The purpose of this handbook is to demonstrate the application of phytoremediation as a cleanup technology for metal contaminated soils, specifically lead. In order for this type remediation strategy to be successful, it is necessary to utilize metal-accumulating plants to extract environmentally important toxic metals, such as Pb, Ni, Cr, Cd and Zn, from the soil.  Certain plants have been identified that not only accumulate metals in the plant root structure, but also translocate the accumulated metals from the root to the leaf and shoot. (Baker et al., 1994)  While many plants do this function, some plants, known as hyperaccumulators, can accumulate extremely high concentrations of metals in their shoots (0.1% to 3% of their dry weight) (USEPA, 1999).  The metal-rich plant material can then be harvested and removed from the site without extensive excavation, disposal costs, and loss of topsoil that is associated with traditional remediation practices.

Sections:

• Requirements of Plants to Remediate Lead
• Hyperaccumulators of Lead
• Enhancements of Lead Uptake

The success of phytoremediation, in general, is dependent on several factors:
 

• First, plants must produce sufficient biomass while accumulating high concentrations of metal.  In some cases, an increased biomass will lower the total concentration of the metal in the plant tissue, but allows for a larger amount of metal to be accumulated overall.
• Second, the metal-accumulating plants need to be responsive to agricultural practices that allow repeated planting and harvesting of the metal-rich tissues.  Thus, it is preferable to have the metal accumulated in the shoots as opposed to the roots, for metal in the shoot can be cut from the plant and removed, which is manageable on a small scale, but impractical on a large scale.  If the metals are concentrated in the roots, the entire plant needs to be removed.  Yet, the necessity of full plant removal not only increases the costs of phytoremediation, due to the need for additional labor and plantings, but also increases the time it takes.  This is because the new plantings need time to establish themselves in the environment and begin accumulation of metals.
• Furthermore, the availability of metals in the soil for plant uptake is another limitation for successful phytoremediation.  For example, lead (Pb²⁺), an important environmental pollutant, is highly immobile in soils.

Lead is known to be “molecularly sticky” since it readily forms a precipitate within the soil matrix, has low aqueous solubility, and, in many cases, is not readily bioavailable.  In most soils capable of supporting plant growth, the soluble Pb2+ levels are relatively low and will not promote substantial uptake by the plant even if it has the genetic capacity to accumulate the metal.  In addition, many plants retain Pb2+ in their roots via sorption and precipitation with only minimal transport to the aboveground harvestable plant portions.  Therefore, it is important to find ways to enhance the bioavailability of Pb2+ or to find specific plants that can better translocate the Pb2+ into harvestable portions (Kumar et al., 1995).

Although there are some challenges associated with the phytoremediation of lead, it remains a very promising strategy, especially in contract to the costliness of feasible alternatives.  An important consideration, however, is that many situations of soil contamination have unique factors that require special evaluation.  For this reason it is important to conduct either laboratory or field testing to determine the optimum set of conditions for a particular site.

Hyperaccumulators of Lead

There are at least 400 known metal hyperaccumulators in the world; however, a limited number of these are Pb2+ hyperaccumulators.  The hyperaccumulation of lead is rare due to the limited free lead (Pb2+) available in soil for absorption.  As previously mentioned, lead is “molecularly sticky” in that is forms Pb complexes with organic matter, sorbs on clay and oxide particles, and precipitates as carbonates, hydroxides, and phosphates (McBride, 1994).  Since lead bonds strongly with soil minerals and organic matter, it is difficult for plants to extract it from the soil and into its roots.  Once lead is absorbed by the plant, it complexes with plants nutrients limiting its ability to be translocated to the harvestable shoots.

 Table 4.2.  Selected Lead Accumulating Plants

Scientific Name	Common Name
Armeria maritima	Seapink thrift
Ambrosia artemisiifolia	Ragweed
Brassica juncea	Indian mustard
Brassica napus	Rape, Rutabaga, Turnip
Brassica oleracea	Flowering/ornamental kale & cabbage, Broccoli
Festuca ovina	Blue/sheep fescue
Helianthus annuus	Sunflower
Thlaspi rotundifolium	Pennycress
Triticum aestivum	Wheat (scout)
Zea mays	Corn

From current research, there is evidence that various plant species have the ability to absorb Pb2+ into the roots and translocate it from the roots to the shoots.  Table 4.2 lists some of the lead accumulating plants found by phytoremediation researchers.  The background concentration of lead in plant tissue is 10 mg/g; therefore the hyperaccumulation of lead is defined as >1000 mg/g.  Vegetation growing in extremely Pb contaminated sites often have less than 50 ppm Pb in the shoots (Cummingham et al., 1995).  Research has shown that even the best Pb2+ accumulating plants could hardly accumulate a shoot Pb2+ concentration greater than 0.1% in their shoots when grown in Pb2+ contaminated soils without the addition of any amendments (Huang et al., 1997a).  The most frequently cited Pb2+ hyperaccumulator is the cultivar Thalspi rotundifolium (L.) Gaud.-Beaup which can obtain a shoot concentration of 8500 mg/g (Reeves and Brooks, 1983).  Unfortunately, this Thalspi species has a small biomass and a slow growth rate, which makes it unsuitable for phytoremediation.  Brassica juncea (L.) Czern. also demonstrated the ability to accumulate lead to a higher degree.  When the plant was grown in a nutrient solution that had a high concentration of soluble lead as much as 1.5 % Pb2+ was found in the shoot tissues (Kumar et al., 1995).  However, this lead-tolerant plant showed little ability to translocate lead in its shoots when it was grown in soils where Pb2+ bioavailability was limited.  Therefore, in immediate applications it will be necessary to harvest the roots as well as the shoots of these plants.

Amendments can be added to either help phytoextract or phytostabilize lead in soils.  The enhancement of lead uptake is discussed in the next section (Section 4.3).  In terms of phytostabilization, the addition of amendments such as alkalizing agents, phosphates, mineral oxides, biosolids, or organic matter can reduce lead solubility.  The more insoluble Pb is, the more unavailable it is to leaching, plant uptake, and mammal ingestion.  These inexpensive amendments can also improve the condition of the soil for plant growth (Berti and Cunningham, 1997).  As these amendments render the contaminant in the soil matrix, the plant secures the stabilized matrix in place preventing water and wind erosion.

Enhancements of Lead Uptake

As discussed previously, there are several different methods through which phytoremediation can occur.  However, in order to maximize the success of a phytoremediation strategy, it is critical to have significant metal bioavailability at a contaminated site as well as a large quantity of plant biomass with high rates of growth.  Metal contaminants that are not soluble, thus not available for plant uptake, may limit the success of phytoremediation.  In most Pb contaminated soils usually less than 0.1% of the total Pb present is bioavailable for plant uptake (Huang and Cunningham, 1996).

The second limitation in Pb phytoextraction is the poor translocation of the metal from the roots to the harvestable shoots.  In the plants that do translocate Pb, translocation is less than 30% (Huang and Cunningham, 1996).  Research is being conducted in the field of Pb phytoremediation to improve both the uptake and translocation of Pb through induced hyperaccumulation, which involves soil pH adjustments and/or the application of synthetic chelates.

In general, the more biomass that the plant has, the more metal that can be accumulated since the metal uptake is a function of the overall biomass.  Thus, the use of fertilizers can help facilitate rapid plant establishment and growth.  For most lead contaminated soil, phosphorous (P) availability is very low due to the precipitation of Pb-P precipitation.  Thus, a foliar P fertilizer spray applied topically to the plant’s leaves and stem increases phosphorous content in the plant, while not confounding the Pb-P binding problem in the soil.  In a study reported by Huang et al.,  (1997b), soil to which phosphate fertilizer was added directly showed diminished lead bioavailability, presumably due to Pb-P precipitation, in contrast to hydroponic uptake.  In contrast, with foliar P (10mM P as KH2PO4, pH 6.0) application the plant dry weight increased by more than 4 fold for both shoots and roots of goldenrod plants within a month after the treatment.

The mobilization of metal contaminants, both in the soil and the plant, is another important factor influencing the success of phytoremediation.  The amount of soluble Pb2+ in the soil appears to be a key factor to the enhancement of Pb2+ uptake by plants (Wu et al., 1999).  There are two main amendment techniques that have been used to increase the bioavailability of lead in soils and the mobility of lead within plant tissue: lowering soil pH and adding synthetic chelates.

Soil pH is a significant parameter in the uptake of metal contaminants.  This is a result of the fact that the soil pH value is one of the principal soil factors controlling metal availability (Alloway, 1990).  By maintaining a moderately acidic pH in the soil through the use of ammonium containing fertilizer or soil acidifiers, lead metal bioavailability and plant uptake has been shown to increase (Huang et al., 1997a; Salt et al., 1995; Smith, 1994).  In a study performed by Chlopecka et al. (1996) on metal contaminated soils in southwest Poland, it was found that soil samples of pH less than 5.6 contained relatively more of all metals in the exchangeable form than in samples where the pH was greater than 5.6.  In addition, at lower pHs the lead in soil has a greater potential to translocate from a plant’s roots into its shoots.  However, it is essential to keep in mind that the ideal pH range for the growth of most plants and grasses is from 5.0-8.0.  Thus, a pH around 5.0 seems to an optimum pH level, since lower values may inhibit plant growth.

Synthetic chelates, such as EDTA (ethylenediaminetetraacetic acid), have been shown to aid in the accumulation of Pb2+ in the plant tissue.  EDTA and other chelates have been used in soils and nutrient solutions to increase the solubility of metal cations and the translocation of Pb into shoots (Wallace et al., 1977; Checkai et al., 1987; Sadiq and Hussain, 1993). Despite an overall increase of Pb in the shoots, there are differences, however, in the extent of accumulation at equivalent chelate levels among various plant species (Huang et al., 1997a).

Chelating agents have been used with great success in non-residential areas, but are not necessarily a requirement.  Furthermore, it should be noted that chelate induced hyperaccumulation might prove fatal to the plant; however, the dying plant can still be harvested and processed for Pb2+ recovery or disposal.  Unfortunately, the downfall associated with these amendment strategies also has to do with the increased bioavailability to other animals and humans.  Increasing Pb solubility increases the chance for it to enter the groundwater.  In addition, the increased bioavailability to plants also makes it easier for Pb to enter the food chain by being ingested by animals eating the plants.  Because of these environmental concerns, if chelates are employed it is necessary to minimize chelate additions.  Huang et al. (1997a) suggested the following field application protocol when growing a high biomass crop that is sensitive to chelates.  After the crop has become well established and is of sufficient biomass, a selected chelate could be applied to the root zone to facilitate rapid Pb accumulation.  The plant should be harvested shortly after chelate addition to reduce environmental risk.

 The physiological and biological mechanisms involved in lead uptake into a plant and root to shoot transport of lead within the plant may require some time to develop and become functional.  Since plant species can differ significantly in lead uptake and translocation, the success of using plants to extract lead from contaminated soils requires the identification of lead accumulating plants that can survive in the presence of co-contaminants, the measurement of the concentration of pollutant in the soil, and knowledge of chemistry (availability or speciation) of the metal in the soil matrix.  The combination of soil amendment/foliar fertilizer application with plants capable of absorbing and translocating lead may be an effective means of remediating an area with varying levels of lead concentration.