Phytoremediation of Lead: What Works, What Doesn’t

Reports of a lead-contaminated water supply in Flint, Michigan, stimulated new public health concerns about lead exposure. According to the Centers for Disease Control and Prevention, there is no safe blood level of lead in children, and the effects of lead exposure cannot be undone. In addition to lead in drinking water, urban gardeners also have concerns about lead in the soil. The United Nations Food and Agriculture Organization says that approximately 800 million people worldwide practice urban agriculture (http://www.fao.org/urban-agriculture/en). Over the last two decades, some researchers and gardeners turned to sunflowers (Helianthus annuus) as a potential tool for phytoremediation, which uses plants or a biologically laden material such as compost to remediate contamination (“Sunflowers to the Rescue!” proclaimed a 2012 article in The Farmer's Almanac about the flowers’ ability to remove lead from a backyard garden). The overarching goal is to render the contaminant not bioavailable—absorbable by humans and wildlife. One type of phytoremediation is phytoextraction, which sunflower ­proponents envisage, whereby contaminant metals are drawn up into plant shoots. From there, the contaminated plant parts can be deposited in designated areas. But despite some promising preliminary studies of sunflowers’ phytoextraction of lead, more extensive research has been disappointing. Since the 1990s, plants, including sunflowers, have been shown to increase accumulation of lead when a chelating agent, such as ethylenediaminetetraacetic acid (EDTA), is added to the soil (see Huang et al. 1997). Although the additive raises lead's solubility in the soil, EDTA is extremely expensive. Moreover, EDTA works in combination with other soil practices, such as adding compost, which are proven, cost-effective responses on their own. Retired US Department of Agri­culture (USDA) scientist Rufus Chaney stresses that no serious study supports significant lead phytoextraction. Chaney, a 2013 inductee to the USDA Agricultural Research Service Science Hall of Fame, has demonstrated plants successfully phytoextracting some metal contamination, notably alpine pennycress extracting cadmium. In addition, Chaney reports that canola is an economically viable possibility for phytoextracting radioactive cesium. But lead is a different story. “It's the chemistry,” Chaney explains. “Lead is adsorbed and precipitated in the soil. It is bound by organic matter; it is bound by iron oxides and clays; and in the presence of enough phosphates to grow plants, the phosphate reacts with the lead and makes it a new compound, making the lead very unavailable for uptake.” Chaney adds that with plants in solutions, lead can move up a plant, but these conditions do not reflect plant growth in the outdoors. Kansas State soil scientist Ganga Hettiarachchi agrees that sunflowers have not proved effective for extracting lead. Hettiarachchi, who in 2016 completed an Environmental Protection Agency (EPA)–funded, 7-year study on brown fields and urban gardens, says, “Unfortunately, we do not have a good plant to phytoextract lead from soil.” That said, another type of phytoremediation, called phytostabilization, does work. With phytostabilization, soil amendments are added, and the contaminant is sequestered, diluted, and prevented from becoming bioavailable. “The ability to phytostabilize lead is very strong and well demonstrated,” Chaney says. “Most lead problems can be solved by treating the soil with phosphate, limestone, and organic matter.” In Kansas City, Hettiarachchi and colleagues found that adding compost “diluted soil total Pb [lead] concentration twenty-nine to fifty-two percent” (Attanayake et al. 2014). Hettiararchchi also found that food grown in urban gardens is safe in many circumstances, but she cautions about humans eating root crops, such as carrots, in areas with 200–250 parts per million or more lead in the soil. Chaney has participated in another study with similar results with urban soils in Baltimore. Another possibility is reflected in Hettiarachchi's current study, a North Atlantic Treaty Organization (NATO) project converting contaminated military sites in NATO partner countries to biofuel crops farms, in part, to reserve healthier soils for food crops. At Fort Riley, in Kansas, where lead contamination comes primarily from bullets, Miscanthus, an ornamental grass, is being planted and studied as a biofuel. Miscanthus, like other plant cover, can also help prevent lead from becoming airborne as dust, another serious concern. Chaney says the real future for making soil safe in urban areas is phytostabilization, with compost as an important ingredient. “As long as we let people know what works, they can do it for themselves,” Chaney says. “It is so inexpensive, it's an easy choice.”