Oxidation treatment of a novel cyanotoxin β-methylamino-L-alanine in Water

• Main Authors: Yi-TingChen, 陳逸廷
• Other Authors: Tsair-Fu Lin
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/tc7qpr

博士 === 國立成功大學 === 環境工程學系 === 106 === Eutrophication of lakes and reservoirs has become a worldwide issue in recent years due to increased nutrient loading from human activities. One issue associated with eutrophication is the overgrowth of cyanobacteria in lakes and reservoirs. This is especially important for the lakes and reservoirs served as drinking water sources, as many cyanobacteria may produce some second metabolites such as cyanotoxins and odorants, posing additional risk to public health and affecting the quality of drinking water. A novel neurotoxin, β-N-Methylamino-L-alanine (BMAA), has been reported to be produced by more than 20 genera of cyanobacteria. The chemical may be biomagnified in the food chain and may cause amyotrophic lateral sclerosis/parkinsonism–dementia complex (ALS/PDC) or Alzheimer's disease. This implies that there is a possibility for public exposure to BMAA if the drinking water sources are with high concentrations of cyanobacteria. However, to date, the removal and fate of BMAA in drinking water systems has never been reported before. As chlorine, ozone, permanganate and OH radical are the most commonly used oxidants in water treatment plants, the reactions and kinetics between BMAA and these oxidants were investigated in this study. The reaction pathway of BMAA, the formation of intermediates and their reaction kinetics during chlorination process were elucidated in the first part of this study. A method based on liquid chromatograph equipped with triple quadrupole mass spectrometry (LC/MS-MS) was developed for the analysis of BMAA and its chlorinated intermediates. Upon chlorination, four chlorinated intermediates, each with 1 or 2 chlorines, were identified. The reaction of BMAA with free chlorine follows a second-order reaction and was pH-dependent. The rate constants k1 increased dramatically 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. The chlorinated intermediates were found to further react with free chlorine, exhibiting a second-order rate constant k3 = 17.75 M-1s-1 under different pH conditions. After all free chlorine was consumed, the chlorinated intermediates auto-decomposed slowly with a first order rate constant k2 = 0.0121 min-1 at pH=5.8, and about 0.0023 - 0.0029 min-1 between pH=7-9.5; when a reductant was added, these chlorinated intermediates were then reduced back to BMAA. BMAA and its chlorinated intermediates can be degraded by ~90% if the CT value = 150 mg/L•min in both deionized and natural water. The oxidation of BMAA with ozone and OH radical also followed the second order reaction rate law. The 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, with similar pH dependency to that with chlorine. 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. Higher hydroxide concentration may accelerate the reaction of ozone and form more reactive oxidants for BMAA. For both permanganate and H2O2 only, the removal of BMAA was negligible. The reaction rate was in the order of OH radicals 〉 ozone 〉〉 chlorine 〉〉permanganate ~ H2O2 ~ direct photolysis. In addition, the natural organic matters was found to slow down the degradation of BMAA if compared with that in deionized water. The results as described shed a useful light to the reactivity, appearance, and removal of BMAA in the oxidation process of a drinking-water system. A method to detect BMAA and its isomers, 2,4-diaminobutyric acid (DAB) and N-(2-aminoethy)glycine (AEG) was also evaluated. A liquid chromatography (LC) coupled with tandem mass spectrometry (MS/MS) was employed for the analysis of BMAA, AEG, and DAB, using derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). The results show that the detection limits of BMAA, AEG, DAB were 2 μg L-1, 2 μg L-1, and 50 μg L-1, respectively. Both AEG and BMAA were detected to be present in the raw water of Rong-Hu Water Treatment Plant (WTP) and the finished water of Tai-Hu WTP. In addition, BMAA and AEG were also found in the laboratory culture samples of Microcystis aeruginosa.