Calcite Dissolution Kinetics Research Articles

Aqueous cadmium uptake by calcite: a stirred flow-through reactor study

Uptake of cadmium ions from solution by a natural Mg-containing calcite was investigated in stirred flow-through reactor experiments. Input NaCl solutions were pre-equilibrated with calcite (pH 8.0) or not (pH 6.0), prior to being spiked with CdCl 2. For water residence times in the reactor less than 0.5 h, irreversible uptake of Cd by diffusion into the bulk crystal had a minor effect on the measured cadmium breakthrough curves, hence allowing us to quantify “fast” Cd 2+ adsorption. At equal aqueous activities of Cd 2+, adsorption was systematically lower for the pre-equilibrated input solutions. The effect of variable solution composition on Cd 2+ adsorption was reproduced by a Ca 2+-Cd 2+ cation exchange model and by a surface complexation model for the calcite-aqueous solution interface. For the range of experimental conditions tested, the latter model predicted binding of aqueous Ca 2+ and Cd 2+ to the same population of carbonate surface sites. Under these circumstances, both adsorption models were equivalent. Desorption released 80 to 100% of sorbed cadmium, confirming that fast uptake of Cd 2+ was mainly due to binding at surface sites. Slow, irreversible cadmium uptake by the solid phase was measured in flow-through reactor experiments with water residence times exceeding 0.7 h. The process exhibited first-order kinetics with respect to the concentration of adsorbed Cd 2+, with a linear rate constant at 25°C of 0.03 h −1. Assuming that diffusion into the calcite lattice was the mechanism of slow uptake, a Cd 2+ solid-state diffusion coefficient of 8.5×10 −21 cm 2 s −1 was calculated. Adsorbed Cd 2+ had a pronounced effect on the dissolution kinetics of calcite. At maximum Cd 2+ surface coverage (∼10 −5 mol m −2), the calcite dissolution rate was 75% slower than measured under initially cadmium-free conditions. Upon desorption of cadmium, the dissolution rate increased again but remained below its initial value. Thus, the calcite surface structure and reactivity retained a memory of the adsorbed Cd 2+ cations after their removal.

Simulation of the development of karst aquifers using a coupled continuum pipe flow model

This paper is intended to provide insight into the controlling mechanisms of karst genesis based on an advanced modeling approach covering the characteristic hydraulics in karst systems, the dissolution kinetics, and the associated temporal decrease in flow resistance. Karst water hydraulics is strongly governed by the interaction between a highly conductive low storage conduit network and a low‐conductive high‐storage rock matrix under variable boundary conditions. Only if this coupling of flow mechanisms is considered can an appropriate representation of other relevant processes be achieved, e.g., carbonate dissolution, transport of dissolved solids, and limited groundwater recharge. Here a parameter study performed with the numerical model Carbonate Aquifer Void Evolution (CAVE) is presented, which allows the simulation of the genesis of karst aquifers during geologic time periods. CAVE integrates several important features relevant for different scenarios of karst evolution: (1) the complex hydraulic interplay between flow in the karst conduits and in the small fissures of the rock matrix, (2) laminar as well as turbulent flow conditions, (3) time‐dependent and nonuniform recharge to both flow systems, (4) the widening of the conduits accounting for appropriate physicochemical relationships governing calcite dissolution kinetics. This is achieved by predefining an initial network of karst conduits (“protoconduits”) which are allowed to grow according to the amount of aggressive water available due to hydraulic boundary conditions. The increase in conduit transmissivity is associated with an increase in conduit diameters while the conductivity of the fissured system is assumed to be constant in time. The importance of various parameters controlling karst genesis is demonstrated in a parameter study covering the recharge distribution, the upgradient boundary conditions for the conduit system, and the hydraulic coupling between the conduit network and the rock matrix. In particular, it is shown that conduit diameters increase in downgradient or upgradient direction depending on the spatial distribution (local versus uniform) of the recharge component which directly enters the conduit system.

The Kinetics of Calcite Dissolution in Dilute Acetic Acid Solutions

The Kinetics of Calcite Dissolution in Dilute Acetic Acid Solutions

A deterministic approach to the coupled analysis of karst springs' hydrographs and chemographs

During the chemically based recession flow phase of karstic springs the carbonate (dissolved limestone) concentration can be expressed as negative power of the flow rate. The empirically determined Conc/ Q relationship allows two parameters ( α and A) to be defined, of which one ( α) depends on the geometric dimensions of the saturated (submerged) karstic network. In this paper we present a deterministic model which simulates the concentration of carbonate at the outlet of a network of circular rectilinear conduits as a function of flow rate. This model, based on hydraulic principles and the calcite dissolution kinetics, allows the sensitivity of the α and A parameters to be studied under different chemical, physical and geometric scenarios. Simulation results show that A is a function of the calcite saturation concentration, whereas α depends on the spatial dimensions of the karstic network (void length and aperture). The deterministic model results were applied to real karstic systems to evaluate the geometric dimensions of submerged karstic networks.

The Effect of Surface Pretreatment with Polymaleic Acid, Phosphoric Acid, or Oxalic Acid on the Dissolution Kinetics of Calcium Carbonate in Aqueous Acid

The kinetics of calcite dissolution in aqueous acidic solution following various surface treatments was investigated using the channel flow cell method with microdisk electrode detection. Pretreatment of calcite with solutions of polymaleic acid, phosphoric acid, or oxalic acid for up to 24 h is shown to provide protective coatings over the calcite, which substantially reduces further acid attack on the surface. Quantitatively each pretreatment was found to lead to a rate constant for proton-induced calcite dissolution of as low as 0.003 cm s−1, an order of magnitude less than that observed for untreated calcite, 0.035 cm s−1 at 21°C.

Modeling of calcite dissolution by oxic respiration in supralysoclinal deep-sea sediments

Abstract Pore-water profiles of CO2, pH, Ca2+ and O2 in situ concentrations were measured at two stations on the upper continental slope off Gabon. The present study evaluates these measurements concerning their implications for the calcium carbonate system in deep-sea sediments. The model CoTReM, which was used to simulate the dynamics of this complex geochemical system, revealed a strong dependence of calcite dissolution on oxic respiration at both sites. All simulated calcite dissolution kinetics reached a dynamic equilibrium with almost equal calcite dissolution rates, but different sub-saturation states and pH values. The latter are mainly dependent on boundary conditions and kinetic rate law parameters. Boundary conditions are of immense importance. They define which pH deviations between measured data and the simulated equilibration (instantaneous kinetics) have still to be fitted by kinetic restrictions per rate law for the equilibration. These pH deviations set up the range of possible values for rate constants in a given rate law to yield a well simulated pH. A failure in the implementation of these boundary conditions may lead to non-linearly flawed rate constants, which fit only one (usually the maximal) pH deviation well. The whole depth distribution of pH deviations has to be fitted very well by a kinetic rate law. Only this procedure secures that the used boundary conditions and rate laws/constants are acceptable. Nevertheless, several kinetic rate laws may be used for good pH fits, featuring clear differences in parameter values for rate constants and reaction orders. The connection between these rate laws with different parameters is examined and the dependence of the rate constant on the given form of the rate law is demonstrated. This study achieves rate constants of 0.038 and 18%/d for a dissolution rate law of 4.5th reaction order and sub-saturation dependence from the term (1−Ω). These results are well within the range of rate constants obtained during former field studies for the same form of the dissolution rate law.

Channel Flow Cell Studies of the Inhibiting Action of Gypsum on the Dissolution Kinetics of Calcite: A Laboratory Approach with Implications for Field Monitoring

The rate of dissolution of surface-treated calcite crystals in aqueous acidic solution has been studied using an adaptation of the channel flow cell method with microdisc electrode detection. Surface treatments of calcite with sulfuric acid lead to the nucleation of gypsum overgrowths, which reduce the rate of dissolution of calcite. Rate constants for untreated calcite and calcite pretreated with sulfuric acid conditions of 0.01 M for 1 h, 0.05 M for 5 h, and 0.1 M for 21 h are found to be 0.035, 0.018, 0.006, and 0.004 cm s−1, respectively. Deterioration of calcite materials caused by acid deposition was investigated by field exposure of untreated and sulfate pretreated calcite rocks under urban conditions for 12 months. The rate constant for both pretreated and untreated calcite exposed to weathering is 0.003 cm s−1. This suggests that calcite self-passivates the surface from further reaction when exposed to acid deposition. However, surface studies indicate that the surface undergoes erosion and dissolution before passivation. Pretreatment of the surface with sulfate protects the surface from acid deposition so it remains less reactive toward acid compared with untreated calcite.

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

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

Simulation of the development of karst aquifers: role of the epikarst

The evolution of a karst aquifer is modelled taking into account the karst groundwater flow as well as the dissolution kinetics of calcite. In particular, infiltration of water from the epikarst is simulated which controls the temporal and spatial distribution of recharge to the phreatic zone. The results show that the evolution of karst conduits is initiated in the spring. The existence of preferential flow paths leads to the evolution of highly conductive so-called dendritic cave systems, i.e., single passages which concentrate the flow and drain the catchment. With time, the amount of undersaturated water flowing directly into the conduit system is increased leading to an acceleration of the conduits enlargement. Three phases are identified for the evolution of karst aquifers: (a) an initiation stage; (b) an enlargement stage; and (c) a stagnation phase.

The kinetics of calcite dissolution in acetic acid solutions

The kinetics of calcite dissolution in acetic acid solutions was investigated over a wide range of pH using a rotating disk apparatus. The results show that the dissolution is influenced by the rate of transport of reactants to the surface, the kinetics of the reversible surface reaction, and the rate of transport of products away from the surface. Below about pH 2.9, the dissolution is influenced by the transport of both reactants and products, while above about pH 3.7, the dissolution is influenced predominantly by the kinetics of the surface reaction. A general model was developed to account for the combined effects of transport and reaction on the rate of dissolution. The effect of acetate ions on the rate of dissolution was investigated in alkaline solutions (pH 8.2 to 14) to eliminate the effects of hydrogen ion attack. The presence of acetate ions was found to have no significant affect on the rate of dissolution when compared to results in potassium chloride and sodium chloride solutions. The rate of dissolution was observed to decrease over this pH range and could not be described by previous reaction mechanisms. Therefore, a surface dissociation mechanism involving water was introduced and was shown to describe the rate of dissolution over the pH range of 8 to 14.

The Influence of Chelating Agents on the Kinetics of Calcite Dissolution

The kinetics of calcite dissolution in the presence of calcium chelating agents was investigated over the pH range of 3.3–12 using a rotating disk apparatus. The results show that the rate of dissolution is increased significantly by the presence of chelating agents such as CDTA, DTPA, and EDTA. The rate of dissolution is influenced by the kinetics of the chelation reactions and varies considerably with pH and type of chelating agent. A surface chelation mechanism was introduced to describe the dissolution. The mechanism involves the adsorption of the chelating agent onto the calcite surface and follows Langmuir–Hinshelwood kinetics. The dissolution is different from conventional hydrogen ion attack in that the chelating agent attacks the calcium component of the lattice rather than the carbonate component. Therefore, the rate of dissolution is enhanced by the influence of hydrogen ion attack at low pH. In addition, the various ionic forms of the chelating agents react with the calcite surface at different rates depending on the number of hydrogen ions associated with the species. In general, the rate of dissolution increases with increasing protonation. The surface complexation mechanism was shown to describe the rate of calcite dissolution in the presence of chelating agents over the pH range of 4–12.

Evidence in support of first-order dissolution kinetics of calcite in seawater

Re-examination of an experimental determination of the dissolution rate of reagent-grade calcite in seawater shows that the original conclusion of rate law with 4.5-order dependence on undersaturation was very sensitive to uncertainties in the saturation state of the seawater with respect to calcite,Ω C. In particular, use of an erroneously high value of the calcite stoichiometric solubility product generated correspondingly low values of the saturation state. Extrapolation of the experimental measurements to a rate of zero dissolution indicates that the calcite solubility was about 20% lower than that used in the original study, similar to more recent estimates. If the lower solubity is used for recalulation of the experimental saturation states, the dissolution rate R d,C (% day −1) is adequately described by the rate experssion: R d, C = 38(1−omega; C ) 1 In situ measurements of pH in the pore waters of calcite-rich sea floor sediments are more consistent with first-order kinetics than with 4.5-order kinetics. Interpretation of pore water pH data using the 4.5-order rate expression requires dissolution rate constants that are different by at least two orders of magnitude and stoichiometric calcite solubility products that are different by several percent between two otherwise similar sites. Application of the first-order dissolution reduces the variability in the rate constant to less than one order of magnitude, and all in situ observations are consistent with a single estimate of calcite solubility. First-order kinetics do not reduce the discrepancy between the laboratory determined rate constants and those based on pore water measurements. Dissolution rate constants constrained by the in situ pH measurements are at least two orders of magnitude less than the laboratory results.

“Mixed” kinetics control of fluid-rock interaction in reservoir production scenarios

We examine scenarios under which mineral-water reaction kinetics exhibit mixed-kinetic control, that is, when both surface reaction rates and hydrodynamic conditions influence rates of dissolution and precipitation from aqueous solutions at mineral-water interfaces. This dependence arises in many engineering situations and imparts a fluid flow velocity dependence on overall rates. Recognition of transport control is critical for extraction of rate data under laboratory conditions; failure to do so can lead to drastically overestimated, or in some cases, underestimated rates of scale formation in production scenarios. Data on gypsum (at 25°C) and calcite (at 100°C) dissolution and precipitation kinetics are examined for relative control of rates by mixed transport/reaction control, and a rate expression is derived that reproduces the different data sets. This rate law takes the form: R = k ( 1 − Ω bulk 1 / 2 + ζ [ 1 − { 1 + 2 ( 1 − Ω bulk 1 / 2 ) / ζ } 1 / 2 ] ) where ζ = ( D m eq ) / 2 δ k where R is rate; k is the rate constant for purely transport control; Ω is the saturation expressed as a ratio of calcium ion concentration in bulk solution to the equilibrium concentration, and the other terms are defined in the text; and ζ is a factor which measures the relative strength of transport or reaction steps as influence concentrations of species at the mineral-water interface. Combined with reaction transport equations, this expression is capable of predicting gypsum and calcite mineral-water reaction rates over a wide variety of saturation and hydrodynamic conditions. Although comprehensive data sets are lacking, we suggest that this equation is applicable to precipitation of other sulfate minerals that are problematic scale formers, such as barite, celestite, and anhydrite.

Weathering as a Topic in an Interdisciplinary Science Course for General Education

An interdisciplinary natural-science core course for general-education students was taught for two years at Indiana-Purdue University Fort Wayne. The course had a modular design and was presented by faculty from mathematics and each of the science departments at the University. It was our goal to give beginning students a chance for in-depth examination of a few subject areas.Students participated in two six-week-long “topic modules,” sandwiched between shorter introductory and capstone modules. This pattern could be repeated (with different topic modules) in a second semester.The geology module focused on the topic of silicate and carbonate mineral weathering. The students worked in teams to design and execute an experiment evaluating the dissolution kinetics of K-feldspar, dolomite, and/or calcite. Experimental results and conclusions were presented as poster sessions evaluated by geoscience faculty. Similar experiments might be of interest to teachers of introductory geology labs, honors courses, freshman seminars, or introductory petrology.

The weathering of aeolian dusts in alpine snows

The chemistry of precipitation (snow and rainfall), snow cover and meltwaters was studied at a French alpine site during the winter-spring seasons of 1986–1987 and 1987–1988. Both acid ( < pH 5.6) and alkaline ( > pH 5.6) deposition events occurred. The strong-acid anions, SO 4 and BO 3, contributed to the acidity of precipitation but NO 3 was the principal anion associated with acidic snowfall. Many alkaline snowfalls are associated with airborne calcareous dusts from regional sources. The most alkaline snowfall, however, was associated with dust from the Sahara Desert. The weathering of dusts in the snow cover during melt leads to the consumption of acidity and an increase in the pH of meltwaters. The results of both field and laboratory experiments show that inputs of calcareous dusts by local aeolian erosion and transport can contribute significantly to the neutralization process. The laboratory experiments also show that the size and distribution of dusts in the snow cover have an effect on the degree of neutralization. Dust in the lower strata of snow cover is more efficient in neutralizing the acidity of meltwaters than dust in the upper strata. The relationship between the distribution of dust and its efficiency to neutralize the acidity of meltwaters can be explained by the kinetics of calcite dissolution under conditions of progressive decreases in the acidity of leached snow and the partial pressures of CO 2 within the snow column during the melt process.

CaCO 3 dissolution in sediments of the Ceara Rise, western equatorial Atlantic

We have used porewater sampling by in situ techniques, including whole-core squeezing, as well as by shipboard sectioning and whole-core squeezing to estimate the rates of sedimentary organic matter oxidation and CaCO 3 dissolution at seven sites on the Ceara Rise in the western equatorial Atlantic Ocean. Porewater NO 3 − profiles at all sites show a pattern indicative of active organic matter oxidation in the upper 15–20 cm of the sediments and in a buried, organic-rich layer. The organic C oxidation rate generally decreases with increasing water depth, from a value of 22 μmol/cm 2/y at the shallowest site (3279 m) to 14 μmol/cm 2/y at the deepest site (4675 m). Over this depth range, the bottomwaters vary from moderately supersaturated with respect to calcite to strongly undersaturated. High-resolution alkalinity profiles, measured in porewaters collected by in situ whole-core squeezing, yield estimated Ca 2+ fluxes of 11 μmol/cm 2/y at a site located at the depth of the calcite saturation horizon, and 7.6 μzmol/cm 2/y at a moderately undersaturated site. Ca 2+ fluxes calculated from profiles in porewaters collected by relatively coarse-resolution in situ sampling methods clearly indicate that there is CaCO 3 dissolution above the calcite saturation horizon. The dissolution of aragonite may contribute to the dissolution flux at the shallowest site. These Ca 2+ fluxes, as well as fluxes estimated from a model of sedimentary organic matter oxidation and calcite dissolution, indicate that 36–66% of the CaCO 3 rain to the seafloor dissolves at sites at and above the calcite saturation horizon, while 52–75% of the rain dissolves at sites below this depth. When these results are incorporated into the oceanic CaCO 3 budget of Milliman 1993, they indicate that 35% of CaCO 3 production is preserved in the deep sea; they suggest a CaCO 3 accumulation rate that is 27% lower than that estimated by Milliman 1993. Our C org oxidation/CaCO 3 dissolution model indicates that a large fraction of the CaCO 3 dissolution that is occurring on the Ceara Rise is attributable to the neutralization of metabolic acids produced during organic matter oxidation. The efficiency with which organic matter oxidation dissolves CaCO 3 (that is, the ratio, CaCO 3 dissolution attributable to organic matter oxidation:organic matter oxidation rate) generally increases as degree of undersaturation of bottomwaters increases. However, there are deviations from the general trend that can be attributed to site-to-site variations in the kinetics of organic matter oxidation and calcite dissolution. This result indicates that the dissolution of CaCO 3 as a result of organic matter oxidation in the deep sea may mask the effects of variations in surface water CaCO 3 productivity and bottomwater chemistry on the accumulation rate of CaCO 3 in deep-sea sediments.

Simulation ofin situ uraninite leaching-part II: The effects of ore grade and deposit porosity

A combined partial equilibrium-mixing cell model has been used to investigate the effects of fluid flow, mineral content, porosity, and lixiviant concentrations onin situ leaching of uraninite. The model couples the rate processes of reactive transport (uraninite and calcite dissolution kinetics and leach solution flow) with solution phase equilibria (acid-base and complexation equilibria). Solution circulation and porosity changes have been explicitly treated in the following way: reacted solution was assumed to be pumped from the system at a constant rate and replaced by fresh lixiviant; the additional void volume resulting from CaCO3 or UO2 dissolution was immediately filled with lixiviant. A solution volume of 1 cm3 was taken for the base, and it was assumed that on each 1200-second increment, loaded solution was removed at the rate of 1.67 × 10-5 cm s-1, equivalent to removal of 2.0 pct of the base volume. The lixiviant considered was NH4HCO3 (NH4)2CO3-H2O2 with reference case concentrations of 1.0 × 10-4, 1.0 × 10-4, and 2.2 × 10-5 mol cm-1. The parameters that were varied in this investigation were the mass fractions of UO2 (0.000 to 0.015) and CaCO3 (0.00 to 0.40) and the initial porosity of the deposit (0.20 and 0.30). Major factors found to affect the uranium content of the solution were UO2 content and initial porosity. Higher UO2 grades were associated with higher U(VI) concentrations, and these were maintained for much longer periods; the consumption of the peroxide oxidant was under mass transfer control. As the leaching reaction slowed, solution replacement began to control the component concentrations, causing decreasing U(VI) concentrations. Higher porosity caused reduced maximum U concentrations and a faster decline. The calcite content had a slight effect on the rate of U leaching; this occurred because high CaCO3 mass fractions led to increased HCO3- concentrations. Early in the leaching process, a lower initial porosity or a higher calcite content led to a higher (less negative) value of the CaCO3 saturation index; however, for the conditions simulated, the solution did not actually become saturated. Also, decreases in the saturation index occurred sooner for higher initial porosities or lower calcite grades. The final porosity was effectively determined by the initial calcite content; dissolution of calcite continued until it had completely reacted, and the uraninite content was too low for it to contribute significantly. Changes in concentrations of the various solution species occurred more rapidly if the ore was more porous, but there were no other significant differences attributable to initial porosity. The H+ concentration was virtually constant throughout leaching if the ore did not contain any calcite; with high calcite contents (40 pct), it remained constant for an extended period following an initial sharp decrease. Changes in the OH-, NH4/+ and NH3 concentrations could be readily predicted from those of H+, and changes in the Ca species concentrations were closely related to those of the Ca and CO3 components. Total U and total H2O2 concentrations behaved oppositely (as required by the reaction stoichiometry), but changes in the concentrations of the minor U(VI) and peroxo species were more complicated. The concentrations of the CO32- and HCO3- species could not readily be predicted from the reaction kinetics, and variations in their concentrations did not reliably indicate pH.

The Dissolution of Calcite in Aqueous Acid: The Influence of Humic Species

The kinetics of proton-induced calcite dissolution in aqueous solution in the presence of humic acids and their sodium salts are reported. In equilibrated acid solutions (pH < 4) there is no inhibition by humic material and dissolution proceeds at a rate simply determined by the solution pH. Contrastingly the sodium salts of humic acids were found to have a significant inhibitory effect on the acid catalyzed dissolution. This was quantified using a novel channel flow cell experiment which employed two electrodes, the upstream of which was used to inject protons into a neutral solution, which also contained sodium salts of humic acid, via electrolytic oxidation of dissolved hydroquinone. The two electrodes were located immediately upstream and downstream of a calcite crystal so that the proton injection served to dissolve the calcite in the (inhibiting) presence of humic salts unequilibrated with the solution pH. The amount of H+ which survived passage to the downstream "detector" electrode was used to quantify the rate of dissolution and hence the inhibitory effects of the humic acid. The latter were found to operate in a manner not inconsistent with Langmuirian adsorption.

Monitoring Particle Sizes with Rotating Disc Electrodes: Measurement of the Dissolution Kinetics of Calcite

The presence of suspended particles in solution causes an enhancement of the mass transport to a rotating disc electrode, the magnitude of which is dependent on the volume fraction of the particulate material. By monitoring the transport limited current due to the reduction/oxidation of a suitable electroactive substrate in the presence of dissolving particles the change in the size of the latter with time may be monitored, thus allowing the rate of dissolution to be inferred. Experiments conducted on the dissolution of calcite at pH 7 are shown to give kinetic data in excellent agreement with that deduced by independent means.

The dissolution of calcite at pH &gt; 7: kinetics and mechanism

A technique is described that allows an assessment of the various candidate rate laws that have been proposed to predict the dissolution kinetics of calcite under high pH conditions. A combination of theoretical modelling and experimentation allows us to choose the following rate law as that which best fits the observed data:rate(molcm−2s−1)=k−k′[Ca2+]s[CO32−]s′, wherek′=k/KspandKspis the solubility product of calcium carbonate. The modelling developed differs from previous studies in that it deals in terms of surface concentrations of reactants,[Ca2+]sand[CO32−], as opposed to those present in bulk solution.