CALCIUM CARBONATE PRECIPITATION: A REVIEW OF THE CARBONATE … · particular. This review hinges on...

87Calcium carbonate precipitation: a review of the carbonate crystallization process and...

© 2016 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 44 (2016) 87-107

Corresponding author: O.M. Suarez, e-mail: [email protected]

CALCIUM CARBONATE PRECIPITATION: A REVIEWOF THE CARBONATE CRYSTALLIZATION PROCESSAND APPLICATIONS IN BIOINSPIRED COMPOSITES

.. 1IGNIU 2. RIZIT EPH O. 9. SVáSIz

Nanotechnology Center for Biomedical, Environmental and Sustainability Applications,P.O. Box 9000, Mayagüez, 00681, Puerto Rico

Received: April 18, 2015

Abstract. Recently, bioinspired materials have received particular interest for their unique prop-erties and applications as well as their non-toxicity, which make them a target for intensiveresearch. The principal interest of this review lies on the study of calcium carbonate and itspolymorphs and how they interact with different substrates, in general, and with biopolymers, inparticular. This review hinges on the synthesis of calcium carbonate particles (aragonite, calciteand vaterite phases) using methodologies such as: gaseous diffusion and reactants mixing.The synthesis of the particles involves some variables which play a decisive role in the formationof calcium carbonate particles. Some of those variables include pH, temperature, concentrationof solutions, concentration of additives, type of additives (organic or inorganic) and the substratesurface roughness. All of them are fundamental in the precipitation of the crystals, motivatingscientists to focus more on the study of the additives on the precipitation of the crystals. Somewidely used additives such as poly(acrylic acid), polyacrylamide and poly(vinyl alcohol) are dis-cussed. Furthermore, since CaCO

3 precipitation can occur on biopolymers or other organic

substrates, these biopolymers have been used to create organic-inorganic compounds exhibit-ing unique properties also discussed in this review. Emphasis on abundant biopolymers suchas chitin, chitosan and cellulose is provided in this review and aims at bettering the understand-ing of potential applications of those organic-inorganic composites. Processes involving cal-cium carbonate precipitation are also explained in detail as well as how the previous variablesplay a pivotal role in the resulting compound.

1. INTRODUCTION

Bioinspired and biomimetic materials are used tofabricate products that have a nature-based struc-ture or are fabricated by mimicking processes orphenomena found in natural materials. These ma-terials possess the advantage of being naturallydesigned as well as adapted or functionalized. Suchfunctionalization provides them with the capacity ofbeing more resistant and elastic compared to theirkin in the original or natural state. The characteris-tics of biomimetic materials are vast. Their

biocompatibility with the environment, non-toxicity,and biodegradability are among the most importantcharacteristics [1,2]. Additionally, they can be pro-duced by low energy consumption means and canrender prolonged useful lifetime to the final product.As a result, the evolution of biomimetic materialshas expanded the range of applications of thesematerials. In recent decades, in particular, scien-tists have focused on the study of biomimetic ma-terials in order to manufacture new compositesmade of biopolymers and calcium carbonate.

88 A. Declet, E. Reyes :nd O. M. Suárez

1.1. Biopolymers

Biopolymers have attained great potential in themarket due to their vast applications in fields suchas “bio-ceramic, bio-sensing, biomedical engineer-ing, bio-nanotechnology and biologically assembly”[3]. As aforementioned, recent and increasing inter-est in these materials is due to their unique charac-teristics such as: biocompatibility, eco-friendly quali-ties and nontoxicity [1,2]. The most commonbiopolymers are chitin, chitosan and cellulose.These biopolymers present many applications in themedical, food, water treatment, industrial and agri-cultural f ields; they also possess goodbiocompatibility with human body tissues and flu-ids [4].

After cellulose, chitin is considered the mostabundant natural polymer. Chitin consists of groupsof -(1,4)N-acetyl glucosamine which are repeatedin the structure of the polymer with multiple hydro-gen bonding [5]. The glucosamine position in thestructure can change to (antiparallel), (parallel)and (combined), which indicates the differencesin packing and polarities of the chains. Of thosestructures, -chitin is the most stable form and canbe dissolved in stronger swelling agents such asaliphatic diamines [1,6]. Additionally, -chitin has ahighly crystalline structure with intra-inter hydrogenbonding, which limits the access of the solvent intothe network [5]. Contrary to -chitin, a more openstructure is obtained for -chitin that makes it sus-ceptible to swelling; polar molecules (water or alco-hol) can penetrate the structure of -chitin and de-stroy the hydrogen bonds of the structure [5]. Con-sequently, N-dimethylacetamide/LiCl is the mostcommon system to dissolve -chitin for its crystal-line structure [6].

Moreover, chitosan is a cationic polysaccharidewith a molecular structure made of hydroxyl andamino groups and the structure consisted of D-glu-cosamine and N-acetyl-D-glucosamine [5,7]. Thisbiopolymer is a derivate of chitin and is producedvia the alkaline N-deacetylation of chitin, which con-sists of removing the acetate group and replacing itwith an amide group in alkaline solutions [2,6]. Thedegree of deacetylation of chitin must be at leastabout 50% to be considered chitosan and to renderit soluble in aqueous acidic media [6]. The mecha-nism of solubility occurs by protonation of the pri-mary –NH

2 group on the C-2 position of the D-glu-

cosamine repeating unit, by which the polysaccha-ride is converted to a polyelectrolyte in acidic me-dia [8]. The properties of chitosan depend on pH,molecular weight, distribution of the acetyl groups

in the structure and the degree of de-acetylation[8]. As mentioned above, at low pH, the amidegroups are protonated, increasing the solubility ofchitosan in dilute aqueous solutions; conversely, athigh pH, the amide groups are deprotonated, de-creasing the solubility of the polymer [6]. Also, ad-vanced functional materials derived from chitosancan be obtained by its chemical modification usingthe different functional groups already present in themolecule. Procedures such as N-alkylation, N-acy-lation, N-carboxyalkylation, and polymer graftinghave been used for the chemical modification ofchitosan [9,10].

As aforementioned, cellulose is the most abun-dant biopolymer in nature, obtained from plant tis-sue after purification [6,11]. This linear syndiotaticpolymer consists of -1,4-glycosidic bonds linkedwith D-glucopyranose units. In the crystalline struc-ture, every monomer is rotated 180° with respect toits neighbors. Additionally, each glucose monomerconsists of three hydroxyl groups (OH) in the C-2,C-3, and C-6 position. The rotational conformationof the hydroxyl group on the C-6 position can alterthe hydrogen bonding pattern and the crystallinitystructure affecting the dissolution of the polymer [11].As in other polymers, some factors affecting thedissolution of cellulose are the length of the poly-mer chains and the degree of polymerization (DP),i.e. the number of glucose units present in the chain.As the glucose units increase, the number of hy-droxyl groups present in the structure also increases.These hydroxyl groups form complex pattern of hy-drogen bonds causing the need for solvents withhigh hydrogen bonding capacity for the cellulosedissolution. The most common solvents used in-clude: dimethylacetamide and lithium chloride(DMAc/LiCl), dimethylsulfoxide andtetrabutylammonium (DMSO/TBAF) and N-methylmorpholine-N-oxide (NMMO). In addition,cellulose can be dissolved in aqueous alkali mediasuch as sodium hydroxide (NaOH/H

2O), sodium

hydroxide/urea and sodium hydroxide/poly(ethyleneglycol) (NaOH/PEG). Furthermore, acidic mediauseful to dissolve it are trifluoroacetic acid,dichloroacetic acid, formic acid, and sulfuric acid.

As mentioned before, these biopolymers caneffectively interact with non-organic elements to cre-ate new composites. One relevant example is theformulation of a calcium carbonate/biopolymer com-posites. These have unique properties inherited fromjoining a plastic organic compound with a hard ce-ramic one. As calcium carbonate has multiple poly-morphs, creating new composites will also dependon what type of CaCO

3 conformation is used. To


Phase Crystallographic Unit Cell Specific Gravity Toughness

calcite hexagonal (rhombohedral) 2.71 g/cm3 brittlevaterite hexagonal 2.65 g/cm3 brittlearagonite orthorhombic 2.93 g/cm3 brittle

Table 1. Properties of the anhydrous crystalline phases, data from [12,14].

further understand these composites behavior andhow to effectively synthesize and functionalize then,a deeper look into calcium carbonate morphologiesis mandatory.

1.2. Polymorphism of calciumcarbonate

In the last decades, investigators have been inten-sively working on the development of ceramic ma-terials capable of being utilized in the preparation of“paint, textiles, plastics, adhesives, tires, ceramicsand industrial paper” [12].Calcium carbonate, nowa-days a ceramic material of high scientific interest,is normally found in the shells of arthropods andmollusks [13]. This carbonate is a polymorphicmaterial that has three anhydrous crystalline phases,i.e. calcite, aragonite and vaterite, in order of in-creasing solubility and decreasing thermodynamicstability, and an amorphous phase. The differenceamong the crystalline phases is due to the carbon-ate ions distribution respect to the calcium cationsinside the unit cell [12]. The carbonate ion is theprincipal unit of the carbonate minerals, possess-ing a rigid structure that contains one carbon atomsurrounded by three oxygen atoms rearranged inan equilateral triangle. The linkage between carbon-ate groups and cations does not affect the 120° CObond angle [14].

The calcite polymorph crystallizes in the hex-agonal system, is stable at room temperature andis the least soluble phase of the polymorphs [12],[14]. In the calcite structure, calcium atoms are ina face-centered rhombohedral unit cell. Aragonite,on the other hand, crystallizes in the orthorhombiccrystal system and is stable at high temperaturesand high pressures [14]. Finally, vaterite crystallizesin the hexagonal crystal system and is the leaststable polymorph [12]. Vaterite is the most solublefor its loose package, as shown in Table 1.

All three phases can be employed in the fabri-cation of organic-inorganic compounds. For in-stance, interesting applications are related to thefabrication of a polymer layer followed by a calciumcarbonate layer. These compounds have unique

applications and deserve a closer look, which isprovided in a later section.

1.3. Some applications of calciumcarbonate

Calcium carbonate particles are appealing due totheir applications in different fields such as: envi-ronmental engineering, chemical engineering,bioengineering among others. CaCO

3 also displays

useful applications in the water purification field. Forexample, Kyong-Soo Hong et al. developed a star-fish-shaped CaCO

3 to remove heavy metal ions such

as Cu+2,Pb+2, Cd+2, Zn+2 and Cr+6 [15]. This starfishCaCO

3 filter bore higher efficiency in removing heavy

metals than other conventional filters such as acti-vated carbon, crab shell, sawdust and other CaCO

3

particles. In another work, Ganiyu Latinwo et al. usedcalcium carbonate particles to improve the mechani-cal properties of polyurethane foam [16]. Theywanted to develop economic foams with high com-pression resistance. Furthermore, calcium carbon-ate has been studied for drug delivery carriers [17].Yaran Zhang et al. studied the biocompatibility ofcalcium carbonate particles on Hela cells. The re-searchers found the minimal and safe dosage ofcalcium carbonate particles for maximizing thera-peutic activity without negatively affecting the bio-system. A drug delivery device of calcium carbon-ate suspensions and calcium alginate hydrogel wasproposed by Brent Lantin and his collaborators [18].They worked on a medically useful hydrogel withcalcium carbonate particles, which can be disinte-grated under the effect of high frequency ultrasonicwaves.

Based on all these applications, it is evident thatcalcium carbonate particles are practicable to de-velop novel materials with potential use in manyfields. However, the precipitation process of the crys-tal is a complex process since it depends on fac-tors affecting the nucleation process and subsequentcrystal growth, which are often difficult to control.Additionally, CaCO

3 precipitation also depends of

its interaction with the substrate surface used forthe fabrication of a composite. All those factors can


affect the final characteristics of the composites aswell as its applications.

2. CALCIUM CARBONATEFORMATION

Because the nucleation and growth of crystals havedifferent pathways, the final structures of the crys-tals depend on the process selected for the crystal-lization. Such crystals can exist as a single crys-tal, iso-oriented crystal, mesocrystal or amorphousphase [19]. For these reasons, it is essential tounderstand the nucleation phenomena and the sub-sequent growth of this ceramic.

2.1. Nucleation and growth process

It is commonly accepted that this carbonate forma-tion begins with a reaction between two most com-mon reactants: calcium chloride (CaCl

2) and am-

monium carbonate (NH4)

2CO

3. Upon the reaction,

calcium cations bond with carbonate ions formingcalcium carbonate embryos in solution or on a cata-lytic polymer surface. The embryos grow to form acritical nucleus which, if the local conditions arefavorable, grow to form a primary nanoparticle. Thesenanoparticles can be formed into a single crystalwith the addition of other ions or can result in aniso-oriented (or equiaxed) phase with the additionof atoms. When additives are present in the reac-

Fig. 1. Nucleation and growth of calcium carbonate. Reprinted with permission from N.A.J.M. Sommerdijkand G. De With // Chemical Reviews 108 2008) 4499. © 2008 American Chemical Society.

tion, a mesoscaled crystal forms during the pro-cess. Common additives are: polyacrylic acid orpolyvinyl alcohol. These additives create a negativesurface charge on the initial crystal that attractsother particles to the surface. This process contin-ues until equilibrium is reached. Additionally, theassembly of embryos, i.e. not bearing critical nucle-ation size, can lead to the formation of an amor-phous phase that can then form an oriented crystalfavored by the additive present in the solution. Thedifferent pathways of crystallization are presentedin Fig. 1. Since particles can nucleate heteroge-neously on a substrate, another important factor toconsider is the roughness of that substrate. In ef-fect, surface roughness can affect the adhesion ofthe calcium carbonate crystals onto the surface.

2.2. Effects of the polymer substrateon the nucleation of calciumcarbonate crystals

The adhesion of atoms onto a substrate is of ut-most importance because the two materials (sub-strate and ad-atoms) come into contact and form aregion in which they sustain and transfer stresses.For strong adhesive bonds, intimate molecular con-tact and active interactions must be present. Theseinteractions include Van der Waals or other non-covalent interactions as a result of surface and in-terfacial energies.


Naturally, the morphology of the organic matrix(polymer substrate) at the nanoscale and the inter-action between the organic matrix and the formingcrystals does affect the heterogeneous nucleation[20]. Furthermore, as mentioned, this nucleationprocess is affected by surface roughness. In effect,an atom on a smooth surface has high interfacialenergy due to its free bonds. For example, a cubiccrystal (six sides) on a smooth surface has onlytwo sides in contact with the surface and the othersides are in contact with the solution. This atomhas four free bonds, which hamper the atom bond-ing to the surface. It moves from one place to an-other on the surface and finally returns to the solu-tion. On the other hand, if one considers the samecubic crystal on a rough surface, the atom has morethan two sides in contact with the surface. It canadhere to the surface better than in the previouscase because it has less free bonds that reducethe interfacial energy. However, materials do not al-ways behave as explained above. Consider the workby Tabor who studied the effects of surface rough-ness and ductility of materials on the adhesion ofsolids [21]. In this case, adhesion decreased asthe surface roughness increased although this wasattributed to the elastic modulus of the phases incontact. Another study by Chaudhury andWhitesides focused on the surface energies ofpoly(dimethylsiloxane) [PDMS] in air and in mix-tures of water and methanol [22]. In their work theyfound that the interfacial interactions decreased asthe methanol content in the water/methanol mix-ture increased. The results showed that the interfa-cial interactions lowered in solutions of water andmethanol as the methanol content increased. How-ever, a small interaction persisted, even in puremethanol. In both the Tabor and Chaudhury andWhitesides’ experiments, the probes used werehemispherical elastomeric lenses.

Therefore, the number of free bonds can affectthe interfacial energy of the calcium carbonate crys-tals. The energy of the crystals increases as thenumber of free bonds increases, affecting the adhe-sion of the crystal onto the substrate surface. Also,if the number of free bonds decreases so will theinterfacial energy. This promotes a better adhesionof the crystals on the substrate.

Thus far, we have discussed the nucleation andgrowth processes for the calcium carbonate crys-tals as well as the influence of substrate roughnesson the nucleation of the crystals. One must nowconsider the methodology implemented for the pre-cipitation of the crystals. As mentioned earlier, cal-cium carbonate can precipitate in three known crys-

talline phases: aragonite, vaterite and calcite. Pre-cipitation of these phases directly depends on thetechnique employed, i.e. by reaction and by gas-eous diffusion. These two techniques will be dis-cussed in order to predict the favored phase in eachone of the techniques.

3. PREPARATION METHODS FORCALCIUM CARBONATEFORMATION

Calcium carbonate can be produced through vari-ous methods. However, the size and morphology ofthe crystals can be affected by the technique usedin the experimental procedure.

For instance, Hang Wang and his collaboratorsutilized a reactor with the two reactant precursorsfor synthesizing calcium carbonate crystals: cal-cium chloride and sodium carbonate [23]. The re-actor consisted of three cells separated by bafflesand polymeric membranes bearing a pore size of 1mm and a width of 2 cm. While one corner of thereactor had calcium chloride solution and the otherone, a sodium carbonate solution, at the center ofthe reactor there was only water. The reactor wasplaced in a water bath at a constant temperature.Upon increasing the temperature to the desiredvalue, the baffles were removed allowing diffusion inthe liquid phase from the edges of the reactor to thecenter. The precipitation of the calcium carbonatecrystals occurred at the center of the reactor. Toyield effective results with this experiment, controlof the reactants’ diffusion was necessary.This tech-nique favored the precipitation of calcium carbonateas calcite, vaterite and aragonite [23].

In addition, the same researchers managed toprecipitate calcium carbonate using another tech-nique, which mixed the calcium chloride and so-dium carbonate solutions. In contrast with the firsttechnique discussed, the diffusion of the reactantswas not controlled. As a result, the favored phaseswere (in order of appearance) vaterite, calcite andaragonite [23].

Herley Casanova et al. used a different methodto precipitate calcium carbonate with a high pres-sure jet homogenizer to mix the solutions [24]. Inthis technique, pipe A contained a sodium carbon-ate solution and pipe B had a calcium chloride so-lution. Sodium caseinate (or casein) was added tothe sodium carbonate solution and the addition ofan acetic acid solution controlled the pH. The equip-ment was submerged in a thermostatic bath untilthis reached the desired temperature. Immediatelythe reactants started to flow through the conducts


until they met at the mixing point. Finally, the prod-uct obtained from the reaction between both flowsfell into a glass flask [24]. The equipment combinedtwo factors: mixing and fragmentation of the agglom-erates of the calcium carbonate nanoparticles. Theflow of the solutions was turbulent and improved thesolution mixing. The high pressure led to a betterdispersion and less agglomerates. The techniquewas successful in forming calcium carbonatenanoparticles, specifically, vaterite and amorphousagglomerates [24].

Xurong Xu et al. achieved precipitation (hetero-geneous nucleation) of calcium carbonate on a sub-strate by going from gaseous diffusion to liquid dif-fusion [25]. They prepared a solution containingcalcium chloride and poly(acrylic acid) and a flaskwith solid ammonium carbonate. After they placedboth flasks in a desiccator, the ammonium carbon-ate started to decompose releasing carbon dioxideand ammonia vapors. The vapors were contained inthe desiccator and made contact with the calciumchloride solution, causing the precipitation of cal-cium carbonate crystals. In this method, thepolyacrylic acid provided a negative charge to thesubstrate surface. Subsequently, the calcium ionsadhered to the substrate surface followed by thecarbonate ions and thus the precipitation of calciumcarbonate occurred.

While the interaction between ions that consti-tuted the reactants was the key factor in this tech-nique, the diffusion and reaction time representedrelevant variables. As expected, higher calcium car-bonate precipitation was achieved at longer diffu-sion times whereas the amorphous phase was ob-tained at lower diffusion times. As a result, onlyvaterite and calcite formed after 16 hours of CO

2

diffusion [25].Rajeev Agnihotri and coworkers used another

method to precipitate calcium carbonate via a cal-cium hydroxide and pure CO

2 gas reaction [26]. The

researchers utilized a Pyrex™ reactor containing acalcium hydroxide solution whereas the gas passedthrough the bottom of the reactor. They added an-ionic surfactants to the calcium hydroxide solutionto modify the surface of the particles. The setuphad a pH meter and a thermocouple to monitor thesevariables. After the precipitation, the aqueous sus-pension was filtered and dried at room temperature.This procedure resulted in different particle sizesas a consequence of the variations in the calciumhydroxide and anionic surfactant concentrations[26].

These techniques were efficient for calcium car-bonate precipitation and favored a specific calcium

carbonate phase. Yet, in most of the techniquesdepending on gaseous diffusion, the predominantphase was aragonite. On the other hand, vateriteand calcite phases were obtained via reaction. It isimportant to note that there are other factors thataffect the final phase obtained; this is not solelydue to the implemented technique. These other fac-tors affecting nucleation and growth processes ofthe crystals include solution pH, temperature andthe presence of additives. The next section dis-cusses these important factors for calcium carbon-ate precipitation.

4. VARIABLES AFFECTING CALCIUMCARBONATE HETEROGENEOUSNUCLEATION OR PRECIPITATION

As aforementioned, numerous variables intervenein the precipitation of the calcium carbonate crys-tals. This requires a further review in order to draw amap of the optimal conditions required for specifictypes of precipitation. Variables that affect the pre-cipitation process are listed below to better under-stand its effects on calcium carbonate precipitation.

4.1. PH and temperature effects on thepolymer surface

We already mentioned that calcium carbonate crys-tallization is affected by the hydrogen potential pro-duced during the reaction. The pH impacts the de-gree of dissociation of the carboxylic groups in thepoly(acrylic acid) structure and the amine groupspresent on the chitosan film. In general, the surfacecharge density of the chitosan and poly(acrylic acid)(PAA) changes at different pH values. PAA existsas a polyanionic chain in the solution at values ofpH above 6.3. At this pH level, the chitosan surfaceis close to be a neutral polymer; a great number of–COO- are bonded on the chitosan surface. At pHvalues below 4.5, PAA exists as a polycation –COOH) and the chitosan surface consists of NH

3+

groups. As the pH decreases, the PAA becomesmore protonated and less charged and thereforedecreases the precipitation of calcium carbonate.Additionally, the precipitation of the crystals is af-fected at pH values above 11.0. At such pH values,the surface charge density decreases and most ofthe hydrogen bonds are destroyed [27]. In general,a minimum number of carbonate groups bind tocalcium cations to form the particles. As the num-ber of –COO- groups increases, so does the pre-cipitation of calcium carbonate crystals on the sur-face.


Fig. 2. Optical micrographs of calcium carbonatecrystal on chitosan polymer surface, considering0.004 wt.% PAA and different pH values; (a) pH 8.5,(b) pH 9.0, (c) pH 9.5, (d) pH 10.0, (e) pH 10.5, (f)pH 11.0, (g) pH 11.5, and (h) pH 12.0. Reprintedwith permission from S.R. Payne, M. Heppenstall-Butler and M.F. Butler // Crystal Growth & Design 7 2007) 1262. © 2007 American Chemical Society.

Simon Payne and a coworker studied the pHeffects of the calcium carbonate precipitation onchitosan films, which were prepared by dissolvingthe polymer in an acetic acid aqueous solution [28].Then, the solution was placed in petri dishes anddried. The acetic acid was neutralized, and the filmwas washed with distilled water and dried. Then thefilm was soaked in a PAA aqueous solution to modifythe surface charge, and the pH value was set at10.5. The film was immersed into a solution of 0.025M CaCl

2, 0.050 M NaHCO

3 and PAA; this solution

was mechanically stirred and the pH constantlymonitored and adjusted using sodium hydroxide.The experiment performed at room temperature wasconducted at different polymer and PAA concentra-tions, and pH values.

Payne et al. studied how the PAA concentrationmodified the polymer surface [28]. They analyzedthe surface of the polymer with infrared spectros-copy. The infrared spectrum shows that for neutral

and basic pH values, the 0.004 wt.% PAA concen-tration produced the most intense band for –COO-

and NH3

+, the functional groups present on thechitosan surface. Furthermore, for a 0.001 wt.% PAAconcentration intense bands were produced. Theexperiment was performed for neutral and 10.5 pHvalues, because this affects the presence of COO-

groups. Above 10.5 pH value, the solution becomessaturated with –COO- groups, affecting the calciumcarbonate precipitation.

Additionally, the effect of PAA concentration wasanalyzed by optical microscopy [28]. It was observedthat calcium carbonate precipitation decreased atlower poly(acrylic acid) concentrations but higherPAA levels favored the precipitation. The increasein PAA facilitated the surface supersaturation withcalcium, which caused the nucleation and growthof calcium carbonate crystals. At low PAA levels,the amount of PAA adsorbed on the chitosan sur-face was low. This cut down the amount of calciumcations on the chitosan surface and lowered theamount of calcium carbonate precipitation on thesurface. In general, the calcium carbonate nucle-ation resulted from a balance between the numberof carboxylate groups available at the film surfaceand the number of ionized carboxylate groups. Ac-cording to the optical microscopy analysis, the op-timum PAA concentration was 0.004 wt.% in whicha maximum nucleation density was obtained. Basedon infrared spectral analysis and optical microscopy,a poly(acrylic acid) concentration of 0.004% wasset to analyze the effects produced by a neutral orbasic solution.

In addition, Payne et al. studied the pH effectson the calcium carbonate precipitation. The experi-mental results consisted of large and well definedspherulites obtained at pH ranging from 8.5 to 9.0[28]. Their morphology was found to be affected asthe pH increased above 9.0, as shown in Fig. 2. Asthe pH rose, the solution became more basic andfavored the carbonate precipitation. At pH > 10.5,the conversion rate of hydrogen carbonate ions intocarbonate ions increased. The calcium cations insolution have been sequestrated, which reduced thecalcium carbonate precipitation.

In another work, Jiaguo Yu et al. also analyzedthe behavior of calcium carbonate precipitation atdifferent pH levels [29]. Nevertheless, Yu and col-laborators did not use a polymeric matrix as Payneet al. Firstly, stock solutions of calcium chloride(0.5 M) and sodium carbonate (0.5 M) were pre-pared for the calcium carbonate precipitation. Then,sodium carbonate solution was injected into an aque-ous PAA solution (1.0 g/L). The pH was set to 10.0


using HCl or NaOH solutions. Then, calcium chlo-ride was mixed with the previous solution under vig-orous mechanical stirring. The authors analyzeddifferent pH values, PAA concentrations, calciumcarbonate levels and temperatures to study theireffects on the precipitation of the crystals. This isan extensive and important publication but for thesake of brevity, in the present review we shall focuson the analysis of the pH effects on the calciumcarbonate precipitation.

The experimental results at 25 °C revealed thatuniform calcium carbonate particles were obtainedby lowering the pH from 10 to 9. High pH values ledto a formation of round and irregular particles. At apH of 12, a reduction in particle size occurred whilea maximum particles size was obtained at a pH of9. Higher pH values promoted the complete proto-nation of carboxylic groups and a polyanionic chainformed. Furthermore, higher pH values raised thesolution supersaturation and the nucleation rate,which naturally lessened the particle size. More-over, the cubic morphology of the particles was notaffected at pH values from 9 to 11 at 80 °C, as shownin Fig. 3. At 80 °C and a pH of 12, smaller irregularparticles were obtained.

Summarizing, the pH of the solution does playan important role in the calcium carbonate precipi-tation. In some cases, the pH is used to modify thestructure of the substrate surface. As mentionedearlier, the substrate is utilized to create a com-

Fig. 3. SEM micrographs of CaCO3 crystals at 80 °C after 24 hours, considering 8.0 mM CaCO

3, 1.0 g/L

PAA, and different pH values; (a) 9, (b) 10, (c) 11, and (d) 12. Reprinted with permission from J. Yu, M. Lei,B. Cheng and X. Zhao // Journal of Solid State Chemistry 177 2004) 681. © 2004 Elsevier.

pound based on an organic-inorganic layer. Thismodification is of great importance since calciumcarbonate precipitation increases proportionally withthe formation of –COO- groups. Additionally, theadditives affected the precipitation of the crystals.Different types of additives exist, with magnesiumbeing one of the most used. As a consequence,two important factors for nucleation and growth aremagnesium concentration and reaction time.

4.2. Magnesium effects on calciumcarbonate crystallization

In biological systems, magnesium stabilizes amor-phous calcium carbonate phases, which form de-pending on the magnesium/calcium ratio and theincubation time of calcium carbonate crystals, i.e.calcite, vaterite and aragonite. Low magnesium/cal-cium ratios favor calcite precipitation whereas highmagnesium/calcium ratios favor aragonite precipi-tation [30].

Loste and her coworkers studied these Mg ef-fects on calcium carbonate precipitation by analyz-ing the influence of the magnesium/calcium ratioon the carbonate precipitation [30]. Their experi-ments consisted of combining sodium hydrogencarbonate solutions with calcium chloride and mag-nesium chloride solutions. The calcium chlorideconcentration was set constant during the experi-ment according to Table 2.


Mg:Ca 0:1 1:1 2:1 3:1 4:1 10:1

CaCl2 (M) 0.06 0.06 0.06 0.06 0.06 0.06

MgCl2 (M) 0 0.06 0.12 0.18 0.24 0.60

Table 2. Composition of precipitation experiments,see [30].

Mg:Ca ratios Time Calcium carbonate phase

0:1 and 1:1 0 min amorphous2-60 min calcite and vaterite

2:1 0-10 min amorphous30-60 min amorphous and calcite(traces)24 hours calcite, aragonite(traces) and calcium carbonate

monohydrate(traces)3:1 0 min amorphous

2 min amorphous and calcite(traces)14 hours calcite and aragonite(traces)48-72 hours calcium carbonate monohydrate

4:1 0 min amorphous10-60 min calcite24-72 hours aragonite72 hours-6 days aragonite(traces), calcite and calcium carbonate monohydrate6 days - nesquehonite

10:1 14 hours amorphous24 hours amorphous and calcite(traces)14 days nesquehonite, calcium carbonate monohydrate and

magnesium calcite

Table 3. X-ray diffraction (XRD) results obtained for each experiment at different times, data from [30].

The researchers analyzed the precipitations ofcalcium carbonate at different times and discoveredthat amorphous calcium carbonate (ACC) precipi-tated in all their experiments [30]. As mentioned ina prior section, this amorphous phase is the mostsoluble and least stable. The stability of the amor-phous phase was discovered to depend on the Mg/Ca ratio in the solution whereas ACC was favored athigher Mg concentrations in the solution. Further-more, higher concentrations of magnesium delayedthe transformation of the amorphous phase and, asa consequence, retarded the transformation ontoother phases.

In the experiments, the precipitation of calciumcarbonate began with the amorphous phase, whichthen transformed to other crystalline phases suchas calcite, vaterite, aragonite, calcium carbonatemonohydrate (CaCO

3.H

2O), nesquehonite

(MgCO3.3H

2O) and magnesium calcite [Table 3]. In

the first minutes, the amorphous phase was favoredin all the experiments. After the precipitation of theamorphous phase, calcite particles were obtained

in all the experiments. However, vaterite phase wasfavored only in the first two experiments (ratios of0:1 and 1:1) after 2 minutes. The experiments withMg/Ca ratios of 2:1, 3:1, 4:1 and 10:1 facilitated theprecipitation of calcite, aragonite and calcium car-bonate monohydrate. Additionally, nesquehonitecame up in the experiments with Mg/Ca ratios of4:1 and 10:1. In the experiment with 10:1, magne-sium calcite was favored.

Furthermore, the same authors analyzed theprecipitates obtained at the top and bottom of thepetri dishes and found that calcium carbonate par-ticles precipitated in the bulk solution and thensettled on the bottom of the petri dish [30]. The par-ticles at the top of the petri dishes were in contactwith the air/solution interface. Conversely, at thebottom of the petri dishes, the particles were in con-tact with the solution for a longer time. According tothe results, the morphology of the crystals changedas the magnesium concentration increased in thesolution whereas significant changes in the calciumcarbonate precipitation occurred in all the experi-ments above 2:1. Calcium carbonate at the bottomof the petri dishes consisted of small calcite par-ticles. However, a thin film of calcite particles wasobtained at the air/solution interface in which theoxygen ions have interacted with the solution. Thisinteraction contributed to the precipitation of cal-cium carbonate particles until a continuous layer ofparticles formed.


The addition of magnesium and reaction timeconsiderably affected calcium carbonate formation.Magnesium altered the saturation of the solution,which means that Mg raised the presence of ions inthe solution. The increasing saturation affected thealready precipitated phases in which the amorphousphase was the first one to form followed by its crys-tallization into other phases as the reaction timeincreased.

There are other additives that are also used forprecipitating calcium carbonate, the most commonones being poly(acrylic) acid, polyacrylamide andpoly(vinyl alcohol). In general, these additives bearupon the substrate surface and interact with thecalcium ions present in the solution. As a conse-quence, it is of great importance to further analyzethe interactions occurring among the additives, thesubstrate, and the calcium and carbonate ions.

4.3. Effects of additives on a polymersurface

The synthesis of inorganic-organic layers bybiomineralization methods has become of mount-ing interest to scientists and researchers. In thiscase, the ceramic nucleation process can be af-fected by several factors such as the degree of satu-ration of a solution and the surface charge on thepolymer surface, issues already addressed in thepresent review. Additives can be used to produce anegative charge on the polymer surfaces, e.g. PAA,poly(vinyl alcohol), poly(ethylene glycol),poly(acrylamide), polypeptides and glycols-polyols.

The negative charge on the surface of the poly-mer change the initial polymer structure [28]. Ofour particular interest is chitosan, which is, as pre-viously mentioned, a cationic polymer with aminegroups (NH

2) and poly(acrylic acid) a polyanion with

carboxylic groups (R-COOH), which interacts elec-trostatically with the amine groups forming a poly-electrolyte complex. Then, calcium cations are at-tracted to the negative charge and the carbonateions bond to the calcium cations. After the reac-tion, precursor calcium carbonate molecules arecreated on the biopolymer structure. The polyelec-trolytes provide cohesion and adhesion between theinorganic and organic layers. When dealing withpolyelectrolytes, variables like pH, composition,conformation, charge density, and solvents becomerelevant [31]. At this point, we need to underscorethat the polymeric additives facilitate calcium car-bonate nucleation and promote the formation of aspecific phase.

4.3.1. Poly(acrylic acid)

As aforementioned, PAA is an anionic polyelectro-lyte with its negative charge resulting from carboxy-late ( COO-) functional groups. According to the lit-erature, PAA is one of the most widely used addi-tives and functionalizes the surface of the substrateforming a polyelectrolyte complex that assists inthe deposition of calcium ions.

He et al. studied the PAA effects during calciumcarbonate precipitation on chitosan films [32]. Theirexperiment consisted of two types of films: chitosanand acetylated chitosan films. The chitosan filmswere obtained by dissolving the polymer in an ace-tic acid aqueous solution. This solution was driedand annealed to remove both water and solvent pro-ducing films with 8% of degree of acetylation. Theacetylated process was performed to raise theacetylation degree to 80%. The resulting acetylatedfilms were obtained using the chitosan films with-out annealing. After soaking the chitosan films withmethanol, they were immersed in a solution of ace-tic anhydride and methanol. Finally, the films werewashed with methanol and dried. Afterwards, thecalcium carbonate precipitation on chitosan filmstook place in a closed box containing two vessels.The first one held a chitosan film or acetylatedchitosan film with calcium chloride solution. Thesecond one contained ammonium carbonate in solidstate. Calcium carbonate precipitation occurred viagaseous diffusion [32].

The PAA concentration was studied by the Fou-rier-transform infrared (FTIR) spectroscopy analy-sis. The results showed that the intensity of thebands diminished as the PAA concentration in thesolution intensified. According to the FTIR analy-sis, the absorption band at 875 and 745 cm-1 corre-sponded to a vaterite phase and the one at 857cm-1, to an aragonite phase [32].

Calcium carbonate phases were affected by thePAA concentration in the solution and the degree ofacetylation (DA). Vaterite was favored at lowpoly(acrylic acid) concentration and 8% DA. How-ever, aragonite formed at low PAA concentration and80% DA. At PAA concentrations higher than 1x10-2

g/L, vaterite appeared with 8% or 80% of DA. It isapparent, therefore, that the degree of acetylationplayed an important role in the calcium carbonateprecipitation at low concentration of the additive [32].

At low PAA concentrations two phenomena oc-curred. PAA exists as a polyelectrolyte compositewith calcium ions, i.e. PAA-Ca+2. The supersatura-tion resulted as a consequence of the electrostaticforces between the ester group -COO- and the am-


Fig. 4. SEM micrographs of CaCO3 crystals at 80 °C, considering 8.0 mM CaCO3, pH:10 and different PAA

concentrations; (a) 0.2, (b) 0.5, (c) 1.0, (d) 2.0. and (e) 5.0 g/L. Reprinted with permission from J. Yu, M. Lei,B. Cheng and X. Zhao // Journal of Solid State Chemistry 177 2004) 681. © 2004 Elsevier.

monia group NH3+. Also, the interaction between the

hydrogen bonds and the -COO-, -OH and -NH2

groups increase the solution supersaturation aroundthe polymer matrix. On another part, PAA-Ca+2 canremain as a mobile polyelectrolyte in the solutionand the CO

2 molecules are absorbed increasing the

solution supersaturation. These phenomena furtherraised the supersaturation with CaCO

3 [32]. In gen-

eral, the aragonite phase came forth at low PAAconcentrations because it grows on chitosan sur-faces rich in OH groups. On the other hand, vateriteformed at high concentrations because it grew onsurfaces comprised of NH

2 and OH groups [32].

In another research, Jiaguo Yu et al. examinedthe effects of PAA on the precipitation and nucle-ation of calcium carbonate crystals [29]. Initially,stock solutions of calcium chloride (0.5M) and so-dium carbonate (0.5M) were prepared. The solutionof sodium carbonate was injected into the aqueousPAA solution (1.0 g/L) with the pH set to 10.0, us-ing HCl or NaOH solutions. Under vigorous mechani-cal stirring, then calcium chloride was mixed withthe previous solution. Some of the variables consid-ered in this research were PAA concentration, cal-

cium carbonate concentration, temperature, and pH.We will discuss the analysis of the effects of thepolyacrylic acid concentration at pH = 10.0 [29].

A scanning electron microscopy analysis re-vealed that the calcium carbonate morphology hadbeen affected by the additive concentration at 25 °Cas presented in Fig. 4 [29]. Lower PAA levels showedcalcium carbonate crystals with rhombohedral andrectangular shapes. The authors attributed thesemorphologies to the insufficient amount of PAA ionsto bind the calcium cations. On the contrary, athigher concentrations of the additive (1.0 to 2.0 g/L), the calcium carbonate crystals changed theircubic morphology to large irregular spheres. Theseshapes resulted from a strong interaction betweenthe carboxylic groups of the PAA and the calciumcarbonate crystals that had precipitated previously.When the additive concentration reached 5.0 g/L,twinned calcium carbonate particles formed. Onanother segment of this same publication, the au-thors revealed that at 80 °C the morphologies of thecalcium carbonate crystals changed, as shown inFig. 4 [29]. Lower PAA concentrations allowed ob-taining branched irregular particles, rhombohedral


and cubic shapes. As the additive concentrationrose, the branched irregular morphology of the par-ticles changed to cubic. Higher PAA levels allowedforming branched particles. When the additive con-centration reached 1.0 g/L calcite particles (cubic-shaped) resulted. As the additive concentrationreached 5.0 g/L, the cubic particles became rounderand had more aggregates of small cubic particles.High temperature also affected the calcium carbon-ate precipitation favoring the formation ofmonodispersed cubic particles [29].

Another critical study on the PAA effects on cal-cium carbonate precipitation is due to Ouhenia etal. [33]. Their experimental procedure consisted ofpreparing three solutions: calcium chloride (CaCl

2),

potassium carbonate (K2CO

3), and PAA. First, the

calcium chloride and poly(acrylic acid) solutionswere mixed at different temperatures. Afterwards,the ensuing solution was mixed with potassiumcarbonate. Finally, the researchers collected theprecipitates by filtration through a cellulose mem-brane and dried them [33]. Once again the resultscorroborated that the temperature did affect the cal-cium carbonate particles precipitation. The studyanalyzed the volume fraction of the polymorphs (cal-cite, vaterite and aragonite) obtained with and with-out the presence of poly(acrylic acid). Furthermore,temperature effects were considered. For calciteparticles, the presence of the additive reduced thevolume fraction of the crystals formed at 25 °C inthe solution compared to those without PAA. Also,higher temperatures lowered the volume fraction ofcalcite particles for both cases. Contrarily, the pres-ence of the additive raised the volume fraction ofvaterite particles compared to those without PAA.At 50 °C the volume fraction of vaterite particles washigher than calcite and aragonite; however, it de-creased above 80 °C for both cases. Additionally,the presence of the additive increased the volumefraction of aragonite crystals compared to thosewithout PAA. At temperatures above 50 °C arago-nite particles formed. Even more, higher volume frac-tion of aragonite particles were obtained at 80ºCthan calcite and vaterite particles [33].

At this point one can safely conclude thatpoly(acrylic) acid has been one of the most studiedadditives by researchers and scientists. In general,the presented studies considered distinct variablessuch as additive concentration, acidity of the solu-tion and temperature. Additionally, the above stud-ies employed different techniques such as gaseousdiffusion and reaction. By controlling these variables,precipitation of the three calcium carbonate crys-talline phases was achieved although all of them

could not been obtained in the same experiment.Just as these researchers decided to study the ef-fects of poly(acrylic) acid, other investigators focusedon studying another important additive: polyacryla-mide.

4.3.2. Polyacrylamide

Polyacrylamide (PAM) is another anionic polyelec-trolyte commonly used to modify or adjust the mor-phology of calcium carbonate. For instance, Wanget al. used the carbonation in combination withPAM, as an organic substrate and the only modifier[34]. In their experimental procedure they prepareda polyacrylamide gel and mixed it with calcium hy-droxide particles (Ca[OH]

2) at different temperatures.

Then, they bubbled CO2 and N

2 into the said solu-

tion while controlling the pH. The resulting calciumcarbonate precipitates were rinsed with distilledwater and dried [34]. Wang et al. identified the mor-phology of the crystals by the Fourier-transform in-frared (FTIR) spectroscopy analysis. Cubic calciteprecipitated without polyacrylamide, but needle-shaped aragonite and calcite crystals were obtainedin the presence of the additive. The FTIR spectrashowed the characteristic peak of calcite crystalsat 875 cm-1 when no polyacrylamide was added while1082 cm-1 for aragonite crystals in presence of poly-acrylamide.

Additionally, differential thermogravimetric analy-sis demonstrated the decomposition of calciumcarbonate and polyacrylamide depending on an or-ganic/inorganic interfacial interaction [34]. At loworganic concentrations, aragonite amount raised.Nevertheless, the aragonite fraction started to de-crease at organic concentrations higher than0.75x10-7 mol/l. The authors attributed this phenom-enon to two possible mechanisms: 1) Based on theOstwald step rule, they claimed that the least stablepolymorph formed first and then transformed intothe more stable phase; and 2) the polyacrylamidesubstrate inhibited the formation of aragonite, prob-ably due to the –NH

2)groups adsorbed on the crys-

tal, which blocked further calcium carbonate growth[34]. Furthermore, the crystal size of calcite becamesmaller for larger concentrations of the additive.Based on these findings, one can conclude that thepolyacrylamide could have delayed the nucleationof calcite particles and stabilized the aragonitephase [34].

Wang et al. determined the fraction of aragonitecrystals at different temperatures. Aragonite crys-tals were not obtained at temperatures below 60 °Cwhile a higher aragonite fraction came along at 80


Fig. 5. Influence of temperature on crystal sizes.Reprinted with permission from C. Wang, J. Zhao,X. Zhao, H. Bala and Z. Wang // Powder Technol-ogy 163 2006) 134. © 2006 Elsevier.

Methanol/Water PAAm (%)0 0.50 1.0 5.0

0:10 C C C C1:9 - C, (V) - -2:8 - C, (V) - -3:7 - C, (V) - -4:6 C V - -

Table 4. Calcium carbonate precipitates (0.10 M)using polyacrylamide from methanol/water solu-tions at a temperature of 23-24 °C after 9 days, datafrom [35].

A = aragonite, C = calcite, V = vaterite; symbol inparentheses indicates that the phase is a minoramount.

°C; yet, the amount of aragonite particles decreasedabove that temperature. Once again it was corrobo-rated that temperature affects the crystal size ofboth phases, i.e. aragonite and calcite, as presentedin Fig. 5. Larger calcite particles formed at 20 °Cbut the size decreased within the 20 °C to 80 °Crange and increased above 80 °C. When the sizesof calcite and aragonite particles were compared,the researchers found that calcite particles werelarger than aragonite ones. A maximum particle sizeof aragonite was obtained at a temperature of 90°C. In general, the crystal size of aragonite and cal-cite particles increases above 80 °C. Wang et al.stated that the NH

2 groups of the additive could ad-

sorb onto the crystal surface, which increased athigher temperatures. The nucleation of calcite crys-tals could be affected by the supersaturation of car-bonate ions on the crystal surface causing as theparticle size to decrease as the supersaturation rose.

By mixing two solutions, Won Kim et al. investi-gated the effects of some nonionic polymers addi-tives on the calcium carbonate precipitation [35].The first solution was calcium chloride while thesecond one consisted of ammonium carbonate andpolyacrylamide (PAAM). The calcium carbonateconcentration was different, but equal amounts ofthe main reactants were mixed. The calcium car-bonate precipitate was separated from the solutionby centrifugation, and washed with distilled waterand acetone to eliminate the residues [35].

The researchers were able to analyze the effectsof the PAAM amount and the methanol/water ratioon the calcium carbonate precipitation [Table 4].Calcite particles were obtained for all polyacryla-mide levels in a water solution and in a 4:6 metha-nol/water solution at 0% of PAAM. Nonetheless,the addition of methanol caused the precipitation ofvaterite and calcite at 0.50% of PAAM. Vaterite par-ticles were obtained at a methanol/water ratio of4:6. Some characteristics of PAAM include its abil-ity to form hydrogen bonds and its poor adsorptionrate because water dissolved the organic additive.As result, calcite particles are precipitated in a wa-ter solution. However, the addition of methanol intothe water solution retarded the vaterite transforma-tion into calcite [35].

The studies presented considered thepolyacrilamide concentration effects, the methanol:water ratios, and the temperature effect. By follow-ing the same line as with the study of PAA, theseresearchers managed to precipitate different calciumcarbonate phases. However, there was always aspecific phase that was favored in the experiments.Apart from the studies made on PAA and PAAM,there is another important additive worth mention-ing that is also used in the CaCO

3 precipitation: the

poly(vinyl alcohol).

4.3.3. Poly(vinyl alcohol)

This polyalcohol is obtained from vinyl acetate by apolymerization process and further base-inducedhydrolysis. It is well known that poly (vinyl alcohol)is an anionic polyelectrolyte with a negative chargearising from the –OH groups. It is an anionic, watersoluble additive that enhances the calcium carbon-ate nucleation. Nucleation of calcium carbonatecrystals depended on the contact between the crys-tals and the polymer. In general, the surface of thepolymer underwent a treatment with an additivewhich then were adsorbed by the polymer surface.Fig. 6 shows how the additive produced some chargeinto the polymer allowing the growth of the calciumcarbonate crystals [36].


Lakshminarayanan et al. further explored theeffects of poly(vinyl alcohol) on the growth and nucle-ation of calcium carbonate crystals. Crystal growthwas studied in two types of polymers: Nylon 66®and Kevlar 29®. Also, this study considered thepretreatment performed with a hydrochloric acidsolution and an alkaline (NaOH) solution on bothtypes of polymers. Then, the polymer fibers weresubmerged into the additive solution, which wasadsorbed by the fiber surface. These treated fiberswere then submerged into a calcium bicarbonatesolution where calcium carbonate crystals (vaterite,calcite and aragonite) began to grow adhered to thepolymer surface by passing CO

2 to the solution [36].

The results showed that the fibers pretreatmentwith acid and alkaline media influenced the natureof the calcium carbonate phases formed on the fi-bers surface. The presence of 2000 ppm of poly-acrylamide in the polymer favored the growth of ara-

Fig. 6. Mechanisms of nucleation and growth of calcium carbonate crystals. Reprinted with permissionfrom R. La]shminarayanan, S. Valiyaveettil, and G.L. Loy // Crystal Growth & Design 3 2003) 953. © 2003American Chemical Society.

Description Calcium Carbonate Polymorphs200 ppm 2000 ppm

Untreated Kevlar 29 vaterite and calcite vateriteUntreated Nylon 66 vaterite and calcite vateriteAcid-treated Kevlar 29 aragonite and vaterite aragoniteAlkali-treated Kevlar 29 aragonite aragoniteAcid-treated Nylon 66 aragonite and calcite aragoniteAlkali-treated Nylon 66 aragonite aragonite

Table 5. Calcium carbonate precipitation, before and after be treated the fibers with poly (vinyl alcohol)additive (PVA), data from [36].

gonite while calcite crystals formed in the absenceof the additive [36].

For untreated fibers, i.e. not exposed to the acidand alkaline media, facilitated the formation ofvaterite and calcite for a lower concentration of theadditive (200 ppm). When this concentration wasraised to 2000 ppm, vaterite formed on both untreatedfibers. Also, the results revealed that the pretreat-ment favored the aragonite phase on both polymers.For Kevlar 29 treated with an acid medium, the ara-gonite and vaterite phases formed for a low additiveconcentration. However, a higher concentration pro-moted aragonite formation. In Nylon 66 treated withan acid medium, the favored phases were aragoniteand calcite using a low concentration of the addi-tive. Higher concentration of PVA produced arago-nite [Table 5]. While the pretreatment increased thenumber of carboxylate and amino groups on the fi-ber surfaces, these polar groups interacted with the


PVA molecules through the hydrogen bonds. Asresult, the PVA molecules accumulated on the fibersurface facilitating the nucleation of aragonite par-ticles. On another hand, the vaterite phase formedon untreated fibers.

In summary, in this study the fibers were treatedwith an acid or alkaline solution prior to the precipi-tation of the calcium carbonate crystals, with theadditive concentration taken into account too. In thesame line as the studies presented, they managedto precipitate the three phases even though not ev-ery phase formed under the same conditions. It isworth it to notice how the nucleation and growthprocesses of the crystals have been affected by ahandful of variables like the ones presented in thisreview. The precipitation of these crystals is com-plex but very interesting since researchers can carryout different studies that permit to select a specificprocess to form one of the three calcium carbonatephases. Following the same trend set by prior in-vestigations, we decided to study calcium carbon-ate precipitation in order to formulate an organic-inorganic composite. To this purpose, our researchwork focused on variables such as PAA concentra-tion, the addition and concentration of magnesiumchloride (MgCl

2) and the reaction time.

5. SYNTHESIS OF CHITIN/CALCIUMCARBONATE COMPOSITES

As mentioned earlier, calcium carbonate has beenused in the fabrication of organic-inorganic compos-ites. These materials, particularly those based onchitin, have gained interest from researchers due totheir applications in different areas such as environ-mental engineering, biomedical engineering, andbionanotechnology.

5.1. Chitin film preparation

We prepared the chitin solution by dissolving 0.75wt.% of the polymer in an N,N-Dimethylacetamide(DMAc) solvent and lithium chloride (LiCl) (5 wt.%),which enhanced the chitin solubility in the solventsystem. After the polymer was dissolved, the chitinsolution (15 mL) was cast into a petri-dish and co-agulated with 2-propanol to promote the gelling. Theresulting sol gel was used for the calcium carbon-ate precipitation without the drying process. Fig. 7shows the resulting chitin surface with some rough-ness. Also bubbles that came up due to the coagu-lation process of the chitin solution are present.

Our investigation started with the fabrication ofchitin matrix because we would later use it for the

Fig. 7. SEM micrograph of a chitin film.

production of composites. After fabricating the poly-mer, we worked on the inorganic phase, i.e. cal-cium carbonate. In producing the carbonate, a fewvariables were considered to optimize the crystalsprecipitation, based on the literature review. Of par-ticular importance was the poly(acrylic) acid con-centration.

5.2. Calcium carbonate precipitationvia reaction

The chitin sol gel was prepared by dissolving thepolymer into an N,N-dimethylacetamide (DMAc) andlithium chloride (LiCl) (5 wt.%). After that, the chitinsolution was coagulated with 2-propanol and theresulting sol gel was used for the calcium carbon-ate precipitation.

We soaked the chitin sol gel into a 0.4 wt.%PAA solution to alter the charge density of the poly-mer film and then, submerged the film into a mix-ture of CaCl

2 (0.25M) and (NH

4)2CO

3 (0.25M) in which

the calcium carbonate precipitated.Upon the first part of this research, calcium car-

bonate precipitated using 0.25 M of CaCl2 and

(NH4)

2CO

3 and 0.4 wt.% PAA. As shown in Fig. 8, a

smaller amount of particles formed on the chitinsurface but the particles had a defined morphology.The precipitation process of calcium carbonate isaffected by the concentration of ammonium carbon-ate and calcium chloride, as shown in Figure 8. Inour research, we worked with 0.25 M ammoniumcarbonate concentration not enough to raise thesolution pH. It is well known that lower pH values(pH < 10) decrease the crystal precipitation. At highpH values (pH > 11), the nucleation rate increasedand shortened the induction time which is requiredfor the formation and subsequent transformation ofthe calcium carbonate particles.


In addition, the PAA concentration affected theprecipitation of CaCO

3 crystals, as presented in Fig.

8. PAA concentrations above 0.4 wt.% inhibited thecrystal growth. Initially, 0.4 wt.% PAA was used,which resulted in a low precipitation. As mentionedbefore, the authors found that the amount of cal-cium carbonate particles decrease on the chitosansurface at lower PAA concentrations. Additionally,they indicated that high PAA concentrations (above0.004 wt.%) simply increased the amount of PAA inthe solution, affecting its surface adsorption.

At this point we could conclude that the PAAconcentration and the ammonium carbonate affectedthe precipitation process of calcium carbonate.Since we worked with a 0.25 M ammonium carbon-ate concentration, i.e. not enough to raise the solu-tion pH, we started using a 0.50 M concentration of(NH

4)

2CO

3 instead. Similarly, a 0.4 wt.% concen-

Fig. 8. Calcium carbonate precipitation on chitin surface: vaterite (spherical crystals), calcite (cubic crys-tals) and amorphous particles. Calcium carbonate phase precipitated at 0.25 M CaCl

2, 0.25 M (NH

4)

2CO

3,

and 0.4 wt.%PAA.

tration of PAA inhibited the crystal growth. There-fore, we decided to realize an additional study ofthe effects PAA on calcium carbonate precipitationsfor our experimental conditions using 0.001 wt.%and 0.004 wt.% PAA.

5.3. Effects of poly(acrylic acid) oncalcium carbonate precipitation

From the previous sections, one can infer that addi-tives have a great deal of importance in the precipi-tation of calcium carbonate crystals. In this sec-tion, we discuss the effects of PAA concentration inthe CaCO

3 precipitation to select better conditions

for the precipitation of the crystals.With that purpose in mind, we selected two PAA

levels: 0.001 wt.% and 0.004 wt.%. For the com-posite fabrication, the chitin sol gel was immersed


Fig. 9. Calcium carbonate precipitation under (a) 0.25M calcium chloride, (b) 0.50M ammonium carbonateand 0.001 wt.% poly(acrylic acid).

Fig. 10. Calcium carbonate precipitation under 0.25M calcium chloride, 0.50M ammonium carbonate and0.004 wt.% poly(acrylic acid).

in an aqueous solution of PAA (0.001 wt.% or 0.004wt.%, as stated) followed by the addition of CaCl

2

(0.25 M) dissolved in ethanol. An ammonium car-bonate solution (0.50 M) was added to a calciumchloride and poly(acrylic acid) solution. The sol gelwas immersed into the solution for 24 hours and letit dry at room temperature. Figs. 9 and 10 presentthe PAA effect on the nucleation and growth pro-cesses of the calcium carbonate crystals. As theadditive concentration increased, the precipitationof the calcium carbonate crystals on the chitin sur-face increased. These CaCO

3 particles were later

found to be amorphous.In the precipitation process, PAA interacted with

the calcium ions to form polyelectrolytes (PAA-Ca+2),which then reacted with the polymer and becameadhered onto its surface. Next, CO

3-2 ions reacted

with the polyelectrolytes, forming the crystals. Ad-ditionally, some amount of polyelectrolytes remainedin the solution without interacting with the polymer,thus, inhibiting the nucleation and growth of the crys-tals. In summary, the precipitated carbonate, whichare initially amorphous then transformed into cal-cite, vaterite or aragonite as the reaction timepassed. This time interval is related to how muchtime the polymer is in intimate contact with the so-lution containing the reactants used for the carbon-ate precipitation.

After studying the PAA effects in the formationof CaCO

3, we turned our attention to the effects of

another well-known additive: MgCl2 (under gaseous

diffusion conditions). Magnesium chloride promotesthe crystal precipitation, as mentioned before.

5.4. Calcium carbonate precipitationvia vapor diffusion

The chitin sol gel was prepared by dissolving thepolymer in a solvent of DMAc and LiCl (5 wt.%).After the polymer dissolution, the solution was co-agulated with 2-propanol. This sol gel was used forthe calcium carbonate precipitation.

It was mentioned before that vapor diffusion isanother way of precipitating calcium carbonate crys-tals as aragonite phase. This precipitation is affectedby the concentration of MgCl

2 and the diffusion time.

In our case, the crystallization was achieved by NH3

and carbon dioxide diffusion from the decomposi-tion of ammonium carbonate, i.e. (NH

4)

2CO

2. The

gaseous diffusion experiment was conducted in aclosed box where the chitin sol gel was immersedin a solution of calcium chloride (0.25 M), PAA (0.004wt.%) and magnesium chloride. Solid ammoniumcarbonate was placed in an open vial inside theclosed box. Our results demonstrated that magne-sium chloride indeed promoted CaCO

3 precipitation

via gaseous diffusion. The aragonite phase precipi-tated at low MgCl

2 concentrations while, on the other

hand, high MgCl2 concentrations favored amorphous

CaCO3 formation. We deem important to underscore

the role of the diffusion time on the carbonate pre-cipitation. High diffusion time promoted the assem-


bling of the crystals until a CaCO3 layer is formed,

as shown in Fig. 11c. This phenomenon can occuras a consequence to a supersaturation degree inthe solution containing CaCl

2 and MgCl

2. When both

CaCl2 and MgCl

2 concentrations rose, an increase

of calcium and magnesium cations occurred. Thesecations reacted with the carbonate ions to form ei-ther calcium carbonate or magnesium carbonate.Next, the incipient crystals started to interact withthe polymer and appeared joined together on thesurface of the polymeric matrix. Magnesium cat-ions could also interact with the CaCO

3 crystals

and incorporate themselves in the crystalline struc-ture.

As shown above, MgCl2 and the diffusion time

affected the CaCO3 precipitation. However, diffusion

Fig. 11. Precipitation of calcium carbonate crystals: (a) 0.10 M MgCl2 for 2 days, (b) 0.50 M MgCl

2 for 2

days, (c) 0.10 M MgCl2 for 4 days.

Fig. 12. Calcium carbonate precipitation via vapor diffusion: (a) small desiccator for 2 days, (b) large desic-cator for 2 days, (c) small desiccator for 4 days and (d) large desiccator for 4 days.

times depend on the dimensions of the system forthe gaseous diffusion process. As consequence,we centered our attention on the desiccator size.

5.5. Effects of the close box size onthe calcium carbonateprecipitation

For this experimental segment we use desiccatorsof different sizes. The area of the larger desiccatorwas 3534 cm3, while the smaller desiccator con-sisted of 155.5 cm3. The sol gel was laid into a petridish with calcium chloride (0.25M), magnesiumchloride (0.1 M) and PAA (0.001 wt.%). Each petridish was placed in a given desiccator containingpieces of solid ammonium carbonate. Naturally, the


carbonate decomposed into ammonia and carbondioxide, which promoted the precipitation of the crys-tals. Fig. 12 shows the effects of the desiccatorsize on the carbonate precipitation.

When comparing the calcium carbonate precipi-tation in a small desiccator at 2 days and 4 days,we can observe a high amount of particles at 4 days.The particles accumulated until a non-uniform layerformed and the particles present an irregular mor-phology. Moreover, the same behavior is observedin a large desiccator at 2 days and 4 days. Thecrystal present a similar cubic morphology for bothtimes.

Once again we underscore that the precipitationof the crystals depends on the desiccator size, asshown in Fig. 12. When comparing the precipita-tion occurring in a small and a large desiccator af-ter 2 days, one can appreciate a marked morphol-ogy difference. In the small desiccator, the amor-phous phase is favored, while a cubic morphologyis observed in the large one. For a diffusion time of4 days, the precipitation of calcium carbonate be-haved similar than for 2 days. In this case, an amor-phous phase precipitated at 2 days and a cubicmorphology was observed after 4 days.

We were able to verify that the precipitation ofcalcium carbonate crystals depended on variablessuch as the calcium chloride, ammonium carbon-ate and poly(acrylic acid) concentration, the diffu-sion time and the size of the system where the pre-cipitation of the crystals under vapor diffusion takesplace. Initially, we found that the precipitation ofcalcium carbonate crystals decreased when thesame concentration of CaCl

2 and (NH

4)

2CO

3 was

used. At the same concentration, the solution wasless alkaline; this promotes less CaCO

3 precipita-

tion.Furthermore, the additive concentration affected

the precipitation of the crystals. A higher PAA con-centration increased the nucleation and growth ofthe crystals. However, PAA above 0.004 wt.% de-creased the formation of the crystals. In this case,more polyelectrolytes (PAA-Ca+2) remained in thesolution without interacting with CO

3-2 ions, which

is necessary in the precipitation of the crystals. Onanother hand, the precipitation of the crystals in-creased with the addition of MgCl

2. The inorganic

additive increased the supersaturation degree of thesolution, promoting the precipitation of the amor-phous phase. Supersaturation leads to the forma-tion of more carbonate phases, as a result of morecalcium and magnesium ions present in the solu-tion. Additionally, the size of the system in whichthe vapor diffusion takes place affected the precipi-

tation of the crystals. The amorphous phase wasfavored in the small desiccator. In closing, this workconsisted of an extensive study of the principal vari-ables that affected the precipitation of the inorganicphase, which were then thoroughly discussed.

6. SUMMARY

Once again, it should be underscored that the con-centration of additives, i.e. PAA, polyacrylamide andPVA, strongly affects the nucleation and precipita-tion of calcium carbonate crystals. Normally, twomethods for the CaCO

3 precipitation are used: gas-

eous diffusion and reaction. Some research groupsas well as our studies demonstrated how the PAAaffects the formation of the carbonate particles viachemical reaction and gaseous diffusion. As pre-sented previously, Payne et al. considered differentPAA concentrations at a specific pH value (pH =10.5) [28]. On another hand, He and coworkers useddifferent poly (acrylic acid) levels and two percent-ages of deacetylation (8%DA and 80%DA) but didnot evaluate the pH of the solution [32]. In our case,we used two PAA concentrations, 0.001 wt.% and0.004 wt.%. Even though the variables studied weredifferent, the conclusions were very similar.

Low PAA concentrations yielded two possibleoutcomes [28,32]. First, a polyelectrolyte (PAA-Ca+2) can be formed in the solution of calcium chlo-ride and PAA, which then interacts with COO- andNH+

3 groups by electrostatic forces. These interac-

tions lead to the formation of COOH, OH and NH2

groups on the chitosan surface, which increasesthe supersaturation degree of calcium carbonatecrystals in the solution. These particles then ad-here onto the chitosan surface. In the second out-come, PAA remains as a carboxylic anion in theform of PAA-Ca+2 in the solution and inhibits theprecipitation of the crystals.

High concentrations of PAA render different re-sults. At high PAA concentrations, NH

2 groups are

transformed into NH+3 groups. The NH+

3 groups then

interact with PAA molecules adsorbed on thechitosan surface. These PAA molecules react withCa+2 ions in the calcium chloride solution and PAA-Ca+2 polyelectrolytes are formed. After mixing am-monium carbonate into the CaCl

2 solution, CO

2 and

NH+3 groups diffuse into the CaCl

2 solution, and the

saturation degree increases, leading to the forma-tion of amorphous particles. Eventually these amor-phous particles are adsorbed on the chitosan sur-face and transformed into another crystalline phase.

In addition, the pH of the solution is also foundto have a great impact on the precipitation of CaCO

3


crystals. Yu and Payne studied the effects of pH onCaCO

3 precipitation by evaluating different pH val-

ues at specific PAA concentration values [28,29].They found that for pH < 10, that the particles havea regular morphology and larger size when com-pared to the particles obtained at pH > 10. Also, thenucleation rate is minimal at lower pH values. How-ever, high pH values (pH>11) trigger high nucleationrates of calcium carbonate particles with irregularmorphology.

In systems with weak polyelectrolytes relatedto additives such as PAA, polyacrylamide and PVA,the pH controls the surface charge on the polymer.The interaction between a weak polyelectrolyte anda weak acidic polymer surface produces a pH dif-ference in the solution that subsequently affects thesurface charge density on the polymer. At higherpH values, the protonation degree decreases andthe surface charge density increases affecting thenucleation rate.

The addition of magnesium (Mg+2) also have apronounced effect on the calcium carbonate pre-cipitation. Eva Loste and our research group variedthe magnesium concentration in the solution wherethe precipitation takes place [30]. Both studies foundthat magnesium (Mg+2) ions promoted the forma-tion of crystals such as magnesium carbonate andcalcium carbonate and inhibit the nucleation of cal-cium carbonate on a calcite phase, which is themost stable phase compared to vaterite and arago-nite. Magnesium adheres strongly to the carbonateions which then inhibit the growth and nucleationprocesses of CaCO

3 in the aqueous solution.

Some researchers also use acid or basic solu-tion as a pretreatment to modify the structure ofpolymer surfaces such as He et al. study with hy-drochloric acid (HCl) and sodium hydroxide (NaOH)[32]. The researchers found that the fibers pretreat-ment with acid and alkaline media affected the cal-cium carbonate phases precipitated on the fiberssurface. In general, modifying the structure of apolymer’s surface is achieved by manipulating manyvariables like the ones summarized in this review.The impact of these variables have on the precipita-tion process will vary differently from one to anotherbut nevertheless have a decisive impact on thesystem’s outcome.

Calcium carbonate is a widely studied com-pound with many applications in the field of materi-als’ science. Its precipitation, as described earlier,depends on numerous variables such as tempera-ture, pH, additive and reactive concentrations. Thesevariables are studied in order to observe their ef-fects in the precipitation of the crystals. An inter-

esting study is increasing the concentration of cal-cium chloride above 0.5 M at a specific PAA con-centration, pH value and temperature. On anotherhand, the effects of two additives at the same con-ditions could be studied. In this case, the betteradditive is identified. Additionally, the precipitationcan be conducted in desiccators with different sizes,controlling pH value, additive concentration and tem-perature. In addition, the gaseous diffusion of CO

2

using decomposition of ammonium carbonate anda constant flow of gaseous CO

2 represent an inter-

esting study. Depending on the desired end result,manipulation of these variables is of utmost impor-tance.

ACKNOLEDGEMENTS

This work is supported by the National ScienceFoundation under grant Nº0833112 CREST pro-gram). Furthermore, the authors would also like tothank the invaluable assistance of the EngineeringResearch and Development Center of the UnitedStates Army Corps of Engineers in Vicksburg, MS,and the technical personnel of the Materials Labo-ratories and Power Electronics Laboratories of theUniversity of Puerto Rico-Mayagüez Campus. Theauthors would like to thank Luis D. Ruiz for his as-sistance during the first stage of this manuscript.

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CALCIUM CARBONATE PRECIPITATION: A REVIEW OF THE CARBONATE … · particular. This review hinges on...

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