Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite

Dissimilatory metal reducing bacteria (DMRB) can influence geochemical processes that affect the speciation and mobility of metallic contaminants within natural environments. Most investigations into the effect of DMRB on sediment geochemistry utilize various synthetic oxides as the Fe^III source (e.g., ferrihydrite, goethite, hematite). These synthetic materials do not represent the mineralogical composition of natural systems, and do not account for the effect of sediment mineral composition on microbially mediated processes. Our experiments with a DMRB (Shewanella putrefaciens 200) and a divalent metal (Zn^II) indicate that, while complexity in sediment mineral composition may not strongly impact the degree of “microbial iron reducibility,” it does alter the geochemical consequences of such microbial activity. The ferrihydrite and clay mineral content are key factors. Microbial reduction of a synthetic blend of goethite and ferrihydrite (VHSA-G) carrying previously adsorbed Zn^II increased both [Zn^II-aq] and the proportion of adsorbed Zn^II that is insoluble in 0.5 M HCl. Microbial reduction of Fe^III in similarly treated iron-bearing clayey sediment (Fe-K-Q) and hematite sand, which contained minimal amounts of ferrihydrite, had no similar effect. Addition of ferrihydrite increased the effect of microbial Fe^III reduction on Zn^II association with a 0.5 M HCl insoluble phase in all sediment treatments, but the effect was inconsequential in the Fe-K-Q. Zinc k-edge X-ray absorption spectroscopy (XAS) data indicate that microbial Fe^III reduction altered Zn^II bonding in fundamentally different ways for VHSA-G and Fe-K-Q. In VHSA-G, ZnO₆ octahedra were present in both sterile and reduced samples; with a slightly increased average Zn-O coordination number and a slightly higher degree of long-range order in the reduced sample. This result may be consistent with enhanced Zn^II substitution within goethite in the microbially reduced sample, though these data do not show the large increase in the degree of Zn-O-metal interactions expected to accompany this change. In Fe-K-Q, microbial Fe^III reduction transforms Zn-O polyhedra from octahedral to tetrahedral coordination and leads to the formation of a ZnCl₂ moiety and an increased degree of multiple scattering. This study indicates that, while many sedimentary iron minerals are easily reduced by DMRB, the effects of microbial Fe^III reduction on trace metal geochemistry are dependent on sediment mineral composition.     

Redox potential measurements and Mössbauer spectrometry of Fe adsorbed onto Fe (oxyhydr)oxides

The redox properties of Fe^II adsorbed onto a series of Fe^III (oxyhydr)oxides (goethite, lepidocrocite, nano-sized ferric oxide hydrate (nano-FOH), and hydrous ferric oxide (HFO)) have been investigated by rest potential measurements at a platinum electrode, as a function of pH (−log₁₀[H⁺]) and surface coverage. Using the constant capacitance surface complexation model to describe Fe^II adsorption onto these substrates, theoretical values of the suspension redox potential (E_H) have been computed, under the assumption that Fe^II adsorption occurs at crystal growth sites of the substrate surface. Good agreement between calculated and experimental E_H values is observed for nano-FOH and HFO, however the redox potentials measured for lepidocrocite and goethite are significantly more oxidizing than predicted. Mössbauer spectroscopic analysis of ⁵⁷Fe^II adsorbed onto HFO and goethite shows that in both cases the adsorbed ⁵⁷Fe^II is incorporated into the crystal structure of the substrate, in broad agreement with the thermodynamic model, but is almost completely oxidized to ⁵⁷Fe^III. The mechanism by which the adsorbed ⁵⁷Fe^II is oxidized is not resolved in this work, but is thought to be due to electron transfer to the substrate, rather than a net oxidation of the suspension. The disagreement between experimental and calculated rest potential measurements in the goethite and lepidocrocite systems is thought to be due to the poor electrochemical equilibration of these suspensions with the platinum electrode, rather than a failure of the thermodynamic model. The model developed for the redox potential of adsorbed Fe^II allows direct assessment of the reactivity of this species towards oxidized pollutants.     

Experimental study on the competitive adsorption of metal ions onto minerals

The study on the competitive adsorption shows that the magnitude order of metal ions adsorbed onto oxide and silicate minerals in near-neutral solution with low ionic strength is in mole/nm² as follows: CaCO₃ > quarte > hydromuscovite > kaolinite > Ca-montmorillonite > goethite > gibbsite. These minerals can be divided into three groups according to their surface equilibrium constantsK _M of the adsorption reactions, which are the function of the dielectric constants ε of the absorbent minerals. The relationships between constantsK _M and mineral dielectric constants ε are described as follows: lgK _M ¹ = 7.813-26.15/ε lgK _M ² = 9.030-26.15/ε lgK _M ³ =11.63-26.15/ε for the adsorption reaction: >SO^- + Mⁿ⁺≥SOM^n-1)+ (n = 1, 2, 3) The first group of minerals include quartz, goethite, 1:1 phyllosilicates and other oxide minerals; the second: gibbsite, brucite and 2:1 phyllosilicates; the third: carbonate, sulphate and phosphorate minerals. The appearance reaction constants have a variation of magnitude ±0.5 for different metal ions with the same mineral. This project was financially supported by the National Natural Science Foundation of China (No. 49572091).     

Surface stabilization of organics on hematite by conversion from terminal to bridging adsorption structures

Insight into the complexation of organic molecules on hematite surfaces was obtained from molecular-level studies of a simple probe molecule (methanol) with the R-cut surface of hematite. The R-cut crystal orientation of hematite, designated in this paper as α-Fe₂O₃(012), has two stable surface structures under ultrahigh vacuum (UHV) conditions based on low-energy electron diffraction (LEED) measurements. These are a (1×1) structure consisting of a bulk terminated arrangement of undercoordinated Fe³⁺ and O²⁻ surface sites and a (2×1) reconstructed structure with unknown atomic structure. Whereas the (1×1) surface is essentially free of Fe²⁺, the (2×1) surface possesses a high surface concentration of Fe²⁺ sites based on electronic structure measurements using electron energy loss spectroscopy (EELS). Methanol adsorbs dissociatively on the (1×1) surface by coordination of the molecule’s oxygen atom at a Fe³⁺ site followed by transfer of the alcohol proton to a bridging O²⁻ surface site, resulting in terminal OCH₃ and bridging OH groups. Most of the dissociated methanol molecules recombine during heating and desorb in vacuum as methanol at 365 and 415 K for the (1×1) and (2×1) surfaces, respectively. However, a significant amount of the terminal OCH₃ and bridging OH groups interchange as the surface is heated above room temperature (RT), resulting in bridging OCH₃ and terminal OH groups. The bridging OCH₃ groups are retained on the surface to higher temperature than the terminal OCH₃ groups, but eventually decompose at about 550 K via a disproportionation reaction that forms gaseous CH₃OH and H₂CO. As a result of the disproportionation reaction, some surface Fe³⁺ sites are reduced to Fe²⁺ sites. The exchange process competes more successfully with recombinative desorption of methanol (from reaction of terminal OCH₃ and bridging OH groups) on the (2×1) surface, despite the fact that this surface is already partially reduced, because terminal OCH₃ groups are more stable on this surface than on the (1×1) surface. Based on these molecular-level findings, extensive exchange terminal organic ligands and bridging OH groups may play a significant role in stabilizing organics on hematite mineral surfaces. Such exchange processes may also play a role in destabilizing hematite surfaces toward reductive dissolution.     

Therm odynamics of Diagenetic Fluid and Fluid／Mineral Reactions in the Eogene Xingouzui Formation,Oil Field T,Jianghan Basin

This study focuses on the thermodynamics of diagenetic fluid from the Eogene Xingouzui Formation which represents the most important reservoir in Field Oil T in the Jianghan Basin. The measured homogenization temperatures (110–139 °C) of fluid inclusions in diagenetic minerals fell within the range of 67 –155 °C at the middle diagenetic stage. The pressure of diagenetic fluid is estimated at 10.2 –56 MPa. The activity of ions in the fluid shows a tendency of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ > Fe³⁺ > Fe²⁺ for cations, and HCO ₃ ⁻ > SO ₄ ²⁻ > F⁻ > Cl⁻ > CO ₃ ²⁻ for anions. For the gaseous facies, there is a tendency of CO₂> CO> H₂S> CH₄> H₂. According to the thermodynamic calculations, the pH and Eh of the fluid are 5.86–6.47 and −0.73–−0.64V, respectively. As a result of the interaction between such a diagenetic fluid and minerals in the sediments, feldspars were dissolved or alterated by other minerals. The clay mineral kaolinite was instable and hence was replaced by illite and chloritoid. This project was jointly funded by the National Natural Science Foundation of China (49133080) and the Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.     

Antimony adsorption on kaolinite in the presence of competitive anions

Antimony (Sb) emissions to the environment are increasing, and there is a dearth of knowledge regarding Sb fate and behavior in natural systems. In natural systems, the presence of competitive anions may compete with Sb for adsorption sites on mineral surfaces, hence increasing its potential bioavailability. Accordingly, the adsorption of Sb(III) on kaolinite was investigated in the presence of competitive anions. Kinetic studies suggest that adsorption reaction of Sb(III) on kaolinite is rapid initially and becoming slow after 12 h both in binary Sb(III)–kaolinite system and in ternary Sb(III)-competitive anion–kaolinite system. The presence of PO₄ ^3? has a much stronger and more obvious promotive effect on the adsorption of Sb(III) on kaolinite compared with the other two anions. The adsorption data of Sb(III) on kaolinite in the absence and presence of competitive anions at three temperatures were successfully modeled using Langmuir (r ² > 0.95) and Freundlich (r ² > 0.95) isotherms. Accompanied the adsorption of Sb(III) on kaolinite, significant oxidation of Sb(III) to Sb(V) had occurred under the experimental conditions used in this study. The presence of kaolinite which has a larger specific surface area could increase the contact area between the adsorbed Sb(III) and oxygen in the bulk solution, which promoted the oxidation rate of Sb(III) to Sb(V).     

高岭石和埃洛石-7Å对稀土元素吸附特征的实验研究

针对赣南风化淋积型稀土矿中稀土元素的分异现象,通过研究该稀土矿的主要矿物成分——高岭石和埃洛石-7的矿物特征及其在不同条件下的吸附特征,探讨了该类型稀土矿的成矿机制和稀土元素分异机理。高岭石和埃洛石-7吸附稀土元素的能力受体系p H值以及金属阳离子钾、钠的的影响,且埃洛石-7吸附稀土元素的能力高于高岭石。横向对比实验结果表明当体系中赋存有K+时,两种粘土矿物对稀土元素的吸附呈现出分化趋势,其中高岭石主要吸附中-重稀土元素,埃洛石-7反之,因此风化淋积型稀土矿风化过程中释放出的K+可能会使高岭石和埃洛石-7呈现出差异吸附特征,进而反过来影响稀土矿中稀土元素的配分。     

Amino acid adsorption by clay minerals in distilled water

The adsorption of 15 protein amino acids from dilute (~ 10 μM) distilled water solutions onto organic-free kaolinite and montmorillonite clay minerals (1 wt% suspensions) was determined at room temperature over a 48 hour period. The systems came to steady state within 2 hours. Basic (positively charged) amino acids were strongly adsorbed (40–80% removal) by both clay minerals. Neutral (uncharged) amino acids were taken up appreciably (10–15%) by montmorillonite, but little if any (<5%) by kaolinite. Acidic (negatively charged) amino acids were adsorbed (20–35%) only by kaolinite. These adsorption patterns appear to be related in part to electrostatic interactions between the clay mineral surfaces and the different amino acid types. The measured extents and selectivities of adsorption onto these clay minerals are sufficiently great to potentially affect the distributions and reactions of free amino acids in natural environments.     

Chloritoid stability in manganese rich low-grade metamorphic rocks,Venn-Stavelot Massif,Ardennes

The Southern Venn-Stavelot Massif is characterized by Ordovician and Devonian rocks very rich in manganese and aluminum, which are attacked by a low grade regional metamorphism. The assemblages 1 (phengite, paragonite, chlorite, chloritoid, garnet, quartz, hematite, rutile) and 2 (phengite, paragonite, chlorite, kaolinite (andalusite, pyrophyllite), garnet, quartz, hematite, rutile) are of basic interest for the formation of chloritoid. As the two rock types are isofaciell and quasi-identical in chemistry except for the iron oxides, there is clear evidence for the influence of on the chloritoid formation at its lower p-T stability limit. This can be shown by a discussion of the phase relations of chloritoid, garnet, kaolinite, chlorite and phengite in respect to the oxidation ratio mol 2 Fe₂O₃x 100/2 Fe₂O₃+ FeO of the host rocks. Especially chloritoid and chlorite change their chemistry in a characteristic way with rising oxidation ratio in getting richer and richer in manganese and magnesium (chloritoid) and magnesium (chlorite). A simultaneous increase in trivalent iron in these phases is supposed. At an oxidation ratio of 85–90 the stability limit of chloritoid is reached. The increasing substitution of manganese and magnesium up to this limit should have a stabilizing effect. In a rough estimate the oxygen partial pressure is supposed to be in the order of 10⁻¹⁰ atm at the stability limit of chloritoid assuming a temperature of metamorphism between 360–400° C. Rocks with oxidation ratios between 90 and 100 are characterized by the presence of kaolinite. If the oxidation ratio is still higher (all iron as Fe³⁺, parts of the manganese in the trivalent state), the rocks belong to assemblage 3 (phengite, paragonite, chlorite, viridine, (kaolinite), (garnet), quartz, hematite, braunite, rutile). Dedicated to Prof. Dr. K. Jasmund at his 60. birthday.     

Parameters controlling the partitioning of tributyltin (TBT) in aquatic systems

In the present study the distribution of TBT between solid and water phase as a function of several parameters was determined. Two types of clay minerals (Na-montmorillonite SWy and kaolinite KGa) and quartz sand were used as sorbents in conventional batch experiments. Sorption coefficients (K_d) followed the order montmorillonite (89 l/kg) > kaolinite (51 l/kg) > quartz (25 l/kg), while for sorption coefficients normalized to the surface area (Kd′) an opposite trend was observed, with the lowest value determined for montmorillonite (2.79 × 10⁻³ l/m²) and the highest for quartz sand (8.04 × 10⁻² l/m²). The results demonstrate that numerous environmental parameters influence the adsorption process of TBT, such as solid/solution ratio, clay content and salinity. Another important factor governing TBT adsorption is pH, because it affects both the TBT species in the water phase as well as the surface properties of the mineral phase. The maximum of TBT adsorption onto clays was always around pH 6–7. According to the data, it is evident that the content of organic matter in the solid phase plays an important role on TBT adsorption, either as particulate organic matter (POM) or organic matter adsorbed to mineral particles (AOM). Experiments were carried out with well characterized organic matter and the results showed a linear increase of K_d from 51 up to 2700 l/kg upon the addition of 5% of particulate organic matter to pure phased kaolinite. TBT adsorption onto mineral surfaces, which were previously enriched with adsorbed organic matter, was investigated at different pH. The present study points to the importance of identifying and characterizing sorbents and envrionmental conditions, in order to predict and model TBT distribution in natural systems.     

Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants

Recent studies show that ferrous iron (Fe^II), which is often abundant in anaerobic soil and groundwater, is capable of abiotically reducing many subsurface contaminants. However, studies also demonstrate that Fe^II redox reactivity in geochemical systems is heavily dependent upon metal speciation. This contribution examines the influence of hydroxamate ligands, including the trihydroxamate siderophore desferrioxamine B (DFOB), on Fe^II reactions with nitroaromatic groundwater contaminants (NACs). Experimental results demonstrate that ring-substituted NACs are reduced to the corresponding aniline products in aqueous solutions containing Fe^II complexes with DFOB and two monohydroxamate ligands (acetohydroxamic acid and salicylhydroxamic acid). Reaction rates are heavily dependent upon solution conditions and the identities of both the Fe^II-complexing hydroxamate ligand and the target NAC. Trends in the observed pseudo-first-order rate constants for reduction of 4-chloronitrobenzene (k_obs, s⁻¹) are quantitatively linked to the formation of Fe^II species with standard one-electron reduction potentials, (Fe^III/Fe^II), below −0.3 V. Linear free energy relationships correlate reaction rates with the (Fe^III/Fe^II) values of different electron-donating Fe^II complexes and with the apparent one-electron reduction potentials of different electron-accepting NACs, (ArNO₂). Experiments describing a redox auto-decomposition mechanism for Fe^II-DFOB complexes that occurs at neutral pH and has implications for the stability of hydroxamate siderophores in anaerobic environments are also presented. Results from this study indicate that hydroxamates and other Fe^III-stabilizing organic ligands can form highly redox-active Fe^II complexes that may contribute to the natural attenuation and remediation of subsurface contaminants.     

Sorption and transport of aqueous FeII in a goethite-coated sand column under anoxic conditions

Experiments were conducted under static batch and dynamic flow conditions to evaluate the sorption of Fe^II onto three goethites (G1, G2 and G3) having different crystal habits, morphologies and surface properties. Results reveal that G1 exhibited the highest Fe^II sorption extent and lowest kinetic rate constant, which may result from higher surface site density, surface roughness and edge surface faces. Surface complexation modeling parameters derived from batch experiments were combined with hydrodynamic parameters to simulate breakthrough curves in goethite-coated sand packed columns. The total sorbed amount of Fe^II at complete breakthrough was in agreement with that expected from the batch experiments, except for G1. Sorption breakthrough predictions that make use of surface complexation parameters accurately predicted Fe^II mobility in G2 and G3 columns, but poorly in G1 column. Experiments at various flow rates in G1 columns represented different amounts of Fe^II sorbed at complete breakthrough, thereby underscoring the impact of kinetic sorption. Moreover, Fe dissolution/re-precipitation or Fe^II-induced transformation of goethite was suspected at the lowest flow rate in the G1 column. The influence of goethite phase specific reactivity on Fe^II sorption under batch versus advective–dispersive flow is herein demonstrated. These findings have strong implications to assess transport of Fe^II and environmental contaminants both in natural and engineered systems.     

Fe and Al oxides distribution in some ultisols and inceptisols of southeastern Nigeria in relation to soil total phosphorus

Ten highly weathered soils in southeastern Nigeria were sampled from their typical A and B horizons for analyses. The objectives were to determine the different forms of Fe and Al oxides in the soils and relating their occurrence to phosphate availability and retention in the soils. The soils are deep and often physically degraded but are well drained and coarse in the particle size distribution. They are mostly dominated by kaolinite in their mineralogy with very high values of SiO₂. The soils are acidic with low soil organic carbon (SOC) contents. The elements in the exchange complex are also low thus reflecting in the low CEC of the soil. Available phosphorus (P) in the soils are generally low while total P ranged from 157 to 982 mg kg⁻¹ with an overall average of 422 mg kg⁻¹. Total Fe in the soil is highest and their order represented as follows: Fe_t > Fe_d > Fe_ox ≥ Fe_p. The pyrophosphate extractable Fe was always higher in the top soil than in the subsoil and was attributed to the fact that these forms of Fe are associated with organic matter which is more abundant in topsoil than in subsoil. Like in Fe forms, the order of Al occurrence could generally be presented as; Al_t > Al_d > Al_ox > Al_p. More Fe and Al oxides in the soils are strongly crystalline while a small quantity is poorly crystalline Fe forms. The amorphous forms of both Fe and Al are very low in the soils when compared with the crystalline forms. The oxides that show very strong affinity to total P are Fe_d–Fe_ox, Fe_d, Al_d, Fe_t, Fe_ox and Al_ox/Al_d. To overcome this problem of P retention in the soil, we recommend constant liming of these soils to neutralize them, application of organic matter and of high dosage of phosphate fertilizer to the soils.     

安徽庐枞盆地酸性蚀变岩帽地质地球化学特征研究

酸性蚀变岩帽是浅成低温热液系统演化的产物,形成于酸性高氧化性流体的化学条件下;在高硫化型浅成低温热液金矿床中广泛发育,是该类矿床的显著识别特征。通过对酸性蚀变岩帽的野外地质特征、矿物共生组合和地球化学特征研究,能较好阐明浅成低温成矿热液系统的特征、性质、发生和发展演化及成矿作用过程。庐枞矿集区是长江中下游成矿带重要的矿集区之一,盆地内广泛发育以明矾石为特征蚀变矿物的酸性蚀变岩帽,面积超过30km~2,指示盆地内高硫化浅成低温热液系统的存在。目前为止,前期工作主要针对明矾石矿床地质特征和明矾石资源储量进行,该酸性蚀变岩帽的地质地球化学特征研究尚未开展。本次工作通过对酸性蚀变岩帽系统的野外采样、全岩地球化学分析和短波红外光谱测试分析技术(PNIRS测试)分析,确定其主要赋存在砖桥组火山岩中,组成矿物为石英、明矾石、高岭石、地开石,此外有少量绢云母、伊利石、珍珠陶土、叶蜡石、褐铁矿,极少数的叶腊石和黄钾铁矾等,在钻孔深部存在浸染状和半自形粒状黄铁矿。由于受到地表风化剥蚀和不同热热中心的影响,水平方向从矾山明矾石矿床向外围发育石英+明矾石带、石英+高岭石/地开石+明矾石带、石英+高岭石/地开石带、硅化带以及最外围的泥质带即高岭石±绢云母±伊利石带。根据酸性蚀变岩帽的矿物组合和主量元素特征,可将其分为三类:硅质蚀变岩、明矾石蚀变岩和粘土蚀变岩。硅质蚀变岩中SiO_2含量发生明显的富集作用,其余主量元素(K_2O、Na_2O、Al_2O_3、Fe_2O_3、P_2O5)含量显著降低;明矾石蚀变岩和粘土蚀变岩具有相似的地球化学特征,SiO_2、Al_2O_3、Fe_2O_3、P_2O_5元素含量范围变大,K_2O和Na_2O含量降低,且Na_2O降低更加明显;而钛为不活泼元素,在岩石发生蚀变过程中TiO_2含量变化很小。矾山地区的酸性蚀变岩帽的产状、蚀变类型、地球化学特征受构造和地层的双重控制。     

Structure, charge distribution, and electron hopping dynamics in magnetite (Fe3O4) (1 0 0) surfaces from first principles

For the purpose of improving fundamental understanding of the redox reactivity of magnetite, quantum-mechanical calculations were applied to predict Fe²⁺ availability and electron hopping rates at magnetite (1 0 0) surfaces, with and without the presence of adsorbed water. Using a low free energy surface reconstruction (½-Fe_tet layer relaxed into the Fe_oct (1 0 0) plane), the relaxed outermost layer of both the hydrated and vacuum-terminated surfaces were found to be predominantly enriched in Fe²⁺ within the octahedral sublattice, irrespective of the presence of adsorbed water. At room temperature, mobile electrons move through the octahedral sublattice by Fe²⁺-Fe³⁺ valence interchange small polaron hopping, calculated at 10¹⁰-10¹² hops/s for bulk and bulk-like (i.e., near-surface) environments. This process is envisioned to control sustainable overall rates of interfacial redox reactions. These rates decrease by up to three orders of magnitude (10⁹ hops/s) at the (1 0 0) surface, and no significant difference is observed for vacuum-terminated versus hydrated cases. Slower hopping rates at the surface appear to arise primarily from larger reorganization energies associated with octahedral Fe²⁺-Fe³⁺ valence interchange in relaxed surface configurations, and secondarily on local charge distribution patterns surrounding Fe²⁺-Fe³⁺ valence interchange pairs. These results suggest that, with respect to the possibility that the rate and extent of surface redox reactions depend on Fe²⁺ availability and its replenishment rate, bulk electron hopping mobility is an upper-limit for magnetite and slower surface rates may need to be considered as potentially rate-limiting. They also suggest that slower hopping mobilities calculated for surface environments may be amenable to Fe²⁺-Fe³⁺ site discrimination by conventional spectroscopic probes.     

Ferric iron in sediments as a novel CO2 mineral trap: CO2–SO2 reaction with hematite

Thermodynamic simulations of reactions among SO₂-bearing CO₂-dominated gas, water and mineral phases predict that Fe^III in sediments should be converted almost entirely to dissolved Fe^II and siderite (FeCO₃), and that SO₂ should simultaneously be oxidized to dissolved sulfate. The reactions are however, subject to kinetic constraints which may result in deviation from equilibrium and the precipitation of other metastable mineral phases. To test the prediction, a laboratory experiment was carried out in a well stirred hydrothermal reactor at 150 °C and 300 bar with hematite, 1.0 m NaCl, 0.5 m NaOH, SO₂ in quantity sufficient to reduce much of the iron, and excess CO₂. The experiment produced stable siderite and metastable pyrite and elemental S. Changes in total dissolved Fe are consistent with nucleation of pyrite at ∼17 h, and nucleation of siderite at ∼600 h. Dissolution features present on elemental S at the conclusion of the experiment suggest nucleation early in the experiment. The experiment did not reach equilibrium after ∼1400 h, as indicated by coexistence of hematite with metastable pyrite and elemental sulfur. However, the results confirm that Fe^III can be used to trap CO₂ in siderite if partly oxidized S, as SO₂, is present to reduce the Fe with CO₂ in the gas phase.     

Electron paramagnetic resonance spectroscopy of Fe<Superscript>3+</Superscript> ions in amethyst: thermodynamic potentials and magnetic susceptibility

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe³⁺ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E ₁′. However, the two broad signals do not correspond to any known Fe³⁺ centers in the quartz lattice, but are most likely attributable to Fe³⁺ clusters on surfaces. The absolute numbers of spins of the Fe³⁺ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy E _a, entropy of activation ΔS and enthalpy of activation ΔH for the Fe³⁺ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe³⁺ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.     

Adsorption of arsenic by natural aquifer material in the San Antonio-El Triunfo mining area, Baja California, Mexico

 Several experiments of arsenic (As) adsorption by aquifer material of the San Antonio-El Triunfo (SA-ET) mining area were conducted to test the feasibility of this material acting as a natural control for As concentrations in groundwater. This aquifer material is mineralogically complex, composed of quartz, feldspar, calcite, chlorite, illite, and magnetite/hematite. The total iron content (Fe₂O₃) in the fine fraction is ∼12%, whereas Fe₂O₃ in the coarse fraction is <10 wt%. The experimental percent total As adsorbed vs. pH curves obtained match the topology of total As adsorbed onto iron oxi-hydroxides surface (arsenate + arsenite; high adsorption at low pH, low adsorption at high pH). A maximum of about 80% adsorbed in the experiments suggests the presence of arsenite in the experimental solutions. The experimental adsorption isotherm at pH 7 indicates saturation of surface sites at high solute concentrations. Surface titration of the aquifer material indicates a point of zero charge (PZC) for the adsorbent of about 8 to 8.5 (PZC for iron oxyhydroxides =7.9–8.2). Comparison between experimental and modeled results (using the MICROQL and MINTEQA2 geochemical modeling and speciation computer programs) suggests that As is being adsorbed mostly by oxyhydroxides surfaces in the natural environment. Based on an estimated retardation factor (R), the travel time of the As plume from the SA-ET area to La Paz and Los Planes is about 700 to 5000 years. Received: 17 March 1997 · Accepted: 8 September 1997     

Insights for size-dependent reactivity of hematite nanomineral surfaces through Cu sorption

Macroscopic sorption edges for Cu²⁺ were measured on hematite nanoparticles with average diameters of 7 nm, 25 nm, and 88 nm in 0.1 M NaNO₃. The pH edges for the 7 nm hematite were shifted approximately 0.6 pH units lower than that for the 25 nm and 88 nm samples, demonstrating an affinity sequence of 7 nm > 25 nm = 88 nm. Although, zeta potential data suggest increased proton accumulation at the 7 nm hematite surfaces, changes in surface structure are most likely responsible for the preference of Cu²⁺ for the smallest particles. As Cu²⁺ preferentially binds to sites which accommodate the Jahn-Teller distortion of its coordination to oxygen, this indicates the relative importance of distorted binding environments on the 7 nm hematite relative to the 25 nm and 88 nm particles. This work highlights the uniqueness of surface reactivity for crystalline iron oxide particles with decreasing nanoparticle diameter.