KINETICS OF METAL EXCHANGE BETWEEN SOLIDS AND SOLUTIONS IN SEDIMENTS AND SOILS INTERPRETED FROM DGT MEASURED FLUXES

Our understanding of geochemical processes in sediments and soils has been limited by a lack of simple procedures to measure the kinetics of transfer from solid phase to solution. Diffusive Gradients in Thin-fi lms (DGT) is an in situ technique which can be used to measure porewat er concentrations and remobilisation fluxes of trace-metals, in sedime nts and soils. The dynamics of the sediment/DGT system were investigat ed using two dimensional modelling to ensure the correct interpretatio n of DGT measured fluxes, investigate the kinetics of the resupply fro m metal sorbed to particles, and estimate the magnitude of the resuppl y from particles to porewater in volumetric terms. When porewater conc entrations adjacent to the DGT device are maintained by fast resupply from a large reservoir of metal sorbed to the solid phase (the sustain ed case), DGT measurements can be interpreted directly as porewater co ncentrations. When there is significant resupply from the solid phase, DGT can be used to measure kinetic parameters. If porewater concentra tions are measured independently by an alternative technique, DGT meas urements can be expressed in terms of a ratio R of DGT estimated to ac tual porewater concentration (0 < R < 1). Our model predicts a relatio nship between R, the kinetics of the resupply process, and the availab le reservoir of sorbed metal (expressed as a K-d value). If, as found previously for Cd and Zn in sediments, R greater than or equal to 0.95 , the response time (T-c) of the (de)sorption process must be less tha n or equal to 0.8 s and K-d (the distribution coefficient between soli d and dissolved metal) must be greater than or equal to 1.1 X 10(5) cm (3) g(-1). For any measured value of R, T-c can be estimated either pr ecisely or within Limits, depending on what is known about K-d. Publis hed DGT measurements for Cu and Fe lead us to estimate response times for the sorption process of 30 mins and 19 mins. If K-d is known preci sely, the apparent 1(st) order rate constants for the sorption process can be determined. Multiple DGT deployments with varying diffusion la yer thicknesses can be used to estimate porewater concentrations. The DGT device depletes the reservoir of available metal sorbed to the sol id phase. This depletion decreases with distance from the device. A si mple relationship was developed to estimate, from the DGT measured flu x, the mass of metal released from unit volume of particles. Copyright (C) 1998 Elsevier Science Ltd.