Environ. Sci. Technol. 1999, 33, 4528-4531
Sedimentation Field-Flow Fractionation Coupled Online to Inductively Coupled Plasma Mass SpectrometrysNew Possibilities for Studies of Trace Metal Adsorption onto Natural Colloids MARTIN HASSELLO ¨ V , * ,† B E N N Y L Y V EÄ N , † A N D R O N A L D B E C K E T T ‡ Analytical and Marine Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden, and CRC for Freshwater Ecology, Water Studies Centre and Department of Chemistry, Monash University, Melbourne, Australia
This work presents a new approach for studies of trace metal adsorption processes onto natural particles by coupling of a sedimentation field-flow fractionation system on-line to an inductively coupled plasma mass spectrometer (ICPMS). This enables size dependent separation of clay sediment particles with continuous determination of elemental composition in the different size fractions. Major elements Al, Si, Fe, and Mn were determined simultaneously together with the trace elements Cs, Cd, Cu, Pb, Zn, and La. Adsorption experiments where different metal loadings have been added to the particle populations under different pH conditions are also presented. The aim of these experiments was to distinguish between weaker and stronger binding sites as well as between surface complexation adsorption and ion-exchange mechanisms.
Introduction The chemical speciation of trace metals in natural waters is receiving increasing attention owing to the need to understand and predict the transport, cycling, and fate of metals in aquatic systems (1). Interest is mainly focused on toxic and radioactive pollutant metals due to their environmental impact, the aim being to gather enough experimental data for characterization of colloids (2, 3) and of their metal binding properties, to enable more accurate modeling of adsorption and transport of the metals (4-6). A key aspect of metal speciation that is poorly understood is the distribution of metals between the dissolved, colloidal, and particulate phases (1, 7-9). To obtain a clearer understanding of the uptake process, much effort has been devoted to laboratory studies of metal adsorption and desorption processes. Sorption experiments on sediments, pure clays, or pure metal oxides are generally performed as batch experiments where metals are added, a reaction time is allowed, and particulate and dissolved phases are separated (10, 11). A combination of these measurements with studies of the acid-base behavior of colloid surfaces led to the development of surface complexation theories for * Corresponding author phone: +46-31-772 2286; fax: +46-31772 2785; e-mail:
[email protected]. † Go ¨ teborg University. ‡ Monash University. 4528
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 24, 1999
trace metal uptake, which provide a convincing description of uptake onto pure solid phases (7). However, natural colloids do not show such well-defined adsorption edges as pure phases, and there is often significant uptake of certain elements at low pH. This cannot be explained by surface complexation mechanisms but may be due to ion exchange processes. In addition, it is known that natural colloids also include organic material, part of which has been shown to form an organic coating on natural inorganic colloids (12, 13) and thereby significantly alter the surface properties of the colloids (14, 15). Conventional procedures have involved relatively coarse methods of determining which size fraction the metals are associated with (e.g. filtering, centrifugation, cross-flow ultrafiltration) (3, 4, 16). In this work a different approach has been adopted to determine the extent and potentially the mechanisms of metal adsorption at much higher size resolution using field-flow fractionation (FFF) online coupled to inductively coupled plasma mass spectrometry (ICPMS). Field-flow fractionation is a family of separation techniques in which an applied field forces the sample colloids toward the wall of a thin channel along which there is a laminar fluid flow. This field force is counteracted by the concentrated driven diffusion of the colloids, and consequently the colloid concentration is exponentially decreased by the distance from the accumulation wall with a mean cloud thickness dependent on the properties of the field driving the flux of colloids. The colloids with the mean cloud thickness furthest away from the wall are experiencing the highest axial flow rate and are eluted first. The major subtechniques are sedimentation (SedFFF), flow, and thermal FFF according to the applied field: these methods are described in detail together with the relevant theory in papers by Giddings (17, 18). In this work sedimentation FFF has been used since it has the highest resolution and an operating range most appropriate for sediment particles. SedFFF is separating colloids according to their buoyant mass, and with a known colloid density the concentration as a function of the equivalent spherical diameter can be measured. SedFFF thus yields the size distribution of the colloids, and by coupling the FFF system to an ICPMS, continuous determinations of a variety of metals as a function of size are possible. Previous reports of this combination of techniques have largely concerned offline applications with determination of major elements (e.g. Si, Al, Fe, Mg) in soil and riverine colloids (19, 20): trace element measurements have been reported only for Rb (21). The aim of the work was to explore the application of coupled FFF-ICPMS to studies of trace metal adsorption processes. Adsorption experiments with different amounts of metal have been carried out in order to find out whether the adsorption first occurs at a certain size range with high affinity sites and thereafter on other size fractions with weaker binding sites. Furthermore, experiments at different pH were conducted in order to distinguish between surface complexation (pH dependent) and ion exchange adsorption (pH independent). Measurements were carried out on the colloidal fraction (
