plants are able to absorb pollutants like oils and some minerals into their leafy tissue. some plants like grasses and grass-like plants are better at this process then others. Grasses, (rushes and sedges also) by their nature help filter heaver solids like dirt small debris from water flowing through them.

Because these plants would at times be in suspended water, the right mix of plants must be chosen that can survive in wet and dry conditions and help filter the amount of pollutnats flowing through the swale. any nursery or landscape professional should be able to assist in chosing plants. do a web search for "phytoremediation" or "bioremediation" and you should get more information on this technique and others.