各種氧化劑對水體中 BMAA 的去除效能及在不同水體中的反應動力學研究

本部分研究已完成論文之撰寫，即將投稿中，詳見於後續 p.35-p.48 5-1 次氯酸鈉與 BMAA 於不同 pH 值及天然水體中之反應動力模式

關於氯與 BMAA 之反應動力學模式，已詳述於 3-2 BMAA 氧化動力模式介 紹中。由 4-1 節的研究結果得知，BMAA 與氯反應可分為三個階段，分別是自由 餘氯與 BMAA 反應、含氯中間產物的自體降解以及含氯中間產物再度被自由餘 氯降解(Scheme 4-1)。此部分的研究，則接續 4-1 節的內容，針對氯在不同 pH 值 及天然水體中與 BMAA 的反應動力進行更深入的研究和探討。

在自由餘氯與 BMAA 反應的部分，同樣使用 4-6 DCR 做為競爭劑，由競爭 實驗的方式求出二階反應速率，分別於 pH5.5、7、9 及蘭潭水庫的湖水(pH=8.5) 進行實驗。結果顯示(圖 5-1)，在不同 pH 值及天然水中的擬合程度均相當好 (R²>0.97)，且 BMAA 和自由餘氯的反應速率會隨著 pH 值的增加而變大(表 5-1)。

值得注意的是，BMAA 和自由餘氯在蘭潭湖水為背景液中的反應速率常數較在 pH=7 中小，可以推測天然水中的天然有機物( Natural organic matter)同時也會與 BMAA 競爭系統中的自由餘氯，而減低了反應速率。

BMAA 的含氯中間產物在 BMAA 初始濃度遠大於氯的條件下進行實驗，並 假定在反應開始 5 分鐘後系統中的自由餘氯濃度已不足以繼續和 BMAA 或 BMAA 的含氯中間產物反應。此時於樣品中加入抗壞血酸，使 BMAA 的含氯中 間產物被還原成 BMAA，與未加入抗壞血酸樣品中所測得 BMAA 之濃度差，即 可視為系統中 BMAA 含氯中間產物的濃度，再利用擬一階反應(公式 4-3)，即可 求出 BMAA 含氯中間產物之自體降解速率 k2。從圖 5-2 中可看出，pH=7-9 範 圍中(包含蘭潭湖水)，其自體降解速率的差異並不大，而於酸性條件下的自體降 解速率則較快(表 5-1)。此部分結果與 Vikesland 等人在 2001 年觀測一氯胺於不 同 pH 值中的自體降解速率結果相似，而水體中的天然有機物並不會影響其自體 降解的速率。

自由餘氯與 BMAA 含氯中間產物之反應速率常數 k3，此部分則以氯的初始 濃度遠大於 BMAA 的條件下進行實驗。由於 BMAA 同樣會在極短的時間內與自 由餘氯反應完畢，因此採用反應進行五分鐘後被還原的 BMAA 變化量來代表 BMAA 含氯中間產物的變化量，同樣可利用 4-2 式求出二階反應速率常數。此部 分實驗除了在 pH=5.5, 7, 9 條件下進行，另有蘭潭湖水(LT water, DOC = 1.4 mg/L) 及成功湖水(CKL water, DOC = 3.7 mg/L)。實驗結果發現，不論是 pH 值或是水體 中天然有機物的含量對於此部分的反應速率常數皆無顯著影響( 表 5-1)，

k3=17.75 M

^-1s^-1，且在五組不同條件下的實驗數據整體擬合結果相當好(圖 5-3，

R² =0.97)。而在 CT 值為 150 mgL^-1•min 時，90%以上的 BMAA 及其含氯中間產

物可以有效的被氯降解。

5-2 過錳酸鉀與 BMAA 之氧化反應

本部分實驗分別以濃度為 3、4、5mg/L 之高錳酸鉀，氧化初始濃度皆為 0.5 mg/L 之 BMAA，採樣時間為 0、5、10、30、60、90、120、180 分鐘，同樣加入 抗壞血酸做為中止劑停止氧化反應進行，並分析過錳酸鉀及 BMAA 之剩餘濃度。

從圖 5-4 中可看出，過錳酸鉀濃度並沒有隨時間而下降，在反應時間 180 分 鐘後仍有超過 90%的過錳酸鉀存在於反應器中。相同的結果可以對應至剩餘 BMAA 比例上，至反應結束時同樣也還有超過 95%的 BMAA，顯示過錳酸鉀並 沒有氧化 BMAA 的能力，在水體中也幾乎不與 BMAA 反應，其反應速率常數只 有 0.25±0.23 M^-1S^-1。由於過錳酸鉀的氧化機制，主要是針對碳-碳雙鍵的破壞，

而 BMAA 的分子結構中並沒有碳-碳雙鍵，因此推測可能是過錳酸鉀幾乎不與 BMAA 反應的原因。相較其他常見的藍綠藻毒，過錳酸鉀對魚腥藻毒(anatoxin-a) 的反應速率相當大，二階反應速率可達 6.4 × 10⁵ M^-1s^-1，微囊藻毒則為 400-470 M^-1s^-1，至於柱孢藻毒則只有 0.3 M^-1s^-1。

5-3 臭氧與 BMAA 之氧化反應

臭氧與 BMAA 的反應速率相當快，在前置實驗中發現一分鐘內就會與 BMAA 反應完畢，因此同樣使用競爭實驗來求得 BMAA 與臭氧之反應速率。在 此部分實驗用使用 4-氯-2-甲基苯酚(4-chloroguaiacol, 4-CG)做為競爭劑。由於臭 氧分子在 pH <7 的情況下在水中很容易分解同時產生氫氧自由基，而為了要區別 此部分的實驗是由臭氧分子直接與 BMAA 反應，因此在各條件下皆有一組於反 應器中添加異丙醇(2-propanol)之對照組，以了解氫氧自由基是否有參與反應。

實驗結果顯示，在各條件下添加異丙醇與否對臭氧與 BMAA 的反應速率並 沒有顯著的差別(圖 5-5)，顯示在臭氧與 BMAA 反應完成的極短時間中，並沒有 太多臭氧分子於水體中分解成氫氧自由基，因此可直接由競爭實驗的結果得到臭 氧對 BMAA 的反應速率常數。參考 Benitez 等人於 2000 年所發表之 4-CG 於不 同 pH 值下和臭氧的反應速率，並利用 3-6 式求得 BMAA 和臭氧的反應速率常 數 (表 5-2)。從表中可看出，pH 值會影響臭氧對 BMAA 的反應速率常數，pH 值越高，反應速率越快，最快為在蘭潭湖水中(pH=8.5)，其反應速率為 3.1 × 10⁹ M⁻¹ s⁻¹。參考其他文獻，臭氧對微囊藻毒、柱孢藻毒及魚腥藻毒的反應速率常數 約為 6.4 × 10⁴- 4.1 × 10⁵ M⁻¹ s⁻¹，顯示 BMAA 較容易受到臭氧的破壞。

5-4 BMAA 在 H2O2/UV 系統下之氧化反應

此實驗 H2O2 初始濃度為 10 mg/L 和 60 mg/L，在 UV 光強度設定為 57.4 Wm

-由基濃度。於不同 pH 值條件下及蘭潭湖水的背景液中，範圍約介於 5.3 × 10^-14- 3.3 × 10^-13M，若與文獻報導高級氧化處理程序中，氫氧自由基濃度在 10^-10M 至 10^-12M (Esplugas et al. 2002)相比較，本實驗系統氫氧自由基生成濃度較低；由於 本研究使用之紫外光波段能量強度並不高，因此較低的自由基產生量是可預期的 (游，2014；Xuo et al., 2015)。

為了要確認 BMAA 是否會直接和 H2O2 產生反應，各條件下皆有一組 BMAA=0.5mg/L, H2O2 = 60 mg/L 不照射 UV 光的對照組。從圖 5-6 可看出，在 實驗時間的 180 分鐘內，各對照組中 BMAA 的濃度幾乎維持恆定，顯示 H2O2和 BMAA 的反應速度相當慢，在 180 分鐘內並無法對 BMAA 做有效降解，同時能 由此結果確認在 UV 燈照射下，是由氫氧自由基進行對 BMAA 的降解。

圖 5-6 為 BMAA 在氫氧自由基做用下於各不同條件水體中隨時間降解的情 形。從圖中可以看出，當 pH 值越高時，相同時間內被降解的 BMAA 量就越多，

代表氫氧自由基在 pH 值較高的情況下與 BMAA 的反應速較快；而同樣在 pH 值 為 8.5 的條件下，在 DI water 中 BMAA 被降解的比在蘭潭湖水中多。由氫氧自 由基濃度代入公式 3-9 後可計算出氫氧自由基對 BMAA 的反應速率常數(表 5-2)，與氯及臭氧相同，pH 值越高反應速率常數越大，在 pH=8.5 的情況下可達 1.05±0.16 × 10¹⁰ M⁻¹ s⁻¹，也是所有氧化劑中反應速率最快的。另外，氫氧自由基 在蘭潭湖水中的反應速率約為在 DI water 中的一半，顯見水體中的天然有機物會 影響並降低對 BMAA 的降解速率。

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0 0.5 1 1.5 2 2.5

pH=9.0 pH=7 pH=5.8 LT Ln (C T/C 0, DCR)

Ln (C

T/C

0, BMAA)

y=0.198X+0.007 R²=0.978 y=0.276X+0.006 R²=0.98 y=0.657X+0.019 R²=0.991

y=1.67X+-0.133R²=0.977

圖 5- 1. Competitional reaction of BMAA and DCR. The initial BMAA and DCR concentration were set at 5μM, and the dosages of chlorine were at 33.3, 5.9, 7.7, 9.3, 13.5 and 15 μM, at pH 5.8, 7.0, 9.0 and in LT water (pH=8.5)

表 5- 1. Rate constant of BMAA and chlorine under different pH condition.

k1 (M

^-1s^-1)

k2(min

^-1)

k3 (M

^-1s^-1) 5.8 2.1 × 10³ 0.0121

17.75 7 5.0 × 10⁴ 0.0029

8.5 (LT water) 3.78 × 10⁴ 0.0027 9 1.21 x 10⁵ 0.0023

* k1: rate constant of free chlorine and BMAA

k2: auto-decomposition rate of chlorinated-BMAA

k3: rate constant of free chlorine and chlorinated-BMAA

-6 -5 -4 -3 -2 -1 0 1

0 50 100 150 200 250 300 350 400

pH=5.5-5.8 pH=7.0 pH=9.0

LT water (pH=8.5)

L n ( C/ C

0,

R ev e rse d B MA A)

Time (min)

y=-0.0121X-0.281, R²=0.897

y=-0.0029X+0.09, R²=0.81

y=-0.0027X-0.002, R²=0.85 y=-0.0023X-0.12, R²=0.74

圖 5- 2. The concentrations of chlorinated BMAA intermediates (Re-BMAA) and the fitting with auto-decomposition model. The initial BMAA concentration was set at 5 mg/L, and the dosages of chlorine were at 0.5, and 1 mg/L.

0 0.2 0.4 0.6 0.8 1 1.2

0 0.2 0.4 0.6 0.8 1 1.2

0 50 100 150 200

pH=7

pH=5.5 KMnO4

B MA A ( C /C

0

) K M nO

4

(C/ C

0

)

Time (min) 0

0.2 0.4 0.6 0.8 1 1.2

0 100 200 300 400 500 600

CT

pH=5.5-9.0 LT Water (pH=8.5)

L n (C/ C

0

, R eve rs ed BM A A )

CKL Water (pH=8.3)

圖 5- 3. The reaction rate between the chlorinated-intermediates and free chlorine under different pHs and in natural water. The initial chlorine dosage was set at 5 mg/L, and the concentrations of BMAA were at 0.5 and 1.25 mg/L.

* DOC: LT water- 1.4 mg/L; CKL water- 3.7 mg/L

圖 5- 4. The degradation of BMAA by permanganate under different pH conditions.

The initial permanganate dosage were set at 3,4 and 5 mg/L, and the concentrations of

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

pH=5.5 pH=5.5/ Propanol pH=7

pH=7/ Propanol LT water (pH=8.5) LT water/ Propanol

Ln (C 0/C T, 4-CG)

Ln (C/C

T, BMAA)



圖 5- 5. Competitional reaction of BMAA/Chloroguaiacol. The initial BMAA and 4-chloroguaiacol concentration were set at 5μM, and the dosages of ozone were at 0.27, 0.54, 0.81, 1.08, 1.35 and 1.62 mg/L, at pH 5.8, 7.0, 9.0 and in LT water (pH=8.5).

(Solid symbols- experiments with scanvenger, 2-propanol; Open symbols- experiments without scanvenger, 2-propanol; R² > 0.93 in all cases.)

表 5- 2. Rate constant of BMAA and oxidants under different pH condition.

pH KBMAA (M⁻¹ s⁻¹)

Chlorine KMnO4 Ozone OH radical 5.8 2.1 × 10³ Slow 5.8 × 10⁶ 1.11±026 × 10⁸ (n=6)

7 5.0 × 10⁴ Slow 6.14 × 10⁷ 6.65±2.01 × 10⁹ (n=6)

8.5 1.05±0.16 × 10¹⁰ (n=4)

8.5 (LT water) 3.78 × 10⁴ - 3.10 × 10⁹ 6.88±1.25 × 10⁹ (n=4)

9 1.21 x 10⁵ -

-1

圖 5- 6. The reaction of BMAA and OH radicals. The initial concentration of BMAA were 0.2 and 0.5 mg/L. The dosage of H2O2 were 10 and 60 mg/L, UV light intensity was set at 56.7 Wm^-2.

(▓: Control experiments were conduct under BMAA=0.5 mg/L, H2O2= 60 mg/L condition of each case.)

Removal of cyanobacterial neurotoxin beta-N-methylamino-L-alanine (BMAA) by four oxidants

Introduction

The blooming of cyanobacteria has become a global phenomenon in recent years because of eutrophication of many lakes and reservoirs. As many cyanobacteria are producers of cyanotoxins and taste and odor compounds, excess growth of cyanobacteria may deteriorate water quality of water resources, posing additional risk to public health and public perception for drinking water.

Recently, studies have demonstrated most species of cyanobacteria have the ability to produce a neurotoxin,

β-N-methylamino-L-alanine (BMAA)(Cox et al. 2005), which

has received renewed attention as an environmental risk factor and associated with amyotrophic lateral sclerosis/parkinsonism–dementia complex (ALS/PDC) or Alzheimer's disease (Murch et al. 2004, Pablo et al. 2009, Bradley et al. 2013). In addition, BMAA has been proved to be neurotoxic to chicks, rats and monkeys (Bell, 2009), and may be mis-incorporated into brain proteins, where it recycles, causing slowness of neurodegeneration(Murch et al. 2004, Rao et al. 2006).

The early investigation of the linkage between BMAA and ALS was from the Chamorro people who live on the island of Guam. Chamorro people not only consume the flour made from the seeds of Cycas micronesica but also flying fox and other animals that fed by cycad seeds(Cox and Sacks 2002). Several neurtoxins were identified in cycad seeds, including BMAA (Bell 2009), produced by the cyanobacterium, Nostoc, which was symbiotic in roots of C. Micronesia and biomagnified 10,000-flood in flying foxes (Cox et al. 2003, Banack et al. 2010). High concentrations of BMAA was detected in the brains of Chamorros who died of ALS/DC and Alzheimer’s disease(Murch et al. 2004). Similarly, in Florida, USA, the high

BMAA levels in the brains of the ALS patients in Florida was linked to the high BMAA concentrations present in the carbs and shrimps they consumed (Brand et al. 2010, Mondo et al. 2012).

Chemical oxidation process has been applied in water treatment for the removal of cyanobacteria cells and extracellular cyanotoxins. Westrick et al. reviewed and summarized the effectiveness and application of six oxidants, for the inactivation of the four conventional cyanotoxins, which including microcystin, anatoxin-a, cylindrospermopsin, and saxitoxin (Westrick et al. 2010). Among these oxidants, chlorine is the most commonly used oxidant/disinfectant in drinking water systems, and the reactions between chlorine and cyanotoxins have been extensively documented.

Chlorine can effectively degrade these cyanotoxins except anatoxin-a. Although oxidation of BMAA was studied to a much lesser extent, Chen et al. (2017) examined chlorination of BMAA in water, including formation of intermediates, reaction mechanisms and kinetics. In their report, four chlorinated intermediates, each with one or two chlorines, were identified, and the intermediates may form reversely back to BMAA under a reducing condition (Chen et al. 2017).

Potassium permanganate (KMnO4) is another oxidant commonly used in drinking water treatment,. The oxidant may inhibit the growth of microorganism, remove iron and manganese, control taste and odor and enhance the removal efficiency for coagulation and filtration processes(Rodríguez et al. 2007a). However, KMnO4 cannot effectively degrade cylindrospermopsin and saxitoxin.

Ozone is effective for degradation of many pollutants, and is widely used in advanced water treatment plants. Ozone reacts with alkene groups, activated aromatic and neutral amine functional groups, with great specificity of hydroxyl radicals (·OH), which are formed from ozone decomposition in aqueous solutions, especially under higher pH

Compared to other oxidants, hydrogen peroxide is known for environmental friendliness. H2O2/UV system is one of the advanced oxidation processes which is a newly developed technology in water treatment. The hydroxyl radicals are produced during the processes with the strong oxidative capacity, and it had been demonstrated can control cyanobacteria and degrade microcystin, anatoxin-a and cylindrospermopsin effectively (Westrick et al. 2010, Huo et al. 2015).

As cyanobacteria are an important group of microorganisms present in many source waters, occurrence of BMAA in drinking water systems is very likely. However, the study about removal of BMAA from water is limited to chlorination of BMAA (Chen et al. 2017, Cao and Xian 2016), with the rate constants between chlorine and BMAA being only available at pH<7. As in eutrophic source water, the pH is commonly higher than 8.0 (Xie et al. 2003), it is necessary to understand the influence of pH of the reactions between chlorine and BMAA.

In light of the shortage of current literature on BMAA removal, the aims of this study include (1) a description of the reaction kinetics during the BMAA chlorination processes in natural water and under alkaline condition, (2) an understanding the removal of BMAA by different oxidants, including KMnO4, ozone and H2O2/UV, and (3) to evaluate the reaction kinetics between these oxidants and BMAA.

Materials and Methods

BMAA analyses

The quantification of BMAA was performed using a liquid chromatograph (1260

The quantification of BMAA was performed using a liquid chromatograph (1260

在文檔中 自來水系統中新型藻毒素β-甲氨基-L-丙氨酸之分析、流佈調查及氧化處理研究 (頁 35-57)