Jiwchar Ganor

CHEMICAL WEATHERING RATE LAWS AND GLOBAL GEOCHEMICAL CYCLES

Abstract In this paper, we discuss the recent kinetic work on water-rock interactions. Standard activity-activity diagrams are reinterpreted, using a mass transfer kinetic model and recent data on the relative rates of mineral reactions. The development of a fully integrated rate law is discussed, with special attention to the important effects of deviation from equilibrium on the rates of mineral-water reactions. The combined effects of temperature, pH, ionic strength, and saturation conditions on the overall dissolution and precipitation rates of minerals must be properly described before any seriously quantitative model of coupled fluid flow and chemical reaction can be undertaken. A rate law that integrates these effects is proposed. The functional dependence of the rate on ΔGr, the free energy change for the mineral reaction, is discussed, based on recent experimental work. An important result is the presence of a surface transition in the reaction mechanism leading to a very strong nonlinear dependence of the dissolution rates on ΔGr. The possible role of dislocation defects in this surface transition is discussed. The relation of global weathering rates and geochemical cycles to the recent experimental and theoretical water-rock kinetic work is explored. The temperature effect on the silica content of streams is reevaluated. The variation of silica concentration with runoff in the rivers of the world is quantified, using a coupled fluid flow and reaction model and the full rate law developed for a proto-granite system by the kinetic experiments. Implications of the water-rock kinetic data for the current geochemical cycles models are discussed with especial emphasis on the link between physical weathering and chemical weathering.

Smectite dissolution kinetics at 80°C and pH 8.8

Abstract The kinetics of dissolution of smectite from the Cabo de Gata volcanic deposit was investigated in the present study. Assuming that the sample is composed solely of smectite, the structural formula of the treated smectite was calculated to be K0.19Na0.51Ca0.195Mg0.08(Al2.56Fe0.42Mg1.02)(Si7.77Al0.23)O20(OH)4 Two types of experiments were carried out: batch experiments to obtain equilibrium data and stirred-flow-through experiments to measure the smectite dissolution rate. All experiments were carried out at a temperature of 80°C and pH of 8.8. mAfter more than 2 yr smectite was still dissolving in the batch experiments, but at a very slow rate. The slow dissolution rate indicates that the system is reasonably close to equilibrium with respect to smectite dissolution. Therefore, the average ion activity product (5 ± 4 × 10−53), obtained from the last samples of the batch experiments, is used as a proxy for the equilibrium constant of the smectite dissolution reaction at 80°C given as Smectite+20H 2 O→0.51Na + +0.19K + +0.195Ca 2+ +1.1Mg 2+ +0.42Fe(OH) 4 − +2.79Al(OH) 4 − +7.77H 4 SiO 4 +0.08OH − In the flow-through experiments at steady state, the average Al/Si (0.33 ± 0.03) and Mg/Si (0.15 ± 0.03) ratios were in very good agreement with these molar ratios of the whole rock analysis (0.35 and 0.14, respectively). The major achievements and conclusions of the present study are as follows: For the first time we present a full stoichiometric dissolution of smectite (i.e., stoichiometric dissolution was observed for Al, Si, and Mg), and show that the obtained dissolution rate is a good measure of the smectite dissolution rate. Pretreatment of the smectite surfaces is necessary to obtain reliable and stoichiometric kinetic results. The dissolution rate of the sample reflects the dissolution rate of the montmorillonitic layers. Under the experimental conditions smectite dissolution rate is not inhibited by aluminum. The dissolution rate of smectite decreases as a function of the silicon concentration. This observation may be explained both by the effect of deviation from equilibrium on dissolution rate and by silicon inhibition, expressed as Rate=−8.1×10 −12 ·(1− exp (−6×10 −10 ·( ΔG r RT ) 6 )) and Rate=( 3.7·10 −17 C Si ) respectively. The current data set cannot be used to differentiate between these two possible reaction mechanisms.

The effect of pH on kaolinite dissolution rates and on activation energy

Experiments measuring kaolinite dissolution rates were carried out using a stirred-flow reactor at temperatures of 25, 50, and 80°C and in the pH range 2 to 4.2. All experiments were conducted under far-from-equilibrium conditions (ΔG < −2.9kcal/mol) and with minimum potential catalysts or inhibitors present in the solution. Therefore, the changes in the dissolution rates should be dominantly a function of the pH and the temperature. At 25 and 50°C the dissolution rate is independent of pH in the pH range 2 to 3, while at 80°C the dissolution rate is proportional to aH+0.4±0.2 The same proportionality of the rate to aH+0.4±0.14 was found at 25 and 50°C in the pH range of 3 to 4. In contrast to the results of previous studies, between pH 3 and 4 there is little or no difference between the pH reaction orders at 25 and 50°C (and also no difference at 80°C, based on previous studies. The similarity of reaction orders at different temperatures suggests that there is no pH effect on the activation energy of the kaolinite dissolution reaction, within the analytical error of our data. Using all the pH and temperature data, the activation energy obtained is 7.0 ± 1.1 kcal/mol.

The effect of pH and temperature on kaolinite dissolution rate under acidic conditions

Abstract The main goal of this paper is to demonstrate a new rate law describing the combined effect of pH (0.5 to 4.5) and temperature (25°C to 70°C) on kaolinite dissolution rate, under far from equilibrium conditions, as a step towards establishing the full rate law of kaolinite dissolution under acidic conditions. Dissolution experiments were carried out using non-stirred flow-through reactors fully immersed in a thermostatic water bath held at a constant temperature of 25.0°C, 50.0°C or 70.0°C ± 0.1°C. Kaolinite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of kaolinite dissolution rate. Kaolinite dissolution rates range as a function of temperature and fluid composition from 8 ± 1 × 10 −15 mol m −2 s −1 (at 25°C and pH 4.5) to 1.5 ± 0.2 × 10 −11 mol m −2 s −1 (at 70°C and pH 0.5). In general, dissolution rate increases with temperature and decreases with pH. The combined effect of pH and temperature is modeled by two independent proton promoted reaction paths. The first reaction path controls the overall dissolution rate at pH ≥ 2.5, whereas the second path controls it below pH 0.5. Between pH 0.5 and 2.5 the two reaction paths influence the rate. Using this model the effects of pH and temperature on the overall dissolution rate of kaolinite under acidic condition can be described by: Rate=2·10 2 ·e −22/RT · 2·10 −10 ·e 19/RT ·a H + 1+2·10 −10 ·e 19/RT ·a H + +5·10 7 ·e −28/RT · 1.4·10 −7 ·e 10/RT ·a H + 1+1.4·10 −7 ·e 10/RT ·a H + where R is the gas constant, T is the temperature (K) and a H + is the activity of protons in solution.

Stirring effect on kaolinite dissolution rate

Abstract Experiments were carried out measuring kaolinite dissolution rates using stirred and nonstirred flow-through reactors at pHs 2 to 4 and temperatures of 25°C, 50°C, and 70°C. The results show an increase of kaolinite dissolution rate with increasing stirring speed. The stirring effect is reversible, i.e., as the stirring slows down the dissolution rate decreases. The effect of stirring speed on kaolinite dissolution rate is higher at 25°C than at 50°C and 70°C and at pH 4 than at pHs 2 and 3. It is suggested that fine kaolinite particles are formed as a result of stirring-induced spalling or abrasion of kaolinite. These very fine particles have an increased ratio of reactive surface area to specific surface area, which results in enhancement of kaolinite dissolution rate. A balance between production and dissolution of the fine particles explains both the reversibility and the temperature and pH dependence of the stirring effect. Since the stirring effect on kaolinite dissolution rate varies with temperature and pH, measurement of kinetic parameters such as activation energy may be influenced by stirring. Therefore, standard use of nonagitated reaction vessels for kinetic experiments of mineral dissolution and precipitation is recommended, at least for slow reactions that are surface controlled.

Constraints on effective diffusivity during oxygen isotope exchange at a marble-schist contact, Sifnos (Cyclades), Greece

A smooth sigmoidal δ18O profile is observed at a marble-schist contact within rocks affected by the Miocene greenschist overprint in the Alpine metamorphic complex of the Cyclades on Sifnos. The point of inflection of the profile occurs at the lithological contact, and indicates that advective movement of fluids into the marble was negligible despite previous evidence showing that18O-enriched fluids infiltrated the schist sequence. Least squares model fits of the diffusion equation to the data giveDt products of 0.10 m2 and 0.13 m2 for marbles and schists, respectively (D =effective diffusion coefficient, t =time). To reconcile these figures with quoted free fluid diffusivities one has to propose that either the effective porosities were extremely low and/or the diffusional exchange time was as low as 102–103 years. Low effective porosities in schists are consistent with a model of greenschist evolution in which hydration reactions induced by (18O-enriched) fluids substantially decreased porosity and reduced exchange to fluid diffusional control.

Surface protonation data of kaolinite—reevaluation based on dissolution experiments

The aim of the present study is to compare available surface titration curves of kaolinite, to explain the differences between them, and to constrain their interpretation based on predictions of surface protonation that emerged from dissolution experiments. Comparison of six surface titration curves obtained at 25 degrees C reveals significant discrepancies, both in the shape of the curves and in the pH of the point of zero net proton charge (pH(PZNPC)). Based on an analysis of the different sites available for adsorption on kaolinite surfaces we conclude that different kaolinite samples are expected to have similar pH(PZNPC). Therefore, the major reason for the differences in the observed surface protonation is related to the different ways in which the pH(PZNPC) was determined. To compare the titration curves, some of the curves were recalculated so that the proton surface concentrations of all the titration curves would be zero around pH 5. As a result, we obtained a good agreement between the titration curves. A prediction of the molar fraction of protonated sites was retrieved from modeling of kaolinite dissolution reaction and was compared to the protonation data obtained from surface titration. The model successfully predicts the surface protonation data of most of the surface titrations.

Kinetics of gibbsite dissolution under low ionic strength conditions

Experiments measuring synthetic gibbsite dissolution rates were carried out using both a stirred-flow-through reactor and a column reactor at 25°C, and pH range of 2.5–4.1. All experiments were conducted under far from equilibrium conditions (ΔG < −1.1 kcal/mole). The experiments were performed with perchloric acid under relatively low (and variable) ionic strength conditions. An excellent agreement was found between the results of the well-mixed flow-through experiments and those of the (nonmixed) column experiments. This agreement shows that the gibbsite dissolution rate is independent of the stirring rate and therefore supports the conclusion of Bloom and Erich (1987) that gibbsite dissolution reaction is surface controlled and not diffusion controlled. The Brunauer–Emmett–Teller (BET) surface area of the gibbsite increased during the flow-through experiments, while in the column experiments no significant change in surface area was observed. The agreement between the dissolution rates of the mixed flow-through experiments that were normalized to the final surface area, with these of the column experiments, supports the assumption that the changes in surface area occurred early in the experiment, before the first steady state was approached. The significant differences in the BET surface area between the column experiments and the flow-through experiments, and the excellent agreement between the rates obtained by both methods, enable us to justify the substitution of the BET surface area for the reactive surface area. The dissolution rate of gibbsite varied as a function of the perchloric acid concentration. At pH > 3.5 the dissolution rate increased as a function of acid concentration, while at pH < 3.5 it decreased with acid concentration. We interpret the gibbsite dissolution rate as a result of a combined effect of proton catalysis and perchlorate inhibition. Following the theoretical study of Ganor and Lasaga (1998) we propose specific reaction mechanisms for the gibbsite dissolution in the presence of perchloric acid. The mathematical predictions of two of these reaction mechanisms adequately describe the experimental data.

SIMPLE MECHANISTIC MODELS FOR INHIBITION OF A DISSOLUTION REACTION

Abstract In this paper we examine the effect of inhibition on mineral dissolution rate. We postulate a reaction mechanism that involves catalysis and inhibition. The reaction mechanism consists of fast adsorption of a catalyst and/or an inhibitor on the mineral surface, followed by a slow hydrolysis step. The rate of the hydrolysis, which is the rate-determining step, depends on the adsorbed surface species. Therefore, the rate law includes the adsorption isotherms of the catalyst and the inhibitor. Two endmember mechanisms are analyzed. In the first mechanism the catalyst and the inhibitor compete with each other; i.e., they have a full mutual (negative) dependence; while in the second mechanism we assume that the adsorption of the catalyst and the inhibitor are absolutely independent of each other. A third general mechanism describes the whole range between the two endmember mechanisms. In this model, the degree of dependence of the catalyst adsorption on the inhibitor adsorption varies from 1 to 0, i.e., from complete competition to full independence, respectively. The models are applied to three case studies of kaolinite, albite and K-feldspar dissolution.

Diffusional isotopic exchange across an interlayered marble‐schist sequence with an application to Tinos, Cyclades, Greece

Marble-schist contacts are frequently characterized by fine-scale interlayering which prevents the application of semi-infinite layer width assumptions to advection/diffusion models of isotope profiles. Using a finite layer width boundary condition, we show that the minimum width (w) of a layer required in order that diffusional exchange at both boundaries will not mutually interfere is given by the relation w>k (Dt)1/2, where D is the effective diffusion coefficient, t is the duration of the diffusion and k is a constant determined by the initial compositional step and the noise (i.e., natural homogeneity and/or analytical accuracy). When this condition is not satisfied, a solution of the diffusion equation for a multilayer transition zone containing several schist and marble layers of different, finite, widths is applicable. Application of this refinement of the diffusion model to an oxygen isotope profile from Tinos (Cyclades) Greece gives 10−4≤Dt≤10−2 m2. This Dt range, and a Dt value of 10−1 m2 previously determined for Sifnos, indicates that rocks undergoing the Oligocene-Miocene greenschist facies overprint of Eocene high-pressure metamorphic rocks in the Cyclades were characterized by extremely low porosities. Permeability perpendicular to marble-schist contacts was sufficiently low to prevent advection into the marbles; correspondingly, it is suggested that the layer-parallel schistosity permitted infiltration of the fluids required for the hydration reactions of the metamorphic overprint.