Principles In Soils And Sediments

The theory for measurements by DGT in solution is presented in Chapter 11 of this book [25]. The complications introduced there, with respect to the effects of gel pore size, pH, ionic strength and the measurement of labile species, apply to the use of DGT in sediments and soils. For simplicity they are not treated here where the focus is on the influence of the solid-phase adjacent to the device.

The key effects of the solid-phase were recognised in the first publication on the use of DGT in sediments [2]. The continual removal of metal from the pore-water to the resin sink induces a concentration gradient within the diffusion layer. If the only transport mechanism for solutes in sediment pore-waters is diffusion, the metal becomes depleted in the pore-waters adjacent to the device. This lowering of the solute concentration can mobilize metal from the solid-phase. Therefore, the mass of solute accumulated by the DGT device depends on the initial concentration in the pore-water, the rate of diffusional supply and the extent and rate of release of solute from the solid-phase. Rather than being a simple device for measuring solute concentrations in the bulk pore-water, DGT is best regarded as a tool for conducting in situ perturbation experiments by introducing a localized solute sink. The amount of solute that accumulates in a given time depends on the extent of the perturbation of the soil or sediment dynamics.

Harper et al. [13] have developed a full mathematical treatment of these processes. Initially, there is no concentration gradient at the surface of the binding layer, as the diffusion layer does not contain solute. The gradient increases as the diffusion layer is supplied with metal from the soil or sediment, with a linear gradient being established in the diffusion layer in a few minutes [2]. A typical profile of the concentration of solute, C, through the diffusion layer of thickness Dg and the pore-waters, at time t, is shown in Fig. 16.1.

The large concentration of binding agent with strong binding sites ensures that the concentration of solute at the surface of the binding layer is effectively zero. There is depletion of solute in the pore-waters, with the concentration at the interface between the DGT surface and the pore-waters, Ci, being less than the concentration in the bulk solution of the pore-waters, Csoln. The concentration gradient through the diffusion layer is linear, allowing calculation of the flux of solute, F(t),

Fig. 16.1. Schematic representation of the concentration gradient through a DGT device and the adjacent soil or sediment.

Fig. 16.1. Schematic representation of the concentration gradient through a DGT device and the adjacent soil or sediment.

mm mm towards the resin at this instance of time:

D is the effective diffusion coefficient of labile solute species in the diffusion layer. With increasing deployment time, solutes in the pore-water become progressively depleted, tending to lower Ci. The supply of solutes from the solid-phase to solution counteracts this depletion, slowing down the decline in Ci. A pseudo-steady-state can be achieved for relatively short (~day) deployment times, but the progressive depletion of solute associated with the solid-phase adjacent to the device ensures that Ci declines at longer deployment times. The concentration gradient in the diffusion layer, and consequently the flux, therefore changes with time. The total mass of solute accumulated per unit area of the DGT device, Ma, is given by integrating the flux over the deployment time (T):

Ma is the directly measured quantity that is obtained from a DGT deployment by eluting a known area of the binding gel and measuring the concentration in the eluent. The time-averaged interfacial concentration during the deployment time (CDGT) is given directly by

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