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Photolysis of Triclosan in the Presence of Polyethylene Plastic
Summary
Triclosan, an antibacterial chemical once common in soaps, can stick to plastic bags floating in water — and this new study finds that sunlight converts it into a dioxin-like compound up to 290 times faster when it's on plastic than when it's dissolved in water alone. This matters because plastic pollution in lakes, rivers, and oceans could be quietly speeding up the formation of more toxic byproducts, which may then make their way into the food chain and eventually to humans who eat fish or seafood.
Plastic pollution represents one of the greatest anthropogenic threats to the environment. Plastic debris may absorb potentially toxic compounds of low water solubility in aqueous environments. Aquatic plastic pollution may transfer absorbed compounds to aquatic animals causing premature death. Here we examine how polyethylene (PE) plastic grocery bags affect the photolysis of triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy) phenol) when it is absorbed on the surface of the PE in aqueous solution. The photolytic conversion of TCS to 2,8-dichlorodibenzodioxin (2,8-DCDD) is up to ten times faster on plastic than in solution when irradiated at 300 nm and 290 times faster when irradiated at 350 nm. Under these conditions, 2,8-DCDD is formed from the protonated form of TCS unlike in solution where 2,8-DCDD is formed from the deprotonated anion. Quantum mechanical calculations suggest the photolytic transformation of TCS to 2,8-DCDD is initiated by a decrease in phenol pka from 8.1 to -1.7 in the S1 excited state of TCS which initiates the cyclization reaction to form 2,8-DCDD on the surface of the PE. PE is the most abundant form of plastic found in the environment and the most common form of PE are grocery bags. TCS was chosen as a model compound for these studies given its widespread occurrence in the environment, its photolysis in solution has been studied extensively under a variety of conditions.