Uptake and accumulation of cadmium, lead and zinc by Siam weed [Chromolaena odorata (L.) King & Robinson]
Introduction
Heavy metals contamination in soil is a major environmental problem. Contamination usually results from industrial activities, such as mining and smelting of metalliferous ores, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, and generation of municipal waste (Kabata-Pendias, 2001). Metal-contaminated soils are notoriously hard to remediate. Current technologies resort to soil excavation and either landfilling or soil washing followed by physical or chemical separation of the contaminants (Prasad, 2004). Unfortunately, these techniques are labor-intensive and costly (Ensley, 2000). In contrast, the use of plants to remove heavy metal contaminants from soils, known as “Phytoremediation”, offers economic and environmental advantages and is a promising technique (Salt et al., 1995). The success of phytoremediation depends on plant growth rate and obtaining high metal concentrations in plant shoots. Plant must produce sufficient biomass while accumulating high concentrations of heavy metals.
In recent years, many plant species, usually those found in heavy metal contaminated areas have been identified as hyperaccumulators, i.e., they have the ability to accumulate unusually high concentrations of heavy metals, without impact on their growth and development (Baker and Brooks, 1989, Xiong, 1997). However, most hyperaccumulators identified so far are not suitable for phytoremediation applications (in the field) due to their small biomass and slow growth (Shen et al., 2002).
Chromolaena odorata (L.) King & Robinson (Asteraceae, Eupatorieae), known as Siam weed, is a perennial shrub that forms dense tangled bushes 1.5–2.0 m in height, occasionally reaching 6 m as a scrambler up trees (McFadyen and Skarratt, 1996). Due to its fast growth rate, and prolific, wind-dispersed seed production, the plants can spread very easily (McFadyen and Skarratt, 1996). C. odorata is widespread throughout Southeast Asia, India, Africa, Australia (McFadyen, 1989, Prasad et al., 1996). It is grown best in areas with a pronounced dry season (McFadyen, 1989). In Thailand, C. odorata is widely distributed throughout the country especially in those areas with a pronounced dry season. C. odorata was found growing commonly on disturbed lead contaminated soils around disused lead mines, at Bo Ngam, Kanchanaburi province, 230 km west of Bangkok. The same species was also found in non-contaminated sites. We hypothesized that the population of this species around the lead mines would have adapted to the contaminated habitat and would differ from the population in non-contaminated sites.
In the present study, a field survey was conducted on C. odorata growing on lead contaminated soil around Bo Ngam lead mine. Hydroponic experiments were then carried out to investigate the differences in growth and heavy metals (Pb, Cd, and Zn) accumulation between C. odorata from contaminated and non-contaminated sites.
Section snippets
Collection and preparation of soil and plant samples from field sites
Bo Ngam lead mine is located in Thongphapum district, Kanchanaburi province, western Thailand. The mine is rich in PbCO3 and was previously operated for 25–30 years before the expiry of the concession in 1996. Samples of C. odorata with soil attached to roots were collected from different sites in Bo Ngam lead mine during October, 2004. These were: site A (ore piling area I), site B (ore piling area II), site C (open pit area), site D (ore dressing plant area). In addition, plants were
Soil metal concentrations and metals uptake and accumulation by plants under field conditions
Soil from site D (ore dressing plant area) had significant higher concentration of all metals, 164 332.7, 1.6, and 260.9 mg kg−1 for Pb, Cd, and Zn, respectively, than other sites (Table 1). There was a close correspondence between extractable Pb contents and the total metal concentrations at each site. C. odorata collected from site D showed the highest Pb and Cd contents in roots (4236 and 0.4 mg kg−1 DW, respectively) and shoots (1376.7 and 0.9 mg kg−1 DW, respectively) (Table 1). C. odorata
Discussion
Markert (1994) gave values of the metal concentration of normal plant with which the uptake in a species could be compared, and showed that the normal compositions of Pb, Cd and Zn in plant are 1, 0.05 and 50 mg kg−1 DW, respectively. Pb is a non-essential element and can be toxic to photosynthesis (Skórzyñska-Polit and Baszyñski, 1997), chlorophyll synthesis (Stobart et al., 1985) and antioxidase enzymes (Somashekaraiah et al., 1992), while Cd inhibits root growth and cell division (Jiang et
Acknowledgements
This research work was supported by the grant from the Post-Graduate Education, Training and Research Program in Environmental Science, Technology and Management under Higher Education Development Project of the Commission on Higher Education, Ministry of Education, and Mahidol University, Bangkok, Thailand. We are grateful to Mr. Philip Round for assistance with the proof reading of the manuscript.
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