The Chemical Speciation of Zn in the Sediments of a Lake Impacted by a Zn Smelter.

Jean-Francois Gaillard*, Samuel M. Webb (Department of Civil Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208-3109, USA, email: jf-gaillard@northwestern.edu), and Gary G. Leppard (National Water Research Institute, CCIW, Burlington, Ontario, L7R 4A6, CANADA)

Abstract:

We investigated the chemical speciation of Zn in the sediments of a backwater lake on the Illinois River (Lake DePue) that has been contaminated by a smelter and a fertilizer plant using Analytical Electron Microscopy and X-Ray Absorption Spectroscopy. These techniques allow us to perform direct analyses, with very little perturbation of the samples.

Zinc bearing particles were characterized by different morphologies ranging from near spherical particles (i.e., a few 100 nm), to small colloids intimately associated with biological templates or present as separate amorphous entities. The elemental analysis of individual particles by X-Ray Energy Dispersive Spectrometry revealed the pervasive presence of Zn through the aquatic environment, and its intimate combination with Fe and P in biotic structures, and with S in small amorphous colloids. Q-XAS measurements were used to quantify the various proportions of the different coordination environment of Zn along a contamination gradient. Close to the source, Zn is bound to relatively labile species, such as Zn-PO₄ and ZnCO₃, whereas afar it is predominantly coordinated with sulfides. This suggests that microbial communities, that are driving early diagenetic reactions, have found an efficient way of coping with metal induced stress.

Introduction:

To understand the fate of metals in aquatic systems it is central to determine their chemical speciation. Often, this speciation is determined by means of selective extraction procedures or by performing equilibrium calculations coupled to appropriate surface complexation models (Westall et al., 1976). However, these approaches can be misleading because wet chemical methods are operationally defined, and natural systems are quite frequently out of equilibrium (Brezonik, 1994). In addition, numerous biological systems have evolved a wide variety of mechanisms to respond to metal stress, some of which involving a re-speciation of metals (Silver, 1997). At present we have a relatively poor knowledge of these processes, and therefore, there is a need to develop speciation schemes to determine directly the various chemical and physical forms of the metals in the environment.

Our approach relies on two complementary techniques: analytical electron microscopy (AEM) and X-ray absorption spectroscopy (XAS). These tools are particularly well suited to investigate the speciation of metals in complex matrices since they rely on the direct observations of environmental samples. As a consequence, we believe that they are less biased than wet chemical extraction procedures. For example, AEM of particles carefully embedded in a hydrophilic resin reveals the intimate and original structure of the sample, and elemental associations at the nano-scale on an individual particle basis. Hence one can identify the various particle morphologies that carry the metals of interest. In addition, this preliminary speciation can then be reexamined by XAS, which provides important information about the average local coordination environment of the element of choice. The relative composition of mixtures of phases can then be determined by means of spectral decomposition. All this information is acquired without alteration of the sample, and without relying on the use of operationally defined extractable phases.

Methods and Materials:

Sample Site and Collection: Lake DePue, a backwater lake on the Illinois River, is located in Northern Illinois, east of LaSalle-Peru. Industrial operations, including zinc smelting, sulfuric acid production, and di-ammonium phosphate fertilizer production, began in 1903 and continued through 1992. These activities have led to substantial metal and nutrient contamination of this aquatic system. The lake very shallow (approximately 2 m) and is characterized by a high primary productivity and a high load of suspended solids (secchi depth typically < 20 cm). Meter long sediment cores were collected with a hand-held piston corer at several sampling periods. Sub-cores for XAS and AEM analyses were retrieved in the field, and brought back to the laboratory for analysis.

AEM Methods: Water column particles and sediment subcore samples for AEM investigation were immediately embedded in a hydrophilic Nanoplast resin, following the procedures of Perret et. al., 1991, and Leppard et. al., 1996. Water column samples were separated into 3 size fractions by gravitational settling and centrifugation. 50 µL of each sample was placed on a Teflon plate and mixed with approximately 150 µL of resin. The resin was placed in an oven set at 40 °C for 2 days in the presence of a desiccant and, afterwards, without a desiccant at 60 °C for 2 days. The resin embedded samples were then placed in to a BEEM capsule and backfilled with Spurr’s resin. The polymerized resins were sectioned with a diamond knife mounted in an ultramicrotome (RMC Ultramicrotome MT-7). Ultrathin sections (approximately 80 nm) mounted on Cu grids were examined with a JEOL 1200 EX II TEMSCAN scanning-transmission electron microscope (STEM) operating at 80 keV. Elemental associations of individual particles were determined using STEM-EDS with a Princeton Gamma Tech Si(Li) X-ray detector.

XAS Methods: Sub-cores for XAS analyses were frozen in liquid N₂ in the field to stop any further chemical reaction. The sub-cores were extruded and cut into thin sections (approx. 1 mm) that were placed in between Kapton tape. The samples were maintained in liquid N₂ until placed in the beam. Standards were obtained from Aldrich and prepared in a similar manner by spreading fine powders evenly between Kapton tape.

X-ray absorption measurements were made at the DuPont-Northwestern-Dow Collaborative Access Team 5-BM beamline at the Advanced Photon Source, Argonne National Laboratory. A Si(111) double crystal monochromator was used to vary the X-ray energy from 200 eV below to 750 eV above the absorption K edge of Zn (9659 eV). The incident intensity, I₀, and transmitted intensity, I_T, were measured by appropriately positioned ionization chambers. The fluorescence signal, I_F, was measured with a Lytle detector equipped with a Z-1 filter. The output of the Lytle detector and ionization chambers were collected at 12.5 kHz in Quick-XAS mode while continuously slewing the monochromator between the beginning and ending energy. Nine successive scans were recorded per sample at a rate of 75 seconds per scan.

The composition of the samples was determined after spectral decomposition performed using a quadratic linear programming method. Standards were chosen after a careful geochemical analysis of Lake DePue sediments. Standards included ZnS, ZnO, Zn(OH)₂, ZnCO₃, Zn₃(PO₄)₂, hydrozincite (Zn₃CO₃(OH)₄), iron oxides precipitated in presence of zinc, and aqueous Zn. All EXAFS spectra were k³-weighted to better emphasize the EXAFS at large k. The fitting procedure was calibrated through measuring a known set of mixed standards of ZnS, Zn₃(PO4)₂, and hydrozincite. The FEFF6 (Mustre de Leon et. al., 1991, and Rehr et. al., 1992) ab initio calculation package was used to check the integrity of the standards.

Results:

The AEM results show that zinc is associated mainly with colloidal particles of rather uniform morphology (Webb et al., 2000). Diatoms, and most of the clay particles show little to no zinc present, while only the clay particles rich in iron show appreciable concentrations of zinc. In the upper layer of the sediments and in the water column, zinc is present in small particles, ranging from 40 to 300 nm in diameter. These particles are rich in iron and phosphorus as shown in Figure 1. As these particles are buried in the sediments, they become significantly altered and the contribution of sulfur in the EDS spectra becomes more pronounced.

Figure 2 provides the distribution of the major zinc species calculated as percent of the total zinc using the Q-XAS speciation protocol at the most contaminated site sampled. It shows a large proportion of aqueous zinc throughout the sampled core (25-40%), as well as a high fraction of Zn-CO₃ below 5 cm. This suggests that a large portion of the zinc at this site is relatively labile and easily transportable.

Figure 1. Typical iron-zinc-phosphate rich particle found in Lake DePue contaminated sediments. The EDS spectrum is taken from the point represented by the arrow. The Cu peak marked with the asterisk results from the presence of the Cu grid.

Figure 2: The relative proportion of the different coordination environment of Zn as a function of depth. The core was retrieved from a zone heavily contaminated.

Acknowledgements:

Funding for this work was provided by the National Science Foundation through Grant MCB-9807697, and the Illinois Department of Natural Resources. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) Synchrotron Research Center located at Sector 5 of the Advanced Photon Source. DND-CAT is supported by the E.I. DuPont de Nemours & Co., The Dow Chemical Company, the U.S. National Science Foundation through Grant DMR-9304725 and the State of Illinois through the Department of Commerce and the Board of Higher Education Grant IBHE HECA NWU 96. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research under Contract No. W-31-102-Eng-38.

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