Assessment of metal availability in contaminated soils using chemical speciation (Donnan equilibrium), DGT and phytotoxicity/uptake by wheat

Abstract

Currently, regulations regarding the maximum permitted concentrations of metals in soils are based on measurements of the total concentrations. However, a range of chemical and biological techniques are being developed to predict the bioavailable component of these pollutants. Chemical measurements have focused either on soil solution chemistry, including free ion activities, or operationally defined fractions of metals. Recently, diffusive gradients in thin films (DGT) have been applied to soils. DGT measures an effective concentration, CE, which includes both the soil solution concentration and an additional term, expressed as a concentration, that represents metal supplied from the solid phase. In this study wheat (Triticum aestivum L.) was grown in a diverse range of long-term contaminated soils resulting from either industrial pollution or biosolid amendments. Metal tissue concentrations (copper, zinc, cadmium) and phytotoxicity (shoot weight) of the wheat will be compared to free metal activities in soil pore waters and CE concentrations. Free metal activities were determined using a Donnan dialysis technique that employs a cation exchange membrane and the principle of Donnan equilibrium to measure free hydrated Zn2+, Cd2+ and Cu2+ activity in soil pore water (Salam and Helmke, 1998). The method of Zhang and Davison (1995) was used to determine CE. References: Salam, A.K. and Helmke, P.A., 1998. The pH dependence of free ionic activities and total dissolved concentrations of copper and cadmium in soil solution. Geoderma, 83, 281-291; Zhang, H. and Davison, W. 1995. Performance characteristics of the technique of diffusion gradients in thin-films (DGT) for the measurement of trace metals in aqueous solution. Anal. Chem. 67, 3391-3400.