A new shoot pushing through the soil, could a humble plant unlock secrets of soil remediation?
[COVER PHOTO Bo Jensen/Colourbox.com, Vira Dobosh/Colourbox.com, MODIFIED]
Phytoextraction:
A natural solution to soil pollution?
A natural solution to soil pollution?
Is phytoextraction a viable solution for soil remediation?
In this long-form article I detail the process, benefits, and drawbacks of phytoextraction.
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Soil contaminated by heavy metals can be very damaging to both the plants the soil supports and the humans and animals that feed upon these plants. Since plants uptake nutrients directly from soil or ground water via their roots, any metal contaminants present will also be absorbed. After entering a plant’s root system, contaminants such as cadmium or lead can be translocated by the roots into the above-ground parts of the plant. This results in metal accumulation in the shoots and leaves. These assimilated metals are then passed to humans and animals through ingestion of the contaminated plant (1). This contaminant extraction process, while harmful to the food chain, can also be used to effectively remediate metal-contaminated soil and groundwater.
A representation of one phytoremediation schematic. Soil that is contaminated with cadmium can be remediated through phytoextraction, with the potential to remove the metal for future use. [PHOTO Elizabeth Lies/Unsplash.com GRAPHICS Lindsey McDonald]
Some plants are extremely efficient at extracting contaminants from the soil. This is because they have properties that allow them to resist the normally toxic effects of interacting with highly concentrated metals (2). The process of using specific types of plants to address contaminants in soil and groundwater is called phytoremediation. A type of phytoremediation commonly used with metal contamination is phytoextraction (2). This is when the accumulator plants are harvested, and the biomass safely disposed of or treated. In the latter case, the metal contaminants are re-extracted and recycled into a usable form (3). Phytoextraction is an emerging biological technology that could become integral in the remediation of soil and water bodies (2).
Soil is considered contaminated when there is a concentration of a chemical higher than what would be considered naturally occurring (1). There are multiple pathways to contamination, including point-source pollutants, those that are deposited directly in the soil; and nonpoint-source pollutants, those that are diffuse and usually unintentional. Point-source pollution is commonly anthropogenic in origin: landfill use, fertilizer and pesticide application, wastewater and sludge disposal, mining waste removal, chemical releases at manufacturing sites, and leaky hydrocarbon containment (1). Mining and hydrocarbon refining are of particular concern in that that they can produce high levels of heavy metals in soils, especially in locations that have weak environmental protection laws (1,4). Another source of metal contamination is mining and smelting effluent when it is released into waters adjacent to agricultural activity, as is the case in Silver Bow Creek, Montana (5). The exact causes of nonpoint-source pollution are more difficult to identify due to the lack of a directly connected localized source. One example is the atmospheric deposition of heavy metals. This is when metal particles originating from anthropogenic or natural sources settle on the ground via precipitation, wind, or other means. (1). Urban soils near industrial and manufacturing areas often demonstrate high levels of atmospheric deposition, especially in terms of metals such as cadmium, mercury, copper, lead, and zinc (4). Natural sources of diffuse metal contamination are metalloid rich sedimentary rock weathering, and volcanic activity (6).
Rice is a crop that is sensitive to metal accumulation. Eating cadmium contaminated rice causes serious health problems including Itai-Itai disease. [PHOTO 3938252/Colourbox.com, Maria Ionova/Unsplash.com]
Cadmium, a widespread soil contaminant, is considered a notably high-risk metal due to its toxicity to plants and its role in food-chain contamination (1). According to the US Health and Human Services database, 40% of sites on the 2016 National Priorities List were soils contaminated with cadmium (7). Cadmium accumulates in leaves in large quantities, which can be 10-500 times greater than in plants grown in non-polluted environments (8). In addition to being a human carcinogen with no known biological function (7), cadmium is also very difficult for human bodies to excrete; for example, cadmium has a biological half life of 6 to 8 years in the kidneys, and 4 to 19 years in the liver (7). The most common route for human exposure to cadmium is oral ingestion of contaminated plants, particularly rice (6,7,9), wheat (10), and leafy vegetables such as spinach (11). Cadmium exposure is linked to Itai-Itai disease, endocrine system disruptions, and bone damage (1,9,12). Itai-Itai, a disease associated with the consumption of rice grown in cadmium-contaminated soils (9), is described as chronic cadmium poisoning, presenting as renal tubular dysfunction followed by softening of the bones (6,9).
Various factors such as soil pH, redox potential, soil composition, and competing metal cations (6) determine the bioavailability of cadmium. For example, plants more readily uptake cadmium if soil is acidic and well drained with low nutrient levels (6). Moreover, some plants are more efficient at assimilating metals because they can withstand much higher levels of contamination without experiencing detrimental effects. These plants, called hyperaccumulators, can be used to clean contaminated soil through phytoremediation, specifically, phytoextraction (2). Compared to traditional methods of soil remediation, phytoextraction is relatively inexpensive, less invasive, and more contained (2). For example, excavating and landfilling a 10-acre contaminated site to a depth of 1 foot requires handling roughly 20,000 tons of soil. Phytoextraction of the same site would result in the need to handle about 500 tons of biomass, which is about 1/40 of the mass of the contaminated soil (13). Traditional approaches also create significant amounts of secondary hazardous waste products which must be addressed; soil washing produces toxic effluent, and excavating coupled with landfilling moves the problem to a secondary site where potential leaching can be an issue (2,14).
hese small clumped flowers can be powerhouse accumulators. The alpine pennycress, left, and canola, right, have been shown to accumulate metals. The pennycress has even shown increased growth when exposed to cadmium. [LEFT PHOTO Krzysztof Ziarnek, Kenraiz/Wikimedia Commons, Creative Commons Attribution-Share Alike 4.0 International license. RIGHT PHOTO 10410764/Colourbox.com]
As mentioned earlier, phytoextraction involves growing hyperaccumulator plants which use their extensive root systems to remove metals from the soil and translocate them to the plants’ aerial parts. At this point, the plants can be harvested and the metal extracted from their biomass (2,3,14). Some plants have been found to be natural hyperaccumulators of metals, with different plants exhibiting different metal tolerances. The levels of uptake also vary, depending on the plant species and surrounding ecozone (15). For cadmium, a plant is considered a hyperaccumulator when it can withstand 100 mg/kg cadmium in its biomass; in contrast, a non-hyperaccumulator is poisoned by levels above 0.1 mg/kg (16). Phytoextraction therefore involves selecting specific plants for specific remediation situations (14). Cadmium accumulates well in black nightshade (Solanum nigrum) and Canadian fleabane (Conyza canadensis), although these species are also tolerant of other metals (14). Alpine pennycress (Thlaspi caerulescens J. et C. Presl), a member of the Brassicaceae family, can selectively uptake cadmium from the soil, accumulating up to 3000 mg/kg cadmium in their aerial parts. This plant even exhibits increased root growth in the presence of cadmium (15). However, a plant species’ uptake potential can depend on co-occurring metals, with zinc depressing cadmium uptake in the alpine pennycress (15). Zinc is an essential micronutrient chemically like cadmium and acts as a competing ion; the more zinc in the soil, the less cadmium that can be absorbed into the roots of the alpine pennycress (15,16).
The ability of alpine pennycress roots to mass in pockets of high cadmium concentrations makes T. caerulescens a promising candidate for widespread soil remediation. This is because it could be used to reach cadmium hotspots, and efficiently remove cadmium pollution. Research by Schwartz et al. indicated that planting alpine pennycress and lettuce in succession resulted in less available cadmium in the soil, and smaller cadmium concentrations in the lettuce crop (15). Also of interest is that increased nitrogen concentrations in soils increases cadmium uptake for T. caerulescens although increased concentrations of other fertilizer components has a negative effect (15). As a result, several factors seem to contribute to a plant species’ usefulness in phytoextraction.
A serious limitation of the alpine pennycress is its small stature; that is, a larger plant, having more biomass, should theoretically uptake more cadmium (2,14,15). Soil remediation experiments with Montana’s Silver Bow Creek cadmium-contaminated soil show that on an individual basis, pennycress was more effective at cadmium removal than its Brassica relatives such as Brassica napus. However, due to its size restrictions, the alpine pennycress overall did not perform any better than larger species (5). This suggests that genetic engineering could be useful in producing taller alpine pennycress with greater biomass. Soil enhancements can also increase the uptake of metals such as cadmium; unfortunately, alterations to natural conditions increase the chance of unintended consequences. For example, chemical soil enhancements can harm beneficial soil microbes and alter soil composition making the soil unsuitable for future plant growth. This would outweigh the benefits of enhanced metal uptake (3). Furthermore, chemical additions to soil are designed to increase the solubility of metals, resulting in metal leaching into ground water and downstream contamination (2).
Life relies on soil. When soil is contaminated, only well suited plants can thrive. Finding ways to remediate soil pollution without harming the soil for future use is important for growing food in depleted cropland. [PHOTO Paul Mocan/Unsplash.com, MODIFIED]
Once metals have been absorbed into the plant’s biomass as part of the phytoextraction process, the plants must be harvested and the metals potentially reclaimed. An important step involves reducing the volume and weight of the harvested plants to make transport, disposal, and metal re-extraction more manageable and less expensive. Volume reduction is typically accomplished via composting, compaction, or pyrolysis (3). All methods reduce biomass while creating a secondary toxic leachate or hazardous material (3). Of the three methods, composting results in the largest total dry-mass loss of contaminated biomass but takes the most time (3). Pyrolysis, which results in significantly more volume/mass reduction of contaminated plant biomass (3), turns the biomass into three products: solid bio-char, liquid bio oil, and pyrolytic gas. The latter is contained so it is not released into the atmosphere (3). Though pyrolysis is an expensive process, some municipalities already use it for wastewater treatment, showing potential for dual use of existing facilities.
After treatment, the biomass material is either discarded or subjected to metal re-extraction via incineration, ashing, or liquid extraction (3). Directly discarding the waste is the quickest method of disposal but offers no opportunity to extract metals. It also means adding more toxic waste to landfills. Incineration of plant material destroys the organic components and releases the metals which are then captured in the form of slag or effluent gases (3). Incineration can reduce the dry mass by 90%, but what remains is toxic waste (3). "Ashing" is a similar technology; it involves burning the contaminated biomass with sub-bituminous coal instead of in a furnace, thus reducing the plant material, and leaving behind metal-infused ash. This approach allows the metal to be recovered (3). Advances in chemistry have led to liquid extraction, which uses chelating agents to selectively bind to metals, enabling them to be completely separated from the biomass. The advantages of liquid extraction are that the waste product is non-toxic and chelating chemicals can be reused in multiple extractions (3).
Examples of hyperaccumulator plants. Each has unique properties that could make or break how viable they could be for sustainable soil remediation. [GRAPHICS Lindsey McDonald]
The use of phytoremediation, and specifically phytoextraction, might represent an economic and ecological alternative to traditional methods for the decontamination and stabilization of heavy metal-polluted sites (17). However, there are many factors to consider: cause or source of pollution, current state of the soil, proximity to industry or homes, cost of remediation, and toxicity of the pollutants (1). Although the easy solution is to scoop the problem away, sequester it out of mind, and ignore the problem, there are more environmentally-friendly options available that can be less expensive and allow for further metal extraction (3). While it is true that phytoextraction can still create toxic waste, it is at a greatly reduced mass compared to traditional remediation methods. Also, phytoextraction allows for metal recycling and the opportunity to repurpose harvested biomass as raw materials; for example, as bio-fuel, bio-plastics, cosmetics, cleaning products (18), and fiber-based products such as paper (19).
If toxic metals such as cadmium continue to pollute soil and ground water at ever-increasing rates, innovate biological solutions such as phytoremediation will be required. Unfortunately, cleaning soil after it has become contaminated is not a long-term solution. Most importantly, the causes of soil degradation must be addressed through strict universal regulations, international environmental action plans, targeted messaging, global food security measures, and major societal shifts in attitude.
References
1. Rodriguez Eugenio N, McLaughlin M, Pennock D. Soil Pollution: A hidden reality. Farm and Agriculture Organization of the United Nations. Rome; 2018.
2. Peer WA, Baxter IR, Richards EL, Freeman JL, Murphy AS. Phytoremediation and hyperaccumulator plants. Top Curr Genet. 2005;14 (August 2005).
3. Sas-Nowosielska A, Kucharski R, Małkowski E, Pogrzeba M, Kuperberg JM, Kryński K. Phytoextraction crop disposal - An unsolved problem. Environmental Pollution. 2004;128(3):373–9.
4. Qing X, Yutong Z, Shenggao L. Assessment of heavy metal pollution and human health risk in urban soils of steel industrial city (Anshan), Liaoning, Northeast China. Ecotoxicol Environ Saf. 2015;120:377–85.
5. Ebbs SD, Lasat MM, Brady DJ, Cornish J, Gordon R, Kochian L V. Phytoextraction of Cadmium and Zinc from a Contaminated Soil. J Environ Qual. 1997;26(5):1424–30.
6. Hu Y, Cheng H, Tao S. The Challenges and Solutions for Cadmium-contaminated Rice in China: A Critical Review. Environ Int. 2016;92–93:515–32.
7. Agency for Toxic Substances and Disease Registry. Toxicological Profile: Cadmium. Toxicological Profile for Cadmium. 2012; https://wwwn.cdc.gov/TSP/ToxProfiles/ToxProfiles.aspx?id=48&tid=15
8. Yanqun Z, Yuan L, Jianjun C, Haiyan C, Li Q, Schvartz C. Hyperaccumulation of Pb, Zn and Cd in herbaceous grown on lead-zinc mining area in Yunnan, China. Environ Int. 2005;31(5):755–62.
9. Ogawa T, Kobayashi E, Okubo Y, Suwazono Y, Kido T, Nogawa K. Relationship among prevalence of patients with Itai-itai disease, prevalence of abnormal urinary findings, and cadmium concentrations in rice of individual hamlets in the Jinzu River basin, Toyama prefecture of Japan. Int J Environ Health Res. 2004;14(4):243–52.
10. Liu K, Lv J, He W, Zhang H, Cao Y, Dai Y. Major factors influencing cadmium uptake from the soil into wheat plants. Ectotoxicology and Environmental Safety. 2015;113:207–13.
11. Baldantoni D, Morra L, Zaccardelli M, Alfani A. Cadmium accumulation in leaves of leafy vegetables. Ecotoxicol Environ Saf. 2016;123:89–94.
12. Brzóska MM, Moniuszko-Jakoniuk J. Effect of low-level lifetime exposure to cadmium on calciotropic hormones in aged female rats. Arch Toxicol. 2005;79(11):636–46.
13. United States Department of Agriculture: Natural Resources Conservation Service. Heavy Metal Soil Contamination. Soil Quality - Urban Technical Note 3. 2000;(3).
14. Gratão PL, Prasad MNV, Cardoso PF, Lea PJ, Azevedo RA. Phytoremediation: Green technology for the clean up of toxic metals in the environment. Brazilian Journal of Plant Physiology. 2005;17(1):53–64.
15. Schwartz C, Echevarria G, Morel JL. Phytoextraction of cadmium with Thlaspi caerulescens. Plant Soil. 2003;249(1):27–35.
16. Kirkham MB. Cadmium in plants on polluted soils: Effects of soil factors, hyperaccumulation, and amendments. Geoderma. 2006;137(1–2):19–32.
17. Suman J, Uhlik O, Viktorova J, Macek T. Phytoextraction of heavy metals: A promising tool for clean-up of polluted environment? Front Plant Sci. 2018;871(October):1–15.
18. Sarisky-Reed V. 5 Everyday Products Made from Biomass: A Few May Surprise You [Internet]. Office of Energy Efficiency and Renewable Energy. 2020 [cited 2020 Oct 20]. Available from: https://www.energy.gov/eere/articles/5-everyday-products-made-biomass-few-may-surprise-you
19. Azeez MA. Pulp and Paper Processing. In: Newaz Kazi S, editor. Pulping of Non-Woody Biomass. IntechOpen; 2018.
2. Peer WA, Baxter IR, Richards EL, Freeman JL, Murphy AS. Phytoremediation and hyperaccumulator plants. Top Curr Genet. 2005;14 (August 2005).
3. Sas-Nowosielska A, Kucharski R, Małkowski E, Pogrzeba M, Kuperberg JM, Kryński K. Phytoextraction crop disposal - An unsolved problem. Environmental Pollution. 2004;128(3):373–9.
4. Qing X, Yutong Z, Shenggao L. Assessment of heavy metal pollution and human health risk in urban soils of steel industrial city (Anshan), Liaoning, Northeast China. Ecotoxicol Environ Saf. 2015;120:377–85.
5. Ebbs SD, Lasat MM, Brady DJ, Cornish J, Gordon R, Kochian L V. Phytoextraction of Cadmium and Zinc from a Contaminated Soil. J Environ Qual. 1997;26(5):1424–30.
6. Hu Y, Cheng H, Tao S. The Challenges and Solutions for Cadmium-contaminated Rice in China: A Critical Review. Environ Int. 2016;92–93:515–32.
7. Agency for Toxic Substances and Disease Registry. Toxicological Profile: Cadmium. Toxicological Profile for Cadmium. 2012; https://wwwn.cdc.gov/TSP/ToxProfiles/ToxProfiles.aspx?id=48&tid=15
8. Yanqun Z, Yuan L, Jianjun C, Haiyan C, Li Q, Schvartz C. Hyperaccumulation of Pb, Zn and Cd in herbaceous grown on lead-zinc mining area in Yunnan, China. Environ Int. 2005;31(5):755–62.
9. Ogawa T, Kobayashi E, Okubo Y, Suwazono Y, Kido T, Nogawa K. Relationship among prevalence of patients with Itai-itai disease, prevalence of abnormal urinary findings, and cadmium concentrations in rice of individual hamlets in the Jinzu River basin, Toyama prefecture of Japan. Int J Environ Health Res. 2004;14(4):243–52.
10. Liu K, Lv J, He W, Zhang H, Cao Y, Dai Y. Major factors influencing cadmium uptake from the soil into wheat plants. Ectotoxicology and Environmental Safety. 2015;113:207–13.
11. Baldantoni D, Morra L, Zaccardelli M, Alfani A. Cadmium accumulation in leaves of leafy vegetables. Ecotoxicol Environ Saf. 2016;123:89–94.
12. Brzóska MM, Moniuszko-Jakoniuk J. Effect of low-level lifetime exposure to cadmium on calciotropic hormones in aged female rats. Arch Toxicol. 2005;79(11):636–46.
13. United States Department of Agriculture: Natural Resources Conservation Service. Heavy Metal Soil Contamination. Soil Quality - Urban Technical Note 3. 2000;(3).
14. Gratão PL, Prasad MNV, Cardoso PF, Lea PJ, Azevedo RA. Phytoremediation: Green technology for the clean up of toxic metals in the environment. Brazilian Journal of Plant Physiology. 2005;17(1):53–64.
15. Schwartz C, Echevarria G, Morel JL. Phytoextraction of cadmium with Thlaspi caerulescens. Plant Soil. 2003;249(1):27–35.
16. Kirkham MB. Cadmium in plants on polluted soils: Effects of soil factors, hyperaccumulation, and amendments. Geoderma. 2006;137(1–2):19–32.
17. Suman J, Uhlik O, Viktorova J, Macek T. Phytoextraction of heavy metals: A promising tool for clean-up of polluted environment? Front Plant Sci. 2018;871(October):1–15.
18. Sarisky-Reed V. 5 Everyday Products Made from Biomass: A Few May Surprise You [Internet]. Office of Energy Efficiency and Renewable Energy. 2020 [cited 2020 Oct 20]. Available from: https://www.energy.gov/eere/articles/5-everyday-products-made-biomass-few-may-surprise-you
19. Azeez MA. Pulp and Paper Processing. In: Newaz Kazi S, editor. Pulping of Non-Woody Biomass. IntechOpen; 2018.
