Effect of citrate on the local Fe coordination in ferrihydrite, arsenate binding, and ternary arsenate complex formation

In oxic environments contaminated with arsenate (As(V)), small polyhydroxycarboxylates such as citrate may impact the structure of precipitating ferrihydrite (Fh) and thus the surface speciation of As(V). In this study, ‘2-line’ Fh was precipitated from ferric nitrate solutions that were neutralized to pH 6.5 in the presence of increasing citrate concentrations and in the absence or presence of As(V). The initial citrate/Fe and As/Fe ratios were 0-50 mol% and 5 mol%, respectively. The reaction products, enriched with up to 0.32 mol citrate per mole Fe, were characterized by X-ray diffraction, transmission electron microscopy, and Fe and As K-edge X-ray absorption spectroscopy. Citrate decreased the particle size of Fh by impairing the polymerization of Fe(O,OH)₆ octahedra via edge and corner linkages. In the presence of citrate and As(V), coordination numbers of Fe decreased by up to 28% relative to pure Fh. Citrate significantly reduced the static disorder of Fe-O bonds, implying a decreased octahedral distortion in Fh. Mean bond distances in Fh were not affected by citrate and remained constant within error at 1.98 Å for Fe-O, 3.03 Å for Fe-Fe1, and 3.45 Å for Fe-Fe2. Likewise, citrate had no effect on the As-Fe (3.31 Å) bond distance in As(V) coprecipitated with Fh. The As K-edge EXAFS data comply with the formation of (i) only monodentate binuclear (²C) As(V) surface complexes and (ii) combinations of ²C, monodentate mononuclear (¹V), and outersphere As(V) surface complexes. Our results suggest that increasing citrate concentrations led to a decreasing ¹V/²C ratio and/or that citrate increasingly impaired the formation of outersphere As(V) complexes. Moreover, citrate stabilized colloidal suspensions of Fh (pH 4.3-6.6, I ∼0.45 M) and reduced Fh formation at the expense of soluble Fe(III)-citrate complexes. At initial citrate/Fe ratios ?25 mol%, between 8% and 41% of total Fe was bound in Fe(III)-citrate complexes after Fh formation. Polynuclear Fe(III)-citrate species were found to bind As(V) via surface complexes indistinguishable by EXAFS from those of As(V) adsorbed to or coprecipitated with Fh. Our study implies that low molecular weight polyhydroxycarboxylates may enhance the mobility of As(V) in aqueous systems of high ionic strength (e.g., neutralizing acid mine drainage) by colloidal stabilization of suspended Fh particles and the formation of ternary As(V) complexes.     

Apparent solubilities of schwertmannite and ferrihydrite in natural stream waters polluted by mine drainage

The apparent solubilities of schwertmannite and ferrihydrite were estimated from the H⁺, OH⁻, Fe³⁺, and SO₄²⁻ activities of the natural stream waters in Korea and mine drainage in Ohio, USA. Both chemical composition of the stream waters and the mineralogy of the precipitates were determined for samples from two streams polluted by coal mine drainage. This study combines these new results with previous data from Ohio, USA to redetermine solubilities. The activities of the dissolved species necessary for the solubility determinations were calculated from the chemical compositions of the waters with the WATEQ4F computer code.Laboratory analyses of precipitates indicated that the main minerals present in Imgok and Osheep creek were schwertmannite and ferrihydrite, respectively. The schwertmannite from Imgok creek had a variable chemical formula of Fe₈O₈(OH)_8−2x(SO₄)_x· nH₂O, where 1.74 ≤ x ≤ 1.86 and 8.17 ≤ n ≤ 8.62. The chemical formula of ferrihydrite was Fe₂O₃· 1.6H₂O. With known mineralogy of the precipitates from each stream, the activities of H⁺, OH⁻, Fe³⁺, and SO₄²⁻ in the waters were plotted on logarithmic activity-activity diagrams to determine apparent solubilities of schwertmannite and ferrihydrite. The best estimate for the logarithm of the solubility product of schwertmannite, logK_s, was 10.5 ± 2.5 around 15°C. This value of logK_s constrains the logarithm of the solubility product of ferrihydrite, logK_f, to be 4.3 ± 0.5 to maintain the stability boundary with schwertmannite observed in natural waters.     

Zn isotopic fractionation caused by sorption on goethite and 2-Lines ferrihydrite

Zn isotopic fractionation caused by sorption on 2-Lines ferrihydrite (Fh2L) and goethite was investigated to assess the role of reactions at the Fe-oxyhydroxide/water interface in changes of the isotopic distribution of Zn. Since sorption reactions are ubiquitous in Earth’s surface environments, it is important to evaluate their influence on the isotopic distribution of Zn before it can be used to track and quantify contributions of various sources and/or biogeochemical processes involving this element. Our results show that Zn isotopes are fractionated upon sorption on Fe-oxyhydroxides with an enrichment of the heavy isotopes present on the solid’s surface. This fractionation appears to proceed through an equilibrium mechanism and yields different (Δ^66/64Zn)_{sorbed-aqueous} values for Zn sorption on goethite [(Δ^66/64Zn)_{sorbed-aqueous} around +0.29‰] and Fh2L [(Δ^66/64Zn)_{sorbed-aqueous} around +0.53‰]. These different magnitudes of Zn fractionation are related to structural differences between Zn complexes existing on the surface of goethite (octahedrally coordinated Zn by oxygen atoms) and Fh2L (tetrahedrally coordinated Zn by oxygen atoms), as evidenced by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and CD-MUSIC modeling. These results show the importance of accounting for reactions at the Fe-oxyhydroxide/water interface when dealing with the isotopic distribution of Zn at the Earth’s surface. Considering the large range of other possible sorbents (Mn or Al oxides, phyllosilicates, carbonates, biologic surfaces, etc.) and the importance of reactions at sorbent/water interfaces for other non-traditional stable isotopes (i.e. Cr, Fe, Ni and Cu) that are increasingly used in environmental studies, these results emphasize the need for further experimental studies that are needed to quantify the isotopic fractionation of these elements possibly accompanying their sorption.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

Metal ion complex formation constants of some sedimentary humic acids with Zn(II), Cu(II) and Cd(II)

Metal ion complex formation constants were determined for several sedimentary humic acids (SHA) derived from a fresh water lake and several coastal marine environments, using a method based on size exclusion chromatography. Only one type of binding site was observed in all cases, and conditional log K_f values of between 5 and 7 (at pH 8, I = 0.01 M) were found. Elemental composition of the SHA was similar to soil HA, except that nitrogen content was significantly higher in the SHA. Other chemical properties of the SHA were consistent with those reported by other workers. While spectroscopic measurements indicated that the SHA may have contained significant amounts of polysaccharide compounds which were not removed by conventional separation and purification procedures, analysis indicated only very low levels of polysaccharides were present in the SHA.     

施氏矿物吸附Cu2+及氧化亚铁硫杆菌的实验研究

在金属硫化物的表生氧化过程中,施氏矿物是最常见的一种次生矿物.施氏矿物具有粒度小、比表面积大、表面吸附能高的特点,能够吸附环境流体中的重金属离子和微生物细胞,从而影响重金属元素及微生物的表生地球化学行为.利用化学合成的施氏矿物,开展了施氏矿物吸附Cu2+及氧化亚铁硫杆菌的实验.结果显示:施氏矿物对金属Cu2+及氧化亚铁硫杆菌均有较强的吸附性;施氏矿物对Cu2+的吸附基本符合Langmuir吸附模型,而对氧化亚铁硫的吸附行为不符合Langmuir模型,可用Freundlich模型描述;施氏矿物的存在对流体中微生物的活动性及其地球化学行为有重要影响,可能会降低氧化菌分解金属硫化物的效率.     

A high-resolution TEM-AEM, pH titration, and modeling study of Zn coprecipitation with ferrihydrite

Experiments of Zn²⁺ and Fe³⁺ coprecipitation as a function of pH were conducted in the laboratory at ambient temperature and pressure. X-ray diffraction patterns of the coprecipitates show two broad peaks at 0.149 and 0.258 nm, which is consistent with published patterns for pure 2-line ferrihydrite. Zn²⁺ uptake occurred at pH ≥5 while Fe³⁺ precipitation occurred between pH 3 and 4, although both Zn²⁺ and Fe³⁺ were present in the same solution during the entire range of pH titration. High-resolution transmission electron microscopy shows that the coprecipitates are 2 to 6 nm sized single crystalline particles but aggregated to 50 to 400 nm sized clusters. Analytical electron microscopy indicated that the 5% atomic Zn with respect to Fe was homogeneously distributed. No segregated phases were found in the clusters or at single crystal edges, which is consistent with published extended X-ray absorption fine structure (EXAFS) results at similar Zn/(Zn + Fe) ratios. Hence, occlusion and surface precipitation may be excluded as possible coprecipitation mechanisms. The bulk solution Zn²⁺ sorption edge was fitted to both solid solution and generalized diffuse layer surface complexation models. However, a solid solution model is inconsistent with published EXAFS results that show tetrahedral polydentate Zn²⁺ complexes sharing apices with Fe³⁺octahedra.     

尾矿酸浸液制备氢氧化铁过程中施威特曼石的形成与转变

尾矿酸浸液在制备氢氧化铁的过程中,由于逐渐滴加碱液正好形成了pH值为2.8～3.8的高SO2-4、高Fe环境,因而生成了施威特曼石.施威特曼石是一种亚稳定矿物,随着时间的延长和体系状态改变,它可以转变为更稳定的针铁矿(氢氧化铁).考查了pH值、温度及时间对施威特曼石相转变的影响.结果表明,在60℃条件下,在pH值为12的碱溶液中转化36 h,施威特曼石可完全转变为Fe(OH)3.     

Stable Cu and Zn isotope ratios as tracers of sources and transport of Cu and Zn in contaminated soil

Copper and Zn metals are produced in large quantities for different applications. During Cu production, large amounts of Cu and Zn can be released to the environment. Therefore, the surroundings of Cu smelters are frequently metal-polluted. We determined Cu and Zn concentrations and Cu and Zn stable isotope ratios (δ⁶⁵Cu, δ⁶⁶Zn) in three soils at distances of 1.1, 3.8, and 5.3 km from a Slovak Cu smelter and in smelter wastes (slag, sludge, ash) to trace sources and transport of Cu and Zn in soils. Stable isotope ratios were measured by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) in total digests. Soils were heavily contaminated with concentrations up to 8087 μg g⁻¹ Cu and 2084 μg g⁻¹ Zn in the organic horizons. The δ⁶⁵Cu values varied little (−0.12‰ to 0.36‰) in soils and most wastes and therefore no source identification was possible. In soils, Cu became isotopically lighter with increasing depth down to 0.4 m, likely because of equilibrium reactions between dissolved and adsorbed Cu species during transport of smelter-derived Cu through the soil. The δ⁶⁶Zn_IRMM values were isotopically lighter in ash (−0.41‰) and organic horizons (−0.85‰ to −0.47‰) than in bedrock (−0.28‰) and slag (0.18‰) likely mainly because of kinetic fractionation during evaporation and thus allowed for separation of smelter-Zn from native Zn in soil. In particular in the organic horizons large variations in δ⁶⁶Zn values occur, probably caused by biogeochemical fractionation in the soil-plant system. In the mineral horizons, Zn isotopes showed only minor shifts to heavier δ⁶⁶Zn values with depth mainly because of the mixing of smelter-derived Zn and native Zn in the soils. In contrast to Cu, Zn isotope fractionation between dissolved and adsorbed species was probably only a minor driver in producing the observed variations in δ⁶⁶Zn values. Our results demonstrate that metal stable isotope ratios may serve as tracer of sources, vertical dislocation, and biogeochemical behavior in contaminated soil.     

Goethite adsorption of Cu(II), Pb(II), Cd(II), and Zn(II) in the presence of sulfate: Properties of the ternary complex

Adsorption of Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ onto goethite is enhanced in the presence of sulfate. This effect, which has also been observed on ferrihydrite, is not predicted by the diffuse layer model (DLM) using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry FeOHMSO₄, where FeOH is a surface adsorption site and M²⁺ is a cation, the effect of SO₄²⁻ on cation adsorption was accurately predicted for the range of cation, goethite and SO₄²⁻ concentrations studied. While the DLM does not provide direct molecular scale insights into adsorption reactions there are several properties of ternary complexes that are evident from examining trends in their formation constants. There is a linear relationship between ternary complex formation constants and cation adsorption constants, which is consistent with previous spectroscopic evidence indicating ternary complexes involve cation binding to the oxide surface. Comparing the data from this work to previous studies on ferrihydrite suggests that ternary complex formation on ferrihydrite involves complexes with the same or similar structure as those observed on goethite. In addition, it is evident that ternary complex formation constants are larger where there is a stronger metal-ligand interaction. This is also consistent with spectroscopic studies of goethite-M²⁺-SO₄²⁻ and phthalate systems showing surface species with metal-ligand bonding. Recommended values of ternary complex formation constants for use in SO₄-rich environments, such as acid mine drainage, are presented.     

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.     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

样品量的大小对铜锌同位素测定值的影响

利用多接收电感耦合等离子体质谱法测定铜、锌同位素比值时,样品量的大小对其同位素测定值存在一定的影响。本研究中使用不同量的实验室标准溶液(CAGS-Cu、CAGS-Zn)过柱,并以该标准溶液为基准,采用标准-样品交叉法测定铜、锌同位素比值。结果表明,当铜、锌的样品量小到一定程度时,其同位素测定值偏离真值;当铜、锌的样品量足够大时,其同位素测定值趋于真值。流程本底对同位素测定值的影响可忽略不计,同时也不存在同质异位素干扰,说明这一现象极可能来自所用试剂和离子交换树脂的杂质组分产生的基质效应。在本文报道的实验条件下,当铜的样品量≥4μg、锌的样品量≥1μg时才能够确保铜、锌同位素测定结果的准确性。这一现象是否在其他同位素体系的测试过程存在,是个值得注意的问题。     

The transport and fate of Fe,Mn, Cu,Zn, Cd,Pb and SO4 in a groundwater plume and in downstream surface waters in the Coeur d'Alene Mining District,Idaho, U.S.A.

The controls on metal concentrations in a plume of acidic (pH 3.29–5.55) groundwater in the Moon Creek watershed in Idaho, U.S.A., were investigated with the use of property-property plots. A plot of Ca vs S demonstrated that a plume of contaminated groundwater was being diluted by infiltration of rain and creek water at shallow depths and by ambient groundwater near bedrock. The small amount of dissolved Fe (2.1 mg/l) was removed while dissolved Pb was added, reaching a maximum concentration of 0.37 mg/l. The other metals (Zn ≤ 16, Al ≤ 6.2, Cu ≤ 2.1 and Cd ≤ 0.077 mg/l) in the shallow groundwater were essentially conserved until they emerged as a seep along the creek bank. Upon mixing with the creek water, groundwater was diluted by factors between 11 and 50, and the pH of the mixture became neutral. Metals originating from the contaminated groundwater were removed in the creek in the following order: Fe > Al > Pb ≫ Cu > Mn > Zn = Cd.Pb and Cu continued to be removed from solution even as the creek passed adjacent to a tailings pile. In contrast, Zn concentrations in the creek increased adjacent to the tailings area, presumably as a result of the reemergence of the upgradient plume as the creek lost elevation.Below the tailings dam, contaminated creek water (400–800 μg Zn/l) was diluted by both smaller side streams and a creek of equal flow. The presence of 3 distinctive water masses required the use of two tracers (dissolved Si and S) to distinguish between mixing and geochemical reactions. The removal of metals was greater during low flow conditions. Pb was removed to the greatest extent, falling below detection limits (0.5 μ/l) at the first sampling location. Copper and Mn were removed to a lesser extent during low flow conditions and approached conservative behavior during high flow conditions. During a 5-km journey through two hydrological regimes, less than 10% of the dissolved Zn and Cd was lost.     

Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite: II. XANES analysis and simulation

X-ray absorption near-edge spectroscopy (XANES) analysis of sorption complexes has the advantages of high sensitivity (10- to 20-fold greater than extended X-ray absorption fine structure [EXAFS] analysis) and relative ease and speed of data collection (because of the short k-space range). It is thus a potentially powerful tool for characterization of environmentally significant surface complexes and precipitates at very low surface coverages. However, quantitative analysis has been limited largely to “fingerprint” comparison with model spectra because of the difficulty of obtaining accurate multiple-scattering amplitudes for small clusters with high confidence.In the present work, calculations of the XANES for 50- to 200-atom clusters of structure from Zn model compounds using the full multiple-scattering code Feff 8.0 accurately replicate experimental spectra and display features characteristic of specific first-neighbor anion coordination geometry and second-neighbor cation geometry and number. Analogous calculations of the XANES for small molecular clusters indicative of precipitation and sorption geometries for aqueous Zn on ferrihydrite, and suggested by EXAFS analysis, are in good agreement with observed spectral trends with sample composition, with Zn-oxygen coordination and with changes in second-neighbor cation coordination as a function of sorption coverage. Empirical analysis of experimental XANES features further verifies the validity of the calculations. The findings agree well with a complete EXAFS analysis previously reported for the same sample set, namely, that octahedrally coordinated aqueous Zn²⁺ species sorb as a tetrahedral complex on ferrihydrite with varying local geometry depending on sorption density. At significantly higher densities but below those at which Zn hydroxide is expected to precipitate, a mainly octahedral coordinated Zn²⁺ precipitate is observed. An analysis of the multiple scattering paths contributing to the XANES demonstrates the importance of scattering paths involving the anion sublattice. We also describe the specific advantages of complementary quantitative XANES and EXAFS analysis and estimate limits on the extent of structural information obtainable from XANES analysis.     

Cu diffusivity in granitic melts with application to the formation of porphyry Cu deposits

We report new experimental data of Cu diffusivity in granite porphyry melts with 0.01 and 3.9 wt% H₂O at 0.15–1.0 GPa and 973–1523 K. A diffusion couple method was used for the nominally anhydrous granitic melt, whereas a Cu diffusion-in method using Pt₉₅Cu₅ as the source of Cu was applied to the hydrous granitic melt. The diffusion couple experiments also generate Cu diffusion-out profiles due to Cu loss to Pt capsule walls. Cu diffusivities were extracted from error function fits of the Cu concentration profiles measured by LA-ICP-MS. At 1 GPa, we obtain \({D_{{\text{Cu, dry, 1 GPa}}}}=\exp \left[ {( - {\text{13.89}} \pm {\text{0.42}}) - \frac{{{\text{12878}} \pm {\text{540}}}}{T}} \right],\) and \({D_{{\text{Cu, 3}}{\text{.9 wt\% }}{{\text{H}}_{\text{2}}}{\text{O}},{\text{ 1 GPa}}}}=\exp \left[ {( - 16.31 \pm 1.30) - \frac{{{\text{8148}} \pm {\text{1670}}}}{T}} \right],\) where D is Cu diffusivity in m²/s and T is temperature in K. The above expressions are in good agreement with a recent study on Cu diffusion in rhyolitic melt using the approach of Cu₂S dissolution. The observed pressure effect over 0.15–1.0 GPa can be described by an activation volume of 5.9 cm³/mol for Cu diffusion. Comparison of Cu diffusivity to alkali diffusivity and its variation with melt composition implies fourfold-coordinated Cu⁺ in silicate melts. Our experimental results indicate that in the formation of porphyry Cu deposits, the diffusive transport of magmatic Cu to sulfide liquids or fluid bubbles is highly efficient. The obtained Cu diffusivity data can also be used to assess whether equilibrium Cu partitioning can be reached within certain experimental durations.     

Re-partitioning of Cu and Zn isotopes by modified protein expression

Cu and Zn have naturally occurring non radioactive isotopes, and their isotopic systematics in a biological context are poorly understood. In this study we used double focussing mass spectroscopy to determine the ratios for these isotopes for the first time in mouse brain. The Cu and Zn isotope ratios for four strains of wild-type mice showed no significant difference (δ⁶⁵Cu -0.12 to -0.78 permil; δ⁶⁶Zn -0.23 to -0.48 permil). We also looked at how altering the expression of a single copper binding protein, the prion protein (PrP), alters the isotope ratios. Both knockout and overexpression of PrP had no significant effect on the ratio of Cu isotopes. Mice brains expressing mutant PrP lacking the known metal binding domain have δ⁶⁵Cu isotope values of on average 0.57 permil higher than wild-type mouse brains. This implies that loss of the copper binding domain of PrP increases the level of ⁶⁵Cu in the brain. δ⁶⁶Zn isotope values of the transgenic mouse brains are enriched for ⁶⁶Zn to the wild-type mouse brains. Here we show for the first time that the expression of a single protein can alter the partitioning of metal isotopes in mouse brains. The results imply that the expression of the prion protein can alter cellular Cu isotope content.