Assessment of labilities of metal complexes with the dynamic ion exchange technique

Federico Quattrini,Carlos Rey-Castro,Josep Galceran,Jaume Puy,Claude Fortin

doi:10.1071/en18202

Federico Quattrini, Carlos Rey-Castro + Show 3 more

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https://doi.org/10.1071/en18202

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Abstract

Environmental contextIn natural waters, the impact of metals on biota is modulated by their binding with ligands. Ion-exchange techniques can provide information about metal-ligand complexes in solution, which can be linked to metal bioavailability in natural waters. We investigate modelling approaches to interpreting data from ion-exchange experiments to help elucidate the contribution of a particular complex to the overall metal uptake. AbstractThe dynamic ion exchange technique (DIET) is proposed to provide speciation information, which can be used to establish links with metal bioavailability in natural waters. The experimental setup consists of a few milligrams of a sulfonic acid type ion exchange resin packed in a plastic microcolumn that is coupled to a peristaltic pump for a sample to interact with the resin which is subsequently eluted. The evolution of both the accumulated number of moles in the resin and the concentration of the effluent can provide information on the dissociation of different metal-ligand complexes when compared with the transport properties. This information can be converted into the lability degree of a given complex or the DIET concentration cDIET, which accounts for the labile fraction contributing to the metal accumulation by the resin column at the operation conditions. cDIET can be extended to columns containing chelating resins (such as those with Chelex) or to chromatography. A comprehensive modelling of the involved phenomena (such as diffusion, advection, reaction kinetics and electrostatic partitioning) leads to the quantitative interpretation of the accumulation time series (accumulation curves) or effluent evolution (breakthrough curves). Particularly simple analytical expressions can be used for short exposure times, when a (quasi) steady-state is attained. These models have been checked against the results from complexes of Cu and Ni with ligands, such as ethylenediamine, and ethylenediaminetetraacetic, iminodiacetic, glutamic, salicylic, malonic and malic acids, which yield complexes with contrasting charges. Caution is advised when estimating the free metal fraction from DIET measurements, as cDIET and the free metal concentration can be considered to be equal only in the case of extremely inert complexes.

Highlights

Metal speciation provides crucial information for current paradigms of ecotoxicity (such as the Free Ion Activity (Anderson et al 1978) and Biotic Ligand (Paquin et al 2002) Models), and several techniques have been developed along the years for this purpose
The evolution of both, the accumulated moles in the resin and the concentration of the effluent, can provide information on the dissociation of different metal-ligand complexes in comparison with transport properties. This information can be converted into the lability degree of a given complex or the Dynamic Ion Exchange Technique (DIET) concentration cDIET, which accounts for the labile fraction contributing to the metal accumulation by the resin column at the operation conditions. cDIET can be extended to columns containing chelating resins or to chromatography
Ion Exchange Technique (IET) is a column equilibration technique that consists of an accumulation step, where the sample solution is flushed through a column packed with a sulphonic acid type resin until equilibrium is attained

Summary

Introduction

Metal speciation provides crucial information for current paradigms of ecotoxicity (such as the Free Ion Activity (Anderson et al 1978) and Biotic Ligand (Paquin et al 2002) Models), and several techniques have been developed along the years for this purpose. Recent work (Leguay et al 2016;Nduwayezu et al 2016;Rowell et al 2018) has considered the possibility of extracting meaningful information from the time-resolved accumulations in IET columns (that we label as “accumulation curves”) In this new approach, called Dynamic Ion Exchange Technique (DIET), the moles of metal accumulated by the resin (nacc) is recorded as a function of contact time, before equilibrium is reached (see Fig 1a). The initial slope of the accumulation curve (labelled as “accumulation rate”, Racc) has been assumed to be related to the free metal concentration in the sample via an empirical relationship (Nduwayezu et al 2016) In those cases where the effluent concentration is above the quantification limit of the analytical technique, the breakthrough curves (see Fig 1c) can be recorded, to obtain additional information.

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