorination of microcystin-LR in natural water: Kinetics, transformation products, and genotoxicity

Microcystin-LR (MC-LR), a type of cyanotoxin commonly found in natural water bodies (sources of drinking water), poses a threat to human health due to its high toxicity. It is essential to successfully remove this cyanotoxin from drinking water sources. In this study, chlorine was used to oxidize MC-LR in Milli-Q water (MQ) (control test) and natural water collected from Lake Longhu (LLW) as a drinking water source. The removal efficiency, proposed transformation pathways, and genotoxicity were investigated. In the chlorine dose range investigated (4.0 mg L^-1 - 8.0 mg L^-1), the apparent second-order rate constants for MC-LR chlorination varied from 21.3 M^-1s^-1 to 31.9 M^-1s^-1 in MQ, higher than that in LLW (9.06 M^-1s^-1 to 17.7 M^-1s^-1) due to a faster chlorine decay attributed to the water matrix (e.g., natural organic matter) of LLW. Eleven transformation products (TPs) of MC-LR were identified in the two waters. The conjugated diene moieties and benzene ring of Adda moiety (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid), and the double bond of Mdha moiety (N-methyldehydroalanine) were the major susceptible reaction sites. Attacking unsaturated bonds by hydroxyl and chlorine radicals to generate monochloro-hydroxy-MC-LR was the primary initial transformation pathway, followed by nucleophilic substitution, dehydration, and cleavage in MC-LR. Chlorine substitution on the benzene ring was also observed. Based on the bacterial reverse-mutation assay (Ames assay), TPs in treated natural water did not induce genotoxicity/mutagenicity. These findings shed light on the role of chlorination in controlling the risk of cyanotoxins in drinking water treatment plants.

Cyanotoxins dissipation in soil: Evidence from microcosm assays

Cyanobacteria proliferate in warm, nutrient-rich environments, and release cyanotoxins into natural waters. If cyanotoxin-contaminated water is used to irrigate agricultural crops, this could expose humans and other biota to cyanotoxins. However, cyanotoxins may be degraded by the diverse microbial consortia, be adsorbed or otherwise dissipate in agricultural soil. This study investigates the disappearance and transformation of 9 cyanotoxins in controlled soil microcosms after 28 d. Six soil types were exposed to factorial combinations of light, redox conditions and microbial activity that influenced the recovery of anabaenopeptin-A (AP-A), anabaenopeptin-B (AP-B), anatoxin-a (ATX-a), cylindrospermopsin (CYN), and the microcystin (MC) congeners -LR, -LA, -LY, -LW, and -LF. Cyanotoxins estimated half-lives were from hours to several months, depending on the compound and soil conditions. Cyanotoxins were eliminated via biological reactions in aerobic and anaerobic soils, although anaerobic conditions accelerated the biological dissipation of ATX-a, CYN and APs. ATX-a was sensitive to photolytic degradation, but CYN, and MCs were not reduced through photochemical transformation. MC-LR and -LA were recovered after exposure to light, redox conditions and low microbial activity, suggesting that they persisted in extractable forms, compared to other cyanotoxins in soil. Cyanotoxin degradation products were identified using high-resolution mass spectrometry, revealing their potential degradation pathways in soil.

Insights into the role of prechlorination in algae-laden raw water distribution process: Algal organic matter and microcystin-LR release, extracellular polymeric substances (EPS) aggregation, and pipeline biofilm communities

Prechlorination routinely applied for the treatment of algae-laden raw water has received extensive attention due to its influence on water quality and aquatic microbes. In this study, prechlorination experiments with different doses were conducted in sets of model raw water distribution systems. With the elevated dose of chlorine and prolonged hydraulic retention time (HRT), the ratio of intact algal cells decreased, and the stability of water enhanced. Dissolved organic carbon (DOC) and nitrogen (DON) increased when chlorine dose elevated from 0 to 0.5 mg/L but decreased with elevations from 0.5 to 2.0 mg/L, while UV₂₅₄ showed a monotonically increasing tendency. DOC, DON and extracellular microcystin-LR increase initially and decrease thereafter with the prolonged HRT. Notably, the effects of prechlorination on extracellular polymeric substances aggregation behavior on pipe walls and microbial community composition was revealed, providing more profound understanding of the community dynamics in this engineered system. This study helped optimize strategies to improve the stability and efficiency of pretreatment of algae-laden water.

Molecularly imprinted nanoparticle-based assay (MINA) for microcystin-LR detection in water

Microcystins (MCs) are highly toxic peptides produced by cyanobacteria during algal blooms. Microcystin-leucine-arginine (MC-LR) is the most toxic and common MC variant with major effects on human and animal health upon exposure. MC-LR detection has become critical to ensure water safety, therefore robust and reliable analytical methods are needed. This work reports the development of a simple and optimized Molecularly Imprinted Nanoparticle-Based Assay (MINA) for MC-LR detection in water. Molecularly Imprinted Nanoparticles (MINs) were prepared by solid-phase polymerization on glass beads conjugated to MC-LR through (3-aminopropyl) triethoxysilane (APTES) via amide bonding. APTES-modified glass beads were obtained under optimized conditions to maximize the density of surface amino groups available for MC-LR conjugation. Two quinary mixtures of acrylic monomers differing in charge, polarity, and functionality were selected from molecular docking calculations and used to obtain MINs for MC-LR recognition using N,N'-methylene-bis-acrylamide (BIS) as the crosslinking agent. MINs were immobilized by physical adsorption onto 96-well polystyrene microplate and evaluated as per their rebinding capacity toward the analyte by using a covalent conjugate between MC-LR and the enzyme horseradish peroxidase (HRP). Experimental conditions for the MINs immobilization protocol, HRP-MC-LR concentration, and composition of the blocking solution were set to maximize the colorimetric response of the MINs compared to non-treated wells. Optimized conditions were then applied to conduct competitive MINAs with the HRP-MC-LR conjugate and the free analyte, which confirmed the preferential binding of MC-LR to the immobilized MINs for analyte concentrations ranging from 1 × 10^-5 nmol L^-1 to 100 nmol L^-1. The best competitive MINA showed a limit of detection of 2.49 × 10^-4 nmol L^-1 and coefficients of variation less than 10% (n = 6), which are auspicious for the use of MINs as analytical tools for MC-LR detection below the permissible limits issued by WHO for safe water consumption (1.00 nmol L^-1). This assay also proved to be selective to the analyte in cross-reactivity studies with two analogous microcystins (MC-RR and MC-YR). Analyses of lagoon and drinking water samples enriched with MC-LR revealed strong matrix effects that reduce the MINA response to the analyte, thus suggesting the need for sample pretreatment methods in future development in this subject.

Development of a Potentially New Algaecide for Controlling Harmful Cyanobacteria Blooms Which is Ecologically Safe and Selective

Harmful cyanobacterial blooms (HCBs) caused by Microcystis aeruginosa are of great concern as they negatively affect the aquatic environment and human health. Chemical methods could rapidly eradicate HCBs and have been used for many decades. However, many chemical reagents are not recommended to eliminate HCBs in the long term, given the possible destructive and toxic effects of the chemicals employed on non-target aquatic organisms. We developed a new algaecide, 2-((1,3,4-thiadiazol-2-yl)thio)-N-(4-chlorophenyl) acetamide (Q2), to control harmful cyanobacteria while being environmentally friendly and selective. In our study, Q2 effectively inhibited cyanobacterial growth, especially of M. aeruginosa, but did not affect eukaryotic algae in test concentrations. A critical mechanism was revealed by transcriptome and metagenomic results showing that Q2 affects multiple cellular targets of cyanobacteria for HCB control, including the destruction of organelles, damage in the photosynthesis center, as well as inhibition of gas vesicle growth, and these changes can be highly relevant to the decrease of quorum-sensing functional KEGG pathways. Furthermore, Q2 did not affect the microbial composition and could recover the disrupted aquatic functional pathways in a short period. This is different from the impact on ecosystem functioning of the traditionally used harmful algaecide diuron. All these results verified that Q2 could be friendly to the aquatic environment, providing a new directional choice in managing HCBs in the future.

Development, Validation and Application of a Targeted LC-MS Method for Quantification of Microcystins and Nodularin: Towards a Better Characterization of Drinking Water

Cyanotoxins can be produced in surface waters by cyanobacterial blooms, mostly during summer and early autumn. Intoxications would result from consumption of water contaminated with the potent hepatotoxins, microcystins and nodularin. Therefore, the WHO has set a guideline value for drinking water quality concerning one congener of microcystin. Consequently, the design of a validated, public reference method to detect and quantify the hepatotoxins in drinking water is necessary. During this study, a method was developed to quantify cyanotoxins (eight microcystin congeners and nodularin) in water using liquid chromatography coupled with tandem mass spectrometry. Additionally, bottled and tap water samples were tested for the presence of cyanotoxins. No cyanotoxins were detected in any of the collected water samples. However, quality controls and the results of a proficiency test show the validity of the method.

Evaluation of ultraviolet/peracetic acid to degrade M. aeruginosa and microcystins -LR and -RR

The reactivity of peracetic acid (PAA) alone, and PAA exposed to ultraviolet radiation (UV), was investigated on Microcystis aeruginosa cells, and on microcystin-LR and -RR. Reaction rates between PAA and MC-LR (k = 3.46 M^-1 s^-1) and MC-RR (k = 2.67 M^-1 s^-1) were determined in an unbuffered acidic solution, and they are approximately 35-45 times lower than a previously reported reaction rate between MC-LR and chlorine at pH 6. Peracetic acid reacted with M. aeruginosa cells as a function of PAA and cell concentrations, with 10 mg/L PAA resulting in 1-log reduction of total MC-LR within 15 min. Advanced oxidation by UV/PAA readily degraded MC-LR and MC-RR, outperforming UV/H₂O₂ at pH 7.7 by > 50% on an equimolar basis. Indirect photolysis at this pH is due to ^•OH and organic radicals, as determined by trials in the presence of excess tert-butanol to scavenge ^•OH. The process is less effective when the pH departs from neutral conditions (5.9 or 10.6) due to the decreased effects of both radicals. These findings suggest that PAA alone might be a viable option for cyanobacteria and microcystins control in preoxidation applications and that UV/PAA is an effective process for degrading MC-LR and MC-RR at neutral pH.

Degradation kinetics of prometryn and formation of disinfection by-products during chlorination

Prometryn is a herbicide that is widely used and frequently detected in aqueous environment and soil. Prometryn is chemically stable, biologically toxic, and easily to accumulate in living bodies, which can cause accumulate in the environment and acute and chronic toxicity to living creatures. In this study, factors affecting the degradation kinetics of prometryn chlorination were studied, including solution pH, bromide and ammonium concentrations, and temperature. Prometryn reacted quickly with aqueous chlorine following the pseudo-first-order kinetics. The maximum pseudo-first-order rate constant (k_app) appeared at pH 5 with the observed rate constant (k_obs) as 190. 08 h^-1; the minimum value of k_app reached at pH 9 with k_obs as 5.26 h^-1. The presence of Br^- and increase of temperature both accelerated the degradation rate of prometryn during chlorination. The activation energy was calculated as 31.80 kJ/mol. Meanwhile 6 disinfection by-products (DBPs) were detected, namely: chloroform (CF), trichloroacetonitrile (TCAN), dichloroacetonitrile (DCAN), dichloroacetone, trichloronitromethane (TCNM), and trichloroacetone. Solution pH significantly affected the formation and distribution of DBPs. CF was the most formed carbonated DBP (C-DBP) with the maximum of 217.9 μg/L at pH 8, and its formation was significantly higher in alkaline conditions. For nitrogenated DBPs (N-DBPs), the yields of DCAN and TCAN were significantly higher in acidic conditions, while the maximum of TCNM achieved in neutral conditions. Because the toxicity of N-DBPs is higher than that of C-DBPs, the pH should be controlled in neutral or slight alkaline conditions during prometryn chlorination to effectively control DBP formation and reduce the related toxicity.

Stability issues of microcystins, anabaenopeptins, anatoxins, and cylindrospermopsin during short-term and long-term storage of surface water and drinking water samples

Reproducible analytical procedures and rigorous quality control are imperative for an accurate monitoring of cyanobacterial toxins in environmental water samples. In this study, the short-term and long-term storage stability of diverse cyanotoxins (anatoxins, cylindrospermopsin, anabaenopeptins, and 12 microcystins) was evaluated in water samples, under different scenarios. Transport controls were performed at three monitoring sites in spiked ultrapure water and lake water to investigate short-term stability issues. Medium-term storage stability was evaluated for up to 14-28 days in ultrapure water, chlorine-treated drinking water (amended with reductant), and surface water (filtered and unfiltered) stored at different temperatures (20 °C, 4 °C, and -20 °C). Substantial decreases of cylindrospermopsin and anabaenopeptins were observed in tap water (20 °C) and unfiltered surface water (20 °C or 4 °C). Regardless of matrix type, cyanotoxin recoveries generally remained within an 80-120% range when the water samples were kept frozen. After a prolonged storage duration of 365 days at -20 °C, most cyanotoxins experienced decreases in the range of 10-20%. The notable exception was for the tryptophan-containing MC-LW and MC-WR, with more substantial variations (30% to 50% decrease) and conversion to N-formylkynurenine analogs. Reanalysis of field-collected surface waters after long-term storage at -20 °C also indicated significantly decreasing trends of cyanotoxins (between 6% and 23% decrease). In view of the above, short sample hold times should be favored as recommended in EPA methods.

Chlorination and bromination of olefins: Kinetic and mechanistic aspects

Hypochlorous acid (HOCl) is typically assumed to be the primary reactive species in free available chlorine (FAC) solutions. Lately, it has been shown that less abundant chlorine species such as chlorine monoxide (Cl₂O) and chlorine (Cl₂) can also influence the kinetics of the abatement of certain organic compounds during chlorination. In this study, the chlorination as well as bromination kinetics and mechanisms of 12 olefins (including 3 aliphatic and 9 aromatic olefins) with different structures were explored. HOCl shows a low reactivity towards the selected olefins with species-specific second-order rate constants <1.0 M^-1s^-1, about 4-6 orders of magnitude lower than those of Cl₂O and Cl₂. HOCl is the dominant chlorine species during chlorination of olefins under typical drinking water conditions, while Cl₂O and Cl₂ are likely to play important roles at high FAC concentration near circum-neutral pH (for Cl₂O) or at high Cl^- concentration under acidic conditions (for Cl₂). Bromination of the 12 olefins suggests that HOBr and Br₂O are the major reactive species at pH 7.5 with species-specific second-order rate constants of Br₂O nearly 3-4 orders of magnitude higher than of HOBr (ranging from <0.01 to >10³ M^-1s^-1). The reactivities of chlorine and bromine species towards olefins follow the order of HOCl < HOBr < Br₂O < Cl₂O ≈ Cl₂. Generally, electron-donating groups (e.g., CH₂OH- and CH₃-) enhances the reactivities of olefins towards chlorine and bromine species by a factor of 3-10², while electron-withdrawing groups (e.g., Cl-, Br-, NO₂-, COOH-, CHO-, -COOR, and CN-) reduce the reactivities by a factor of 3-10⁴. A reasonable linear free energy relationship (LFER) between the species-specific second-order rate constants of Br₂O or Cl₂O reactions with aromatic olefins and their Hammett σ⁺ was established with a more negative ρ value for Br₂O than for Cl₂O, indicating that Br₂O is more sensitive to substitution effects. Chlorinated products including HOCl-adducts and decarboxylated Cl-adduct were identified during chlorination of cinnamic acid by high-performance liquid chromatography/high resolution mass spectrometry (HPLC/HRMS).

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 (S₂O₃^2-), 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 (k_HOCl) and OCl^- (k_OCl-) 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 (k_app,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 (k_app,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.

Transformation of X-ray contrast media by conventional and advanced oxidation processes during water treatment: Efficiency, oxidation intermediates, and formation of iodinated byproducts

X-ray contrast media (ICM), as the most widely used intravascular pharmaceuticals, have been frequently detected in various environmental compartments. ICM have attracted increasingly scientific interest owing to their role as an iodine contributor, resulting in the high risk of forming toxic iodinated byproducts (I-BPs) during water treatment. In this review, we present the state-of-the-art findings relating to the removal efficiency as well as oxidation intermediates of ICM by conventional and advanced oxidation processes. Moreover, formation of specific small-molecular I-BPs (e.g., iodoacetic acid and iodoform) during these processes is also summarized. Conventional oxidants and disinfectants including chlorine (HOCl) and chloramine (NH₂Cl) have low reactivities towards ICM with HOCl being more reactive. Iodinated/deiodinated intermediates are generated from reactions of HOCl/NH₂Cl with ICM, and they can be further transformed into small-molecular I-BPs. Types of disinfectants and ICM as well as solution conditions (e.g., presence of bromide (Br^-) and natural organic matters (NOM)) display significant impact on formation of I-BPs during chlor(am)ination of ICM. Uncatalyzed advanced oxidation process (AOPs) involving ozone (O₃) and ferrate (Fe(VI)) exhibit slow to mild reactivities towards ICM, usually leading to their incomplete removal under typical water treatment conditions. In contrast, UV photolysis and catalyzed AOPs including hydroxyl radical (HO^•) and/or sulfate radical (SO₄^.-) based AOPs (e.g., UV/hydrogen peroxide, UV/persulfate, UV/peroxymonosulfate (PMS), and CuO/PMS) and reactive chlorine species (RCS) involved AOPs (e.g., UV/HOCl and UV/NH₂Cl) can effectively eliminate ICM under various conditions. Components of water matrix (e.g., chloride (Cl^-), Br^-, bicarbonate (HCO₃^-), and NOM) have great impact on oxidation efficiency of ICM by catalyzed AOPs. Generally, similar intermediates are formed from ICM oxidation by UV photolysis and AOPs, mainly resulting from a series reactions of the side chain and/or C-I groups (e.g. cleavage, dealkylation, oxidation, and rearrange). Further oxidation or disinfection of these intermediates leads to formation of small-molecular I-BPs. Pre-oxidation of ICM-containing waters by AOPs tends to increase formation of I-BPs during post-disinfection process, while this trend also depends on the oxidation processes applied and solution conditions. This review summarizes the latest research findings relating to ICM transformation and (by)products formation during disinfection and AOPs in water treatment, which has great implications for the practical applications of these technologies.

Toxicity of chlorinated algal-impacted waters: Formation of disinfection byproducts vs. reduction of cyanotoxins

Seasonal algal blooms in surface waters can impact water quality through an input of algal organic matter (AOM) to the pool of dissolved organic matter as well as the release of cyanotoxins. The formation and speciation of disinfection byproducts (DBPs) during chlorination of algal-impacted waters, collected from growth of Microcystis aeruginosa were studied. Second-order rate constants for the reactions of microcystins (MCs) with chlorine and bromine were determined. Finally, the toxicity of chlorinated algal-impacted waters was evaluated by Chinese hamster ovary (CHO) cytotoxicity and genotoxicity assays. Under practical water treatment conditions, algal-impacted waters produced less regulated trihalomethanes (THMs) and haloacetic acids (HAAs), haloacetonitriles (HANs), and total organic halogen (TOX) than natural organic matter (NOM). For example, the weight ratios of DBP formation from AOM to NOM (median levels) were approximately 1:5, 1:3, 1:2 and 1:3 for THMs, HAAs, HANs, and TOX, respectively. Increasing initial bromide level significantly enhanced THM and HAN concentrations, and therefore unknown TOX decreased. The second-order rate constant for the reactions of MC-LR (the most common MC species) with chlorine was 60 M^-1 s^-1 at pH 7.5 and 21 °C, and the rate constants for MC congeners follow the order: MC-WR > MC-LW > MC-YR > MC-LY > MC-LR ≈ MC-RR. The reaction rate constant of bromine with MC-LR is two orders of magnitude higher than that of chlorine. Unchlorinated algal-impacted waters were toxic owning to the presence of MCs, and chlorination enhanced their cytotoxicity and genotoxicity due to the formation of toxic halogenated DBPs. However, the toxicity of treated waters depended on the evolution of cyanotoxins and formation of DBPs (particularly unknown or emerging DBPs).

Comparing the effects of chlorination on membrane integrity and toxin fate of high- and low-viability cyanobacteria

Occurrence of toxic cyanobacterial blooms in natural freshwaters could impair drinking water quality. Chlorine was often employed as an oxidant to treat algal-laden source waters in drinking water treatment plants. However, previous studies only focused on high-viability cyanobacteria at exponential phase. Whether the change of cell-viability of cyanobacteria could affect chlorination was unknown. Here, high- and low-viability Microcystis were collected from a whole life cycle of cyanobacteria in lab-scale, and effects of chlorination on membrane integrity and toxin fate of high- and low-viability Microcystis were subsequently investigated. Results showed chlorine exposure was lower for low-viability cells than high-viability cells with the same initial chlorine dosage, but low-viability cells were less resistant to chlorination, leading to higher rate of membrane damage (k_loss) and intracellular toxin release (k_i). For high-viability cells, there was no increase of extracellular toxin with sufficient chlorine exposure whereas it showed a continuous increase for low-viability cells mainly due to its lower rate of extracellular toxin degradation (k_e, 26 ± 8 M^-1 s^-1) than intracellular toxin release (k_i, 110 ± 16 M^-1 s^-1) (k_e < k_i). Besides, total toxin could be completely oxidized for high-viability cells with sufficient chlorine exposure (>30 mg min L^-1) whereas chlorination could not work well for low-viability cells even with chlorine exposure of as high as 36 mg min L^-1. These findings indicated chlorination may not be a feasible option to treat low-viability cyanobacteria during decline stage of cyanobacterial blooms.

Delayed Release of Intracellular Microcystin Following Partial Oxidation of Cultured and Naturally Occurring Cyanobacteria

Oxidation processes can provide an effective barrier to eliminate cyanotoxins by damaging cyanobacteria cell membranes, releasing intracellular cyanotoxins, and subsequently oxidizing these toxins (now in extracellular form) based on published reaction kinetics. In this work, cyanobacteria cells from two natural blooms (from the United States and Canada) and a laboratory-cultured Microcystis aeruginosa strain were treated with chlorine, monochloramine, chlorine dioxide, ozone, and potassium permanganate. The release of microcystin was measured immediately after oxidation (t ≤ 20 min), and following oxidant residual quenching (stagnation times = 96 or 168 h). Oxidant exposures (CT) were determined resulting in complete release of intracellular microcystin following chlorine (21 mg-min/L), chloramine (72 mg-min/L), chlorine dioxide (58 mg-min/L), ozone (4.1 mg-min/L), and permanganate (391 mg-min/L). Required oxidant exposures using indigenous cells were greater than lab-cultured Microcystis. Following partial oxidation of cells (oxidant exposures ≤ CT values cited above), additional intracellular microcystin and dissolved organic carbon (DOC) were released while the samples remained stagnant in the absence of an oxidant (>96 h after quenching). The delayed release of microcystin from partially oxidized cells has implications for drinking water treatment as these cells may be retained on a filter surface or in solids and continue to slowly release cyanotoxins and other metabolites into the finished water.

Effects and mechanism on the removal of neurotoxin β-N-methylamino-l-alanine (BMAA) by chlorination

β-N-Methylamino-l-alanine (BMAA), a new cyanobacterial toxin, is found in different aquatic ecosystems worldwide and is to threaten the human nervous system. Therefore, it is important for water plants to develop feasible methods to counter the effects of BMAA. In this study, the removal of BMAA by chlorine, as well as its intermediate products, at different pH values and the mechanism of pH on the removal BMAA were investigated. The results showed that the chlorination of BMAA is in accordance with the second-order kinetics model. The reaction rate of chlorinated BMAA increased with the increase in the concentration of chlorine. The pH of the solution significantly affected the reaction rate. The apparent kinetic constant (k_app) decreased from 6.00 × 10³ M^-1·min^-1 to 35.5 M^-1·min^-1 when the pH increased from 4.5 to 9 in the chlorine concentration of 32.23 μM. It is probable that the species distribution and proportion of BMAA and chlorine at different pH values were the main causes of this phenomenon. Additionally, the chlorination reaction consisted of four elementary reactions and hydrogen ions were beneficial to the reaction. The temperature also affected the reaction rate and the activation energy of the reaction was 16.6 ± 1.99 kJ·M^-1. A variety of degradation products were detected and the path of degradation was speculated. Chlorination, dechlorination, and decarboxylation were the main processes of oxidative degradation. Furthermore, the composition of the degradation products was the same at different pH values.

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.

Oxidation of Microcystic-LR via the solar/chlorine process: Radical mechanism, pathways and toxicity assessment

Microcystin-LR (MC-LR) is the most widely distributed and harmful variant toxins released by cyanobacteria, which poses potential threaten to people and aquatic animals when entering natural water. In our research, solar/chlorine process was comprehensively investigated to degrade and detoxify MC-LR. Under the chlorine concentration of 1.0 mg L^-1, MC-LR (1.0 μM) was decreased by 96.7%, 26%, and 9% by solar/chlorine process, chlorination, and solar irradiation respectively. Quenching experiments confirmed that reactive chlorine species (RCS) and hydroxyl radical (HO) were the predominant reactive species in solar/chlorine process at neutral condition, and ozone was generated because of the participation of triplet-state oxygen (O(³P)). The respective contributions of each reactive species were calculated with the order as: RCS, HO, ozone, and solar irradiation. The presence of HCO₃^- and natural organic matter in water inhibited the degradation efficiency of MC-LR. Moreover, the transformation products of MC-LR generated during the solar/chlorine process were identified and a possible pathway was proposed. The hepatotoxicity of MC-LR and its transformation products was compared using protein phosphatase 2A. Our experimental results revealed that the concentration and hepatotoxicity of MC-LR both significantly decreased, and most products were not hepatoxic. Overall, the solar/chlorine process is a promising alternative technology to degrade MC-LR during eutrophication.

Decomposition of β-N-methylamino-L-alanine (BMAA) and 2,4-diaminobutyric acid (DAB) during chlorination and consequent disinfection byproducts formation

β-N-methylamino-L-alanine (BMAA) and 2,4-diaminobutyric acid (DAB) are two newly identified algal toxins, and they may react with chlorine to undergo decomposition and generate disinfection byproducts (DBPs) during pre-chlorination as well as chlorine disinfection. In this study, the decomposition of BMAA and DAB during chlorination and the consequent DBPs formation were investigated. The BMAA and DAB concentrations in source waters were determined, the decomposition kinetics of BMAA and DAB and the formation of DBPs during chlorination were studied, the formation pathways of DBPs from BMAA and DAB were explored, and the factors which may affect the decomposition and DBPs formation were examined. The results revealed that BMAA and DAB were commonly detected in source waters from Taihu Lake, and the highest level of BMAA reached 230.8 ng/L, while the concentrations of DAB were generally around 2.0 ng/L. The decomposition of BMAA and DAB during chlorination both followed pseudo-first-order decay while the decomposition rate constant of DAB was significantly higher than that of BMAA. Trihalomethanes (THMs), haloacetic acids (HAAs), and haloacetonitriles (HANs) were all generated during the chlorination of BMAA and DAB with relatively high yields. Notably, the THMs, HAAs, and HANs yields of each carbon atom from BMAA and DAB were significantly higher than that from other organic precursors, and the formation of HANs from DAB was significantly higher than that from BMAA. The formation pathways of DBPs from BMAA and DAB were tentatively proposed and verified through theoretical calculations. Of note, the proposed formation pathways of THMs and HAAs from BMAA were similar to that from DAB, while the proposed formation pathways of HANs from BMAA and DAB showed some differences. Chlorine dose, pH and temperature all affected the decomposition of BMAA and DAB and DBPs formation during chlorination.

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.

Application of passive sampling for sensitive time-integrative monitoring of cyanobacterial toxins microcystins in drinking water treatment plants

Calibrated adsorption-based passive samplers were used for time-integrative monitoring of microcystins (MCs) in three full-scale drinking water treatment plants (DWTPs) in the Czech Republic during two vegetation seasons (Jun-Nov), in parallel with traditional discrete sampling. MCs were detected in epilimnetic water samples at concentrations up to 14 μg/L, but their levels in raw water in DWTPs were below 1 μg/L WHO guideline value for drinking water. Conventional treatment technologies (coagulation/filtration) eliminated cyanobacteria and intracellular toxins but had a limited removal efficiency for extracellular toxins. MCs were regularly detected in final treated water, especially in DWTPs equipped only with the conventional treatment, but their concentrations were below the quantitation limit of discrete sampling (<25 ng/L). Passive samplers in combination with LC-MS/MS analysis provided excellent sensitivity allowing to detect time-weighted average (TWA) concentrations of MCs as low as 20-200 pg/L after 14-d deployment. Median MC TWA concentrations in the treated water from the individual DWTPs were 1-12 ng/L, and most likely did not present significant health risks. Passive samplers well reflected spatiotemporal variations of MCs, actual concentrations of extracellular toxins, MC removal efficiency in DWTPs, and toxin concentrations in the treated water. Passive sampling can be effectively used for assessment and management of MC health risks during DWTP operation.

Genotoxicity effects of Phanerochaete chrysosporium against harmful algal bloom species by micronucleus test and comet assay

Algal blooms and toxins have become serious ecological problems. White-rot fungi have been demonstrated to be a feasible means of control, but the genotoxicity mechanisms involved have not been reported. In this study, Cryptomonas obovata FACHB-1301, Oscillatoria sp. FACHB-1083, and Scenedesmus quadricauda FACHB-507 were co-cultured with Phanerochaete chrysosporium under optimal conditions of 250 mg-l at 25 °C with DO 7.0 mg-l for 1, 3, 5 and 7 d. Compared to the control groups, the values for tadpoles exposed to algae treated with Phanerochaete chrysosporium were only increased from 1.95 ± 0.09, 2.78 ± 0.08 and 2.37 ± 0.13 to 2.45 ± 0.07, 3.56 ± 0.08 and 2.54 ± 0.10, and the frequency of nuclear anomalies reached 6.45 ± 0.06, 11.14 ± 0.05 and 7.85 ± 0.10 to 7.68 ± 0.08, 13.12 ± 0.06 and 8.57 ± 0.12 in the experimental groups after 7 d. What's more, the tail lengths were only increased to 36.77 ± 0.54, 41.58 ± 0.78 and 35.38 ± 0.66, and the comet length reached 55.67 ± 0.68, 68.56 ± 0.85 and 51.43 ± 0.82. The results demonstrated that Phanerochaete chrysosporium effectively decreased genotoxicity effects in Fejervarya multistriat tadpoles. These results could provide new ideas for inhibiting water blooms, and lay a theoretical foundation for promoting the deepening of water eutrophication.

Elimination kinetics and detoxification mechanisms of microcystin-LR during UV/Chlorine process

Microcystin-LR (MC-LR), a toxin produced by cyanobacteria, is very toxic and poses a threat to public health when entering water treatment works. In this study, UV/chlorine process, as an advanced oxidation process (AOP), has been demonstrated for effective elimination of MC-LR levels and associated toxicity. At a chlorine dose of 3.0 mg L^-1 and UV fluence of 125 mJ cm^-2, MC-LR (initial concentration 1.0 μM) was reduced by 92.5%, which was much higher than 20.3% removal under UV irradiation alone and 65.1% removal during dark chlorination. Enhanced degradation was attributed by hydroxyl radicals (HO) and reactive chlorine species (RCS), mainly Cl₂^- and ClO. Increasing chlorine doses or lowering pH favored MC-LR removal. Increased bicarbonate and natural organic matter concentrations inhibited MC-LR removal, but bromide ions enhanced MC-LR removal instead. MC-LR elimination rates in natural waters were roughly two times smaller than those in ultrapure water. The reactive radicals promoted hydroxylation of both diene of Adda moiety and double bond of Mdha moiety in MC-LR. UV exposure enhanced the dechlorination of chloro-MC-LR via the cleavage of CCl bond. The toxicity was evaluated by a protein phosphatase (PP2A) inhibition assay. At a chlorine dose of 3.0 mg L^-1 and UV fluence of 125 mJ cm^-2, the toxicity of the treated water was reduced by 75.0%, which was also higher than 25.7% and 46.7% removal under UV irradiation alone and during dark chlorination, respectively. These results highlight UV/chlorine is an efficient AOP for MC-LR degradation and detoxification.

Electrochemical oxidation of Microcystis aeruginosa using a Ti/RuO2 anode: contributions of electrochemically generated chlorines and hydrogen peroxide

Electrochemical oxidation was proposed as a promising technology for algal control in drinking water treatment. To be effective, the electrogenerated oxidants should have long half-lives and could continually inhibit the growth of algae. In this study, we used the electrochemical system equipped with a Ti/RuO₂ anode which focus on generating long half-life chlorines and H₂O₂. We explored the impact of electrical field and electrogenerated oxidants on algal inhibition, and we investigated the production of electrogenerated reactive species and their contributions to the inhibition of Microcystis aeruginosa (M. aeruginosa) in simulated surface water with low Cl^- concentrations (< 18 mg/L). We developed a kinetic model to simulates the concentrations of chlorines and H₂O₂. The results showed that electrical field and electrogenerated oxidants were both important contributors to algal inhibition during electrochemical oxidation treatment. The Ti/RuO₂ anode mainly generates chlorines and H₂O₂ from Cl^- and water. During the electrolysis at current density of 20 mA/cm², when initial Cl^- concentrations increased from 0 to 18 mg/L (0-5.07 × 10^-4 mol/L), the chlorines increased from 0 to 3.62 × 10^-6 mol/L, and the H₂O₂ concentration decreased from 3.68 × 10^-6 to 1.15 × 10^-6 mol/L. Our model made decent predictions of other Cl^- concentrations by comparing with experiment data which validated the rationality of this modeling approach. The electrogenerated chlorine species were more effective than H₂O₂ at an initial Cl^- concentration of 18 mg/L.

Oxidation of cyanobacterial neurotoxin beta-N-methylamino-L-alanine (BMAA) with chlorine, permanganate, ozone, hydrogen peroxide and hydroxyl radical

Beta-N-methylamino-L-alanine (BMAA), a new cyanobacterial neurotoxin produced by more than 20 genera of cyanobacteria, has been associated with amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC) or Alzheimer's disease. Although BMAA has been shown to be removed in drinking water treatment plants (DWTPs), studies regarding the reactions between BMAA and the commonly used oxidants in DWTPs are limited to chlorine under specific conditions. In this study, the reaction kinetics between BMAA and five oxidants commonly used in DWTPs, including chlorine, potassium permanganate, ozone, hydrogen peroxide and hydroxyl radical were investigated. The oxidation of BMAA by chlorine, ozone or OH radical followed the second order reaction rate law, and the reaction rate was in the order of OH radicals > ozone >> chlorine. The rate constants increased by 20 times from 2 × 10³ M^-1s^-1 at pH 5.8 to 4.93 × 10⁴ M^-1s^-1 at pH 7, and kept in a relatively stable level at pH 7-9.5; rate constants of OH radicals were 1.11 × 10⁸ M^-1s^-1 at pH 6.5 and 5.51 × 10⁹- 1.35 × 10¹⁰ M^-1s^-1 at pH > 6.5. For both permanganate and H₂O₂ only, the removal of BMAA was negligible. The pH dependency of chlorine and the OH radical may be attributed to the neutral form of BMAA with free lone pair electrons readily to be attacked by oxidants. However, for ozonation of BMAA, the rate constants were 1.88 × 10⁶-3.72 × 10¹⁰ M^-1s^-1, with a linear dependency on pH, implying that the hydroxide concentration governs the reaction. In addition, the rate of BMAA degradation was found to be slower in natural water if compared with that in deionized water.

Susceptibility of the Algal Toxin Microcystin-LR to UV/Chlorine Process: Comparison with Chlorination

Microcystin-LR (MC-LR), an algal toxin (cyanotoxin) common in sources of drinking water, poses a major human health hazard due to its high toxicity. In this study, UV/chlorine was evaluated as a potentially practical and effective process for the degradation of MC-LR. Via mass spectrometry analysis, fewer chlorinated-MC-LR products were detected with UV/chlorine treatment than with chlorination, and a transformation pathway for MC-LR by UV/chlorine was proposed. Different degrees of rapid degradation of MC-LR were observed with varying pH (6-10.4), oxidant dosage (0.5-3 mg L^-1), natural organic matter (0-7 mg L^-1), and natural water sources. In contrast to the formation of primarily chloroform and dichloroacetic acid in deionized water where MC-LR serves as the only carbon source, additional chlorinated disinfection byproducts were produced when sand filtered natural water was used as a background matrix. The UV/chlorine treated samples also showed quantitatively less cytotoxicity in vitro in HepaRG human liver cell line tests than chlorination treated samples. Following 16 min (96 mJ cm^-2) of UV irradiation combined with 1.5 mg L^-1 chlorine treatment, the cell viability of the samples increased from 80% after exposure to 1 mg L^-1 MC-LR to 90%, while chlorination treatment evidenced no reduction in cytotoxicity with the same reaction time.

In-situ electrochemical Fe(VI) for removal of microcystin-LR from drinking water: comparing dosing of the ferrate ion by electrochemical and chemical means

Harmful algal blooms (HAB) release microtoxins that contaminate drinking water supplies and risk the health of millions annually. Crystalline ferrate(VI) is a powerful oxidant capable of removing algal microtoxins. We investigate in-situ electrochemically produced ferrate from common carbon steel as an on-demand alternative to crystalline ferrate for the removal of microcystin-LR (MC-LR) and compare the removal efficacy for both electrochemical (EC) and chemical dosing methodologies. We report that a very low dose of EC-ferrate in deionized water (0.5 mg FeO₄^2- L^-1) oxidizes MC-LR (MC-LR₀ = 10 μg L^-1) to below the guideline limit (1.0 μg L^-1) within 10 minutes' contact time. With bicarbonate or natural organic matter (NOM), doses of 2.0-5.0 mg FeO₄^2- L^-1 are required, with lower efficacy of EC-ferrate than crystalline ferrate due to loss of EC-ferrate by water oxidation. To evaluate the EC-ferrate process to concurrently oxidize micropollutants, coagulate NOM, and disinfect drinking water, we spiked NOM-containing real water with MC-LR and Escherichia coli, finding that EC-ferrate is effective at 10.0 mg FeO₄^2- L^-1 under normal operation or 2.0 mg FeO₄^2- L^-1 if the test water has initial pH optimized. We suggest in-situ EC-ferrate may be appropriate for sporadic HAB events in small water systems as a primary or back-up technology.

Varied influence of microcystin structural difference on ELISA cross-reactivity and chlorination efficiency of congener mixtures

Enzyme-linked immunosorbent assay (ELISA) is an antibody-based analytical method that has been widely applied in water treatment utilities for the screening of toxic cyanobacteria metabolites such as microcystins (MCs). However, it is unknown how the minor structural difference of MCs may impact their chlorination kinetics and measurement via ELISA method. It was found in this study that, regardless of the experimental conditions (n = 21), there was no MC-YR or MC-LY residual, while different removal rates of other MCs were observed (MC-RR > MC-LR > MC-LA ∼ MC-LF) as measured by liquid chromatography tandem mass spectrometry (LC-MS/MS), which was consistent with the relative reactivity of the amino acid variables with free chlorine. The removal of total MCs was generally lower as measured by ELISA than by LC-MS/MS. By incorporating both analytical results, existence of ADDA-containing byproducts or byproducts that had a higher sensitivity toward the ELISA kit was demonstrated, after excluding the contribution of the cross-reactivity of the parent MCs. It should be noted, however, that the cross-reactivities of MCs could be influenced not only by MC congeners, but also by other conditions such as mixtures and the applied ELISA kit.

Photocatalytic degradation of microcystin-LR with a nanostructured photocatalyst based on upconversion nanoparticles@TiO2 composite under simulated solar lights

In this work, we prepared advanced upconversion nanoparticle coated with TiO₂ photocatalysts (NaYF₄:Yb,Tm@TiO₂) to utilize not only UV energy but also the large portion of NIR energy in order to improve the utilization efficiency of solar lights. The MC-LR (10 μg/mL) degradation rate can be approached 100% within 30 min at the concentration of NaYF₄:Yb,Tm@TiO₂ 0.4 mg/mL and initial pH value 4, while 61%, using pure TiO₂ (P25) under simulated solar lights. The reaction processes were studied and fitted with the pseudo-first-order kinetic model. Highly reactive hydroxyl radicals (•OH) were found to be the major reactive species. Meanwhile, seven degradation intermediates of MC-LR were examined by liquid chromatography/mass spectrometry and the degradation mechanism was analyzed. The main degradation pathways were proposed based on the molecular weight of the intermediates and the reaction mechanism are hydroxylation on the diene bonds and the aromatic ring of Adda. The products were evaluated to be nontoxic based on the construction of the intermediates. This study demonstrated that the NIR energy can be used as the driving source for photocatalysis besides the UV and the NIR-responsed photocatalysis had a high-efficiency and potential for MC-LR degradation.

Effect of oxidant demand on the release and degradation of microcystin-LR from Microcystis aeruginosa during oxidation

In this research, the release and degradation of intracellular microcystin-LR (MC-LR) due to oxidation of Microcystis aeruginosa (M. aeruginosa) was examined kinetically. Brief exposure to free chlorine with no measureable oxidant exposure was demonstrated to be sufficient to induce rapid release of intracellular MC-LR from M. aeruginosa. Thus, in a water treatment plant, there is currently no level of prechlorination that can be assumed to be safe, since very low preoxidation prior to filtration and no measureable free chlorine residual may still observe the release and buildup of extracellular MC-LR. Higher chlorine dosages resulting in a measureable exposure or CT (concentration times contact time) cause more rapid release and oxidation of the intracellular toxins. Further, the rate of release of MC-LR with intermediate oxidant dosages were shown to be initially rapid, but then slowed to a lower release rate due to an as yet undetermined mechanism. While free chlorine was reactive with the extracellular MC-LR, the monochloramine resulting from the consumption of the free chlorine by ammonia was not. Consideration of the ammonia concentration and the chlorine dosage relative to the chlorination breakpoint dosages is important for utilities assessing the impact of prechlorination of water containing cyanobacteria. MC-LR, once released, was rapidly oxidized by permanganate resulting in only negligible buildup of extracellular toxins.

Cyanobacterial Toxins in Freshwater and Food: Important Sources of Exposure to Humans

A recent ecological study demonstrated a significant association between an increased risk of nonalcoholic liver disease mortality and freshwater cyanobacterial blooms. Moreover, previous epidemiology studies highlighted a relationship between cyanotoxins in drinking water with liver cancer and damage and colorectal cancer. These associations identified cyanobacterial blooms as a global public health and environmental problem, affecting freshwater bodies that are important sources for drinking water, agriculture, and aquafarms. Furthermore, as a result of climate change, it is expected that our freshwater environments will become more favorable for producing harmful blooms that produce various cyanotoxins. Food is an important source of cyanotoxin exposure to humans, but it has been less addressed. This paper synthesizes information from the studies that have investigated cyanotoxins in freshwater and food on a global scale. We also review and summarize the health effects and exposure routes of cyanotoxins and candidates for cyanotoxin treatment methods that can be applied to food.

Reaction Pathways and Kinetics of a Cyanobacterial Neurotoxin β-N-Methylamino-L-Alanine (BMAA) during Chlorination

β-N-Methylamino-L-alanine (BMAA), a probable cause of the amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC), or Alzheimer's disease, has been identified in more than 20 cyanobacterial genera. However, its removal and fate in drinking water has never been reported before. In this study, the reaction of BMAA with chlorine, a common drinking-water oxidant/disinfectant, was investigated. A liquid chromatograph coupled with a triple quadrupole mass spectrometer was employed to quantify BMAA and its intermediates. Upon chlorination, four chlorinated intermediates, each with one or two chlorines, were identified. The disappearance of BMAA caused by chlorine follows a second-order reaction, with the rate constant k₁ is 5.0 × 10⁴ M^-1 s^-1 at pH ∼7.0. The chlorinated intermediates were found to further react with free chlorine, exhibiting a second-order rate constant k₃ = 16.8 M^-1 s^-1. After all free chlorine was consumed, the chlorinated intermediates autodecomposed slowly with a first order rate constant k₂ = 0.003 min^-1; when a reductant was added, these chlorinated intermediates were then reduced back to BMAA. The results as described shed a useful light on the reactivity, appearance, and removal of BMAA in the chlorination process of a drinking-water system.

Impact of chlorine on the cell integrity and toxin release and degradation of colonial Microcystis

The occurrence of toxic cyanobacteria in drinking water sources is problematic for water authorities as they can impair drinking water quality. Chlorine as a commonly used oxidant in water treatment plants has shown the potential to lyse cyanobacterial cells, resulting in the release of secondary metabolites which are hard to be removed during conventional water treatment processes. The majority of cyanobacterial species such as Microcystis, often occur in colonial forms under natural conditions. However, previous studies have mainly focused on the influence of chlorination on individual cyanobacterial cells due to technique limitations. A syringe dispersion method combined with a fluorescence technique (SYTOX Green stain with flow cytometry), was successfully developed for the evaluation of cell integrity of colonial Microcystis. Chlorination of Microcystis-laden water was conducted at different chlorine dosages for different colonial sizes (<37, 37-270 and 270-550 μm). The results indicated that colonial Microcystis cells were more resistant to chlorine oxidation than individual cells, which may be attributed to protection from the cell-bound mucilage. There was a lag phase before cell rupture occurred and a Delayed Chick Watson Model describes the experimental data very well for the kinetics of cyanobacterial cell rupture. The growing colonial size caused increases in the lag phases but decreases in the cell lysis rates. Chlorination also induced the release of microcystins (MCs) from colonial Microcystis cells. In particular, increased levels of dissolved MCs were observed in Cheng Kung Lake (CKL) water. In summary, the reaction of chlorine with colonial cyanobacteria is more complicated than with individual cells. The efficiency of chlorine oxidation could be reduced by the cell-bound mucilage and natural water matrix. These observations may provide insights for water authorities to assess the risk to drinking water quality posed by chlorination under natural conditions.

An insight of disinfection by-product (DBP) formation by alternative disinfectants for swimming pool disinfection under tropical conditions

Sodium hypochlorite (NaClO) is the most commonly used disinfectant in pool treatment system. Outdoor pools usually suffer from the strong sunlight irradiation which degrades the free chlorine rapidly. In addition, more pools start to adopt the recirculation of swimming pool water, which intensifies the disinfection by-product (DBP) accumulation issue. Given these potential drawbacks of using NaClO in the tropical environment, two alternative organic-based disinfectants, trichloroisocyanuric acid (TCCA, C3Cl3N3O3) and bromochlorodimethylhydantoin (BCDMH, C5H6BrClN2O2), were investigated and compared to NaClO in terms of their self-degradation and the formation of DBPs, including trihalomethanes (THMs) and haloacetic acids (HAAs), under simulated tropical climate conditions. The result reveals that halogen stabilizer, TCCA, had the advantages of slower free chlorine degradation and lower DBP concentration compared to NaClO, which makes it a good alternative disinfectant. BCDMH was not recommended mainly due to the highly reactive disinfecting ingredient, hypobromous acid (HBrO), which fails to sustain the continuous disinfection requirement. Total disinfectant dosage was the main factor that affects residual chlorine/bromine and THM/HAA formation regardless of different disinfectant dosing methods, e.g. shock dosing (one-time spiking) in the beginning, and continuous dosing during the whole experimental period. Two-stage second-order-kinetic-based models demonstrate a good correlation between the measured and predicted data for chlorine decay (R(2) ≥ 0.95), THM (R(2) ≥ 0.99) and HAA (R(2) ≥ 0.83) formation. Higher temperature was found to enhance the DBP formation due to the temperature dependence of reaction rates. Thus, temperature control of pools, especially for those preferring higher temperatures (e.g. hydrotherapy and spa), should take both bather comfort and DBP formation potential into consideration. It is also observed that chlorine competition existed between different precursors from natural organic matters (NOM) in filling water and body fluid analogue (BFA). Among the composition of BFA, uric acid, citric acid and hippuric acid were found to be the main precursors for HAA formation.

Colonial cell disaggregation and intracellular microcystin release following chlorination of naturally occurring Microcystis

Colonial cell disaggregation and release of intracellular microcystin were evaluated following chlorine treatment of naturally occurring Microcystis. Microscopic observations of water samples collected from Lake Mead, Nevada, USA, confirmed the presence of colonial Microcystis with cells protected by an outer sheath up to 30 μm thick. During chlorination, two stages of cell decomposition were observed, stage 1: colonial cell disaggregation, and stage 2: unicellular degradation. Following a [Cl2]0:DOC0 ratio of 0.15 (t = 20 min, pH = 8.2-8.5) in unfiltered Lake Havasu samples, total particle count increased from (1.0 ± 0.11) × 10(5) to 4.2 × 10(5) particles/mL and fluorescent particle count increased from (1.2 ± 0.50) × 10(4) to 1.2 × 10(5) particles/mL, illustrating colonial cell disaggregation. Although total and fluorescent particles increased, the concentration of chlorophyll-a (Chl-a) decreased from 81 μg/L to 72 μg/L, and continued to decrease at higher [Cl2]0:DOC0 ratios. The preliminary second order rate constant for the reaction between Microcystis and chlorine in natural waters was estimated using either Chl-a (k = 15 M(-1) s(-1)) or fluorescence particle count (k = 38 M(-1) s(-1)) as an indicator of cell damage following colonial disaggregation (i.e., at [Cl2]0:DOC0 ratio ≥0.15). Complete release of intracellular microcystin-LR (MC-LR) was observed in both Lake Havasu and Lake Mead samples when applying a [Cl2]0:DOC0 ratio of 0.30 (t = 20 min), which was equivalent to a chlorine exposure of 8 min-mg/L for Lake Havasu samples. With chlorination, DOC increased by 3-18% indicating release of either colony-bound or cell-bound DOC. The results demonstrated the ability of chlorine to disaggregate/inactivate natural Microcystis colonies, and identified oxidation conditions resulting in complete release of intracellular MC-LR.

Chlorine/UV Process for Decomposition and Detoxification of Microcystin-LR

Microcystin-LR (MC-LR) is a potent hepatotoxin that is often associated with blooms of cyanobacteria. Experiments were conducted to evaluate the efficiency of the chlorine/UV process for MC-LR decomposition and detoxification. Chlorinated MC-LR was observed to be more photoactive than MC-LR. LC/MS analyses confirmed that the arginine moiety represented an important reaction site within the MC-LR molecule for conditions of chlorination below the chlorine demand of the molecule. Prechlorination activated MC-LR toward UV254 exposure by increasing the product of the molar absorption coefficient and the quantum yield of chloro-MC-LR, relative to the unchlorinated molecule. This mechanism of decay is fundamentally different than the conventional view of chlorine/UV as an advanced oxidation process. A toxicity assay based on human liver cells indicated MC-LR degradation byproducts in the chlorine/UV process possessed less cytotoxicity than those that resulted from chlorination or UV254 irradiation applied separately. MC-LR decomposition and detoxification in this combined process were more effective at pH 8.5 than at pH 7.5 or 6.5. These results suggest that the chlorine/UV process could represent an effective strategy for control of microcystins and their associated toxicity in drinking water supplies.

Effect of chlorination on the protein phosphatase inhibition activity for several microcystins

Microcystins are of particular concern due to their toxicity to both humans and animals and may be the most prominent cyanotoxin observed in freshwater. Although a number of studies have investigated the fate of microcystins and other algal toxins through drinking water treatment facilities, measurement of their potential for toxic activity after chlorination, a popular form of treatment in the United States, has not been investigated. In this study, six microcystin variants are subjected to chlorine oxidation. The degradation of each microcystin variant is measured by liquid chromatography/mass spectrometry simultaneously with protein phosphatase inhibition (PPI) response over reaction time with chlorine. Results show that inhibition is dependent on the incorporated amino acid residues, their placement within the microcystin structure, as well as pH. This pH dependence may have practical implications to such activities such as drinking water treatment when the pH is usually adjusted to around 8. Namely, at this pH, even with chlorine addition for disinfection, PPI activity may not be totally eliminated even when the initial MYCs are eliminated.

Toxic cyanobacteria and drinking water: Impacts, detection, and treatment

Blooms of toxic cyanobacteria in water supply systems are a global issue affecting water supplies on every major continent except Antarctica. The occurrence of toxic cyanobacteria in freshwater is increasing in both frequency and distribution. The protection of water supplies has therefore become increasingly more challenging. To reduce the risk from toxic cyanobacterial blooms in drinking water, a multi-barrier approach is needed, consisting of prevention, source control, treatment optimization, and monitoring. In this paper, current research on some of the critical elements of this multi-barrier approach are reviewed and synthesized, with an emphasis on the effectiveness of water treatment technologies for removing cyanobacteria and related toxic compounds. This paper synthesizes and updates a number of previous review articles on various aspects of this multi-barrier approach in order to provide a holistic resource for researchers, water managers and engineers, as well as water treatment plant operators.