Aquatic colloids relevant to radionuclide migration: characterization by size fractionation and ICP-mass spectrometric detection

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Abstract

The application of the flow-field flow fractionation (FFFF) combined with on-line ICP-mass spectrometry (ICP-MS) to the characterization of aquatic colloids is described. The capabilities and drawbacks of the technique are discussed with the aid of two examples. (1) The size distribution of smectitic colloids dispersed from a natural bentonite is determined by FFFF and laser light scattering (LLS) and compared with the size information obtained by ICP-MS detection. Due to the pronounced size dependency of the scatter light intensity, the LLS detection tends to overestimate the larger sized particles. Therefore, the FFFF-ICP-MS fractogram delivers more reliable size information. (2) Groundwater humic/fulvic colloids and the humic matter extracted from the corresponding sediment, both taken from the Gorleben aquifer (Lower Saxony, Northern Germany), are analysed by FFFF-ICP-MS. The location of REE, U, Th and Ca in different colloid size fractions appears to be very similar in both samples. The element specific fractograms suggest in agreement with earlier studies the location of Th and the REE mainly in inorganic colloids>17 nm containing also Fe and/or Al. U and Ca appear to be distributed between larger colloids and the fulvic/humic acid fraction with a size <3 nm. The results demonstrate that even in groundwater/sediment systems with high humic/fulvic content, inorganic colloids may play an important role as carrier for polyvalent metal ions. The consequences of this finding on the applicability of in-situ Kd values for U, Th and REE, taken as naturally abundant representatives of the nuclear waste derived actinides, for the assessment of laboratory sorption data is discussed.

Introduction

Interaction with aquatic colloids considerably alters the geochemical characteristics and mobility of radionuclides and trace metal ions in natural aquifer systems. From a number of investigations it is known that natural inorganic and organic colloids are able to strongly adsorb actinide ions and to transport them through porous and fractured media [1], [2]. The nature of radionuclide binding to colloids, the stability and the size of colloids are identified as important parameters determining the relevance of colloid-mediated radionuclide migration [3]. Appropriate characterization methods are required to obtain relevant information on aquatic colloid properties. The on-line combination of ICP-mass spectrometry with the flow field-flow fractionation (FFFF) [4], [5], [6], [7], [8] and the size exclusion chromatography (SEC) [9], [10], [11] are discussed as interesting methods for the characterization of natural colloids. The techniques do not only provide the colloid size distribution but also give an insight into the interaction of metal ions with different colloid size fractions.

The present paper describes two examples for the application of FFFF-ICP-MS: (1) the size distribution of smectitic colloids in a low mineralized granitic groundwater is studied by FFFF-ICP-MS. The size is compared with that obtained by using a static laser light scattering detector (LLS). The smectitic colloids are washed out from a natural bentonite and are assumed to play a role in the migration of radionuclides in the near field of a nuclear waste repository in granitic host rock at the bentonite barrier system/groundwater interface [12]. (2) The interaction of the trace metal ions U, Th and rare earth elements (REE) with humic colloids in a groundwater and humic matter extracted from the accessory sandy aquifer sediment is studied by FFFF-ICP-MS. The aim of the experiment is to decide if the solid/liquid distribution of the naturally abundant U, Th and REE (‘in-situ Kd’) is determined by complexation to humic matter in the sediment and humic/fulvic acid in the groundwater. It is discussed whether or not in-situ Kd-values can be taken to predict the sorption behaviour of nuclear waste derived actinides in such system at a long-term. In order to reflect the sorption behaviour of actinides, in-situ Kd values must be related to the trace elements in those phases, which are accessible for the interaction with groundwater dissolved species and not to those fractions contained in the bulk mineral phases [13], [14]. The ‘accessible’ phases in the sediment are estimated in the present work by using an extraction method. Solid humic matter is thought to be a very important component of the sediment with regard to metal ion sorption [15]. The alkaline extraction with a 0.1 M NaOH solution is frequently applied to extract the sedimentary humic matter [16]. Humic matter bound trace elements are co-extracted and thus can be determined by element analysis of the extract. Groundwater and humic matter rich sediment samples are taken from the Gorleben site (Lower Saxony, Northern Germany).

Section snippets

Samples

The groundwater is taken from the Gorleben aquifer (Gohy-2227). More detailed information on the anaerobic sampling conditions as well as the chemical and physico-chemical properties of these groundwaters can be found in Ref. [17]. The composition of the groundwater is given in Table 1. The corresponding sediment has been sampled from a bore core and stored under ambient conditions. The sandy sediment originates from 128 m depth, i.e. from the level where the Gohy-2227 groundwater has been

Size characterization of smectitic colloids

The results of the fractionation of the smectite dispersions with a solid content of 3.8 g l−1 is plotted in Fig. 1. According to the structural formula of the investigated bentonite ([Si7.66Al0.34][Al2.68Fe0.34Mg0.91]X0.81O20(OH)4) (X: Halogenide) [20] Si, Al, and Fe are the main components of the clay. However, only Mg, Al and Fe can be detected by FFFF-ICP-MS with the identical shape of the fractograms. The Si-signal is heavily affected by the high background in the effluent due to bleeding

Acknowledgements

The support of the Adenauer foundation by granting a Ph.D. stipend for T. Ngo Manh is gratefully acknowledged. The authors would like to thank Mr F.W. Geyer for assistance with the ICP-MS measurements, Mrs T. Kisely for her help in performing the laboratory work and the carbon analysis, Dr B. Luckscheiter for the XRD and Mrs S. Rabung for the XRF analysis.

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