Glass Dissolution Rate Research Articles

Dissolution of basalts and peridotite in seawater, in the presence of ligands, and CO 2: Implications for mineral sequestration of carbon dioxide

Steady-state silica release rates ( r Si) from basaltic glass and crystalline basalt of similar chemical composition as well as dunitic peridotite have been determined in far-from-equilibrium dissolution experiments at 25 °C and pH 3.6 in (a) artificial seawater solutions under 4 bar pCO 2, (b) varying ionic strength solutions, including acidified natural seawater, (c) acidified natural seawater of varying fluoride concentrations, and (d) acidified natural seawater of varying dissolved organic carbon concentrations. Glassy and crystalline basalts exhibit similar r Si in solutions of varying ionic strength and cation concentrations. Rates of all solids are found to increase by 0.3–0.5 log units in the presence of a pCO 2 of 4 bar compared to CO 2 pressure of the atmosphere. At atmospheric CO 2 pressure, basaltic glass dissolution rates were most increased by the addition of fluoride to solution whereas crystalline basalt rates were most enhanced by the addition of organic ligands. In contrast, peridotite does not display any significant ligand-promoting effect, either in the presence of fluoride or organic acids. Most significantly, Si release rates from the basalts are found to be not more than 0.6 log units slower than corresponding rates of the peridotite at all conditions considered in this study. This difference becomes negligible in seawater suggesting that for the purposes of in-situ mineral sequestration, CO 2-charged seawater injected into basalt might be nearly as efficient as injection into peridotite.

Preparation and characterization of antibacterial P2O5–CaO–Na2O–Ag2O glasses

60P₂O₅-20CaO-(20-x) Na₂O-xAg₂O and 60P₂O₅-30CaO-(10-x) Na₂O-xAg₂O glasses, x = 0, 0.5,1, and 2 mol % were prepared using normal glass melting technique. The antibacterial activity of pressed disks of powdered glass (undoped and silver-doped glass) was investigated against S.aureus, P.aeruginosa, and E.coli micro-organisms using agar disk-diffusion assays at 37 °C for 24 h. The antibacterial activity was deduced from the inhibition zone diameter (IZD), zone of no bacterial growth, measured under the stated experimental conditions. The antibacterial activity increases with the increase in IZD and vice versa. Dissolution of glass in water at 37 °C, pH changes of water during glass dissolution, and concentrations of silver ions released from silver-doped glasses into water during their dissolution were determined. An increase in the concentration of silver ions released from silver-doped glasses into water was observed with increasing time of glass dissolution and with increasing Ag₂O content. The tested silver-free and silver-doped glasses demonstrated different antibacterial activity against the tested micro-organisms. For silver-free glasses, an increase in IZD was observed with the increase in the glass dissolution rate and with the decrease in pH of water. Also, the IZD showed an increase with increasing Ag₂O content of silver-doped glasses.

Deep-UV Raman spectroscopic analysis of structure and dissolution rates of silica-rich sodium borosilicate glasses

As part of ongoing studies to evaluate relationships between structure and rates of dissolution of silicate glasses in aqueous media, sodium borosilicate glasses of composition Na 2O·xB 2O 3·(3 − x)SiO 2, with x ≤ 1 (Na 2O/B 2O 3 ratio ≥ 1), were analyzed using deep-UV Raman spectroscopy. Results were quantified in terms of the fraction of SiO 4 tetrahedra with one non-bridging oxygen (Q 3) and then correlated with Na 2O and B 2O 3 content. The Q 3 fraction was found to increase with increasing Na 2O content, in agreement with studies on related glasses, and, as long as the value of x was not too high, this contributed to higher rates of dissolution in single pass flow-through testing. In contrast, dissolution rates were less strongly determined by the Q 3 fraction when the value of x was near unity, and appeared to grow larger upon further reduction of the Q 3 fraction. Results were interpreted to indicate the increasingly important role of network hydrolysis in the glass dissolution mechanism as the BO 4 tetrahedron replaces the Q 3 unit as the charge-compensating structure for Na + ions. Finally, the use of deep-UV Raman spectroscopy was found to be advantageous in studying finely powdered glasses in cases where visible Raman spectroscopy suffered from weak Raman scattering and fluorescence interference.

Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering

In order to use lithium isotopes as tracers of silicate weathering, it is of primary importance to determine the processes responsible for Li isotope fractionation and to constrain the isotope fractionation factors caused by each process as a function of environmental parameters (e.g. temperature, pH). The aim of this study is to assess Li isotope fractionation during the dissolution of basalt and particularly during leaching of Li into solution by diffusion or ion exchange. To this end, we performed dissolution experiments on a Li-enriched synthetic basaltic glass at low ratios of mineral surface area/volume of solution ( S/ V), over short timescales, at various temperatures (50 and 90 °C) and pH (3, 7, and 10). Analyses of the Li isotope composition of the resulting solutions show that the leachates are enriched in 6Li (δ 7Li = +4.9 to +10.5‰) compared to the fresh basaltic glass (δ 7Li = +10.3 ± 0.4‰). The δ 7Li value of the leachate is lower during the early stages of the leaching process, increasing to values close to the fresh basaltic glass as leaching progresses. These low δ 7Li values can be explained in terms of diffusion-driven isotope fractionation. In order to quantify the fractionation caused by diffusion, we have developed a model that couples Li diffusion with dissolution of the glassy silicate network. This model calculates the ratio of the diffusion coefficients of both isotopes ( a = D 7/ D 6), as well as its dependence on temperature, pH, and S/ V. a is mainly dependent on temperature, which can be explained by a small difference in activation energy (0.10 ± 0.02 kJ/mol) between 6Li + and 7Li +. This temperature dependence reveals that Li isotope fractionation during diffusion is low at low temperatures ( T < 20 °C), but can be significant at high temperatures. However, concerning hydrothermal fluids ( T > 120 °C), the dissolution rate of basaltic glass is also high and masks the effects of diffusion. These results indicate that the high δ 7Li values of river waters, in particular in basaltic catchments, and the fractionated values of hydrothermal fluids are mainly controlled by precipitation of secondary phases.

Do carbonate precipitates affect dissolution kinetics? 1: Basaltic glass

Basaltic glass dissolution rates were measured in mixed-flow reactors at basic pH and at 25 °C and 70 °C in aqueous solutions supersaturated with respect to calcite for up to 140 days. Inlet solutions were comprised of NaHCO 3 ± CaCl 2 with ionic strengths > 0.03 mol kg − 1 . Scanning Electron Microscope images show that significant CaCO 3 precipitated during these experiments. This precipitate grew on the basaltic glass in experiments performed in Ca-free inlet solutions, but nucleated and grew independently of the glass surfaces in experiments performed in Ca-bearing inlet solutions. In those experiments where CaCO 3 precipitated on the glass surface, it grew as discrete crystals; no pervasive CaCO 3 layers were observed. The lack of structural match between glass and calcium carbonate favors CaCO 3 nucleation and growth as discrete crystals. Measured basaltic glass dissolution rates based on either Si, Al, or Mg were both 1) independent of time during the experiments, and 2) equal to that of corresponding control experiments performed in NaHCO 3-free inlet solutions. Taken together, these observations show that basaltic glass dissolution rates are unaffected by the precipitation of secondary CaCO 3 precipitation. It seems therefore likely that carbonate precipitation will not slow basaltic glass dissolution during mineral sequestration efforts in basaltic rocks.

On the influence of stirring conditions on the dissolution rate of chalcogenide glasses and films

The local dissolution rates of chalcogenide films based on arsenic sulfide have been investigated in solutions of two types, i.e., alkaline aqueous solutions and amine solutions. The contribution of diffusion and hydrodynamic components to the total dissolution rate has been studied. It has been demonstrated that the possible contribution of the diffusion stages to the total rate of the interaction between the films and alkaline solutions is associated with the slow desorption of reaction products from the interface. The accumulation of interaction products in dipentylamine solutions favors an increase in the dissolution rate of the films, whereas the reaction products in the reaction with alkalis have an inhibiting effect. The difference between the used solvents of dipentylamine can lie in the difference between their abilities to form intermediate compounds with sulfur that have different nucleophilicities.

Influence of the Chemical Composition on Nature and Activity of the Surface Layer of Zn-Substituted Sol−Gel (Bioactive) Glasses

Two Zn-doped sol―gel glasses with the same ZnO content (5 wt %; 4% mol) but different overall composition have been synthesized and characterized, in comparison with a bioactive Zn-free reference glass. The role of ZnO in modifying the bioactivity of sol―gel glasses was investigated by soaking the glasses in a simple tris(hydroxymethyl)aminomethane-buffered solution (TRIS-BS), so as to maximize the solubility and to minimize back-precipitation phenomena, which will depend only on the nature and concentration of dissolved glass components. Glass dissolution/ions release in TRIS-BS was monitored by ion coupled plasma emission spectroscopy, whereas modifications of surface composition upon reaction were checked by X-ray photoelectron spectroscopy (XPS). The deposition of a Ca―P layer and the consequent crystallization to hydroxy―apatite (HA) and/or hydroxy―carbonate―apatite (HCA) at the glass surface were investigated by X-ray diffraction and Raman, Fourier transform infrared (FTIR), and XPS spectroscopies. Glass dissolution rate, back-precipitation of silica gel, and formation/crystallization of an apatite-like layer on Zn-containing glasses were found to be either inhibited or delayed, according to the overall glass composition, in that the presence of the network former ZnO component enhances glass reticulation, with the consequent formation ofSi-O-Zn units. The presence of a ZnO component has no effect per se, but its influence depends on the overall composition of the glass and, in particular, on the CaO/SiO 2 and ZnO/CaO ratios, which determine the nature/structure of Zn and Ca surface species. Glass surface features were investigated by the combined use of in situ FTIR spectroscopy and adsorption microcalorimetry. The role played by surface Ca species, thought to be the most hydrophilic sites, was found to be a decisive factor in both glass dissolution mechanism and formation of an apatite-like surface layer: (i) the scarce dissolution in aqueous media of a (non bioactive) low-Ca and high-silica glass is due to the high reticulation caused by the scarce population of Ca 2+ cations in the role of network modifiers; and (ii) the amount of the latter species is, instead, much larger in the corresponding (moderately bioactive) high-Ca and low-silica glass, which dissolves more, although exhibiting a larger durability in aqueous solution than the Zn-free glass.

A 25-year laboratory experiment on French SON68 nuclear glass leached in a granitic environment – First investigations

We have investigated a 25.75-year old leaching experiment to improve our understanding of the mechanisms controlling glass dissolution in geological disposal conditions. A SON68 glass block was leached in slowly renewed synthetic groundwater (at 90 °C, 100 bar) in contact with some pieces of granite and Ni–Cr–Mo alloy as environmental storage materials. One hundred and sixty-three samplings were carried out over the entire duration of the experiment and were used to calculate the mean thickness of the altered glass (28 (±9) μm) and the glass dissolution rate. After few months, the rate remained very constant at 6 × 10 −3 g m −2 d −1 which is about 20 times higher than the residual rate measured in a batch reactor at the same temperature. At the end of the experiment, mainly SEM analyses were performed on the entire glass block. Surprisingly, the glass alteration layer has neither a homogeneous thickness, nor a homogeneous morphology. The location of the sampling valve (at half height of the glass block) seems to divide the glass block into two parts. In the upper half (above the sampling valve), the general morphology of the alteration layer consists in a relatively simple and uniform gel and some secondary phases which are rare-earth phosphates. The mean measured thickness of this alteration layer is 6.7 (±0.3) μm. However, in the lower half of the glass block, the gel is globally larger and frequently contains rounded shapes which are rare-earth phosphates. This section is edged by secondary phases bearing Mg, Na, Zn and Ni. The mean measured thickness is 81.3 (±1.1) μm in the lower half. In this experiment, the flow rate which leads to the hydrodynamic transport of the soluble species must be a key factor for the local glass alteration process. We have also shown that this unexpected behavior is likely due to heterogeneities of the chemistry of the solution. This conclusion is supported by the behavior of Mg. This element, supplied by the inlet solution, precipitates with Si and forms clay minerals and therefore weakens the passivating properties of the gel. Mg-rich clay minerals are only observed in the lower half of the glass block. Further investigations are necessary to better understand the coupling between the hydrodynamics and chemistry in this experiment. However, based on this study, we can conclude that glass in disposal should be very sensitive to the water renewal near the glass surface.

Novel phosphate glasses with different amounts of TiO2 for biomedical applications: Dissolution tests and proof of concept of fibre drawing

The aim of this work was to develop a range of phosphate glasses having different solubilities and with suitable characteristics to be drawn into fibres. Three glasses with molar compositions 50P2O5–30CaO–9Na2O–3SiO2–3MgO–(5−x)K2O–xTiO2 (x=0, 2.5, 5) were prepared and characterized in terms of thermal properties, density and dissolution rate. The dissolution tests were conducted in bi-distilled water (pH 6.0±0.5), Tris–HCl (pH 7.4±0.1) and citric acid buffer solutions (pH 3.0±0.1) in order to study the influence of pH on the dissolution process.TiO2 had a stabilizing effect on the glass, causing an increase in the glass transition temperature and density and a decrease in the glass dissolution rate. Acidic medium was found to strongly enhance the glass dissolution. The pH decrease caused by the acidic degradation products and the consequent enhancement of the glass weight loss were attenuated with the TiO2 increase.The fabrication of glass fibres using a preform drawing approach was investigated. Control and versatility of the process are demonstrated through the production of several tens of meters of fibre with diameters ranging from 37±2μm up to 173±7μm. Preliminary analysis reveal pristine fibres, free of crystallization, showing adequate flexibility for prospective biomedical applications.

ChemInform Abstract: Dissolution Rates of Silicate Glasses in Water at pH 7

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The effect of aqueous sulphate on basaltic glass dissolution rates

Steady-state dissolution rates of basaltic glass were measured in mixed-flow reactors at 50 °C and 3 < pH < 10 in HCl–NaCl–NH 4OH bearing solutions having an ionic strength of 0.01 M, or higher, as a function of aqueous sulphate concentration. Sulphate was added to reactive aqueous solutions in the form of Na 2SO 4. Measured dissolution rates increase with increased sulphate concentration in acid conditions, but little effect was found in basic conditions. At pH 5, the presence of 0.01 mol/kg sulphate doubled the dissolution rate while 0.10 mol/kg of sulphate tripled the dissolution rate compared to that measured in sulphate-free solutions. This rate increase is found to be consistent with that calculated using an equation previously proposed by Gislason and Oelkers (2003), where the observed rate increase stems from the formation of aqueous Al-sulphate complexes. As the release of divalent cations is thought to be the rate limiting step for CO 2 mineralization in basalt, adding aqueous sulphate to injected CO 2 could accelerate carbonization processes in this rock during carbon sequestration efforts.

Effect of silver concentration on the silver-activated phosphate glass

The effects of silver concentration on the structure and properties of silver-activated phosphate glasses, with the nominal molar compositions xAg 2O·(1 − x)(30Na 2O·10Al 2O 3·60P 2O 5) and 0 ≤ x ≤ 10 mol%, were studied. Increasing the Ag 2O-content decreases the glass transition temperature ( T g), increases the thermal expansion coefficient (TEC) and increases the glass dissolution rate in water. 31P nuclear magnetic resonance (NMR) and Raman spectroscopies indicate that the addition of Ag 2O leads to the formation of chain-terminating P-tetrahedra, and 27Al NMR spectra indicate that Al-octahedra are the preferred structural moiety. Optical spectroscopy indicates that Ag 2O-additions shift the UV-absorption edge to longer wavelengths. Irradiating glasses with ≤1.0 mol% Ag 2O with 60Co γ-rays creates a photoluminescence (PL) center that emits near 605 nm when excited with UV light (337.1 nm). The intensity of this PL center is proportional to the radiation dosage (up to 200 Gy), and the relative sensitivity is maximized in glasses with 0.05 mol% Ag 2O. When x > 1 mol% Ag 2O, a second PL center, emitting at 470 nm, is activated. The formation of this second PL center is associated with the loss of radiation sensitivity for glasses with greater Ag 2O-contents.

Mineral sequestration of carbon dioxide in basalt: A pre-injection overview of the CarbFix project

In this paper we describe the thermodynamic and kinetic basis for mineral storage of carbon dioxide in basaltic rock, and how this storage can be optimized. Mineral storage is facilitated by the dissolution of CO 2 into the aqueous phase. The amount of water required for this dissolution decreases with decreased temperature, decreased salinity, and increased pressure. Experimental and field evidence suggest that the factor limiting the rate of mineral fixation of carbon in silicate rocks is the release rate of divalent cations from silicate minerals and glasses. Ultramafic rocks and basalts, in glassy state, are the most promising rock types for the mineral sequestration of CO 2 because of their relatively fast dissolution rate, high concentration of divalent cations, and abundance at the Earth's surface. Admixture of flue gases, such as SO 2 and HF, will enhance the dissolution rates of silicate minerals and glasses. Elevated temperature increases dissolution rates but porosity of reactive rock formations decreases rapidly with increasing temperature. Reduced conditions enhance mineral carbonation as reduced iron can precipitate in carbonate minerals. Elevated CO 2 partial pressure increases the relative amount of carbonate minerals over other secondary minerals formed. The feasibility to fix CO 2 by carbonation in basaltic rocks will be tested in the CarbFix project by: (1) injection of CO 2 charged waters into basaltic rocks in SW Iceland, (2) laboratory experiments, (3) studies of natural analogues, and (4) geochemical modelling.

Measurement of initial dissolution rate of P0798 simulated HLW glass by using micro-reactor flow-through test method

AbstractWe applied a new type of flow-through test method using micro-reactor consisting of a simple test apparatus with compact size to measurement of the dissolution rate of a Japanese type of simulated waste glass (P0798 glass). In this test method, a solution flows through a micro-channel (20 mm length, 2 mm width, 0.16 mm depth) in contact with a face of coupon shaped glass specimen, and the output solution is retrieved at certain intervals to be analyzed for determination of the glass dissolution rate. By using this test method the initial dissolution rate of glass matrix or forward dissolution rate was measured as a function of pH (3 to 11) and temperature (25°C to 90°C). The present test results indicated that the initial dissolution rate has ‘V-shaped’ pH dependence, and the effect of pH on the dissolution rate decreases with increase in temperature similar to the results measured by using the Single-pass flow-through (SPFT) method. The present test results also indicated that the dissolution of B is controlled by diffusion process and that of Si is controlled by surface reaction process.

Impact of Silicon Migration Through Buffer Material on the Lifetime of Vitrified Waste

AbstractA sensitivity analysis was conducted to evaluate the impact of silicon migration through buffer material on the lifetime of vitrified waste. The results indicate that the lifetime depends on a combination of the dissolved glass fraction in the non-steady-state phase controlled by the silicon pore diffusion coefficient (Dp) and the silicon distribution coefficient (Kd) in the buffer material and the steady-state dissolution rate defined by Dp and the groundwater flow rate (Q) in the excavation disturbed zone. In the case where the glass dissolution rate reaches the steady-state dissolution rate, the sensitivity of the lifetime to Dp and Q varies according to the magnitude relationship between Dp and Q. We also discuss the impact on the lifetime of glass hydration, which proceeds simultaneously with glass matrix dissolution. The results show that glass hydration is less important for the lifetime than glass matrix dissolution in an open system and it can be concluded that silicon migration through the buffer material will be an important process for estimating the lifetime of the vitrified waste. A preliminary calculation of the long-term waste behavior with realistic assumptions indicates the importance of the silicon migration parameters Kd and Dp, which control the dissolution behavior of the vitrified waste in the non-steady-state phase, for evaluating radionuclide release.

Surface Morphological Changes Accompanying Dissolution of Bioactive Glass: Effect of Residual Stress

We report our observations concerning the time evolution of surface morphology occurring during the in vitro immersion of bioactive glass surfaces in contact with phosphate buffer solution. We compare regions under intentionally produced residual stresses via micro-indentation to those where no indentation was performed. The sign of the residual stress is shown to be important for predicting dissolution behaviour; compression retards dissolution, whereas tension enhances dissolution. We analyze our results with a simple model for the work of bond dissociation. We report that a highly constrained residual compressive stress state, such as in an indent, leads to a work deficit in comparison to tension, which accounts for the slower dissolution rate of compressed bioactive glass. Such a mechanochemical effect suggests that the presence of residual stresses from the manufacture of biomedical implants and devices could lead to accelerated or delayed dissolution and that careful control of residual stresses should be sought for predictable performance in dissolvable materials.

Theoretical consideration on the application of the Aagaard–Helgeson rate law to the dissolution of silicate minerals and glasses

The kinetic laws derived from the work of Aagaard and Helgeson are discussed, notably those applied to aluminosilicate or borosilicate glasses. Aagaard and Helgeson extended the kinetic formalism of an elementary reaction in a homogeneous medium to overall alteration processes in heterogeneous media by assuming they consist of a series of elementary steps. The dissolution rate of a mineral phase can thus be expressed as follows: (1) r ϕ = k ϕ ∏ i a i − n i j ( 1 − exp ( − A σ R T ) ) where k ϕ is the kinetic constant of hydrolysis of the mineral ϕ, a i the activity of the reactants i in the limiting elementary step j, n ij the stoichiometric coefficient for reactant i in the limiting reaction j, A the chemical affinity of the overall dissolution reaction, σ the average stoichiometric number of the overall reaction, R the ideal gas constant and T the temperature. We first illustrate the relation between transition state theory and a kinetic law such as Eq. (1) initially associated with an elementary reaction, using the simple example of hydriodic acid synthesis. We then discuss the extension of Eq. (1) to overall processes, showing that there is no obvious relation between the elementary limiting step and the contents of the affinity function and that this reflects a problem of scale in the Aagaard–Helgeson law between the kinetic constant (based on microscopic theories) and the affinity term (a macroscopic entity derived from classical thermodynamics). The discussion shows the difficulties encountered in attempting to determine the activity of the reactants in the elementary limiting step, particularly in the case of surface groups, and highlights the limited validity of extending a chemical affinity law of the type (1 − exp(− Δ G/ RT))—which is theoretically valid for an elementary reaction near equilibrium for an overall process. Our article then reviews Grambow's reasoning and the difficulties he encountered in applying the Aagaard–Helgeson law to the dissolution of nuclear borosilicate glasses. This example shows that the concepts of the Aagaard–Helgeson law are not simple to use, notably for determining the content of the affinity function and calculating the activity of the surface species. With complex (basaltic or nuclear) glasses, formulating the hypothetical series of elementary reactions becomes unrealistic, and the notion of an equilibrium constant remains a difficult problem. From those considerations, one can conclude that the classical first order rate law appears to be a more empirical than theoretical equation. Moreover, even if the affinity function with respect to the silicate network stability is a point to account for the rate drop, other phenomena like slow diffusion of reactive species through the hydrated layer or precipitation of secondary minerals (smectite, zeolite) are likely more important to predict the long-term dissolution rate of natural or nuclear glasses in most of the confined environments.

Phthalic acid complexation and the dissolution of forsteritic glass studied via in situ FTIR and X-ray scattering

Multiple Internal Reflection Fourier Transform Infra-Red (MIR-FTIR) spectroscopy was developed and used for in situ flow-through experiments designed to study the process of organic acid promoted silicate dissolution. In tandem with the FTIR analysis, ex situ X-ray scattering was used to perform detailed analyses of the changes in the surface structure and chemistry resulting from the dissolution process. Phthalic acid and forsteritic glass that had been Chemically Vapour Deposited (CVD) onto an internal reflection element were used as reactants, and the MIR-FTIR results showed that phthalic acid may promote dissolution by directly binding to exposed Mg metal ion centers on the solid surface. Integrated infrared absorption intensity as a function of time shows that phthalic acid attachment apparently follows a t 1/2 dependence, indicating that attachment is a diffusive process. The diffusion coefficient of phthalic acid was estimated to be approximately 7 × 10 −6 cm 2 s −1 in the solution near the interface with the glass. Shifts in the infrared absorption structure of the phthalate complexed with the surface compared to the solute species indicate that phthalate forms a seven-membered ring chelate complex. This bidentate complex efficiently depletes Mg from the glass surface, such that after reaction as much as 95% of the Mg may be removed. Surface depletion in Mg causes adsorbate density to fall after an initial attachment stage for the organic ligand. In addition, the infrared analysis shows that silica in the near surface polymerizes after Mg removal, presumably to maintain charge balance. X-ray reflectivity shows that the dissolution rate of forsteritic glass at pH 4 based on Mg removal in such flow-through experiments was equal to 4 × 10 −12 mol cm −2 s −1 (geometric surface area normalized). Reflectivity also shows how the surface mass density decreases during reaction from 2.64 g cm −3 to 2.2 g cm −3, consistent with preferential loss of Mg from the surface. Auxiliary batch experiments with forsteritic glass films deposited onto soda glass were also completed to add further constraints to the mechanism of reaction. By combining reflectivity with diffuse scatter measurements it is shown that the primary interface changes little in terms of atomic-scale roughness even after removal of several hundred angstroms of material. These measurements unequivocally show how a dicarboxylic acid bonds to and may chelate the dissolution of a magnesium-bearing silicate. At the molecular level the solid surface retreat may best be described by a depinning model where Mg is preferentially removed and residual silica tetrahedra polymerize and act to episodically “pin” the surface.

Volcanogenic sediment–seawater interactions and the geochemistry of pore waters

Four volcanic ash-bearing marine sediment cores and one ash-free reference core were examined during research cruise RV Meteor 54/2 offshore Nicaragua and Costa Rica to investigate the chemical composition of pore waters related to volcanic ash alteration. Sediments were composed of terrigenous matter derived from the adjacent continent and contained several distinct ash layers. Biogenic opal and carbonate were only minor components. The terrigenous fraction was mainly composed of smectite and other clay minerals while the pore water composition was strongly affected by the anaerobic degradation of particulate organic matter via microbial sulphate reduction. The alteration of volcanic matter showed only a minor effect on major element concentrations in pore waters. This is in contrast to prior studies based on long sediment cores taken during the DSDP, where deep sediments always showed distinct signs of volcanic ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawater-derived pore fluids. Dissolved silica concentrations were, however, significantly enriched in ash-bearing cores and showed no relation to the low but variable contents of biogenic opal. Hence, the data suggest that silica concentrations were enhanced by ash dissolution. Thus, the dissolved silica profile measured in one of the sediment cores was used to derive the in-situ dissolution rate of volcanic glass particles in marine sediments. A non-steady state model was run over a period of 43 kyr applying a constant pH of 7.30 and a dissolved Al concentration of 0.05 μM. The kinetic constant ( A A) was varied systematically to fit the model to the measured dissolved silica-depth profile. The best fit to the data was obtained applying A A = 1.3 × 10 −U9 mol of Si cm − 2 s − 1 . This in-situ rate of ash dissolution at the seafloor is three orders of magnitude smaller than the rate of ash dissolution determined in previous laboratory experiments. Our results therefore imply that field investigations are necessary to accurately predict natural dissolution rates of volcanic glasses in marine sediments.

The effect of aqueous sulphate on basaltic glass dissolution rates

AbstractSteady-state dissolution rates of basaltic glass were measured in mixed-flow reactors at 50ºC at pH 3 and 4 as a function of aqueous sulphate concentration. Dissolution rates in the presence of 0.1 moles/kg SO42- were found to be ~3 times greater than those in corresponding SO42- free solutions. This rate increase is found to be approximately consistent with that calculated using a rate equation previously proposed by Gislason and Oelkers (2003). These results suggest that the addition of sulphate to injected CO2 may facilitate CO2 sequestration in basalts by accelerating basaltic glass dissolution rates thus more rapidly releasing aqueous Ca and Mg to solution.