Kinetics and Mechanism of ^•OH Mediated Degradation of Dimethyl Phthalate in Aqueous Solution: Experimental and Theoretical Studies

The hydroxyl radical (^•OH) is one of the main oxidative species in aqueous phase advanced oxidation processes, and its initial reactions with organic pollutants are important to understand the transformation and fate of organics in water environments. Insights into the kinetics and mechanism of ^•OH mediated degradation of the model environmental endocrine disruptor, dimethyl phthalate (DMP), have been obtained using radiolysis experiments and computational methods. The bimolecular rate constant for the ^•OH reaction with DMP was determined to be (3.2 ± 0.1) × 10⁹ M^–1s^–1. The possible reaction mechanisms of radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) were considered. By comparing the experimental absorption spectra with the computational results, it was concluded that the RAF and HAT were the dominant reaction pathways, and OH-adducts (^•DMPOH₁, ^•DMPOH₂) and methyl type radicals ^•DMP­(-H)­α were identified as dominated intermediates. Computational results confirmed the identification of transient species with maximum absorption around 260 nm as ^•DMPOH₁ and ^•DMP­(-H)­α, and these radical intermediates then converted to monohydroxylated dimethyl phthalates and monomethyl phthalates. Experimental and computational analyses which elucidated the mechanism of ^•OH-mediated degradation of DMP are discussed in detail

Association of Toxin-Producing <i>Clostridium botulinum</i> with the Macroalga <i>Cladophora</i> in the Great Lakes

Avian botulism, a paralytic disease of birds, often occurs on a yearly cycle and is increasingly becoming more common in the Great Lakes. Outbreaks are caused by bird ingestion of neurotoxins produced by <i>Clostridium botulinum</i>, a spore-forming, gram-positive, anaerobe. The nuisance, macrophytic, green alga <i>Cladophora</i> (Chlorophyta<i>; mostly Cladophora glomerata L.</i>) is a potential habitat for the growth of <i>C. botulinum</i>. A high incidence of botulism in shoreline birds at Sleeping Bear Dunes National Lakeshore (SLBE) in Lake Michigan coincides with increasingly massive accumulations of <i>Cladophora</i> in nearshore waters. In this study, free-floating algal mats were collected from SLBE and other shorelines of the Great Lakes between June and October 2011. The abundance of <i>C. botulinum</i> in algal mats was quantified and the type of botulism neurotoxin (<i>bont)</i> genes associated with this organism were determined by using most-probable-number PCR (MPN-PCR) and five distinct <i>bont</i> gene-specific primers (A, B, C, E, and F). The MPN-PCR results showed that 16 of 22 (73%) algal mats from the SLBE and 23 of 31(74%) algal mats from other shorelines of the Great Lakes contained the <i>bont</i> type E (<i>bont/E</i>) gene. <i>C. botulinum</i> was present up to 15 000 MPN per gram dried algae based on gene copies of <i>bont/E</i>. In addition, genes for <i>bont/A</i> and <i>bont/B</i>, which are commonly associated with human diseases, were detected in a few algal samples. Moreover, <i>C. botulinum</i> was present as vegetative cells rather than as dormant spores in <i>Cladophora</i> mats. Mouse toxin assays done using supernatants from enrichment of <i>Cladophora</i> containing high densities of <i>C. botulinum</i> (>1000 MPN/g dried algae) showed that <i>Cladophora</i>-borne <i>C. botulinum</i> were toxin-producing species (BoNT/E). Our results indicate that <i>Cladophora</i> provides a habitat for <i>C. botulinum</i>, warranting additional studies to better understand the relationship between this bacterium and the alga, and how this interaction potentially contributes to botulism outbreaks in birds