Lead in environmental health

(Third of three parts)

Part 3:  Remediation of lead-polluted soils

Lead poisoning can be prevented by effectively controlling their environmental sources (like leaded petrol, paints, pipes, ceramics, cosmetics, etc.), especially when coupled with public education and enforceable regulations.

Lead-polluted soils represent the single largest pool of this toxic metal in the environment. From mining sites to manufacturing operations to urban infrastructure, soil-borne lead can disperse and pose health threats to humans. However, other than to children, lead in soil does not pose any health concern to the rest of the population if the soil is properly managed.

Generally, lead in soil is hardly taken up by plants. Even in farm soils amended with sewage sludge, this metal is minimally taken up by roots — the reasons being the presence of organic matter (OM sequesters the metal to an unavailable form) and the high pH of soil (typically around pH 5.5 to seven). However, extreme edaphic (distinguished by soil rather than by climate) and environmental conditions, such as those present in derelict mine sites, landfills, battery recycling sites, etc., can promote solubility, dispersion, and transport of the metal to unwanted areas. Unlike in farms, soils in these areas are characteristically low in OM, highly acidic with low buffering capacity, infertile, and therefore unfit to grow plants. Acidic leachates (“acid mine drainage” in mining jargon) are produced and these can be transported to the groundwater or to nearby bodies of surface water like streams and lakes.

Soil pollution by lead is generally confined in the surface layer — true in areas impacted by smelting, urban areas from air pollution, and neighborhoods with leaded paints. Remediation (also referred to as reclamation in metal industry) seems easy to execute in these areas since only a few centimeters of the top soil are polluted; however, when the total land area is taken into account, it becomes a formidable task.

There are regulatory guidelines when remediation should be undertaken. When the soil level of lead (the US Environmental Protection Agency uses the Toxicity Characteristic Leaching Procedure, or TCLP, an extraction method) is suspected to cause ecological and/or human effects, a risk assessment is called for. This can be either an ecological or human health risk assessment. The former is usually preferred because it is more cost-effective as it does not involve any human subject.

When risk-based remediation is deemed necessary, remediation can take the form of conventional, engineering-based technologies (like soil excavation, soil flushing or washing, etc) or ecological-type approaches (like phytoremediation, in situ chemical stabilization, etc). The former technologies are typically more expensive and invasive than the latter; however, the latter are limited to only surficial —[sur(face) + (super)ficial] — treatments and becomes inapplicable when pollution has migrated deeply (greater than one meter).

Phytoremediation refers to a group of approaches that involves the use of certain green plants to aid in stabilizing the soil structure against wind and water erosion by providing ground cover (i.e., phytostabilization). The plants usually have dense root systems and the aerial parts protect the ground from the forces of wind and raindrops, keeping the soil in place. Certain plants have abilities to take up unusually high amounts of the metal from the soil, accumulating it in the foliage (i.e., phytoextraction). This requires plant species that have dense aboveground vegetation. Other plants have extensive root systems to intercept and sequester soluble contaminants from water and waste streams (i.e., rhizofiltration). These species are generally aquatic. Others have been modified by biotechnology to uptake, sequester and eventually volatilize organo-metallic compounds into the atmosphere (i.e., phytovolatilization).

To clean up a site using phytoremediation, a combination of two or more approaches may be necessary. A mining or smelting site is a complex scenario where erosion, leaching/runoff, and high amounts of soluble lead may be present. The issue is exacerbated when there are communities around it and safe drinking water needs to be ensured. In this case, a combination of phytoremediation and in situ chemical stabilization may be opted.

In situ chemical stabilization requires treatment of the soil on-site where amendment materials are used by mixing them with the surface soil. Like phytoremediation, this approach is limited to only the surface layer. Alkaline biosolids (a form of sewage sludge compost), coal fly ash, livestock manure compost, and phosphate rocks can be used as ameliorants.

Phosphate rocks (or hydroxy-apatite) are mined in North Carolina and Florida and other parts of the world. They are rather inexpensive and can provide soluble phosphates to form stable mineral complexes with the lead (known as lead pyromorphite). This renders the soil lead quite immobile and less bioavailable. However, a mere treatment of impacted soil with phosphate rock does not guarantee stability of the soil and more importantly, the soil remains infertile to support plant life. This is where combining phytoremediation with in situ chemical treatment becomes attractive for soil cleanup. This approach has been used to remediate Superfund and other hazardous sites in the US.

The main goal in phytoextraction (also called phytomining) is to recycle the lead from the soil profile to the plant biomass. It is ideal to employ hyperaccumulator species since they take up metals in large amounts, or greater uptake can be induced by adding chelating agents in the soil to solubilize the metal. One major drawback with this method is that the roots cannot take up all the solubilized metal as the chelated metal could move beyond the root zone.

Prof. W. Wenzel from Vienna, Austria and I patented a cleanup technology where we combine phytoremediation and in situ chemical stabilization that allows long-term treatment of polluted sites. Metals from below ground are extracted by roots and recycled onto the ground surface as fallen biomass. Metals released from decomposed biomass are sequestered in the soil surface by the apatite minerals. Plants don’t have to be harvested to process and recover the metals from the biomass as metals concentrate in the apatite minerals. This combined technology is more conducive to promote sustainable environment.

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Domy Adriano is a professor emeritus at the University of Georgia’s College of Agricultural and Environmental Sciences, and the Savannah River Ecology Lab. He has had academic stints at Kansas State University, University of California, Riverside, and Michigan State University. As complement, see Adriano (2001), Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability and Risks of Metals. Springer, N.Y., 866 p. (cited over 900 times according to Google Scholar). E-mail him at [email protected].