Transformation of jarosite during simulated remediation of a sandy sulfuric soil.

Aeration of wetland soils containing iron (Fe) sulfides can cause strong acidification due to the generation of large amounts of sulfuric acid and formation of Fe oxyhydroxy sulfate phases such as jarosite. Remediation by re-establishment of anoxic conditions promotes jarosite transformation to Fe oxyhydroxides and/or Fe sulfides, but the driving conditions and mechanisms are largely unresolved. We investigated a sandy, jarosite-containing soil (initial pH = 3.0, Eh ~600 mV) in a laboratory incubation experiment under submerged conditions, either with or without wheat straw addition. Additionally, a model soil composed of synthesized jarosite mixed with quartz sand was used. Eh and pH values were monitored weekly. Solution concentrations of total dissolved organic carbon, Fe, S, and K as well as proportions of Fe2+ and SO42- were analysed at the end of the experiment. Sequential Fe extraction, X-ray diffraction, and Mössbauer spectroscopy were used to characterize the mineral composition of the soils. Only when straw was added to natural and artificial sulfuric soils, the pH increased up to 6.5, and Eh decreased to approx. 0 mV. The release of Fe (mainly Fe2+), K, and S (mainly SO42-) into the soil solution indicated redox- and pH-induced dissolution of jarosite. Mineralogical analyses confirmed jarosite losses in both soils. While lepidocrocite formed in the natural sulfuric soil, goethite was formed in the artificial sulfuric soil. Both soils showed also increases in non-sulfidized, probably organically associated Fe2+/Fe3+, but no (re-)formation of Fe sulfides. Unlike Fe sulfides, the formed Fe oxyhydroxides are not prone to support re-acidification in the case of future aeration. Thus, inducing moderately reductive conditions by controlled supply of organic matter could be a promising way for remediation of soils and sediments acidified by oxidation of sulfuric materials.

Imaging of Al/Fe ratios in synthetic Al-goethite revealed by nanoscale secondary ion mass spectrometry.

RATIONALE: Aluminium (Al)-substituted goethite is ubiquitous in soils and sediments. The extent of Al-substitution affects the physicochemical properties of the mineral and influences its macroscale properties. Bulk analysis only provides total Al/Fe ratios without providing information with respect to the Al-substitution of single minerals. Here, we demonstrate that nanoscale secondary ion mass spectrometry (NanoSIMS) enables the precise determination of Al-content in single minerals, while simultaneously visualising the variation of the Al/Fe ratio. METHODS: Al-substituted goethite samples were synthesized with increasing Al concentrations of 0.1, 3, and 7 % and analysed by NanoSIMS in combination with established bulk spectroscopic methods (XRD, FTIR, Mössbauer spectroscopy). The high spatial resolution (50-150 nm) of NanoSIMS is accompanied by a high number of single-point measurements. We statistically evaluated the Al/Fe ratios derived from NanoSIMS, while maintaining the spatial information and reassigning it to its original localization. RESULTS: XRD analyses confirmed increasing concentration of incorporated Al within the goethite structure. Mössbauer spectroscopy revealed 11 % of the goethite samples generated at high Al concentrations consisted of hematite. The NanoSIMS data show that the Al/Fe ratios are in agreement with bulk data derived from total digestion and demonstrated small spatial variability between single-point measurements. More advantageously, statistical analysis and reassignment of single-point measurements allowed us to identify distinct spots with significantly higher or lower Al/Fe ratios. CONCLUSIONS: NanoSIMS measurements confirmed the capacity to produce images, which indicated the uniform increase in Al-concentrations in goethite. Using a combination of statistical analysis with information from complementary spectroscopic techniques (XRD, FTIR and Mössbauer spectroscopy) we were further able to reveal spots with lower Al/Fe ratios as hematite.

Reference spectra of important adsorbed organic and inorganic phosphate binding forms for soil P speciation using synchrotron-based K-edge XANES spectroscopy.

Direct speciation of soil phosphorus (P) by linear combination fitting (LCF) of P K-edge XANES spectra requires a standard set of spectra representing all major P species supposed to be present in the investigated soil. Here, available spectra of free- and cation-bound inositol hexakisphosphate (IHP), representing organic P, and of Fe, Al and Ca phosphate minerals are supplemented with spectra of adsorbed P binding forms. First, various soil constituents assumed to be potentially relevant for P sorption were compared with respect to their retention efficiency for orthophosphate and IHP at P levels typical for soils. Then, P K-edge XANES spectra for orthophosphate and IHP retained by the most relevant constituents were acquired. The spectra were compared with each other as well as with spectra of Ca, Al or Fe orthophosphate and IHP precipitates. Orthophosphate and IHP were retained particularly efficiently by ferrihydrite, boehmite, Al-saturated montmorillonite and Al-saturated soil organic matter (SOM), but far less efficiently by hematite, Ca-saturated montmorillonite and Ca-saturated SOM. P retention by dolomite was negligible. Calcite retained a large portion of the applied IHP, but no orthophosphate. The respective P K-edge XANES spectra of orthophosphate and IHP adsorbed to ferrihydrite, boehmite, Al-saturated montmorillonite and Al-saturated SOM differ from each other. They also are different from the spectra of amorphous FePO4, amorphous or crystalline AlPO4, Ca phosphates and free IHP. Inclusion of reference spectra of orthophosphate as well as IHP adsorbed to P-retaining soil minerals in addition to spectra of free or cation-bound IHP, AlPO4, FePO4 and Ca phosphate minerals in linear combination fitting exercises results in improved fit quality and a more realistic soil P speciation. A standard set of P K-edge XANES spectra of the most relevant adsorbed P binding forms in soils is presented.

Characterization of ferrihydrite-soil organic matter coprecipitates by X-ray diffraction and Mössbauer spectroscopy.

In soils and sediments ferrihydrite often precipitates from solutions containing dissolved organic matter, which affects its crystallinity. To simulate this process we prepared a series of 2-line ferrihydrite-organic matter coprecipitates using water extractable organic matter (OM) from a forest topsoil. The products were characterized byX-ray diffraction, Mössbauer spectroscopy, N2-gas adsorption and transmission electron microscopy. With increasing C/Fe ratios of the initial solution the d-spacings of the two major XRD peaks increased, while peak shoulders at 0.22 and 0.16 nm weakened. The asymmetry of the 0.26 nm peak decreased and disappeared at a C/Fe ratio of 0.78. The quadrupole splitting of the Mössbauer spectra at 300 K increased from 0.78 to 0.90 mm s(-1), the mean magnetic hyperfine field at 4.2 K dropped from 49.5 to 46.0 T, and the superparamagnetic collapse of the magnetic hyperfine splitting was shifted toward lower temperatures. These data reflect a strong interference of OM with crystal growth leading to smaller ferrihydrite crystals, increased lattice spacings, and more distorted Fe(O,OH)6 octahedra. Even small amounts of OM significantly change particle size and structural order of ferrihydrite. Crystallinity and reactivity of natural ferrihydrites will therefore often differ from their synthetic counterparts, formed in the absence of OM.