amidoximated starch beads and CaCO3

Journal Pre-proofs Composite materials based on chitosan/amidoximated starch beads and CaCO3 Diana Felicia Loghin, Claudiu-Augustin Ghiorghita, Oana Maria Munteanu Blegescu, Marcela Mihai PII: DOI: Reference:

S0022-0248(19)30489-0 https://doi.org/10.1016/j.jcrysgro.2019.125274 CRYS 125274

To appear in:

Journal of Crystal Growth

Received Date: Revised Date: Accepted Date:

15 February 2019 29 August 2019 6 October 2019

Please cite this article as: D.F. Loghin, C-A. Ghiorghita, O.M.M. Blegescu, M. Mihai, Composite materials based on chitosan/amidoximated starch beads and CaCO3, Journal of Crystal Growth (2019), doi: https://doi.org/ 10.1016/j.jcrysgro.2019.125274

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© 2019 Published by Elsevier B.V.

Composite materials based on chitosan/amidoximated starch beads and CaCO3 Diana Felicia LOGHIN,1 Claudiu-Augustin GHIORGHITA,1 Oana Maria MUNTEANU BLEGESCU,2 and Marcela MIHAI1* 1“Petru

Poni” Institute of Macromolecular Chemistry, 41 A Grigore Ghica Voda Alley, 700487

Iasi, Romania. * [email protected] 2S.C.

Azomures S.A., Gheorghe Doja Str. 300, Tirgu Mures, 540237, Mures, Romania

Abstract. Herein, beads based on chitosan and amidoximated starch were used, for the first time, as support materials for CaCO3 crystallization. The CaCO3 crystal growth was conducted by using different carbonate sources (Na2CO3, dimethyl- and diethyl-carbonates) and different strategies (fast, alternate or slow diffusion). The interaction of the obtained composite beads with Cu(II) ions was also investigated. The morphology of composite beads, before and after interaction with Cu(II) ions, was followed by SEM, while the formed polymorph types was determined by FTIR spectroscopy. Highlights  Composite beads were used as support materials for CaCO3 crystal growth  Three carbonate sources (sodium, diethyl and ammonium carbonate) for CaCO3 growth  Ratio between the formed crystals depend on the applied crystallization method  Interaction of Cu(II) ions with the beads with crystallized CaCO3 was investigated

Keywords: A1. Characterization; A2. Growth from solutions; B1. Biological macromolecules; B1. Calcium compounds 1. Introduction In nature, the formation of biominerals may occur by biological induced or controlled inorganic crystallization using different organic templates or guiding molecules, sometimes in organic preformed compartments. The crystal growth rate, the particle size, morphology and stability of inorganic particles, are often influenced by the presence of macromolecules. Thus, a wide variety of organic additives have been already tested for their influence on the calcium carbonate crystallization [1-6]. Polysaccharides, such as starch, pectin and dextran, represent an important group of organic macromolecular additives for optimizing the calcium carbonate

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properties [1,7-10]. Also, it is assumed that chitosan plays an important role on the formation of inorganic–organic structure of natural formed shells [11]. In synthetic systems, the crystallization process was studied using a large range of strategies, inspired by the biological mechanisms [12]. Among them, calcium carbonate-based materials are the most studied systems [13]. For instance, ion exchanger beads were used for CaCO3 growth, the crystallization mechanism depending on the functional groups of the polymeric beads [14, 15]. The further interaction of CaCO3–modified ion exchangers with Cu2+ ions depended both on the polymer chemical structure and the CaCO3 polymorphs. Beads derived from polysaccharides represent cost-effective and eco-friendly alternatives to synthetic ion exchangers, being of extremely interest for various applications, such as bio-sorbents, carriers, and target-release containers [16]. Among known minerals, carbonates are well known as effective materials in removing or retaining heavy metals, most probably by a combined mechanism of ion-exchange and precipitation on the carbonate surface [17]. Heavy metals ions presence induced a permanent toxic effect of contaminated waters. Due to their high reactivity, the carbonate and bicarbonate ions often control the presence of metal ions traces in natural and waste waters [18]. Previously performed sorption experiments have shown that different divalent cations are strongly adsorbed by the calcium carbonate surface, the capacity for heavy metals retention being very high. The investigation of the interactions between trace heavy metals and carbonate minerals is important for understanding their role in the crystal growth and/or growth inhibition processes. In this context, some organic beads obtained by ionotropic gelation/covalent cross-linking of chitosan and amidoximated starch [19] were used as support materials for CaCO3 crystal growth, applying different methods for crystal growth, namely the adding protocol of inorganic partners (rapid mixing, alternate dipping or slow diffusion) or the carbonate source (Na2CO3, dimethyl- and diethyl-carbonates). Also, the interaction of the obtained composite beads with Cu(II) ion was investigated. The morphology of the new composites, before and after interaction with copper ions, was investigated by SEM, while the CaCO3 polymorphs content was assessed by FTIR spectroscopy. 2. Experimental Section 2.1. Materials CaCl2 ∙ 2 H2O, Na2CO3 and CuSO4 · 5 H2O from Sigma-Aldrich, as well as dimethyl- and diethyl-carbonates (DMC and DEC) from Merck were used as received. Millipore grade water -2-

with a conductivity of 0.055 µS/m was used throughout all experiments. Chitosan/modified starch beads were obtained using a previously described method [19]. Briefly, for beads formation, ionotropic gelation was performed with sodium tripolyphosphate, while the covalent cross-linking was performed with epichlorohydrin, by applying two strategies: (i) through mixing of already prepared amidoximated starch with chitosan (samples code: A), and (ii) by mixing the potato starch-g-poly(acrylonitrile) (PS-g-PAN) copolymer with chitosan solution, the amidoximation of the nitrile groups taking place inside the beads (samples code: D). 2.2. Preparation of CaCO3 based composite microparticles The CaCO3 crystal growth on beads was carried out at 22 oC using different carbonate sources, applying the following protocols: (1) the beads were immersed in 0.2 M CaCl2 aqueous solution for 1h, followed by rapid addition of an equal volume of 0.2 M Na2CO3 aqueous solution and vigorous stirring for 1 min, followed by 20 min in static conditions. (2) the beads were immersed in 0.2 M CaCl2 aqueous solution for 20 min, then rinsed with distilled water, followed by a similar procedure with 0.2 M Na2CO3 aqueous solution, each step repeating for 4 times. (3) the beads were introduced in 50 mL aqueous solutions of 0.2 M CaCl2, and then 1.5 mL of DEC were added under constant stirring. The CaCO3 precipitation started after adding 10 mL of NaOH solution (1 M) to the reaction medium at room temperature. The mixture was stirred for 1 min on a magnetic stirrer, at room temperature, and then kept under static conditions for 30 min. (4) the protocol is similar to (3), except that instead of DEC we used 1 mL DMC. (5) the beads were immersed in 50 mL of 0.2 M CaCl2 aqueous solutions, followed by the addition of 5 mL of 25% NH4OH solution and 1.5 mL of DEC, under stirring for 1 min. Then the reaction vessel was hermetically closed and kept for 24 h under static conditions. (6) the protocol is similar to (5), except that instead of DEC we used 1 mL DMC. The obtained composite materials were intensively washed with water and finally dried for 24 h at room temperature. Henceforth, the samples are coded Xn, where X represents beads type (A or D), and n is the number of crystallization protocol. 2.3. Cu(II) interaction with CaCO3/beads composites The reactivity of CaCO3/beads composites with copper ions was tested using a batch equilibrium procedure, using approximately 10 mg of dry CaCO3/beads and 10 mL aqueous

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solution of 100 mg·L-1 CuSO4·5H2O with pH = 6, at room temperature, for 5 h. The resulting composites were thoroughly washed with distilled water. 2.4. Characterization methods The surface morphology of the composite beads, before and after interaction with copper ions, was examined using an FEI Environmental Scanning Electron Microscope type Quanta 200, operating at 30 kV with secondary electrons, in high vacuum mode. The chemical composition of the composite beads, before and after CaCO3 crystallization was investigated by infrared spectroscopy, in attenuated total reflectance (ATR) mode, using a Vertex 70 Bruker FT-IR spectrometer. The polymorph content has been determined from FTIR-ATR spectra following the method proposed by Vagenas et al. [20], taking into account the absorption peaks at 745 and 713 cm-1. The crystal phases (in mg) has been calculated applying the following relations: CV = A745/αV745

(1)

CC = A713/αC713

(2)

where subscripts C and V denoted calcite and vaterite phases, respectively, Axxx is the measured absorption, and α is the calculated absorptivity for each polymorph and absorption band [20]: αv745= 21.8 mm2 mg-1, αC713= 63.4 mm2 mg-1. The percentile polymorph content was then calculated and discussed. 3. Results and Discussion The controlled inorganic crystallization by different organic templates or guiding molecules was intensively studied in recent years, mainly for understanding the natural occurring similar processes. Herein, beads obtained by embedding in chitosan matrix some micron-size particles of either amidoximated starch or PS-g-PAN were used as templates for CaCO3 growth [19]. The initial beads are spherical microparticles with a medium diameter of about 0.9 – 1.5 mm, their morphology depending on their synthesis strategy: even if the surface of both beads reveals relatively dense structures, the inner morphology of D-type beads was characterized by interconnected pores and a less dense morphology. The growth of CaCO3 on beads is expecting to take place by the interaction of the amidoxime groups of modified starch and/or amine group from chitosan with Ca2+ ions by chelation, thus providing the nucleation centers upon which crystallization can occur. To deeply investigate the growth of CaCO3 on beads, different carbonate sources (Na2CO3, DEC or DMC) and reaction duration were applied, Figures 1 and 2 showing the SEM images obtained on the beads surface for all used crystallization methods.

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Figure 1. SEM micrographs of A - CaCO3 composites.

Figure 2. SEM micrographs of D - CaCO3 composites. As shown in Figures 1 and 2, all the applied methods for CaCO3 growth on the beads were effective, irrespective of carbonate source (Na2CO3, DEC or DMC) or the beads synthetic strategy, all samples showing a layer of CaCO3 microparticles grown on beads surface. A correlation between the bead synthesis strategy and the formed CaCO3 crystals can be observed, the beads prepared by in situ amidoximation of PS-g-PAN (type D beads) leading to the

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formation of larger CaCO3 crystals than the beads obtained by the mixing of amidoximated starch with chitosan (type A beads). The applied crystallization method also influences the size of the crystals grown on the beads. Thus, applying the most usual method, mixing rapidly CaCl2 and Na2CO3 (method 1), the formation of small crystals on beads surface took place, irrespective of beads synthesis protocol. Using the same reactants but alternately dipping the beads in the corresponding solutions (method 2), led also to the formation of an excess of CaCO3 microparticles, as well as a thin inorganic film on the beads surface. When DEC or DMC were used, i.e. when the precipitation of CaCO3 took place as a result of the hydrolysis of di(m)ethylcarbonate in alkaline medium (methods 3-6) by the release CO2, the formed CaCO3 microparticles layer depended both on ethyl or methyl carbonate type and the mode of carbonate formation. Thus, when the fast method was applied (methods 3 and 4) CaCO3 layer on beads surface grew less uniform as compared to corresponding crystals layer based on Na2CO3 (method 1), irrespective of beads synthetic route. When the slow method was used (methods 5 and 6), the controlled precipitation of CaCO3 by in situ hydrolysis of di(m)ethyl carbonate in CaCl2 solution took place, which simulates the slow formation of CaCO3. In this case, due to the slow generation of carbonate ions through the hydrolysis of di(m)ethyl carbonates in the alkaline solution (pH ~ 10), the formation of CaCO3 was retarded to several hours, thus ensuring a controllable precipitation of CaCO3 like in biomimetic synthesis. Thus, nice calcite rhombohedra appear on the external surfaces of beads, as a consequence of the decreasing supersaturation at the end of the crystallization process. Also, the 24h long reaction enables the growth of large crystals on beads surface, D-type beads being more uniformly covered by CaCO3 crystals. To evidence the polymorph selection on the CaCO3 mineralization on beads surface, following the above described crystallization pathways, FTIR spectra were recorded and CaCO3 polymorphs were quantified taking into account the absorption peaks at 745 and 713 cm-1 (Figure 3).

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100 80 60 40 20 0 A1

A2

A3

A4

A5

A6

D1

Vaterite

Calcite

D2

D3

D4

D5

D6

Figure 3. FTIR-ATR spectra of A and D beads and corresponding CaCO3 composites and the calculated calcite and vaterite content The FTIR spectra of CaCO3 composite beads presented in Figure 3 show some fundamental bands of CaCO3 calcite and vaterite polymorphs [20], beside some characteristic groups of cross-linked polymers. As our previous studies shows [21], the selection of polymorphs depends on different factors such as temperature, environmental pH, concentration of individual components, over-saturation, ionic strength or impurities. At currently aplied reaction conditions, during the initial CaCO3 crystallization stage unstable amorphous phase is probably formed, which subsequently transforms into vaterite, a metastable phase, and calcite, a stable phase. Vaterite is evidenced by the peaks at 1088 cm-1 – the symmetric stretch (ν1), and at ~745 cm-1 – the carbonate in-plane bending absorption (ν4). The bands at 841-852 (ν2) and 712 (ν4) cm-1 are characteristic for calcite. Thus, the FTIR spectra show that the samples -7-

contain a mixture of calcite and vaterite crystals, with less calcite obtained by the DEC based crystallization method, irrespective of beads type. As shown in Figure 3, the polymorph content is influenced mainly by the method of introducing carbonate ions in the crystallization systems and less by the beads synthesis pathway. The higher content in calcite was obtained for the methods with the longest reaction time (methods 2, 5 and 6). At applied reaction conditions (high supersaturation) both vaterite and calcite nucleate simultaneously, according to their solubility constants [22]. Vaterite is the less stable phase, its nucleation rate being faster than that of calcite. At high supersaturation, spontaneous precipitation of calcium carbonates from supersaturated solutions led to the formation of vaterite, which further agglomerate as spherical particles. However, vaterite is not thermodynamically stable, and therefore could dissolve in the solution and re-precipitate as calcite. As the particles were separated and dry in about 1h, the vaterite particles are stable. In the diffusion-controlled experiments (methods 5 and 6) the supersaturation increased gradually by the diffusion of the solute into the crystallization medium, favoring thus the transformation of vaterite crystals to calcite ones. Thus, calcite is preferentially formed in the diffusion-controlled experiment. Cu(II) interaction with CaCO3/beads composites As the previous study shows [19], A and D type beads have high affinity for Cu(II) ions, the interaction taking place both by chelation of the amidoximated groups of modified starch and of the functional groups of chitosan. Also, copper usually precipitates from sulfate solutions on solid CaCO3 in the form of basic sulfates [14,15]. Therefore, CaCO3 crystals grown on beads surface would increase the reaction capacity of composites vs pristine beads, as a synergic effect of chelation properties of beads and of CaCO3 characteristics, given by the amount of inorganic content on beads surface and the formed polymorphs. The newly formed patterns on the CaCO3/beads after interaction with Cu(II) have been followed by SEM (Figures 4 and 5).

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Figure 4. SEM micrographs of A-CaCO3 composites after interaction with Cu(II) ions

Figure 5. SEM micrographs of D-CaCO3 composites after interaction with Cu(II) ions The differences between the morphologies of the A and D beads and the CaCO3 polymorph ratio influence their interaction with Cu(II) ions. As SEM images in Figures 4 and 5 show, seems that part of CaCO3 has been removed from the beads surface and part was transformed by copper-calcium exchange reaction. Previous studies [14,15,23] suggested that during the interaction of CaCO3 with Cu(II) ions, both the transformation of vaterite to calcite and the formation of copper-carbonate complexes could take place by adsorption of Cu(II) at the -9-

CaCO3-water interface with rapid dehydration and formation of coordinated monodentate complexes, accumulation of Cu(II) complexes at the CaCO3 surface, and the incorporation of Cu(II) ions into the CaCO3 crystals lattices. Also, the chemical exchange reaction between CuSO4 and CaCO3 can take place with the formation of different compounds, part of them precipitating also on the beads surface.

Figure 6. FTIR-ATR spectra of A and D beads and corresponding CaCO3 composites after interaction with Cu(II) ions According to Schosseler et al [24] different chemical processes could occur upon addition of Cu(II) ions to CaCO3 surface: dehydration due to the adsorption of the trace metal ions from solution; accumulation of square-planar or square-pyramidal copper complexes at exposed surface sites, integration into the calcite lattice with the tetragonal distortion axis between three possible orientations. Herein, after interaction with CuSO4 solution, the formation of azurite [Cu3(CO3)2(OH)2] and CaSO4 dihydrate (gypsum) is sustained by the FTIR-ATR spectra of samples A and D with CaCO3 after interaction with Cu(II) (Figure 6): ~1025 cm-1 (stretching OH bands) and at ~ 870 cm−1 (CO bending) of azurite; ~ 620, 670 and 1155 cm-1 correspond to the stretching of the sulphate group in gypsum [24]. Actually, the characteristic peaks of azurite can be found in the 1600 to 1400 cm-1 spectral region, due to the ν3 stretching vibrations of the CO32-, and therefore it is difficult to discriminate the calcium and copper carbonates in this spectral region.

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Conclusions The present study follows the growth of CaCO3 crystals on beads based on chitosan and modified starch, by using different carbonate sources (Na2CO3, dimethyl- and diethylcarbonates). It was found that when the precipitation of CaCO3 was promoted by the hydrolysis of di(m)ethylcarbonate in alkaline medium, the formed inorganic layer grew thicker and more uniform. FTIR spectra evidenced a mixture of calcite and vaterite crystals, irrespective of beads type, the ratio between the formed crystals depending on the applied crystallization method. After interaction with Cu(II) ions the composite beads surface was changed, probably due to the incorporation of copper ions into the lattice of CaCO3 crystals and the formation of azurite and gypsum. Our results support the concept of a dynamic calcium carbonate surface, by continuously forming and dissolving CaCO3 polymorphs as a function of environmental changes up to the equilibrium is reached (when the formation and dissolution rates are equals). Also, the structural information obtained for Cu(II) interaction by the dynamic exchange of copper, calcium and carbonate ions between the solid and aqueous media forming different structured surface layer on polymer beads, provide a better understanding of its interaction with calcium carbonate minerals. Acknowledgements: This work was supported by a grant of the Ministry of Research and Innovation, CNCS-UEFSCDI, Project number PN-III-P1.1.PD-2016-1313, within PNCDI III. References 1. J. Kontrec, D. Kralj, Lj. Brecevic, G. Falini, Influence of some polysaccharides on the production of calcium carbonate filler particles, J. Crystal Growth 310, 4554–4560, 2008 2. F. Tewes, O. L. Gobbo, C. Ehrhardt, A. M. Healy, Amorphous Calcium Carbonate Based-Microparticles for Peptide Pulmonary Delivery, ACS Appl. Mater. Interfaces 82, 1164-1175, 2016 3. S. Utech, Al. R. Boccaccini, A review of hydrogel-based composites for biomedical applications: enhancement of hydrogel properties by addition of rigid inorganic fillers, J. Mater. Sci. 51, 271–310, 2016 4. Y. Xu, K.C.H. Tijssen, P.H.H. Bomans, A. Akiva, H.Friedrich, A.P.M. Kentgens, N.A.J.M. Sommerdijk, Microscopic structure of the polymer-induced liquid precursor for calcium carbonate, Nature Commun., 9, Article number 2582, 2018

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