Original ArticlesKinetics of metal exchange between solids and solutions in sediments and soils interpreted from DGT measured fluxes
Introduction
There is a large reservoir of trace-metals in the sediments of aquatic systems. Remobilisation of these metals may be important in determining porewater concentrations and fluxes to overlying waters. When fresh material is recruited to surface sediments, metals may be released from decomposing organic matter or from the surfaces of iron and manganese oxides as they undergo reductive dissolution. Very steep concentration gradients or sharp concentration maxima may result in the porewaters (Zhang et al., 1995b). Other localised concentration maxima may be associated with organic decomposition occurring in microniches (Davison et al., 1997). When metals are released within sediments where there is no convection they are transported diffusionally to a sink. The processes that remove dissolved metals to the solid phase include adsorption, absorption, and surface precipitation, often referred to collectively as sorption (Honeyman and Santschi, 1988). Similarly, if porewater concentrations are lowered, metals may be remobilised by desorption, the reverse processes.
The characteristic times for (de)sorption processes vary, between milliseconds (for surface complex formation) and weeks (for some sorption processes in natural systems; Honeyman and Santschi, 1988). Previous studies on the particle-water interaction of metals in natural aquatic systems Comber et al 1996, Jannasch et al 1988, Muller and Kester 1991, Nyffeler et al 1984 have calculated rate constants for some sorptive processes, but their experimental methodology has generally prevented them from investigating what appear to be significant processes (Jannasch et al., 1988) operating over timescales faster than a few minutes. Furthermore their results are commonly derived from dilute homogenised solutions of particles and may not reflect the processes in settled sediments. The calculated sorption rate constants from such studies are exponential functions of particle concentration Honeyman et al 1988, Honeyman and Santschi 1987.
Recently the technique of Diffusional Gradients in Thin-films (DGT) has been developed (Davison and Zhang, 1994) to enable the simultaneous in situ measurement of concentrations and fluxes of several metals in porewaters at high spatial resolution. DGT operates by inducing a controlled perturbation into the sediment, and the resultant measurements reflect the response of the sediment to that perturbation. DGT measurements have been interpreted to provide in situ information on labile metal species in seawater (Davison and Zhang, 1994); remobilisation fluxes and concentration profiles at high resolution (1 mm) in surficial freshwater sediments (Zhang et al., 1995b); ultra-high resolution (100 μm) profiles in microbial mats (Davison et al., 1997); and remobilisation fluxes in soils (Zhang et al., 1998).
A full description of the DGT procedure is given by Zhang and Davison (1995) and Zhang et al. (1995a). A typical DGT probe is depicted in Fig. 1 and comprises two layers of polyacrylamide gel: a diffusion gel layer and a resin-impregnated gel layer containing Chelex ion-exchange resin (the resin layer). These are placed on a plastic backing plate with the resin layer in contact with the plate. On top of these are placed a filter and finally a plastic front plate with an exposure window. During deployment dissolved metal in the porewater diffuses through the filter and gel diffusion layer. On contacting the resin layer the metal is removed from solution by binding to the resin. This sets up a concentration gradient in the diffusion layer which determines the rate of accumulation of metal in the resin. After deployment, typically for 1 day, the filter and gel diffusion layer are discarded and the mass of metal in the resin layer determined. The quantity measured directly is, therefore, the mass of metal accumulated per unit area of the resin; this may be divided by the deployment time to give a time averaged flux to the resin from the porewater.
The theoretical interpretation of DGT measurements as porewater concentrations relies on several assumptions including a rapid resupply from solid phase to solution. Where these conditions do not hold the interpretation has been limited to describing the areal flux to the DGT assembly as a remobilisation flux from solid to solution phase. However, remobilisation fluxes are more sensibly expressed volumetrically so that they can be related to the mass of the solid phase. The current theory for DGT cannot do this. In this paper the limiting conditions of the DGT theory required to calculate concentrations are assessed, and the theory is further developed to enable the quantitative interpretation of areal fluxes in terms of the kinetics of transfer from solid phase to solution. Previously reported DGT measurements are used to illustrate the application of the developed theory.
Section snippets
Modelling approaches to investigating DGT performance
The experimental performance of DGT probes in stirred solutions has largely been assessed (Zhang and Davison, 1995). However, the validity of the associated theory for deployment in sediments or soils depends on transport within both the DGT assembly and the porewaters as well as interactions with the sediment or soil. To describe these processes quantitatively requires a numerical model, which can be used to investigate the in situ operation of DGT. The model must be dynamic, to describe the
Principles of DGT
Figure 2 illustrates the general principles of DGT operation at pseudo steady-state. It is not meant to suggest any specific type of deployment. The Chelex resin within the resin layer binds the metal that contacts that layer. This creates a concentration gradient between the resin layer and the sediment porewater and causes metal to diffuse from the porewater through the diffusion layer, to the resin layer, where it is removed from solution. Thus the DGT assembly is continually supplied by a
Model description
The DGT induced fluxes in sediments model (DIFS) consists essentially of three compartments: the resin; the dissolved phase; and the sorbed phase. The relationship between these is shown in Fig 3. Transport in the dissolved phase is assumed to be by molecular diffusion alone and obeys a modified version of Fick’s 2nd Law of Diffusion (Berner, 1980) where porosity is constant (Eqn. 5). ∇2Cd is the second spatial derivative of the dissolved concentration Cd (mol cm−3) in the
Results
Previously (Zhang et al., 1995b) DGT modelling has been limited to 1D (effectively the domain comprises the horizontal axis in Fig. 2). In general, a 2D domain better represents an in situ DGT deployment. The greatest difference occurs where there is effectively no resupply (k1 = k−1 = 0). This case was modelled in 1D by Zhang et al. (1995b) and after 24 h deployment the ratio of DGT estimated to initial porewater concentration was 0.06, whereas our 2D approach yields a ratio of 0.10. However,
Conclusions
Our interpretations of DGT measurements in sediments require that the simple model chosen describes satisfactorily the interaction between sediment solid phase and porewater. The sediment should be at a sorptive equilibrium, adequately described by Kd, prior to DGT deployment. This is most likely to be the case for relatively uncontaminated sediments and soils where there is only a low occupancy of binding sites. Incorporating only one sorption process may not provide the best description of
Acknowledgements
NERC and BBSRC provided financial support. This paper was improved significantly by the perceptive comments of the three anonymous referees.
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