Kinetics and pathways of anatoxin-a phototransformation during water treatment with UV

Abstract

In consequence of global weather change and eutrophication, the intensity of algal bloom in fresh water has been increased which lead to the high cyanobacteria production. Cyanobacteria can produce toxins such as anatoxin-a and microcystin which are responsible for the livestock and human poisoning. Anatoxin-a is one of the toxins mainly secreted by cyanobacteria Anabaena flos-aquae. In this study, the transformation of anatoxin-a during ultraviolet (UV) photolysis at 254nm in the absence and presence of dissolved organic matter (DOM) and various inorganic ions was investigated in a quasi-collimated beam system. To clarify the major photo-transformation pathway of anatoxin-a, several scavengers were used such as tert-butanol (T-BuOH), phenol, furfuryl alcohol (FFA) and sodium azide for separate quenchings of various photo-chemical produced reactive species including OH radical, excited triplet state of DOM, and singlet oxygen (1O2), respectively, while ascorbic acid was used for suppressing of electrons transfer toward DOM reaction. All the samples were analyzed immediately after UV exposure by using high-performance liquid chromatography with UV detection (HPLC/UV). The result showed that the transformation of anatoxin-a followed a pseudo-first-order kinetics with respect to UV fluence in the range of 0-1500 mJ/cm2. This study indicated that anatxin-a transformation was pH dependence which acidification of the anatoxin-a stabilized while alkalisation accelerated its degradation. The elimination of anatoxin-a was enhanced by DOM at high concentration and was inhibited at the high initial concentration of anatoxin-a. The resulting pseudo-first-order rate constant of of anatoxin-a (kanatoxin-a) was 1.4E-03 cm2/mJ, 1.2E-03 cm2/mJ and 1.6E-03 cm2/mJ in the pure water matrix, in the presence of inorganic ions water matrix and in the presence of DOM water matrix, respectively. The degradation of anatoxin-a in the presence of inorganic ions such as Mg2+, K+, Ca2+, Cl-, SO42- was lightly decreased. In the presence at low concentration of DOM (fulvic acid was used as source), no significant effect was observed. In all three water matrix, indirect photolysis is the main photodegradation pathway of anatoxin-a via the reaction with phototransformation intermediate product of anatoxin-a, which counted for around 50-60% of process. UV photolysis of anatoxin-a was found to produce 1O2 itself. The steady state concentrations and quantum yields of 1O¬2 was 2.96E-12 M and 0.18, respectively, which could be calculated from the loss of furfural alcohol.