Accumulation of Pb, Cu, and Zn in native plants growing on a contaminated Florida site

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Abstract

Contamination of heavy metals represents one of the most pressing threats to water and soil resources as well as human health. Phytoremediation can be potentially used to remediate metal-contaminated sites. This study evaluated the potential of 36 plants (17 species) growing on a contaminated site in North Florida. Plants and the associated soil samples were collected and analyzed for total metal concentrations. While total soil Pb, Cu, and Zn concentrations varied from 90 to 4100, 20 to 990, and 195 to 2200 mg kg 1, those in the plants ranged from 2.0 to 1183, 6.0 to 460, and 17 to 598 mg kg 1, respectively. None of the plants were suitable for phytoextraction because no hyperaccumulator was identified. However, plants with a high bioconcentration factor (BCF, metal concentration ratio of plant roots to soil) and low translocation factor (TF, metal concentration ratio of plant shoots to roots) have the potential for phytostabilization. Among the plants, Phyla nodiflora was the most efficient in accumulating Cu and Zn in its shoots (TF = 12 and 6.3) while Gentiana pennelliana was most suitable for phytostabilization of sites contaminated with Pb, Cu and Zn (BCF = 11, 22 and 2.6). Plant uptake of the three metals was highly correlated, whereas translocation of Pb was negatively correlated with Cu and Zn though translocation of Cu and Zn were correlated. Our study showed that native plant species growing on contaminated sites may have the potential for phytoremediation.

Introduction

Heavy metals are currently of much environmental concern. They are harmful to humans, animals and tend to bioaccumulate in the food chain. Activities such as mining and smelting of metal ores, industrial emissions and applications of insecticides and fertilizers have all contributed to elevated levels of heavy metals in the environment (Alloway, 1994). The threat that heavy metals pose to human and animal health is aggravated by their long-term persistence in the environment.

Several technologies are available to remediate soils that are contaminated by heavy metals. However, many of these technologies are costly (e.g. excavation of contaminated material and chemical/physical treatment) or do not achieve a long-term nor aesthetic solution (Cao et al., 2002, Mulligan et al., 2001). Phytoremediation can provide a cost-effective, long-lasting and aesthetic solution for remediation of contaminated sites (Ma et al., 2001). One of the strategies of phytoremediation of metal-contaminated soil is phytoextraction, i.e. through uptake and accumulation of metals into plant shoots, which can then be harvested and removed from the site. Another application of phytoremediation is phytostabilization where plants are used to minimize metal mobility in contaminated soils.

More than four hundreds plants are known as hyperaccumulators of metals, which can accumulate high concentration of metals into their aboveground biomass. These plants include trees, vegetable crops, grasses and weeds. Based on Baker and Brooks (1989), hyperaccumulators are defined as plants that accumulate > 1000 mg kg 1 of Cu, Co, Cr, Ni or Pb, or > 10,000 mg kg 1 of Mn or Zn. Hyperaccumulators of Co (26 species), Cu (24), Mn (8), Ni (145), Pb (5), and Zn (4) have been reported (Baker and Brooks, 1989). The five hyperaccumulators of Pb include Armeria martima, Thlaspi rotundifolium, Thlaspi alpestre, Alyssum wulfenianum, and Polycarpaea synandra. Plant metal uptake is influenced by soil factors including pH, organic matter, and cation exchange capacity as well as plant species, cultivars and age. The mobility and availability of heavy metals in the soil are generally low, especially when the soil is high in pH, clay and organic matter (Jung and Thornton, 1996, Rosselli et al., 2003).

It is important to use native plants for phytoremediation because these plants are often better in terms of survival, growth and reproduction under environmental stress than plants introduced from other environment. There has been a continuing interest in searching for native plants that are tolerant to heavy metals; however, few studies have evaluated the phytoremediation potential of native plants under field conditions (Shu et al., 2002, Mcgrath and Zhao, 2003). Heavy metals can cause severe phytotoxicity, and may act as powerful force for the evolution of tolerant plant populations. Therefore, it is possible to identify metal-tolerant plant species from natural vegetation in field sites that are contaminated with various heavy metals.

The overall objectives of this research were: 1) to determine the concentrations of Pb, Cu and Zn in plant biomass growing on a contaminated site; 2) to compare metal concentrations in the aboveground biomass to those in roots and in soils, and 3) to assess the feasibility to use these plants for phytoremediation purpose. Information obtained from this study should provide insight for using native plants to remediate metal-contaminated sites.

Section snippets

Site characterization

The plant and soil samples used in this study were collected from a known metal-contaminated site located in an urban area of northwest Jacksonville, Florida. The site has been vacant, occupies approximately 4047 m2, and is covered mainly by grasses. Past industrial activities, including a gasoline station, salvage yard, auto body shop, and recycling of lead batteries, have contributed to elevated metal concentrations in this site. Contamination of heavy metals was mainly concentrated in the top

Soil properties and metal concentrations

Previous research has shown that this soil has relatively high organic matter (3.91%) and high pH (6.95) (Cao et al., 2003), which is not typical of Florida soils where low organic matter and pH are common (Chen et al., 1999). The source of the elevated organic matter was probably from the urban waste deposited on the site over the years. The use of lime to reduce soil pH probably caused alkaline pH of this soil (Cao et al., 2001).

Selected properties of the 10 collected soil samples are listed

Conclusion

This study was conducted to screen plants growing on a contaminated site to determine their potential for metal accumulation. Only species with both BCFs and TFs greater than one have the potential to be used for phytoextraction. Among the 36 plant samples of 17 plant species screened, no plant species were identified as metal hyperaccumulators. However, several plants had BCFs or TFs greater than one. G. pennelliana was most effective in taking up all three metals, with BCFs ranging from 1.1 to

Acknowledgments

This research was supported in part by the Florida Institute of Phosphate Research. The authors would like to thank Dr. Greg MacDonald for his assistance in plant identification and Mr. Thomas Luongo for his assistance in chemical analysis and proof-reading of the manuscript.

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