Non-equilibrium water/rock interactions—I. Model for interface-controlled reactions

The results of established crystal growth theory and silicate dissolution experiments are combined in developing a new model for mineral/water reactions controlled by surface processes. The overall reaction rate at steady-state is determined by coupling equations for the velocities of mass transport and interface detachment processes. Non-steady state processes can be successfully treated when interface reactions control the rate. For most sparingly soluble minerals, diffusion through the solution can be neglected as a rate-determining factor.Many surface processes are driven by the total interface under saturation, but only processes facilitating detachment contribute to dissolution. Other, non-detachment related, surface reactions result in lower dissolution rates. Slow rates of many mineral/solution reactions are attributed to the surface processes which consume the energy that would otherwise drive detachment.An analysis of the time dependence of interface reaction velocities indicates that linear rate laws apply when uniform detachment or layer-source generation mechanisms such as screw dislocations control the dissolution rate. At low interfacial undersaturations, first-order, logarithmic rate laws prevail. A parabolic time dependence occurs if surface detachment parameters vary as a function of (time)^{$\text{1}\text{2}$}.     

Dissolution kinetics and the weathering of mafic minerals

Fayalite, hypersthene, basalt, and obsidian were dissolved in buffered solutions (25°C; pH 4.5 and 5.5) under air, N₂ or O₂ atmospheres, in order to follow the kinetics of dissolution. Each dissolved more rapidly at lower pH values, dissolving most rapidly in the initial few days, followed by slower dissolution for periods up to six months. Dissolution was more rapid when air was excluded. In oxygen atmospheres an Fe(OH)₃ precipitate armors mineral surfaces, thus inhibiting further dissolution, and further affects the solution by scavenging dissolved silica and cations. Dissolution reactions include initial exchange between cations and H⁺, incongruent dissolution of silicate structures, oxidation of Fe²⁺ in solution, precipitation of Fe(OH)₃, and scavenging of dissolved silica and cations by Fe(OH)₃. Dissolution kinetics may explain weathering of mafic rocks and minerals at the Earth's surface, the formation of Fe-oxide coatings on mineral grains, weathering of submarine mafic rocks and intrastratal solution of mafic minerals in buried sandstones. Early Precambrian weathering would have been more rapid before the appearance of large amounts of oxygen in the atmosphere, and continental denudation rates may have been higher than at present because of this effect and the predominance of mafic igneous rocks at an early stage of continent formation and growth.     

Investigation of time-dependent reactions of H+ ions with variable and constant charge soils: a comparative study

A sampling-separation method and a dynamic monitoring method were used to investigate the time-dependent reactions of H⁺ ions with two contrasting types of soil, variable charge soils (VCS) and constant charge soils (CCS), by directly evaluating H⁺ ion consumption and other relevant consequences. The results for both CCS and VCS show that H⁺ ion consumption, increase in positive surface charge and increase in soluble Al are all characterized by a rapid step followed by a slow one. The higher the content of free Fe oxides in the soil, the larger the increase in positive surface charge and in H⁺ ion consumption in the initial rapid step. This is due mainly to protonation on external surfaces. The gradual increase in positive surface charge in the slow step for the 3 VCSs is a result of H⁺ ion diffusion to the reactive sites of Fe–OH on internal surfaces. The very low content of free Fe oxides on internal surfaces of the 2 CCSs render a negligible increase in positive surface charge in the slow step. For the 3 VCSs, the gradual consumption of H⁺ ions in the slow process is the result of protonation, Al dissolution and/or transformation into exchangeable acidity. For the 2 CCSs, however, the gradual consumption is mainly the result of Al dissolution and/or transformation into exchangeable acidity. The time-dependent Al dissolution from both VCS and CCS is influenced by several factors such as mineral components, solubility and dissolution rates of the soils, and H⁺ ion concentration in soil suspensions.     

Structure and dynamics of water on Li-, Na-, K-, Cs-, H3O-exchanged muscovite surfaces: A molecular dynamics study

The distribution and dynamics of water molecules and monovalent cations (Li⁺, Na⁺, K⁺, Cs⁺, and H₃O⁺) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs⁺ and Cl⁻ ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li⁺ ions near the muscovite surface. For Na⁺, K⁺, Cs⁺, and H₃O⁺ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li⁺, Na⁺, K⁺, and H₃O⁺ ions adsorbed on the muscovite surfaces. The water molecules around Cs⁺ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs⁺ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li⁺ ions and decreased with an increase in the ionic radius.     

Processes controlling acid attenuation in the unsaturated zone of a Triassic sandstone aquifer (U.K.), in the absence of carbonate minerals

Continuous core samples were taken through the unsaturated zone at three sites on the outcrop of Permo-Triassic sandstone in the British West Midlands. Sample sites were chosen for lack of recent, direct anthropogenic disturbance, and for differing vegetation: heathland, birch woodland and conifer forest. Interstitial water was extracted and analyzed for 32 major and trace elements. Solid phases were analyzed for exchangeable cations and mineralogy. The rate of recharge calculated using a Cl mass balance method was three times greater below heathland than below afforested sites owing to higher evapotranspiration rates in the woodlands.Carbonate minerals were absent from the unsaturated zone at each site. Soil solutions were acidic and soils at the woodland sites were more acidic (pH 4.0) than those at the heathland site (pH 4.5). Acidic interstitial water solutions were found to up to 5.0 m depth in the unsaturated zone and are partially neutralized by two aluminosilicate mineral reactions in the unsaturated zone: cation exchange and K-feldspar dissolution. The rate at which these acid neutralizing reactions act to neutralize acidity is revealed by the rate of depletion of base cations from the unsaturated zone in recharge solutions; K⁺ (dissolution), Ca⁺² + Mg⁺² (cation exchange). The total base cation depletion rate was greatest below heathland; this can be attributed mainly to the greater rate of SO₄ assimilation by the woodland biome.     

Modelling Zn(II) sorption onto clayey sediments using a multi-site ion-exchange model

In environmental studies, it is necessary to be able to predict the behaviour of contaminants in more or less complex physico-chemical contexts. The improvement of this prediction partly depends on establishing thermodynamic models that can describe the behaviour of these contaminants and, in particular, the sorption reactions on mineral surfaces. In this way, based on the mass action law, it is possible to use surface complexation models and ion exchange models. Therefore, the aim of this study is (i) to develop an ion-exchange model able to describe the sorption of transition metal onto pure clay minerals and (ii) to test the ability of this approach to predict the sorption of these elements onto natural materials containing clay minerals (i.e. soils/sediments) under various chemical conditions. This study is focused on the behaviour of Zn(II) in the presence of clayey sediments. Considering that clay minerals are cation exchangers containing multiple sorption sites, it is possible to interpret the sorption of Zn(II), as well as competitor cations, by ion-exchange equilibria with the clay minerals. This approach is applied with success to interpret the experimental data obtained previously in the Zn(II)–H⁺–Na⁺–montmorillonite system. The authors’ research team has already studied the behaviour of Na⁺, K⁺, Ca²⁺ and Mg²⁺ versus pH in terms of ion exchange onto pure montmorillonite, leading to the development of a thermodynamic database including the exchange site concentrations associated with montmorillonite and the selectivity coefficients of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Zn²⁺ versus H⁺.     

Diffusion control of quartz and forsterite dissolution rates

An established engineering model is used to identify conditions where diffusion controls the dissolution of quartz and forsterite in packed beds. The model shows that diffusion control is favored at low advection flux, large grain size, high temperature, and high pH (if the reaction consumes H⁺). Quartz dissolution is chemical reaction controlled for most geochemically reasonable combinations of temperature, grain size, and flow rate. On the other hand, forsterite dissolution rates can be diffusion controlled for typical advection fluxes, grain sizes, temperatures, and pH’s. The apparent activation energy for diffusion-controlled reactions in a packed bed is much higher than the <∼20 kJ/mol value that is often used to identify diffusion controlled reactions. The models are quite general and can be adapted to deal with other mineral dissolution and precipitation reactions.     

Ion exchange properties of oceanic ferromanganese nodules and enclosing pelagic sediments

Results of the experimental study of ion exchange properties of deep-sea pelagic sediments and related ferromanganese nodules (FMN) are considered. The exchange complex of sediments and nodules includes Na⁺, K⁺, Ca²⁺, and Mg²⁺ cations. The FMNs also include Mn²⁺ cations. Series of reactivity of metal cations during exchange reactions in different types of pelagic clayey sediments and diagenetic-sedimentary FMN are compiled. Series of exchange capacity of the sediments and FMN for alkali and heavy metal cations are also presented. The exchange capacity of FMN is always higher than that of enclosing sediments. Sediments are characterized by reversible equivalent sorption of cations of both alkali and heavy metals. Irrespective of the mineral composition, the FMNs are characterized by the reversible equivalent sorption of alkali metal cations, whereas sorption of heavy metal cations is only partly reversible. More over, alkali metal cations do not replace heavy metal cations. The results obtained refine the role of ion exchange processes in the redistribution of heavy metal cations at the water-bottom sediment interface during the diagenetic-sedimentary formation of ferromanganese nodules.     

Pyrite oxidation in the tertiary sands of the London Basin aquifer

Possible groundwater quality changes related to pyrite oxidation during artificial groundwater recharge and its storage in the Tertiary sands of the London Basin are investigated. Pyrite textures in the Tertiary sands are examined by scanning electron microscopy while an experimental approach is used to study mechanisms of pyrite oxidation and of some associated chemical reactions. In the Tertiary sands of the London Basin aquifer, pyrite occurs as aggregates made of discrete individual crystals 0.5–5 μm in size or, in a cryptocrystalline form, often as pseudomorphs of biogenic debris. It can expose a very large specific surface area to porefluids. Although ferric iron, which can be an oxidising agent of pyrite, is abundant in the solid phase of the Tertiary sands, it does not appear to take a significant part in this case. Pyrite oxidation seems to rely on a supply of oxygen. Leaching experiments using a 0.001 M H₂SO₄ solution were carried out to examine interactions between mildly acidic groundwater resulting from pyrite oxidation at a moderate rate and the host-sediment. In the presence of CaCO₃ in the solid phase, H⁺ is rapidly buffered by CaCO₃ dissolution. Oscillations of this reaction around equilibrium appear to trigger cation-exchange reactions on clay mineral surfaces, resulting in the release of major cations (e.g. K and Mg) into solution. In the absence of CaCO₃ in the solid phase, H⁺ buffering occurs less efficiently solely through exchange of cations for H⁺ on clay minerals surfaces. If the rate of pyrite oxidation in the Tertiary sands becomes high enough for the buffering capacity of the system to be exceeded, the groundwater pH begins to fall. Interactions between low pH (2) groundwaters and the host sediments were examined by leaching solid material in 0.01 M and 0.1 M H₂SO₄ solutions. Concentrations of Fe, Mg and K increase in solution throughout the experiment, indicating partial dissolution of clay minerals. The composition of the porefluid thus depends on the geochemical composition and surface area of the different clay minerals present.     

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.     

Groundwater Evolution and Mineral Alteration Reactions in the Basaltic Rock Sequence of Mt. Wasserkuppe, Germany: A Case Study

Groundwater sampling was accomplished in the basaltic sequence of the Rh?n mountain range, Germany, in order to investigate hydrochemical groundwater evolution and to delineate mineral alteration reactions involved in natural weathering. The hydrochemical compositions of near-surface groundwaters indicate a Ca/Mg–HCO₃ type with near-neutral pH and evolve to a Na–HCO₃ type with high pH at greater depth. Column experiments were performed with basaltic and phonolitic rock samples to determine individual mineral alteration reactions. The basic reactions could be related to the alteration of olivine, Ca-pyroxene, plagioclase, pyrrhotite, and feldspathoids under formation of secondary clay minerals (smectites, illite) and goethite. The mineral alteration reactions deduced from the leaching experiments by inverse modelling were found to be consistent with the mineral reactions associated with the natural groundwaters. The reactions calculated for groundwater evolution involve the alteration of primary and secondary minerals to produce low-T mineral phase. The conversion of secondary Na-beidellite to illite occurs at a later stage of groundwater evolution, reducing the concentrations of K⁺ and Mg²⁺. Near-surface groundwaters do not indicate significant cation exchange. Initial cation exchange requires elevated pH values, with Mg²⁺ removed from solution preferred to Ca²⁺. Na-alkalisation of the groundwaters at greater depth suggests the exchange of Na⁺ for Mg²⁺ and Ca²⁺ on Na-beidellite, supported by cation exchange on coatings of iron hydroxides as alteration products. Among the mature high-pH groundwater at greater depth, the dissolution of anorthite and albite has significant effect on groundwater composition.     

Nature of Co-bearing ferromanganese crusts of the Magellan Seamounts (Pacific Ocean): Communication 2. Ion exchange properties of ore minerals

The results of experimental studies of ion exchange properties of Co-bearing ferromanganese crusts in the Magellan Seamounts (Pacific Ocean) are discussed. Maximum reactivity in reactions with the participation of manganese minerals (Fe-vernadite, vernadite) is typical of Na⁺, K⁺, and Ca²⁺ cations, whereas minimum activity is recorded for cations Pb²⁺ and Co²⁺. The exchange complex of ore minerals in crusts is composed of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Mn²⁺ cations. The exchange capacity of manganese minerals increases from the alkali metal cations to rare and heavy metal cations. Peculiarities of the affiliation of Co²⁺, Mn²⁺, and Mg²⁺ cations in manganese minerals of crusts are discussed. In manganese minerals, Co occurs as Co²⁺ and Co³⁺ cations. Metal cations in manganese minerals occur in different chemical forms: sorbed (Na⁺, K⁺, Ca²⁺, Mn²⁺, Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺); sorbed and chemically bound (Mg²⁺, Ni²⁺, Y³⁺, La³⁺, and Mo⁶⁺); and only chemically bound (Co³⁺). It is shown that the age of crust, its preservation time in the air-dry state, and type of host substrate do not affect the ion exchange indicators of manganese minerals. It has been established that alkali metal cations are characterized by completely reversible equivalent sorption, whereas heavy metal cations are sorbed by a complex mechanism: equivalent ion exchange for all metal cations; superequivalent, partly reversible sorption for Ba²⁺, Pb²⁺, Co²⁺, and Cu²⁺ cations, relative to exchange cations of manganese minerals. The obtained results refine the role of ion exchange processes during the hydrogenic formation of Co-bearing ferromanganese crusts.     

An EFTEM/HRTEM high-resolution study of the near surface of labradorite feldspar altered at acid pH: evidence for interfacial dissolution-reprecipitation

Using an approach combining high-resolution and energy-filtered transmission electron microscopy (HRTEM and EFTEM), we have studied with Å to nm-spatial resolution the interfacial region that delimits the near-surface altered zone and non-altered labradorite feldspar after dissolution under acid pH conditions. The interface is characterized by extremely sharp and spatially coincident changes in structure and chemistry. The 500-nm-thick altered zone is depleted in interstitial cations (Ca, Na, K) and Al, a framework element, whereas it is enriched in H, O, and Si. Modeling H⁺-alkali interdiffusion within a 500-nm-thick altered zone shows that volume interdiffusion cannot reproduce the sharp chemical interfaces measured by EFTEM. Based on these new data, we propose that the near-surface altered zone is a result of interfacial dissolution-reprecipitation, and not of preferential leaching of cations and interdiffusion with H⁺. This implies an intrinsic dissolution process that is stoichiometric, where the breaking of bonds and release of interstitial cations and framework elements (Al, Si, and O) to solution occur contemporaneously at equal relative rates from the original fluid–mineral interface.     

Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals,gases and an aqueous phase

The compositional characteristics of many geochemical systems which involve the interaction of natural aqueous solutions with minerals and gases are conveniently described using the following thermodynamic components: Cl^?, SO₄⁼, HS^?, CO₃⁼, H⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Fe⁺⁺, Zn⁺⁺, Cu⁺, Al⁺⁺⁺, SiO₂ and H₂O. A set of mass balance and mass action equations equal in number to the number of components plus the number of saturated minerals (and gases) is defined for a specified temperature, pressure and bulk composition. The mass balance equations include terms for minerals, gases and the molalities of aqueous complexes and dissociated species. This set of non-linear equations can be solved with the aid of a computer using'a Newton-Raphson technique. The calculation takes account of aqueous ion complexing, oxidation-reduction equilibria, activity coefficients, non-unit water activity and solid solutions. The use of H⁺, SO₄⁼, HS^? and H₂O as components allows straightforward treatment of processes involving oxidation-reduction, evaporation, boiling and changes of total aqueous H⁺ due to hydrolysis, mineral reaction or temperature change. One product of this approach is a technique for calculating pH at high temperature from measurement of pH at room temperature.By linking a series of discrete overall heterogeneous equilibrium calculations in which incremental changes of bulk composition, temperature or pressure are made, dynamic geochemical processes can be modeled. Example calculations for two such processes are given. These are the heating of seawater from 25° to 300°C and the isothermal irreversible reaction of rhyolite with an aqueous solution at 250°C.     

Thermodynamic equilibrium, kinetics, activation barriers, and reaction mechanisms for chemical reactions in Karst Terrains

Chemical reactions pertinent to karst systems divide broadly into (a) speciation reactions within aqueous solutions, (b) dissolution/precipitation and other acid/base reactions between aqueous solutions and solid minerals, and (c) redox reactions involving various carbon and sulfur-bearing species. As a backdrop against which other chemistry can be evaluated, selected phase diagrams and equilibrium speciation diagrams were calculated for the system Ca—Mg—O—H—C—S. The kinetics of reactions within this system span time scales from milliseconds for homogeneous reactions in solution through hundreds of hours for carbonate mineral dissolution reactions, to geologic time scales for reactions such as the aragonite/calcite inversion or the oxidation/reduction of native sulfur. In purely inorganic systems, kinetic barriers, typically on the order of tens of kJ/mole, are set by nucleation processes and by activated complex formation. Biological processes impact the purely inorganic chemistry by the following mechanisms: (a) Secretions and waste products from biological activity or consumption of CO₂ by organisms changes the chemistry in the microenvironments of reaction surfaces. Oxidation potentials, pH, and ion activities may be modified, thus shifting equilibria. (b) Reaction rates may be increased due to modification of activated complexes and thus the activation barriers to reaction. (c) Organic compounds or microorganisms may act as substrates, thus lowering nucleation barriers. The preservation of microorganisms in cave deposits does not necessarily prove a cause and effect relationship.     

Dissolution Kinetics of Calcite in 0.1 M NaCl Solution at Room Temperature: An Atomic Force Microscopic (AFM) Study

Atomic force microscopy (AFM) was used to study the rates of migration of the (10¯1 4) plane of a single-crystal of calcite dissolving in 0.1 M NaCl aqueous solutions at room temperature. The solution pH and P_{CO 2} controlled in the ranges 4.4 < pH < 12.2 and 0 < P_{CO 2} < 10^-3.5 atm (ambient), respectively. Measured step velocities were compared with the mineral dissolution rates determined from the calcium fluxes. The step velocity is defined as the average of the velocities of the obtuse and acute steps. Rates of step motion increased gradually from 1.4(±0.2) at pH 5.3 to 2.4(±0.3) nm s^-1 at pH 8.2, whereas the rates inverted and decreased to the minimum value of 0.69(±0.18) nm s^-1 at pH 10.8. For pH > 10.8, only the velocity of the obtuse steps increased as pH increased, whereas that of acute steps gradually decreased.The dissolution rate of the mineral can be calculated from the measured step velocities and average slope, which is proportional to the concentration of exposed monomolecular steps on the surface. The average slope of the dissolving mineral, measured at pH 5.6 and 9.7, was 0.026 (±0.015). Using this slope, we calculate bulk dissolution rates for 5.3 < pH < 12.2 of 4.9(±3.0) × 10^-11 to 1.8(±1.0) × 10^-10 mol cm^-2 s^-1. The obtained dissolution rate can be expressed by the following empirical equation:R_dss = 10^{-4.66(±0.13)}[H⁺] + 10^{-3.87(±0.06)}[HCO₃ ^-] + 10^{-7.99(plusmn; 0.08)}[OH^-]We propose that calcite dissolution in these solutions is controlled by elementary reactions that are similar to those that control the dissolution of other amphoteric solids, such as oxides. The mechanisms include the proton-enhanced hydration and detachment of calcium-carbonate ion pairs. The detachments are enhanced by the presence of adsorbed nucleophiles, such as hydroxyl and bicarbonate ions, and by protons adsorbed to key oxygens. A molecular model is proposed that illustrates these processes.     

Hydrogeochemical Analysis of Processes Through Modeling of Seawater Intrusion Impacts in Biscayne Aquifer Water Quality,USA

Hydrogeochemical processes that accompany seawater intrusion in coastal aquifers can alter the resulting water quality and are important ingredients in coastal aquifer management. The presence of dissolution–precipitation reactions and ion exchange in the mixing zone of the Biscayne aquifer (FL, USA) are suggested based on changes in major ion concentrations and mineral saturation indices (SI). Major ion concentrations from 11 groundwater samples are compared with theoretical mixing between freshwater and seawater. PHREEQC code was used to calculate saturation indices of the samples with respect to common phases in the Biscayne aquifer. High Ca²⁺ and HCO₃ ^? content of the samples is typical of waters in contact with carbonate aquifers. Water quality of the samples is mainly attributed to mixing and precipitation–dissolution reactions with calcite and dolomite. The samples were saturated with calcite (SI ~ 0) and undersaturated for dolomite (SI < 0), while a few samples showed dolomite saturation. Because gypsum and halite SI could be predicted by theoretical mixing, reactions with those minerals, if present, are thought to be insignificant. In the active intrusion areas, cation exchange also appears to modify water quality leading to excess Ca²⁺, but depleted Na⁺, Mg²⁺ and K⁺ concentrations. On the other hand, samples from previous intrusion areas plotted very close to the theoretical mixing line and approached equilibrium with the seawater.     

Evidence for ligand hydrolysis and Fe(III) reduction in the dissolution of goethite by desferrioxamine-B

Desferrioxamine-B (DFOB) is a bacterial trihydroxamate siderophore and probably the most studied to date. However, the manner in which DFOB adsorbs at mineral surfaces and promotes dissolution is still under discussion. Here we investigated the adsorption and dissolution reactions in the goethite-DFOB system using both in situ infrared spectroscopic and quantitative analytical methods. Experiments were carried out at a total DFOB concentration of 1 μmol/m², at pH 6, and in the absence of visible light. Our infrared spectroscopic results indicated that the adsorption of DFOB was nearly complete after a 4-h reaction time. In an attempt to determine the coordination mode at the goethite surface, we compared the spectrum of adsorbed DFOB after a 4-h reaction time to the spectra of model aqueous species. However, this approach proved too simplistic in the case of such a complex ligand as DFOB, and we suggest that a more detailed investigation (IR in D₂O, EXAFS of adsorbed model complexes) is needed to elucidate the structure of the adsorbed siderophore. Between a 4-h and 4-day reaction time, we observed the growth of carboxylate stretching bands at 1548 and 1404 cm⁻¹, which are indicators of DFOB hydrolysis. Acetate, a product of DFOB hydrolysis at its terminal hydroxamate group, was quantified by ion chromatography. Its rate of formation was linear and nearly the same as the rate of Fe(III) dissolution. The larger hydrolysis product, a hydroxylamine fragment, was not detected by LC-MS. However, a signal due to the oxidized form of this fragment, a nitroso compound, was found to increase linearly with time, which is an indirect indication for Fe(III) reduction. Based on these findings, we propose that DFOB undergoes metal-enhanced hydrolysis at the mineral surface followed by the reduction of surface Fe(III). While Fe(II) was not detected in solution, this is likely because it remains adsorbed at the goethite surface or becomes buried in the goethite crystal by electron conduction. Taking into account the extent and similarity between the rates of hydrolysis and dissolution, we suggest that a reductive mechanism could play an important part in the dissolution of goethite by DFOB. This possibility has not been considered previously in the absence of light and at circumneutral pH.     

Direct determinations of the rates of rhyolite dissolution and clay formation over 52,000 years and comparison with laboratory measurements

Four porous, glass-dominated rhyolites from Kozushima Island, different in age and extent of weathering, were studied. Because the four rhyolites are homogeneously weathered to considerable depth, and because their initial chemical compositions were equal, the different rock characteristics can provide information about rates of rhyolite dissolution and clay mineral formation over ∼52,000 yr. Because glass surfaces retreat without surface roughening, surface area (measured by Brunauer-Emmett-Teller method; BET) was assumed to be approximately constant over time. The field dissolution rate, as inferred from the rate of loss of Si, was ∼6 × 10⁻¹⁹ mol cm⁻² s⁻¹. The estimated clay mineral formation rate was ∼1 × 10⁻¹⁹ mol cm⁻² s⁻¹. About 20% of dissolved Si precipitated as clays. In order to investigate the factors affecting the field dissolution rate, dissolution experiments that used powdered and block rhyolite samples were conducted. Under relevant field conditions (20°C and pH 6∼7), the rates were ∼5 × 10⁻¹⁷ and ∼5 × 10⁻¹⁸ mol cm⁻² s⁻¹ for powdered rhyolite and blocks, respectively. The dissolution rates obtained in this study decrease in the order powder > block > field. Because all surface areas were directly measured by BET, the differences are not attributable to the errors in surface area. The most plausible explanations of the slower rates are the lower degree of flushing and resultant high-solution saturation states in the pores (both in the field and in the rhyolite blocks used in experiments) plus the formation of alteration/hydrated layers at the glass surface.