Leonid Lakshtanov - Academia.edu (original) (raw)
Papers by Leonid Lakshtanov
This work is a continuation and extension of our studies of the kinetics of recrystallization and... more This work is a continuation and extension of our studies of the kinetics of recrystallization and precipitation of calcite in the presence of biopolymers, as well as precipitation of calcite on the surface of biogenic calcite (chalk). Our results show that the dependence of calcite precipitation rate on supersaturation does not obey the parabolic law that is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation-mediated growth. Rate of calcite precipitation on chalk surfaces at the lowest supersaturation studied in this work is 3 orders of magnitude lower than on pure calcite surfaces. The presence of non-polar organic compounds, such as biomarkers, decreases surface energy but does not change the mechanism of calcite precipitation. On the contrary, the presence of adsorbed polysaccharides and polyamino acids on the surface determines the mechanism of calcite growth and precipitation. This is confirmed by the results of measur...
Геохимия, 2019
Strontium adsorption has been studied by the method of acid-base potentiometric titrations at thr... more Strontium adsorption has been studied by the method of acid-base potentiometric titrations at three different temperatures: 25, 50, 75C. The effect of pH, ionic strength, sorbate/sorbent ratio, and temperature on adsorption was investigated. Experimental data were simulated using two various surface complexation models, with two different electrostatic descriptions of the interface: the constant capacitance model (CCM) and the triple-layer model (TLM). Although the both models used are able to account for the acid-base reactions and surface complexation of strontium on birnessite, we consider that the TLM is more applicable for a description of heterophaseous system H+ MnOH Sr2+. Under conditions of low ionic strength and negatively charged surface, Sr2+ ions compete with the electrolyte ions and form outer-sphere complexes along with inner-sphere complexes. Consequently, using the CCM for description of strontium adsorption data could be mathematically satisfactory, but physically ...
Geochemistry International, 2019
Strontium adsorption was studied by acid-base potentiometric titration at various pH, ionic stren... more Strontium adsorption was studied by acid-base potentiometric titration at various pH, ionic strength, the sorbate/sorbent ratio, and temperatures (at 25, 50, and 75°C). The experimental data were interpreted using two models of surface complexation with two different electrostatic models of the interface: the constant capacitance (CCM) and triple-layer (TLM) models. Although both models are able to take into account acid-base reactions and surface complexation of Sr on birnessite, we believe that TLM is more suitable for the description of the H +->MnOH-Sr 2+ heterogeneous system. At a low ionic strength and negatively charged surface, Sr 2+ ions compete with electrolyte ions and form both inner-and outer-sphere complexes. Although the application of CCM in describing Sr adsorption may be mathematically satisfactory, it has little physical sense. We suggest a model that involves both inner-sphere (>MnOHSr 2+ , >MnOSr + , >MnOSrOH 0) and outer-sphere ([>MnO-Sr 2+ ] +) surface complexes. The corresponding constants of formation of these surface complexes were calculated at 25, 50, and 75°C.
Crystal Growth & Design, 2018
Recrystallization requires dissolution in pore fluids and precipitation on existing particle surf... more Recrystallization requires dissolution in pore fluids and precipitation on existing particle surfaces in a process known as Ostwald ripening, where the smallest particles feed growth on the larger ones. Recrystallization conditions are optimized in industry when commercial products require larger crystals, and it is the dominant process in burial diagenesis, which turns sediments into rock. In chalk, the original coccolith elements are often still clearly distinguishable, which is a surprise considering the rapid rates of pure calcite recrystallization. We studied the rates of calcite precipitation on the surface of the particles in various chalk samples, using the constant composition method over a wide range of supersaturations. Our results show that dependence of calcite precipitation rate on supersaturation does not obey the parabolic law, which is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation-mediated growth. The rates of calcite precip...
Crystal Growth & Design, 2018
The influence of biopolymers, alginate (Alg) and polyaspartate (pAsp), on the kinetics of calcite... more The influence of biopolymers, alginate (Alg) and polyaspartate (pAsp), on the kinetics of calcite precipitation was studied using the constant composition method for supersaturation states ranging from 2.4 to 4.5, at pH = 8.5. Biopolymer presence changes the mechanism of calcite precipitation and its inhibition depending on the system history. In a system without polymers, calcite precipitates by spiral growth. Introducing polymers during growth, inhibits step movement by pinning at a number of kink sites along step edges. Polymer adsorption induces growth by two-dimensional nucleation. Both polymers inhibit calcite growth. Inhibition is stronger at higher concentration and lower solution supersaturation. The interfacial free energy, a key parameter in the control of nucleation and growth, was estimated from the analysis of the precipitation rates, as well as data obtained from vapour adsorption, are quite identical for calcite with either alginate or polyaspartate adsorbed. This is confirmed by electrokinetic measurements, which show similar ζ-potential values for calcite with each of the polymers. Increasing polymer concentration and adsorption time led to a progressive decrease of the effective interface free energy, which could explain the much lower supersaturation needed for the onset of surface nucleation.
Crystal Growth & Design, 2016
Organic molecules control calcite growth and crystal morphology, influence biomineralization proc... more Organic molecules control calcite growth and crystal morphology, influence biomineralization processes and offer clues for optimizing antiscalants for industry. Here we quantified the effect of amino acid monomers, aspartic acid (Asp 1) and glycine (Gly 1), and their polymers (Asp n , Asp 5 and Gly 5), on calcite growth rate, in a constant composition setup. Asp 1 and its polymers inhibit growth, with rate decreasing as amino acid chain length, i.e., number of carboxyl groups, increases. For example, the addition of 2 mM Asp 1 reduced growth by 54%, while it required only 0.0012 mM Asp n to reduce growth by 94%. Gly 1 and Gly 5 only marginally affect growth (<10%), indeed they slightly promote growth at most tested concentrations. Fitting of adsorption isotherms (Langmuir, Langmuir-Freundlich, Flory-Huggins) confirmed that Asp polymers adsorb strongly, explaining their strong control on calcite growth but Gly 1 and Asp 1 adsorb less due to competition with carbonate ions (∆G ads energies: Asp n =-39 kJ/mol, Asp 5 =-50 kJ/mol , Asp 1 =-21 kJ/mol and Gly 1 =-22 kJ/mol). Calcite morphology was equally affected by the amino acids. Crystal edges became rougher and corners more rounded.
Current methods for extracting oil from chalk leave behind as much as 75% of the total oil availa... more Current methods for extracting oil from chalk leave behind as much as 75% of the total oil available. It remains as small droplets in pore spaces or as thin films on particle surfaces. This oil is inaccessible at present, but if it could be extracted, the impact on global ...
Acta Physica Hungarica, 1989
One-dimensional diffusion in a membrane is studied by solution of the according Euler-Lagrange eq... more One-dimensional diffusion in a membrane is studied by solution of the according Euler-Lagrange equations derived from Gyarmati's governing principle of dissipative processes. In the linear case the exact solution is found for ternary ideal systems (dilute ideal solution in contact with the membrane). In the quasilinear case of a binary ideal system solutions are searched by Stark's dual field methods.
Journal of Contaminant Hydrology, 2014
Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption ... more Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption studies. The goal was to understand the difference in sorption behavior between synthetic and biogenic calcite. Experiments at atmospheric partial pressure of CO2, in solutions equilibrated with calcite and chalk and pH ranging from 7.7 to 8.8, explored the influence of initial concentration and the amount and type of sorbent on Ni uptake. Adsorption increases with increased surface area and pH. A surface complexation model describes the data well. Stability constants for the Ni surface complex are log KNi=-1.12 on calcite and log KNi=-0.43 and -0.50 on the two chalk samples. The study confirms that synthetic calcite and chalk both take up nickel, but Ni binds more strongly on the biogenic calcite than on inorganically precipitated, synthetic powder, because of the presence of trace amounts of polysaccharides and clay nanoparticles on the chalk surface.
The acid/base properties of the hematite/water interface and surface complexation with gold has b... more The acid/base properties of the hematite/water interface and surface complexation with gold has been studied by potentiometric titrations in the four component system H + -≡FeOH -AuCl − 4 -Cl − . Equilibrium measurements were performed in NaCl media at 298.2 K. In the evaluation of equilibrium model from experimental data the constant capacitance model was applied. The acid/base properties were investigated in 0.1 M NaCl in the range 2.6 ≤ pH ≤ 7.4. The resulting intrinsic constants for protonation and deprotonation of hydroxyl groups at the surface were log β s 1,1,0,0(int) = 7.10 ± 0.06 and log β s −1,1,0,0(int) = −7.80 ± 0.06. The density of proton active surface sites was 2.85 nm −2 and the specific capacitance 2.5 C V −1 m −2 . In the investigation of surface complexation of gold, the potentiometric titrations were performed in the range 2.0 ≤ pH ≤ 10.2. Titration data was supplemented with analysis of Au in the aqueous phase by atomic absorbance spectrometry. The equilibrium model proposed consists of the following monodentate surface complexes: ≡FeOHAuCl 3 (log β s 0,1,1,−1 = 1.45 ± 0.03), ≡FeOHAuCl 2 OH (log β s −1,1,1,−2 = −3.89 ± 0.02), ≡FeOHAu(OH) 3 (log β s −3,1,1,−4 = −21.94 ± 0.05). A slightly better fit could be obtained by assuming formation of a bidentate complex with the composition (≡FeO) 2 Au(OH)H 2 O. However, based on structural arguments this complex was rejected.
Crystal Growth & Design, 2015
ABSTRACT Recrystallization is one of the dominant processes active in early diagenesis. There is ... more ABSTRACT Recrystallization is one of the dominant processes active in early diagenesis. There is ample evidence in the literature that polysaccharides significantly inhibit growth of calcium carbonate, but little is known about their effect on recrystallization at low degrees of supersaturation. In this study, we monitored recrystallization in calcite saturated 0.1 M NaCl solutions using H14CO3- as a tracer. Alginate inhibits calcite recrystallization as expected, but for the same alginate concentrations inhibition is weaker than for growth. We explain these results by extending the interpretation of data from precipitation experiments and by understanding that alginate acts in two opposite directions. On the one hand, alginate pins steps, effectively inhibiting calcite growth. On the other hand, alginate promotes heterogeneous calcite nucleation and subsequent growth, decreasing the interface free energy and serving as an effective template for calcite nucleation and growth. To get closer to an assessment of the effective free surface energy, gamma', in this study, we measured the energy of the alginate-water interface, gamma(aw) by using vapor adsorption. The values of interfacial surface tension between pure calcite and water, gamma(cw), and calcite with adsorbed alginate and water, gamma(aw), were found to be equal to 76 and 115 mJ m(-2). gamma(aw) > gamma(cw), which contributes to a lower energy barrier for calcite nucleation on adsorbed alginate. The high value of gamma(aw) testifies that alginate molecules prefer to create new calcite-alginate interfaces than to maintain alginate-water interfaces. In growth experiments, we take into account only inhibition because inhibition and subsequent nucleation are separated by specific induction times. For recrystallization, the time scale for induction is much longer than for growth, and the two oppositely directed processes act almost together and simultaneously, resulting in weaker overall inhibition than for simple calcite growth alone.
Radiochimica Acta, 2000
Secondary minerals have the potential to sequester escaped actinides in the event of a radioactiv... more Secondary minerals have the potential to sequester escaped actinides in the event of a radioactive waste repository failure, but currently, data to define their maximum uptake capacity are generally lacking. To estimate a maximum limit for solid solution in calcite, we took advantage of the behavioural similarities of the 4 f-orbital lanthanides with some of the 5 f-orbital actinides and used rare Earth element (REE) concentration as an analogue. A suite of 65 calcite samples, mostly pure single crystals, was assembled from a range of ...
Mineralogical Magazine, 2008
Mineralogical Magazine, 2008
Nickel can cause allergic reactions so it is often considered to be a toxic metal. In some areas ... more Nickel can cause allergic reactions so it is often considered to be a toxic metal. In some areas of Denmark and northern Europe, where drinking water comes predominantly from chalk aquifers, Ni concentrations in groundwater are higher than safety limits. The contamination is natural in the sense that it comes from oxidation of small grains of pyrite in the chalk that contain Ni as a trace element . Because of Danish water-treatment practice, the most politically acceptable water-supply protocol is not to use secondary water treatment, so in order to minimize allergy risks for consumers, high-Ni water is either mixed with low-Ni water to bring concentrations below the Danish limit, or new, uncontaminated wells are drilled. Some recent efforts focused on finding a method to minimize nickel concentration while the water is still in the ground, before it is pumped to the primary treatment facility and distributed.
Journal of Contaminant Hydrology, 2003
A lack of information in databases for contamination risk assessment about the transport behaviou... more A lack of information in databases for contamination risk assessment about the transport behaviour of the trivalent f-orbital elements in groundwater systems where calcite is at equilibrium motivated this study of Eu(3+) uptake. The free drift technique was used to examine the effects of Eu(3+) concentration, presence of Na(+) or K(+) and temperature, as well as calcite nucleation and precipitation kinetics, on the partitioning of calcite. Changes in surface composition and morphology resulting from exposure of single crystals of Iceland spar to Eu(3+)-bearing solutions were observed with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). First results confirm that calcite has high affinity for Eu(3+). Rates of nucleation and precipitation strongly affect the extent of uptake but the presence of Na(+) and K(+) has no effect, suggesting formation of solid solution as CaCO(3)-EuOHCO(3). Surface-sensitive techniques prove that Eu(3+) is adsorbed to calcite even when the surface is dissolving and adsorption is not accompanied by precipitation of a separate Eu(3+)-solid phase. Adsorbed Eu modifies calcite's dissolution behaviour, roughening terraces and rounding step edges, and producing surface morphology where some surface sites appear blocked. Results imply that Eu(3+) concentrations in natural calcites are limited by Eu(3+) availability rather than by a lack of ability to fit into calcite's atomic structure. This behaviour can probably be expected for other trivalent rare Earth elements (REE), actinides and fission products whose behaviour is similar to that of Eu(3+). These elements are likely to be incorporated within the calcite bulk in systems where it is precipitating and the demonstrated strong partitioning ensures some uptake even where calcite is at or under saturation.
Journal of Colloid and Interface Science, 1999
Acid-base reactions and surface complexation of Sr(II) at the hematite/water interface have been ... more Acid-base reactions and surface complexation of Sr(II) at the hematite/water interface have been studied by means of potentiometric titrations at three different temperatures: 25, 50, and 75°C. Equilibrium measurements were performed in 0.1 M NaCl. In the evaluation of equilibrium models for the acid-base reactions and complexation reactions in the three-component system H ؉ O('FeOH)OSr 2؉ , the constant capacitance model was applied. During the titrations with Sr, aliquots of the suspension were sampled at in several points. The aqueous concentrations of Sr were analyzed by atomic absorption spectrometry. Treatment of data included tests for formation of both inner-sphere and outersphere complexes of different stoichiometric composition. The proposed equilibrium model consists of the following surface complexes of inner sphere type: 'FeOHSr 2؉ and 'FeOSrOH. Besides the stability constants for the surface complexes, the thermodynamic parameters ⌬H and ⌬S were evaluated. The combined effect of a decrease in pH pzc with increasing temperature and positive enthalpies of surface complex formation favors adsorption of Sr at elevated temperatures.
Geochimica et Cosmochimica Acta, 2011
The kinetics of calcite precipitation in the presence of alginate was investigated using the cons... more The kinetics of calcite precipitation in the presence of alginate was investigated using the constant composition technique. In the concentration range investigated (0.0002-0.005 g L À1 ), alginate inhibits calcite precipitation. The extent of inhibition increased with increased alginate concentration and decreased solution supersaturation. Alginate adsorption, derived from normalized calcite precipitation rates, is described satisfactorily by the Langmuir adsorption model. At lowest supersaturation, alginate adsorption onto calcite probably reaches its maximal uptake of 7.5E-4 g m À2 , corresponding to surface coverage of one molecule for each 200-300 nm 2 , depending on the molecular mass of alginate. This means that one alginate molecule can be bound over 100-150 Ca surface sites. Initially, on the surface of the inhibited calcite, XPS identified alginate but after further time in solution, when the system had recovered, XPS demonstrated that it disappeared from the surface, presumably buried under the newly formed calcite. The alginate affinity constant decreases with increasing supersaturation, evidence for incomplete adsorption. A simple model based on competition between growth and desorption effectively describes the observed change in the adsorption constant.
Geochimica et Cosmochimica Acta, 2004
ABSTRACT Partitioning of Eu(III) in calcite, CaCO3, was evaluated with the aim of collecting data... more ABSTRACT Partitioning of Eu(III) in calcite, CaCO3, was evaluated with the aim of collecting data on partition coefficients and to enhance understanding of the incorporation mechanisms. This information will aid in the interpretation of geological processes from rare Earth element (REE) data and in the use of Eu(III) as a chemical analogue for the trivalent actinides, particularly Am(III) and Cm(III). Coprecipitation experiments were carried out by the constant addition method at 25°C and PCO2 = 1 atm. Eu(III) was strongly partitioned from the solution into calcite. For dilute solid solutions (XEu < 0.001), Eu partition coefficients were estimated to be 770 ± 290 and found to be independent of calcite precipitation rate in the range of 0.02 to 2.7 nmol mg−1 min−1. This could be explained by the approximately equal values of the Eu partition and adsorption coefficients. Several solid solution models were tested. A vacancy model for Eu2(CO3)3-CaCO3 is consistent with the experimental results and constraints on geometry for Eu fit in the calcite lattice. For low Eu content, vacancy density is independent of Eu concentration in the solid so logarithm of the ion activity product, log (Eu)2(CO32−)3, depends linearly on log XEu2. The fit of the data to such a model is good evidence that Eu(III) is taken up as a true solid solution, not simply by physical trapping. A model using EuOHCO3-CaCO3 is also consistent with the uptake stoichiometry, but EuOH2+ substitution for Ca2+ would be expected to distort the calcite structure more than is compatible with such a high KD. Several other models, including EuNa(CO3)2-CaCO3, were abandoned because their stoichiometric relationships did not fit the experimental data.
This work is a continuation and extension of our studies of the kinetics of recrystallization and... more This work is a continuation and extension of our studies of the kinetics of recrystallization and precipitation of calcite in the presence of biopolymers, as well as precipitation of calcite on the surface of biogenic calcite (chalk). Our results show that the dependence of calcite precipitation rate on supersaturation does not obey the parabolic law that is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation-mediated growth. Rate of calcite precipitation on chalk surfaces at the lowest supersaturation studied in this work is 3 orders of magnitude lower than on pure calcite surfaces. The presence of non-polar organic compounds, such as biomarkers, decreases surface energy but does not change the mechanism of calcite precipitation. On the contrary, the presence of adsorbed polysaccharides and polyamino acids on the surface determines the mechanism of calcite growth and precipitation. This is confirmed by the results of measur...
Геохимия, 2019
Strontium adsorption has been studied by the method of acid-base potentiometric titrations at thr... more Strontium adsorption has been studied by the method of acid-base potentiometric titrations at three different temperatures: 25, 50, 75C. The effect of pH, ionic strength, sorbate/sorbent ratio, and temperature on adsorption was investigated. Experimental data were simulated using two various surface complexation models, with two different electrostatic descriptions of the interface: the constant capacitance model (CCM) and the triple-layer model (TLM). Although the both models used are able to account for the acid-base reactions and surface complexation of strontium on birnessite, we consider that the TLM is more applicable for a description of heterophaseous system H+ MnOH Sr2+. Under conditions of low ionic strength and negatively charged surface, Sr2+ ions compete with the electrolyte ions and form outer-sphere complexes along with inner-sphere complexes. Consequently, using the CCM for description of strontium adsorption data could be mathematically satisfactory, but physically ...
Geochemistry International, 2019
Strontium adsorption was studied by acid-base potentiometric titration at various pH, ionic stren... more Strontium adsorption was studied by acid-base potentiometric titration at various pH, ionic strength, the sorbate/sorbent ratio, and temperatures (at 25, 50, and 75°C). The experimental data were interpreted using two models of surface complexation with two different electrostatic models of the interface: the constant capacitance (CCM) and triple-layer (TLM) models. Although both models are able to take into account acid-base reactions and surface complexation of Sr on birnessite, we believe that TLM is more suitable for the description of the H +->MnOH-Sr 2+ heterogeneous system. At a low ionic strength and negatively charged surface, Sr 2+ ions compete with electrolyte ions and form both inner-and outer-sphere complexes. Although the application of CCM in describing Sr adsorption may be mathematically satisfactory, it has little physical sense. We suggest a model that involves both inner-sphere (>MnOHSr 2+ , >MnOSr + , >MnOSrOH 0) and outer-sphere ([>MnO-Sr 2+ ] +) surface complexes. The corresponding constants of formation of these surface complexes were calculated at 25, 50, and 75°C.
Crystal Growth & Design, 2018
Recrystallization requires dissolution in pore fluids and precipitation on existing particle surf... more Recrystallization requires dissolution in pore fluids and precipitation on existing particle surfaces in a process known as Ostwald ripening, where the smallest particles feed growth on the larger ones. Recrystallization conditions are optimized in industry when commercial products require larger crystals, and it is the dominant process in burial diagenesis, which turns sediments into rock. In chalk, the original coccolith elements are often still clearly distinguishable, which is a surprise considering the rapid rates of pure calcite recrystallization. We studied the rates of calcite precipitation on the surface of the particles in various chalk samples, using the constant composition method over a wide range of supersaturations. Our results show that dependence of calcite precipitation rate on supersaturation does not obey the parabolic law, which is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation-mediated growth. The rates of calcite precip...
Crystal Growth & Design, 2018
The influence of biopolymers, alginate (Alg) and polyaspartate (pAsp), on the kinetics of calcite... more The influence of biopolymers, alginate (Alg) and polyaspartate (pAsp), on the kinetics of calcite precipitation was studied using the constant composition method for supersaturation states ranging from 2.4 to 4.5, at pH = 8.5. Biopolymer presence changes the mechanism of calcite precipitation and its inhibition depending on the system history. In a system without polymers, calcite precipitates by spiral growth. Introducing polymers during growth, inhibits step movement by pinning at a number of kink sites along step edges. Polymer adsorption induces growth by two-dimensional nucleation. Both polymers inhibit calcite growth. Inhibition is stronger at higher concentration and lower solution supersaturation. The interfacial free energy, a key parameter in the control of nucleation and growth, was estimated from the analysis of the precipitation rates, as well as data obtained from vapour adsorption, are quite identical for calcite with either alginate or polyaspartate adsorbed. This is confirmed by electrokinetic measurements, which show similar ζ-potential values for calcite with each of the polymers. Increasing polymer concentration and adsorption time led to a progressive decrease of the effective interface free energy, which could explain the much lower supersaturation needed for the onset of surface nucleation.
Crystal Growth & Design, 2016
Organic molecules control calcite growth and crystal morphology, influence biomineralization proc... more Organic molecules control calcite growth and crystal morphology, influence biomineralization processes and offer clues for optimizing antiscalants for industry. Here we quantified the effect of amino acid monomers, aspartic acid (Asp 1) and glycine (Gly 1), and their polymers (Asp n , Asp 5 and Gly 5), on calcite growth rate, in a constant composition setup. Asp 1 and its polymers inhibit growth, with rate decreasing as amino acid chain length, i.e., number of carboxyl groups, increases. For example, the addition of 2 mM Asp 1 reduced growth by 54%, while it required only 0.0012 mM Asp n to reduce growth by 94%. Gly 1 and Gly 5 only marginally affect growth (<10%), indeed they slightly promote growth at most tested concentrations. Fitting of adsorption isotherms (Langmuir, Langmuir-Freundlich, Flory-Huggins) confirmed that Asp polymers adsorb strongly, explaining their strong control on calcite growth but Gly 1 and Asp 1 adsorb less due to competition with carbonate ions (∆G ads energies: Asp n =-39 kJ/mol, Asp 5 =-50 kJ/mol , Asp 1 =-21 kJ/mol and Gly 1 =-22 kJ/mol). Calcite morphology was equally affected by the amino acids. Crystal edges became rougher and corners more rounded.
Current methods for extracting oil from chalk leave behind as much as 75% of the total oil availa... more Current methods for extracting oil from chalk leave behind as much as 75% of the total oil available. It remains as small droplets in pore spaces or as thin films on particle surfaces. This oil is inaccessible at present, but if it could be extracted, the impact on global ...
Acta Physica Hungarica, 1989
One-dimensional diffusion in a membrane is studied by solution of the according Euler-Lagrange eq... more One-dimensional diffusion in a membrane is studied by solution of the according Euler-Lagrange equations derived from Gyarmati's governing principle of dissipative processes. In the linear case the exact solution is found for ternary ideal systems (dilute ideal solution in contact with the membrane). In the quasilinear case of a binary ideal system solutions are searched by Stark's dual field methods.
Journal of Contaminant Hydrology, 2014
Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption ... more Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption studies. The goal was to understand the difference in sorption behavior between synthetic and biogenic calcite. Experiments at atmospheric partial pressure of CO2, in solutions equilibrated with calcite and chalk and pH ranging from 7.7 to 8.8, explored the influence of initial concentration and the amount and type of sorbent on Ni uptake. Adsorption increases with increased surface area and pH. A surface complexation model describes the data well. Stability constants for the Ni surface complex are log KNi=-1.12 on calcite and log KNi=-0.43 and -0.50 on the two chalk samples. The study confirms that synthetic calcite and chalk both take up nickel, but Ni binds more strongly on the biogenic calcite than on inorganically precipitated, synthetic powder, because of the presence of trace amounts of polysaccharides and clay nanoparticles on the chalk surface.
The acid/base properties of the hematite/water interface and surface complexation with gold has b... more The acid/base properties of the hematite/water interface and surface complexation with gold has been studied by potentiometric titrations in the four component system H + -≡FeOH -AuCl − 4 -Cl − . Equilibrium measurements were performed in NaCl media at 298.2 K. In the evaluation of equilibrium model from experimental data the constant capacitance model was applied. The acid/base properties were investigated in 0.1 M NaCl in the range 2.6 ≤ pH ≤ 7.4. The resulting intrinsic constants for protonation and deprotonation of hydroxyl groups at the surface were log β s 1,1,0,0(int) = 7.10 ± 0.06 and log β s −1,1,0,0(int) = −7.80 ± 0.06. The density of proton active surface sites was 2.85 nm −2 and the specific capacitance 2.5 C V −1 m −2 . In the investigation of surface complexation of gold, the potentiometric titrations were performed in the range 2.0 ≤ pH ≤ 10.2. Titration data was supplemented with analysis of Au in the aqueous phase by atomic absorbance spectrometry. The equilibrium model proposed consists of the following monodentate surface complexes: ≡FeOHAuCl 3 (log β s 0,1,1,−1 = 1.45 ± 0.03), ≡FeOHAuCl 2 OH (log β s −1,1,1,−2 = −3.89 ± 0.02), ≡FeOHAu(OH) 3 (log β s −3,1,1,−4 = −21.94 ± 0.05). A slightly better fit could be obtained by assuming formation of a bidentate complex with the composition (≡FeO) 2 Au(OH)H 2 O. However, based on structural arguments this complex was rejected.
Crystal Growth & Design, 2015
ABSTRACT Recrystallization is one of the dominant processes active in early diagenesis. There is ... more ABSTRACT Recrystallization is one of the dominant processes active in early diagenesis. There is ample evidence in the literature that polysaccharides significantly inhibit growth of calcium carbonate, but little is known about their effect on recrystallization at low degrees of supersaturation. In this study, we monitored recrystallization in calcite saturated 0.1 M NaCl solutions using H14CO3- as a tracer. Alginate inhibits calcite recrystallization as expected, but for the same alginate concentrations inhibition is weaker than for growth. We explain these results by extending the interpretation of data from precipitation experiments and by understanding that alginate acts in two opposite directions. On the one hand, alginate pins steps, effectively inhibiting calcite growth. On the other hand, alginate promotes heterogeneous calcite nucleation and subsequent growth, decreasing the interface free energy and serving as an effective template for calcite nucleation and growth. To get closer to an assessment of the effective free surface energy, gamma', in this study, we measured the energy of the alginate-water interface, gamma(aw) by using vapor adsorption. The values of interfacial surface tension between pure calcite and water, gamma(cw), and calcite with adsorbed alginate and water, gamma(aw), were found to be equal to 76 and 115 mJ m(-2). gamma(aw) > gamma(cw), which contributes to a lower energy barrier for calcite nucleation on adsorbed alginate. The high value of gamma(aw) testifies that alginate molecules prefer to create new calcite-alginate interfaces than to maintain alginate-water interfaces. In growth experiments, we take into account only inhibition because inhibition and subsequent nucleation are separated by specific induction times. For recrystallization, the time scale for induction is much longer than for growth, and the two oppositely directed processes act almost together and simultaneously, resulting in weaker overall inhibition than for simple calcite growth alone.
Radiochimica Acta, 2000
Secondary minerals have the potential to sequester escaped actinides in the event of a radioactiv... more Secondary minerals have the potential to sequester escaped actinides in the event of a radioactive waste repository failure, but currently, data to define their maximum uptake capacity are generally lacking. To estimate a maximum limit for solid solution in calcite, we took advantage of the behavioural similarities of the 4 f-orbital lanthanides with some of the 5 f-orbital actinides and used rare Earth element (REE) concentration as an analogue. A suite of 65 calcite samples, mostly pure single crystals, was assembled from a range of ...
Mineralogical Magazine, 2008
Mineralogical Magazine, 2008
Nickel can cause allergic reactions so it is often considered to be a toxic metal. In some areas ... more Nickel can cause allergic reactions so it is often considered to be a toxic metal. In some areas of Denmark and northern Europe, where drinking water comes predominantly from chalk aquifers, Ni concentrations in groundwater are higher than safety limits. The contamination is natural in the sense that it comes from oxidation of small grains of pyrite in the chalk that contain Ni as a trace element . Because of Danish water-treatment practice, the most politically acceptable water-supply protocol is not to use secondary water treatment, so in order to minimize allergy risks for consumers, high-Ni water is either mixed with low-Ni water to bring concentrations below the Danish limit, or new, uncontaminated wells are drilled. Some recent efforts focused on finding a method to minimize nickel concentration while the water is still in the ground, before it is pumped to the primary treatment facility and distributed.
Journal of Contaminant Hydrology, 2003
A lack of information in databases for contamination risk assessment about the transport behaviou... more A lack of information in databases for contamination risk assessment about the transport behaviour of the trivalent f-orbital elements in groundwater systems where calcite is at equilibrium motivated this study of Eu(3+) uptake. The free drift technique was used to examine the effects of Eu(3+) concentration, presence of Na(+) or K(+) and temperature, as well as calcite nucleation and precipitation kinetics, on the partitioning of calcite. Changes in surface composition and morphology resulting from exposure of single crystals of Iceland spar to Eu(3+)-bearing solutions were observed with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). First results confirm that calcite has high affinity for Eu(3+). Rates of nucleation and precipitation strongly affect the extent of uptake but the presence of Na(+) and K(+) has no effect, suggesting formation of solid solution as CaCO(3)-EuOHCO(3). Surface-sensitive techniques prove that Eu(3+) is adsorbed to calcite even when the surface is dissolving and adsorption is not accompanied by precipitation of a separate Eu(3+)-solid phase. Adsorbed Eu modifies calcite's dissolution behaviour, roughening terraces and rounding step edges, and producing surface morphology where some surface sites appear blocked. Results imply that Eu(3+) concentrations in natural calcites are limited by Eu(3+) availability rather than by a lack of ability to fit into calcite's atomic structure. This behaviour can probably be expected for other trivalent rare Earth elements (REE), actinides and fission products whose behaviour is similar to that of Eu(3+). These elements are likely to be incorporated within the calcite bulk in systems where it is precipitating and the demonstrated strong partitioning ensures some uptake even where calcite is at or under saturation.
Journal of Colloid and Interface Science, 1999
Acid-base reactions and surface complexation of Sr(II) at the hematite/water interface have been ... more Acid-base reactions and surface complexation of Sr(II) at the hematite/water interface have been studied by means of potentiometric titrations at three different temperatures: 25, 50, and 75°C. Equilibrium measurements were performed in 0.1 M NaCl. In the evaluation of equilibrium models for the acid-base reactions and complexation reactions in the three-component system H ؉ O('FeOH)OSr 2؉ , the constant capacitance model was applied. During the titrations with Sr, aliquots of the suspension were sampled at in several points. The aqueous concentrations of Sr were analyzed by atomic absorption spectrometry. Treatment of data included tests for formation of both inner-sphere and outersphere complexes of different stoichiometric composition. The proposed equilibrium model consists of the following surface complexes of inner sphere type: 'FeOHSr 2؉ and 'FeOSrOH. Besides the stability constants for the surface complexes, the thermodynamic parameters ⌬H and ⌬S were evaluated. The combined effect of a decrease in pH pzc with increasing temperature and positive enthalpies of surface complex formation favors adsorption of Sr at elevated temperatures.
Geochimica et Cosmochimica Acta, 2011
The kinetics of calcite precipitation in the presence of alginate was investigated using the cons... more The kinetics of calcite precipitation in the presence of alginate was investigated using the constant composition technique. In the concentration range investigated (0.0002-0.005 g L À1 ), alginate inhibits calcite precipitation. The extent of inhibition increased with increased alginate concentration and decreased solution supersaturation. Alginate adsorption, derived from normalized calcite precipitation rates, is described satisfactorily by the Langmuir adsorption model. At lowest supersaturation, alginate adsorption onto calcite probably reaches its maximal uptake of 7.5E-4 g m À2 , corresponding to surface coverage of one molecule for each 200-300 nm 2 , depending on the molecular mass of alginate. This means that one alginate molecule can be bound over 100-150 Ca surface sites. Initially, on the surface of the inhibited calcite, XPS identified alginate but after further time in solution, when the system had recovered, XPS demonstrated that it disappeared from the surface, presumably buried under the newly formed calcite. The alginate affinity constant decreases with increasing supersaturation, evidence for incomplete adsorption. A simple model based on competition between growth and desorption effectively describes the observed change in the adsorption constant.
Geochimica et Cosmochimica Acta, 2004
ABSTRACT Partitioning of Eu(III) in calcite, CaCO3, was evaluated with the aim of collecting data... more ABSTRACT Partitioning of Eu(III) in calcite, CaCO3, was evaluated with the aim of collecting data on partition coefficients and to enhance understanding of the incorporation mechanisms. This information will aid in the interpretation of geological processes from rare Earth element (REE) data and in the use of Eu(III) as a chemical analogue for the trivalent actinides, particularly Am(III) and Cm(III). Coprecipitation experiments were carried out by the constant addition method at 25°C and PCO2 = 1 atm. Eu(III) was strongly partitioned from the solution into calcite. For dilute solid solutions (XEu < 0.001), Eu partition coefficients were estimated to be 770 ± 290 and found to be independent of calcite precipitation rate in the range of 0.02 to 2.7 nmol mg−1 min−1. This could be explained by the approximately equal values of the Eu partition and adsorption coefficients. Several solid solution models were tested. A vacancy model for Eu2(CO3)3-CaCO3 is consistent with the experimental results and constraints on geometry for Eu fit in the calcite lattice. For low Eu content, vacancy density is independent of Eu concentration in the solid so logarithm of the ion activity product, log (Eu)2(CO32−)3, depends linearly on log XEu2. The fit of the data to such a model is good evidence that Eu(III) is taken up as a true solid solution, not simply by physical trapping. A model using EuOHCO3-CaCO3 is also consistent with the uptake stoichiometry, but EuOH2+ substitution for Ca2+ would be expected to distort the calcite structure more than is compatible with such a high KD. Several other models, including EuNa(CO3)2-CaCO3, were abandoned because their stoichiometric relationships did not fit the experimental data.