Kinetics of mineral dissolution and precipitation
Over the last 20 years our concerns with environmental contamination and remediation have led us to advance in the kinetics of water-rock interaction. We have centered our investigation in geochemical processes originated from four sources of anthropogenic contamination:
- (1) nuclear waste
- (2) acid mine drainage (AMD)
- (3) groundwater (de)nitrification
- (4) atmospheric accumulation/geological sequestration of CO2.
A common necessity in the management of these phenomena is the quantitative treatment of the geochemical evolution in the respective environments.
On the one hand, enhancement of the understanding of the mechanisms involving fluid flow and reactions with mineral surfaces requires incorporation of macroscopic to nanoscopic techniques in our studies. We have aimed at characterizing and quantifying the complex processes at the very low spatial scale. On the other hand, to predict the geochemical evolution in natural and artificial environments incorporation of the kinetics of mineral-fluid reactions in the form of mineral rate laws into reactive transport numerical codes is a necessary task. The resulting simulations gain reliability as the numerical codes include the kinetics of mineral dissolution/precipitation reactions coupled to the transport equations.
We constantly need to improve our knowledge of water-rock/mineral interaction. Within this context, along these years, the kinetics of the following minerals/materials has been studied: smectite, kaolinite and biotite (clays); pyrite, marcasite, galena, chalcopyrite and sphalerite (sulphides); biogenic hydroxyapatite (phosphate); periclase (oxide); calcite, aragonite and dolomite (carbonates), fluorite (halide), gypsum (sulphate) and fly ash, synthetic NaP1 zeolite and C-S-H gel of the Portland cement paste (silicates).
(1) NUCLEAR WASTE
- Smectite-rich bentonites have been recognized as suitable clays to be used as a sealant material in the multibarrier systems designed for storage of high-level nuclear waste in burial repositories. Due to its osmotic swelling capacity (and consequently its plasticity and impermeability), smectite impedes groundwater interaction with the metal canisters. The cation exchange reactions immobilize undesirable cations from the radioactive waste and retard its leakage from the canisters towards the local groundwater. However, the durability of the smectite itself under confinement conditions is a key datum that must also be taken into consideration. Laboratory experiments have been conducted to establish the dissolution rate dependence on the degree of saturation of several smectites that have been selected as a suitable material for the engineering barrier surrounding the nuclear water canister, and it is being used accordingly in some European field-scale experiments.
- Our main goals are to determine a reliable long-term stoichiometric dissolution rates of smectite, i.e., one that will be based on the release rate of the cations that occupy the octahedral and tetrahedral sites of the smectite and to examine the effect of the degree of saturation on smectite dissolution rate in the framework of kinetic theory.
Cama and Ayora (1998) Modeling the dissolution behavior in the clayey barrier, Mineralogical Magazine, 62 A, 271-272.
Cama et al. (1999) The effect of deviation from equilibrium on dissolution rate and on apparent variations in the activation energy. Geochimica and Cosmochimica Acta, 63, 17: 2481-2486.
Cama et al. (2000) Smectite dissolution kinetics at 80 °C and pH 8.8. Geochimica and Cosmochimica Acta, 64, 15: 2701-2717.
Rozalén et al. (2008) Experimental study of the effect of pH on the kinetics of montmorillonite dissolution at 25 ˚C. Geochimica and Cosmochimica Acta 72, 4224-4253.
Marty et al. (2011) Dissolution kinetics of synthetic Na-smectite. An integrated experimental approach. Geochimica and Cosmochimica Acta 75, 5849-5864.
Cama J. and Ganor J. (2015) Dissolution kinetics of clay minerals, 101-154. Natural and engineered clay barriers. Developments in clay science 6: Edited by C. Tournassat, I. Bourg, C. Steefel and F. Bergaya. Elsevier.
The use of concrete for storing low- and intermediate-level nuclear waste requires that the durability of this material is optimal for the lifetime of these repositories. This lifetime is directly proportional to the time the radioactive waste needs to reach natural levels of radioactivity. One of the most important processes that puts such durability at risk is the attack of concrete by water of low mineral content and neutral pH. This effect may cause alterations in the microstructure of the concrete, such as decalcification and dissolution of the cement phases, increased porosity and loss of barrier properties. Therefore, it is necessary to know the reactivity of cement and concrete to be able to assess the durability of these materials. Calcium silicate hydrate (C–S–H) is the main binding phase in all Portland cement-based systems.
- We have carried out flow-through experiments to study the dissolution kinetics of C–S–H gel. The flow of demineralized water causes the dissolution of C–S–H and changes in the composition of the solutions. The changes in Ca and Si concentrations and pH have been monitored during the reaction, allowing the measurement of the variation in the atomic Ca/Si ratio of the solution, the calculation of dissolution rates, the assessment of the effect of the solution saturation state on the rates and the derivation of a C–S–H dissolution rate law. Additionally, C–S–H compositional and microstructural changes during dissolution have been analyzed using several techniques (XRD, SEM, EPMA and 29Si NMR).
Trapote-Barreira et al. (2014) Dissolution kinetics of C-S-H gel. Flow-through experiments. Physics and Chemistry of the Earth 70-71, 17-31.
Trapote-Barreira et al. (2015) Structural changes of C-S-H gel during dissolution: SANS-USANS and 29Si-NMR characterization. Cement and Concrete Research 72, 76-89.
Trapote-Barreira et al. (2016) Degradation of mortar under advective flow: column experiments and reactive transport modeling. Cement and Concrete Research 81, 81-93.
The C-S-H gel dissolves incongruently when the Ca/Si ratio is high and congruently as the Ca/Si ratio decreases to the tobermorite stoichiometric Ca/Si ratio of 0.83. Deconvolution of the 29Si-NMR spectra of unreacted C-S-H.
(2) ACID MINE DRAINAGE (AMD)
2.2) Materials for water remediation
2.2.1) hydroxyapatite (Apatite II)
- Apatite II has been used in in column experiments to study its removal capacity of some divalent cations (Fe(II), Zn(II), Mn(II) and Pb(II)) at acid pH range (3–5.6), emulating large scale passivation systems. The authors show that the efficiency of Apatite IITM increases by decreasing water acidity.
- We measured the solubility of the Apatite II and compare it with different apatites’ solubility that can be found in the literature, investigated the hydroxyapatite (Apatite II) dissolution kinetics and its pH dependence reactivity of biogenic in the pH range between 2 and 7, and investigated the mechanisms of divalent metal removal by Apatite IITM (Pb, Zn, Mn, Cd and Cu). The kinetic and solubility data obtained are applied to the reactive transport simulation of column experiments of Apatite II dissolution and Cu retention. The results improve the knowledge of hydroxyapatite kinetics and overall process of removing undesired cations from acid waters by using suitable water treatments (e.g., Passive Remediation Barriers (PRB)).
Oliva et al. (2010) The Use of Apatite II™ to Remove divalent metal ions Zinc(II); Lead(II); Manganese(II) and Iron(II) from Water in Passive Treatment Systems: Column Experiments. Journal of Hazardous Materials 184, 364-374.
Oliva et al. (2011) Removal of cadmium (II), copper(II), nickel (II), cobalt (II) and mercury(II) from aqueous solutions by Apatite II™: Column Experiments. Journal of Hazardous Materials 194, 312-323.
Oliva et al. (2012) Biogenic hydroxyapatite (Apatite IITM) dissolution kinetics and metal removal from acid mine drainage. Journal of Hazardous Materials 213-214, 7-18.
2.2.2) synthetic zeolite
Cama J., Ayora C., Querol X. and Ganor J. (2005) Dissolution kinetics of synthetic zeolite NaP1 and its implication to zeolite treatment of contaminated waters. Environmental Science and Technology 39, 4871-4877.
SEM image of zeolite particles, zeolite dissolution rate-pH dependence and zeolite dissolution rate law.
- With the aim to advance in the current knowledge of mineral dissolution kinetics we decided to investigate the dissolution mechanisms that drive fluorite dissolution at low temperature and acidic pH. The reasons were: (1) fluorite has a simple chemistry and crystal morphology; (2) the current availability of techniques such as atomic force microscopy (AFM) and vertical scanning interferometry (VSI) allows direct investigation of the water–mineral interface and (3) the most comprehensive studies on fluorite dissolution tackled the reaction kinetics on the basis of aqueous chemistry obtained from fluorite powder dissolution experiments.
- Additionally, there is no direct knowledge about pit nucleation on the (1 1 1) surface of fluorite at different pH values. VSI and AFM allow the observation of surfaces that range from tens to hundreds of thousands of square microns. This large scale gives precise dissolution information relevant to surface mechanisms over characteristic areas of the reacted surface. Also, the nanoscale precision of the vertical measurements allows accurate determination of variation in size and shape of reactive surface features, such as etch pits, steps and terraces. Mineral dissolution rates can be measured directly from variations in surface height with time, using either the total surface explored or at specific locations.
- Lastly, fluorite has been investigated as a host for REE in magmatic and hydrothermal deposits. In the context of water-mineral interaction, dissolution rates of fluorite and the release of REE are not fully known.
Cama et al. (2010) Fluorite dissolution in acidic pH: in situ AFM and ex situ VSI experiments and Monte Carlo simulations. Geochimica and Cosmochimica Acta, 74, 4298-4311.
(3) GROUNDWATER DENITRIFICATION
It is necessary to determine, clarify and quantify the role of pyrite as an electron donor in the bacterial mediated denitrification process in order to assess its feasibility for nitrate remediation in contaminated groundwater, as well as to characterize nitrogen and oxygen isotope fractionation for pyrite-driven denitrification by T. denitrificans in order to evaluate the magnitude of the isotopic fractionation expected in nitrate-contaminated aquifers.
- To accomplish it, two types of experiments with powdered pyrite were performed: (1) batch experiments inoculated with pure culture of denitrificans to study the overall reaction and determine isotope fractionation and (2) long-term flow-through experiments to evaluate the performance of the denitrification process over time and under flow conditions.
Torrentó et al. (2010) Denitrification of groundwater with pyrite and Thiobacillus denitrificans. Chemical Geology 278, 80-91.
Torrentó et al. (2011) Enhanced denitrification in groundwater and sediments from a nitrate-contaminated aquifer after addition of pyrite. Chemical Geology 287, 90-101.
Torrentó et al. (2012) Characterization of attachment and growth of Thiobacillus Denitrificans on pyrite surfaces. Geomicrobiology Journal 29, 379-388.
(4) GEOLOGICAL CO2 STORAGE
Reactivity of limestone, sandstone and marl
Calcite dissolution in acidic waters which react with the rock minerals (sulphides, silicates, carbonates, etc.) is expected to occur in the presence of divalent cations. Our interest is to study calcite dissolution in sulphuric waters as it occurs in several environmental scenarios. Calcite (limestone) has been proposed to treat Acid Mine Drainage (AMD) effluents using passive treatment systems. In a different context, calcite may react with sulphate produced in deep saline aquifers as sequestered CO2 reacts with evaporate minerals (e.g., gypsum). Therefore calcite dissolution in acidic, sulphate-rich waters with divalent metals (Pb, Cu, Zn and Cd among others), when gypsum coating takes place, is a likely scenario.
- It is under such conditions that the calcite dissolution was studied using the vertical scanning interferometry (VSI) technique, allowing us (1) to elucidate the surface mechanism(s) that control calcite dissolution in acidic pH in a wide range of sulphate and metal concentrations, (2) to obtain the calcite dissolution rates at different pH and solution composition and (3) to determine the influence of gypsum and metal-sulphate coatings on calcite dissolution rate and morphology. Also, the energy-dispersive X-ray fluorescence (EDXRF) has been applied to measure the precipitation rate of metal-sulphate coatings on the cleavage surface of calcite.
Atanassova et al. (2013) Calcite interaction with acidic sulphate solutions: a vertical scanning interferometry and energy-dispersive XRF study. European Journal of Mineralogy 25, 331-351.
Garcia-Rios et al. (2014) Interaction between CO2-rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical CO2 conditions: experiments and modeling. Chem. Geol. 383, 107–122.
Garcia-Rios et al. (2015) Influence of the flow rate on dissolution and precipitation features during percolation of CO2-rich sulfate solutions through fractured limestone samples. Chemical Geology 414, 95–108.
Dávila et al. (2016) Interaction between a fractured marl caprock and CO2-rich sulfate solution under supercritical CO2 conditions. Int. J. Greenh. Gas Control 48, 171-185.