nanoparticles

International Journal of Biological Macromolecules 140 (2019) 749–760

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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Thermal degradation of calcium and sodium alginate: A greener synthesis towards calcium oxide micro/nanoparticles Patrícia dos Santos Araújo a, Gabriela Bertoni Belini b, Giovanni Pimenta Mambrini c, Fabio Minoru Yamaji a,b, Walter Ruggeri Waldman a,b,c,d,⁎ a

Graduate Program of Materials Science, Federal University of São Carlos – campus Sorocaba, Brazil Graduate Program of Planning and Use of Renewable Resources, Federal University of São Carlos – Campus Sorocaba, Brazil Physics, Chemistry and Matemathics Department, Federal University of São Carlos – campus Sorocaba, Brazil d Graduate Program of Biotechnology and Environmental Monitoring, Federal University of São Carlos – Campus Sorocaba, Brazil b c

a r t i c l e

i n f o

Article history: Received 28 May 2019 Received in revised form 6 August 2019 Accepted 12 August 2019 Available online 13 August 2019 Keywords: Alginate Metal oxide Micro/nanoparticles

a b s t r a c t Processes for nanoparticle synthesis often use toxic solvents under aggressive conditions. A greener alternative is the burning of self-organized alginate systems. We followed the influence of the CaCl2 concentrations during gelation of sodium alginate and the heating rate on the synthesis of nanoparticles by the combustion method using TGA as a reactor vessel. Samples were collected after each main process of mass loss and characterized using the Scanning Electron Microscopy, Infrared Spectroscopy, and X-ray Diffraction. Samples treated at 50 °C·min−1 presented porous structures at temperatures more than 500 °C lower than the treatments at 10 °C·min−1. All calcium alginate samples presented changing from a predominantly amorphous to crystalline structures such as Ca(OH)2, CaCO3 in the calcite phase and CaO as a function of the temperature, while sodium alginate produced mainly Na2CO3, NaOH and NaO. We observed two main correlations: 1) between the porosity and the heating rate, and 2) between the formation of crystalline structure in intermediate temperatures and the CaCl2 concentration in the gelation step. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Metal oxide micro/nanoparticles are important due to the broad range of technological applications of these materials, such as the use as a catalyst [1], as semiconductors in electronic boards [2], and magnetic particles used in medicine [3]. There are several processes for obtaining nanoparticles such as solvothermal oxidation [4], precipitation [5], sonochemistry [6] or the synthesis supported by other polymers such as polymethyl methacrylate (PMMA) [7]. In most of these cases, the processes make use of solvents or reagents that are not environmentally friendly or are complex, multi-step, and time-consuming [8]. An environmentally friendly alternative is the use of the micro/nanostructures produced by the polysaccharides to distribute the metals heterogeneously before burning/ oxidizing the matrix to form micro/nanoparticles of metal oxides. Examples of this process already developed are cellulose fibers which can provide a hydrophilic substrate for heterogeneous nucleation of ZnO [9] or starch that can form helices in water offering a hydrophobic inner core and a hydrophilic outer surface [10]. Despite being an ⁎ Corresponding author at: Graduate Program of Materials Science, Federal University of São Carlos – Campus Sorocaba, Brazil. E-mail addresses: [email protected], [email protected] (W.R. Waldman).

https://doi.org/10.1016/j.ijbiomac.2019.08.103 0141-8130/© 2019 Elsevier B.V. All rights reserved.

energy-intensive method, the greenness of this alternative lies in other aspects of the Green Chemistry as stated by Anastas and Warner [30]: a safer process of production (Principle #3: Less Hazardous Chemical Synthesis); the decrease of the use of reactants or solvents that do not incorporate in the product (Principle #2: Atom Economy); and avoid the use of solvents and auxiliaries (Principle #5: Safer Solvents and Auxiliaries). One example of a polysaccharide with a structure for the sacrificial template to produce nanoparticles is alginate. Extracted from algae, alginate is a linear, water-soluble copolymer composed of two monomer building blocks, β-D-mannuronic acid (M) and α-L-guluronic acid (G). The monomers are linked by 1–4 glycosidic bonds and may form Mblocks, G-blocks or MG-blocks. The sequence of the monomers may vary along the chain and influence their properties [11,12]. This polysaccharide has the ability to form gels, due to the affinity of alginates with certain divalent ions, such as Calcium, Barium, Zinc, Copper, among others, and the ability to bind to these ions in a cooperative and selective way [11–13]. From a given divalent ion concentration, which varies with the nature of the ion and with the G/M ratio of the alginate, a more intense interaction occurs between divalent ions of G-blocks forming the structures known as egg-boxes. (Fig. 1). Furthermore, the increase of ions concentration promotes a co-operative binding mechanism among egg-boxes [14].

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P. dos Santos Araújo et al. / International Journal of Biological Macromolecules 140 (2019) 749–760

Fig. 1. Schematic representation of the three steps of egg-box formation: A) alginate chains before the contact of calcium ions; B) the beginning of interaction between the G block and the calcium ions; C) the egg-box structure after the reaction with enough calcium ions.

The gelling property allows the use of alginate as a template for the synthesis of micro/nanoparticles of Calcium oxide. In this process, only water is used as a solvent, and mainly CO2 and H2O gases are generated as final byproducts. The technique is based on the affinity between the G-block domains and divalent ions, which provides the formation of egg-boxes promoting the heterogeneous distribution of metal in the polymer matrix. The process of using the egg-boxes as sacrificial templates for the synthesis of oxide nanoparticles lies on the more favorable oxidation of the amorphous region, made mainly of the mannuronic blocks, and the further oxidation of the egg-boxes resulting in the oxides [12,15]. The presence of the amorphous Mannuronic-rich domains between the egg-boxes is important to obtain smaller sizes of structures or particles of oxides after the degradation of the polymeric matrix. The dispersion of the ions inside the polysaccharide matrix and the low thermal degradation temperature of the biopolymer may lead to the formation of a larger number of smaller oxide crystals [16]. Therefore, with control of material concentration, reaction time and temperature, the protocol can be adjusted and directed to the shape and size desired for the final product [12]. Literature presents many examples of nanoparticles synthesis methods based on alginate template intermediate. [17] obtained nanoparticles of silver modified zinc oxide, with a nanocube format with a mean size and strong activity against gram-negative bacteria using alginate as a sacrificial template. Alginate also assisted in the formation of high purity monophasic nickel oxide (cubic phase) nanoparticles with a size of 20 to 30 nm from the thermal decomposition process of nickel alginate beads [18]. The influence of the matrix was investigated during the synthesis of Y2O3 nanoparticles [19], where there were obtained crystals with the size of 23.0 ± 1.0 nm, while the conventional method obtained the size of 30.2 ± 1.0 nm. During the synthesis of the nanoparticles, the final stage where the polymer matrix is removed occurs by degradation of this material through combustion treatment. During degradation, well-defined mass loss processes occur and can be observed in steps. Some authors have identified up to 4 stages of mass loss during degradation by thermogravimetry of their samples [13,20,21]. These steps have been attributed, first to the sample water loss, followed by the decomposition or decarboxylation of the sample (second step), decomposition of carbonaceous residue (third step) and the formation of ash and formation of oxide with the release of CO2 (fourth step). In the literature, authors usually study the protocols for the synthesis of the micro/nanoparticles using polymers as a sacrificial template but without considering the mechanism of degradation of polymers in the synthesis of micro/nanoparticles. We will explore in this work two factors which influence the mechanism of degradation, concentration of the divalent ions and the heating rate, to assess the potential of this approach for the green synthesis of micro/nanoparticles of metal oxides. The study also offered a characterization of each major step of mass loss through the analysis of morphology, chemical, and crystalline nature.

2. Materials and methods 2.1. Preparation of alginate beads We used sodium alginate (Protanal SF 120 - Batch G3703902 – extracted from Laminaria Hyperborea – 36% M block, 38% of MG + GM block, and 26% G block), provided by FMC Biopolymer, and CaCl2.2H2O by Dinâmica Ltda to prepare, respectively, solutions of 1 wt% sodium alginate and CaCl2 0.01 and 1 mol.l−1 in distilled water. Before solubilization, sodium alginate was dried until constant weight to remove moisture from the sample. The dripping of alginate solution was performed using a 50 ml burette with a distance of 2 cm between the tip of burette and the surface of the calcium solution. After dripping, we kept the beads under gentle stirring for 3 h to prevent agglomeration of particles, and more 21 h of resting (24 h total). After the crosslinking period, the calcium alginate beads were collected and washed with distilled water three times, interrupting the crosslinking process and avoiding the presence of sodium chloride residue on the surface of dried beads. The beads were dried in an oven at 100 °C until constant mass. After the drying period, the samples were washed again to remove possible calcium chloride residue from the used reagent, dried and stored.

2.2. Thermogravimetric analysis Calcium alginate gelled in CaCl2 solutions of 0.01 mol·l−1 and 1 mol·l−1 and sodium alginate were studied using the Perkin Elmer TGA with a heating rate of 10 °C and 50 °C·min−1. Gas flow of synthetic air of 20 ml.min−1 was used. The first test was performed by heating the samples to 900 °C. After the analysis of the TG curve obtained, new tests were carried out to collect the samples at the significant points of mass loss. The tests were done in duplicates, and an average initial mass of 31 ± 7 mg in alumina crucible was used. To facilitate the discussion of sample results, they were named by codes (Table 1), according to the concentration of the cation in the preparation of the gelling solution and the rate of heating used during the test by thermogravimetry.

Table 1 Codes for sodium and calcium alginate samples after thermogravimetric treatment. Ion Sodium Sodium Calcium Calcium Calcium Calcium

Concentration (mol.l−1)

Rate of heating (°C.min−1)

Sample code

– – 0.01 0.01 1 1

10 50 10 50 10 50

Na-10 na-50 Ca-0.01–10 Ca-0.01–50 Ca-1-10 Ca-1-50

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# Sample Code

0

751

% Residue Temperature (°C) 900°C 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850

1 Na-10

18

27

2 Na-50*

14

3 Ca-0.01-10

7

5 Ca-1-10 6 Ca-1-50*

14

3

32

17 5

4 Ca-0.01-50*

12

21 16

24

9

6

12

4

23 13

7 38

20.8 ±0.2

1st step

24.9 ±4.4

2nd step

11.8 ±1.7

3rd step

18.1 ±3.1

last step

11.1 ±0.2 11

19.2 ±1.4

* Samples did not finish weight loss. There was no stable poron at 900 ° C. Fig. 2. Comparison of the main mass loss processes of samples Na-10, Na-50, Ca-0.01-10, Ca-0.01-50, Ca-1-10 and Ca-1-50 in synthetic air. The number within each process is the percentage of mass loss relative to this process. The sum of the mass loss of all processes and the final residue is not 100% because the integral was only of the main processes of mass loss.

2.3. Infrared spectroscopy The analysis of the chemical groups was performed on the Nicolet IR-200 spectrophotometer in the range between 4000 and 400 cm−1, in absorbance mode with 4 cm−1 resolution in 64 scans. FTIR pellets were prepared in 10:1 dilution in KBr (by Labsynth Ltda). 2.4. X-ray diffraction XRD patterns were obtained by using XRD-6100 Shimadzu, with CuKα radiation, at 40 kV and 30 mA. The 2θ angle range was 5 to 75° at 2°, and the analysis was performed in continuous scan mode at 2°/min and 0.02° step. 2.5. Scanning electron microscopy The morphological analysis of the samples was performed using the Scanning Electron Microscopy (SEM) model, Quanta 650 FEG, from the CNPEM-LNNano laboratory. Samples were observed in several magnifications, from 200× to 200,000×, with acceleration voltage between 5 and 20 kV, according to the magnification. Samples were metalized in a Bal-Tec SCD 005 Coater, for 60 s at 40 mA. 3. Results and discussion In this work, we studied the transformations occurred with calcium alginate during the thermogravimetric analysis, collecting samples after each major process of mass loss. We varied the concentration of CaCl2 solution (0.01 and 1 mol·L−1) used in the preparation of the calcium alginate, and the heating rate (10 and 50 °C·min−1) during the TGA analysis. At each mass loss step, we stopped the heat treatment and removed the sample from the equipment for FTIR, SEM, and XRD analysis. In this section, we will first present an overview of the thermogravimetric results associated with the morphology, and then we will present the characterization by FTIR and X-ray diffractometry after each step of the thermal treatment.

temperature of the beginning of mass loss was around the same for all samples and heating rates, this extension can be attributed to the higher temperature gradient between the surface of the sample and its interior caused by the higher heating rate [22]. Despite the higher temperature range in which water loss occurs for the heating rate of 50 °C.min−1, we observed a lower amount of water lost in this rate, comparing lines 1 and 2, 3 and 4, and 5 and 6, in Fig. 2. It happened probably due to the longer time the sample is in the evaporation/boiling temperature range, having more time for this mass loss to occur before the next mass loss process begins. Probably, part of the water evaporation took place overlapped with the following process of mass loss for the heating rate of 50 °C·min−1. Regarding the different samples, the amount of water present was higher in sodium alginate (19–21%) than in calcium alginate gelled with 1 mol·l−1 CaCl2 solution (11–15%) and finally in calcium gel solution with 0.01 mol·l−1 CaCl2 solution (4.5–6%). This order was probably due to the difference in conditions for the water to permeate the polymer bulk. Sodium alginate, without any crosslinked egg-box structure, had the best conditions for diffusion of water. Among calcium alginates, the best condition for the formation of egg-box structures was the lower concentration of CaCl2, with better conditions to establish equilibrium in the formation of egg-box structures, bringing the chains closer and leaving less space for the diffusion of the water through the interior of the polymer mass. In Fig. 2 it is also observed that for the Ca-0.01–10 and Ca-0.01–50 there are two distinct processes that occurred between 200 and 350 °C because two distinct processes occurred in sequence. To discuss it better, we plotted the derivative of the thermogravimetric curves for 10 °C·min−1 rates compared with sodium alginate (Fig. 3). The mass

3.1. Thermogravimetric analysis - general overview Fig. 2 shows a summary of the main mass loss processes of the thermogravimetric curves obtained from the samples (All curves are available in Supplementary Material – Fig. 1S). The first mass loss process for all samples is usually attributed to the loss of water due to the hydrophilic nature of the alginate. We observed that this step ended at higher temperatures for the highest heating rate - 180 to 230 °C (Fig. 2 - lines 2, 4 and 6) compared to the samples heated to 10 °C·min−1 - 130 to 200 °C (Fig. 2 - lines 1, 3 and 5). Since the

Fig. 3. Comparison between DTG of Ca-0.01–10 and Na-10.

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Fig. 4. Thermogravimetric curves of Na-10 (above) and Na-50 (below) and micrographs of samples taken at each temperature indicated.

P. dos Santos Araújo et al. / International Journal of Biological Macromolecules 140 (2019) 749–760

loss processes started at around the same temperature for both samples, but for calcium alginate we observed two processes. One hypothesis for this behavior is that only part of the processes of mass loss was shifted for higher temperatures due to the presence of calcium and the eggbox structures associated with it. Regarding the final residues, no influence of CaCl2 concentration in the gelification was observed since no significant difference was observed for both heating rates, at 10 °C·min−1 (Fig. 2 - lines 3 and 5) and 50 °C·min−1 (Fig. 2 - lines 4 and 6). On the other hand, a higher quantity of residues was observed in the samples tested at 50 °C·min−1 (18–19%) compared with the samples tested at 10 °C·min−1 (11–12%), showing the influence of the heating rate in the formation of final residues. This probably was due to the shorter exposure time at higher temperatures that leads to a less efficient burning, and therefore a major quantity of solid residues at the end. Regarding Figs. 2 and 3, it is an important highlight that the sum of the mass loss of all processes and the final residue is not 100%. It happens because we calculated the integral only for the main processes of mass loss, while between processes there is also a mass loss since the values of the derivative are different from zero, which can be observed in the Supplementary material – Fig. 2S. 3.2. Morphology after each step of mass loss 3.2.1. Sodium alginate Fig. 4 shows the mass loss evolution of sodium alginate as a function of temperature for two heating rates, 10 and 50 °C·min−1, and the micrographs obtained by SEM analysis after the end of each stage of mass loss. The first process of mass loss, attributed to the evaporation of water, was observed up to 210 and 230 °C, respectively to Na-10 and Na-50. Similar materials were produced, with the beads still intact at lower magnifications and without significant formation of discontinuities or pores at higher magnifications. The second process was the main process of mass loss, observed between 210/230 to 310 °C. Both heating rates presented an agglomeration process, without the presence of the fibers and granules of the starting material. The difference is that at the lower heating rate micropore formation of 5 to 10 μm is observed with some impurities with approximately 1 μm size, whereas in the higher heating rate, there was the formation of plates with cracks and fractures. The end of the heating in thermobalance was at 700 °C for Na-10 and at 900 °C Na-50. For the sample Na-10, particle agglomerates with network structure and predominance of openings and pores of different shapes and sizes were observed. For the sample Na-50, we observed some sites with roughed textured, stick-shaped particles and apertures and pores. The different temperatures for the different heating rates experiments were due to the impregnation of the residue in the alumina crucible. It was not possible the removal for analysis, so the samples were collected at 700 °C. On the TG curves, both rates presented similar behavior with some differences: DTG peak of the second mass loss process for the sample Na-50 was more intense, less broad and at a higher temperature. Only the sample with the lowest heating rate presented a DTG peak between 600 and 700 °C, and at the end of the processing Na-50, there was no plateau formation in the DTG curve, indicating that the mass loss process was not completed. 3.2.1. Calcium alginate 0,01 mol·l−1 The thermogravimetric analysis at both rates (Fig. 5) showed approximately 5 to 7% of moisture loss during the first process. In the observed micrographs of the material collected after the process of moisture loss, the samples obtained in both heating rates presented the preserved format, with a smooth texture and without porosity. The sample Ca-0.01–10 presented two distinct processes in the range between 200 and 350 °C, while in the sample Ca-0.01–50 the

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same processes presented a shorter range of temperature, 230-340 °C, and an overlapping without a clear delimitation between the processes. For the samples under thermal treatment at 10 °C·min−1, samples were collected at 265 and 350 °C, since the delimitation between the mass loss processes was clear. It was observed that there was a crack formation in the mass loss process between 265 and 350 °C. For the Ca-0.0150, porosities at 340 °C were observed in the same temperature range, which was not apparent in the samples Ca-0.01-10 evaluated at 350 °C. After 350 °C, both heating ratios showed slow and continuous mass loss processes until the end of the thermal treatment. It was observed an event at 560 °C in all the replicates for Ca-0.01-10 (all the replicates in Supplementary Material – Fig. 3S). We have collected material after this event and observed internal pores while externally the sample had a heterogeneous surface, with continuous regions with a flat surface and other regions with a rougher surface. Additionally to the morphological characterization, we can also conclude about the different mass loss mechanisms each sample can be lead according to the different conditions of thermal treatment. At the end of the heat treatment, we can observe the influence of the heating rate not only in the mechanisms of mass loss but also in the formation of the inorganic products at 900 °C. The particles produced at 50 °C.min−1 have structures with edges or stems, being predominant the shape of stems of 15 to 20 μm with angles of approximately 90° between them, while the particles produced at 10 °C·min−1 presented the shape of small granules and stems between 1 and 4 μm. 3.2.1. Calcium alginate 1 mol·l−1 We observed three mass loss processes in TG curves obtained for Ca1-10 and Ca-1-50 (Fig. 6). Both presented approximately 13 and 14% of moisture mass loss respectively during the first process. The second mass loss step for the two heating rates was divided into three overlapped processes, observable in the first derivative of the TG curve. For Ca-1-10, this process started at a lower temperature and had a longer range. At the end of the experiment, the lower heating rate presented a plateau in the thermogravimetric curve, while the sample Ca-1-50 still showed mass variation, indicating that there was still a mass loss process in progress. After the loss of moisture, the samples in the two heating ratios maintained the spherical shape, with signs of breakage-forming degradation that could later become pores. The texture of this material is heterogeneous with rough spots and wrinklings in several directions. After the main step of mass loss, sample Ca-1-10 showed increased roughness, with more frequent and larger openings. In sample Ca-150 collected at 340 °C, the material has a continuous and rough textured surface on the outer face and internal rounded pores of dimensions between 500 nm and 10 μm. At 900 °C no pores were found in both samples. The sample Ca-1-10 shows a rough and continuous surface and the sample Ca-1-50 shows less apparent roughness and more cracks. 3.3. Infrared spectroscopy We obtained FTIR spectra from the samples Na-10 (Fig. 7-A), Na-50 (Fig. 7-B), Ca-0.01-10 (Fig. 7-C), Ca-0.01-50 (Fig. 7-D), Ca-1-10 (Fig. 7E) and Ca-1-50 (Fig. 7-F), all collected after the different processes of mass loss. The main absorptions regions were between 3460 and 3485 cm−1, assigned to the OH bond; between 1640 and 1650 cm−1, assigned to the asymmetric C_O bond of carboxyl groups; and, for the most of the samples collected at the end of the thermal treatment, at 1420 cm−1 corresponding to the symmetric C_O [23]. The bands between 3450 and 3480 cm-1 (hydroxyls) indicate the presence of water or the production of calcium hydroxide, Ca(OH)2, the remaining component of the carbonation process [24–26]. The calcium hydroxide is also produced through the reaction between calcium oxide, CaO, and the water released during the oxidation of the alginate [27]. The band between 1645 and 1420 cm-1 is attributed to the

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Fig. 5. Thermogravimetric curves of samples Ca-0.01–10 (above) and Ca-0.01–50 (below) and micrographs of samples taken at each temperature indicated.

P. dos Santos Araújo et al. / International Journal of Biological Macromolecules 140 (2019) 749–760

Fig. 6. Thermogravimetric curves of the samples Ca-1-10 (above) and Ca-1-50 (below), and the micrographs of the samples removed at each temperature indicated.

755

756

P. dos Santos Araújo et al. / International Journal of Biological Macromolecules 140 (2019) 749–760

Absorbance (a.u.)

3464

3484 3470

700 °C

C=O 1648

310 °C

1642

210 °C

1642

B

1420

3500

3000

2500

2000

1500

Sodium alginate - 50 °C.min

3470

C=O

900 °C 3484

1645

310 °C 3470

1000

500

4000

Starting material 1645

3500

3000

-1

D

C=O ou C-O ( CaCO3) 1645 1456

560 °C

Absorbance (a.u.)

3483

-1

Calcium alginate 0,01 mol.l - 10 °C.min 3475

CaCO3

880

3450

350 °C

3480

265 °C 3470

1645

1429

1625

1429 1095 - 1026

C=O

200 °C

OH*

Starting material 1625 1430

3500

3000

2500

2000

1500 -1

900 °C

Starting material 1625

OH*

1430

1000

500

4000

3500

3000

2500

F

-1

3465

Starting material

3500

1448

1630

1433 1090 - 1025

-1

3000

2500

2000

-1

1500

Wavenumber (cm )

500

500

1642

1433

1642 3465 OH*

4000

3500

CaCO3

879

1433

340 °C

*H2O e Ca(OH2) 1000

-1

C=O ou C-O ( CaCO3)

900 °C

1445

C=O

*H2O e Ca(OH2) 4000

1642

1000

Calcium alginate 1 mol.l - 50 °C.min

3465

1420

200 °C 2370

1500 -1

Absorbance (a.u.)

Absorbance (a.u.)

OH*

2000

Wavenumber (cm )

1642

400 °C

1090 - 1030

*H2O e Ca(OH2)

C=O ou C-O ( CaCO3)

3475

1420

C=O 1642

230 °C

3464

3470

1648

-1

1468

340 °C

3464

1090 - 1030

-1

900 °C

500

C=O ou C-O ( CaCO3) 1642 1642

Calcium alginate 1 mol.l - 10 °C.min

3460

1000

-1

Wavenumber (cm )

E

1500

Calcium alginate 0,01 mol.l - 50 °C.min

3464

C-O

1645

*H2O e Ca(OH2) 4000

2000

Wavenumber (cm )

Absorbance (a.u.)

C

2500

-1

Wavenumber (cm )

3475

1645

230 °C

OH

-1

1645

3464

Starting material 1645

OH

4000

-1

Absorbance (a.u.)

A

-1

Sodium alginate - 10 °C.min

3470

1648

230 °C

1090 - 1025

1630 Starting material 1445 3000

2500

2000

1500

-1

1000

500

Wavenumber (cm )

Fig. 7. FTIR spectra of samples of Na-10 (A) and Na-50 (B), Ca-0.01–10 (C) and Ca-0.01–50 (D), Ca-1-10 (E) and Ca-1-50 (F), obtained at different temperatures during the thermal treatment.

carbonyl of the carboxyl group of the alginate [28]. Some authors consider that, in the case of the sample containing calcium, the bands between 1400 and 1500 cm-1 correspond to the CO bond of the

carboxylate, as well as the band at 880 cm-1, originated from the formation of CaCO3 [24–26]. Absorption between 1000 and 1100 cm-1 can be considered C\\O stretch [29]. In some spectra, the weak absorption in

P. dos Santos Araújo et al. / International Journal of Biological Macromolecules 140 (2019) 749–760

the region in 2359 cm-1 is related to the presence of atmospheric CO2. In Fig. 7 it is observed that with the increase of temperatures and the decrease of the heating rate, the band between 1400 and 1450 cm-1 (carbonate group), increases in comparison with the band at 1640 cm-1, (carbonyl from alginate), evidencing the conversion of the polymer in an inorganic salt as a function of the thermal treatment. At the final temperatures, Ca-0.01–10 and Ca-1-50 showed the band related to the carbonate, 880 cm−1. The formation of carbonate is relevant since the CO2 released in the degradation of the polymer can react with the oxide formed, generating CaCO3. 3.4. X-rays diffraction The results of X-ray diffraction were analyzed using the JCPDS (Joint Committee Powder Diffraction Standards) database for the attribution of crystalline phases and structures. 3.4.1. Sodium alginate The diffractograms of sodium alginate samples calcined at 10 °C. min−1 (Fig. 8-A) and 50 °C·min−1 (Fig. 8-B) show predominantly amorphous structure after the loss of water (210 and 230 °C, respectively) and the beginning of a crystallographic peak formation at 35° for both heating rates after the main mass loss process. In the final step, 700 °C for the lower heating rate and 900 °C for the higher heating rate, the samples presented peaks of Na2CO3 (standard n° 86-281), Na2O (standard n° 2-1285), and Na(OH) (standard n° 88-2129), with different intensities. 3.4.2. Calcium alginate The diffractograms obtained from calcium alginate samples gelled in CaCl2 solutions at concentrations of 1 mol·l−1 and 0.01 mol·l−1 at different heating rates at 10 °C and 50 °C·min−1, are shown in Fig. 9. A distinction can be seen in the evolution of the crystalline composition of the gelled alginates at different concentrations of CaCl2. For alginates gelled in 0.01 mol·l−1 CaCl2 solution, peaks at 32° and 46° are already observed at 340–350 °C, while for the gelled samples in 1 mol·l−1 CaCl2 solution the diffractogram is of a predominantly amorphous substance. At higher heating rates, one consequence is that the samples are exposed less time at higher temperatures, leading to lower efficiency in the conversion of CaCO3 to CaO. This behavior can be seen in Fig. 9 (B) and (D), at the rate of 50 °C.min−1, where there is a more complex composition associated with the Ca(OH)2 (standard n° 50-8), CaCO3 (standard n° 86-2340) and CaO (standard n° 37-1497) mixture. At

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lower heating rates of decomposition, Fig. 9 (A) and (C), the material formed after heating to 900 °C has a more definite composition and closer to CaO. It is possible to observe that for Ca-0.01-10 (Fig. 9-A) there is already formation of diffraction peaks related to Ca(OH)2 at 350 °C, which evolved at the temperature of 560 °C for peaks associated with Ca (OH)2 and CaCO3, and finally at 900 °C presents a diffractogram associated with CaO and Ca(OH)2. Making an association with the results obtained from SEM, the samples with the most complex morphology at the end of the thermal treatment, Ca-0.01-10 and Ca-0.01-50 (Fig. 5), were also the more complex regarding the chemical composition (Fig. 9-A and B). 3.4.3. Origin of Ca(OH)2 Concerning Ca(OH)2 in the diffractogram obtained with the final sample at 900 °C (Fig. 9-A, B, and D), its presence may be due to two possible origins: 1) To the reaction of the CaO obtained from the humidity of the atmosphere with which the oxide had contact during the handling and storage or even the reaction between CaO and H2O formed during the oxidation of alginate during thermal treatment; 2) Ca(OH)2 residual of the thermal process, which has not yet evolved/ reacted to CaO. Evidence in this work that may indicate what the most likely hypothesis is the result of burning Ca-1-10 (Fig. 9-C) which formed CaO without the presence of Ca(OH)2 in the final product at 900 °C, or as an intermediate during the major mass loss process. As all samples were stored and handled in the same way, if the first hypothesis were the most likely, Ca(OH)2 peaks would also appear in this sample. It was not possible to understand why the sample produced with 1 mol. l−1 leads to the formation of CaO without Ca(OH)2 as an intermediate or contamination in the final product. 4. Conclusions The gelling conditions influenced the thermogravimetric results: the lower the calcium concentration, lower the moist in the beads, and better is the separation of the processes of mass loss. Samples of calcium alginate produced more final residues than sodium alginate in the same conditions. Porous structures were found at 340 °C in both the samples treated at 50 °C·min−1 while at a lower heating rate a porous product was

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found only at the final temperatures. This is relevant information if alginate is used as a support for catalysis or as an adsorbent. In the diffraction study, it was observed that, during the burning, the calcium alginate changed from a predominantly amorphous structure to form crystalline structures such as Ca(OH)2, CaCO3 in the calcite phase and CaO in the final stages, while sodium alginate changed also from a predominantly amorphous structure to Na2CO3, NaOH and NaO. In summary, we presented here a well-detailed study about the synthetic steps of calcium oxide by the alginate decomposition method. Thermogravimetry results in addition to spectrometric and diffraction analysis provided a data set that allowed to investigate each step in the synthetic method proposed in this work.

Acknowledgements FMY acknowledges CNPq for the grant 407044/2013-2, WRW acknowledges São Paulo Research Foundation (FAPESP) for the grant 2016/24936-3, and PSA acknowledges Capes for the fellowship 1658297. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2019.08.103.

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