Lead from outdoor based paint can end up in your soil, contaminating vegetables and potentially lead poisoning children. Is there a way to remove lead from soil? One way being investigated to fulfill this role is phytoextraction. Phytoextraction is the use of plants to clean-up contaminants from soils, sediments, or water.1
So, how does it work?
The plants absorb contaminants through the roots and carry them up into the stems or leaves; leaving less of the contaminants in the soil after the plant has been harvested2. If there are still some heavy metals in the soil after the first round, the cycle can be repeated until the levels are minimized. An interesting tidbit is that the impurities that wind up in the plant are concentrated into a much smaller volume in the plant than was in the soil initially3.
There are two types of phytoextraction that have been studied. The first is using plants that naturally accumulate impurities from the soil and the second consists of adding EDTA or another chelating agent to assist in absorbing the contaminants from the soil4.
The following provides information on plants that are showing promise in removing specific contaminants from soil.
To Remove LEAD - Sunflower5, Indian mustard (Brassica juncea), maize (Zea mays)6, tall fescue (Festuca arundinacea Schreb.)7, Hemidesmus indicus8 Scientific research supports the use of sunflowers as a way to remove lead from the soil19 20. In fact sunflower, Indian mustard, tobacco, rye, spinach, and corn have been studied for their ability to remove lead from water, with sunflower having the greatest ability. In one study, after only one hour of treatment, sunflowers reduced lead concentrations significantly21. In addition, a Daimler Chrysler factory used Sunflowers and mustard plants to clean up 10 years worth of lead accumulation in their soil. Within a year, 25% of the toxin had been removed using the phytoextraction, saving the company $1 million in clean up costs22.
To Remove ARSENIC - Sunflowers9, Ladder brake (Pteris vittata L.)10, the fern Pteris vittata L11 brake fern12
To Remove URANIUM - Sunflowers13
To Remove CADMIUM - Alpine Pennycress (patent filed by University of Maryland)14, Willow15, Indian mustard (Brassica juncea)16
To Remove MERCURY - Indian mustard (Brassica juncea)16, Mercury removal is considered experimental and has shown promise using genetically modified plants that vaporize mercury17.
To remove DDT - Pumpkin and zucchini plants18. The DDT and other pesticides tend to accumulate in the flowers.
Improving Soil Health to Reduce Lead Uptake
While there have been quite a few studies reviewing the types of plants capable of removing specific contaminants, there have only been few laypeople practicing this approach. Willow Rosenthal of Alameda County used this approach to help limit the amount of lead in the vegetables she grows. She conducted several experiments to determine which plants would help her to remove lead from her soil and stumbled upon an interesting find. If she used compost in her soil, she actually reduced the amount of lead uptake in her vegetables23; so while the soil still contained the contaminants, her plants absorbed less of them. Another find was adding rock phosphate to phosphate deficient soils - the phosphate binds the lead so plants have trouble absorbing it.
There are some commercial test kits on the market that help test soil for contaminants. Two of those are the Lead Inspector and Safe to Play’s Arsenic soil removal kit
Where does the contaminated soil go?
Disposal of the contaminated plants is tricky. They cannot be recycled as green material and incineration is considered to be the most environmentally sound and economical method24. According to the EPA, If plants are incinerated, the ash must be disposed of in a hazardous waste landfill and the volume of ash will be less than 10% of the volume that would be created if the contaminated soil itself were dug up for treatment25.
Pros and cons
Advantages include low cost and eco-friendly way of removing contaminants from the soil. Some of the disadvantages are it can take a long time (several years) to fully remove a contaminant from the soil and unfavorable climate can limit a plant's growth. There was some concern regarding whether consumption of the contaminated plants by wildlife would affect them; however some studies have found that the animals stay away from these plants because they taste bad.