Degradation of neurotoxin β-N-methylamino-L-alanine by UV254 activated persulfate: Kinetic model and reaction pathways

https://doi.org/10.1016/j.cej.2020.127041Get rights and content

Highlights

  • Degradation of BMAA was investigated in the UV/PS system.

  • Pseudo steady-state model can predict BMAA degradation by UV/PS system in pure water.

  • The forms of BMAA at different pH values significantly affect the reaction rate.

  • HCO3 enhanced BMAA degradation, while Cl and NOM suppressed the degradation.

Abstract

In this study, the degradation of β-N-methylamino -L-alanine (BMAA; a novel neurotoxin produced by cyanobacteria) in the UV254/persulfate (PS) system was investigated. The degradation of BMAA satisfied the pseudo-first-order kinetic model. Moreover, the effects of reaction parameters (PS concentration, BMAA concentration, and pH) and water matrix (natural organic matter (NOM) and some anions) were evaluated. The second-order rate constants between BMAA and SO4radical dot or radical dotOH at pH 7 were 4.75 × 10 9 M −1·s −1 and 5.49 × 10 9 M −1·s −1, respectively, as measured by a competition kinetics experiment. The results of the steady-state kinetics model and scavenging experiments indicated that SO4radical dot was the major radical contributing to BMAA degradation in the UV254/PS system. Moreover, pH significantly affected the degradation rate of BMAA, and the highest rate was obtained at pH 8 with a pseudo-first-order rate constant (kobs) of 1.039 min−1. However, the degradation of BMAA was inhibited in aqueous solution due to the influence of the water matrix. The kobs decreased in the presence of NOM and chloride ions (Cl) and increased in the presence of bicarbonate (HCO3). Given that the degradation rate of total organic carbon (TOC) was lower than that of BMAA, the transformation products and a possible degradation pathway were investigated. The C-N bond on BMAA may be attacked by radicals, and BMAA may be transformed into 2,3-diaminopropionic acid or L-alanine. Moreover, BMAA could also be converted to 2-hydroxy-3-methylamino propanoic acid through hydroxylation. Three different BMAA transformants were further oxidized to generate pyruvic acid, after which decarboxylation leads to the formation of acetic acid from pyruvate.

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 (radical dotOH), which can be generated through chemical, sonochemical, photochemical, and electrochemical processes [13]. Recently, sulfate radical (SO4radical dot)-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]. SO4radical dot has a higher redox potential (E0(SO4radical dot/SO42−) = 2.5–3.1 V) [21] than HOradical dot (E0 (HOradical dot/H2O) = 1.9–2.7 V) [22]. Additionally, the rate constants between SO4radical dot and the water matrix (NOM, CO32−/HCO3 and Cl) are lower than those of radical dotOH (Table S1). Therefore, compared to radical dotOH, SO4radical dot has a weaker scavenging effect and a longer half-life, which favors the removal of organic compounds [23]. The second-order rate constants between SO4radical dot and organic compounds ranged from 107 to 1010 M−1·s−1 [21].

SO4radical dot 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 SO4radical dot [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 SO4radical dot and radical dotOH; (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, SO4radical dot and radical dotOH 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 SO4radical dot was the main ROS. The following were our main conclusions:

  • (1)

    The second-order rate constants between BMAA and SO4radical dot or radical dotOH 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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