In response to my latest video, “Are We Gardening in a Toxic World” viewers have asked a very important question- one that I did not address (but should have) in the video.
That question was, “Phytoremediation (the use of living plants to reduce, degrade or remove toxic residue from the soil or groundwater) sounds great—but what do we do with those plants once they’ve sucked up all those toxins?”
The answer to that question, unfortunately, is not cut and dry. From an industrial standpoint, there has been a call for more research into phytoremediation management. It seems there has been a significant amount of research done on which plants work as phytoremediators and which mechanisms they function by… not so much on the management end. As it stands, one of the most common ways to deal with phytoremediators currently is to turn the biomass (plant matter) into biofuel—obviously not an option for most backyard gardeners (though I have heard from folks dabbling in homemade biofuels). The blanket recommendation for smaller scale phytoremediation sites (those which are utilizing annual plants), is to compost or burn the biomatter. Clearly- you can see the potential problems here. For the most part composting or controlled burning releases some toxins back into the environment, but per the research it’s at an ‘acceptable’ level- much less than what existed before. Burning also poses a problem for the home gardener, as it is typically recommended that the burning be done within an enclosed system and the remaining ashes potentially be disposed of at a biohazard waste facility.
Management also varies from plant to plant, based on which specific method each species utilizes to remove pollutants or draw up toxins. There are 4 main mechanisms by which phytoremediation functions, and plants may use one or a combination of these:
“Degradation through metabolism by plants or enhanced microbial action; Vaporization as the plant transpires; Extraction—accumulation, then collection, recycling or disposal; Containment by adsorption or otherwise reducing movement or availability”
The types of phytoremediation utilizing these mechanisms are:
“PHYTOEXTRACTION—high biomass metal-accumulating plants and appropriate soil amendments are used to transport and concentrate metals from the soil to the roots and/or above-ground shoots, which are then harvested. These plant parts are dried, incinerated or composted. Metals are recycled (perhaps for a net profit) or disposed of as hazardous waste. Soil amendments, such as metal chelators, facilitate uptake by releasing metals such as lead which are bound to soil components. Example: Brake fern (Pteris vittata) hyperaccumulates arsenic in its above-ground shoots to concentrations 200 times higher than those found in the soil. Other target metals include lead, cadmium, chromium, nickel, zinc, and various radionuclides.
RHIZOFILTRATION—Plant roots grown in aerated water precipitate and concentrate heavy metals from polluted effluents. Example: Hydroponically-grown sunflowers treat for lead, copper, uranium, strontium, cesium, cobalt and zinc. Native wetland plants in Georgia are being evaluated for their ability to remediate acidic contaminated runoff. [*author's note: Plants used for rhizofiltration are typically managed the same way as those used for phytoextraction]
PHYTOSTABILIZATION—Plants control movement of toxins from the site either by controlling erosion or movement by water, or by binding them tightly to their roots, rendering them unavailable and thus harmless. The contaminants are not removed from the site. Example: Poplars.
PHYTOVOLATILIZATION—Plants extract volatile metals from the soil and volatize them from the foliage. Example: Mercury and selenium can be removed in this way. Less research has been conducted in the areas of phytostabilization and phytovolatilization than on other methods.
PHYTODEGRADATION—The plants absorb hydrocarbons and other complex organic molecules, then metabolize or mineralize them in chemical reactions energized by sunlight. Example: Some enzymes break down ammunition wastes (explosives), chlorinated solvents or herbicides.
RHIZODEGRADATION (rhizosphere bioremediation)—Microorganisms in the rhizosphere consume and digest organic substances for nutrition and energy. Substances released by the plant roots feed the microorganisms enhancing their activity. This is plant-enhanced bioremediation.
PHYTOVOLATILIZATION—Plants extract contaminants from the soil and release the contaminant or a modified form thereof into the atmosphere via evapotranspiration from foliage. Examples: Targets of this technology include PCBs (polychlorinated biphenyls), TCEs (trichloroethylenes), PAHs (polyaromatic hydrocarbons), pesticide residues and various explosives. Poplar trees have been shown to volatilize 90% of the TCE taken up.”
More information on the individual phytoremediation mechanisms here: Phytoremediation Resource Guide (epa.gov)
Some of the most common phytoremediators used by a backyard gardener or homeowner are likely to be annual plants like sunflowers and hemp, both of which typically function as phytoextractors. This would leave homeowners the option of composting or burning to manage the resulting biomass. Often the process has to be repeated multiple times to get the desired results. As mentioned prior, burning is not the best option for most of us, and composting does pose its challenges.
But the good news is, the composting process, as well as the overall efficiency of the phytoremediation itself, can be made more effective by the addition of certain soil amendments. As per a 2021 study, “supplying soil with organic amendments facilitates the biochelators which increases the bioavailability and mobility of soil pollutants thus increasing the phytoextraction efficiency.”
The addition of biochar is one amendment option that is accessible for small scale growers. It is believed that biochar assists in multiple ways. It is an “excellent surface adsorbent” and thanks to properties such as high pH and cation exchange capacity it can enhance the effectiveness of phytoremediation processes. Enhanced phytoremediation strategy for sustainable management of heavy metals and radionuclides - ScienceDirect
Biochar also increases the microbial activity which degrades some pollutants. Frontiers | Influences of Biochar on Bioremediation/Phytoremediation Potential of Metal-Contaminated Soils (frontiersin.org)
The addition of compost may also help. According to a 2021 study: “the addition of compost improved the ability of the plants to reduce the availability of Cu, Zn, Cd, and Pb in the soil”. Effect of Manure and Compost on the Phytostabilization Potential of Heavy Metals by the Halophytic Plant Wavy-Leaved Saltbush - PMC (nih.gov)
It is important to note that, while some studies indicate the resulting compost obtained from plants used for phytoremediation may be introduced back into living ecosystems, I would highly advise against using any such compost in or near a garden containing food plants. A realistic small-scale setup might be having dedicated perennial phytoremediation plantings in a certain area (plants such as willows and poplars, for example) and utilizing compost from annual phytoremediator plants in those areas only (*note-- this is purely speculation on my part). Another option would be solely to stick with plants which function via phytostabilization and are not removed or disposed of, such as poplars.
There are also additional methods of remediation that I did not touch on in my video. Both bioremediation (the use of microbes to clean contaminated soil or water) and mycoremediation (the use of fungi mycelium to break down contaminants in the soil) show promise, both as independent methods AND in conjunction with phytoremediation. The addition of compost and utilizing biochar are both methods which are tapping into bioremediation-- part of their effectiveness is due to the introduction of more microbes into the soil.
Find more on bioremediation here: Green Remediation Best Management Practices: Bioremediation (epa.gov)
Find more on mycoremediation here: Mycoremediation: Expunging environmental pollutants - ScienceDirect
Vermiremediation (yes, you guessed it, utilizing earthworms to restore organically contaminated soils) also shows promise, but more research is needed, and is recommended as a secondary measure after initial remediation processes, or only for low levels of contamination.
While all of these methods of remediation are quite nuanced, they do show promise for the improvement of contaminated soil and water. I also strongly recommend that anyone with concerns over soil or water contamination have a professional lab assessment conducted. As a small-scale gardener or homeowner, it can be challenging to know exactly what you are dealing with. And always err on the side of caution. Avoid growing any food crops or raising animals on land which might contain toxins.
I hope that this serves to clear up some of the questions about phytoremediation, and if anyone has any additional information or resources on this topic they can share- please leave a comment below!