A structural model for the geometry of Fe(III) octahedra near the surface of finely divided ferrihydrite was elaborated based on the bond-valence theory and by considering the interaction of water molecules in the 2 nearest hydration spheres. In contrast to bulk Fe atoms, which are bonded to bridging oxo (O) and hydroxo (OH) ligands, surface Fe atoms are also octahedrally coordinated to H2O ligands forming the 1st hydration shell ((H2O)I). In the wet state, external water molecules of the 2nd hydration shell ((H2O)II) are singly H-bonded to (H2O)I, while they are doubly coordinated in the dry state. Accordingly, wet ferrihydrite contains twice as many sorbed water molecules as dry ferrihydrite, and the structural difference due to the 2nd hydration shell accounts quantatively for the 15% increase of ferrihydrite weight experimentally measured in moist atmosphere. The interaction of surface Fe atoms with their 2 nearest hydration spheres modifies the geometry of surface Fe octahedra as compared to bulk octahedra, and idealized Fe-OH and Fe-H2O bond lengths in the wet and dry state were evaluated by the bond-valence theory. Our structural model provides a sound crystal-chemical basis to describe many apparent incongruities of Fe X-ray absorption near edge structure (K-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic data that have led to differing interpretations of the coordination environment of Fe in ferrihydrite by various investigators.

Mn-substituted goethite was synthesized and the interaction between arsenite (As(III)) and Mn-substituted goethite was investigated by both solution chemistry and X-ray adsorption near edge structure (XANES) spectroscopy. Results indicate that the oxidation of As(III) is favored by Mn-substituted goethite. This reaction was more sensitive to temperature than to pH. Different reaction mechanisms may account for As(III) oxidation. Since As(III) is more mobile and toxic than As(V), the oxidation reaction of As(III) with Mn-substituted goethite may decrease arsenic toxicity under some conditions.

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.

X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

This paper compares the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (AlIV). The highest relative proportion of AlIV, 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlIV/Altotal ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low AlIV concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of AlVI and especially site multiplicity. AlVI XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique AlVI site by comparison with crystalline references. The presence of only one AlVI site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AlIV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework alumino-silicates, and less positive than those of sheet silicates, which are typically in the range 65–75 ppm. 27Al-H1 CPMAS NMR spectra suggest that both AlIV and AlVI have Al-O-H linkages.

Chromium(VI) in the environment is of particular concern because it is toxic to both plants and animals, even at low concentrations. As a redox-sensitive element, the fate and toxicity of chromium is controlled by soil reduction-oxidation (redox) reactions. In-situ remediation of chromium combines reduction of Cr(VI) to Cr(III) and immobilization of chromium on mineral surfaces. In this study, Fe-rich smectite, montmorillonite, illite, vermiculite, and kaolinite were examined to determine reactivity in sorption-reduction of Cr(VI). The clays were compared to forms that were reduced by sodium dithionite. Clays containing Fe(II) efficiently removed soluble Cr(VI) from solution. Chromium K-edge X-ray absorption near edge structure (XANES) suggested that clays containing Fe(II) reduced Cr(VI) to Cr(III), immobilizing Cr at the clay/water interface. Adsorption of Cr(VI) by the Fe(II)-containing clay was a prerequisite for the coupled sorption-reduction reaction. Sodium dithionite added directly to aqueous suspensions of non-reduced clays reduced Cr(VI) to Cr(III), but did not immobilize Cr on clay surfaces. The capacity of clays to reduce Cr(VI) is correlated with the ferrous iron content of the clays. For dithionite-reduced smectite, the exchangeable cation influenced the sorption reaction, and thus it also influenced the coupled sorption-reduction reaction of Cr(VI). The pH of the aqueous system affected both the amount of Cr(VI) reduced to Cr(III) and the partition of Cr(III) between aqueous and adsorbed species. A plot of pH vs. amount (adsorption envelope) adsorbed for the coupled sorption-reduction reaction of Cr by reduced smectite exhibited a similar pattern to that of typical anion-sorption.

A reservoir rock is a porous geological formation in contact with 2 liquids, brine and oil. An improved knowledge of rock wettability is of primary importance to estimate the amount of crude oil in underground resources. The petroleum industries have observed that wettability contrasts in sedimentary reservoir rocks are largely correlated to the presence of clays, illite and/or kaolinite in the rocks’ intergranular space.

More precisely, the grain surfaces of illite show a preference for brine. Kaolinite preferentially adsorbs oil, which imparts its hydrophobic characteristics to the mineral surface. Using X-ray absorption spectroscopy (XAS) and Fourier transform infrared (FTIR) spectroscopy, we studied the adsorption process of asphaltenes in the presence of water at the microscopic level. We demonstrate experimentally that the wettability contrasts observed in kaolinite and illite are related to structural differences between these 2 clays, and we show the role of the grain surface hydroxyls. With clay materials, the purity of the samples is the most important limitation of the quantitative use of extended X-ray absorption fine structure (EXAFS).

Excavated bentonite from two large iron-bentonite field experiments at Äspö Hard Rock Laboratory in Sweden was investigated with respect to iron redox chemistry and mineralogy. The iron redox chemistry was studied by Fe K-edge X-ray absorption near edge structure spectroscopy and the mineral phases were studied using X-ray diffraction. Bentonite is to be used as a buffer material in high-level radioactive waste repositories to protect the waste containers from their surroundings. Montmorillonite, which is responsible for the sealing properties in the bentonite, is susceptible to redox reactions. A change in the montmorillonite iron redox chemistry may affect its layer charge and hence its properties. The experiments included are the first Alternative Buffer Material test (ABM1) and the Temperature Buffer Test (TBT). The clays were heated to a maximum of ~130°C (ABM1) or ~150°C (TBT) for 2.5 and 7 y, respectively. In the central part of the compacted clay blocks was placed an iron heater and the distance from the heater to the rock was ~10 cm (ABM1) and ~50 cm (TBT), respectively. Eleven different clay materials were included in the ABM1 experiment and five were analyzed here. In the ABM1 experiment, the Fe(II)/Fe(III) ratio was increased in several samples from the vicinity of the heater. Kinetic data were collected and showed that most of the Fe(II)-rich samples oxidized rapidly when exposed to atmospheric oxygen. In the TBT experiment the corrosion products were dominated by Fe(III) and no significant increase in Fe(II) was seen. In ABM1, reducing conditions were achieved, at least in parts of the experiment; in TBT, reducing conditions were not achieved. The difference was attributed to the larger scale of the TBT experiment, providing more oxygen after the installation, and to the longer time taken for water saturation; oxidation of the samples during excavation cannot be ruled out. Minor changes in the bentonite mineral phases were found in some cases where direct contact was made with the iron heater but no significant impact on the bentonite performance in high-level radioactive waste applications was expected as a result.

Twelve nontronites and two ferruginous smectites have been characterized with respect to Fe3+ occupancy of tetrahedral sites. The techniques used were near infrared, Fe-K X-ray absorption near-edge and X-ray absorption fine-structure spectroscopies, along with two X-ray diffraction techniques. The results show that calculations of the structural formulae of many nontronites should be adjusted to include Fe3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (∼44% Fe2O3) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe3+. Samples with chemical compositions similar to Garfield nontronite (∼36.5% Fe2O3) may have small amounts (<5% of total Fe3+) of tetrahedral Fe3+. Tetrahedral Fe3+ is unlikely to be present in samples containing less than ∼34% Fe2O3.

The As sorption capacity of a natural Mn and Fe mineral-containing sample from the Iron Quadrangle province, Brazil, was investigated. A detailed mineralogical identification was obtained by combining X-ray diffraction analyses (with Rietveld refinement), X-ray fluorescence spectroscopy, optical microscopy, and scanning electron microscopy coupled with X-ray energy dispersive spectrometry-EDS. The oxidation state of the adsorbed As species was determined by X-ray absorption near edge structure spectroscopy. The results demonstrate that the presence of naturally occurring Mn oxides promotes the effective oxidation of As (III) to As(V). Also, the Mn minerals show a significant uptake of both the trivalent and pentavalent As species. This study demonstrates that the combined influences of As(III) depletion by oxidation and adsorption on a natural oxide sample consisting of Mn minerals and Fe oxides may effectively contribute to the reduction the As concentration in waters.

Among all iron oxides, hematite (α-Fe2O3), goethite (α-FeOOH), and ferrihydrite (FeOOH⋅nH2O) are the most common mineral species. While immobilization of Mo6+ by surface adsorption on ferric oxides has been studied extensively, the mechanisms of incorporation in their structure have been researched little. The objective of this study was to investigate the relation between Mo content and its structural incorporation in hematite, goethite, and six-line ferrihydrite by a combination of X-ray absorption spectroscopy (XAS), powder X-ray diffraction (pXRD), and inductively-coupled plasma optical emission spectrometry (ICP-OES). Synthesized in the presence of Mo, the hematite, goethite, and six-line ferrihydrite phases incorporated up to 8.52, 0.03, and 17.49 wt. % Mo, respectively. For hematite and goethite, pXRD analyses did not indicate the presence of separate Mo phases. Refined unit-cell parameters correlated with increasing Mo concentration in hematite and goethite. The unit-cell parameters indicated an increase in structural disorder within both phases and, therefore, supported the structural incorporation of Mo in hematite and goethite. Analysis of pXRD measurements of Mo-bearing six-line ferrihydrites revealed small amounts of coprecipitated akaganéite. X-ray absorption near edge structure (XANES) measurements at the Mo L3-edge indicated a strong distortion of the MoO6 octahedra in all three phases. Fitting of extended X-ray absorption fine structure (EXAFS) spectra of the Mo K-edge supported the presence of such distorted octahedra in a coordination environment similar to the Fe position in the investigated specimen. Incorporation of Mo6+ at the Fe3+-position for both hematite and goethite resulted in the formation of one Fe vacancy in close proximity to the newly incorporated Mo6+ and, therefore, charge balance within the hematite and goethite structures.

Kaznakhtite, ideally Ni6Co3+2(CO3)(OH)16⋅4H2O, is a new member of the hydrotalcite group within the hydrotalcite supergroup. The mineral was discovered at the Kaznakhtinskiy ultrabasic massif, Altai Republic, SW Siberia, Russia. It occurs as powdery aggregates forming flattened lenses up to 1.5 × 0.5 cm and veinlets up to 1 cm long and up to 1 mm thick in aggregates of chrysotile, lizardite, stichtite and dolomite. Other associated minerals include brucite, chromite, heazlewoodite, manganochromite, magnetite and magnesioferrite. Kaznakhtite aggregates are composed of tiny platy grains up to 0.01 mm across. Kaznakhtite is light green and translucent in aggregates. It has an earthy lustre and white streak. Cleavage is micaceous on {001}. Dcalc = 2.864 g cm–3. The mineral is optically uniaxial (–) with ɛ = 1.657(3) and ω = 1.676(3), and weakly pleochroic in greenish hues, ω > ɛ. Chemical composition (wt.%, electron microprobe, Co valence state determined by XANES spectroscopy, CO2 and H2O calculated by stoichiometry) is: MgO 2.15, NiO 47.40, ZnO 0.22, Al2O3 0.16, Cr2O3 0.98, Co2O3 17.42, Cl 0.63, CO2 5.05, H2O 24.60, –O=Cl –0.14, total 98.47. The empirical formula calculated based on the sum of all metal cations = 8 apfu is (Ni5.54Mg0.47Zn0.02)Σ6.03(Co3+1.83Cr0.11Al0.03)Σ1.97C1.00O2.99(OH)15.84Cl0.16⋅4H2O. Infrared spectroscopy confirmed the presence of CO32– anions, OH– groups and H2O molecules. The crystal structure was refined by the Rietveld method with RB = 0.19%. Kaznakhtite is trigonal, space group R͞3m, a = 3.0515 (3), c = 23.180 (3) Å, V = 186.93 (4) Å3 and Z = 3/8. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.72 (100) (003); 3.863 (24) (006); 2.630 (4) (101); 2.576 (10) (012); 2.294 (6) (015); 1.950 (4) (018); 1.526 (4) (110); and 1.497 (4) (113). Kaznakhtite is a Co3+ analogue of reevesite, Ni6Fe3+2(CO3)(OH)16⋅4H2O. The mineral is named after its type locality.

We have synthesized graphene oxide (GO) using Hummer's method which was subsequently reduced (rGO) by hydrazine hydrate. The synthesized GO was coated with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) conducting polymer (CP) to obtain CP-GO which was also further reduced using hydrazine hydrate to form CP-rGO. Scanning electron microscopy, Raman spectroscopy, X-ray diffraction, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy, X-ray absorption near-edge structure (XANES) techniques were used to study the electronic and structural properties of GO, rGO, CP-GO, and CP-rGO nanocomposites for biomedical applications. The superconducting quantum interference device method was used to investigate the magnetic properties of the nanocomposites. The electrical conductivity of the CP-GO nanocomposites was found to be ~104 times higher than that of GO due to an increase in sp2 content and subsequent decrease in oxygen functional groups. In rGO, we observed an improved paramagnetic saturation magnetization of approximately 5.6 × 0−3 emu/g at 2 K. The electronic and magnetic behavior of PEDOT-PSS-coated nanocomposites, as a result, were successfully tuned for potential biological and biomedical applications.

Eudialyte-group minerals (EGM) attract global interest as potential resources for high-field-strength elements (e.g. Zr, Nb, Ta, and rare-earth elements), i.e. critical materials for modern technologies. They are particularly valued for their relative enrichment in the most critical lanthanides, i.e. Nd and heavy rare earth elements (Gd–Lu). However, rare earth element (REE) substitution mechanisms into the EGM structure are still poorly understood. Light and heavy REE may occupy different sites and there may be ordering and/or defect clustering in the structure. This study uses X-ray absorption spectroscopy to determine the structural state of REE in EGM from prospective eudialyte-bearing complexes. Yttrium K-edge and Nd L3-edge spectra were collected as proxies for heavy and light REE, respectively, and compared to natural and synthetic REE-bearing standards. Extended X-ray absorption fine structure data yield best fits for Y in six-fold coordination with Y–O distances of 2.24–2.32 Å, and a second coordination sphere comprising Fe, Na, Ca, Si and O at radial distances of 3.6–3.8 Å. These findings are consistent with dominant Y3+ substitution for Ca2+ on the octahedral M1 site in all the samples studied, and exclude preferential substitution of Y3+ onto the smaller octahedral Z site or the large low-symmetry N4 site.

Using lattice strain theory, we constructed relative partitioning models to predict site preferences of lanthanides we have not measured directly. The models predict that all REE are favoured on the Ca-dominant M1 site and that preferential partitioning of heavy over light REE increases in EGM containing significant Mn in the M1-octahedral rings (oneillite subgroup). Thus, the flat REE profiles that make EGM such attractive exploration targets are not due to preferential partitioning of light and heavy REE onto different sites. Instead, local ordering of Mn- and Ca-occupied M1 sites may influence the capacity of EGM to accommodate heavy REE.

A series of titanium silicate glasses along the composition joins TiO2-SiO2, TiO2-Na2SiO3, TiO2-K2SiO3 and TiO2-CaSiO3, has been examined using oxygen K-edge X-ray absorption near edge structure spectroscopy (XANES) confined to the near-surface region. Sharp pre-edge features in the spectra can be used to determine the Ti coordination in the glasses. The presence of [4]Ti is indicated by a pre-edge peak at ∼534 eV while [5]Ti is indicated by a peak at ∼533 eV. Titanium exists in all these glasses as [4]Ti and [5]Ti with no [6]Ti being present. For alkali-containing glasses the [5]Ti site becomes more prevalent with increasing TiO2. TiO2-K2SiO3 compositions contain a greater proportion of [4]Ti than comparable Na2O compositions. This is consistent with earlier Ti L-edge findings. The TEY spectra for the TiO2-CaSiO3 compositions indicate the presence of significant amounts of [5]Ti at high TiO2 contents; however, comparison of TEY (near surface) and FY (bulk sample) XANES shows that the [5]Ti is confined to the surface of the glass sample with the bulk of the glass containing [4]Ti.

Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeoenvironmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2%) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5% dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg–O bond distances of ∼2.12 Å, which are much smaller than the Ca–O bond distance of 2.35 Å but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain nanodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.08 Å) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite

The coupled thermal oxidation and deprotonation in air of iron-rich biotite (FeO + Fe2O3 = 34%) has been investigated by EXAFS and XANES spectroscopy at the Fe-K edge and by XANES spectroscopy at the Ti-K edge. Samples annealed for 5 h at temperatures between 250° to 600°C have been studied. Distortions mainly of the Fe-Fe correlation within the octahedral layers are reflected in increasing Debye-Waller factors of the Fe-Fe correlation peak proportional to the annealing temperature. Unchanged Fe-O nearest-neighbour and Fe-Fe next-nearest-neighbour coordination numbers show that these distortions, nonetheless, do not change the structural topology of the octahedral layers. A model is introduced to demonstrate that increasing distortions are compatible with the expected heterogenous deprotonation mechanism in biotite. Titanium occurs in octahedral coordination. It was found to be unaffected by the coupled oxidation/deprotonation process. Both the coordination number and the valence state stay constant during the annealing process, in spite of dramatic changes of the Fe2+/Fe3+ ratio. Thermally activated hopping conduction involving Ti according to Fe2+Ti4+ → Fe3+Ti3+ is, therefore, not a significant process during thermal deprotonation and oxidation in biotite.

The colours of natural and synthetic beryl (ideally Be3Al2Si6O18) have been attributed either tothe presence of chromophore ions partially replacing Al and/or Be (Cr3+, Fe2+/3+, Mn2+) or to the occurrence of colour centres related to the presence of multi-atomic groups. However, no full explanation has been proposed for the blue colour in natural beryl gemstones (aquamarine) despite it being well established that bluish-green synthetic aquamarines containing Fe turn blue when heated for 1 h at 400°C.

A X-ray absorption near-edge spectroscopy (XANES) study at the Fe K-edge was performed on blue beryl crystals from Licungo pegmatite (Mozambique) to ascertain the speciation state of this colouring element, reported in blue sapphire (ideally Al2O3) as a mixture of Fe2+ and Fe3+ in octahedral coordination. The general trend of the spectra is the same when synchrotron radiation (SR) impinges along the c axis of the hexagonal crystal, as well as along the a axis with c vertical. Conversely, a shoulder in the absorption edge is noticed when the incident beam is perpendicular to c, with this axis horizontal in the polarization plane of the SR beam, a reverse in the relative intensities of the two post-edge features is simultaneously observed. Pre-edge features accounting for Fe speciation (valence plus coordination) are identical in all three situations. Considering previous approaches which account for polarization, an explanation is suggested for the special effects observed on the Fe K-edge XANES spectra of analysed blue beryl crystals. Ideas on the origin of the blue colour of this mineral are also presented.

The oxidation of chalcopyrite and pyrite was examined using Fe-K- and Cu-K-edge REFLEXAFS spectroscopy. The Fe XANES of the pyrite proved to be a very sensitive indicator of oxidation, revealing the development of a goethite-like surface species; the EXAFS data showed an increasing O:S ratio with the degree of oxidation and gave Fe–O distances of c. 1.9 Å. On the oxidized chalcopyrite surfaces, the development of Fe-O and Cu-O species was observed, with both the XANES and EXAFS revealing the progressive development of these species with oxidation. Differences in the sensitivity of the XANES and EXAFS to the degree of oxidation can be related to the degree of long range order and changes in the intensity of the pre-edge feature of the Fe are a function of its oxidation state and coordination geometry in the surface species.

The weathering and oxidation of mine tailings has the potential to contaminate water and soil with toxic elements. To understand the mechanisms, extent and products of the long-term weathering of complex Bolivian tailings from the Cerro Rico de Potosí, and their effects on As, Pb, P and Sb cycling, three-year long laboratory column experiments were carried out to model 20 years of dry- and wet-season conditions in the Pilcomayo basin. Chemical analysis of the leachate and column solids, optical mineralogy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, microscale X-ray absorption near edge structure spectroscopy, Bureau Commun de Référence sequential extraction and water-soluble chemical extractions, and speciation modelling have shown that the weathering of As-bearing pyrite and arsenopyrite, resulted in a loss of 13–29% of the original mass of As. By contrast, Pb and Sb showed much lower mass losses (0.1–1.1% and 0.6–1.9%, respectively) due to the formation of insoluble Pb- and Sb(V)-rich phases, which were stable at the low pH (~2) conditions that prevailed by the end of the experiment. The experiment also demonstrated a link between the cycling of As, Sb, and the oxidation of Fe(II)-bearing sphalerite, which acted as a nucleation point for an Fe-As-Sb-O phase. Phosphorus was relatively immobile in the tailings columns (up to 0.3% mass loss) but was more mobile in the soil-bearing columns (up to 10% mass loss), due to the formation of soluble P-bearing minerals or mobilization by organic matter. These results demonstrate the influence of mine tailings on the mobility of P from soils and on the potential contamination of ecosystems with As, and strongly suggest that these materials should be isolated from fluvial environments.