This is a complicated question because there are many kinds of pollutants. If a pollutant is in the ground, having more roots can help the plant to take in more of a pollutant. Some plants hold onto certain pollutants more. Some break down certain pollutants better. Some take in certain pollutants better. So the species of plant is also very important in determining whether plants can be used to clean up a site. You are right that the size and amount of roots matter. Roots with more surface area will be better for picking up pollutants in general. Surface area is an important concept in biology. Smaller objects have PROPORTIONATELY more surface area than big objects. For example, picture a log; now picture sticks that are tied together to make a bundle, that is the same size as the log. If you took the bark off the log and the bark off of each stick, there would be a lot more bark from the sticks, meaning that they have a greater surface area. The surface is where the important work of taking in nutrients and water takes place. This is why roots get smaller and smaller in diameter.

If the pollutant is in the air, the surface area of the leaves is very important. Plants take in carbon as carbon dioxide (CO₂) through their leaves, not their roots. They also take in other pollutants. So the surface area of the plants matter, but the type of pollutant and the type of plant are important too.

I work for an organization that helps students with science fair project ideas called Science Buddies.They have a lot of information on some science fair projects related to the idea you had questions about. Please check out these websites to give you some information on the questions your asking, and ideas for how to test your questions:

* Here is an "Ask a Scientist" page on "how to test air pollutants and which plants clean air best:"

* Here is a project idea on "

* Here is a project idea on "Do plants promote pesticide breakdown?"

I think you'll need to narrow down which pollutants you are interested in because plants interact with different chemicals in different ways. In 2009 it was found that some pollutants, specifically PCBs, could be broken down by plant roots. Here is an article that discusses this research:

Tree and plant roots act as water filters for groundwater, trapping pollutants and contaminants. I guess that the size of the roots is important. The longer the roots, the more pollutants and contaminants can be trapped. Please go to the next interesting link:

This link depicts the wetland construction process and the different kind of plants that act as natural filters for pollutants and contaminants in the soil.