Calcium Carbonate Nucleation Investigated in a Double Pulse

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Calcium carbonate nucleation investigated in Double Pulse experiment Barbara M. Liszka, R. Martijn Wagterveld, Geert -Jan Witkamp, and Otto Cees Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00058 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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Crystal Growth & Design

Calcium carbonate nucleation investigated in a Double Pulse experiment Barbara M. Liszka1,2, R. Martijn Wagterveld 2, Geert-Jan Witkamp3, Cees Otto1 1

Department of Medical Cell Biophysics, MIRA institute, University of Twente, Enschede, The

Netherlands 2

Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9,

8911 MA Leeuwarden, The Netherlands 3

Delft University of Technology Biotechnology Department, Delft, The Netherlands

KEYWORDS Nucleation calcium carbonate, Double Pulse experiment, growth rate

ABSTRACT The nucleation rate is essential in a number of research fields in order to control crystal formation. The purpose of this study is to test and optimize Double Pulse procedure as a method to investigate nucleation of calcium carbonate The induction time, interpreted as time of formation of postcritical nuclei was used to separate a stage in which nucleation is a main process from a stage in which formed nuclei mainly grow. . The induction time was defined for a model mineralization solution by recording the pH profile of the supersaturated solution representing the desaturation curve. In the double pulse procedure nucleation was quenched during the induction time at several time points and existing nuclei could grow until a size detectable by Scanning Electron Microscopy. It was observed, under applied super saturation

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conditions S= 4, that postcritical nuclei formed directly when the saturation level of the solution was achieved. It is proposed here that that the growth of crystals occurs due to the agglomeration of nuclei. Introduction

Precipitation of calcium carbonate (CaCO3) is of evident importance in a variety of fields. In nature, precipitation of CaCO3 occurs as bio-mineralization1 during which e.g. shells and mollusks are formed. Precipitation of minerals is also a crucial process in industrial production such as the synthesis of drugs 2 or additives for paints and rubbers.3,4 The precipitation of CaCO3 is also an undesired process, when crystalline scale forms in heat exchangers, installations for water purification or oil and gas recovery.

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Understanding the mechanisms underlying the

formation of early solid particles in solution is of considerable importance in order to predict, design and control crystal formation. There are well established nucleation

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and crystal

growth theories 11 available, but the actual nucleation mechanism is not fully described. The classical nucleation theory (CNT)

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assumes that nuclei form in a homogenous

supersaturated solution as a result of stochastic collisions of the dissolved constituents. A free energy change (∆ G) is required for the formation of nuclei. The critical size of a nucleus is the point at which the surface free energy reduction is compensated by the volume free energy increase. The newly formed crystalline lattice can either grow or re-dissolve, but the process should decrease the free energy of the particle. This means that the critical size is the minimum size required for a postcritical particle.12 Based on this theory the nucleation rate, , can be calculated from equation 1 as follows: = (− / ) exp (−∆ / ) (1)


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Where (− / ) is related to the kinetic barrier, activation energy, is Boltzmann

constant, is absolute temperature, ( −∆ / ) is the thermodynamic barrier.

The kinetic barrier is usually neglected and energetic excess ∆ can be expressed in terms of the affinity, ∅:

∅ =

" (2) !

Where IAP represents the ion activity, ! is the bulk solubility product of the nucleating phase,

and / ! represents the saturation ratio.

The previous equations enable calculation of the nucleation rate at a given saturation level. Nevertheless, due to limited abilities of CNT to describe “real” systems, the calculated value can differ from the experimentally obtained data.13 The limitations rise from the following assumptions: (1) The growth of nuclei is due to addition of one monomer at the time: collision between existing nuclei or particle dissolution due to size-dependent stability are ignored. (2) The bulk values of ! might not be applicable at the level of nuclei as the final crystal could be formed through a gradual conversion of thermodynamic unfavorable polymorphs into more favorable ones.12 (3) The kinetic part of the nucleation barrier is neglected in CNT as it is difficult to quantify (see equation 1) In recent years, attempts have been made to modify the CNT based on experimental observation. Empirical data have provided evidence of the occurrence, called pre-nucleation clusters (PNCs)14 15

, for several bio-minerals such as calcium carbonate, calcium phosphate

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and silica. The

existence of postcritical PNCs in calcium carbonate saturated and under-saturated solutions, was shown in studies that combined potentiometric titration and analytical ultracentrifugation


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(AUC).20 The findings were confirmed in an investigation with cryo-TEM.21 There, it was suggested that the aggregation process leads to nucleation as various sizes of clusters existed in the population. In addition, so called pre-clusters were observed to persist after nucleation. Here we investigate the early phase of CaCO3 crystal growth by using the double pulse technique, which was proposed by Tammann in 1922 and further developed by Kashchiev et al.. 22 23,24

The technique is based on the experimental possibility to quench nucleation in a

supersaturated solution and allow only existing postcritical nuclei to grow until they reach a detectable size. Scanning electron microscopy (SEM) was chosen as a method to detect nuclei. The method is focused on quantification of nuclei formed over time in a model mineralizing solution. The following points are investigated (vide infra) using the double pulse technique: (1) Does the double pulse experiment enable to determine from the counted crystals the real number of nuclei formed over time in a mother liquor?, (2) Can the double pulse experiment be used to calculate the nucleation rate during the induction time?, and (3) Does the number of crystals obtained during a double pulse experiment provide information on the nucleation mechanism?

Theory Double pulse technique The double pulse technique enables a rapid decrease of the supersaturation of a solution and ideally stops the nucleation. During the first pulse at a high supersaturation level nuclei are


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formed. During a second pulse at a reduced supersaturation level, no new nuclei are formed and only growth of existing nuclei to crystals occurs. The postcritical nuclei will act as seeds. The technique has been successfully employed in electro-crystallization 23 and has also been used to determine the growth rate of the ferritin protein.25 In this work we have implemented the method to determine the nucleation rate of calcium carbonate in solution. The first order nucleation in a supersaturated solution was investigated. In order, to modify the supersaturation of a solution it is important to determine the desaturation curve of the mother liquor and determine the duration of the induction time. In this study the induction time was determined for our model mineralization solution by recording the pH profile of the solution.26 Nucleation and induction time The time period, which elapses between achievement of supersaturation and the appearance of crystals is known as an induction time. The induction time strongly depends on the level of supersaturation, the presence of impurities, the agitation state etc. The induction time may be defined as the sum of the relaxation time, tr, i.e. the time which is required to achieve “quasisteady” distribution of molecular clusters and the time to form a postcritical nucleus, tn, and the time to grow nuclei to a detectable size tg.11 $%&' = $( + $& + $* A schematic desaturation curve is shown in Figure 1, where the induction time is marked as A, the fast desaturation as B and the equilibrium as C. The induction time is related to the formation of a minimum detectable volume of a newly formed material, the end of the time is marked as D in Figure 1A. During desaturation, growth of crystals become a dominating process. On the basis of this theory the induction time can be used to separate dominant nucleation, from dominant growth. In general, induction time is associated with unseeded precipitation

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but can also be


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observed when seeds are added into saturated solution.11 When seeds are present in solution, the induction time will be shorter than in the unseeded solution. In case of a double pulse experiment postcritical nuclei, which were formed during the induction time, will act as seeds when the solution suddenly is diluted to a lower supersaturation level. A test for that assumption is a comparison of the induction time of the original solution S=a, an original solution with S=b, and the double pulse solution, which is the solution S=a diluted to S=b. When the induction time for a double pulse solution is longer than the induction time for the original solution with S= a but shorter than for the original solution with S=b at the moment of dilution of the mother solution, postcritical nuclei were present. A schematic representation of this situation is shown in Figure 1B.

Figure 1 A) schematic desaturation curve with induction time (A), fast desaturation zone (B), equilibrium zone (C), inflection point (D). Fig. 1 B) A schematic representation showing a


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comparison of the expected duration of induction times for original solution S=a, diluted solution S=a to S=b, and S=b in double pulse experiment (red dotted line).

Assumptions for the interpretation of the induction time: 1) Supersaturation is reached instantaneously in the solution and remains constant during the induction time, 2) Nucleation rate is not too fast or too slow otherwise the solvent – crystal equilibrium could not be achieved, 3) Postcritical nuclei will act as seeds when the solution is diluted to a lower supersaturation level, 4) When seeds (nuclei) are present in saturated solution, the duration of the induction time will be limited mainly by crystal growth 5) The induction time for solution S=b must be significantly longer than the induction time in the double pulse experiment that diluted from state S=a to state S=b, so that at the moment of dilution the number of primary nuclei from the solution S=b is negligible to the number of secondary nuclei from solution S=a.28 pH measurement The following reactions occur in solution: 234

+,-. + +/0 -1 56 +,+/0 ↓ 29

8- +/0 ↔ 8+/0 1 + 8 . 2;

8+/0 1 ↔ 8 . ++/ -1 0


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The formation of CaCO3 crystals in solution involves the release of protons which leads to a decrease of pH. When the inflection point appears on a pH curve, crystals are big enough to be detected by pH changes (0.01). At point D (Figure 1) the induction time is reached Solution thermodynamics The driving force for crystal formation is the supersaturation ratio C DE 1F 1F G is the gas constant, CG is the absolute temperature, ∆μ C DE 1F G is

the change in chemical potential and ? is the number of ions in the formula units. For ionic solutions, such as a solution for CaCO3 mineralization, the supersaturation ratio is best expressed in terms of ion activities, with ?=2:

11

18 MΩ cm. The 4 mM NaHCO3 solution was prepared in a 250 mL volumetric flask. The ionic strength (I) of the final solution was maintained at 0.1 by adding KCl to the solution of NaHCO3. The 8 mM


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CaCl2 solution was prepared in a 250 mL volumetric flask. The pH was adjusted to the desired value by adding NaOH to the CaCl2 solution. A mother mineralization solution was prepared by rapidly mixing of solutions of CaCl2 and NaHCO3 in ration 1:1 in a 500 ml glass container. All glassware had been cleaned with 1M H2SO4 and flashed multiple times with milliQ water. The mixture was stirred with a magnetic stirring bar with 400 rpm. The reaction was performed at room temperature (295 K). During the reaction the container was open to atmosphere. The change of pH in the electrolyte over time was recorded and presented as desaturation curves. A mineralizing solution with diluted conditions was prepared similar as is described above. The final concentration of components and starting parameters for both solutions are presented in Table 1. The supersaturation ratio of solution and chemical speciation was calculated in the Visual MinteQ from the total concentrations of ions in solution (Cl-, Ca2+, Na+, H+, CO32-, K+). The program computes the result based on the mass-action equation, mass balance equations, charge balance equations and Bronsted-Guggenheim-Scatchard specific ion interaction theory (SIT) model for calculating activity coefficients. 29-31

Table 1. Final characteristics and calculated parameters of mineralization solution CaCl2 [mM] 4.0 3.5c

NaHCO3 [mM] 2.0 1.7

NaOH [mM] 0.200 0.175

KCl [M]

S* [-]

I [M]

pHa

pHb

pHc

0.086 0.076

4 3.7

0.1 0.088

8.831 8.848

8.415 8.348

8.920 8.860

* with respect to calcite, a CO2 pp not included, b CO2 pp included, c solution composition after dilution of the mother mineralization solution, c measured pH

Multi-channel on-line pH - and temperature recording A five channels system for simultaneous on-line pH measurement was home-built. The system was made from Phidgets electronic elements: Phidget Inter_face_Kit 8/8/8, Phidgets ORP/pH


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Adapter 1130 and temperature sensor: TSic-506F/501F. For recording the temperature and pH in time a program was made in LABVIEW12. The pH Orion 8102BN Ross combination electrodes were purchased from Thermo Scientifics Orion Ross. The precision of the electrodes is 0.01 pH and the drift

Calcium Carbonate Nucleation Investigated in a Double Pulse

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