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Biopolymer control on calcite precipitation O. N. Karaseva, L. Z. Lakshtanov, D. V. Okhrimenko, D. A. Belova, J. Generosi, and S. L. S. Stipp Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00096 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018
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Biopolymer control on calcite precipitation O.N. Karaseva2, L.Z. Lakshtanov,1,2,*, D.V. Okhrimenko1, D.A. Belova1, J. Generosi1, and S.L.S. Stipp 1 1
Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark 2 Institute of Experimental Mineralogy RAS, 142432 Chernogolovka, Russia
Corresponding author *
E-mail:
[email protected] Abstract 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.
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Introduction The kinetics of growth, precipitation and nucleation of calcite is of fundamental importance for understanding many natural and anthropogenic geochemical systems. It has long been known that at near Earth surface conditions, calcite often has a major impact on the chemistry and evolution of such diverse processes as diagenesis of modern marine sediments,1 the degradation of arid agricultural soils2 and the effectiveness of water treatment methods for the removal of trace metals3 but the precise mechanisms that enhance or inhibit calcite growth and the growth of minerals in general, are still not well enough understood to allow reliable prediction. Inhibitors of calcite precipitation are of interest in a number of fields, ranging from engineering sciences to medicine, biomineralisation and geochemistry. Inhibition could be primarily of importance for the extremely slow rates of recrystallization in chalk, which can be composed of more than 90% calcite. The preservation of large quantities of microfossils suggests that something inherent in the chalk dramatically decreases the rate of recrystallization.4 In particular, there is evidence that biopolymers, particularly polysaccharides and polypeptides, significantly inhibit crystallization of calcium carbonate.510
Highly carboxylated proteins and polysaccharides are major components of the organic
matrices associated with calcification in many organisms. The mechanism of calcite growth in the presence of biopolymers is therefore important in mineralogy, biology, environmental geochemistry, crystal growth and materials science. Many studies have investigated the influence of additives on the crystallization of calcium carbonate.8,9,11-13 Interaction of biogenic species, including polypeptides and polysaccharides, with the calcite surface has been observed to affect dissolution and precipitation rates.14,15 It has been demonstrated in many studies that the electron donating moieties of these biopolymers are responsible for chelating aqueous Ca2+ cations.16 Moreover, they specifically
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interact with crystallographic features of calcite15,17 and can affect the kinetics of calcite growth and nucleation.15 It is generally accepted, that the strong inhibitory effect of polymers on precipitation kinetics is related to their preferential adsorption on mineral surfaces. On one hand, adsorption should lead to a decrease of the interface free energy and, as a result, to increased nucleation rate. On the other hand, each adsorbed particle can impede attachment of lattice ions during growth as well as can be an obstacle for the advancing step (the “steppinning” mechanism18), which retards crystal growth. The presence of impurities can lead to different mechanisms for calcite precipitation and its inhibition depending on the system history. When an impurity is introduced into the system during crystal growth, it can inhibit step movement by pinning at a number of kink sites along the step edge.19 A calcite surface, wetted in the presence of an inhibitor, activates growth by the formation of surface nuclei.19 Clearly, this process has received insufficient attention, in spite of recognition that the calcite surface history exerts an important control on surface properties20 and inhibition mechanisms by biopolymers, that are operative in natural systems. This paper reports on our continued efforts to explore the interaction between biopolymers and calcite. To refine our understanding of the inhibitory mechanisms of biopolymer-calcite interaction, we investigated the effects of alginate and polyaspartate on calcite precipitation, using the constant composition method.8,10,12,13,21,22 Alginate and polyaspartate were selected as our model biopolymers because of their demonstrated presence in nature and their well characterized chemistry. Our experimental efforts focused on the influence of the calcite surface history on the inhibitory mechanism, in other words, biopolymer was introduced to the system either during the growth of calcite or calcite growth was initiated in the presence of adsorbed biopolymer. For further insight into the interaction of the biopolymers with the calcite surface, we also determined the ζ potential of calcite samples that had been overgrown
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in the presence of the biopolymers. The surface free energy is a key parameter that controls nucleation and growth, and in the case of the three phase contact (calcite-biopolymer-aqueous solution), surface free energy is a function of several contributing aspects. To get closer to an assessment of the effective free surface energy in this study, we measured the interface energy at the calcite-biopolymer-solution interface using vapour adsorption and compared it with that obtained from the analysis of the precipitation rates.
Materials and Methods Materials Reagent grade calcite powder, supplied by Sigma-Aldrich, was used for all experiments. The powder was recrystallised through several treatments of deionised water saturated with CO2, by a method modified from Stipp and Hochella,23 to homogenize particle size and form. This resulted in calcite crystal dimensions of ~10 µm and surface area of 0.2 m2 g-1. All chemicals were reagent grade or better, purchased from Sigma-Aldrich and Merck P.A. and all solutions were made with MilliQ deionized water (resistivity 18.2 MΩ cm, Millipore water purification system). In the precipitation experiments, we used freshly made solutions of 0.1 M CaCl2, 0.1 M Na2CO3 and 0.1 M NaHCO3 with concentrations of Ca2+, CO32- and HCO3- verified by atomic absorption spectroscopy (Perkin Elmer AAS Analyst 800) and titration (Metrohm 809 Titrano). Two biopolymer stock solutions were prepared using ultrapure deionised water and alginic acid sodium salt (Alg, 0.5 g L-1) and poly-(α,β)-DLaspartic acid sodium salt (pAsp, 68 mg L-1). Calcite samples with adsorbed polymers were prepared for precipitation experiments as well as for vapour adsorption measurements. 2 g of calcite powder was placed into 50 mL of calcite saturated solution at pH 8.5 (adjusted by NaHCO3 and Na2CO3) and the desired
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amount of polymer. After the specific time for adsorption (2 or 24 h), the calcite-solution mixture was filtered with a vacuum filter, rinsed with calcite saturated solution and freezedried.
Precipitation experiments Calcite precipitation rates were studied using the constant-composition method (pH-start titration system), described in detail elsewhere.8,10,13,24,25 The experiments were made in a double-walled glass reaction vessel with an overhead propeller (Metrohm 802), where temperature was maintained at 25 ⁰C ± 1 ⁰C. The supersaturated working solution, with total volume of 50 mL, was prepared prior to each experiment by mixing various aliquots of the 0.1 M CaCl2 and 0.1 M NaHCO3 solutions with 0.1 M NaCl solution, to reach the desired supersaturation state with respect to calcite. Solution pH was adjusted to 8.5 with few drops of 0.1 M NaOH. Air space above the working solution in the reaction vessel was kept to a minimum to maintain constant CO2 partial pressure. This was verified by constant pH in the supersaturated solution for 1 hour prior the experiment. An experiment began when 0.15 g of treated calcite powder was added to the the working solution in the reaction vessel, initiating precipitation. Calcite precipitation resulted in solution pH drop, which was immediately compensated by the addition of CaCl2 and Na2CO3 / NaHCO3 titrant solutions (with twice the concentration of the carbonate in the supersaturated solution) using a peristaltic pump (Ismatech) to maintain steady state conditions in the vessel. We used NaCl solution in all experiments to maintain the ionic strength at the desired value. The added volume of titrant solution was continuously recorded during the entire experiment and used to calculate calcite precipitation rate. As verification of
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constant composition, random sampling and analysis for Ca2+ verified that solution composition and the supersaturation degree remained constant within ±5%. Supersaturation, S, was defined as 1/ 2
IAP S= K sp
aCa 2+ aCO32 − = K sp
1/ 2
,
(1)
where IAP stands for the ion activity product; aion represents the ion activity and Ksp represents the thermodynamic solubility product of calcite. We used the PHREEQC26 geochemical speciation code to calculate saturation index with respect to calcite and solution speciation. The overall precipitation rate, R, is determined from:
R=
[Ca ]titr dV [Ca ]titr R′ , = mA dt mA
(2)
where R′ represents the solution addition rate; [Ca]titr stands for the Ca concentration in the titrant CaCl2 solution, m - the initial mass of the seed crystals; A - the specific surface area and V - the volume of the titrant added. To study the effect of the surface history on calcite growth inhibition, precipitation experiments were carried out in two ways. In Case 1, biopolymer was injected into the reaction vessel after 10-15 min of calcite growth in the pure system, when calcite precipitation rate had become approximately constant. Several samples made in experiments with different polymer loadings were checked using X-ray diffraction to verify that calcite was the only phase being produced. Calculations with expected intensities for other CaCO3 phases indicated that within the 1% detection limit, no crystalline materials other than calcite were found. In Case 2, calcite seed was exposed to a polymer solution prior to the precipitation experiment. Calcite powder (0.15 g) was first placed into 50 mL of calcite saturated solution with the desired amount of biopolymer. After a specific time of interaction (2 or 24 h), the
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supernatant was removed and calcite seeds were gently washed with a calcite saturated solution and immediately used in a precipitation experiment.
Solid characterization Solid samples were collected at several stages during the precipitation experiments and analyzed with atomic force microscopy (AFM) and scanning electron microscopy (SEM) to monitor the change in calcite surface topography and crystal morphology. At the desired time, a small amount (~3 mL) of suspension was removed from the reaction vessel, filtered with a vacuum filter and quickly rinsed with calcite saturated solution. Immediately after, a part of the calcite powder was left to dry at room temperature for further SEM analysis, while another part was resuspended in CaCO3 saturated solution. A droplet of the suspension was quickly put on a clean, stainless steel substrate, covered by ozone cleaned aluminium foil, and the sample solution was swept from the sample with a gentle nitrogen stream and examined with AFM within the same day. To study surface topography, AFM images were taken in air, in noncontact mode with a MFP-3D AFM from Asylum Research, Santa Barbara, USA. Commercially available silicon cantilevers (Olympus, Japan) with nominal spring constant of 2 N m-1 were used. We collected images from flat surfaces of many individual calcite crystals to confirm reproducibility and representability of experimental data. The images were treated using the AFM software codes, Igor Pro and Gwyddion. SEM images were used to investigate calcite crystal morphology. The images were taken with a Quanta 3D FEG 200/600 SEM under high vacuum, with an accelerating voltage of 2 kV and a beam current of 8.53 pA.
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Vapour adsorption Surface energy was determined for pure calcite and for the calcite samples with adsorbed Alg or pAsp using vapour adsorption and the approach described by Schlangen et al.27 In this method, the surface pressure, πSV, of the adsorbed vapour film can be found by integrating the adsorption isotherm from low pressure to saturation. Close to saturation, at the relative pressure, P/P0 = 1, the adsorbed amount of vapour, Г, is equal to Г∞, which can be represented as the microscopically thin film of adsorbatewhich properties are similar to the bulk liquid. In this case the surface pressure is equal to the work of spreading, Ws: ߨ ௌ ሺГஶ ሻ = ܹௌ = ߛ ௌ − ሺߛ ௌ + ߛ ሻ ,
(3)
where γL and γS stand for the surface tension for the liquid and the solid and γSL, represents the interfacial tension between solid and liquid. Based on πSV (Гஶ ), tone can calculate the work of adhesion, WA, which is the work of separation the solid-liquid interface: ܹ = ߛ ௌ + ߛ − ߛ ௌ = 2ߛ + ߨ ௌ ሺГஶ ሻ .
(4)
Using Owens and Wendt approach28 the work of adhesion can be represented as a sum of geometric mean of the dispersive and polar components:
WA = 2(γ Sdγ Ld )1/2 + 2(γ Spγ Lp )1/2 ,
(5)
where γSd and γSp represent surface energy dispersive and polar components of the solid and γLd and γLp, stand for the surface energy dispersive and polar components of the liquid.
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In this work we used water (MQ) and ethanol (99.5% purity, ≤ 0.1% water) as the vapour probes, as described earlier.29 For water, γLd = 21.8 mJ m-2 and γLp = 51.0 mJ m-2; for ethanol, γLd = 18.8 mJ m-2 and γLp = 2.6 mJ m-2.30 When the work of adhesion for minimum two liquids on a solid is known, the surface energy dispersive and polar components of the of that solid can be calculated. The total solid surface energy, γSt, is then calculated as the sum of the dispersive and polar components. Vapour adsorption isotherms were recorded using a Quantachrome Autosorb-1 Sorption Analyser at 20 °C, over the interval 1·10-3 ≤ P/P0 ≤ 0.9. Prior for the vapour adsorption measurements, the samples were degassed for 24 hours at room temperature in vacuum ( 8,5, both the calcite surface and the deprotonated carboxylic groups of the polymers are negatively charged, and the polymers adsorb independently of their charge, the electrokinetic measurements offer evidence of the specific, coordinative interaction between the carboxylic groups of both biopolymers and the calcite surface. Such specific adsorption is the main reason for strong inhibition of calcite precipitation by these organic substances. Moreover, at pH > 8 where the side chain carboxylic groups of both polymers can be considered completely dissociated, the ζ potential of the calcite surface with adsorbed Alg or with adsorbed pAsp becomes identical (Fig. 10). This could mean an identical adsorption density of both polymers as well as a very similar number of surface ionogenic groups exposed to the solution. Thus, results of the electrokinetic measurements are consistent with our findings of similar effects of Alg and pAsp on calcite precipitation.
Interfacial free energy determined by vapour adsorption measurements
The results of the determination of surface energy, γs, and the solid-water interfacial surface energy, γsw, are given in Table 3. The relative uncertainty in estimating the amount of adsorbed vapour, and thus the work of wetting, is 2%. For the pure calcite sample, the determined total surface energy, γs, is in good agreement with our previous findings29 but
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higher than reported in the literature60 for a similar system, 170 mJ m-2. Interfacial solidwater surface energy, γsw, was obtained from modified Eq. 2:
γୗ = γୗ + γ -W.
(23)
We estimated the calcite-water interfacial surface energy to be 129.3 mJ m-2, which is close to that found by Söhnel and Mullin61 (120 mJ m-2). It should be noted that the vapour adsorption method for determining surface energy can be affected by the choice of the probe vapour. Thus the results for total surface energy and the distribution of its polar and dispersive components depends on the choice of vapour.
Table 3. Surface energies for calcite and calcite with adsorbed Alg and pAsp.
Sample Pure calcite
Calcite + Alg
Calcite + pAsp
Conc., g L-1 0.013 0.1 0.17 0.013 0.1 0.1 0.17 0.17 0.0043 0.0087 0.025 0.0435 0.0435 0.0043 0.0087 0.025 0.0435 0.87
Time of adsorption days 2 2 2 24 24 24 24 24 2 2 2 2 2 24 24 24 24 24
Surface energy ys, mJ m-2 dispersive
polar
total
Interface energy ysw, mJ m-2
2.9 81.5 12.3 76.9 53.0 91.0 90.0 39.3 18.3 33.0 49.7 36.0 31.1 36.8 18.9 25.6 56.0 93.9 1.8
328.3 211.9 331.6 204.3 261.8 199.2 246.3 262.4 327.1 260.3 176.7 218.3 236.0 285.8 234.8 159.9 164.5 102.4 947.5
331.2 293.4 343.9 281.3 314.8 290.2 336.3 301.8 345.4 293.3 226.4 254.2 267.1 322.7 253.7 185.6 220.4 196.3 949.4
129.3 74.0 123.9 68.0 88.5 72.3 96.3 84.6 119.9 82.0 43.5 60.1 68.4 97.3 67.1 30.5 40.2 34.1 569.8
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When polymer was adsorbed on calcite, we observed a decrease of the total surface energy and a change in the distribution of the dispersive and polar components. The polar component decreased, while the dispersive component grew. The effect was more pronounced if the polymer was allowed to adsorb for a longer time or from a higher polymer concentration. Decrease of the interfacial free energy can be seen in Fig. 11. The effect is more pronounced for pAsp than for Alg and at higher adsorption times. For example interfacial solid-water surface free energy drops from 129.3 mJ m-2 for pure calcite to 30.5 mJ m-2 when pAsp (8.7 mg L-1) was adsorbed for 24 hours. However, at even much higher concentration of polyaspartic acid (870 mg L-1), the surface energy was extremely high, which resulted from high water uptake by the sample. This phenomenon can be explained by the fact that at high concentration of adsorbed pAsp on calcite, full coverage was achieved and the calcite-water interface no longer existed. In this case, we observed only the interaction of water molecules with pAsp, which is known for forming hydrogels for retaining water62 and high water uptake leads to incorrect surface energy values.
a
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b
Fig. 11. The effective interface free energy, γeff, for calcite with adsorbed polymers, a) Alg and b) pAsp (obtained by several methods) vs. polymer concentration: triangles, from precipitation kinetics; squares, from vapour adsorption measurements (24 hours polymer adsorption); rhombi, from vapour adsorption measurements (2 hours polymer adsorption); cross, from vapour adsorption measurements (pure calcite); filled square, theoretical value, γcw, for a pure calcite-water interface.52
The effective interface free energy derived from vapour adsorption measurements differs from that obtained from calcite precipitation kinetics data. We recall from the discussion above, that the effective interface free energy, γeff, for calcite nucleation on a polymer, consists of two parts. The first part is the surface free energy of the calcite-water interface (γcw), for the surface regions that are free of the adsorbed polymer. The second part is the free energy of the three phase interface, i.e. calcite-polymer-water, γ’. In the method of vapour adsorption, these two terms in Eq. 18 or 21 remain unchanged but an additional term associated with the energetics of the polymer-water interface, γpw, should be included because water (or alcohol, as the probe vapour) also adsorbs directly on the polymer chains. At low surface coverage, the polymer can adsorb as straight, fully extended chains, where carboxyl groups interact with surface calcium ions. The chain backbone is quite rigid, indicating that most of the segments attach to the surface and are not free to rotate.63 Polymer molecules spread and arrange themselves over the surface in flat, facedown positions, leaving only a small fraction of free functional groups.
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With increasing surface coverage, some parts of the chain attach flat on the surface but some extend into the solution in loops or clusters, where water is adsorbed (such as for polyacrylic acid20 and the so called “train-loop” model64). Moreover, some chains can be anchored on the surface at its carboxyl terminals, exposing more hydrophobic parts to the solution and serving as anchors for other organic molecules65. From general considerations, the number of chain parts, that are not attached to the surface, should increase nonlinearly with increasing surface coverage, i.e. greatly enhanced when there is little free space left on the surface. At relatively low surface coverage, the hydrophilic carboxyl groups of Alg and pAsp react with the calcite surface, exposing more of the hydrophobic parts of the polymer molecule on the surface for interacting with water. Thus, in the vapour adsorption method, with increasing polymer concentration and decreasing adsorption time, the polymer-water interface is more and more affected and therefore the effective interface free energy increases, tending in the limit to the value of γpw. This pattern can be observed in Fig. 11. With increasing polymer concentration, γeff first decreases, in accordance with Eq. 21, and then increases because of the predominance of the polymer-water interface. This is the main difference from data obtained by precipitation kinetics, where the effective interface free energy, γeff, changes from γcw for pure calcite to γ’ for the complete surface coverage, θ = 1. Based on literature data,10,66 one can suppose that for Alg, this condition is surely fulfilled for C > 0.005 g L-1, whereas for pAsp, it is filled for C > 0.01 g L-1. Therefore, data at higher concentrations obtained with the vapour adsorption method correspond solely with the free energy for the polymer-water interface γpw. Considering this, it can be argued that the method of vapour adsorption gives reasonable values for the interfacial free energy, γ, which are quite similar to those obtained from the analysis of calcite precipitation kinetics in the presence of biopolymers.
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Concluding remarks
The interaction of biopolymers with minerals is a distinctive characteristic of biogeochemical systems.The presence of biopolymers results in a change of mechanism for calcite precipitation and its inhibition, depending on the system history. Calcite growth without polymers proceeds by a second order surface reaction mechanism. When polymer is introduced to the system during the growth, it inhibits step movement by pinning at a number of kink sites along the step length. A calcite surface, wetted in the presence of an organic polymer, activates growth controlled by two-dimensional nucleation. Both alginate (Alg) and polyaspartate (pAsp) inhibit calcite growth, more strongly as their concentration increases and as solution supersaturation decreases. When alginate is introduced to a system during layer growth on calcite that was previously lacking in organic compounds (Case 1), inhibition is more effective than when calcite begins to grow on samples that had been exposed to alginate (Case 2) over the entire range of supersaturation studied. Opposite was observed for the behaviour of polyaspartate. Only when S > 4, the effectiveness of inhibition in Case 1 becomes higher. This has important implications in biomineralisation. The sequence of providing organic molecules on a growing mineral surface gives organisms another button to turn for engineering the size and shape of their biominerals, to fit them for their purpose. It also gives new clues for synthesis of biomimetic materials. The interfacial free energy, a key parameter in controlling nucleation and growth, that was estimated from analysis of the precipitation kinetics data as well as from vapour adsorption data are quite identical for both alginate and polyaspartate. These are indirectly confirmed by the electrokinetic measurements, which show similar ζ potential values for calcite with each of the polymers adsorbed. Increasing polymer concentration and adsorption time led to a
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progressive decrease of γeff, which might result in much lower supersaturation values to be required for the onset of surface nucleation.
ACKNOWLEDGMENTS We thank Keld West for helping to build the constant composition set up. The project was funded by Maersk Oil and Gas A/S and the Danish Advanced Technology Foundation (HTF) through Nano-Chalk, by a Frame Grant from the UK EPSRC (Engineering and Physical Sciences Research Council) called MIB and by the Russian Foundation for Basic Research (16-05-00234).
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References (1) Bathurst, R. G. C. Carbonate Sediments and Their Diagenesis; 2nd Edition, Elsevier, 1975. (2) Buol, S. W.; Hole, F. D.; McCracken, R. J. Soil Genesis and Classification; Iowa State University Press, Ames., 1973. (3) Reddy, M. M. Kinetic inhibition of calcium carbonate formation by wastewater constituents. In Chemistry of Wastewater Technology; Rubin, A.J., Ed .; Ann Arbor Science, 1978; pp 3l-58. (4) Belova, D.A.; Johnsson, A.; Bovet, N.; Lakshtanov, L.Z.; Stipp, S.L.S. The effect on chalk recrystallization after treatment with oxidizing agents. Chem. Geol. 2012, 291, 217– 223. (5) Gunthorpe, M. E.; Sikes, C. S. Potent natural inhibitors of CaCO3. Crystallization from chalk deposits. Ohio J. Sci. 1986, 86, 106–110.
(6) Welch, S.A.; Barker, W.W.; Banfield, J.F. Microbial extracellular polysaccharides and plagioclase dissolution. Geochim. Cosmochim. Acta 1999, 63, 1405–1419.
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Biopolymer control on calcite precipitation O.N. Karaseva, L.Z. Lakshtanov,, D.V. Okhrimenko, D.A. Belova, J. Generosi, and S.L.S. Stipp.
Synopsis 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.
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