Chlorine/UV Treatment of Anatoxin-a
Chlorination followed by UV irradiation (Cl/UV) is a newly developed advanced oxidation process (AOP) that has several advantages over other AOPs. Specifically, as compared to the UV/H2O2 process, Cl/UV can provide the required chlorine residual in potable water systems, and avoids the additional chlorine demand to quench residual H2O2. Also, for research purposes, UV254 emitted from low-pressure mercury lamp could more efficiently yield free radicals from HOCl and OCl- and decrease 30-75% of the energy requirement. Cyanotoxins, as one of the most threatening toxic substances in our freshwater, have been investigated regarding their degradation, including through the use of AOPs. However, in most recent studies, radical chemistry has been considered primarily in treating many hazardous compounds including cyanotoxins, but the effect of chlorination in activating hazardous chemical toward UV treatment has barely been investigated. Many cyanotoxins, including neurotoxins (e.g., Anatoxin-a) and hepatotoxins (e.g., Microcystin- LR) have one or more primary and secondary amines as their functional groups. Chlorination of amine groups is generally rapid, yielding an N-Cl bond by electrophilic substitution, and can activate cyanotoxins toward subsequent UV exposure by increasing the product of ε⋅Φ. Anatoxin- a (ANTX-a), was selected as a representative cyanotoxin to investigate the effect Cl/UV treatment because it is a secondary amine. A critical review of the available literature for chlorination, UV, and Cl/UV treatment was conducted. Objectives of this review were to: (1) compile and review cyanotoxin-related treatment processes and their fundamental knowledge; (2) compare methods for degradation of cyanotoxins, especially ANTX-a; (3) evaluate current efficacy in cyanotoxin treatment; and (4) identify the potential of Cl/UV treatment for degradation of ANTX-a. Briefly, many AOPs have been applied for treatment for ANTX-a, but many conflicts remained in either the chlorination or the direct UV photolysis. Despite direct UV treatment, AOPs such as UV/VUV and UV/H2O2 have also been carried out and identified to be more efficient than direct treatment. There is also a lack of knowledge on the behavior of ANTX-a in chlorination, as it was hypothesized to be a fast process; however, one study indicated slow chlorination in ANTX-a. The Cl/UV process has not been previously investigated for degradation of ANTX-a. Chapter 2 and Chapter 3 provide descriptions of experiments that were conducted to define the efficiency of 3 types of treatment: chlorination, direct UV photolysis, and Cl/UV treatment. The study objectives were to: (1) identify the behavior of ANTX-a and its surrogate compound dimethylamine (DMA) in chlorination; (2) identify the effect of direct UV exposure on degradation of these compounds; (3) identify the change in efficiency attributable to the Cl/UV process; and (4) evaluate the value of Cl/UV process as compared to other AOPs. Generally, the results described herein are in disagreement with previous studies that suggested that the chlorination of ANTX-a is a slow process. On the contrary, the behavior of the ANTX-a was quite like DMA in chlorination, which was a very fast process. The fact that quenching the solution with sodium thiosulfate did not only quench free chlorine, but also quenched N-chlorinated compound, was an indication that N-chlorination occurred during the chlorination of ANTX-a. Since the behaviors of ANTX-a and DMA are quite similar in chlorination, it was possible to prepare N-chlorinated solutions of ANTX-a and DMA, which allowed further investigation of the effect of the N-Cl bond in increasing the activity of amine compounds under UV254 exposure. The quantum yield of N-Cl bond was obtained as 0.624 ±0.011 mol⋅E-1 from the direct photolysis on N-chlorodimethylamine (CDMA). Cl/UV photodegradation, or the photodegradation of N-chloro-ANTX-a, was also identified to be a faster treatment than direct photolysis of ANTX-a. For comparison, a quantum yield of 0.100 mol⋅E-1 and a product of molar absorptivity and quantum yield (ε⋅Φ) of 383 L⋅cm-1⋅E-1 were obtained. As compare to 243 L⋅cm-1 E-1 for ANTX-a itself, the increase in ε⋅Φ represents great activation by prechlorination. However, although the linear regression indicated a correlation coefficient of more than 99%, as fumaric acid always existed in the solution of ANTX-a (Fumarate), we did not regard that the photodegradation on N-chloro-ANTX-a was pseudo-first- order. We supposed that in pure ANTX-a solution, the performance of UV/Cl process could be even more efficient.
Blatchley, Purdue University.
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