Phytoremediation Potential of Thlaspi caerulescens and Bladder Campion for Zinc‐ and Cadmium‐Contaminated Soil

Metal-tolerant hyperaccumulator plants may be useful to phytoremediate contaminated soils. To evaluate agronomic management practices to maximize phytoremediation, two metallophytes, Thlaspi caerulescens J. and C. Presl (Zn hyperaccumulator) and bladder campion [Silene vulgaris (Moench) Garcke L. ] (an indicator) compared to ’Rutgers’ tomato (Lycopersicon esculentmn L.) in a pot study to assess Zn and Cd uptake patterns in relation to soil pH. Soils used for the study were gathered at three different sites in the vicinity of an old Zn smelter in Palmerton, PA, and contained 48 000, 4100, and 2100 mg kg-1 Zn and 1020, 37.4, and 35.2 mg kg-1 Cd, respec, tively. Each soil was adjusted to three pH levels ranging from 5.06 to 7.04. Thlaspi caerulescens showed much greater tolerance to the metals than the other plants (up to 18 455 mg kg1 Zn and 1020 mg kg~ Cd dry shoots without yield reduction) with metal stress apparent only in the low pH treatments of the two most contaminated soils. In all treatments except for the farm Soil (least contaminated) at 5.06, T. caerulescens had higher concentrations of both Zn and Cd than bladder campion and tomato. Thlaspi caerulescens was also more effective at translocating both Zn and Cd from soil to plant shoots. A variety of soil extractions were used to evaluate the correlation of shoot metal concentrations with quantitative measures of "available" soil metals. Concentrations f Cd measured in several common extractants (DTPA, water, 0.01 M Ca(NO3)z, and 1.0 M NH~NO3) significantly correlated with Cd concentrations i tissue of each plant. Shoot Zn concentrations of bladder campion and tomato were significantly correlated with Zn extracted by the neutral salt ext~’actants for all soils. For T. caerulescens, the neutral salt extractable Zn was significantly correlated with shoot Zn only in the two more contaminated soils. No extractant predicted shoot Zn concentration for T. caerulescens in the least contaminated soil. p ROBLEMS associated with the heavy metal contamination of soils have been well documented (Takijima and Katsumi, 1973; Beyer, 1988; Simms and Morgan, 1988). In particular, excess Cd and Zn in soil have been the cause of human health effects, animal fatalities, and the disruption of natural ecosystems. Although contamination can be geochemically derived (calamine and other mineral-rich soils), soil contamination is often the unintentional result of anthropogenic activities. Although the consequences of contamination are becoming better understood, effective and economic technologies for remediation of these sites remain elusive. It has been suggested that highly specialized plants that have evolved the ability to accumulate and tolerate very high concentrations of metals from soils may provide the basis for remediation of these sites (Chaney, 1983; Baker et al., 1991). Hyperaccumulator species have been found that contain >10000 mg kg-1 Zn and S.L. Brown and J.S. Angle, Dep. of Agronomy, Univ. of Maryland, College Park, MD 20742; R.L. Chaney, Environmental Chemistry Lab, USDA-ARS, Beltsville, MD 20705; and A.J.M. Baker, Dep. of Animal and Plant Sciences, Univ. of Sheffield, Sheffield, S10 2UQ UK. Received 27 Jan. 1993. *Corresponding author. Published in J. Environ. Qual. 23:1151-1157 (1994). >100 mg kg-~ Cd shoot biomass (Baker and Brooks, 1989; Baker et al., 1994). By successive croppings of these plants, metals could be translocated from soil to shoot tissue. Plant tissue could then be harvested and smelted to recycle soil metals. However, before phytoremediation can be effectively exploited on contaminated sites, a better understanding of metal uptake by hyperaccumulator species is necessary. Thlaspi caerulescens has been identified as a Zn and Ni hyperaccumulator (Rascio, 1977; Reeves and Brooks, 1983; Reeves et al., 1983; Reeves and Baker, 1984; Ingrouille and Smirnoff, 1986). Bladder campion has ecotypes that are able to tolerate a variety of metals and falls under the indicator plant classification (Baker et al., 1983; Verkleij et al., 1990; Schat and Klaff, 1992). Although bladder campion is not a suitable candidate for phytoremediation, its increased Zn and Cd tolerance makes it a suitable species for recolonization and stabilization of contaminated sites. Much of the previous works on T. caerulescens has involved the collection of field samples and measurement of metal concentration in aboveground dry matter. Recently, Baker et al. (1994) found mean.shoot Zn and Cd concentrations as high as 21 000 and 164 mg kg -1, respectively, for plants growing on metalliferous mine wastes at sites in the Southern Pennines, UK. Extractable metals were not correlated with metals in plant shoots. Reeves and Brooks (1983) tested the Zn concentrations of a variety of herbarium specimens from "calcareous, schistose, serpentinitic, siliceous, porphyritic, argillaceous, gneissic and granitic" soils. While all plants had >1000 mg kg-1 Zn, no relationship between soil Zn and plant Zn was described. For phytoremediation to be economically viable, optimum management practices must be identified. To this end, it is desirable to identify quantitative relationships between total and/or extractable soil metals with plant metal uptake. Numerous tudies have attempted to define such a relationship for economically important plants grown on sewage sludge-amended soils. Past studies have also examined the behavior and phytoavailability of heavy metals in soil systems. Solubility of Zn and Cd in a soil system will largely be controlled by pH and the extent of contamination in relation to the specific adsorption capacity of the soil. Specific adsorption (adsorption in the presence of 0.01 M Ca) of heavy metals by oxide minerals and organic matter has been well documented (Tiller et al., 1984; McBride, 1989): Metal adsorption in soils is strongly pH dependent (McBride and Blasik, 1979; Sanders, 1983). Solubility is also function of the nature of the metal, and the length of equilibration time between soil and metal (Adams and Sanders, 1985; Korcak and Fanning, 1985; Bell et al., 1991). Patterns of uptake for hyperaccumulator plants may differ from conventional economic species. High shoot