Kinetics and mechanism of natural fluorapatite dissolution at 25 °C and pH from 3 to 12

The dissolution rates of natural fluorapatite (FAP), Ca₁₀(PO₄)₆F₂, were measured at 25 °C in mixed-flow reactors as a function of pH from 3.0 to 11.7, and aqueous calcium, phosphorus, and fluoride concentration. After an initial preferential Ca and/or F release, stoichiometric Ca, P, and F release was observed. Measured FAP dissolution rates decrease with increasing pH at 3 ? pH ? 7, FAP dissolution rates are pH independent at 7 ? pH ? 10, and FAP dissolution rates again decrease with increasing pH at pH ? 10. Measured FAP dissolution rates are independent of aqueous Ca, P, and F concentration at pH ≈ 3 and pH ≈ 10.Apatite dissolution appears to be initiated by the relatively rapid removal from the near surface of F and the Ca located in the M1 sites, via proton for Ca exchange reactions. Dissolution rates are controlled by the destruction of this F and Ca depleted surface layer. The destruction of this layer is facilitated by the adsorption/penetration of protons into the surface at acidic conditions, and by surface hydration at neutral and basic conditions. Taking into account these two parallel mechanisms, measured fluorapatite forward dissolution rates can be accurately described using

     

Surface chemistry of disordered mackinawite (FeS)

Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K_s^app = {Fe²⁺} · {H₂S(aq)} · {H⁺}⁻² = 10^+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ∼7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10⁻³ mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10⁻³ mol/g FeS, determines surface charge changes. Total site density is 4 sites nm⁻². The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.     

Humic substance charge determination by titration with a flexible cationic polyelectrolyte

The anionic charge of humic substances (HS) plays a major role in the interaction of HS with other components. Therefore, the potential of the polyelectrolyte titration technique to obtain the charge density of HS in simple 1-1 electrolyte solutions has been investigated. Titrations are carried out with an automatic titrator combined with the “Mütek particle charge detector” which allows determination of the Mütek potential and the pH as a function of the added amount of titrant which is a solution of poly-diallyldimethylammonium chloride (polyDADMAC), a cationic strong polyelectrolyte. When the Mütek potential reverses its sign the iso-electric point (IEP) of the polyDADMAC-HS complex is reached. The polyDADMAC/HS mass ratio at the IEP gives information on the HS charge density and from the pH changes in solution an estimate of the charge regulation in the HS-polyDADMAC complex can be obtained. In general, for polyDADMAC-HS complexes an increase in the dissociation of the acid groups of HS is found (charge regulation). The charge regulation decreases with increasing concentration of 1-1 background electrolyte. Cation incorporation can be neglected at 1-1 electrolyte concentrations ? 1 mmol L⁻¹ and a 1-1 stoichiometry exists between the polyDADMAC and HS charge. However, at these low salt concentrations the charge regulation is substantial. A detailed analysis of purified Aldrich humic acid (PAHA) at pH 5 and a range of KCl concentrations reveals that the anionic charge of PAHA in the complex increases at 5 mmol L⁻¹ KCl by 30% and at 150 mmol L⁻¹ KCl by 12%. On the other hand, increasing amounts of K⁺ become incorporated in the complex: at 5 mmol L⁻¹ KCl 5% and at 150 mmol L⁻¹ KCl 24% of the PAHA charge is balanced by K⁺. By comparing at pH 5 the mass ratios polyDADMAC/PAHA in the complex at the IEP with the theoretical mass ratios of polyDADMAC/PAHA required to neutralize PAHA in the absence of charge regulation and K⁺ incorporation, it is found that at 50 mmol L⁻¹ KCl the extra negative charge due to the interaction between polyDADMAC and PAHA is just compensated by K⁺ incorporation in the complex. Therefore, a pseudo 1-1 stoichiometry exists at about 50 mmol L⁻¹ 1-1 electrolyte concentration and only at this salt concentration polyDADMAC titrations and conventional proton titrations give identical results. Most likely this is also true for other HA samples and other pH values. For FA further study is required to reveal the conditions for which polyDADMAC and proton titrations give identical results.     

Surface chemistry of kaolinite and Na-montmorillonite in aqueous electrolyte solutions at 25 and 60 °C: Experimental and modeling study

The aqueous interfacial chemistry of kaolinite and Na-montmorillonite samples was investigated by potentiometric measurements using acid/base continuous titrations and batch experiments at 25 and 60 °C. Using the batch experimental method, a continuous drift of pH was observed reflecting the mineral dissolution. Consequently, the continuous titration method appears to be the best way of studying solid surface reactions. For each clay mineral, the net proton surface excess/consumption was calculated as a function of pH and ionic strength (0.025, 0.1 and 0.5 M). At 25 °C, and according to the literature data, the pH corresponding to zero net proton consumption for montmorillonite appears to depend on ionic strength, whereas the value for kaolinite is constant and close to 5. Similar results are obtained at 60 °C, which suggests that the point of zero net proton consumption for clay minerals does not depend on temperature, at least up to 60 °C. On the other hand, the temperature rise induces a slight increase of the net proton surface excess. Finally, the diffuse double layer formalism (DDLM) is used to model the experimental data. The model involves two processes: the protonation/deprotonation of two types of edge sites (aluminol and silanol) and H⁺/Na⁺ exchange reactions on basal surfaces, while a tiny proportion of the negative structural charge remains uncompensated. This last process maintains a negative surface potential whatever the pH of the solution, which is in agreement with electrokinetic data.     

The effect of pH and ionic strength on proton adsorption by the thermophilic bacterium Anoxybacillus flavithermus

Numerous studies have utilized surface complexation theory to model proton adsorption behaviour onto mesophilic bacteria. However, few experiments, to date, have investigated the effects of pH and ionic strength on proton interactions with thermophilic bacteria. In this study, we characterize proton adsorption by the thermophile Anoxybacillus flavithermus by performing acid-base titrations and electrophoretic mobility measurements in NaNO₃ (0.001-0.1 M). Equilibrium thermodynamics (Donnan model) were applied to describe the specific chemical reactions that occur at the water-bacteria interface. Acid-base titrations were used to determine deprotonation constants and site concentrations for the important cell wall functional groups, while electrophoretic mobility data were used to further constrain the model. We observe that with increasing pH and ionic strength, the buffering capacity increases and the electrophoretic mobility decreases. We develop a single surface complexation model to describe proton interactions with the cells, both as a function of pH and ionic strength. Based on the model, the acid-base properties of the cell wall of A. flavithermus can best be characterized by invoking three distinct types of cell wall functional groups, with pK_a values of 4.94, 6.85, and 7.85, and site concentrations of 5.33, 1.79, and 1.42 × 10⁻⁴ moles per gram of dry bacteria, respectively. A. flavithermus imparts less buffering capacity than pure mesophilic bacteria studied to date because the thermophile possesses a lower total site density (8.54 × 10⁻⁴ moles per dry gram bacteria).     

Solubility of nanocrystalline scorodite and amorphous ferric arsenate: Implications for stabilization of arsenic in mine wastes

Solubility experiments were performed on nanocrystalline scorodite and amorphous ferric arsenate. Nanocrystalline scorodite occurs as stubby prismatic crystals measuring about 50 nm and having a specific surface area of 39.88 ± 0.07 m²/g whereas ferric arsenate is amorphous and occurs as aggregated clusters measuring about 50–100 nm with a specific surface area of 17.95 ± 0.19 m²/g. Similar to its crystalline counterpart, nanocrystalline scorodite has a solubility of about 0.25 mg/L at around pH 3–4 but has increased solubilities at low and high pH (i.e. <2 and >6). Nanocrystalline scorodite dissolves incongruently at about pH > 2.5 whereas ferric arsenate dissolution is incongruent at all the pH ranges tested (pH 2–5). It appears that the solubility of scorodite is not influenced by particle size. The dissolution rate of nanocrystalline scorodite is 2.64 × 10⁻¹⁰ mol m⁻² s⁻¹ at pH 1 and 3.25 × 10⁻¹¹ mol m⁻² s⁻¹ at pH 2. These rates are 3–4 orders of magnitude slower than the oxidative dissolution of pyrite and 5 orders of magnitude slower than that of arsenopyrite. Ferric arsenate dissolution rates range from 6.14 × 10⁻⁹ mol m⁻² s⁻¹ at pH 2 to 1.66 × 10⁻⁹ mol m⁻² s⁻¹ at pH 5. Among the common As minerals, scorodite has the lowest solubility and dissolution rate. Whereas ferric arsenate is not a suitable compound for As control in mine effluents, nanocrystalline scorodite that can be easily precipitated at ambient pressure and temperature conditions would be satisfactory in meeting the regulatory guidelines at pH 3–4.     

The hydrolysis and precipitation of Pd(II) in 0.6 mol kg NaCl: A potentiometric, spectrophotometric, and EXAFS study

The soluble and insoluble hydrolysis products of palladium were investigated in aqueous solutions of 0.6 mol kg⁻¹ NaCl at 298.2 K. Potentiometric titrations of millimolal palladium(II) solutions were used to monitor hydrolysis reactions of the mononuclear PdCl₃OH²⁻ and species. Spectrophotometric titrations were also used to corroborate the speciation change and to extract the correlative molar absorption coefficients for the PdCl₃OH²⁻ species in the 210-320 nm range. Longer-term potentiometric titrations systematically yielded precipitates which matured over a period of 6 weeks and resulted in a more extensive release of protons to the solution. Precipitation experiments in the 3-11 pH range showed the dominant precipitating phase to be Pd(OH)_1.72Cl_0.28. EXAFS measurements yielded an average of 3.50 O and 0.50 Cl atoms per Pd atom with a Pd-O distance of 2.012 Å and a Pd-Cl distance of 2.185 Å. Speciation modeling of proton and palladium mass balance data of experiments for palladium concentrations ranging from 0.047 to 10.0  mmol kg⁻¹ required the presence of polynuclear complexes containing 3-9 palladium atoms. The existence of such complexes is moreover supported by previous investigations of palladium hydroxide chains of the type [Pd(OH)_1.72Cl_0.28]_n, that are coiled and/or aggregated into nanometer-sized (15-40 Å) spheroids.     

Experimental study of igneous and sedimentary apatite dissolution: Control of pH, distance from equilibrium, and temperature on dissolution rates

Apatite dissolution experiments were conducted using both a fluidized bed and stirred tank reactor over a range of pH, temperature, solution saturation state, and on non-carbonated and carbonated apatite compositions: igneous fluorapatite (FAP) and sedimentary carbonate fluorapatite (CFA), respectively. From 2 <pH <6, the rate of release from dissolution of all apatite components [calcium (Ca), phosphorus (P), and fluoride (F)] increased with decreasing pH for FAP. From 6 < pH < 8.5, the FAP dissolution rate is pH independent. Measuring apatite dissolution rates at pH > 8.5 were not possible due to detection limits of the analytical techniques used in this study and the high insolubility of FAP. For the CFA compositions studied, the dissolution rate decreased with increasing pH from 4 < pH < 7. During early stages of the dissolution reaction for both FAP and CFA, mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P and Ca:F being greater than mineral stoichiometric ratios, suggesting that Ca was preferentially released compared to P and F from the mineral structure during the early stages of dissolution. An increase in reacted solution pH accompanies this early elevated release of Ca. As the dissolution reaction proceeded to steady state, dissolution became congruent. When normalized to BET measured surface area, FAP dissolved faster from 4 < pH < 7 compared to CFA. The apparent Arrhenius activation energy (E_a) of FAP dissolution over the temperature range of 25-55°C at pH = 3.0, I = 0.1, and pCO₂ = 0 is 8.3 ± 0.2 kcal mol⁻¹. Both the apparent exchange of solution H⁺ for solid-bound Ca at low pH in the early stage of dissolution and the E_a of dissolution suggest a surface and not a diffusion controlled dissolution reaction for FAP and CFA. The degree of undersaturation of the solution, ΔG_R, with respect to FAP was important in determining the dissolution rate. At pH = 3.0, I = 0.1, and pCO₂ = 0, the dissolution rate of FAP was ∼ 5× greater in the far-from-equilibrium region compared to the near-equilibrium slope region.A simple apatite weathering model incorporating the experimental results from this study was constructed, and numerical calculations suggest that during the Phanerozoic both the surface area of igneous rock available for weathering and the average global temperature were important factors in determining the P weathering flux from apatite dissolution. It is possible that elevated global temperatures coupled with relatively high surface area of igneous rock during the early- to mid-Paleozoic resulted in elevated P weathering fluxes, which along with climatic evolutionary pressures of the Neoproterozoic, facilitated the radiation of multicellular organisms, large-scale phosphorite deposition, and abundance of calcium phosphate shelled organisms during the early Cambrian.     

Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsO4 · 2H2O) and their application to arsenic behavior in buried mine tailings

Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 10^4.04 (FeH₂AsO₄²⁺), 10^9.86 (FeHAsO₄⁺), and 10^18.9 (FeAsO₄). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log K_sp) are −23.0 ± 0.3 and −25.83 ± 0.07, respectively, at 25 °C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2-8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log K_sp of ferric arsenate decreased from −22.80 to −24.67, while that of FO (as Fe(OH)₃) increased from −39.49 to −33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2-3. The same general As(V) behavior is observed in the pore waters of neutralized tailings buried for 5 yr at depths of up to 32 m in the JEB tailings management facility (TMF), where arsenic in the pore water decreases to 1-2 mg/L with increasing age and depth. In the TMF, average apparent log K_sp values for ferric arsenate and ferric hydroxide are −25.74 ± 0.88 and −37.03 ± 0.58, respectively. In the laboratory tests and in the TMF, the increasing crystallinity of scorodite and the amorphous character of the coexisting FO phase increases the stability field of scorodite relative to that of the FO to near-neutral pH values. The kinetic inability of amorphous FO to crystallize probably results from the presence of high concentrations of sulfate and arsenate.     

High- and low-affinity binding sites for Cd on the bacterial cell walls of Bacillus subtilis and Shewanella oneidensis

Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3-9 that requires the presence of four distinct sites, with pK_a values of 3.3 ± 0.2, 4.8 ± 0.2, 6.7 ± 0.4, and 9.4 ± 0.5, and site concentrations of 8.9(±2.6) × 10⁻⁵, 1.3(±0.2) × 10⁻⁴, 5.9(±3.3) × 10⁻⁵, and 1.1(±0.6) × 10⁻⁴ moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1-200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.     

X-ray photoelectron spectroscopic studies of dolomite surfaces exposed to undersaturated and supersaturated aqueous solutions

Cleaved surfaces of dolomite were studied using ex-situ X-ray photoelectron spectroscopy (XPS) following exposure of the surfaces to various experimental conditions. Dolomite samples exposed to air, to a highly undersaturated solution (0.1 M NaCl, pH = 9), and to solution with a supersaturation (−Δμ/kT) of 5.5 (pH = 9) were investigated with semiquantitative methods of analysis to ascertain the degree of non-stoichiometry resulting at the dolomite surface from reactive conditions. It was found that the dolomite cleavage surface in undersaturated solution was not altered significantly from the stoichiometric surface termination. The composition of the cleaved surface after exposure to supersaturated solution, a surface known to have self-limiting growth characteristics under similar conditions, was found to be Ca²⁺ rich (Ca_xMg_2 − x(CO₃)₂, 1.7 > x > 1.3). The observations, while underscoring differences in hydration/dehydration kinetics of the two alkaline earth cations, suggest that achievement of equilibrium at dolomite-water interfaces may be subject to significant barriers from both undersaturated and supersaturated solutions.     

Humic acid protein complexation

Interactions of purified Aldrich humic acid (PAHA) with lysozyme (LSZ) are investigated. In solution LSZ is moderately positively and PAHA negatively charged at the investigated pH values. The proton binding of PAHA and of LSZ is determined by potentiometric proton titrations at various KCl concentrations. It is also measured for two mixtures of PAHA-LSZ and compared with theoretically calculated proton binding assuming no mutual interaction. The charge adaptation due to PAHA-LSZ interaction is relatively small and only significant at low and high pH. Next to the proton binding, the mass ratio PAHA/LSZ at the iso-electric point (IEP) of the complex at given solution conditions is measured together with the pH using the Mütek particle charge detector. From the pH changes the charge adaptation due to the interaction can be found. Also these measurements show that the net charge adaptation is weak for PAHA-LSZ complexes at their IEP. PAHA/LSZ mass ratios in the complexes at the IEP are measured at pH 5 and 7. At pH 5 and 50 mmol/L KCl the charge of the complex is compensated for 30-40% by K⁺; at pH 7, where LSZ has a rather low positive charge, this is 45-55%. At pH 5 and 5 mmol/L KCl the PAHA/LSZ mass ratio at the IEP of the complex depends on the order of addition. When LSZ is added to PAHA about 25% K⁺ is included in the complex, but no K⁺ is incorporated when PAHA is added to LSZ. The flocculation behavior of the complexes is also different. After LSZ addition to PAHA slow precipitation occurs (6-24 h) in the IEP, but after addition of PAHA to LSZ no precipitation can be seen after 12 h. Clearly, PAHA/LSZ complexation and the colloidal stability of PAHA-LSZ aggregates depend on the order of addition. Some implications of the observed behavior are discussed.     

Gold solubility in oxidized and reduced, water-saturated mafic melt

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g⁻¹ to 0.64 μg g⁻¹ at FMQ + 4, and 0.54 μg g⁻¹ to 0.1 μg g⁻¹ at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g⁻¹). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO₂ values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.     

Limitations of the potentiometric titration technique in determining the proton active site density of goethite surfaces

Density of proton active surface sites at mineral surfaces is a property of fundamental importance in equilibrium modeling of surface complexation reactions. In this article, methods for an experimental determination of these sites at the surface of α-FeOOH (goethite) are explored. It is shown that previously obtained saturation data of goethite with respect to protons do not yield a site density that can be considered as an intrinsic sorbent property: the results are below crystallographically expected values and values for different ionic media in terms of composition and concentration yield different numbers—for example, chloride would yield higher values than nitrate at the same concentration, and higher electrolyte concentration would favor higher apparent maxima. Although site saturation might be explained by electrostatic repulsion, which is more efficient at high electrolyte concentration or for certain ions, further independent experimental results show that no saturation occurs on goethite down to ph ≡ −log[H⁺] = 2.2 and possibly to ph = 1.0 in 0.6 M NaCl. For those very low pH values, the experimental charging curve was obtained by coulometric back titration (using the Gran plot) or titrations with tris (hydroxymethyl)-aminomethane of the supernatant of acidified goethite suspension. These experimental data are to our knowledge the first high quality data at such low pHs. However, small errors in the determination of proton concentrations (1%) are shown to strongly affect the shape of the charging curve for ph < 2. Furthermore, goethite dissolution (proton consumption and iron reduction in coulometric titrations) and liquid junction effects interfere at low ph, hampering the straightforward application of coulometric Gran titrations over the whole pH range. From these experiments, it can nonetheless be ascertained that a minimum of 2.5 protons/nm² can be adsorbed at the goethite surface from the point of zero charge (ph 9.4) to pH 0.9. Although these studies are restricted to goethite, those studies in which titrations with excess acid and base have been used for the determination of proton active site concentrations of sorbents should be reconsidered.     

A surface structural model for ferrihydrite I: Sites related to primary charge, molar mass, and mass density

A multisite surface complexation (MUSIC) model for ferrihydrite (Fh) has been developed. The surface structure and composition of Fh nanoparticles are described in relation to ion binding and surface charge development. The site densities of the various reactive surface groups, the molar mass, the mass density, the specific surface area, and the particle size are quantified. As derived theoretically, molecular mass and mass density of nanoparticles will depend on the types of surface groups and the corresponding site densities and will vary with particle size and surface area because of a relatively large contribution of the surface groups in comparison to the mineral core of nanoparticles. The nano-sized (∼2.6 nm) particles of freshly prepared 2-line Fh as a whole have an increased molar mass of M ∼ 101 ± 2 g/mol Fe, a reduced mass density of ∼3.5 ± 0.1 g/cm³, both relatively to the mineral core. The specific surface area is ∼650 m²/g. Six-line Fh (5-6 nm) has a molar mass of M ∼ 94 ± 2 g/mol, a mass density of ∼3.9 ± 0.1 g/cm³, and a surface area of ∼280 ± 30 m²/g. Data analysis shows that the mineral core of Fh has an average chemical composition very close to FeOOH with M ∼ 89 g/mol. The mineral core has a mass density around ∼4.15 ± 0.1 g/cm³, which is between that of feroxyhyte, goethite, and lepidocrocite. These results can be used to constrain structural models for Fh. Singly-coordinated surface groups dominate the surface of ferrihydrite (∼6.0 ± 0.5 nm⁻²). These groups can be present in two structural configurations. In pairs, the groups either form the edge of a single Fe-octahedron (∼2.5 nm⁻²) or are present at a single corner (∼3.5 nm⁻²) of two adjacent Fe octahedra. These configurations can form bidentate surface complexes by edge- and double-corner sharing, respectively, and may therefore respond differently to the binding of ions such as uranyl, carbonate, arsenite, phosphate, and others. The relatively low PZC of ferrihydrite can be rationalized based on the estimated proton affinity constant for singly-coordinated surface groups. Nanoparticles have an enhanced surface charge. The charging behavior of Fh nanoparticles can be described satisfactory using the capacitance of a spherical Stern layer condenser in combination with a diffuse double layer for flat plates.     

Fluorapatite and fluorite solubility controls on geothermal waters in Yellowstone National Park

Potential solubility controls on phosphorus in Yellowstone National Park geothermal waters were investigated using the analytical phosphate estimates of Stauffer and Thompson (1978), the computer program, WATEQF, and adopting the equilibrium constant: $\text{log}\text{K}{}_{\mathrm{25°}}\text{= ?61.4}$ for fluorapatite (FAP = Ca₅(PO₄)₃F) dissolution. The near-boiling high-Cl geyser and spring effluents are at or near saturation with respect to (with) FAP. The sixteen representative springs in this category had FAP saturation indices ($\text{S.I. =}\text{log}\text{IAP/K}{}_{\mathrm{r}}$) ranging from ? 3.2 to +4.9 and averaging +0.9. The strongly positive indices were all associated with the highly alkaline conditions resulting from adiabatic cooling in the near surface environment. Hot spring waters indicating extensive dilution (reduced Cl) by meteoric water have lower pH's, and despite PO₄ and Ca concentrations an order of magnitude higher than the geysers, are still frequently undersaturated with FAP. The travertine-depositing “Mixed-water” springs are invariably supersaturated with FAP at ground surface and at or near saturation with hydroxylapatite. Supersaturation may result from kinetic inhibition of apatite crystallization by the elevated Mg²⁺, HCO₃^?, and lower temperatures in these springs. The phosphates may be residuals of the meteoric dilution water.Separately, if Strübel's temperature-dependent estimates of fluorite (CaF₂) solubility are adopted, the high-Cl geysers and springs on “Geyser Hill” and at Norris are consistently undersaturated with CaF₂ at the 90–100° orifice temperatures. The apparent disequilibrium may reflect fluorite equilibration at the much higher temperatures (> 200°C) in the deeper enthalpy reservoirs.     

Surface complexation modelling of Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) adsorption onto kaolinite

Proton binding constants for the edge and basal surface sites of kaolinite were determined by batch titration experiments at 25 °C in the presence of 0.1 M, 0.01 M and 0.001 M solutions of NaNO₃ and in the pH range 3-9. By optimizing the results of the titration experiments, the ratio of the edge sites to the basal surface sites was found to be 6:1. The adsorption of Cd(II), Cu(II), Ni(II), Zn(II) and Pb(II) onto kaolinite suspensions was investigated using batch adsorption experiments and results suggested that in the lower pH range the metallic cations were bound through non-specific ion exchange reactions on the permanently charged basal surface sites (X⁻). Adsorption on these sites was greatly affected by ionic strength. With increasing pH, the variable charged edge sites (SOH) became the major adsorption sites and inner-sphere specifically adsorbed monodentate complexes were believed to be formed. The effect of ionic strength on the extent of adsorption of the metals on the variable charged edge sites was much less than those on the permanently charged sites. Two binding constants, log K_(X2Me) and log K_(SOMe), were calculated by optimizing these constants in the computer program FITEQL. A model combining non-specific ion exchange reactions and inner-sphere specific surface complexations was developed to predict the adsorption of heavy metals onto kaolinite in the studied pH range. Linear free energy relationships were found between the edge site binding constants and the first hydrolysis constants of the metals.     

Enthalpies and entropies of proton and cadmium adsorption onto Bacillus subtilis bacterial cells from calorimetric measurements

We used titration calorimetry to measure the bulk heats of proton and Cd adsorption onto a common Gram positive soil bacterium Bacillus subtilis at 25.0 °C. Using the 4-site non-electrostatic model of Fein et al. [Fein, J.B., Boily, J.-F., Yee, N., Gorman-Lewis, D., Turner, B.F., 2005. Potentiometric titrations of Bacillus subtilis cells to low pH and a comparison of modeling approaches. Geochim. Cosmochim. Acta69 (5), 1123-1132.] to describe the bacterial surface reactivity to protons, our bulk enthalpy measurements can be used to determine the following site-specific enthalpies of proton adsorption for Sites 1-4, respectively: −3.5 ± 0.2, −4.2 ± 0.2, −15.4 ± 0.9, and −35 ± 2 kJ/mol, and these values yield the following third law entropies of proton adsorption onto Sites 1-4, respectively: +51 ± 4, +78 ± 4, +79 ± 5, and +60 ± 20 J/mol K. An alternative data analysis using a 2-site Langmuir-Freundlich model to describe proton binding to the bacterial surface (Fein et al., 2005) resulted in the following site-specific enthalpies of proton adsorption for Sites 1 and 2, respectively: −3.6 ± 0.2 and −35.1 ± 0.3 kJ/mol. The thermodynamic values for Sites 1-3 for the non-electrostatic model and Site 1 of the Langmuir-Freundlich model of proton adsorption onto the bacterial surface are similar to those associated with multifunctional organic acid anions, such as citrate, suggesting that the protonation state of a bacterial surface site can influence the energetics of protonation of neighboring sites. Our bulk Cd enthalpy data, interpreted using the 2-site non-electrostatic Cd adsorption model of Borrok et al. [Borrok, D., Fein, J.B., Tischler, M., O’Loughlin, E., Meyer, H., Liss, M., Kemner, K.M., 2004b. The effect of acidic solutions and growth conditions on the adsorptive properties of bacterial surfaces. Chem. Geol.209 (1-2), 107-119.] to account for Cd adsorption onto B. subtilis, yield the following site-specific enthalpies of Cd adsorption onto bacterial surface Sites 2 and 3, respectively: −0.2 ± 0.4 and +14.4 ± 0.9 kJ/mol, and the following third law entropies of Cd adsorption onto Sites 2 and 3, respectively: +57 ± 4 and +128 ± 5 J/mol K. The calculated enthalpies of Cd adsorption are typical of those associated with Cd complexation with anionic oxygen ligands, and the entropies are indicative of inner sphere complexation by multiple ligands. The experimental approach described in this study not only yields constraints on the molecular-scale mechanisms involved in proton and Cd adsorption reactions, but also provides new thermodynamic data that enable quantitative estimates of the temperature dependence of proton and Cd adsorption reactions.     

Genesis of arsenic/fluoride-enriched soda water: A case study at Datong,northern China

The high As and F⁻ groundwaters from Datong Basin are mostly soda waters with a Na/(Cl+SO₄) (meq) ratio greater than unity, As and F⁻ up to 1550 μg/L and 10.4 mg/L, respectively, and with pH between 7.6 and 9.1. Geochemical modeling indicates that the waters are oversaturated with respect to calcite and clay minerals such as kaolinite, and undersaturated with respect to primary rock-forming minerals such as anorthite and albite. The water chemistry also is affected by evapotranspiration. The degree of evaporative enrichment is up to 85 in terms of Cl⁻. Results of the hydrogeochemical studies indicate that the occurrence of soda water at Datong is the result of incongruent dissolution of aluminosilicates at one stage of their interaction with groundwater when the water is oversaturated with respect to calcite and evapotranspiration-related salt accumulation is not too strong. Studying the genesis of soda waters provides new insights into mechanism of As and F⁻ enrichment in the aquifer system. Due to CaF₂ solubility control and OH⁻–F⁻ exchange reactions, F⁻ can be enriched in soda water. And the high pH condition of soda water favors As desorption from oxyhydroxide surfaces, thereby increasing the concentration of As in the aqueous phase.     

Uptake of uranium and trace elements in pyrite (FeS2) suspensions

Pyrite dissolution and interaction with Fe^(II), Co^(II), Eu^(III) and U^(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]_aq/[Fe]_aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm⁻². Concentration isotherms for Eu^(III) and U^(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu^(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10⁻⁶ mol g⁻¹. In the U^(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10⁻⁸ mol g⁻¹. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO₂(s).Scanning electron microscopy and micro-Raman spectrometry of U^(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO_2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe^(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U^(VI).