Dissolution and crystallization rates of silicate minerals as a function of chemical affinity (original) (raw)
Related papers
Hydrometallurgy, 2014
The kinetics of the dissolution of silicates, aluminosilicates and quartz is described by a novel theory of dissolution. The experimental data for the rate of dissolution of these minerals shows a remarkable pattern: for many of these minerals, the order of reaction with respect to H + is close to 0.5 in the acidic region, and the order of reaction with respect to OH − is close to 0.5 in the alkaline region. It is proposed that the site of bond breaking in the rate-determining step of dissolution is the weakest bond, and this is frequently the metal-oxygen bond because of the higher bond energy of silicon-oxygen bonds. Alternatively, the least number of silicon-oxygen bonds is broken. This means that silicate groups react intact as a unit. Both metal atoms and silicate groups react and are removed independently. The rates of these independent processes are coupled by the potential at the surface. In the acid region, H + reacts with silicate groups at the surface. In the alkaline region, OH − ions react with the metal atom at the surface. The proposed theory of dissolution correctly predicts the observed orders of reaction with respect to H + ions and OH − ions in solution. The order of reaction of forsterite with respect to H + changes from 0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. The proposed mechanism suggests that the reason for the change in order of reaction is that the H + needs to be positioned at the inner Helmholtz plane to be effective in alkaline solutions.
Hydrometallurgy, 2014
The kinetics of the dissolution of silicates, aluminosilicates and quartz is described by a novel theory of dissolution. The experimental data for the rate of dissolution of these minerals shows a remarkable pattern: for many of these minerals, the order of reaction with respect to H + is close to 0.5 in the acidic region, and the order of reaction with respect to OH − is close to 0.5 in the alkaline region. It is proposed that the site of bond breaking in the rate-determining step of dissolution is the weakest bond, and this is frequently the metal-oxygen bond because of the higher bond energy of silicon-oxygen bonds. Alternatively, the least number of silicon-oxygen bonds is broken. This means that silicate groups react intact as a unit. Both metal atoms and silicate groups react and are removed independently. The rates of these independent processes are coupled by the potential at the surface. In the acid region, H + reacts with silicate groups at the surface. In the alkaline region, OH − ions react with the metal atom at the surface. The proposed theory of dissolution correctly predicts the observed orders of reaction with respect to H + ions and OH − ions in solution. The order of reaction of forsterite with respect to H + changes from 0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. The proposed mechanism suggests that the reason for the change in order of reaction is that the H + needs to be positioned at the inner Helmholtz plane to be effective in alkaline solutions.
On the Mechanism of the Dissolution of Quartz and Silica in Aqueous Solutions
ACS Omega, 2017
Quartz and silica are common materials, and their dissolution is of significant interest to a wide range of scientists. The kinetics of the dissolution of quartz and silica have been measured extensively, yet no clear theory of dissolution is available. A novel theory of dissolution and crystallization has recently been proposed that envisages the removal of material from the surface to form ions in solution leaving behind a charged surface vacancy. These vacancies create a potential difference across the Stern layer that accelerates or retards the removal of ions. In this way, the surface potential difference is caused by and influences the rate of the removal of ions. From this theory, a model of quartz dissolution is derived that predicts the observed orders of reaction. This prediction of the orders of reaction fits a data set consisting of 285 experiments. The model also describes the effect of Na + , K + , and Li + ions, as well as the effect of heavy water. A significant component of the model is its ability to describe the zeta potential of the quartz−water interface. The model successfully predicts a transient period at the beginning of the reaction when the rate could either increase or decrease.
Hydrometallurgy, 2014
The kinetics of the dissolution of silicates, aluminosilicates and quartz is described by a novel theory of dissolution. The experimental data for the rate of dissolution of these minerals shows a remarkable pattern: for many of these minerals, the order of reaction with respect to H + is close to 0.5 in the acidic region, and the order of reaction with respect to OH − is close to 0.5 in the alkaline region. It is proposed that the site of bond breaking in the rate-determining step of dissolution is the weakest bond, and this is frequently the metal-oxygen bond because of the higher bond energy of silicon-oxygen bonds. Alternatively, the least number of silicon-oxygen bonds is broken. This means that silicate groups react intact as a unit. Both metal atoms and silicate groups react and are removed independently. The rates of these independent processes are coupled by the potential at the surface. In the acid region, H + reacts with silicate groups at the surface. In the alkaline region, OH − ions react with the metal atom at the surface. The proposed theory of dissolution correctly predicts the observed orders of reaction with respect to H + ions and OH − ions in solution. The order of reaction of forsterite with respect to H + changes from 0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. The proposed mechanism suggests that the reason for the change in order of reaction is that the H + needs to be positioned at the inner Helmholtz plane to be effective in alkaline solutions.
A New Hypothesis for the Dissolution Mechanism of Silicates
The Journal of Physical Chemistry C, 2012
A novel mechanism for protonating bridging O atoms (O br ) and dissolving silica is proposed that is consistent with experimental data and quantum mechanical simulations of the α-quartz (101)/water interface. The new hypothesis is that H + -transfer occurs through internal surface H-bonds (i.e., SiOH−O br ) rather than surface water H-bonds and that increasing ionic strength, I, favors formation of these internal H-bonds, leading to a larger pre-exponential factor, A, in the Arrhenius equation, k = A exp(−ΔE a /RT), and higher rates of dissolution. Projector-augmented planewave density functional theory (DFT) molecular dynamics (MD) simulations and static energy minimizations were performed on the α-quartz (101) surface and with pure water, with Cl − , Na + , and Mg 2+ . Classical molecular dynamics were performed on α-quartz (101) surface and pure water only. The nature of the H-bonding of the surface silanol (SiOH) groups with the solution and with other surface atoms is examined as a test of the above hypothesis. Statistically significant increases in the percentages of internal SiOH−O br H-bonds, as well as the possibility of O br protonation with H-bond linkage to silanol group, are predicted by these simulations, which is consistent with the new hypothesis. This new hypothesis is discussed in relation to experimental data on silicate dissolution.
Geochimica et Cosmochimica Acta, 2011
The dissolution-precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH) 3 solutions with pH 8.2-9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol À1 of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol À1 are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q 1 or Q 2 silicon sites, respectively. The dissolution mechanism varies with DG such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q 1 sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q 1 reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q 2 rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q 3 sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.
Kinetics of rock-water reactions
1989
Dissolution rates for aluminosilicate minerals typically show a "U"-shaped dependence of the rate on the solution pH that is, the minimum rate is at some intermediate pH and the rate increases as the pH changes from this minimum. Previous attempts to explain the position of this minimum and the overall reaction order with respect to [H'] were not based on realistic reaction mechanisms for hydrolysis of Si-0 and Al-0 bonds. In this thesis, reaction mechanisms consisting of sets of elementaly reactions were developed for the dissolution of quartz and albite (feldspar). Hydrolysis of Si-0 and Al-0 bonds is catalyzed by [H+] at low pH and by [OH-] at high pH. For quartz, the catalyzed hydrolysis is interpreted as following an SN2 reaction mechanism. The fractional order of the dissolution rate with respect to catalyst activity is shown to be a result of the stoichiometry of the elementary reactions. The pH dependence of the albite dissolution rate was shown to depend upon ...