Phytoremediation is a process that uses plants to remove, degrade, or contain contaminants in soil, water, or air. Plants are used in phytoremediation because they have the natural ability to absorb and accumulate contaminants from the environment. The roots of the plants uptake the contaminants from the soil or water, and the contaminants are then either stored in the plant tissues or broken down by the plant's enzymes. This process helps to clean up contaminated sites by reducing the concentration of pollutants in the environment.