Cadmium and zinc bioaccumulation by Phytolacca americana from hydroponic media and contaminated soils

Hydroponic, greenhouse and field experiments were conducted to explore the potential of pokeweed (Phytolacca americana L.) to accumulate Zn and Cd from nutrient solutions and contaminated soils. The hydroponic results confirmed that this native species is a strong Zn and Cd bioaccumulator that does not experience severe phytotoxicity until quite high root and shoot concentrations, approaching 4000 and 1600 mg kg^-1 of Zn, and 1500 and 500 mg kg^-1 of Cd, respectively. These high Zn and Cd concentrations were accompanied by increased sulfur and lower manganese in both shoots and roots. However, in field and greenhouse trials with soils historically contaminated by a number of heavy metals including Zn and Cd, concentrations of Zn and Cd in shoots of P. americana reached concentrations less than 30% and 10%, respectively, of those achieved with hydroponics. The main constraint to phytoremediation of soils by P. americana was the low concentrations of Zn and Cd in soil solution. Pretreatment of the metal-contaminated soil by oxalic acid increased soluble Cd and Zn but failed to increase plant uptake of either metal, a possible result of higher solubility of competing metal ions (Cu, Mn) or low bioavailability of Cd and Zn-oxalate complexes.