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

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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 Kd value). If, as found previously for Cd and Zn in sediments, R ≥ 0.95, the response time (Tc) of the (de)sorption process must be ≤0.8 s and Kd (the distribution coefficient between solid and dissolved metal) must be ≥1.1 × 105 cm3 g−1. For any measured value of R, Tc can be estimated either precisely or within limits, depending on what is known about Kd. Published DGT measurements for Cu and Fe lead us to estimate response times for the sorption process of 30 mins and 19 mins. If Kd is known precisely, the apparent 1st 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.

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). ∂Cd∂t=D∇2Cd2Cd 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.

References (29)

  • R.A. Berner

    Early Diagenesis

    (1980)
  • C.L. Bielders et al.

    Particle density of volcanic soils as measured with a gas pycnometer

    Soil Sci. Soc. Amer. J.

    (1990)
  • W. Davison et al.

    In situ speciation measurements of trace components in natural-waters using thin-film gels

    Nature

    (1994)
  • W. Davison et al.

    Performance-characteristics of gel probes used for measuring the chemistry of pore waters

    Environ. Sci. Technol.

    (1994)
  • Cited by (0)

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