Kinetics of metal exchange between solids and solutions in sediments and soils interpreted from DGT measured fluxes

Abstract

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-films (DGT) is an in situ technique which can be used to measure porewater concentrations and remobilisation fluxes of trace-metals, in sediments and soils. The dynamics of the sediment/DGT system were investigated using two dimensional modelling to ensure the correct interpretation of DGT measured fluxes, investigate the kinetics of the resupply from metal sorbed to particles, and estimate the magnitude of the resupply from particles to porewater in volumetric terms. When porewater concentrations adjacent to the DGT device are maintained by fast resupply from a large reservoir of metal sorbed to the solid phase (the sustained case), DGT measurements can be interpreted directly as porewater concentrations. When there is significant resupply from the solid phase, DGT can be used to measure kinetic parameters. If porewater concentrations are measured independently by an alternative technique, DGT measurements can be expressed in terms of a ratio R of DGT estimated to actual porewater concentration (0 < R < 1). Our model predicts a relationship between R, the kinetics of the resupply process, and the available reservoir of sorbed metal (expressed as a K d value). If, as found previously for Cd and Zn in sediments, R ≥ 0.95, the response time (T c) of the (de)sorption process must be ≤0.8 s and K d (the distribution coefficient between solid and dissolved metal) must be ≥1.1 × 10 5 cm 3 g −1. For any measured value of R, T c can be estimated either precisely or within limits, depending on what is known about K d. Published 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 precisely, the apparent 1 st order rate constants for the sorption process can be determined. Multiple DGT deployments with varying diffusion layer thicknesses can be used to estimate porewater concentrations. The DGT device depletes the reservoir of available metal sorbed to the solid phase. This depletion decreases with distance from the device. A simple relationship was developed to estimate, from the DGT measured flux, the mass of metal released from unit volume of particles.