Basaltic Glass Dissolution Research Articles

Effect of Cation Chloride Concentration on the Dissolution Rates of Basaltic Glass and Labradorite: Application to Subsurface Carbon Storage

The steady-state dissolution rates of basaltic glass and labradorite were measured in the presence of 10 to 700 × 10−3 mol·kg−1 aqueous NaCl, KCl, CaCl2, and MgCl2 at 25 °C. All rates were measured in mixed flow reactors, and at pH~3.6 by the addition of HCl to the reactive fluids. The steady-state basaltic glass dissolution rates, based on Si release, increased by ~0.3 log units in the presence of 10−3 mol·kg−1 of either CaCl2 or MgCl2 compared to their rates in 10−3 mol·kg−1 of NaCl or KCl. In contrast, the steady-state dissolution rates of labradorite decreased by ~0.4 log units in the presence of 10−3 mol·kg−1 of either CaCl2 or MgCl2 compared to their rates in 10−3 mol·kg−1 of NaCl or KCl. These contrasting behaviours likely reflect the varying effects of these cations on the stability of rate controlling Si-rich activated complexes on the surface of the dissolving solids. On average, the Si release rates of these solids are similar to each other and increase slightly with increasing ionic strength. As the pH of water charged with 10 to 30 bars CO2 is ~3.6, the results of this study indicate that both basaltic glass and labradorite dissolution will likely be effective at increasing the pH and adding Ca to the aqueous phase in saline fluids. This observation supports potential efforts to store carbon through its mineralization in saline aquifers containing Ca-bearing feldspar and in submarine basalts.

Reaction of pseudowollastonite with carbonate-bearing fluids: Implications for CO2 mineral sequestration

The kinetics of silicate carbonation in aqueous solutions are typically sluggish, especially at neutral to alkaline conditions. This hampers the complete understanding of the mechanisms and parameters that control mineral carbonation during carbon capture and storage (CCS). Here we study the hydrothermal dissolution and carbonation of pseudowollastonite (psw; α-CaSiO3), one of the most reactive silicates known, under a range of geochemical conditions ranging from acidic to strongly alkaline pH, presence/absence of different background alkali metal ions and carbonate sources (K2CO3 and Na2CO3, pH ~13, or NaHCO3 and KHCO3, pH ~9). We show that in addition to amorphous silica precipitation, the formation of secondary Na + Ca- or K + Ca-silicates in the presence of Na+ and K+ background ions, respectively, fosters the progress of psw carbonation. However, the formation of Ca-containing secondary crystalline silicates and Ca-containing amorphous silica is shown to be a strong handicap for a fully effective carbonation. In all cases a higher conversion into CaCO3 (up to ~70 mol%) is achieved when using bicarbonate salts (i.e., lower initial pH). By using a reactor with a pressurized CO2-solution, with and without Na+ or K+ background ions, rapid and nearly complete conversion of psw with a CaCO3 yield ~92 mol% is achieved because, in addition to the initial low pH (~3.7) that favored α-CaSiO3 dissolution, abundant Ca-free non-passivating amorphous silica formed along with calcite. These results imply that the presence (e.g., use of sea water during CO2 injection or mixing with saline formation solutions) or the release of different alkali metal ions (e.g., after feldspar and/or basaltic glass dissolution) in combination with a reaction-induced pH increase during in situ CCS scenarios may strongly limit carbonation due to the capture of alkaline-earth metals in secondary silicates and a reduction in reaction rates. In turn, our results show that the high conversion achieved in pure CO2-aqueous systems, while relevant for ex situ CCS, may not reflect the actual conversion in multicomponent natural systems following reactive transport during in situ CCS. Moreover, the precipitation of secondary silicate and calcium carbonate phases have a direct cementing effect, which could be detrimental for in situ CCS, as it would likely reduce host rock permeability, but would be relevant and beneficial for the setting of novel CaSiO3-based non-hydraulic cements with reduced CO2 footprint.

Reaction path modelling of in-situ mineralisation of CO2 at the CarbFix site at Hellisheidi, SW-Iceland

Results from injection of 175 tonnes of CO2 into the basaltic subsurface rocks at the CarbFix site in SW-Iceland in 2012 show almost complete mineralisation of the injected carbon in less than two years (Matter et al., 2016; Snæbjörnsdóttir et al., 2017). Reaction path modelling was performed to illuminate the rate and extent of CO2-water-rock reactions during and after the injection. The modelling calculations were constrained by the compositions of fluids sampled prior to, during, and after the injection, as reported by Alfredsson et al. (2013) and Snæbjörnsdóttir et al. (2017). The pH of the injected fluid, prior to CO2 dissolution was ∼9.5, whereas the pH of the background waters in the first monitoring well prior to the injections was ∼9.4. The pH of the sampled fluids used in the modelling ranged from ∼3.7 at the injection well to as high as 8.2 in the first monitoring well. Modelling results suggest that CO2-rich water-basalt interaction is dominated by crystalline basalt dissolution along a faster, high permeability flow path, but by basaltic glass dissolution along a slower, pervasive flow path through which the bulk of the injected fluid flows. Dissolution of pre-existing calcite at the onset of the injection does not have a net effect on the carbonation, but does contribute to a rapid early pH rise during the injection, and influences which carbonate minerals precipitate. At low pH, Mg, and Fe are preferentially released from crystalline basalts due to the higher dissolution rates of olivine, and to lesser extent pyroxene, compared to plagioclase and glass (Gudbrandsson et al., 2011). This favours the formation of siderite and Fe-Mg carbonates over calcite during early mineralisation. The model suggests the formation of the following carbonate mineral sequences: siderite at pH < 5, Mg-Fe-carbonates and Ca-Mg-Fe-carbonates at pH > 5, and calcite at higher pH. Other minerals forming with the carbonates are Al- and Fe-hydroxides and chalcedony, and zeolites and smectites at elevated pH. The most efficient carbonate formation is when the pH is high enough for formation of carbonates, but not so high that zeolites and smectites start to form, which compete with carbonates over both cations and pore space. The results of reaction path modelling at the CarbFix site in SW-Iceland indicate that this “sweet spot” for mineralisation of CO2 is at pH from ∼5.2 to 6.5 in basalts at low temperature (20–50 °C).

Numerical simulations of CO2 sequestration in basaltic rock formations: challenges for optimizing mineral-fluid reactions

Abstract Numerical simulations were carried out for determining the chemical reactions relevant for the sequestration of CO2 in basaltic rock formations. The mineralogy of natural geological systems consists of silicate minerals such as the phyllosilicates and zeolites that form complex solid solutions. Using the GEMS code package based on Gibbs energy minimization, combined with the new MINES database, we can now simulate the solubility of these multicomponent and multisite mineral solid solutions in basaltic rocks. This study explores the varying effects of CO2 partial pressures, basaltic glass dissolution kinetics and reaction time on the complex chemistry of the overall CO2-water-basalt reaction path. The simulations indicate four reaction progress stages with the competing reactions between smectites (di- and trioctahedral) and Ca-Fe-Mg-carbonates controlling the amount of CO2 mineralized. A better understanding of these key mineral-fluid reactions and improvement of their thermodynamic models is critical for making more acurate predictive calculations. This comprises the basis for extending the simulations to reactive transport models, and for the assessment of the feasibility of long-term CO2 storage in basaltic rock formations.

Electrolyte effects on surface chemistry of basaltic glass in the initial stages of dissolution

For an understanding of the effect of solution composition on the dissolution rate of basaltic glass detailed knowledge of surface chemistry is important. Here the zeta potential (ζ) as a characteristic parameter of the magnitude of surface charge at the solid–liquid interface was used to determine ionic effects on surface chemistry in initial stages of basaltic glass dissolution. In a systematic approach powdered synthetic basaltic glass was dispersed in solutions of different cations (NO3− salts of Na+, K+, Mg2+, Ca2+, Ba2+, Zn2+, and Al3+) and anions (Na+ salts of F−, Cl−, I−, NO3−, SO42−, C2O42−, HPO42−), each in concentrations of 0.1, 0.5, 1.0, 2.5, and 5.0mmol/L. ζ was traced in time sequences up to 12,000h at ideally circumneutral pH. Ion affinities to glass surfaces were characterized by sorption isotherms. A change of glass chemical composition by the formation of altered layers was determined by depth profiling using secondary neutral mass spectrometry (SNMS). The dissolution of the glass was quantified by the amount of Si released after 4000h.A marked decrease of ζ in deionized water within the first 3h reaction time is assigned to the desorption of alkali and alkaline earth metal cations from the glass surface and formation of negatively charged SiO− sites. The addition of anions resulted in stronger negative initial ζ values in comparison with the experiment in deionized H2O indicating marked anion adsorption on surface sites, most obvious for F−, C2O42− and HPO42−. The initial ζ was increased upon the addition of divalent cations indicating neutralization of negatively charged surface sites. Over time a striking shift from negative to positive ζ was obtained, most markedly for Ca2+ and Zn2+. The addition of trivalent Al3+ resulted directly in positive ζ indicating a strong adsorption on glass surfaces. With the progress of the experiment the sign of ζ reversed to negative values again. The reason for charge reversal is not fully understood and might be related with cation adsorption exceeding the negative surface charge and a concentration of Fe oxides at the glass surface. After an ~2000h reaction time ζ adjusted for most electrolyte additions to slightly negative ζ until the end of the experiment, indicating that a final state in the composition of surface sites was reached. The presence of monovalent Na+ and K+ in solution suppressed Si release from the glass, whereas it is accelerated by bivalent cations. It appears that the neutralization of deprotonated SiO− sites by monovalent cations – their preferential binding is also indicated by chemical analysis – favors polymerization resulting in slower Si release. Upon the addition of Al3+ it is likely that SiOAlOSi bonds are formed, which can suppress Si release. The presence of F−, C2O42−, and HPO42− clearly enhances glass dissolution, most probably by increasing the coordination of network forming cations, hereby weakening bonds. The observed generation of positive ζ on basaltic glass surfaces is remarkable, and can improve in natural systems the adsorption capability of the basaltic glass surface for negatively charged compounds from pore solution, anions, dissolved organic matter and also bacterial cell walls.

Dissolution behaviour of model basalt fibres studied by surface analysis methods

New concepts of surface modifications aimed at the enhancement of alkali resistance of basalt fibres require research work on chemical composition of interacting surface layers as well as knowledge about fundamental processes of basaltic glass dissolution. Therefore, two model basalt fibres manufactured out of subalkaline and alkaline rock material were leached in NaOH solution at a temperature of 80°C for up to 11 days. The formation of a corrosion shell was observed in both cases and was analyzed by SEM/EDX. The model fibres out of subalkaline rocks show dissolution kinetic, which is two-staged, whereas the more alkaline fibre reflects a linear one. The complex composition of basalt fibre is detected by EDX and XPS. The surface of basalt fibres is rich in Si and Al. XPS high resolution spectra provide information on oxidation state of iron.

An experimental study of basaltic glass–H2O–CO2 interaction at 22 and 50 °C: Implications for subsurface storage of CO2

A novel high pressure column flow reactor was used to investigate the evolution of solute chemistry along a 2.3m flow path during pure water- and CO2-charged water–basaltic glass interaction experiments at 22 and 50°C and 10−5.7 to 22bars partial pressure of CO2. Experimental results and geochemical modelling showed the pH of injected pure water evolved rapidly from 6.7 to 9–9.5 and most of the iron released to the fluid phase was subsequently consumed by secondary minerals, similar to natural meteoric water–basalt systems. In contrast to natural systems, however, the aqueous aluminium concentration remained relatively high along the entire flow path. The aqueous fluid was undersaturated with respect to basaltic glass and carbonate minerals, but supersaturated with respect to zeolites, clays, and Fe hydroxides. As CO2-charged water replaced the alkaline fluid within the column, the fluid briefly became supersaturated with respect to siderite. Basaltic glass dissolution in the column reactor, however, was insufficient to overcome the pH buffer capacity of CO2-charged water. The pH of this CO2-charged water rose from an initial 3.4 to only 4.5 in the column reactor. This acidic reactive fluid was undersaturated with respect to carbonate minerals but supersaturated with respect to clays and Fe hydroxides at 22°C, and with respect to clays and Al hydroxides at 50°C. Basaltic glass dissolution in the CO2-charged water was closer to stoichiometry than in pure water. The mobility and aqueous concentration of several metals increased significantly with the addition of CO2 to the inlet fluid, and some metals, including Mn, Cr, Al, and As exceeded the allowable drinking water limits. Iron became mobile and the aqueous Fe2+/Fe3+ ratio increased along the flow path. Although carbonate minerals did not precipitate in the column reactor in response to CO2-charged water–basaltic glass interaction, once this fluid exited the reactor, carbonates precipitated as the fluid degassed at the outlet. Substantial differences were found between the results of geochemical modelling calculations and the observed chemical evolution of the fluids during the experiments. These differences underscore the need to improve the models before they can be used to predict with confidence the fate and consequences of carbon dioxide injected into the subsurface.

Dynamics of basaltic glass dissolution – Capturing microscopic effects in continuum scale models

The method of ‘multiple interacting continua’ (MINC) was applied to include microscopic rate-limiting processes in continuum scale reactive transport models of basaltic glass dissolution. The MINC method involves dividing the system up to ambient fluid and grains, using a specific surface area to describe the interface between the two. The various grains and regions within grains can then be described by dividing them into continua separated by dividing surfaces. Millions of grains can thus be considered within the method without the need to explicity discretizing them. Four continua were used for describing a dissolving basaltic glass grain; the first one describes the ambient fluid around the grain, while the second, third and fourth continuum refer to a diffusive leached layer, the dissolving part of the grain and the inert part of the grain, respectively.The model was validated using the TOUGHREACT simulator and data from column flow through experiments of basaltic glass dissolution at low, neutral and high pH values. Successful reactive transport simulations of the experiments and overall adequate agreement between measured and simulated values provides validation that the MINC approach can be applied for incorporating microscopic effects in continuum scale basaltic glass dissolution models. Equivalent models can be used when simulating dissolution and alteration of other minerals.The study provides an example of how numerical modeling and experimental work can be combined to enhance understanding of mechanisms associated with basaltic glass dissolution. Column outlet concentrations indicated basaltic glass to dissolve stoichiometrically at pH 3. Predictive simulations with the developed MINC model indicated significant precipitation of secondary minerals within the column at neutral and high pH, explaining observed non-stoichiometric outlet concentrations at these pH levels. Clay, zeolite and hydroxide precipitation was predicted to be most abundant within the column.

Dynamics of basaltic glass dissolution – Capturing microscopic effects in continuum scale models.

Dynamics of basaltic glass dissolution – Capturing microscopic effects in continuum scale models.

Mineralogical aspects of CO2 sequestration during hydrothermal basalt alteration — An experimental study at 75 to 250 °C and elevated pCO2

The alteration of basaltic glass was studied experimentally at elevated CO2 pressures (~10–25bar) and hydrothermal conditions (75–250°C) to determine the effects of temperature and extent of reaction (ξ) on the secondary mineral formation and compositional evolution of alteration assemblages resulting from CO2–water–basalt interaction. At <100°C, the alteration products consisted of concentric layers of ankerite and dolomite–ankerite solid solutions (Ca–Mg–Fe carbonates) and amorphous silica. At ≥150°C, mixed Ca–Mg–Fe smectites and chlorite, calcite, amorphous silica and zeolites formed instead of Ca–Mg–Fe carbonates. Competing reactions between carbonates and clays for major divalent cations (Ca, Mg and Fe) were affected by the extent of reaction with smectites formed initially and progressively being replaced by calcite and chlorite. The basaltic glass dissolution rate and mechanism were also affected by temperature and reaction time. At lower temperatures (≤150°C), a hydrated leached layer formed on the glass surface and the mass fluxes in the system were largely controlled by the dissolution rate and mechanism of the glass, whereas at higher temperature (250°C) the dissolution rate of the basaltic glass was fast with the reactions primarily driven by secondary mineral replacement and growth. We conclude that the driving force for carbonate mineralization in basaltic glass at elevated CO2 conditions is linked to the geochemical behavior and mobility of Ca, Mg and Fe together with the availability of Si for Al–Si-mineral formation. At <100°C, the divalent cations are available for Ca–Mg–Fe carbonate formation, whereas at ≥150°C abundant clays limit the availability of Mg and Fe resulting in calcite being the only carbonate formed. This implies a quantitatively more effective fixation of CO2 at <100°C associated with the formation of Ca–Mg–Fe carbonates.

Does the presence of heterotrophic bacterium Pseudomonas reactans affect basaltic glass dissolution rates?

Far-from-equilibrium steady-state basaltic glass dissolution rates were measured using newly developed Bacterial Mixed-Flow Reactors (BMFR) at 25°C. Experiments were performed in aqueous pH 4, 6, 8, and 10 buffer solutions with and without nutrients and in: 1) the absence of bacteria, 2) the presence of 0.1–0.4gwet/L dead Pseudomonas reactans, and 3) in the presence of 0.9–19.0gwet/L live P. reactans extracted from a deep subsurface oxygen-bearing basaltic aquifer. The BMFR allows steady-state rate measurements at constant concentrations of live or dead bacteria. Experiments ran for 30–60days on a single basaltic glass powder, initially at bacteria- and nutrient-free conditions until Si steady-state glass dissolution was achieved. Then, in 2–4 steps, either live or dead P. reactans were added via nutrient-rich or nutrient-free inlet solutions, and the flux of Si and other mineral constituents from the basaltic glass were measured until a new chemical steady state was attained. Scanning Electron Microscope and X-ray Photoelectron Spectroscopic techniques verified the presence of live bacteria on the basaltic glass surfaces through biofilm formation, bacterial dissolution imprints, and enrichment of surface layers in C and N. The presence of either live or dead P. reactans lowers constant pH steady-state basaltic glass dissolution rates by no more than ~0.5log units which is close to combined analytical and experimental uncertainties of the experiments. The presence of bacteria did not cause any significant modification in trace element release rates from dissolving basaltic glass within the uncertainty of the measurements. Experiments in nutrient-rich solutions yielded close to stoichiometric release of all major elements from basaltic glass at pH 6–8 likely as a result of organic ligand complexation with aqueous Al and Fe, which would otherwise form hydroxy-precipitates. As the results of this work suggest, at most, a small inhibiting effect of live or dead P. reactans on basaltic glass dissolution rates, geochemical modeling of mineral reactivity in basaltic aquifers likely does not require explicit provision for the presence of heterotrophic bacteria on basaltic glass reactivity.

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.

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.

Low-temperature alteration of basalts – the effects of temperature, acids and extent of reaction on mineralization and water chemistry

The weathering and low-temperature alteration of basaltic glass was studied by geochemical modelling in order to gain an insight into the effects of temperature, acid supply and extent of reaction on the secondary mineralogy and water chemistry. Basaltic glass was dissolved in dilute water at 10–150°C in a closed system and secondary minerals commonly observed in nature allowed to precipitate when saturated. The weathering of basalts in the presence of CO$_2$ was observed to go through three stages; initially simple insoluble Al and Fe hydroxides were formed. Upon progressive basaltic glass dissolution imogolite, allophane and/or kaolinite and Ca–Mg–Fe smectites predominated, decreasing the mobility of Al, Fe, Si, Ca and Mg whereas extensive weathering and alkaline pH values resulted in the formation of smectites, zeolites, calcite and SiO$_2$ minerals. Under low-temperature geothermal conditions and in strong H$_2$ SO$_4$ acid solutions (pH\&lt;4) amorphous silica, kaolinite, Al–Fe oxyhydroxides and sulphur-containing minerals were most important and most cations like Na, K, Ca and Mg were observed to be mobile. Under mildly acid conditions in CO$_2$ enriched waters (pH 5–7) kaolinite, chalcedony, Ca–Mg–Fe smectites and Mg–Fe–Ca carbonates predominated and Fe, Al and Si were found to be immobile, whereas Mg and Ca mobility depended on the mass of carbonate formed and water pH. Under alkaline conditions (pH\&gt;8) that resulted from a low acid supply and/or extensive basaltic glass dissolution chalcedony, celadonite, Ca–Mg–Fe smectites, zeolites and calcite were found to form, greatly reducing the mobility of most dissolved elements. The dominant factor determining the weathering and low-temperature alteration of basaltic glass and the associated elemental mobility is the pH of the water. In turn, the pH value is determined by the input and type of acid and their ionization constant and quantity of basaltic glass dissolution and secondary minerals formed that increases with the extent of the reaction and the temperature. The weathering mineralogy observed associated with basaltic glass in Iceland is typical of low to moderate degree of alteration under low (atmospheric) CO$_2$ conditions. The low-temperature regional geothermal alteration commonly observed in Iceland is also typical for low CO$_2$ alteration and the reaction order of celadonite and chalcedony, followed by mixed clays and chlorite and eventually zeolites and calcite, indicate increased water–rock interaction at pH&gt;8 with temperature being less important on the overall reaction path.

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.

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.

Mechanism, rates, and consequences of basaltic glass dissolution: II. An experimental study of the dissolution rates of basaltic glass as a function of pH and temperature

This study is aimed at quantifying surface reaction controlled basaltic glass dissolution rates at far-from-equilibrium conditions. Towards this aim, steady-state basaltic glass dissolution rates were measured as a function of pH from 2 to 11 at temperatures from 6° to 50°C, and at near neutral conditions to 150°C. All rates were measured in open system titanium mixed flow reactors. Measured dissolution rates display a common pH variation; dissolution rates decrease dramatically with increasing pH at acid conditions, minimize at near neutral pH, and increase more slowly with increasing pH at basic conditions. The pH at which basaltic glass dissolution minimizes decreases with increasing temperature. Dissolution rates were interpreted within the context of a multioxide dissolution model. Constant temperature rates are shown to be consistent with their control by partially detached Si tetrehedra at the basaltic glass surface. Regression of far-from-equilibrium dissolution rates obtained in the present study and reported in the literature indicate that all data over the temperature and pH range 6° < T < 300°C and 1 < pH < 11 can be described within uncertainty usingr+,geo = AA exp−(EA/RT)aH+3aAl3+1/3where r +,geo signifies the geometric surface area normalized steady-state basaltic glass dissolution rate at far-from-equilibrium conditions, A A refers to a constant equal to 10 −5.6 (mol of Si)/cm 2/s, E A , designates a pH independent activation energy equal to 25.5 kJ/mol, R stands for the gas constant, T signifies temperature in K, and a i represents the activity of the subscripted aqueous species.

In situ determination of long-term basaltic glass dissolution in the unsaturated zone

Maximum in situ weathering rates of basaltic glass measured at the El Malpais National Monument in New Mexico are on the order of 2–5×10 −19 mol/cm 2 s. Rates were calculated from backscattered electron (BSE) imaging of weathered porosity and are equivalent to 1.7–5% of the surface per 1000 years. Weathering is independent of glass composition but appears to increase with flow elevation at El Malpais. Measured rates represent weathering over 3000 years and are substantially lower than glass dissolution rates measured in the laboratory over much shorter time spans. Basaltic glass is a close chemical analogue to glass hosts proposed for encapsulation of high-level nuclear wastes. Radionuclide release rates predicted from the basis of in situ field rates are substantially less than those predicted from short-term laboratory experiments.

The mechanism, rates and consequences of basaltic glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25°C and pH = 3 and 11

Steady state basaltic glass dissolution rates were measured as a function of aqueous aluminum, silica, and oxalic acid concentration at 25° C and pH 3 and 11. All rates were measured in mixed flow reactors, performed in solutions that were strongly undersaturated with respect to hydrous basaltic glass, and exhibited stoichiometric Si versus Al release. Rates are independent of aqueous silica activity, but decrease with increasing aqueous aluminum activity at both acidic and basic conditions. Increasing oxalic acid concentration increased basaltic glass dissolution rates at pH 3, but had little affect at pH 11. All measured rates can be described within experimental uncertainty using r=kaH+3aAl+30.35r signifies the surface area normalized basaltic glass steady state dissolution rate, k refers to a rate constant equal to 10−11.65 (mol of Si)/cm2/s, and ai represents the activity of the subscripted aqueous species.The observation that all rates obtained in the present study can be described by a single regression equation supports strongly the likelihood that basaltic glass dissolution is controlled by a single mechanism at both acidic and basic pH and in both the presence and absence of organic acids. Taking account of the dissolution mechanisms of similarly structured and compositioned minerals, and previously published studies of basaltic glass dissolution behavior, basaltic glass dissolution likely proceeds via 1) the relatively rapid and essentially complete removal of univalent and divalent cations from the near surface; 2) aluminum releasing exchange reactions between three aqueous H+ and Al in the basaltic glass structure; followed by 3) the relatively slow detachment of partially liberated silica. The breaking of Al-O bonds does not destroy the glass framework; it only partially liberates the silica tetrahedral chains by removing adjoining Al atoms. Basaltic glass dissolution rates are proportional to the concentration of partially detached framework Si tetrahedra near the surface, which is linked through the law of mass action for the Al/proton exchange reaction to aqueous aluminum activity.

Dissolution kinetics of basaltic glasses: control by solution chemistry and protective effect of the alteration film

Basaltic glasses are considered as natural analogs for industrial nuclear aluminoborosilicate glasses. Alteration experiments were conducted in closed and open systems at 90°C with a synthetic basalt glass doped with 1% lithium (dissolution tracer). The evolution of the alteration kinetics over time was assessed by comparison of reaction progress at different degrees in closed system experiments. The maximum dissolution rate (initial rate, r 0) was comparable to the value observed for an SON68-type nuclear glass; the basaltic glass alteration rate subsequently dropped by four orders of magnitude. The kinetic models currently proposed in the literature to account for the alteration kinetics of basaltic glasses, nuclear glasses or aluminosilicate minerals are based on the concept of chemical affinity: the chemical affinity alone is assumed to control the dissolution kinetics. When applied to the experimental data for the closed system tests with basaltic glass, these models failed to account for the low measured rates. An inhibiting effect of dissolved silica was then investigated through open system basaltic glass alteration experiments with silicon-enriched solutions. The basaltic glass dissolution rate dropped by a factor not exceeding 200 (or about two orders of magnitude) compared with r 0 at the high imposed silicon concentrations (130 ppm). A protective effect of the alteration film was advanced to account for the four-orders-of-magnitude rate drop observed in closed system experiments, based notably on an examination of natural basaltic glasses ranging in age from several thousand to a few million years. The mean alteration rates can be estimated from the measured palagonite thicknesses, taking into account the age of the natural glasses; the rates were comparable to those measured in the laboratory for high reaction progress.