Michael J. Blaylock

Phytoremediation: a novel strategy for the removal of toxic metals from the environment using plants.

Toxic metal pollution of waters and soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is an emerging technology called phytoremediation. Three subsets of this technology are applicable to toxic metal remediation: (1) Phytoextraction—the use of metal-accumulating plants to remove toxic metals from soil; (2) Rhizoflltration—the use of plant roots to remove toxic metals from polluted waters; and (3) Phytostabilization—the use of plants to eliminate the bioavailability of toxic metals hi soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Phytoremediation: a novel approach to an old problem

Abstract There is a great need for cost effective technology to remediate soils and bodies of water contaminated with heavy metals and radionuclides. Phytoremediation, an emerging technology of using plants for the removal of pollutants could provide an affordable way to restore the economic value of contaminated land. Two main subsets of phytoremediation have been developed: phytoextration, which is based on using high biomass crop plants in combination with a system of soil amendments to extract heavy metals from soil, and rhizofiltration, a technology which employs plants to remove contaminants from aqueous streams.

Method for detecting selenium speciation in groundwater.

To better understand the potential toxicity of Se, it is necessary to know the concentration of different Se ionic species (e.g., SeO 3 2- and SeO 4 2- ). The hydride generation atomic absorption spectrophotometry (HGAAS) method of Se analysis cannot separate Se into individual ionic species. Ion chromatography (IC) can determine SeO 3 2- and SeO 4 2- concentrations simultaneously ; however, common anions, such as sulfate (S0 4 2- ), in groundwater interferes with SeO 3 2- and SeO 4 2- speciation. The purpose of this study was to measure the concentration of ionic SeO 3 2- and SeO 4 2- species in groundwater, thereby determining the chemical speciation of dissolved Se. Three groundwater samples with high concentrations of Mg 2+ and SO 4 2- were used in this study. The ionic SeO 3 2- and SeO 4 2- species in groundwatersamples were selectively adsorbed onto copper oxide (CuO) particles by lowering the pH to 5.5. These ionic species were desorbed from the surface of CuO particles by increasing the pH to 12.5. Subsequently, the concentrations of SeO 3 2- and SeO 4 2- ionic species in solutions were determined with HGAAS and IC. The effect of divalent cations (e.g., Mg 2+ ) on the concentration of SeO 4 2- in aqueous solutions was also evaluated. The dissolved Se concentration in three groundwater samples ranged from 22 to 151 μg/L. The CuO particles extracted 97% of SeO 3 2- from groundwater samples, suggesting that Se(lV) concentrations were dominated by the SeO 3 2- ion. However, CuO particles extracted 80% of SeO 4 2- from groundwater samples. These results suggest that Se(Vl) concentrations consisted of SeO 4 2- and metal SeO 4 2- solution species. The dissolved Mg 2+ in groundwater samples formed a strong neutral ion pair with SeO 4 2- (MgSeO 4 0 ), which was not adsorbed by the CuO particles. Overall chemical speciation of dissolved Se, extracted with CuO particles, suggests that groundwater samples consisted of SeO 3 2- (6-36%), SeO 4 2- (32-65%), organic Se species (14-23%), and neutral ion pairs (9-16%). An important aspect of the proposed method is that CuO can be used in the field to extract both SeO 3 2- and SeO 4 2- ionic species from groundwater samples, and these species could be desorbed from CuO and measured using HGAAS or IC methods, depending upon the concentrations of these species.

Modeling selenite sorption in reclaimed coal mine soil materials

Selenite (SeO2-3) sorption in soils has been correlated with pH, soil mineralogy, and soil solution composition, factors that are often highly variable with respect to mine soil materials. Selenite equilibrium and adsorption batch studies were conducted with four mine soil materials to determine adsorption parameters that could be used to develop a model to predict Se retention. Initial mass, Freundlich, Langmuir, and other relationships were explored to describe adsorption and retention of Se in these soils. For equilibrium and adsorption studies, 25 ml of solution was added to 2.5 g of soil in a polyethylene centrifuge tube. Time-dependent analysis consisted of duplicate treatments of two SeO2-3 levels and reaction times, of 2, 6, 24, 48, 168, 336, and 504h. Adsorption studies were arranged in a 3 X 10 X 4 factorial design (three replications, 10 SeO2-3 concentrations, four soils) and equilibrated for 14 d. Selenite sorption as a function of pH in each material was also examined. Selenite sorption of 10 μg Se/g soil was not greatly affected by pH between pH 4 and 8, except in one sample where sorption decreased at pH 6. Initial mass isotherms were very similar for Se additions up to 20 mg/kg for all soils and predicted Se sorption very similar to the experimental data for these and 12 additional soils. The Freundlich and Langmuir isotherms did not effectively predict Se sorption.

Enhanced Degradation of TCE on a Superfund Site Using Endophyte-Assisted Poplar Tree Phytoremediation

Trichloroethylene (TCE) is a widespread environmental pollutant common in groundwater plumes associated with industrial manufacturing areas. We had previously isolated and characterized a natural bacterial endophyte, Enterobacter sp. strain PDN3, of poplar trees, that rapidly metabolizes TCE, releasing chloride ion. We now report findings from a successful three-year field trial of endophyte-assisted phytoremediation on the Middlefield-Ellis-Whisman Superfund Study Area TCE plume in the Silicon Valley of California. The inoculated poplar trees exhibited increased growth and reduced TCE phytotoxic effects with a 32% increase in trunk diameter compared to mock-inoculated control poplar trees. The inoculated trees excreted 50% more chloride ion into the rhizosphere, indicative of increased TCE metabolism in planta. Data from tree core analysis of the tree tissues provided further supporting evidence of the enhanced rate of degradation of the chlorinated solvents in the inoculated trees. Test well groundwater analyses demonstrated a marked decrease in concentration of TCE and its derivatives from the tree-associated groundwater plume. The concentration of TCE decreased from 300 μg/L upstream of the planted area to less than 5 μg/L downstream of the planted area. TCE derivatives were similarly removed with cis-1,2-dichloroethene decreasing from 160 μg/L to less than 5 μg/L and trans-1,2-dichloroethene decreasing from 3.1 μg/L to less than 0.5 μg/L downstream of the planted trees. 1,1-dichloroethene and vinyl chloride both decreased from 6.8 and 0.77 μg/L, respectively, to below the reporting limit of 0.5 μg/L providing strong evidence of the ability of the endophytic inoculated trees to effectively remove TCE from affected groundwater. The combination of native pollutant-degrading endophytic bacteria and fast-growing poplar tree systems offers a readily deployable, cost-effective approach for the degradation of TCE, and may help mitigate potential transfer up the food chain, volatilization to the atmosphere, as well as direct phytotoxic impacts to plants used in this type of phytoremediation.