Abstract

This study investigated the capability of vacuum UV to reduce the concentration of cyanobacterial toxin microcystin-LR (MC-LR) using low-pressure Hg lamps emitting 185 nm and 254 nm light. A collimated beam setup was used to irradiate samples of MC-LR solutions prepared in Milli-Q® water. The impact of competing water compounds was tested using solutions containing dissolved organic carbon (DOC), alkalinity (NaHCO3), and chloride (NaCl). Results showed that MC-LR in pure water at typical concentrations found in cyanobacterial bloom waters (17 and 40 μg/L) could be reduced below detection limits (0.5 μg/L) within one minute of irradiation time by a UV dose less than 40 mJ/cm2. A solution with a much higher initial concentration of MC-LR (870 μg/L) did show a reduced degradation rate. The presence of competing compounds does appear to reduce observed MC-LR degradation rates with the greatest impact caused by DOC followed by alkalinity followed by chloride. MC-LR degradation appears to occur by both direct photolysis by 254 nm photons and by advanced oxidation by hydroxyl radicals generated from 185 nm photons. Vacuum UV has shown promising capability at reducing MC-LR concentration.

INTRODUCTION

Cyanobacteria are photosynthetic prokaryotic organisms that are common components of many freshwater systems (Rippka et al. 1979; Chorus & Bartam 1999). Environmental conditions, such as water temperature, light exposure, and nutrient concentration, can lead to rapid growth of cyanobacteria and promote the formation of blooms (Chorus & Bartram 1999). A number of studies have found that cyanobacterial blooms are occurring more frequently in surface waters around the world, and many reports indicate that the effects of climate change are promoting this phenomenon (Edwards et al. 2006; O'Neil et al. 2012; Paerl & Paul 2012; Taranu et al. 2015).

The presence of cyanobacteria in drinking water sources can be harmful to human health. Many types of cyanobacteria produce toxic compounds known as cyanotoxins, of which the most prevalent are microcystins (Sharma et al. 2012). Microcystins are hepatotoxic compounds that can accumulate in the liver and promote the growth of tumours (Falconer 1999). The most commonly occurring and toxic microcystin is microcystin-LR (MC-LR) (Sharma et al. 2012). Studies on the toxicity of microcystins led the World Health Organization to establish a provisional guideline of 1.0 μg/L for MC-LR in drinking water (Chorus & Bartram 1999).

In cyanobacterial blooms, toxins are present within the cyanobacteria cells and dissolved in the surrounding water (Hitzfeld et al. 2000). During conventional treatment, intracellular toxins can be removed through physical separation of cells by filtration, flotation, and/or sedimentation following coagulation/flocculation. During cell removal there is potential for cell lysis and the release of intracellular toxins into the water (Roegner et al. 2014). Extracellular toxins are typically addressed by chemical oxidation. Chlorine, permanganate and ozone all show effective reduction in dissolved MC-LR concentration (Acero et al. 2005; Onstad et al. 2007; Rodriguez et al. 2007a). However, kinetic studies show that the hydroxyl radical has a significantly higher reaction rate constant with MC-LR than chlorine, permanganate and ozone (Rodriguez et al. 2007b).

Vacuum UV (VUV) is a chemical-free method of generating hydroxyl radicals in aqueous solutions. Photons emitted at wavelengths below 200 nm instigate a series of radical-forming reactions through the photochemical homolysis and ionization of water molecules (Gonzalez et al. 2004). These two reactions result in the formation of hydroxyl radicals, hydrogen atoms and solvated electrons. Dissolved oxygen scavenges the hydrogen atoms and solvated electrons, producing intermediate radical species that ultimately contribute to hydroxyl radical formation. The details of the complex reaction mechanisms during vacuum UV irradiation of aqueous systems can be found elsewhere (Gonzales et al. 2004). Vacuum UV has the advantage of providing advanced oxidation without requiring any chemical addition, and may be particularly suitable for small, remote communities where maintaining a consistent chemical supply can be difficult.

The purpose of this study is to determine the capability of vacuum UV to reduce the concentration of MC-LR in water. Experiments were conducted using different initial concentrations of MC-LR. The impact of alkalinity, chloride, and natural organic matter (NOM) on MC-LR degradation was also tested.

MATERIAL AND METHODS

Sample preparation

Water solutions were prepared using Milli-Q® water spiked with stock MC-LR purchased from Enzo Life Sciences Inc. Suwannee River NOM (isolate 2R101N) purchased from the International Humic Substances Society was used to add dissolved organic carbon (DOC) to solutions. Sodium bicarbonate (NaHCO3) was used to add alkalinity to solutions. Sodium chloride (NaCl) was used to add chloride to solutions.

Experimental setup

Experiments were conducted on a vacuum UV collimated beam setup (Figure 1), containing a low-pressure ozone-generating mercury lamp purchased from Light Sources Inc. Such lamps typically have light emissions of approximately 10–20% 185 nm, 80–90% 254 nm, and a very small percentage of emissions in the visible range (Linden & Mohseni 2014). The term ozone-generating refers to the reaction between 185 nm photons and oxygen in the air to produce ozone–a reaction that reduces the transmission of the 185 nm photons. To allow 185 nm transmission, the lamp is housed in a sealed polyvinyl chloride (PVC) enclosure that is continuously purged with nitrogen gas to remove oxygen. At the top of the enclosure is a window made from Suprasil quartz upon which a reaction vessel with a Suprasil quartz bottom can sit. Solutions are exposed to radiation through the bottom of the reaction vessel. Vessel diameter is 45 mm and typical sample volumes were 40 mL. Further details about the setup can be found in a paper by Duca et al. (2014). The 254 nm incident irradiation was determined by iodide/iodate actinometry as described by Rahn (1997). Correction factors described by Bolton & Linden (2003) were applied to convert incident irradiance to average irradiance of 254 nm for each water sample. Experiments were conducted in triplicate, at room temperature (20°C) and neutral pH (6–8) conditions.

Figure 1

Vacuum UV collimated beam experimental apparatus (Duca et al. 2014).

Figure 1

Vacuum UV collimated beam experimental apparatus (Duca et al. 2014).

Analytical methods

The concentration of MC-LR in water samples was determined using a Dionex Ultimate 3000 High Performance Liquid Chromatography (HPLC) system with a C18 (4 μm particle diameter) reversed-phase analytical column and UV detector. A mobile phase of phosphate buffer (pH 2.8) and methanol (50:50%, v/v) was used at a flow rate of 1 mL/min. The column temperature was maintained at 35°C and UV detection was at a wavelength of 240 nm. To measure low concentrations of MC-LR, HPLC analysis was preceded with solid-phase extraction (SPE). A SUPELCO Visiprep™ SPE Vacuum Manifold apparatus by Sigma-Aldrich was used. The MC-LR detection limit was approximately 0.5 μg/L. The DOC concentration of samples was measured using a Sievers M9 TOC Analyzer by General Electric. The 254 nm UV absorbance of samples was measured using an Agilent Cary 100 UV-Vis spectrophotometer. The alkalinity of samples was measured by titration using 0.02 N sulphuric acid as per Standard Method 2320B from the American Water Works Association. The concentration of chloride in water samples was measured using a Dionex ICS-1100 ion chromatography system with an IonPac® AS22-Fast analytical column and an ASRS-300 (4 mm) suppressor. The mobile phase was an aqueous solution of 4.5 mM carbonate and 1.4 mM bicarbonate pumped at a flow-rate of 1.2 mL/min.

RESULTS AND DISCUSSION

MC-LR degradation in pure water

Natural waters affected by cyanobacterial blooms have shown a range of toxin concentrations that typically fall between 1 and 80 μg/L, with highly contaminated waters containing concentrations up to 300 μg/L (Svrcek & Smith 2004). To test the degradation of MC-LR by vacuum UV, solutions of MC-LR in Milli-Q® water were irradiated for different time intervals. Solutions of 17 μg/L and 40 μg/L were tested to observe if differences in initial toxin concentration within the normal bloom water range affected degradation rates. The results (Figure 2) show that MC-LR concentration of both waters was reduced below detectable levels within one minute of exposure. The lowest UV dose required to achieve undetectable MC-LR in the 17 μg/L and 40 μg/L solutions was 30 mJ/cm2 and 40 mJ/cm2, respectively.

Figure 2

Impact of initial MC-LR concentration on degradation by vacuum UV.

Figure 2

Impact of initial MC-LR concentration on degradation by vacuum UV.

A third solution of 870 μg/L MC-LR was also tested to observe if very high initial MC-LR concentration would impact the rate of degradation by vacuum UV. As shown in Figure 2, the higher toxin concentration did result in a lower degradation rate; however, the concentration of MC-LR was still reduced by 70% after 1.6 min of irradiation, which corresponded to a UV dose of less than 65 mJ/cm2. The reduction in MC-LR degradation rate is likely due to hydroxyl radical generation becoming a limiting factor.

Initial toxin concentration did appear to have an effect on vacuum UV degradation kinetics, however the effect appeared to be minimal at toxin concentrations typically found in natural bloom waters. In the absence of competing compounds vacuum UV was capable of effectively degrading MC-LR at relatively low UV doses.

Relative impact of 254 nm and 185 nm

Vacuum UV lamps emit photons at 254 nm and 185 nm wavelengths, allowing for two potential methods of toxin degradation: advanced oxidation by hydroxyl radicals generated when 185 nm photons react with water, and direct photolysis by absorption of 254 nm photons by the toxin. In order to observe the relative contribution of each wavelength to MC-LR degradation, samples of the 870 μg/L solution were irradiated under two different conditions: vacuum UV and by a UVC lamp that only emitted 254 nm photons. Figure 3 shows the degradation of MC-LR under both conditions. The results showed MC-LR does undergo degradation by direct photolysis when exposed to 254 nm photons. Exposure to vacuum UV led to greater toxin degradation which can be attributed to hydroxyl radicals generated via 185 nm photons.

Figure 3

Degradation of 870 μg/L MC-LR solution by direct photolysis (254 nm) and direct photolysis with advanced oxidation (254 nm + 185 nm).

Figure 3

Degradation of 870 μg/L MC-LR solution by direct photolysis (254 nm) and direct photolysis with advanced oxidation (254 nm + 185 nm).

Impact of competing compounds on MC-LR degradation rate

When competing compounds are present in solution, MC-LR degradation can be impeded. Two possible means of impedance are scavenging of hydroxyl radicals that could otherwise degrade the target toxin, and absorbance of 254 nm photons to reduce the UV dose applied to the total sample volume. Common constituents of natural waters, such as the carbonate ion, chloride ion, and DOC, are typically present in much higher concentrations than cyanotoxins and therefore compete for available hydroxyl radicals. Hydroxyl radicals are non-selective and impedance of MC-LR degradation due to radical scavenging is based on the reaction rate constants between the hydroxyl radical and the competing compounds.

In addition to scavenging hydroxyl radicals, DOC can absorb 185 nm and 254 nm photons. To test the robustness of vacuum UV to degrade MC-LR in the presence of organics, MC-LR solutions made with 1.5 mg/L and 2 mg/L DOC were irradiated and compared to a solution with no DOC.

Figure 4 shows the impact of DOC on the reduction of MC-LR. The presence of DOC reduced the rate of MC-LR degradation, and therefore a longer irradiation time was required to achieve undetectable MC-LR levels. The degradation rates of the solutions containing DOC were similar, and though the difference in DOC concentration was not very large, the solution with 1.5 mg/L DOC received a higher average irradiance because the solution had a lower UV absorbance and therefore required less time to receive the same UV dose as the solution with 2 mg/L DOC.

Figure 4

Impact of DOC on MC-LR degradation by vacuum UV.

Figure 4

Impact of DOC on MC-LR degradation by vacuum UV.

Although NaHCO3 and NaCl do not contribute significantly to the 254 nm absorbance of a solution, the presence of alkalinity and chloride does impede MC-LR degradation. Table 1 shows UV absorbance at 254 nm and observed first-order rate constants (kobs) for MC-LR solutions containing 2 mg/L DOC, 50 mg/L HCO3/CO32− and 50 mg/L Cl, as well as the observed rate constant for an MC-LR solution in pure water. The results show that DOC has the most impact on MC-LR degradation rates, followed by HCO3/CO32− and Cl. Duca (2015) found similar results, where the presence of bicarbonate, and DOC reduced the vacuum UV degradation of the herbicide atrazine through hydroxyl radical scavenging.

Table 1

UV absorbance and observed rate constants of MC-LR solutions with and without competing compounds

SolutionUV254 absorbance (cm−1)kobs (min−1)
17 μg/L MC-LR in pure water <0.0001 4.4 
15 μg/L MC-LR + 50 mg/L Cl 0.0005 ± 0.00006 3.6 
18 μg/L MC-LR + 50 mg/L HCO3/CO32− <0.0001 1.9 
16 μg/L MC-LR + 2 mg/L DOC 0.0638 ± 0.00008 1.2 
SolutionUV254 absorbance (cm−1)kobs (min−1)
17 μg/L MC-LR in pure water <0.0001 4.4 
15 μg/L MC-LR + 50 mg/L Cl 0.0005 ± 0.00006 3.6 
18 μg/L MC-LR + 50 mg/L HCO3/CO32− <0.0001 1.9 
16 μg/L MC-LR + 2 mg/L DOC 0.0638 ± 0.00008 1.2 

CONCLUSIONS

This study investigated the capability of vacuum UV to degrade the cyanotoxin MC-LR. Vacuum UV showed effective degradation of MC-LR in pure water, reducing typical bloom concentrations of 17 μg/L and 40 μg/L to below the detection limit of 0.5 μg/L within 1 min of irradiation (UV doses of 30 and 40 mJ/cm2, respectively). The high toxin concentration of 870 μg/L had a slower degradation rate than the solutions with typical bloom concentration, but was still reduced by 70% after 1.6 min of irradiation and a UV dose less than 65 mJ/cm2. MC-LR degradation appears to occur by both direct photolysis and advanced oxidation. The experiment to determine the relative contributions of each degradation method found that over 50% MC-LR reduction could be attributed to direct photolysis by 254 nm photons, while advanced oxidation contributed an additional 20% MC-LR reduction. The presence of DOC impeded MC-LR degradation, likely by absorbing 254 nm photons and scavenging hydroxyl radicals. The presence of alkalinity also reduced the observed MC-LR degradation rate to lesser extent. Chloride also affected MC-LR degradation, but showed the least impact. Vacuum UV has shown to be a promising technology for reducing MC-LR concentration to below guideline levels in both pure water and in the presence of competing compounds.

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