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

In this study, the degradation of beta-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 pseudofirst-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 SO4 center dot(-) or center dot OH at pH 7 were 4.75 x 10(-9) M-1.s(-1) and 5.49 x 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 SO4-center 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 (k(obs)) of 1.039 min(-1). However, the degradation of BMAA was inhibited in aqueous solution due to the influence of the water matrix. The k(obs) 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.

Automated summary

The degradation of BMAA in the UV254/persulfate system follows a pseudofirst-order kinetic model, with SO4· identified as the major radical contributing to the degradation. The pH significantly affects the degradation rate, with the highest rate achieved at pH 8. However, the presence of water matrix components inhibits the degradation process.