Photodegradation of Metolachlor in Natural and Constructed Wetlands

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2008-06

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The Ohio State University

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Abstract

Agricultural wetlands receive a variety of nutrients and synthetic organic contaminants (SOCs), such as pesticides, from agricultural activities. Constructed wetlands have been proposed as a cost-effective and sustainable treatment method for the retention and degradation of SOCs in agricultural ecosystems. Since wetlands have a high ability to retain and process material, it is reasonable to assume that constructed wetlands can act as a buffer between agricultural areas and receiving waters, thereby mitigating the impact of pesticides in runoff. One important pathway in wetlands, photolysis, is capable of degrading these substances through sunlight driven reactions. Naturally occurring substances, such as iron, dissolved organic matter (DOM), and nitrate, may accelerate the photolytic degradation of SOCs through a photosensitized process called indirect photolysis. The purposes of this study were: (1) to determine the nature and abundance of photosensitizers (DOM, nitrate, and iron) present in one natural wetland and three constructed wetlands, (2) to investigate the sunlight induced degradation of metolachlor in the four wetland waters, and (3) to promote the photodegradation of metolachlor in constructed agricultural wetlands through the addition of iron to wetland waters. Water samples were collected from one natural and three constructed wetlands (Old Woman Creek, Olentangy River Wetland Research Park, Waterman Farm Wetland, and Defiance County Wetland) and were analyzed for pH, UV-Vis absorbance, total organic carbon, nitrate, and total iron concentration. Photodegradation of the herbicide metolachlor was carried out in the wetland waters to examine the effects of natural photosensitizers on the photofate of metolachlor. Metolachlor degradation was enhanced 58 to 141 times at pH 4 and 2 to 11 times at pH 8 in wetland waters compared to direct photolysis in deionized water, with Old Woman Creek water acting most effectively. The addition of 2 to 10 µM of iron promoted metolachlor degradation in Old Woman Creek water by 37-100% at pH 4, while no significant enhancements were observed for other wetland waters. Current studies are optimizing the photodegradation through altering the concentrations of different photosensitizers.

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metolachlor, constructed agricultural wetlands, photodegradation

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