Numerous man-made chemicals are deposited in wastewater treatment plants (WWTPs), where biological and chemical treatments often result in the formation of various transformation products via reactions such as conjugation and alkylation. When wastewater and biosolids are applied for agricultural use, such transformation products may be released to the soil and come in contact with soil microorganisms, plants and faunas (e.g., earthworm). Using the high-production volume antibacterial agent triclosan as a model compound, we tested the hypothesis that methyl triclosan, the primary transformation product of triclosan, can be converted back to the parent compound in soil, earthworm, and plants. Upon 42-d incubation in soil without earthworm, triclosan was detected in a sandy soil (0.12 µg/g) following an initial treatment of methyl triclosan at 1.0 µg/g, suggesting microbial involvement. The back conversion was also observed in earthworm, and reached 3.3 µg/g (tissue) for a sandy soil and 0.89 µg/g for a clay soil. The back conversion in earthworm was further confirmed by direct methyl triclosan exposure in earthworm under soil-free conditions. The back conversion also took place in plants, including <i>Arabidopsis thaliana</i> cells, lettuce and carrot seedlings, and mature cucumber and cherry radish plants. Root tissues usually exhibited the highest triclosan levels. These results showed that the product-to-parent back conversion is prevalent for methyl triclosan in different systems and appears to be biologically mediated. Given that many emerging contaminants have phenolic structures and alkylation is a common transformation pathway, it is highly probable that similar back conversion happens also for many other contaminants and should be considered in achieving a full understanding of their environmental fate and risks.

Back conversion from product to parent: Methyl triclosan to triclosan in soil, earthworms, and plants

2019 Spring National Meeting

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