Proton-induced X-ray Emission

Black particles, collected by filtration (1.2~pm pore size) from the anoxic waters of a soft-water lake, were examined by a scanning proton microprobe which permitted quantitative elemental analysis by proton-induced X-ray emission (PIXE) and Rutherford backscattering (RBS). There was a uniform distribution of sulfur across the filter, but Fe, and to a lesser extent, Mn, was localized in -5-pm-diameter clusters. Elemental analysis with 1 -pm-diameter beams revealed that the Fe clusters were mainly comprised of iron oxides. Iron sulfide material not in the Fe clusters had stoichiometric proportions of Fel.oSo.60Po.60Cao.~~~.14. Although a purely biogenie origin for P, Ca, and K cannot be ruled out, the composition is consistent with the particles originating as authigenic iron oxides which react with sulfide as they sink through the water column. The iron sulfide particles are richer in Cu (4,000 ppm) and Zn (6,000 ppm) than the iron oxides, suggesting that these elements are also concentrated as their insoluble sulfides. The coexistence of iron oxides and sulfides indicates that either the supply of sulfide is limiting or that some iron oxide particles are unreactive. Black particles of iron sulfide have been clearly observed when the bottom waters of seasonally anoxic lakes are filtered (Davison and Heaney 1978; Cook 1984). Evidence for their iron sulfide composition has come from the ion activity product, IAP = aFe2+ x aHS-/(H+), which in many lakes has been found to be constant with depth, indicating control of the concentration of solution species by the solid phase (Davison 199 1). For a wide variety of waters the IAP falls in a narrow range (pIAP = 2.6-3.22) which agrees with the measured solubility product for amorphous ferrous sulfide of p&, = 2.95kO.l. These iron sulfide particles have been poorly characterized, partly because of the Acknowledgments We thank John Hamilton-Taylor for comments on an earlier draft and the Natural Environment Research Council for funding this work. The operation of the scanning proton microprobe facility is supported in part by the Wellcome Trust. Characterization of lacustrine iron sulfide particles with difficulties of performing measurements on samples that are very sensitive to oxygen. When the ratio of Fe(II) to S(-II) has been measured calorimetrically or polarographically, a stoichiometric excess of Fe(II) has been observed in both larger (>0.8 pm) (Davison and Dickson 1984) and colloidalsized (co.45 pm) (Buffle et al. 1987) particles. Mossbauer spectroscopy has confirmed this imbalance and led to the suggestion that the particles may comprise iron oxyhydroxides with a surface coating of sulfide (Davison and Dickson 1984). For a hard-water lake it was hypothesized that the stoichiometric excess of Fe may be due to the presence of siderite (Buffle et al. 1987), but Davison and Heaney (1978) failed to find any carbonate in their similar samples from a soft-water lake. Recent studies have shown a direct association between the geochemical cycling of selected trace metals in lakes and the active redox cycling of Fe and Mn (Morfett et al. 1988; Balistrieri et al. 1992). However, to our knowledge, there are no reports of the trace element composition of iron sulfide particles. This note attempts to rectify some of the above deficiencies by examining iron sulfide particles with nuclear microscopy (Grime and Watt 1990) at the Oxford University scanning proton microprobe (SPM) facility (Grime et al. 1991). This technique uses a beam of high-energy (> 2 MeV) protons focused to a diameter of 1 pm and is capable of elemental mapping and quantitative microanalysis for elements throughout the periodic table by means of various ion-solid interactions. The analytical techniques used in this study are PIXE (proton-induced X-ray emission) (Johansson and Campbell 1988) which gives ppm detection limits for all elements with 2 > 12, and RBS (Rutherford backscattering) (Chu et al. 1978) which uses the measured energy of recoiling protons to determine the concentrations and