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How do plants trap or convert pollutants?
Answer 1:

There are interesting articles that can give answers to your questions on pollutants and plants. These articles can be found in the next links:

plants can break down pollutants
natural filters or VSF

From the first link I found the importance of wetlands in trapping pollutants. Wetlands protect water quality by trapping sediments and retaining excess nutrients and other pollutants such as heavy metals. Sediments, nutrients, and toxic chemicals enter wetlands primarily by way of "runoff," a term used to describe the rain and stormwater that travels over land surfaces on its way to receiving waters. Sediments, which are particles of soil, settle into the gravel of streambeds and disrupt or prevent fish from spawning, and can smother fish eggs. Other pollutants -- notably heavy metals -- are often attached to sediments and present the potential for further water contamination. Wetlands remove these pollutants by trapping the sediments and holding them. The slow velocity of water in wetlands allows the sediments to settle to the bottom where wetland plants hold the accumulated sediments in place.

Toxic chemicals reach surface waters in the same way as nutrients, and can cause disease, death, or other problems upon exposure to plants and animals (including humans). In a function similar to nutrient removal, wetlands trap and bury these chemicals or may even convert some of them to less harmful forms. Scientists are continuing to study what happens to toxic chemicals when they enter wetlands, and they warn us that even if the toxins are buried, they are still potentially dangerous.

From the second link I found that plants can break down persistent pollutants like PCBs. (from Wikipedia :
"PCB: polychlorinated biphenyl is any of the 209 configurations of organochlorides with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for a PCB is C12H10-xClx. 130 of the 209 different PCB arrangements and orientations are used commerciallyPCBs were widely used as dielectric and coolant fluids, for example in transformers, capacitors, and electric motors. Due to PCBs' toxicity and classification as a persistent organic pollutant, PCB production was banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants in 2001)"

Plants can chemically change the persistent contaminants PCBs into other forms. The enzymes in the roots of a poplar tree can alter the chemical structure of a common type of PCB pollutant, rendering it less harmful than the original chemical.Enzymes in the tree roots changed the chemical structure of the PCB. Enzymes are molecules that modify, make or break down chemicals in a similar manner in all life from plants to bacteria to insects to people.

The third link speaks about VFS or vegetative filter strips. A VFS is an area of vegetation that is planted intentionally to help remove sediment and other pollutants from running water.

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