Introduction

Ion exchange reactions at mineral electrolyte interfaces belong to the most important processes that control the distribution of toxic metal ions between liquid and solid phases in the environment. While the equilibrium properties (adsorption isotherms, adsorption edges, equilibrium constants, etc.) have been widely investigated, less information is available on the kinetics of such processes. Most work in the latter field has been focused on the kinetics in the slow time domain (minutes to days). In general, the processes observed here are controlled by diffusion in the macropores of highly aggregated or compacted colloid particles or in concentrated suspensions [1—4], The reactions observed are slow because of long diffusion paths. However, many authors report on fast reactions, whose equilibria are established instantaneous ly compared to their accessible time domain. These fast reactions at the solid — electrolyte interface with half lives of less than 1 min have hardly been examined. By use of the pressure jump [5] and the stopped flow techniques [6] it was possible to investigate fast ion exchange processes at clay minerals. However, only few investigations have been performed on the fast exchange kinetics of heavy metals with layer-silicates in suspension.

Compared to reactions in solution, the interpretation of kinetic data at solid-electrolyte interfaces is often difficult because of the heterogenity of suspended particles. In general, the kinetic data recorded (concentration as a function of time) are interpreted by use of a special model like first- and second order kinetics, Elovich's equation or a ]/Tlaw [1], These models imply some special experimental constraints, e.g., a limited concentration and time range. In Elovich's equation an exponentially de-

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Elovich's Eq. __ experimental data 1st order

a: affinity spectrum a: affinity spectrum c o

log Keq or ( AG01 kJ mol"1)

b: kinetic spectrum

Fig. 1 Amount exchanged of Cd2+, n, ([Cd2+]total: 1.5 X 10-5 mol dm~3) at Mg2+-montmorillonite (0.05 g dm-3). Crosses are experimental data, the lines are least-square-fits with Elovich's equation and the pseudo-first-order rate law, respectively creasing affinity is assumed that depends on the degree of occupancy of surface binding sites, and the j/Tlaw requires a uniform space geometry, i.e., pores of equal shape and size. In reality, those conditions are rarely met. As an example, Fig. 1 shows the reaction of 1.5 X 10~5 mol dm"3 Cd2+ with 50 mg dm"3 Mg2+-montmorillonite (2.4 x 10"5 mol exchange sites per dm3) at pH = 6.7 and T = 25 °C. Crosses are experimental data (see below) and the lines show the best fits of Elovich's equation and a pseudo-first-order reaction, respectively. It can be clearly seen that in both cases the experimental data cannot be described correctly. The interpretation with Elovich's equation shows greater deviations at longer times, whereas the first-order law shows systematic deviations over the whole time range. Other fitting attempts like a second-order reaction and the j/Raw also fail, but are not shown here.

In this work a method of data evaluation is applied that does not require an a priori choice of a definite model. In 1983, Shuman and Olsson proposed to apply the so-called kinetic spectrum method on the reactions of metals with dissolved humic material [7], The basic idea is that binding sites on a homologous complexant [8] (like humic acid or suspended colloid particles) are heterogeneous. The system should not be described with a definite, discrete affinity (expressed by an equilibrium constant), but rather with a spectrum of affinities. That means at the surface of the homologous complexant there exists as very large number of exchange sites and each of them has its own affinity to a certain metal ion. The contribution of each site to the total occupancy can be plotted versus its affinity, expressed as the logarithm of its specific equilibrium constant or Gibb's function. This plot is call c o 3

b: kinetic spectrum c o 3

Fig. 2 Schematic drawing of the affinity spectrum (a) and the corresponding kinetic spectrum (b) for an arbitrary system log (k/s1)

Fig. 2 Schematic drawing of the affinity spectrum (a) and the corresponding kinetic spectrum (b) for an arbitrary system ed the affinity spectrum. As an example, Fig. 2a shows such an affinity spectrum for an arbitrary system[8].

For the kinetics in such a system this means each site should be described by its own rate constant, kt. This rate constant implies, for example, its special energy of activation or its accessibility in porous media. Figure 2b shows the kinetic spectrum of an arbitrary system. The changes of this spectrum can be investigated as a function of temperature, pH or concentrations to give further information and knowledge about a certain system. As an example, in this work the ion exchange reaction of Cd2+ with Mg2+-montmorillonite is investigated and interpreted for the first time by the kinetic spectrum method.

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