Using Phytoextraction & Phytofiltration

Heavy metal cleanup is typically done through nanofiltration methods (present in water treatment plants), which occurs, ideally, before the water gets to us. However, animals and plants that live in these water sources -- some of which we consume -- still feel the effects of heavy metal contamination.

One solution to this is implementing plants capable of phytoextraction along the banks of the lake in pollution-concentrated areas. 

Phytoextraction is a biochemical process that's been used to remove metals such as lead and cadmium from contaminated soil. There is also potential for it to be used to remove metals from bodies of water. 

Most plants osmose water through their roots via root pressure differences and capillary action. The water in the soil has high potential energy due to the higher pressure and solute potential in the soil. The water in the roots has lower potential energy, as the pressure and solute potential is lower within the roots. Water always seeks an area of lower potential, so it moves into the roots from the soil. After that, capillary action (the tendency of liquid to move up against gravity when confined within a narrow tube) in the veins of the plant carries the water molecules up into the plant itself.

The equation modeling how plants absorb any liquid from the soil is the hydraulic variation of Ohm's Law, most simply put as:

Flow = Δψ / R

where the flow rate (volume of liquid passing through a point per unit of time, the units can vary) equals the pressure difference between the two spaces (measured in PSI, and would be between the soil and the roots) divided by the resistance to the flow (due to friction in or geometry of the roots, measured in mass/sec). Using this equation, one can find the rate at which different plants absorb liquid from the soil.

This absorption process can also be used to remove heavy metals from the soil. Most heavy metals are not soluble (do not dissolve in water) due to the other elements they’re bonded to, but certain plants are able to release root exudates – fluids intended to reduce harmful microorganisms in the soil – that can increase the solubility of those metals and allow them to dissolve into the water in the soil. Thus, when the plants absorb that water, they also absorb those metals into their bodies the same way.

A combination of phytoextraction and phytofiltration in the marshlands around the Great Lakes is a low-energy way to remove heavy metals such as lead, mercury, and cadmium -- and is a solution applicable around the world.