Degradation of neurotoxin β-N-methylamino-L-alanine by UV254 activated persulfate: Kinetic model and reaction pathways
Graphical abstract
Introduction
β-N-methylamino-L-alanine (BMAA) was first extracted from Cycad seeds in 1967 [1] and is considered a novel neurotoxin that can cause neurodegenerative diseases [2]. BM AA concentrations as low as 10 μM have been linked to neuronal damage and neurodegenerative diseases [3]. However, unlike other cyanotoxins such as microcystins and nodularin, which are restricted to certain cyanobacteria, BMAA can be synthesized by multiple cyanobacterial strains [4]. Excessive nutrient loading often leads to cyanobacterial blooms worldwide, and BMAA may be released into aquatic environments via cell lysis [5]. Therefore, BMAA is ubiquitous in eutrophic waters [6]. Moreover, due to BMAA bioaccumulation [7], high concentrations of this compound have been reported in common food items such as oysters, mussels, and fish [8]. This may lead to an increased risk of toxin dietary exposure, and therefore the implications of BMAA pollution to human health must be further investigated.
Chemical oxidation is a feasible BMAA removal method that can be implemented in drinking water treatment plants. The degradation behaviors of BMAA by chlorine and potassium permanganate have been previously investigated [9], [10]. Potassium permanganate cannot remove BMAA, whereas chlorine can effectively degrade BMAA in pure water. However, our previous work showed that the efficiency of BMAA chlorination would drop dramatically in aqueous solution, and the reaction time would be prohibitively lengthy for drinking water plants to achieve a degradation rate of 90% or higher [11]. For BMAA-contaminated water, chlorination may not guarantee the safety of water intended for end consumers and may lead to the formation of chlorinated disinfection byproducts. Therefore, a more efficient BMAA treatment method is urgently needed.
Advanced oxidation processes (AOPs) are a technique for the decomposition of organic contaminants by generating highly efficient radical species [12]. Compared with chemical oxidation, AOPs may be able to degrade BMAA more efficiently. Traditional AOPs are based on hydroxyl radicals (OH), which can be generated through chemical, sonochemical, photochemical, and electrochemical processes [13]. Recently, sulfate radical (SO4−)-based AOPs have been widely studied. This constitutes a newly developed and promising water treatment process for removing organic contaminants including polycyclic aromatic hydrocarbon (PAHs) [14], dyes [15], pharmaceutical compounds [16], [17], [18], pesticides [19], and bacteria [20]. SO4− has a higher redox potential (E0(SO4−/SO42−) = 2.5–3.1 V) [21] than HO (E0 (HO/H2O) = 1.9–2.7 V) [22]. Additionally, the rate constants between SO4− and the water matrix (NOM, CO32−/HCO3− and Cl−) are lower than those of OH (Table S1). Therefore, compared to OH, SO4− has a weaker scavenging effect and a longer half-life, which favors the removal of organic compounds [23]. The second-order rate constants between SO4− and organic compounds ranged from 107 to 1010 M−1·s−1 [21].
SO4− can be generated from the activation of persulfate or peroxymonosulfate through heat [15], [16], [24], incorporation of chemical additives in homogeneous and heterogeneous media [25], [26], [27], or by UV radiation [28], [29]. Among these activation methods, UV-mediated PS activation is an especially effective method to generate SO4− [30], and the UV254/PS process has been reported to efficiently degrade a range of organic contaminants including cyanotoxins, pesticides, pharmaceuticals, and antibiotics [28], [29], [31], [32], [33], [34], [35]. Considering the high efficiency of the UV254/PS system, UV254/PS may have a good performance in the removal of BMAA, but there is no information available to date on this issue.
This study sought to investigate the removal of BMAA by the UV254/PS system. Our main objectives were to (1) evaluate the degradation efficiency of BMAA in the UV254/PS system; (2) determine the reaction rate constant of BMAA with SO4− and OH; (3) investigate the effects of different reaction conditions (pH, PS dose, and BMAA initial concentration) on the oxidation performance via photodegradation kinetics; (4) evaluate and compare the influence of NOM, Cl−, and CO32−/HCO 3− on the degradation of BMAA; (5) develop pseudo-steady-state kinetic models to forecast the decomposition of BMAA under different conditions; and (6) identify the transformed products and propose a potential reaction path.
Section snippets
Materials
All reagents used in this study were of analytical grade or higher; more details are provided in Text S1.
Experimental procedures
All experiments were performed in a batch reactor (Fig. S1) and a UV lamp (28 W, 254 nm, Jiguang, Shanghai) with a quartz sleeve placed vertically on a 100 mL quartz beaker. The UV lamp was turned on 30 min before the beginning of the experiments to obtain a stable UV irradiation. The volume-based irradiance (I0) into the solution was determined to be 9.29 × 10 −7 Einstein·L −1·s −1 by
Degradation efficiency of BMAA in the UV254/PS system
As shown in Fig. 1, the degradation of BMAA was very slow by direct PS oxidation or direct UV254 photolysis, and the removal ratios were only 12.69% and 5.53% at 10 min for BMAA by PS oxidation and UV254 photolysis, respectively. The degradation rate of BMAA increased remarkably in the presence of PS and UV254, and therefore the removal of BMAA was mainly attributed to radicals.
As stated above, SO4− and OH coexist in the UV254/PS processes. To understand the main reactive oxygen species (ROS)
Conclusion
BMAA degradation was investigated in the UV254/PS system under various experimental conditions and a steady-state kinetic model. BMAA could be effectively degraded by the UV254/PS system, and SO4− was the main ROS. The following were our main conclusions:
- (1)
The second-order rate constants between BMAA and SO4− or OH at pH 7 were 4.75 × 109 M −1·s −1 and 5.49 × 109 M −1·s −1, respectively. The kobs was proportional to the concentration of PS and inversely proportional to the concentration of BMAA.
- (2)
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by National Natural Science Foundation of China (Grant No. 51508130), Natural Science Foundation for Youth of Heilongjiang Province of China (No. QC2016073) Study on the Safety Control and Management of Cyclops of Zooplankton in Micropolluted Water
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