Natural speciation of Ni, Zn, Ba, and As in ferromanganese coatings on quartz using X-ray fluorescence, absorption, and diffraction

The mineralogy of natural ferromanganese coatings on quartz grains and the crystal chemistry of associated trace elements Ni, Zn, Ba, and As were characterized by X-ray microfluorescence, X-ray diffraction, and EXAFS spectroscopy. Fe is speciated as ferrihydrite and Mn as vernadite. The two oxides form alternating Fe- and Mn-rich layers that are irregularly distributed and not always continuous. Unlike naturally abundant Fe-vernadite, in which Fe and Mn are mixed at the nanoscale, the ferrihydrite and vernadite are physically segregated and the trace elements clearly partitioned at the microscopic scale. Vernadite consists of two populations of interstratified one-water layer (7 Å phyllomanganate) and two-water layer (10 Å phyllomanganate) crystallites. In one population, 7 Å layers dominate, and in the other 10 Å layers dominate. The three trace metals Ni, Zn, and Ba are associated with vernadite and the metalloid As with ferrihydrite. In vernadite, nickel is both substituted isomorphically for Mn in the manganese layer and sorbed at vacant Mn layer sites in the interlayer. The partitioning of Ni is pH-dependent, with a strong preference for the first site at circumneutral pH and for the second at acidic pH. Thus, the site occupancy of Ni in vernadite may be an indicator of marine vs. continental origin, and in the latter, of the acidity of streams, lakes, or soil pore waters in which the vernadite formed. Zinc is sorbed only in the interlayer at vacant Mn layer sites. It is fully tetrahedral at a Zn/Mn molar ratio of 0.0138, and partly octahedral at a Zn/Mn ratio of 0.1036 consistent with experimental studies showing that the ^VIZn/^IVZn ratio increases with Zn loading. Barium is sorbed in a slightly offset position above empty tetrahedral cavities in the interlayer. Arsenic tetrahedra are retained at the ferrihydrite surface by a bidentate-binuclear attachment to two adjacent iron octahedra, as commonly observed. Trace elements in ferromanganese precipitates are partitioned at a few, well-defined, crystallographic sites that have some elemental specificity, and thus selectivity. The relative diversity of sorption sites contrasts with the simplicity of the layer structure of vernadite, in which charge deficit arises only from Mn⁴⁺ vacancies (i.e., no Mn³⁺ for Mn⁴⁺ substitution). Therefore, sorption mechanisms primarily depend on physical and chemical properties of the sorbate and competition with other ions in solution, such as protons at low pH for Ni sorption.     

Ba and Ni speciation in a nodule of binary Mn oxide phase composition from Lake Baikal

The partitioning and incorporation mechanism of Ni and Ba in a ferromanganese nodule from Lake Baikal were characterized by X-ray microfluorescence, microdiffraction, and absorption spectroscopy. Fe is speciated as goethite, and Mn as romanechite (psilomelane) and 10 Å-vernadite (turbostratic buserite) with minor 7 Å-vernadite (turbostratic birnessite). Barium is associated with romanechite and Ni with vernadite in distinct and irregularly distributed layers, and each type of Mn oxide is separated from the other type by goethite. The binary Mn oxide banding pattern is interpreted by a two-mode accretionary model, in which the variation in Ba flux induced by hydrothermal water pulses determines whether a tectomanganate (romanechite) or phyllomanganate (vernadite) is formed during the ferromanganese nodule accretion. Consistent with the dependence of Ni sorption on pH and the circumneutral pH of the lake water, nickel is mainly substituted isomorphically for Mn in the manganese layer, and is not sorbed at vacant Mn layer sites in the interlayer.     

Chemical and structural control of the partitioning of Co, Ce, and Pb in marine ferromanganese oxides

The oxidation state and mineral phase association of Co, Ce, and Pb in hydrogenetic, diagenetic, and hydrothermal marine ferromanganese oxides were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical extraction. Cobalt is trivalent and associated exclusively with the Mn oxide component (vernadite). Cerium is tetravalent in all genetic-type oxides (detection limit for Ce(III) ∼ 5 at. %), including Fe-rich areas (ferrihydrite) of hydrogenetic oxides, and is associated primarily with vernadite. Thus, the extent of a Ce anomaly does not result from variations in redox conditions, but appears to be kinetically controlled, decreasing when the growth rate increases from hydrogenetic to diagenetic to hydrothermal oxides. Lead is divalent and associated with Mn and Fe oxides in variable proportions. According to EXAFS data, Pb is mostly sorbed on edge sites at chain terminations in Fe oxide and at layer edges in Mn oxide (ES complex), and also on interlayer vacancy sites in Mn oxide (TCS complex). Sequential leaching experiments, spectroscopic data, and electrochemical considerations suggest that the geochemical partitioning in favor of the Mn oxide component decreases from Co to Ce to Pb, and depends on their oxidative scavenging by Mn and Fe oxides.     

Composition of manganese nodules and manganese carbonates from Loch Fyne,Scotland

Manganese nodules and manganese carbonate concretions occur in the upper 10–15 cm of the Recent sediments of Loch Fyne, Argyllshire in water depths of 180–200 m. The nodules are spherical, a few mm to 3 cm in diameter, and consist of a black, Mn-rich core and a thin, red, Fe-rich rim. The carbonate occurs as irregular concretions, 0.5–8 cm in size, and as a cement in irregular nodule and shell fragment aggregates. It partially replaces some nodule material and clastic silicate inclusions, but does not affect aragonitic and calcitic shell fragments.The nodules are approximately 75% pure oxides and contain 30% Mn and 4% Fe. In the cores, the principal mineral phase is todorokite, with a Mn/Fe ratio of 17. The rim consists of X-ray amorphous Fe and Mn oxides with a Mn/Fe ratio of 0.66. The cores are enriched, relative to Al, in K, Ba, Co, Mo, Ni and Sr while the rims contain more P, Ti, As, Pb, Y and Zn.The manganese carbonate has the composition (Mn_47.7 Ca_45.1 Mg_7.2) CO₃. Apart from Cu, all minor elements are excluded from significant substitution in the carbonate lattice.Manganese nodules and carbonates form diagenetically within the Recent sediments of Loch Fyne. This accounts for the high Mn/Fe ratios in the oxide phases and the abundance of manganese carbonate concretions. Mn concentrations in the interstitial waters of sediment cores are high (ca. 10 ppm) as also, by inference, are the dissolved carbonate concentrations.     

Zinc mobility and speciation in soil covered by contaminated dredged sediment using micrometer-scale and bulk-averaging X-ray fluorescence, absorption and diffraction techniques

The mobility and solid-state speciation of zinc in a pseudogley soil (pH = 8.2-8.3) before and after contamination by land-disposition of a dredged sediment ([Zn] = 6600 mg kg⁻¹) affected by smelter operations were studied in a 50 m² pilot-scale test site and the laboratory using state-of-the-art synchrotron-based techniques. Sediment disposition on land caused the migration of micrometer-sized, smelter-related, sphalerite (ZnS) and franklinite (ZnFe₂O₄) grains and dissolved Zn from the sediment downwards to a soil depth of 20 cm over a period of 18 months. Gravitational movement of fine-grained metal contaminants probably occurred continuously, while peaks of Zn leaching were observed in the summer when the oxidative dissolution of ZnS was favored by non-flooding conditions. The Zn concentration in the <50 μm soil fraction increased from ∼61 ppm to ∼94 ppm in the first 12 months at 0-10 cm depth, and to ∼269 ppm in the first 15 months following the sediment deposition. Higher Zn concentrations and enrichments were observed in the fine (<2 μm) and very fine (<0.2 μm) fractions after 15 months (480 mg kg⁻¹ and 1000 mg kg⁻¹, respectively), compared to 200 mg kg⁻¹ in the <2 μm fraction of the initial soil. In total, 1.2% of the Zn initially present in the sediment was released to the environment after 15 months, representing an integrated quantity of ∼4 kg Zn over an area of 50 m². Microfocused X-ray fluorescence (XRF), diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy techniques were used to image chemical associations of Zn with Fe and Mn, and to identify mineral and Zn species in selected points-of-interest in the uncontaminated and contaminated soil. Bulk average powder EXAFS spectroscopy was used to quantify the proportion of each Zn species in the soil. In the uncontaminated soil, Zn is largely speciated as Zn-containing phyllosilicate, and to a minor extent as zincochromite (ZnCr₂O₄), ^IVZn-sorbed turbostratic birnessite (δ-MnO₂), and Zn-substituted goethite. In the upper 0-10 cm of the contaminated soil, ∼60 ± 10% of total Zn is present as ZnS inherited from the overlying sediment. Poorly-crystalline Zn-sorbed Fe (oxyhydr)oxides and zinciferous phyllosilicate amount to ∼20-30 ± 10% each and, therefore, make up most of the remaining Zn. Smaller amounts of franklinite (ZnFe₂O₄), Zn-birnessite and Zn-goethite were also detected. Further solubilization of the Zn inventory in the sediment, and also remobilization of Zn from the poorly-crystalline neoformed Fe (oxyhydr)oxide precipitates, are expected over time. This study shows that land deposition of contaminated dredged sediments is a source of Zn for the covered soil and, consequently, presents environmental hazards. Remediation technologies should be devised to either sequester Zn into sparingly soluble crystalline phases, or remove Zn by collecting leachates beneath the sediment.     

Zinc sorption to biogenic hexagonal-birnessite particles within a hydrated bacterial biofilm

Biofilm-embedded Mn oxides exert important controls on trace metal cycling in aquatic and soil environments. The speciation and mobility of Zn in particular has been linked to Mn oxides found in streams, wetlands, soils, and aquifers. We investigated the mechanisms of Zn sorption to a biogenic Mn oxide within a biofilm produced by model soil and freshwater Mn^II-oxidizing bacteria Pseudomonas putida. The biogenic Mn oxide is a c-disordered birnessite with hexagonal layer symmetry. Zinc adsorption isotherm and Zn and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy experiments were conducted at pH 6.9 to characterize Zn sorption to this biogenic Mn oxide, and to determine whether the bioorganic components of the biofilm affect metal sorption properties. The EXAFS data were analyzed by spectral fitting, principal component analysis, and linear least-squares fitting with reference spectra. Zinc speciation was found to change as Zn loading to the biosorbent [bacterial cells, extracellular polymeric substances (EPS), and biogenic Mn oxide] increased. At low Zn loading (0.13 ± 0.04 mol Zn kg⁻¹ biosorbent), Zn was sorbed to crystallographically well-defined sites on the biogenic oxide layers in tetrahedral coordination to structural O atoms. The fit to the EXAFS spectrum was consistent with Zn sorption above and below the Mn^IV vacancy sites of the oxide layers. As Zn loading increased to 0.72 ± 0.04 mol Zn kg⁻¹ biosorbent, Zn was also detected in octahedral coordination to these sites. Overall, our results indicate that the biofilm did not intervene in Zn sorption by the Mn-oxide because sorption to the organic material was observed only after all Mn vacancy sites were capped by Zn. The organic functional groups present in the biofilm contributed significantly to Zn removal from solution when Zn concentrations exceeded the sorption capacity of the biooxide. At the highest Zn loading studied, 1.50 ± 0.36 mol Zn kg⁻¹ biosorbent, the proportion of total Zn sorption attributed to bioorganic material was 38 mol%. The maximum Zn loading to the biogenic oxide that we observed was 4.1 mol Zn kg⁻¹ biogenic Mn oxide, corresponding to 0.37 ± 0.02 mol Zn mol⁻¹ Mn. This loading is in excellent agreement with previous estimates of the content of cation vacancies in the biogenic oxide. The results of this study improve our knowledge of Zn speciation in natural systems and are consistent with those of Zn speciation in mineral soil fractions and ferromanganese nodules where the Mn oxides present are possibly biogenic.     

X-ray absorption spectroscopy study on the effect of hydroxybenzoic acids on the formation and structure of ferrihydrite

Organic ligands are known to interfere with the polymerization of Fe(III), but the extent of interference has not been systematically studied as a function of structural ligand properties. This study examines how the number and position of phenol groups in hydroxybenzoic acids affect both ferrihydrite formation and its local (<5 Å) Fe coordination. To this end, acid Fe(III) nitrate solutions were neutralized up to pH 6.0 in the presence of 4-hydroxybenzoic acid (4HB), 2,4-dihydroxybenzoic acid (2,4DHB), and the hydroquinone 3,4-dihydroxybenzoic acid (3,4DHB). The initial molar ligand/Fe ratios ranged from 0 to 0.6. The precipitates were dialyzed, lyophilized, and subsequently studied by X-ray absorption spectroscopy and synchrotron X-ray diffraction. The solids contained up to 32 wt.% organic C (4HB ∼ 2,4DHB < 3,4DHB). Only precipitates formed in 3,4DHB solutions comprised considerable amounts of Fe(II) (Fe(II)/Fe_tot ≤ 6 mol%), implying the abiotic mineralization of the catechol-group bearing ligand during Fe(III) hydrolysis under oxic conditions. Hydroxybenzoic acids decreased ferrihydrite formation in the order 4HB ∼ 2,4DHB ? 3,4DHB, which documents that phenol group position rather than the number of phenol groups controls the ligand’s interaction with Fe(III). The coordination numbers of edge- and double corner-sharing Fe in the precipitates decreased by up to 100%. Linear combination fitting (LCF) of Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra revealed that this decrease was due to increasing amounts of organic Fe(III) complexes in the precipitates. Although EXAFS derived coordination numbers of Fe in ferrihydrite remained constant within error, all organic ligands decreased the coherently scattering domain (CSD) size of ferrihydrite as indicated by synchrotron X-ray diffraction analysis (4HB < 2,4DHB ? 3,4DHB). With decreasing particle size of ferrihydrite its Fe(O,OH)₆ octahedra became progressively distorted as evidenced by an increasing loss of centrosymmetry of the Fe sites. Pre-edge peak analysis of the Fe K-edge XANES spectra in conjunction with LCF results implied that ferrihydrite contains on an average 13 ± 3% tetrahedral Fe(III), which is in very good agreement with the revised single-phase structural model of ferrihydrite (Michel, F. M., Barron, V., Torrent, J., Morales, M. P. et al. (2010) Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism. Proc. Natl. Acad. Sci. USA107, 2787-2792). The results suggest that hydroxybenzoic acid moieties of natural organic matter (NOM) effectively suppress ferrihydrite precipitation as they kinetically control the availability of inorganic Fe(III) species for nucleation and/or polymerization reactions. As a consequence, NOM can trigger the formation of small ferrihydrite nanoparticles with increased structural strain. These factors may eventually enhance the biogeochemical reactivity of ferrihydrite formed in NOM-rich environments. This study highlights the role of hydroquinone structures of NOM for Fe complexation, polymerization, and redox speciation.     

Seasonal variations of Zn, Cu, As and Mo in arsenic-rich stream at the Mokrsko gold deposit, Czech Republic

Monthly sampling of slightly alkaline arsenic-rich stream in the Mokrsko gold deposit revealed seasonal variations in dissolved Zn, Cu, As and Mo. Concentrations of trace metal cations (Zn, Cu) increased as much as 330 and 178%, respectively, from minimum mean values at autumn to maximum mean values at spring. In contrast, concentrations of trace element oxyanions (As, Mo) revealed opposite seasonal pattern with increase to 189% (As) and 123% (Mo) during summer–autumn, indicating that in-stream biogeochemical process(es) played the main role in controlling the seasonal variations of these trace elements. The trace elements were mainly scavenged by low crystalline Mn oxyhydroxide and Fe oxyhydroxide (ferrihydrite). Results are consistent with sorption and coprecipitation processes controlling seasonal variations of dissolved Zn and Cu, while As and Mo dynamics appear linked to Mn redox reactions. The sorption processes and Mn redox processes are attributed to the changes of pH and oxic/anoxic conditions on the surface of oxyhydroxides, respectively, which are themselves controlled by the balance between photosynthesis and respiration. Under the geochemical conditions of the stream, inferred Mn redox reactions can only be explained by microbial activity.     

Proton release during adsorption of heavy metal ions by a hydrous manganese dioxide

During the adsorption of Pb, Cu, Mn or Zn by birnessite, 1 mol of H⁺ is released from the surface for each mole of metal ion sorbed. The potassium appearing in solution during adsorption is not directly involved in the reaction. This resolves some apparently conflicting published results.     

Synthesis and phase transformations involving scorodite, ferric arsenate and arsenical ferrihydrite: Implications for arsenic mobility

Scorodite, ferric arsenate and arsenical ferrihydrite are important arsenic carriers occurring in a wide range of environments and are also common precipitates used by metallurgical industries to control arsenic in effluents. Solubility and stability of these compounds are controversial because of the complexities in their identification and characterization in heterogeneous media. To provide insights into the formation of scorodite, ferric arsenate and ferrihydrite, series of synthesis experiments were carried out at 70 °C and pH 1, 2, 3 and 4.5 from 0.2 M Fe(SO₄)_1.5 solutions also containing 0.02-0.2 M Na₂HAsO₄. The precipitates were characterized by transmission electron microscopy, X-ray diffraction and X-ray absorption fine structure techniques. Ferric arsenate, characterized by two broad diffuse peaks on the XRD pattern and having the structural formula of FeAsO₄·4-7H₂O, is a precursor to scorodite formation. As defined by As XAFS and Fe XAFS, the local structure of ferric arsenate is profoundly different than that of scorodite. It is postulated that the ferric arsenate structure is made of single chains of corner-sharing Fe(O,OH)₆ octahedra with bridging arsenate tetrahedra alternating along the chains. Scorodite was precipitated from solutions with Fe/As molar ratios of 1 over the pH range of 1-4.5. The pH strongly controls the kinetics of scorodite formation and its transformation from ferric arsenate. The scorodite crystallite size increased from 7 to 33 nm by ripening and aggregation. Precipitates, resulting from continuous synthesis at pH 4.5 from solutions having Fe/As molar ratios ranging from 1 to 4 and resembling the compounds referred to as ferric arsenate, arsenical ferrihydrite and As-rich hydrous ferric oxide in the literature, represent variable mixtures of ferric arsenate and ferrihydrite. When the Fe/As ratio increases, the proportion of ferrihydrite increases at the expense of ferric arsenate. Arsenate adsorption appears to retard ferrihydrite growth in the precipitates with molar Fe/As ratios of 1-4, whereas increased reaction gradually transforms two-line ferrihydrite to six-line ferrihydrite at Fe/As ratios of 5 and greater.     

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to ²¹⁰Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS₂ aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.     

Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (O_layer) with three-layer Mn atoms (Mn_layer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3O_layer-□-3Mn_layer). ^TCZn complexes replace interlayer Mn²⁺ (Mn_inter²⁺) and protons. ^TCZn and ^TCMn_inter³⁺ together balance the layer charge deficit originating from Mn_layer⁴⁺ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three O_layer and one water molecule (^[IV]TC complex: (H₂O)-^[IV]Zn-3O_layer). At high loading, zinc is predominantly octahedrally coordinated to three O_layer and to three interlayer water molecules (^[VI]TC complex: 3(H₂O)-^[VI]Zn-3O_layer), as in chalcophanite (^[VI]ZnMn₃⁴⁺O₇·3H₂O). Sorbed Zn induces the translation of octahedral layers from −a/3 to +a/3, and this new stacking mode allows strong H bonds to form between the ^[IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (O_next): O_next…(H₂O)-^[IV]Zn-3O_layer. Empirical bond valence calculations show that O_layer and O_next are strongly undersaturated, and that ^[IV]Zn provides better local charge compensation than ^[VI]Zn. The strong undersaturation of O_layer and O_next results not only from Mn_layer⁴⁺ vacancies, but also from Mn³⁺ for Mn⁴⁺ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, ^[IV]Zn,Mn_layer³⁺, and Mn_next³⁺ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu²⁺ forms a Jahn-Teller distorted ^[VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb²⁺ is not oxidized to Pb⁴⁺ and forms predominantly ^[VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing (^[VI]TE) interlayer complexes (Pb-3O_layer-3Mn), as in quenselite (Pb²⁺Mn³⁺O₂OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mn_layer³⁺, which is responsible for the formation of less abundant interlayer complexes, such as ^[IV]Zn TC in ZnBi and ^[VI]Pb TE in PbBi.     

Decoupling of arsenic and iron release from ferrihydrite suspension under reducing conditions: a biogeochemical model

High levels of arsenic in groundwater and drinking water are a major health problem. Although the processes controlling the release of As are still not well known, the reductive dissolution of As-rich Fe oxyhydroxides has so far been a favorite hypothesis. Decoupling between arsenic and iron redox transformations has been experimentally demonstrated, but not quantitatively interpreted. Here, we report on incubation batch experiments run with As(V) sorbed on, or co-precipitated with, 2-line ferrihydrite. The biotic and abiotic processes of As release were investigated by using wet chemistry, X-ray diffraction, X-ray absorption and genomic techniques. The incubation experiments were carried out with a phosphate-rich growth medium and a community of Fe(III)-reducing bacteria under strict anoxic conditions for two months. During the first month, the release of Fe(II) in the aqueous phase amounted to only 3% to 10% of the total initial solid Fe concentration, whilst the total aqueous As remained almost constant after an initial exchange with phosphate ions. During the second month, the aqueous Fe(II) concentration remained constant, or even decreased, whereas the total quantity of As released to the solution accounted for 14% to 45% of the total initial solid As concentration. At the end of the incubation, the aqueous-phase arsenic was present predominately as As(III) whilst X-ray absorption spectroscopy indicated that more than 70% of the solid-phase arsenic was present as As(V). X-ray diffraction revealed vivianite Fe(II)₃(PO₄)₂.8H₂O in some of the experiments. A biogeochemical model was then developed to simulate these aqueous- and solid-phase results. The two main conclusions drawn from the model are that (1) As(V) is not reduced during the first incubation month with high Eh values, but rather re-adsorbed onto the ferrihydrite surface, and this state remains until arsenic reduction is energetically more favorable than iron reduction, and (2) the release of As during the second month is due to its reduction to the more weakly adsorbed As(III) which cannot compete against carbonate ions for sorption onto ferrihydrite. The model was also successfully applied to recent experimental results on the release of arsenic from Bengal delta sediments.     

Geochemistry and specific features of manganese ore formation in sediments of oceanic bioproductive zones

Processes of authigenic manganese ore formation in sediments of the northern equatorial Pacific are considered on the basis of study of the surface layer (<2 mm) of ferromanganese nodule and four micronodule size fractions from the associated surface sediment (0–7 cm). Inhomogeneity of the nodule composition is shown. The Mn/Fe ratio is maximal in samples taken from the lateral sectors of nodule at the water-sediment interface. Compositional differences of nodules are related to the preferential accumulation of microelements in iron oxyhydroxides (P, Sr, Pb, U, Bi, Th, Y, and REE), manganese hydroxides (Co, Ni, Cu, Zn, Cd, Mo, Tl, W), and lithogenous component trapped during nodule growth (Ga, Rb, Ba, and Cs). The Ce accumulation in the REE composition is maximal in the upper and lower parts of the nodule characterized by the minimal Mn/Fe values. The compositional comparison of manganese micronodules and surface layers of the nodule demonstrated that the micronodule material was subjected to a more intense reworking during the diagenesis of sediments. The micronodules are characterized by higher Mn/Fe and P/Fe ratios but lower Ni/Cu and Co/Ni ratios. The micronodules and nodules do not differ in terms of contents of Ce and Th that are least mobile elements during the diagenesis of elements. Differences in the chemical composition of micronodules and nodules are related not only to the additional input of Mn in the process of diagenesis, but also to the transformation of iron oxyhydroxides after the removal of Mn from the close association with Fe formed in the suspended matter at the stage of sedimentation.     

The effect of phytostabilization on Zn speciation in a dredged contaminated sediment using scanning electron microscopy, X-ray fluorescence, EXAFS spectroscopy, and principal components analysis

The maintenance of waterways generates large amounts of dredged sediments, which are deposited on adjacent land surfaces. These sediments are often rich in metal contaminants and present a risk to the local environment. Understanding how the metals are immobilized at the molecular level is critical for formulating effective metal containment strategies such as phytoremediation. In the present work, the mineralogical transformations of Zn-containing phases induced by two graminaceous plants (Agrostis tenuis and Festuca rubra) in a contaminated sediment ([Zn] = 4700 mg kg⁻¹, [P₂O₅] = 7000 mg kg⁻¹, pH = 7.8), untreated or amended with hydroxylapatite (AP) or Thomas basic slag (TS), were investigated after two yr of pot experiment by scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS), synchrotron-based X-ray microfluorescence (μ-SXRF), and powder and laterally resolved extended X-ray absorption fine structure (μ-EXAFS) spectroscopy. The number and nature of Zn species were evaluated by principal component (PCA) and least-squares fitting (LSF) analysis of the entire set of μ-EXAFS spectra, which included up to 32 individual spectra from regions of interest varying in chemical composition. Seven Zn species were identified at the micrometer scale: sphalerite, gahnite, franklinite, Zn-containing ferrihydrite and phosphate, (Zn-Al)-hydrotalcite, and Zn-substituted kerolite-like trioctahedral phyllosilicate. Bulk fractions of each species were quantified by LSF of the powder EXAFS spectra to linear combinations of the identified Zn species spectra.In the untreated and unvegetated sediment, Zn was distributed as ∼50% (mole ratio of total Zn) sphalerite, ∼40% Zn-ferrihydrite, and ∼10 to 20% (Zn-Al)-hydrotalcite plus Zn-phyllosilicate. In unvegetated but amended sediments (AP and TS), ZnS and Zn-ferrihydrite each decreased by 10 to 20% and were replaced by Zn-phosphate (∼30∼40%). In the presence of plants, ZnS was almost completely dissolved, and the released Zn bound to phosphate (∼40-60%) and to Zn phyllosilicate plus (Zn,Al)-hydrotalcite (∼20-40%). Neither the plant species nor the coaddition of mineral amendment affected the Zn speciation in the vegetated sediment. The sediment pore waters were supersaturated with respect to Zn-containing trioctahedral phyllosilicate, near saturation with respect to Zn-phosphate, and strongly undersaturated with respect to (Zn,Al)-hydrotalcite. Therefore, the formation of (Zn,Al)-hydrotalcite in slightly alkaline conditions ought to result from heterogeneous precipitation on mineral surface.     

Arsenic and metal mobility from Au mine tailings in Rodalquilar (Almería,SE Spain)

In the old mining area of Rodalquilar, mine wastes, soil and sediments were characterized and the results revealed high concentration of Au, Ag, As, Bi, Cu, Fe, Mn, Pb, Se, Sb and Zn in tailings and sediments. The contaminant of greatest environmental concern is As. The mean concentration in the tailings was 679.9, and 345 mg/kg in the sediments of Playazo creek. The groundwater samples from the alluvial aquifer showed high concentration of Al, As, Cd, Fe, Hg, Mn, Ni, Pb, Se, Sb and Zn and very high concentration of chloride and sulfate, which were above the concentration defined in the European standards for drinking water. The presence of As in groundwater may be caused by the oxidation of arsenian pyrite, the possible As desorption from goethite and ferrihydrite and the jarosite dissolution. Groundwater concentrations of Cd, Fe, Mn, and possibly Cu, were associated with low values of Eh, indicating the possible dissolution of oxy-hydroxides of Fe and Mn. The mobility of metals in the column experiments show the release of Al, Fe, Mn, Cr, Cu, Ni, V and Zn in significant concentrations but below the detected values in groundwater. However, As, Cd, Sb, Se Pb and Au, are generally mobilized in concentrations above the detected values in groundwater. The possible mass transfer processes that could explain the presence of the contaminants in the aquifer and the leachates was simulated with the PHREEQC numerical code and revealed the possible dissolution of the following mineral phases: jarosite, natrojarosite, arsenian pyrite, alunite, chlorite, kaolinite and calcite.     

Natural speciation of Mn, Ni, and Zn at the micrometer scale in a clayey paddy soil using X-ray fluorescence, absorption, and diffraction

The natural speciation of Mn (0.19 g/kg), Ni (46 mg/kg), and Zn (42 mg/kg) in the argillic horizon (120 cm depth, pH = 5.6) of an Ultisol from a paddy soil in northern Taiwan was investigated by advanced X-ray synchrotron techniques. Microchemical associations were imaged by synchrotron-based X-ray microfluorescence, host minerals were identified by standard and micrometer-resolved X-ray diffraction, and the local coordination environment of Mn, Ni, and Zn was probed using extended X-ray absorption fine structure (EXAFS) spectroscopy on a powdered sample and a soil thin section, and polarized EXAFS spectroscopy on a highly textured self-supporting clay film from the <2 μm fraction of the soil. Manganese was concentrated in Fe-Mn soft mottles (44.4 g/kg) as turbostratic hexagonal birnessite and lithiophorite having Mn³⁺/Mn⁴⁺ atomic ratios of ∼20% and 50%, respectively. Quantitative analysis of high-order scattering paths of the EXAFS spectrum for natural and synthetic lithiophorite revealed that Mn³⁺ and Mn⁴⁺ are ordered in the layer. A structural model is proposed, in which Mn⁴⁺ and Mn³⁺ are ordered similarly to Al and Li in the layer, with Mn³⁺ cations being surrounded by six Mn⁴⁺, and Mn⁴⁺ cations by three Mn³⁺ and three Mn⁴⁺. Similar cation ordering in the manganese and aluminum layers likely provides a more homogeneous local balance of the excess and deficit of charges in each layer and increases the stability of lithiophorite. Ni (r = 0.70 Å) substitutes for Mn (r(Mn⁴⁺) = 0.54 Å, r(Mn³⁺) = 0.65 Å) in the manganese layer in the natural lithiophorite. In contrast, Zn (r = 0.74 Å) fills vacant sites in the gibbsitic layer of natural lithiophorite, in a similar manner as lithium (r = 0.74 Å) in synthetic lithiophorite. The partitioning of Ni and Zn between the two layers is a result of the general preference of Ni, whose size is intermediate between those of Mn³⁺ and Li⁺, for slightly smaller sites. In contrast with nickel, which is detected only where there is lithiophorite, the Zn-lithiophorite association found in Fe-Mn mottles is not representative of the bulk soil. The combined use of X-ray diffraction, and powder and polarized EXAFS spectroscopy revealed that Zn is predominantly bound to hydroxy-Al interlayers sandwiched between 2:1 vermiculite layers in the fine soil matrix. The incorporation of Zn in the gibbsitic layer of both lithiophorite and vermiculite helps increase the stability of these minerals by providing positive charge to balance the negative charge from the 2:1 phyllosilicate layer and the layer of lithiophorite. This binding environment for zinc is probably the main mechanism by which zinc is sequestered in acidic to near-neutral aluminum-rich clayey soils.     

Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II)

Due to the strong reducing capacity of ferrous Fe, the fate of Fe(II) following dissimilatory iron reduction will have a profound bearing on biogeochemical cycles. We have previously observed the rapid and near complete conversion of 2-line ferrihydrite to goethite (minor phase) and magnetite (major phase) under advective flow in an organic carbon-rich artificial groundwater medium. Yet, in many mineralogically mature environments, well-ordered iron (hydr)oxide phases dominate and may therefore control the extent and rate of Fe(III) reduction. Accordingly, here we compare the reducing capacity and Fe(II) sequestration mechanisms of goethite and hematite to 2-line ferrihydrite under advective flow within a medium mimicking that of natural groundwater supplemented with organic carbon. Introduction of dissolved organic carbon upon flow initiation results in the onset of dissimilatory iron reduction of all three Fe phases (2-line ferrihydrite, goethite, and hematite). While the initial surface area normalized rates are similar (∼10⁻¹¹ mol Fe(II) m⁻² g⁻¹), the total amount of Fe(III) reduced over time along with the mechanisms and extent of Fe(II) sequestration differ among the three iron (hydr)oxide substrates. Following 16 d of reaction, the amount of Fe(III) reduced within the ferrihydrite, goethite, and hematite columns is 25, 5, and 1%, respectively. While 83% of the Fe(II) produced in the ferrihydrite system is retained within the solid-phase, merely 17% is retained within both the goethite and hematite columns. Magnetite precipitation is responsible for the majority of Fe(II) sequestration within ferrihydrite, yet magnetite was not detected in either the goethite or hematite systems. Instead, Fe(II) may be sequestered as localized spinel-like (magnetite) domains within surface hydrated layers (ca. 1 nm thick) on goethite and hematite or by electron delocalization within the bulk phase. The decreased solubility of goethite and hematite relative to ferrihydrite, resulting in lower Fe(III)_aq and bacterially-generated Fe(II)_aq concentrations, may hinder magnetite precipitation beyond mere surface reorganization into nanometer-sized, spinel-like domains. Nevertheless, following an initial, more rapid reduction period, the three Fe (hydr)oxides support similar aqueous ferrous iron concentrations, bacterial populations, and microbial Fe(III) reduction rates. A decline in microbial reduction rates and further Fe(II) retention in the solid-phase correlates with the initial degree of phase disorder (high energy sites). As such, sustained microbial reduction of 2-line ferrihydrite, goethite, and hematite appears to be controlled, in large part, by changes in surface reactivity (energy), which is influenced by microbial reduction and secondary Fe(II) sequestration processes regardless of structural order (crystallinity) and surface area.     

Zinc surface complexes on birnessite: A density functional theory study

Biogeochemical cycling of zinc is strongly influenced by sorption on birnessite minerals (layer-type MnO₂), which are found in diverse terrestrial and aquatic environments. Zinc has been observed to form both tetrahedral (Zn^IV) and octahedral (Zn^VI) triple-corner-sharing surface complexes (TCS) at Mn(IV) vacancy sites in hexagonal birnessite. The octahedral complex is expected to be similar to that of Zn in the Mn oxide mineral, chalcophanite (ZnMn₃O₇·3H₂O), but the reason for the occurrence of the four-coordinate Zn surface species remains unclear. We address this issue computationally using spin-polarized density functional theory (DFT) to examine the Zn^IV-TCS and Zn^VI-TCS species. Structural parameters obtained by DFT geometry optimization were in excellent agreement with available experimental data on Zn-birnessites. Total energy, magnetic moments, and electron overlap populations obtained by DFT for isolated Zn^IV-TCS revealed that this species is stable in birnessite without a need for Mn(III) substitution in the octahedral sheet and that it is more effective in reducing undersaturation of surface O at a Mn vacancy than is Zn^VI-TCS. Comparison between geometry-optimized ZnMn₃O₇·3H₂O (chalcophanite) and the hypothetical monohydrate mineral, ZnMn₃O₇·H₂O, which contains only tetrahedral Zn, showed that the hydration state of Zn significantly affects birnessite structural stability. Finally, our study also revealed that, relative to their positions in an ideal vacancy-free MnO₂, Mn nearest to Zn in a TCS surface complex move toward the vacancy by 0.08-0.11 Å, while surface O bordering the vacancy move away from it by 0.16-0.21 Å, in agreement with recent X-ray absorption spectroscopic analyses.     

Geochemical and equilibrium trends in mine pit lakes

Chemical composition and equilibrium trends in mine pit lakes were examined to provide guidance for the application of geochemical models in predicting future lake water quality at prospective open pit mines. Composition trends show that elevated solute levels generally occur only at the extremes of acidic and alkaline pH conditions. Concentrations of cationic metals (Al, Cd, Cu, Fe, Mn, Pb, and Zn) are elevated only in acidic pit lakes, whereas anionic metalloids (As and Se) are generally elevated only in alkaline pit lakes. These trends are indicative of sulfide mineral oxidation and evapoconcentration for acidic and alkaline conditions, respectively.For nearly all pit lakes, SO₄ is the dominant solute, but is limited by gypsum solubility. Fluorite, calcite, and barite are also important solubility controls. Well-defined solubility controls exist for the major metals (Al, Fe, Mn), including jurbanite and alunite for Al, ferrihydrite for Fe, and manganite, birnessite, and, possibly, rhodochrosite for Mn. Determinations of definite controls for the minor metals are less distinct, but may include otavite for Cd, brochantite and malachite for Cu, cerrusite and pyromorphite for Pb, and hydrozincite and Zn silicates for Zn. Concentrations of As and Se appear to be limited only by adsorption, but this control is sharply diminished by increased pH and SO₄ concentration. In general, the concentrations of minor metals in pit lakes are not well represented by the theoretical solubilities of pure-phase minerals contained in the thermodynamic databases. Hence, modeling efforts will generally have to rely on empirical data on the leaching characteristics of pit wall-rocks to predict the concentrations of minor metals (Cd, Cu, Pb, Zn) in mine pit lakes.Methodologies for predicting pit lake water chemistry are still evolving. Geochemical and equilibrium trends in existing pit lakes can provide valuable information for guiding the development and application of predictive models. However, mineralogical studies of pit lake sediments, suspended particles, and alteration assemblages and studies of redox transformations are still needed to validate and refine the representations of geochemical processes in water quality models of mine pit lakes.