Surface-controlled Dissolution Research Articles

Temperature and basinal fluid controls on feldspar diagenesis, Lower Cretaceous sandstones, Scotian Basin, Canada

The passive-margin Scotian Basin accumulated thick Upper Jurassic–Lower Cretaceous deltaic sandstones and shales, which were influenced by high heat flow in the mid-Cretaceous and deformed by salt tectonics. This study takes advantage of the complex thermal history and record of saline fluid flow in the basin to determine the relative importance of burial depth, temperature, salinity of basinal brines, overpressure, and sandstone permeability in controlling the dissolution of detrital feldspar and growth of authigenic K-feldspar and albite. Feldspar grains and volcanic lithic clasts from two transects, with differing thermal and salt tectonic histories, were studied by scanning electron microscope (SEM) backscattered electron images, energy-dispersive spectroscopy, SEM cathodoluminescence (CL) imaging, and hot-cathode CL microscopy. Published fluid inclusion measurements are integrated with burial history curves and timing of deformation on salt detachment surfaces to provides a new synthesis of the post-Jurassic thermal and saline fluid flow history of the basin. Surface-controlled dissolution of K-feldspar and growth of authigenic K-feldspar rims occur from 1.5 to 3.1 km. More destructive transport-controlled dissolution, down to 3.9 km, is more effective in thick-bedded permeable sandstones that acted as fluid pathways. Albitization of detrital K-feldspar and sodic plagioclase, and of feldspars in volcanic lithic clasts was facilitated by high Na+ content of formation waters migrating preferentially through such pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Authigenic albite partially fills pores created by K-feldspar dissolution. Overpressured conditions enhance albite authigenesis, as a result of slower fluid advection. Episodic release of hot, saline, overpressured fluids in the zone of surface-controlled dissolution of K-feldspar favoured albitization but had no detectable effect on K-feldspar dissolution. Basin hydrology plays a definitive role in feldspar diagenesis, producing patterns that cannot be interpreted as a simple consequence of temperature and activity of Na+.

The Early Stages of Mimetite Dissolution in EDTA Studied with Atomic Force Microscopy and Scanning Electron Microscopy.

A mineral, mimetite Pb5(AsO4)3Cl, is one of the most insoluble minerals and continues to be considered a viable remedial strategy for immobilization of Pb and As from contaminated soils. It has been recognized that many well-known, naturally-occurring, and synthetic chelators strongly influence dissolution processes in near-surface geological environments. In this study, crystals of mimetite were observed in scanning electron microscopy (SEM) and atomic force microscopy (AFM) before and after dissolution in EDTA (ethylene diamine tetra-acetic acid) solution. Direct in situ observations at room temperature made in an AFM fluid cell revealed that the grain surface roughness has increased due to development of etch pits. Both hexagonal and prismatic walls developed dissolution features between 0.6 and 1.2 µm, respectively, during duration of the experiment. AFM observations suggest surface-controlled dissolution dominated step retrieval on both prismatic and hexagonal surfaces. SEM observations showed the development of rounded edges on hexagonal walls and elongated, oval etch pits on the prismatic wall. These results, representing early dissolution patterns on mimetite surfaces, might suggest that low pH conditions in soils containing organic acids similar to EDTA might contribute to remobilization of Pb and As from mimetite when applied to stabilization of these toxic metals in contaminated soils.

Surface-controlled dissolution rates: a case study of nanoceria in carboxylic acid solutions.

Nanoparticle dissolution in local milieu can affect their ecotoxicity and therapeutic applications. For example, carboxylic acid release from plant roots can solubilize nanoceria in the rhizosphere, affecting cerium uptake in plants. Nanoparticle dispersions were dialyzed against ten carboxylic acid solutions for up to 30 weeks; the membrane passed cerium-ligand complexes but not nanoceria. Dispersion and solution samples were analyzed for cerium by inductively coupled plasma mass spectrometry (ICP-MS). Particle size and shape distributions were measured by transmission electron microscopy (TEM). Nanoceria dissolved in all carboxylic acid solutions, leading to cascades of progressively smaller nanoparticles and producing soluble products. The dissolution rate was proportional to nanoparticle surface area. Values of the apparent dissolution rate coefficients varied with the ligand. Both nanoceria size and shape distributions were altered by the dissolution process. Density functional theory (DFT) estimates for some possible Ce(IV) products showed that their dissolution was thermodynamically favored. However, dissolution rate coefficients did not generally correlate with energy of formation values. The surface-controlled dissolution model provides a quantitative measure for nanoparticle dissolution rates: further studies of dissolution cascades should lead to improved understanding of mechanisms and processes at nanoparticle surfaces.

Influence of temperature on the dissolution kinetics of synthetic LaPO4-monazite in acidic media between 50 and 130 °C

Single-phase monazite-type ceramics are discussed as waste forms for the safe disposal of surplus plutonium or separated minor actinides. To gain a deeper insight into the mechanism governing the dissolution of monazite-type compounds, the dissolution kinetics of synthetic LaPO4-monazite was systematically studied by dynamic dissolution experiments carried out in the temperature range from 50 to 130 °C in acidic media (0.01 M HNO3). The dissolution rates at far from equilibrium conditions increased from 3.2 × 10−5 g m−2 d−1 at 50 °C to 2.5 × 10−4 g m−2 d−1 at 130 °C. Two different temperature regions were observed, in which the normalised dissolution rates of LaPO4 have a diverging temperature dependence, indicating two different mechanisms of dissolution: namely surface-controlled dissolution (T = 50–90 °C; Ea = 44 kJ mol−1) and transport-controlled dissolution (T = 90–130 °C; Ea = 7.5 kJ mol−1). Complementary thermodynamic modelling studies of the dissolution of LaPO4 at the experimental conditions show that La-rhabdophane (LaPO4·0.667H2O) is the thermodynamically favoured phosphate phase in aqueous environments below about 100 °C. Apparently, the hydration of monazite and the formation of a thin surface layer consisting of rhabdophane is an intermediate step controlling the dissolution kinetics of the LaPO4 monazite ceramics in low temperature aqueous environments.

Kinetics of conversion of brushite coatings to hydroxyapatite in alkaline solution

This work describes the kinetics of conversion of brushite coatings, produced by chemical deposition, to hydroxyapatite. The conversion was performed in alkaline solution at three temperatures: 50°C, 55°C, and 60°C. The evolution of the transformation was assessed by X-ray diffraction, and the phase fractions were determined by Rietveld refinement. The Johnson–Mehl–Avrami equation was applied to the data in order to describe the conversion mechanisms. The value of 42kJ/mol obtained for the conversion activation energy is of the same order as found in similar surface-controlled dissolution processes. The Avrami exponent was n=3.5, which is consistent with an interface-controlled growth of three-dimensional HA crystals with decreasing nucleation rate.

Current collectors as reducing agent to dissolve active materials of positive electrodes from Li-ion battery wastes

This study evaluates the electrochemical effect of mineral, organic and metallic reducing agents to promote LiNi1/3Mn1/3Co1/3O2 (NMC) dissolution in acidic media. The main aim is to compare the reactivity of metallic current collectors (Cu and Al) to more conventional reducing agents such as organic acids and hydrogen peroxide. In a more applied perspective, this work points out the economic interest to use current collectors as an alternative chemical reagent for an efficient NMC dissolution as they are inherently present in the fraction treated by an hydrometallurgical approach. It demonstrates that galvanic interactions are suitable to leach the transition metal oxides from spent LIBs without heating and emission unlike carboxylic acid solutions, allowing the development of low ecological footprint processes. In a more fundamental framework, the results obtained clear several parts of the dissolution NMC mechanism. A fast first step is controlled by the solution pH with a low influence of redox reactions on the kinetic. A slower second stage is subjected to the electrochemical processes with a surface-controlled dissolution.

Determination of mineral dissolution regimes using flow-through time-resolved analysis (FT-TRA) and numerical simulation

Flow-through time resolved analysis (FT-TRA) involves subjecting small mineral samples (<10mg) inserted in a miniature flow-through cell (50μL) to controlled flows of eluent analyzed on-line by ICP-MS. In this study, FT-TRA is used to empirically determine the dissolution regimes for the two well-studied minerals forsterite and calcite, representing minerals with relatively slow and fast dissolution kinetics. A proportional increase in steady-state effluent [Mg, Si] concentrations with increasing flow-through cell eluent residence times confirms a dominantly surface-controlled dissolution regime for a powdered forsterite sample at pH2.3, implying that transport limitations are negligible. In contrast, the relationship between flow rates and dissolution rates for single grain calcite samples at pH2.3–4 reveals that transport limitations affect the rate of calcite dissolution. To provide a quantitative and process-based assessment of the effect of diffusive transport limitations, simulations of the calcite experiments were performed with a high resolution, pore-scale model that considers the geometry of the calcite grain and the FT-TRA flow-through reactor. The pore-scale model reproduces the observed effluent [Ca] concentrations for all experimental conditions using a single set of surface kinetic parameters, by accounting for the formation of a diffusive boundary layer (DBL) that varies in thickness as a function of flow rates. These results demonstrate that combining FT-TRA with pore-scale modeling makes it possible to obtain unprecedented insights not achievable by either method separately, including quantification of DBL thicknesses and the determination of transport controls as a function of pH, flow velocity and residence times.

Synergistic effect of reductive and ligand-promoted dissolution of goethite.

Ligand-promoted dissolution and reductive dissolution of iron (hydr)oxide minerals control the bioavailability of iron in many environmental systems and have been recognized as biological iron acquisition strategies. This study investigated the potential synergism between ligands (desferrioxamine B (DFOB) or N,N'-Di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED)) and a reductant (ascorbate) in goethite dissolution. Batch experiments were performed at pH 6 with ligand or reductant alone and in combination, and under both oxic and anoxic conditions. Goethite dissolution in the presence of reductant or ligand alone followed classic surface-controlled dissolution kinetics. Ascorbate alone does not promote goethite dissolution under oxic conditions due to rapid reoxidation of Fe(II). The rate coefficients for goethite dissolution by ligands are closely correlated with the stability constants of the aqueous Fe(III)-ligand complexes. A synergistic effect of DFOB and ascorbate on the rate of goethite dissolution was observed (total rates greater than the sum of the individual rates), and this effect was most pronounced under oxic conditions. For HBED, macroscopically the synergistic effect was hidden due to the inhibitory effect of ascorbate on HBED adsorption. After accounting for the concentrations of adsorbed ascorbate and HBED, a synergistic effect could still be identified. The potential synergism between ligand and reductant for iron (hydr)oxide dissolution may have important implications for iron bioavailability in soil environments.

The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate

The effects of pH and Mg on the crystallization of amorphous calcium carbonate (ACC) to vaterite and/or calcite were studied using a combination of in situ time resolved synchrotron-based techniques and electron microscopy. The experiments showed that Mg increased the stability of ACC and favoured the formation of calcite over vaterite. A neutral (∼7) starting pH during mixing promoted the transformation of ACC into calcite via a dissolution/reprecipitation mechanism. Conversely, when ACC formed in a solution that started with a high initial pH (∼11.5), the transformation to calcite occurred via metastable vaterite, which formed via a spherulitic growth mechanism. In a second stage this vaterite transformed to calcite via a surface-controlled dissolution and recrystallization mechanism. These crystallization pathways can be explained as a consequence of the pH-dependent composition, local structure, stability and dissolution rates of ACC.

Effect of pH, anions and cations on the dissolution kinetics of humic acid particles

The dissolution kinetics of solid humic acid (HA) particles under different conditions is reported and analyzed. The dissolution rate at pH 4 and 5 is very slow and several years may take to achieve equilibration. The rate is strongly increased by increasing pH, and at pH 11 complete dissolution is achieved in 30 min. The dissolution rate is also markedly increased by the presence of monocarboxylic acids and anionic herbicides such as glyphosate and 2,4-D. On the contrary, the rate is decreased by inorganic divalent cations (Mg 2+, Ca 2+, Sr 2+, Ba 2+). The divalent cation paraquat has a dual behavior, decreasing the dissolution rate at pH >7 but increasing it at pH <7. Simple modeling assuming a surface-controlled dissolution kinetics can reproduce reasonably well the effects of anions and cations. The different ions seem to affect the dissolution rate by modifying intermolecular forces between HA molecules located at the surface of the particles.

Comparative solubilisation of potassium carbonate, sodium bicarbonate and sodium carbonate in hot dimethylformamide: application of cylindrical particle surface-controlled dissolution theory

A surface-controlled dissolution of cylindrical solid particles model is applied to potassium carbonate, sodium bicarbonate and sodium carbonate in dimethylformamide at elevated temperatures. Previously published data for the dissolution of potassium carbonate is interpreted assuming a cylindrical rather than a spherical shape of the particles, the former representing a closer approximation to the true shape of the particles as revealed by scanning electron microscopy. The dissolution kinetics of sodium carbonate and sodium bicarbonate in dimethylformamide at 100 degrees C were investigated via monitoring of the deprotonation of 2-cyanophenol with dissolved solid to form the 2-cyanophenolate anion that was detected with UV-visible spectroscopy. From fitting of experimental results to theory, the dissolution rate constant, k, for the dissolutions of potassium carbonate, sodium bicarbonate and sodium carbonate in dimethylformamide at 100 degrees C were found to have the values of (1.0 +/- 0.1) x 10(-7) mol cm(-2) s(-1), (5.5 +/- 0.3) x 10(-9) mol cm(-2) s(-1) and (9.7 +/- 0.8) x 10(-9) mol cm(-2) s(-1), respectively.

Experimental and Theoretical Study of the Surface-Controlled Dissolution of Cylindrical Particles. Application to Solubilization of Potassium Hydrogen Carbonate in Hot Dimethylformamide

In this paper we present a mathematical model for the surface-controlled dissolution of cylindrical solid particles. This is employed to interpret experimental data published previously for the dissolution of potassium bicarbonate in dimethylformamide at elevated temperatures. Significant kinetic differences in assuming cylindrical rather than spherical shapes are reported with the former representing a closer approximation to the true shape of the particles as revealed by scanning electron microscopy. From the fits of experimental data to the cylindrical model for the surface-controlled dissolution, the dissolution rate constant, k, for the dissolution of KHCO(3) in DMF was found to be (9.6 +/- 1.6) x 10(-9) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 34.5 kJ mol(-1) over the temperature range of 60-100 degrees C. Comparison between cylindrical and spherical dissolution theory highlights the importance of considering the particle shapes for realistic modeling of surface-controlled dissolution kinetics.

Heterogeneous Kinetics of the Dissolution of an Inorganic Salt, Potassium Carbonate, in an Organic Solvent, Dimethylformamide

Understanding the mechanisms of solid-liquid systems is fundamental to the development and operation of processes for the production of agrochemicals and pharmaceuticals. The use of a strong inorganic base in an organic solvent, typically, potassium carbonate in dimethylformamide, is often used to facilitate the formation of a required anionic organic nucleophile. In this paper, the dissolution kinetics of potassium carbonate in dimethylformamide at elevated temperatures is studied in the presence of ultrasound, as revealed via monitoring of the deprotonation of 2-cyanophenol by dissolved K2CO3. Two independent experimental methods were employed; the loss of 2-cyanophenol was detected electrochemically at a platinum microdisk working electrode, and the formation of the 2-cyanophenolate anion was monitored via UV/visible spectroscopic analysis. The results were modeled by fitting the experimental data to a theoretical model for the surface-controlled dissolution of solid particles. The dissolution rate constant, k, for the dissolution of K2CO3 in DMF was found to have a value of (1.3 +/- 0.2) x 10(-7) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 44.2 +/- 0.4 kJ mol(-1) over the temperature range of 70-100 degrees C studied.

Reactions at the Solid−Liquid Interface: Surface-Controlled Dissolution of Solid Particles. The Dissolution of Potassium Bicarbonate in Dimethylformamide

We present a mathematical model for the surface-controlled dissolution of solid particles. This is applied to the dissolution of a solid having different particle size distribution functions: those of a monodispersed solid containing particles of all one size, a two-size-particle distribution, and a Gaussian distribution of the particle sizes. The dissolution of potassium bicarbonate in dimethylformamide is experimentally studied indirectly at elevated temperatures. We monitor the dissolution via the homogeneous deprotonation of 2-cyanophenol by dissolved KHCO3. The loss of 2-cyanophenol was detected electrochemically at a platinum microdisk electrode, and separately, the formation of the 2-cyanophenolate anion was monitored via UV-visible spectroscopic analysis. The results presented show that the kinetics of the loss of 2-cyanophenol behaves on one hand as a homogeneous chemical process and on the other hand as a dissolution-rate-controlled process. Initially, predissolved KHCO3 in solution deprotonates the 2-cyanophenol and homogeneous reaction dominates the observed kinetics, and at longer times, the observed kinetics is controlled by the rate of KHCO3 dissolution. Modeling of the experimental results for the surface-controlled dissolution of KHCO3 in dimethylformamide (DMF) yielded a mean value for the dissolution rate constant, k, at elevated temperatures; k was found to have a value of (1.1 +/- 0.3) x 10(-8) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 34.4 +/- 0.4 kJ mol(-1) over the temperature range 60-100 degrees C.

Iron oxide dissolution and solubility in the presence of siderophores

Iron is an essential trace nutrient for most known organisms. The iron availability is limited by the solubility and the slow dissolution kinetics of iron-bearing mineral phases, particularly in pH neutral or alkaline environments such as carbonatic soils and ocean water. Bacteria, fungi, and plants have evolved iron acquisition systems to increase the bioavailability of iron in such environments. A particularly efficient iron acquisition system involves the solubilization of iron by siderophores. Siderophores are biogenic chelators with high affinity and specificity for iron complexation. This review focuses on the geochemical aspects of biological iron acquisition. The significance of iron-bearing minerals as nutrient source for siderophore-promoted iron acquisition has been confirmed in microbial culture studies. Due to the extraordinary thermodynamic stability of soluble siderophore-iron complexes, siderophores have a pronounced effect on the solubility of iron oxides over a wide pH range. Very small concentrations of free siderophores in solution have a large effect on the solution saturation state of iron oxides. This siderophore induced disequilibrium can drive dissolution mechanisms such as proton-promoted or ligand-promoted iron oxide dissolution. The adsorption of siderophores to oxide surfaces also induces a direct siderophore-promoted surface-controlled dissolution mechanism. The efficiency of siderophores for increasing the solubility and dissolution kinetics of iron oxides are compared to other natural and anthropogenic ligands.

Surface chemistry and dissolution kinetics of divalent metal carbonates.

A surface complexation model (SCM) for divalent metal carbonates (Ca, Mg, Sr, Ba, Mn, Fe, Co, Ni, Zn, Cd, and Pb) is developed based on new electrophoretic measurements and correlation between aqueous and surface reactions stability constants. This SCM postulates the formation of the following surface species: >CO3H0, >CO3-, >CO3Me+, >MeOH0, >MeO-, >MeOH2+, >MeHCO30, and MeCO3- within the framework of a constant capacitance of the electric double layer. It can be used to describe the surface-controlled dissolution kinetics of divalent metal carbonates and allows determination of the order of dissolution reactions with respect to rate-controlling protonated carbonate surface groups in acid solutions (>CO3H0) and hydrated metal groups (>MeOH2+) in neutral to alkaline solutions. The reaction order with respect to protonated carbonate groups increases from 2 for MnCO3 and ZnCO3 to 4 for NiCO3, whereas for hydrated surface metals, it augments from 2 for ZnCO3 to approximately 4 for MnCO3 and NiCO3. The dissolution rates at 5 < or = pH < or = 8 increase in the order Ni < Mg < Co < Fe < Mn < Zn < Cd < Sr < or = Ca approximately = Ba approximately = Pb and correlate nicely with water exchange rates from the aqueous solution into the hydration sphere of the corresponding dissolved cations. Such a correlation allows the generation for all carbonates of a model describing their dissolution/precipitation kinetics, including the effect of various ligands, provided that rate constants and their activation volumes for water exchange around Me(II)-ligand dissolved complexes are available.

Geochemical and mineralogical controls on trace element release from the Penn Mine base-metal slag dump, California

Base-metal slag deposits at the Penn Mine in Calaveras County, California, are a source of environmental contamination through leaching of potentially toxic elements. Historical Cu smelting at Penn Mine (1865–1919) generated approximately 200,000 m 3 of slag. The slag deposits, which are flooded annually by a reservoir used for drinking water and irrigation, also may be in contact with acidic ground waters (pH<4) from the adjacent mine area. Slags vary from grey to black, are glassy to crystalline, and range in size from coarse sand to large (0.6×0.7×1.5 m), tub-shaped casts. Metals are hosted by a variety of minerals and two glass phases. On the basis of mineralogy, slags are characterized by 4 main types: fayalite-rich, glassy, willemite-rich, and sulfide-rich. The ranges in metal and metalloid concentrations of 17 slag samples are: As, 0.0004–0.92; Ba, 0.13–2.9; Cd, 0.0014–1.4; Cu, 0.18–6.4; Pb, 0.02–11; and Zn, 3.2–28 wt.%. Leachates from Toxicity Characteristic Leaching Procedure tests (acetic acid buffered at pH 4.93) on two willemite-rich slags contained Cd and Pb concentrations (up to 2.5 and 30 mg/l, respectively) in excess of US Environmental Protection Agency (USEPA) regulatory limits. Analyses of filtered (0.45 μm) water, collected within the flooded slag dump during reservoir drawdown, reveal concentrations of Cd (1.7 μg/l), Cu (35 μg/l), and Zn (250 μg/l) that exceed USEPA chronic toxicity guidelines for the protection of aquatic life. Data from field and laboratory studies were used to develop geochemical models with the program EQ3/6 that simulate irreversible mass-transfer between slag deposits and reservoir waters. These models include kinetic rate laws for abiotic sulfide oxidation and surface-controlled dissolution of silicates, oxides, and glass. Calculations demonstrate that the main processes controlling dissolved metal concentrations are (1) dissolution of fayalite, willemite, and glass; (2) sulfide oxidation; and (3) secondary phase precipitation. Close agreement between model results and measured concentrations of Al, Ba, Cu, Fe, SiO 2, and SO 4 in the slag dump pore waters suggests that the dissolved concentrations of these elements are controlled by solubility equilibrium with secondary phases. Differences between predicted and measured Cd and Pb concentrations imply that field weathering rates of glass and sulfides are approximately two orders of magnitude lower than laboratory rates. Overprediction of Pb release may also reflect other attenuation processes in the natural system, such as sorption or coprecipitation.

The influence of natural mineral coatings on feldspar weathering

Quantification of the rate of weathering of feldspar, the most abundant mineral in the Earth's crust, is required to estimate accurately carbon dioxide fluxes over geological timescales and to model groundwater chemistry. Laboratory dissolution rates, however, are consistently found to be up to four orders of magnitude higher than the ‘natural' rates1,2 measured in the field. Although this discrepancy has been attributed to several factors2, previous research has tended to suggest that the underlying mechanism of feldspar dissolution under acidic pH may differ between the field and the laboratory3. Here we demonstrate that weathered albite surfaces, like laboratory-dissolved samples, are sodium- and aluminium-depleted, indicating that the dissolution mechanism in acidic soils is similar to that in acidic laboratory solutions. We find that microtopography images are consistent with dissolution occurring at specific surface sites — indicative of surface-controlled dissolution dominated by a non-stoichiometric layer. Elevated aluminium and silicon ratios reported previously3,4, and used to suggest a mechanism for field weathering different from laboratory dissolution3, can alternatively be explained by a thin, hydrous, patchy, natural coating of amorphous and crystalline aluminosilicate. This coating, which is largely undetected under scanning electron microscopy after cleaning, but visible under atomic force microscopy, alters surface chemistry measurements and may partially inhibit the field dissolution rate.

Surface-Controlled Reactivity of Metal Carbonates

AbstractMetal carbonate surfaces are a common component of soils, sediments, terrestrial organisms, and many engineered structures. The surface-controlled growth and dissolution of metal carbonate minerals (e.g. calcite) often affects fluid flow in pipes (through scaling) as well as the nanoscale assembly of complex biologic structures (similar reactions control Ca levels in the oceans, and ultimately, long-term CO2 levels). Engineering metal carbonate surface chemistry to enhance or inhibit growth, dissolution, or crystal nucleation is therefore an area with obvious economic impact. Metal carbonate superstructures are rapidly altered by dissolution/re-precipitation reactions, necessitating the measurement of surface chemistry near equilibrium. Measurements of metal sorption using limited residence time reactors point to the importance of adsorbed/exchanged metal ions in controlling surface charge and electrokinetic behavior. The same reactions appear to control growth and dissolution rates as well.

Evolution of Hematite Surface Microtopography Upon Dissolution by Simple Organic Acids

AbstractThe surface microtopography of hematite over the course of dissolution in oxalic and citric acids was examined by in-situ and ex-situ atomic-force microscopy. In-situ imaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed in 2 mM citric acid demonstrated that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situ imaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situ and ex-situ imaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution.