Interest in selenium pollution and remediation technology has escalated during the past two decades. Although not known to be essential for plants, selenium is essential but could be toxic for humans and animals, depending on its concentration. A major selenium controversy in the 1980's emerged in California when the general public and scientific community became aware of selenium's potential as an environmental contaminant. After extensive research on several strategies to reduce loads of mobile Se for entering the agricultural ecosystem a plant-based technology, defined as 'phytoremediation' received increasing recognition, as a low-cost environmentally friendly approach for managing soluble Se in the soil and water environment. Successful long-term field remediation of Se by plants is, however, dependent upon acceptance and widespread use by growers, who are also concerned about potential commercial value from using the plant-based technology. Obtaining products with economic value from plants used in the cleanup of soil would certainly be an additional benefit to phytoremediation, which could help sustain its long-term use.

The possibility of using phytoremediation with weed plant species in Thailand to remove chromium (Cr) from soil was investigated. Six plant species, Cynodon dactylon, Pluchea indica, Phyllanthus reticulatus, Echinochloa colonum, Vetiveria nemoralis, and Amaranthus viridis, were chosen for their abilities to accumulate total chromium (TCr) at tanning industry sites. These plant species were studied in pots at a nursery. Cynodon dactylon and Pluchea indica provided highest TCr accumulation capacities of 152.1 and 151.8 mg/kg of plant on a dry weight basis, respectively, at a pulse hexavalent Cr [Cr(VI)] input of 100 mg Cr(VI)/kg soil. Most of the Cr uptake occurred within 30 days after the input. The TCr accumulation by Pluchea indica was observed in roots, stems, and leaves at 27%, 38%, and 35% of the TCr mass uptake, respectively, whereas 51%, 49% and 0% of the TCr mass uptake accumulated in roots, stems, and leaves of Cynodon dactylon, respectively. The results on Cr accumulation and translocation in plant tissues suggest that Cr was removed mainly via phytoaccumulation and Pluchea indica is more suitable than Cynodon dactylon for the phytoremediation of Cr contaminated soil.

Zinc toxicity thresholds for reclamation plants are largely unknown. As a result, ecological risk assessments often rely on toxicity thresholds for agronomic species, which may differ from those of restoration species. Our objective was to provide Zn toxicity thresholds for forb species that are commonly used in reclamation activities. We used a greenhouse screening study where seedlings of yarrow (Achillea millefolium L.), Bigelow's tansyaster (Machaeranthera bigelovii (Gray) Greene var. bigelovii), blue flax (Linum perenne L. var. Appar), alfalfa (Medicago sativa L. var. Ladak), Palmer's penstemon (Penstemon palmeri Gray), and Rocky Mountain penstemon (Penstemon strictus Benth. var. Bandera) were grown in sand culture and exposed to increasing concentrations of Zn. Lethal concentrations (LC50 - substrate Zn concentration resulting in 50% mortality), effective concentrations (EC50 - substrate Zn concentration resulting in 50% biomass reduction), and phytotoxicity thresholds (PT50 - tissue Zn concentration resulting in 50% biomass reduction) were then determined. Phytotoxicity thresholds and effective concentrations for these reclamation species were relatively consistent between species. Our estimates of PT50-shoot for these species range from 1258 to 3214 mg Zn kg-¹ . Measures of EC50-plant for these restoration forbs ranged from 82 to 214 mg Zn L-¹ . These thresholds might be more useful for risk assessors working on reclamation sites than those based on non-reclamation species that are widely used.

Phytoremediation of pollutants in soils is an emerging technology, using different soil-plant interaction properties. For organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), phytodegradation seems to be the most promising approach. It occurs mostly through an increase of the microbial activity in the plant rhizosphere, allowing the degradation of organic substances, a source of carbon for soil microbes. Despite a large amount of available data in the literature concerning laboratory and short term PAH phytodegradation experiments, no actual field application of such technique was previously carried out. In the present study, a soil from a former coking plant was used to evaluate the feasibility and the efficiency of PAH phytodegradation in the field during a three years trial and following a bioremediation treatment. Before the phytoremediation treatment, the soil was homogenized and split into six independent plots with no hydrological connections. On four of these plots, different types of common plant species were sowed: mixture of herbaceous species, short cut (P1), long cut (P2), ornamental plants (P3) and trees (P4). Natural vegetation was allowed to grow on the fifth plot (P5), and the last plot was weeded (P6). Each year, representative sampling of two soil horizons (0-50 and 50-100 cm) was carried out in each plot to characterize the evolution of PAHs concentration in soils and in soils solution obtained by lixiviation. Possible impact of the phytoremediation technique on ecosystems was evaluated using different eco- and genotoxicity tests both on the soil solid matrix and on the soil solution. For each soil horizon, comparable decrease of soil total PAHs concentrations were obtained for three plots, reaching a maximum value of 26% of the initial PAHs concentration. The decrease mostly concerned the 3 rings PAHs. The overall low decrease in PAHs content was linked to a drastic decrease in PAHs availability likely due to the bioremediation treatment. However, soil solutions concentration showed low values and no signficant toxicity was characterized. The mixture of the herbaceous species seemed to be the most promising plants to be used in such procedure.