664 Environmental Toxicology and Chemistry, Vol. 25, No. 3, pp. 664–670, 2006  2006 SETAC Printed in the USA 0730-7268/06 $12.00 + .00 KINETICS OF ZINC AND CADMIUM RELEASE IN FRESHLY CONTAMINATED SOILS HAO ZHANG,*² W ILLIAM DAVISON,² A NDY M. TYE,‡ NEIL M.J. CROUT,‡ and SCOTT D. YOUNG‡ ²Department of Environmental Science, Lancaster University, Lancaster LA1 4YQ, United Kingdom ‡School of Life and Environmental Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom ( Received 20 December 2004; Accepted 13 April 2005) Abstract—The kinetics of metal release from the solid phase to solution was measured on two sets of 14 freshly contaminated soils with diverse properties. From measurements of metal concentrations in extracted soil pore water, the amount accumulated from the soil by diffusive gradients in thin-film (DGT) devices, and the distribution coefficient for labile metal, K dl , estimated by isotopic exchange, we calculated the response time, T c , of the soil-solution system to the removal of metal by DGT and the rate constant for release from the solid phase, k -1 . Resupply was so fast for Zn that T c (and k -1 ) could be measured only in three of the soils, with either a silty or a sandy loam texture and low to intermediate pH (4.84–5.66). In only six clay soils was resupply of Cd too fast to measure. The generally slower release rates of Cd compared to Zn may reflect the 100-fold lower concentration of Cd, which allowed a greater proportion of it to occupy stronger binding sites with slower release rates. The rate constants derived indicate that supply from the solid phase to solution will not limit uptake of Cd or Zn by plants in clay soils, but it could be a factor in sandy or silty soils with a low pH. These findings suggest that risk assessment of clay soils could be undertaken using measurements of metals in soil solution. However, devices such as DGT, which respond to the kinetics of supply, are necessaryto assess available metal in low pH, sandy, and silty soils. Keywords—Trace metals Kinetics Soil Diffusion Fluxes INTRODUCTION It is now generally accepted that metals are supplied to plant roots from the soil solution [1]. The soil solution is resupplied from the solid phase as the metal concentration is depleted adjacent to the root, according to established models of nutrient uptake [2]. If the supply from the solid phase to solution is kinetically limited, then this may be one of the regulatory factors influencing the amount of metal taken up by the plant. Determination of the rates of metal transfer from the solid phase to solution, on a time scale appropriate to the transfer that occurs in the rhizosphere, provides the basic in- formation needed to appreciate the potential role of this kinetic control. Such understanding is necessary to determine which techniques for measuring metals are appropriate for risk as- sessment. If supply from solid phase to solution is not kinet- ically limited, techniques that estimate the concentration in soil solution should provide a reliable estimate of the metal available to plants. Where kinetic control exists, other ap- proaches that can accommodate this factor may be more ap- propriate for risk assessment. The technique of diffusive gradients in thin films (DGT) continuously removes metal to a resin sink after it has diffused through a gel of well-defined thickness [3]. It mimics the re- moval of metals that occurs at the root–soil interface, inducing metal supply from solid phase to solution on a time scale appropriate to uptake of metals by plants [4]. Several studies have demonstrated good relationships between metal concen- trations in plants and metal accumulated by DGT devices [5– 10]. * To whom correspondence may be addressed (h.zhang@lancaster.ac.uk). Presented at the Symposium on Risk Assessment of Metals in Soils, 14th Annual Meeting, SETAC Europe Meeting, Prague, Czech Republic, April 18–22, 2004. A dynamic, numerical model of metal transfer from the soil system to the DGT device has been developed [11,12]. Known as DGT-induced fluxes in soils (DIFS), it represents exchange of metal between solid phase and solution using first-order rate equations, with the equilibrium partition between the two phas- es described by a distribution coefficient for labile metal, K dl . Measurements of the ratio (R) of the DGT-measured concen- tration to the concentration of metal in the soil solution allow calculation of kinetic parameters and/or K dl . Ernstberger et al. [13] showed that K dl and the kinetic parameters could be de- rived simultaneously from model fits of measurements made by DGT for a range of deployment times. Extension of this approach to five soils that had been incubated for three years after amendment with metals showed that the derived values of K dl for Cd were similar to those measured by isotopic ex- change [14]. For Zn they were similar to values derived from total soil concentrations. When Nowack et al. [8] used the same approach for Cu and Zn in field and homogenized soils, the quality of the fits was more variable, suggesting that several pools of metal may exist, each with different kinetic param- eters. Measurements of R at a single time (usually 24 h) have been used with various estimations of distribution coefficients to calculate kinetic parameters for Zn [7] and As [15]. In all this work, the range of soils studied has been limited, pre- venting interpretation of the kinetic parameters in terms of soil properties. This study reports kinetic data derived from isotopic ex- change measurements of the K dl for Cd and Zn and DGT mea- surements for 14 metal-amended soils that represent a range of textures, pH (4.84–7.05), and organic carbon (1.0–7.1%). Measurements on subsamples of soils that had been either air- dried or maintained moist after sampling provided some as- sessment of the dependency of the kinetics on treatment pro- cesses.