Reduction of Fe(III) colloids by Shewanella putrefaciens: A kinetic model

A kinetic model for the microbial reduction of Fe(III) oxyhydroxide colloids in the presence of excess electron donor is presented. The model assumes a two-step mechanism: (1) attachment of Fe(III) colloids to the cell surface and (2) reduction of Fe(III) centers at the surface of attached colloids. The validity of the model is tested using Shewanella putrefaciens and nanohematite as model dissimilatory iron reducing bacteria and Fe(III) colloidal particles, respectively. Attachment of nanohematite to the bacteria is formally described by a Langmuir isotherm. Initial iron reduction rates are shown to correlate linearly with the relative coverage of the cell surface by nanohematite particles, hence supporting a direct electron transfer from membrane-bound reductases to mineral particles attached to the cells. Using internally consistent parameter values for the maximum attachment capacity of Fe(III) colloids to the cells, M_max, the attachment constant, K_P, and the first-order Fe(III) reduction rate constant, k, the model reproduces the initial reduction rates of a variety of fine-grained Fe(III) oxyhydroxides by S. putrefaciens. The model explains the observed dependency of the apparent Fe(III) half-saturation constant, , on the solid to cell ratio, and it predicts that initial iron reduction rates exhibit saturation with respect to both the cell density and the abundance of the Fe(III) oxyhydroxide substrate.     

K-H3OFe3(SO4)2(OH)6铁矾的XRD研究

根据X射线衍射(XRD)分析发现: A Fe₃(SO₄)₂(OH)₆(A=K⁺、H₃O⁺)系列铁钒的XRD数据十分相近,难以用XRD区别,需通过能谱(EDS)辅助分析,才能区分此类铁矾。另外,此类铁矾的003和107面网间距d随K⁺含量增大而增大,且呈一元三次方程的关系;而033和220面网间距d随K⁺含量增大而减小,呈一元二次方程的关系。对该现象从铁矾晶体结构方面进行解释:K⁺、H₃O⁺离子位于较大空隙中,且沿着Z轴方向排列,当K⁺、H₃O⁺离子之间相互替换时,会导致该铁矾晶体结构在Z轴方向有较明显的变化。     

碱熔共沉淀-电感耦合等离子体质谱法测定橄榄岩中的稀土元素

橄榄岩的稀土元素特征对研究岩石成因、岩浆作用过程具有重要的意义。橄榄岩中的稀土元素含量低(∑REEs=0.1~1μg/g),且存在镁、铁等基体元素的干扰,难以准确测定。前人通常利用高压密闭酸溶-离子交换法处理样品,将稀土元素与镁、铁等基体元素分离,达到了预富集的效果,但耗时长(消解时间接近7天)、操作步骤繁多,不利于大批量样品的分析。本文建立了过氧化钠碱熔、Fe(OH)_3和Mg(OH)_2共沉淀的样品前处理方法,通过离心使溶液与沉淀分离,从而实现了稀土元素与镁、铁等基体元素的快速分离,再采用电感耦合等离子体质谱法测定稀土元素含量。方法检出限为0.17~2.18 ng/g,加标回收率为95%~101%,国家标准物质(GBW07101和GBW07102)的测定值与标准值的相对误差小于20%,相对标准偏差(RSD,n=11)小于10%。该方法既减少了分步沉淀过程中带来的损失,也缩短了分析周期(消解时间仅需一天),操作简便,分析效率高。     

Solid-liquid equilibria of Mg(OH)2(cr) and Mg2(OH)3Cl·4H2O(cr) in the system Mg-Na-H-OH-Cl-H2O at 25°C

The solubility of crystalline Mg(OH)₂(cr) was determined by measuring the equilibrium H⁺ concentration in water, 0.01-2.7 m MgCl₂, 0.1-5.6 m NaCl, and in mixtures of 0.5 and 5.0 m NaCl containing 0.01-0.05 m MgCl₂. In MgCl₂ solutions above 2 molal, magnesium hydroxide converted into hydrated magnesium oxychloride. The solid-liquid equilibrium of Mg₂(OH)₃Cl·4H₂O(cr) was studied in 2.1-5.2 m MgCl₂. Using known ion interaction Pitzer coefficients for the system Mg-Na-H-OH-Cl-H₂O (25°C), the following equilibrium constants at I = 0 are calculated:

     

Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)4

The stable boron isotope ratio (¹¹B/¹⁰B) in marine carbonates is used as a paleo-pH recorder and is one of the most promising paleo-carbonate chemistry proxies. Understanding the thermodynamic basis of the proxy is of fundamental importance, including knowledge on the equilibrium fractionation factor between dissolved boric acid, B(OH)₃, and borate ion, B(OH)₄⁻ (, hereafter α_(B3-B4)). However, this factor has hitherto not been determined experimentally and a theoretically calculated value (Kakihana and Kotaka, 1977, hereafter KK77) has therefore been widely used. I examine the calculations underlying this value. Using the same spectroscopic data and methods as KK77, I calculate the same α_(B3−B4) = 1.0193 at 300 K. Unfortunately, it turns out that in general the result is sensitive to the experimentally determined vibrational frequencies and the theoretical methods used to calculate the molecular forces. Using analytical techniques and ab initio molecular orbital theory, the outcome for α_(B3-B4) varies between ∼1.020 and ∼1.050 at 300 K. However, several arguments suggest that α_(B3-B4) ? 1.030. Measured isotopic shifts in various ¹⁰B-, ²D-, and ¹⁸O-labeled isotopomers do not provide a constraint on stable boron isotope fractionation. I conclude that in order to anchor the fundamentals of the boron pH proxy, experimental work is required. The critics of the boron pH proxy should note, however, that uncertainties in α_(B3-B4) do not bias pH reconstructions provided that organism-specific calibrations are used.     

Dissolution of jarosite [KFe3(SO4)2(OH)6] at pH 2 and 8: Insights from batch experiments and computational modelling

Jarosite [KFe₃(SO₄)₂(OH)₆] is a mineral that is common in acidic, sulphate-rich environments, such as acid sulphate soils derived from pyrite-bearing sediments, weathering zones of sulphide ore deposits and acid mine or acid rock drainage (ARD/AMD) sites. The structure of jarosite is based on linear tetrahedral-octahedral-tetrahedral (T-O-T) sheets, made up from slightly distorted FeO₆ octahedra and SO₄ tetrahedra. Batch dissolution experiments carried out on synthetic jarosite at pH 2, to mimic environments affected by ARD/AMD, and at pH 8, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)₂), suggest first order dissolution kinetics. Both dissolution reactions are incongruent, as revealed by non-ideal dissolution of the parent solids and, in the case of the pH 8 dissolution, because a secondary goethite precipitate forms on the surface of the dissolving jarosite grains. The pH 2 dissolution yields only aqueous K, Fe, and SO₄. Aqueous, residual solid, and computational modelling of the jarosite structure and surfaces using the GULP and MARVIN codes, respectively, show for the first time that there is selective dissolution of the A- and T-sites, which contain K and SO₄, respectively, relative to Fe, which is located deep within the T-O-T jarosite structure. These results have implications for the chemistry of ARD/AMD waters, and for understanding reaction pathways of ARD/AMD mineral dissolution.     

Influence of Mn oxides on the reduction of uranium(VI) by the metal-reducing bacterium Shewanella putrefaciens

The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by the subsurface bacterium Shewanella putrefaciens strain CN32 was investigated using synthetic Mn(III/IV) oxides (pyrolusite [β-MnO₂], bixbyite [Mn₂O₃] and K⁺-birnessite [K₄Mn₁₄O₂₇ · 8H₂O]). In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO₂[s]) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence or in the presence of gibbsite (Al[OH]₃) added as a non-redox-reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43 to 100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. However, in the absence of Mn(III/IV) oxides, UO₂(s) accumulated as copious fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments. However, the accumulation of U(IV) in the cell periplasm may physically protect reduced U from oxidation, promoting at least a temporal state of redox disequilibria.     

Thermodynamic properties of the Sb(III) hydroxide complex Sb(OH)3(aq) at hydrothermal conditions

The standard thermodynamic properties and Helgeson-Kirkham-Flowers (HKF) parameters for Sb(OH)_3(aq) have been estimated. For this purpose, the available solubility data for senarmontite, valentinite, stibnite, and native Sb in a wide range of temperatures (15 to 450°C) and pressures (1 to 1000 bar), and thermodynamic properties of Sb oxides (senarmontite and valentinite) have been critically analyzed. Published data were complimented by results from new experiments performed by solubility and solid-state galvanic cell methods. Both experimental data and thermodynamic calculations show that the hydroxide complex Sb(OH)_3(aq) is primarily responsible for hydrothermal transport of antimony, especially at temperatures above 250°C.     

The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)12(aq) molecule and aqueous solutions

We report rates of oxygen exchange with bulk solution for an aqueous complex, ^IVGeO₄Al₁₂(OH)₂₄(OH₂)₁₂⁸⁺(aq) (GeAl₁₂), that is similar in structure to both the ^IVAlO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (Al₁₃) and ^IVGaO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (GaAl₁₂) molecules studied previously. All of these molecules have ε-Keggin-like structures, but in the GeAl₁₂ molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al₁₃ and GaAl₁₂. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using ¹⁷O-NMR.Apparent rate parameters for exchange of the bound water molecules (η-OH₂) are k_ex²⁹⁸ = 200 (±100) s⁻¹, ΔH^‡ = 46 (±8) kJ · mol⁻¹, and ΔS^‡ = −46 (±24) J · mol⁻¹ K⁻¹ and are similar to those we measured previously for the GaAl₁₂ and Al₁₃ complexes. In contrast to the Al₁₃ and GaAl₁₂ molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl₁₂ species.The two topologically distinct μ₂-OH sites in the GeAl₁₂ molecule exchange at greatly differing rates. The more labile set of μ₂-OH sites in the GeAl₁₂ molecule exchange at a rate that is faster than can be measured by the ¹⁷O-NMR isotopic-equilibration technique. The second set of μ₂-OH sites have rate parameters of k_ex²⁹⁸ = 6.6 (±0.2) · 10⁻⁴ s⁻¹, ΔH^‡ = 82 (±2) kJ · mol⁻¹, and ΔS^‡ = −29 (±7) J · mol⁻¹ · K⁻¹, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ₂-OH sites in the Al₁₃ and GaAl₁₂ molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ₂-OH site with bulk solution for the GeAl₁₂ molecule, which contrasts with Al₁₃ and GaAl₁₂ molecules.     

Free energy of formation for green rust sodium sulphate (NaFe6Fe3(OH)18(SO4)2(s))

In a recent study, sulphate-bearing green rust (GR_SO4) was shown to incorporate Na⁺ in its structure (NaFe^II₆Fe^III₃(OH)₁₈(SO₄)_2(s); GR_Na,SO4). The compound was synthesised by aerial oxidation of Fe(OH)_2(s) in the presence of NaOH. This paper reports on its free energy of formation .Freshly synthesised GR_Na,SO4 was titrated with 0.5 M H₂SO₄ in an inert atmosphere at 25 °C, producing dissolved Fe²⁺ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions:

     

Acid dissociation mechanisms of Si(OH)4 and Al(H2O)6 in aqueous solution

Silicic acid and the hexa-aqua of Al³⁺ are fundamental model aqueous species of chemical importance in nature. In order to investigate their hydroxyl dissociation mechanisms, Car-Parrinello molecular dynamics (CPMD) simulations were carried out, which allow treating the solutes and solvents on the same footing. The method of constraint was employed to trigger the reactions by taking coordination number as the reaction coordinate and the thermodynamic integration was used to obtain the free-energy profiles. The approximate transition states were located and the reactant and product states were also characterized. The free-energy changes of dissociation are found about 15.0 kcal/mol and 7.7 kcal/mol for silicic acid and Al-aqua, respectively. From the simulation results, the first pKas were calculated by using two approaches, which are based on the pristine thermodynamic relation and the RDF (radial distribution function)-free energy relation, respectively. Because of more uncertainties involved in the RDF way, it is suggested that the pristine way should be favored, which shows an error margin of 1 pKa unit. This study provides an encouraging basis for applying the present methodology to predict acidity constants of those groups that are difficult to measure experimentally.     

Rates of oxygen exchange between the Al2O8Al28(OH)56(H2O)26(aq) (Al30) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₆¹⁸⁺(aq) (Al₃₀) are reported at pH≈4.7 and 32-40°C. The Al₃₀ molecule is a useful model for geochemists because it is ≈2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al₃₀ molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ₃-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ₂-OH and μ₃-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of ¹⁷O-enriched water, growth of the ¹⁷O-NMR peak near 37 ppm, which is assigned to μ₂-OH and μ₃-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the ¹⁷O-NMR spectra, assigned to bound water molecules (η-OH₂), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH₂ sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH₂ sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH₂ sites on the Al₃₀.     

Solubility of Fe-ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O)

The solubility of Fe-ettringite (Ca₆[Fe(OH)₆]₂(SO₄)₃ · 26H₂O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe-ettringite was found to be stable up to pH 13.0. At higher pH-values Fe-monosulphate (Ca₄[Fe(OH)₆]₂(SO₄) · 6H₂O) and Fe-monocarbonate (Ca₄[Fe(OH)₆]₂(CO₃) · 6H₂O) are formed. The solubilities of these hydrates at 25 °C are:      

Structural change and mineralogical transformation mechanism of aluminum hydroxide gels from forced hydrolysis Al（Ⅲ） solutions containing AlO4Al12（OH）24（H2O）12^7＋ polyoxycation during aging

The structural change and mineralogy of Al gel during aging time were investigated by using spectroscopy techniques. The results indicated that: 1) the aggregation extent of solution-gel system increases with aging time, and the structure of amorphous gel becomes more short-ordered; 2) after six months, the gel formats nordstrandite and little gibbsite; 3) a marked decrease in the number of (Al-OH)_oh bands occurring at 610 cm⁻¹ and increase in the number of (Al-OH₂)_oh bands occurring at 555 cm⁻¹ indicate that the gel undergoes rearrangement-like process during aging; 4) the gel constantly contains Al-O tetrahedron of Keggin structure, but the signal peak occurring at ≈61×10⁻⁶ of ²⁷Al MAS NMR have a slight shift to downfield with aging time. A mineralogical transformation mechanism for hydrolysis Al(III) solution was proposed.     

The flux of oxygen from the basal surface of gibbsite (α-Al(OH)3) at equilibrium

Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (μ₂-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of ¹⁸O from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in ¹⁸O. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched H₂¹⁸O in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates ¹⁸O into the least-reactive sites, which are presumably μ₂-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the η-OH₂ sites exchange within fractions of a second and should be isotopically normal using this procedure.When suspended in isotopically normal electrolyte solutions, we find that the rates of release of ¹⁸O from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of μ₂-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the μ₂-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution.