Steady-state Dissolution Research Articles

Processes at the magnesium-bearing carbonates/solution interface. II. kinetics and mechanism of magnesite dissolution.

Steady-state dissolution rates of magnesite (MgCO 3) were measured at 25°C as a function of pH (from 0.2 to 12), total dissolved carbonate concentration (10 −5 < ΣCO 2 < 0.1 M), and ionic strength (0.002 < I < 0.5 M) using a mixed-flow reactor. Dissolution rates were found to be pH-independent at 0 < pH < 3, proportional to a H+ at 3 < pH < 5, pH-independent at 5 < pH < 8, and decreasing with increasing pH at pH > 8 and ΣCO 2 > 10 −3 M. In the acid pH region (3 ≤ pH ≤ 5), the rates increase significantly with ionic strength. In the alkaline pH region, carbonate and bicarbonate ions and ionic strength inhibit significantly the dissolution rate even at far from equilibrium conditions. The surface complexation model developed by Pokrovsky et al. (1999a) was used to correlate magnesite dissolution kinetics with its surface speciation. Dissolution rates in the acid pH region are controlled by the protonation of >CO 3 − surface complexes. In neutral and carbonate-rich alkaline solutions, >MgOH 2 + controls the dissolution kinetics. The following rate equation, consistent with transition state theory was used to describe magnesite dissolution kinetics over the full range of solution composition: R (mol/cm 2/s)=[10 7.198·{>CO 3H°} 3.97+10 5.38·{>MgOH 2 +} 3.94]·(1−exp(−4 A/ RT)) where {> i} stands for surface species concentration (mol/m 2), and A refers to the chemical affinity of the overall reaction. This equation reflects the formation of two different precursor-activated complexes which contain four protonated >CO 3H° species in acid solutions and four protonated (hydrated) >MgOH 2 + groups in neutral and alkaline solutions. The very low magnesite dissolution/precipitation rates predicted by this equation, especially at close to equilibrium conditions, are consistent with those deduced from field measurements.

An experimental study of dolomite dissolution rates as a function of pH from −0.5 to 5 and temperature from 25 to 80°C

Steady-state dissolution rates of Haute Vallée de l'Aude dolomite were measured in mixed-flow reactors using rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range −0.39 to 4.44, and at temperatures of 25, 50 and 80°C. Dissolution rates were found to depend on disk rotation speed at all temperatures and pH. Interpretation of these rates using equations reported by Gregory and Riddiford [Gregory, D.P., Riddiford, A.C., 1956. Transport to the surface of a rotating disk. J. Chem. Soc., 33, 37–56] yields steady-state dissolution rates as a function of the solution pH adjacent to dolomite surface (pH surf). Rates at all temperatures and 1<pH surf<5 are found to be consistent with r= k 1 a n 1 H +,surf , where r refers to surface area normalized steady-state dissolution rates, k 1 stands for a rate constant, and a H +,surf designates hydrogen ion activity in the solution adjacent to the dolomite surface, and n 1 denotes a constant equal to 0.63, 0.73 and 0.80±0.05 at 25, 50 and 80°C, respectively. Rates tend towards pH independence at more acidic conditions suggesting the saturation of dolomite surfaces with rate controlling protonated species. Additional experiments performed to assess the effect of the presence of aqueous Ca 2+ and Mg 2+ demonstrate that these species have little to no effect on dolomite dissolution rates at pH=2 and 50°C.

Localised corrosion behaviour of Fe-N model alloys

The present investigations were carried out to understand the role played by nitrogen in affecting the passivation and pitting kinetics of Fe-N model alloys. Synthetic alloys of Fe-N, Fe-10%Ni-N and Fe-10%Ni-3%Mo-N were prepared by surface nitriding process with 0.9 wt% nitrogen on the surface. Pit initiation studies were conducted on pure Fe and Fe-N alloy in an acidic sulphate solution (0.01 M H 2SO 4 and 0.99 M Na 2SO 4) containing 0.0007, 0.001, 0.007 and 0.07 M NaCl at three different electrode potentials, namely, +800, +1000 and +1200 mV (NHE). Pit growth kinetics was assessed by studying the dissolution behaviour of these alloys during potentiostatic transient in an acidic sulfate solution containing 0.5, 1, 2 and 4.5 M NaCl. The steady state dissolution current was considered as the parameter reflecting the pit dissolution behaviour of the alloys. Potentiodynamic anodic polarisation of the alloys was also carried out in the acidic sulphate solution at room temperature. The results of the investigation indicated that as the chloride content increased nitrogen addition delayed the pit initiation as well as reduced the dissolution kinetics of the pits. The polarisation study of the model alloys indicated that nitrogen played a greater role in reducing the corrosion behaviour of the alloys by reducing the active peak current density and widening the passive range in which pitting is less probable.

In vitro stability of a highly crystalline hydroxylapatite coating in a saturated citric acid solution.

A novel pressurized hydrothermal post-plasma-spray process has been developed to convert the crystalline non-HA and amorphous components of plasma-sprayed hydroxylapatite coatings back into crystalline HA. The process, known commercially as MP-1, was used to produce coatings comprising approximately 96% crystalline HA. The in vitro solubility of the coating in saturated citric acid solution has been measured to simulate the effect of implant detoxification procedures, which use citric acid as a cleaning medium. The MP-1 coating solubility in saturated citric acid solution (pH = 1) was compared to that of coatings with crystalline HA contents ranging from 37.5-82%. All coatings showed an initial sharp rise in coating dissolution, which correlated with crystalline HA content, followed by a steady state dissolution rate. After 60 s at 25 degrees C, the MP-1 coating showed a 65% decrease in solubility compared to a highly amorphous coating (AM-2). All coatings showed very similar steady state dissolution rates, except for AM-2, which was significantly higher. SEM analysis showed that the AM-2 coating surface was degraded substantially more than the other coatings, resulting in partial coating exfoliation. A mechanism of coating dissolution is proposed, in which the initial rapid leaching of soluble phases from the coating leaves behind a porous layer of highly crystalline HA at the coating surface. The stability of this porous crystalline layer leads to steady state, diffusion-limited dissolution of the remainder of the coating. The observed two-regime dissolution profile can be accurately represented by a 2-parameter model, which predicts the initial sharp rise in coating dissolution followed by a slower, steady state loss in coating mass. Model parameters were determined from experimental solubility data, and were shown to correlate with the percentage of crystalline HA in the coatings. The present data suggest that the treated coating is significantly more resistant to degradation from aggressive detoxification procedures such as citric acid burnishing.

Experimental study of acidity-consuming processes in mining waste rock: some influences of mineralogy and particle size

Weathering reactions producing and consuming acid in fresh waste rock samples from the Aitik Cu mine in northern Sweden have been investigated. Batch-scale (0.15 kg) acid titrations with waste rock of different particle sizes were operated for 5 months. The pH was adjusted to a nearly constant level, similar to that in effluents from waste rock dumps at the site (pH near 3.5). The reactions were followed by analysing for all major dissolved elements (K, Na, Mg, Ca, Si, Al, SO 4, Cu, Zn, Fe) in aliquots of solution from the reaction vessels. In addition, the solids were physically and chemically characterised in terms of mineralogy, chemical composition, particle size distribution, surface area and porosity. The results show that the alkalinity production is initially dominated by a rapid dissolution of small amounts of calcite and rapidly exchangeable base cations on silicate surfaces. Steady-state dissolution of primary silicate minerals also generates alkalinity. The total alkalinity is nearly balanced by input of acid from the steady-state oxidation of sulphides, such that the pH 3.1–3.4 can be maintained without external input of acid or base. There is a large difference in weathering rates between fine materials and larger waste rock particles (diameters ( d) >0.25 mm) for both sulphides and silicates. As a result particles with d smaller than 0.25 mm contribute to approximately 80% of the sulphide and silicate dissolution. Calcite dissolution can initially maintain a neutral pH but with time becomes limited by intra-particle diffusion. Calcite within particles larger than 5–10 mm reacts too slowly to neutralise the acid produced from sulphides.

Dissolution kinetics of three component non-disintegrating compacts: theophylline, benzoic acid and salicylamide

The dissolution characteristics of three component compacts containing theophylline, benzoic acid and salicylamide were investigated over a wide range of compositions, the dissolution profiles of all three components being monitored. Benzoic acid and theophylline interacted to form a soluble complex with an apparent binding constant of 37.1 l/mol. Reasonable agreement between individual component steady state dissolution rates and those predicted by a three component diffusion controlled dissolution model, with two components interacting to form soluble complexes, was obtained. Benzoic acid rates tended to be higher than predicted in the absence of salicylamide while theophylline dissolution tended to be lower than predicted. Plateaus in the theoretical profiles for salicylamide rate versus composition reflected the absence of complex formation by this component and its lower intrinsic dissolution rate, which resulted in it forming the surface layer over a wider composition range.

Grundsätzliche Probleme der Korrosion von Legierungen

Phenomenological equations are given for the steady state dissolution of alloys and their composition at the surface. The slow component with the smallest rate constant essentially determines the steady state dissolution rate and is enriched in the alloy surface. The phenomenological equations are compared to experimental results both for active and passive alloys. In ideal cases, phenomenological rate constants are independent of composition. Rate constants of pure slow components being much smaller than rate constants of pure fast components are enhanced by alloying with the fast component. Thus, corrosion protection by alloying often is much less efficient than predicted by the ideal case. Deviations from the ideal case can be understood, if dissolution proceeds at kinks in steps on low index planes of a crystalline alloy, or from equivalent sites in glassy metal alloys. Thus, the dissolution rates depend on the rate constants for dissolution from a kink and on the surface concentration of kinks. The surface concentration of kinks is determined by processes parallel to the dissolution from kinks, e.g. by dissolution of atoms from densely packed steps or by 2-dimensional nucleation in low index planes. The probability of such processes grows with the electrode potential and the concentration of a fast component. The resulting increase of the surface concentration of kinks occupied mainly by the slow component enhances its phenomenological rate constant. Steady states are approached at room temperature much slower than expected from diffusion coefficients of solid alloys extrapolated from high temperatures. Consequently, at room temperature changes of composition penetrate deeply into the bulk. The apparent increase of the interdiffusion coefficients is due to vacancy injection connected to the processes of kink generation. Selective corrosion and passivation often are accompanied by large surface stresses which may result in plastic deformation or fracture.

An experimental study of kaolinite dissolution and precipitation kinetics as a function of chemical affinity and solution composition at 150°C, 40 bars, and pH 2, 6.8, and 7.8

Steady-state dissolution and precipitation rates of a pure highly crystalline kaolinite were determined as a function of chemical affinity and aqueous Si and Al concentrations using a mixed flow reactor at 150°C, 40 bars, and a pH of 2, 6.8, and 7.8. Dissolution experiments, performed in solutions undersaturated with respect to all possible solid phases, exhibited stoichiometric Al and Si release. For chemical affinities (A) ranging between 2 and 10 kcal/mol, measured dissolution rates are an inverse function of aqueous Al concentration, and of aqueous Al and silica concentrations at acidic and alkaline pH, respectively. As equilibrium is approached (A ≤ 2 kcal/mol), these rates become increasingly controlled by chemical affinity. The variation of dissolution/crystallization rates with chemical affinity and aqueous Al and Si concentrations are described using a coupled transition state theory (TST)/Langmuir adsorption model by adopting the concept that reaction rate is controlled by the decomposition of a silica rich/aluminum deficient precursor complex. This complex is formed by the exchange of three hydrogen ions for one aluminum atom at the kaolinite surface. At alkaline conditions, this reaction competes with precursor complex condensation and/or aqueous silica adsorption forming unreactive siloxane groups. Taking account of the precursor complex formation reactions, equations were derived to describe kaolinite dissolution and crystallization rates over the full range of chemical affinity and aqueous Al and Si concentrations at both acidic and alkaline pH values. Near to equilibrium, kaolinite dissolution and precipitation rates predicted by these equations are considerably lower thon those obtained from the extrapolation of far from equilibrium rates using previous rate laws also derived from TST but based on incorrect precursor complex formation reactions. The lower rates found in the present study are consistent with those deduced from petrographic observations in sedimentary basins.

Kinetic aspects of basaltic glass dissolution at 90°C: role of aqueous silicon and aluminium

Steady-state dissolution rates of a synthetic basaltic glass were measured in an open-system mixed flow reactor as a function of solution composition at a temperature of 90°C and over the pH range 7.8 to 8.3. The dissolution is a two-step process. The first of these steps involves the release of the cation modifier elements leading to the formation of a hydrated surface gel (HBG) of which the solubility controls the overall dissolution reaction. The glass steady-state dissolution rates were found to be independent of aqueous aluminium and silicium concentration but to depend on the chemical affinity for the overall hydrolysis reaction. The glass is a rapidly reacting solid, whose dissolution induces a dramatic change in solution concentration, which results readily in small chemical affinities for the dissolution reaction. Consequently, conditions of great undersaturation have not been investigated (affinity max. 9.8 kJ/mol). However, our results strongly suggest that the dissolution rates are controlled by the decomposition of a stoichiometric silico-aluminous surface precursor. The variation of the steady-state dissolution rates can be described using a simple expression based on the concept that the precursor is formed by the simple absorption of reactants: R (mol cm −2s −1) = 3 × 10 −10 (OH −) 0.39 (1− Q/8.2 × 10 −5), where Q, the ion activity quotient is equal to: Q = (H 4SiO 4) (Al(OH) 4 −) 0.36 (Fe(OH) 3) 0.18 (OH −) −0.36.

Fundamental aspects of the corrosion of alloys

Phenomenological equations are given for the steady state dissolution of alloys and their composition at the surface. The slow component with the smallest rate constant essentially determines the steady state dissolution rate and is enriched in the alloy surface. The phenomenological equations are compared to experimental results both for active and passive alloys. In ideal cases, phenomenological rate constants are independent of composition. Rate constants of pure slow components being much smaller than rate constants of pure fast components are enhanced by alloying with the fast component. Thus, corrosion protection by alloying often is much less efficient than predicted by the ideal case. Deviations from the ideal case can be understood, if dissolution proceeds at kinks in steps on low index planes of a crystalline alloy, or from equivalent sites in glassy metal alloys. Thus, the dissolution rates depend on the rate constants for dissolution from a kink and on the surface concentration of kinks. The surface concentration of kinks is determined by processes parallel to the dissolution from kinks, e.g. by dissolution of atoms from straight steps or by two-dimensional nucleation in low index planes. The probability of such processes grows with the electrode potential and the concentration of a fast component. The resulting increase of the surface concentration of kinks occupied mainly by the slow component enhances its phenomenological rate constant. Steady states are approached at room temperature much slower than expected from diffusion coefficients of solid alloys extrapolated from high temperatures. Consequently, at room temperature changes of composition penetrate deeply into the bulk. The apparent increase of the interdiffusion coefficients is due to vacancy injection connected to the processes of kink generation. Selective corrosion and passivation often are accompanied by large surface stresses which may result in plastic deformation or fracture.

Feldspar dissolution in acidic and organic solutions: Compositional and pH dependence of dissolution rate

The steady-state dissolution rates of plagioclase feldspars into inorganic acid solutions in a flow-through reactor increased with Al content of the mineral from 1.4 · 10 −11 mol Si/m 2/s for albite to 5.6 · 10 −9 mol Si/m 2/s for bytownite. A similar trend was observed for minerals dissolved in neutral solutions although the rates were lower. The results of these experiments are used to develop a simple empirical equation to describe the dissolution of tectosilicates (quartz + feldspars): R H = k H a H + n H where R H is the dissolution rate of tectosilicates in acid solution, a H + is the activity of H + ion, and k H and n H are dependent on the aluminum fraction in the tectosilicate framework [ A l A l + S i ] : log k H = − 11.24 + 25.98 * [ A l A l + S i ] 2 and n H = − 0.052 + 4.23 * [ A l A l + S i ] 2 . This model, with its strong dependence on Al fraction, suggests that tectosilicate dissolution in acid solution results primarily from attack at Al sites at the mineral surface. In acidic oxalate solutions the steady-state dissolution rates were, in some cases, up to a factor of 10 higher than dissolution rates in inorganic solutions at the same pH and appeared to have a similar dependence on pH and mineral composition, at least away from the extremes in aluminum fraction (quartz and bytownite). On the basis of the results of the experiments with acidic oxalate and previous experiments showing a linear dependence of feldspar dissolution rate on organic ligand concentration, an empirical expression for the ligand-promoted component of tectosilicate dissolution rates as measured by silica release ( R L) is proposed: R L = (κ H L [L] − k H) a H + n + ℝ H L(Si) where the first term describes the effect of competitive proton and ligand attack at Al sites at the mineral surface leading to silica release to solution and R H L(Si) reflects the smaller rate of attack at Si sites (κ H L is a factor depending on the ligand, [L] is the ligand concentration, k H and a H+ are as given above, and n describes the pH dependence of ligand- and proton-promoted dissolution and is taken to be equal to n H away from the extremes of aluminum fraction). The strong dependence of dissolution rate in acidic organic solutions on aluminum fraction indicates that both protons and ligands attack the mineral surface at the same, presumably Al, sites.

A kinetic model for the anodic dissolution of zinc in alkaline electrolyte with sodium metasilicate additions

The anodic dissolution of zinc electrodes in potassium hydroxide solutions containing sodium metasilicate has been investigated experimentally and theoretically in this study. On the basis of the experimental r̀esults, a kinetic model based on a competitive chemisorption approach has been developed and tested. The steady-state dissolution of zinc electrodes in the above-mentioned system has been measured using rotating disc electrode techniques. Good agreement is obtained between the model and the experimental results. The activation energy for the system is close to that for a kinetically controlled mode. This suggests that a surface reaction is rate limiting during the electro-dissolution process. It has been shown that the increase in the Tafel slope for the above-mentioned system in the active dissolution region is due to the adsorption of silicate on the electrode surface. A modified Langmuir model has been found to work reasonably well for the system studied. A change in the reaction order with respect to hydroxide ion has been found experimentally and predicted by the model also.

Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis

Steady-state dissolution rates of anorthite (An 96) were measured as a function of aqueous Si, Al, and Ca concentration at temperatures from 45 to 95°C and over the pH range 2.4 to 3.2 using a Ti mixed-flow reactor. All dissolution experiments exhibited stoichiometric dissolution. The concentration of aqueous Si, Al, and Ca ranged from ∼7 X× 10 −5 to ∼1 × 10 −3 molal, ∼6 × 10 −5 to ∼3.4 × 10 −3 molal, and ∼5 × 10 −5 to ∼0.1 molal, respectively, corresponding to calculated anorthite chemical affinities ranging from ∼ 115 to ∼65 kJ/mol. Measured anorthite dissolution rates at constant temperature are proportional to α H + 1.5, where α H + designates the activity of the hydrogen ion, and consistent with an apparent activation energy of 18.4 kJ/mol. Anorthite dissolution rates are independent of aqueous Al concentration, which is in contrast with the alkali feldspars, whose constant pH, far from equilibrium rates are proportional to α Al +3 −0.33 (Oelkers et al., 1994; Gautier et al., 1994; E. H. Oelkers and J. Schott, unpubl. data). This difference suggests a distinctly different dissolution mechanism. For the case of both types of feldspars it appears that Al is more readily removed than Si from the aluminosilicate framework. Because it has a Si Al ratio of 3, the removal of Al from the alkali feldspar framework leaves partially linked Si tetrehedra. Removal of Si still requires the breaking of SiO bonds, and thus the overall alkali feldspar dissolution rate is controlled by the decomposition of a silica-rich surface precursor. The variation of alkali feldspar dissolution rates with aqueous Al activity stems from the fact that the formation of this precursor requires the removal of Al. In contrast, because it has a Si Al ratio of 1, the removal of Al from the anorthite framework leaves completely detached Si tetrehedra. As a result, the removal of Si does not require the breaking of SiO bonds, the rate controlling precursor complex is not formed by the removal of Al, and the overall dissolution rate is independent of aqueous Al concentration at far from equilibrium conditions. It can be inferred from these results that the variation of far from equilibrium aluminosilicate dissolution rates on aqueous Al depends on the number and relative strength of different bond types that need to be broken for mineral hydrolysis.

Experimental study of K-feldspar dissolution rates as a function of chemical affinity at 150°C and pH 9

Steady state dissolution rates of a K-rich feldspar (K 0.81Na 0.15Ba 0.03Al 1.05Si 2.96O 8) were measured as a function of chemical affinity and aqueous Si and Al concentration in solutions containing 5 × 10 −3 m total K using a titanium mixed flow reactor at a temperature of 150°C and pH of 9.0. All dissolution experiments exhibited stoichiometric dissolution with respect to Al and Si. The concentration of aqueous silica and Al ranged from 1 × 10 −6 to 5 × 10 −4mol/kg and 4 × 10 −7 to 5 × 10 −4mol/kg, respectively, corresponding to K-feldspar chemical affinities ranging from ~90 to ~5 kJ/mol. Logarithms of measured dissolution rates are an inverse linear function of aqueous aluminum concentration, but independent of aqueous silica concentration at all chemical affinities greater than ~20 kJ/mol. These rates become increasingly controlled by chemical affinity as equilibrium is approached. This variation of steady state dissolution rates is consistent with their control by the decomposition of silica rich/aluminum deficient surface precursor complex. Taking account of transition state theory and the identity of reactions to form this precursor complex, an equation was derived to describe the steady state dissolution rates over the full range of chemical affinity. A simplified but less general version of this equation, which can be used to describe the steady state rates ( r) obtained in the present study can be expressed as r=k+ 1 a Al(OH) + −a H+ 1 3 (1 − exp (− A 3RT )) where k + stands for a rate constant equal to 1.7 × 10 −17 mol/cm 2/s, a H+ and aAl( OH) 4 − designate the activities of H + and Al(OH) 4 − , respectively, A refers to the chemical affinity of the overall reaction, R signifies the gas constant and T denotes the temperature in K. Corresponding experiments performed in a batch-type reactor illustrate the consistency between dissolution rates generated in open and closed systems.

The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions

Analysis of aluminosilicate steady-state dissolution/precipitation rates indicate that in contrast to what is commonly assumed, the constant pH rates are not independent of chemical affinity at far from equilibrium conditions. Rather, the logarithm of these rates for albite and kaolinite are linear functions of the logarithm of aqueous Al concentration over wide ranges of saturation states. Consideration of both the steady-state rates and the surface chemistry of these minerals following dissolution indicates that these rates are consistent with their control by the decomposition of an Al-deficient, silica-rich surface precursor complex. Taking account of reactions written to form this complex leads to a rate equation for the dissolution/precipitation of these minerals that accurately describes their variation on pH, aqueous Al concentration, and chemical affinity. By analogy, it appears likely that the rates of numerous other aluminosilicate dissolution/crystallization reactions are also consistent with their control by the decomposition of similar precursor complexes. It follows from these observations that 1. (1) the generation of steady state dissolution rate constants from experiments performed in batch type reactors, 2. (2) the interpretation of the pH dependence of aluminosilicate dissolution reactions requires explicit account of the effects of aqueous Al concentration on these rates.

The effect of natural weathering on dissolution rates

Surface area measurements and dissolution experiments were performed on a naturally weathered mineral assemblage rich in quartz and feldspar. The surface areas of unfractured (612 μm) and ground samples were measured at different levels of surface detail, using a dissecting microscope, SEM, and the BET-krypton method. Roughness factors of the freshly created surfaces by grinding were 2.1 (SEM at 10,000× magnification) and 4.8 (BET), respectively. Roughness factors of the naturally weathered surfaces were 2.8 (SEM at 10,000×, including effects from etch pitting) and 34 (BET), respectively. These data indicate that (1) a large amount of BET surface area was created during natural weathering, and (2) etch pits did not contribute significantly to the BET surface area created during natural weathering. Comparison with data from the literature further suggests that the surface roughness factor of naturally weathered feldspar grains decreases with decreasing grain diameter. Also, most BET surface area additionally created during natural weathering appeared to be nonreactive. At pH 3 and pH 5 HCl, steady-state dissolution rates of Na, K, Ca, Al, and Si (mol·cm −2 of BET surface·s −1) were determined for the unfractured sample and for four ground samples of increasingly smaller average grain diameter. The data from the ground samples were used to extrapolate the dissolution rate of hypothetical fresh-surface grains to the average diameter of the unfractured, naturally weathered sample material. The results showed that, at 612 μm, freshly created BET surfaces would dissolve approximately one order of magnitude faster than their naturally weathered counterparts. Comparison with literature data suggests that this discrepancy in dissolution rate, at equal diameters of freshly created and naturally weathered grains, increases with increasing grain diameter. Dissolution rates of freshly created feldspar surfaces in the laboratory are frequently up to several orders of magnitude higher than those of naturally weathered feldspar surfaces in actual field situations. The findings from this research partly explain such discrepancies. However, other factors (such as imperfect contact between solution and solids, the presence of organic ligands, etc.) should also be considered in explaining the discrepancies.

Dissolution kinetics of heulandite at pH 2–12 and 25°C

The dissolution rate of heulandite depends strongly on pH. Based on silica release, the logarithm of the steady-state dissolution rate at pH 2 is −13.1 mol cm −2 s −1. The logarithm of the rate decreases to −15.8 mol cm −2 s −1 at pH 7.2 and increases again to − 14.6 mol cm −2s −1 at pH 12.2. At low pH, Al is released preferentially to silica; but at intermediate and high pH, the release of silica appears to be congruent relative to Al. The change in dissolution rate with pH indicates that at low pH, the dissolution mechanism is controlled by the detachment of a positively charged Al species, > A1-OH 2 +. Below pH 5, however, a silica-rich surface layer is formed requiring diffusion through the layer. At intermediate and high pH, it is likely that the dissolution rate is controlled by the detachment of a negatively charged silica species, > Si −O −. The reaction order of the hydrogen ion under low pH conditions is 0.7, and the reaction order of the OH − ion is 0.3 at high pH. The measured dissolution rates indicate that a 1 mm heulandite crystal would dissolve in 300,000 yrs if the solution composition is maintained undersaturated.

Free energy dependence of albite dissolution kinetics at 80°C and pH 8.8

Steady-state dissolution rates of albite were measured in aqueous solutions at 80°C and pH 8.8 as a function of solution saturation state (or the Gibbs free energy of reaction, ΔG r) using a continuously-stirred flow-through reactor. Congruent dissolution rates increase from 0.7·10 −12 to 33.2·10 −12 mol m −2 s −1 with increasing undersaturation from −0.4 to −10.5 kcal mol −1. The dissolution rate exhibits a highly nonlinear dependence on solution saturation state over this range in ΔG r that can be expressed by the sum of the rates of two parallel reactions: R DISS =− k 1[1−exp(− ng m1 )]− k 2[1−exp(1− g)] m2 where g≡ ΔG r | RT is a dimensionless parameter; k 1 = (30.46 ± 0.42) · 10 −12; k 2 = (2.73 ± 0.26) · 10 −12 ( k 1, k 2 in mol m −2 s −1; n = (8.4 ± 0.8) · 10 −17; m 1 = 15.0 ± 0.1; and m 2 = 1.45 ± 0.50. Far from equilibrium, the dissolution rate attains a constant maximum value independent of ΔG r for undersaturations ≤ −9 kcal mol −1. Between −6 and −9 kcal mol −1, the dissolution rate increases sharply with decreasing ΔG r. Between −0.4 and −6 kcal mol −1 the dissolution rate increases with decreasing ΔG r, but to a much lesser extent. Near equilibrium, −0.9 kcal mol −1≤ ΔG r≤0 or | ΔG r|≲ RT, the dependence of dissolution rate on solution saturation state is approximately linear and can be expressed by: R diss =(−1.53±0.07)×10 −12 g A rate law with a functional dependence on solution saturation state derived from transition state theory cannot account for this complex dependence of rate on ΔG r. The sharp increase in dissolution rate between −6 and −9 kcal mol −1 can be correlated with theoretical calculations for the critical solution undersaturation required to etch dislocations in albite. SEM observations of albite surfaces confirm that pitting was more extensive at the highest solution undersaturations, relative to solutions closer to equilibrium. The one to two order of magnitude discrepancy between previous laboratory determinations of feldspar dissolution rates (in highly undersaturated solutions) and field-based estimates (presumably in solutions closer to equilibrium) may be explained by the variation of the dissolution rate with ΔG r. However, hydrology and surface characteristics of naturally-weathered vs. experimentally-dissolved feldspars are also important when comparing field and laboratory dissolution rates.

An experimental investigation of nonaqueous phase liquid dissolution in saturated subsurface systems: Steady state mass transfer rates

Results of an experimental investigation into steady state dissolution of nonaqueous phase liquids (NAPLs) entrapped within water saturated porous media are presented. The influence of porous media characteristics, NAPL type, and aqueous phase velocity on NAPL dissolution rates is explored through evaluation of a series of laboratory column studies. For many of the conditions tested, measured organic solute concentrations in the column effluents were below solubility limits, indicating nonequilibrium conditions. Entrapped NAPL distributions are examined and shown to depend upon porous media grading and mean grain size. Experimental results reveal a dependence of dissolution rate on the distribution pattern of entrapped NAPL, as well as upon aqueous phase velocity. A phenomenological model for the mass transfer process is developed which incorporates grain size parameters as surrogate measures of NAPL distribution. An additional set of column experiments, employing another porous medium and NAPL type, confirm the usefulness of the model as a predictor of steady state mass transfer rates in similar systems.

Dissolution of dense chlorinated solvents into groundwater. 2. Source functions for pools of solvent

When a spill of dense chlorinated hydrocarbon (CHC) solvent occurs into the subsurface, penetration into the saturated zone is likely to lead to the accumulation of pools of solvent on the tops of low-permeability layers. Steady-state dissolution from such pools was modeled. The dissolution rate depends on the molecular diffusion coefficient, the vertical dispersivity α v , the groundwater velocity v, the solubility, and the pool length. A value for α v for dissolution of trichloroethylene in a sand-filled physical model was found to be ∼0.00023 m