There are a variety of processes that allow plants to lower the contaminant levels in soil as portrayed in the above graphic.

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An early US EPA report [link] describes the various processes:

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Phytoextraction … is the process of planting a crop of a species that is known to accumulate contaminants in the shoots and leaves of the plants, and then harvesting the crop and removing the contaminant from the site. Unlike the destructive degradation mechanisms, this technique yields a mass of plant and contaminant (typically metals) that must be transported for disposal or recycling. [Our comment: since this EPA report many phytoremediation plants have been used for bioenergy and construction material]. This is a concentration technology that leaves a much smaller mass to be disposed of when compared to excavation and landfilling.

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Rhizofiltration is similar to phytoextraction in that it is also a concentration technology. It differs from phytoextraction in that the mechanism is root accumulation and harvest using hydroponic (soil-less) growing techniques. This is useful for separating metal contaminants from water.

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Volatilization or transpiration through plants into the atmosphere is another possible mechanism for removing a contaminant from the soil or water of a site. [Our comment: this has become far more popular since the EPA report].

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Containment [stabilization] using plants either binds the contaminants to the soil, renders them nonbioavailable, or immobilizes them by removing the means of transport. Physical containment of contaminants by plants can take the form of binding the contaminants within a humic molecule (humification), physical sequestration of metals as occurs in some wetlands, or by root accumulation in nonharvestable plants. Certain trees sequester large concentrations of metals in their roots, and although harvesting and removal is difficult or impractical, the contaminants present a reduced human or environmental risk while they are bound in the roots. Risk reduction may also be achieved by transforming the contaminant into a form that is not hazardous, or by rendering the contaminant nonbioavailable.