Thermal Decomposition of Biomineralized Calcium Carbonate

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Thermal Decomposition of Biomineralized Calcium Carbonate: Correlation between the Thermal Behavior and Structural Characteristics of Avian Eggshell Yoji Tsuboi, and Nobuyoshi Koga ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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ACS Sustainable Chemistry & Engineering

Thermal Decomposition of Biomineralized Calcium Carbonate: Correlation between the Thermal Behavior and Structural Characteristics of Avian Eggshell

Yoji Tsuboi and Nobuyoshi Koga* Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan

*E-mail: [email protected]

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Abstract

This study focused on the thermal decomposition of biomineralized CaCO3, using avian eggshell. Biomineralized CaCO3, which exhibits a specialized structure, is a possible source of CaO used across various applications, including CO2 capture. An understanding of the relation between the thermal decomposition kinetics in producing CaO and the original structure of biomineralized CaCO3 may contribute to the further effective use of biowastes. The thermal decomposition of avian eggshell occurs via two mass-loss processes: the primary thermal degradation of the outer shell membrane and the subsequent thermal decomposition of calcite in the shell matrix. Each mass-loss process is composed of multiple reaction steps. The partially overlapping reaction steps originate from the structural characteristics of the eggshell, in addition to the physico-chemical properties of the reactant in each process. The overlapping features of the component reaction steps were revealed by a detailed kinetic analysis of mass-loss curves; thus determining the contribution and kinetic parameters of each reaction step. The separated reaction steps were correlated with the corresponding reaction region in the eggshell structure by observing morphological changes during the reaction. Spatially, the morphological characteristics of the as-produced CaO varied markedly, that may indicate a variety of functionalities in terms of material use.

Keywords: avian eggshell, structure, thermal decomposition, kinetics, morphology


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INTRODUCTION CO2 capture and storage by means of reversible solid–gas reactions have been studied as a potential method for reducing anthropogenic CO2 and mitigating the greenhouse effect.1-11 As such, alkaline and alkaline earth oxides and hydroxides have long been studied as potential CO2 adsorbents.12-15 Among other processes, CaO carbonation to form CaCO3 and thermal decomposition of the CaCO3 to reproduce CaO are the most extensively studied processes in terms of reaction kinetics, thermodynamics, and morphology: CaO (s) + CO2 (g) ⇄ CaCO3 (s).15-39 Both the carbonation of CaO and the thermal decomposition of CaCO3 are complex heterogeneous processes that are controlled by various physico-geometrical processes and reaction conditions including the partial pressure of CO2. The formation of surface product layers of CaCO3 and CaO during carbonation and decomposition processes, respectively, is the most characteristic physicogeometrical factor controlling the heterogeneous kinetics, and it is generally seen for the reactions in solid–gas systems.40-49 Because the diffusion of CO2 through surface product layers is necessary for CO2 intake and uptake, complete formation of CaCO3 through the carbonation of CaO cannot be achieved in many cases due to the blocking of CO2 diffusion by the surface product layer.26,27 In addition, deterioration of the CO2 absorption capacity is generally observed with repeated carbonation–decarbonation cycles.15 The reaction kinetics in the solid–gas system regulate the morphology of as-produced CaCO3 and CaO in the carbonation and decomposition processes, respectively, as well as the morphological, crystallographic, and chemical characteristics of each reactant solid.29,39 Therefore, the reaction kinetics and morphological characteristics of CaO and CaCO3 control the CO2 absorption capacity of CaO, the decomposition kinetics of CaCO3, and the repeatability of the carbonation–decarbonation cycle. To increase the practical applicability of CO2 sorbents, by increasing their CO2 absorption capability and decreasing their deterioration during carbonation–decarbonation cycles, much effort is being made to improve the sorbents and the 3 ACS Paragon Plus Environment


cyclic processes by modification of the sorbents by doping,50-52 supporting,53 and the use of pretreatments such ball milling and annealing.36,54 Suitable cycling conditions have also been examined in relation to the reaction atmosphere and the heating conditions for each reaction cycling step.55-61 The source of CaO for the capture of CO2 is in many cases mineral CaCO3 such as limestone, although biomineralized CaCO3 is a further possible candidate. Eggshells and shellfish shells typically contain high contents of CaCO3. In addition, a significant amount of biomineralized CaCO3 is present in wastes from households and the food industry. Thus, the reuse of biowaste also contributes to the establishment of sustainable systems for the effective reuse of the material.6265

The CO2 capture capacity of CaO produced by the thermal decomposition of various

biomineralized CaCO3 sources has been tested with reference to that of mineral CaCO3 and by comparison of various biomineralized CaCO3 forms.66-72 In such studies, sufficient CO2 absorption capacity has been reported. Because of the characteristic chemical impurities in biomineralized CaCO3, superior repeatability of carbonation–decarbonation has been observed for the CaO obtained by thermal decomposition of oyster shell.68 In a comparison of the CaO obtained from different biomineralized CaCO3 forms, different CO2 absorption capacities were reported by Castilho et.al.72 Although the comparison was made using CaO obtained by the thermal decomposition of mechanically ground biomineralized CaCO3, CaO morphology changed among the various biomineralized CaCO3 sources. Morphological variation of CaO has been suggested as a possible cause of the differing CO2 capture capacities. It is well recognized that biominerals have specific structural characteristics that yield sophisticated functions in living organisms.73,74 For example, the detailed structural characteristics of avian eggshell have been shown to be constructed of five layers comprising calcite polycrystalline particles and membranes of protein/polysaccharide fibers.75-81 The detailed structure of avian eggshell is illustrated in Figure S1 in the Supporting 4 ACS Paragon Plus Environment

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Information. The eggshell mechanically protects the egg contents (e.g., developing embryo), exchanges respiratory gases between the inside and outside of the egg, offers protection from microorganisms, supplies calcium to the internal embryo, and maintains the water content inside the egg.78 The controlled mineralization of CaCO3 in many living organisms offers a model for researchers to develop a biomimetic mineralization process for realizing CaCO3 polymorph selection, morphological control, and organized composite production.82-85 However, when biomineralized CaCO3 is considered as a possible precursor of CaO, additional treatment processes should be carefully designed. The separation of CaCO3 from the organic membrane in avian eggshells is a typical example,86 as is the grinding of biomineralized CaCO3 before thermal processing.87 During thermal processing, the decomposition of residual organic membranes;88 transformation

of

metastable

CaCO3

phases

including

amorphous

CaCO3,89,90

monohydrocalcite,91,92 aragonite,93,94 and vaterite to calcite;95 and thermal decomposition of calcite occur to produce CaO.87 These thermally induced transformations are heterogeneous processes and the reaction kinetics largely depend on the structural characteristics of the biomineralized CaCO3 and the reaction conditions. Consequently, the morphological characteristics and functional capacity of as-produced CaO are controlled by (i) the original structural characteristics of the biomineralized CaCO3, (ii) pretreatment, and (iii) the conditions of thermal processing. Therefore, detailed knowledge of the correlation between the kinetic/mechanistic features of the thermally induced processes and the structural characteristics of the biomineralized CaCO3 is necessary when designing effective thermal processing conditions. As a case study for the production of CaO from biomineralized CaCO3, this study focuses on the correlation between the kinetic/mechanistic features of the thermal decomposition of avian eggshell and the structure of the shell, as well as the morphological characteristics of the asproduced CaO. To gather information on the correlation with reactant structure, a small piece of 5 ACS Paragon Plus Environment


avian eggshell was subjected to thermal analyses without any previous treatment and the morphology of the intermediate solid at selected reaction steps was observed microscopically. The major reaction process involved during the thermal treatment was the thermal degradation of the membrane and the thermal decomposition of biomineralized calcite. However, the reaction process for each was characterized kinetically as a partially overlapping multistep process. The overlapping reaction steps were separated through detailed kinetic analyses of systematically collected thermoanalytical data. The kinetic features of each separated reaction step were interpreted in relation to the structure of the avian eggshell. The structural and morphological characteristics of the as-produced CaO construction are explained on the basis of the kinetic and mechanistic features of each reaction step. It is believed that this fundamental research on the thermal decomposition of avian eggshell will contribute notably to the effective use of biowastes, not only as material resources but also as precursors for obtaining sophisticated functional materials that take advantage of the specific structure of biomineralized CaCO3. Preliminary to investigating the functionality of each region of CaO produced by the thermal decomposition of biomineralized CaCO3, this article reports in detail on the thermal behavior and kinetics of the thermal decomposition.

EXPERIMENTAL SECTION Samples and their Characterization Hen’s eggs purchased in a supermarket were boiled for 10 min using tap water. After cooling in cold tap water, the eggshell was dehulled and the inner shell membrane was carefully removed from the eggshell, which comprised the calcareous shell and the outer shell membrane. The eggshell was dried in air for one day and stored in a desiccator over silica gel. The morphology of the outside and inside surfaces was observed using a scanning electron microscope (SEM, JSM6510, JEOL) after coating of the surfaces with a platinum thin layer by sputtering (30 mA, 30 s, 6 ACS Paragon Plus Environment

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JFC-1600, JEOL). The elemental composition of the outside and inside surfaces of the eggshell was determined using energy-dispersive X-ray (EDX) spectroscopy (X-act, OXFORD) in conjunction with the aforementioned SEM instrument. A thin section of eggshell was prepared and the structure of the cross section was observed using an optical microscope under polarized light.96,97 The sample was pulverized using an agate mortar and pestle and subjected to powder Xray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). The XRD measurement (Cu-Kα, 40 kV, 20 mA) was performed using a diffractometer (RINT-2200V, RIGAKU) at a scan speed of 4° min−1 in a 2θ range from 5° to 60°. After diluting the sample with KBr, the FTIR spectrum was recorded using a spectrometer (FTIR 8400S, SHIMADZU) by the diffuse reflectance method. A piece of eggshell of approximately 5 mg was weighed into a platinum cell (5 mm in diameter and 2.5 mm in height) and thermogravimetry–differential thermal analysis (TG–DTA, Thermoplus Evo2, RIGAKU) was performed at a heating rate β of 5 K min−1 in flowing N2 (300 cm3 min−1).

Characterization of Thermal Behavior A piece of eggshell (approximately 10 mg) was weighed into a platinum cell (5 mm in diameter and 2.5 mm in height) and TG–DTA measurements (TG-8120, RIGAKU) were performed by heating at β = 10 and 20 K min−1 in flowing He (200 cm3 min−1). During the TG– DTA measurements, portion of outlet gas from the instrument was introduced to a mass spectrometer (MS, M-200QA, ANELVA) via a silica capillary tube (75 μm in internal diameter) heated to 500 K. The mass spectrum of the outlet gas was measured every 10 s at a mass range from m/z =10 to m/z =70 (EMSN : 1.0 A, SEM; 1.0 kV); thus obtaining TG/DTA–MS curves.



The change in the XRD pattern during sample heating was determined using the aforementioned XRD instrument by attaching a programmable heating chamber (PTC-20A, RIGAKU). The ground sample was press-fitted on a platinum plate and heated at β = 10 K min−1 in flowing N2 (100 cm3 min−1). The sample was maintained at various temperatures in the range 473 K to 1073 K, in steps of 50 K, for 15 min. The XRD measurements were performed while the sample was maintained at each temperature. Partially decomposed samples were prepared using a suspension type TG (TGA-50, SHIMADZU). A piece of eggshell (approximately 10 mg or 2.5 mg) was weighed into a platinum cell (6 mm in diameter and 2.5 mm in height). The 10 mg sample was heated to different temperatures ranging from 474 K to 873 K at β = 3 K min−1 in a flowing mixed gas N2–CO2 (80% CO2, 100 cm3 min−1) to obtain the partially reacted sample during the first mass-loss process. Similarly, the 2.5 mg sample was heated to various temperatures ranging from 873 K to 1223 K at β = 3 K min−1 in flowing N2 (80 cm3 min−1) to obtain the partially reacted sample during the second mass-loss process. The partially decomposed samples were cooled in the TG instrument to room temperature and were subsequently observed by SEM using the procedures described above.

Measurement of Kinetic Data TG measurements were performed using the aforementioned suspension type TG and the mass-loss curves were used for kinetic calculation. To record the kinetic data for the first mass-loss process, a piece of eggshell (ca. 10 mg) or ground sample (ca. 10 mg) was weighed into a platinum cell (6 mm in diameter and 2.5 mm in height). The sample was heated in the instrument to 1050 K at different β (0.5 ≤ β/K min−1 ≤ 10) in a flowing mixed N2–CO2 gas (80% CO2, 100 cm3 min−1). Atmospheric CO2 was introduced to separate the temperature regions of the first and second massloss processes because the second mass-loss process (thermal decomposition of calcite) was 8 ACS Paragon Plus Environment

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significantly shifted to higher temperatures with increasing atmospheric CO2. It was also confirmed that the influence of atmospheric CO2 on the rate behavior of the first mass-loss process was negligible. Similarly, a piece of eggshell (ca. 2.5 mg) weighed into the same platinum cell was heated to 1223 K at different rates of β (1 ≤ β/K min−1 ≤ 10) in flowing N2 (80 cm3 min−1) to record the kinetic data of the second mass-loss process. The mass-change traces were also recorded at different constant temperatures (888 ≤ T/K ≤ 928) under conditions that were otherwise identical to those of the measurements under nonisothermal conditions. In addition, the mass-change measurements were performed by regulating the mass-loss rate to different constant C values (2.5 ≤ C/μg min−1 ≤ 20.0) using a technique of sample controlled thermal analysis.98 All the TG instruments used in this study were preliminarily calibrated with respect to the temperature and mass measurements. The detailed calibration procedures for each TG instrument are described in the Supporting Information. Baseline corrections were made to all the mass-change curves before their use as kinetic data.

RESULTS AND DISCUSSION Sample Characterization Figure 1 shows typical SEM images of the outside and inside surfaces of the eggshell. The outside surfaces comprise aggregates of submicron-sized spherical particles (Figure 1a). Many cracks radiate in different directions on the outside surface layer. In fresh eggshell, the outside surface is covered by cuticles; however, in our sample, cuticles were removed during the boiling of the egg. The fibrous protein that makes up the outer shell membrane is dendritically expanded on the inside surface of the eggshell (Figure 1b). Behind the outer shell membrane, mammillary knobs with many nanometer-sized pores are observed. EDX spectra of the outside and inside 9 ACS Paragon Plus Environment


surfaces are shown in Figure S2 in the Supporting Information, and the elemental compositions are summarized in Table S1. Owing to CaCO3, the major elements observed in the outside surfaces are Ca, C, and O (Figure S2a). Mg and P exist as minor components and are understood to be a cationic impurity of crystalline CaCO3 and the anionic impurity PO43− (probably existing as calcium phosphates), respectively. On the inside surfaces (Figure S2b), the major components of C, N, and O are attributed to the amino group of the fibrous protein of the outer shell membrane. The minor component S is interpreted as originated from amino acids.81, 99 Ca appeared as a minor component because the calcic mammillary knobs are covered with the outer shell membrane.

Figure 1. Typical SEM images of the eggshell: (a) outside surface and (b) inside surface.

Figure S3 in the Supporting Information shows the XRD pattern and FTIR spectrum of the eggshell. The XRD pattern is perfectly in agreement with that of calcite (Rhombohedral, S.G. = R−3c, a = b = 4.98900, c = 17.06200, JCPDS 05-0586) (Figure S3a). Because the FTIR spectrum indicates a broad absorption at 3000–3600 cm−1, which is attributed to the O–H stretching vibration mode, the existence of H2O in the sample is expected. The FTIR spectrum also exhibits characteristic absorption bands of calcite, including ν2, ν3, and ν4 modes of CO32− at 876, 1446, and 713 cm−1, respectively.100,101 Figure 2 shows typical TG–DTG–DTA curves for the sample recorded in flowing N2 (300 cm3 min−1). The TG curve indicates two distinguishable mass-loss 10 ACS Paragon Plus Environment

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processes: one at 400–800 K and the other at 850–1000 K. The first mass-loss process is expected to be due to thermal degradation of the outer shell membrane that exists on the inside surface of the eggshell. With reference to the original sample mass, this process yields a mass-loss value of 2.81 ± 0.48%. The second mass-loss step is considered to be the thermal decomposition of calcite and yields a mass-loss of 43.23 ± 0.67% with reference to the sample mass at the temperature where the first mass-loss process was completed, which closely corresponds to the calculated value. CaCO3 → CaO + CO2

(m = 43.97%)

(1)

Figure 2. Typical TG–DTG–DTA curves for the thermal decomposition of eggshell (m0 = 4.982 mg) during heating at β = 5 K min−1 in flowing N2 (300 cm3 min−1).

Thermal Behavior Figure 3 shows a typical result of TG/DTA–MS measurement of the eggshell. Water vapor and CO2 were detected as the major gaseous products, with parent peaks at m/z = 18 and m/z = 44 and the necessary fragment peaks in both mass-loss processes. During the first mass-loss process, the evolution behaviors of water vapor (m/z = 18) and CO2 (m/z = 44) can be correlated with three distinguishable reaction steps (Figure 3a). The evolution rate of water vapor is more significant 11 ACS Paragon Plus Environment


than that of CO2 in the first reaction step. The maximum evolution rates of water vapor and CO2 during the first mass-loss process are observed in the second reaction step. In the third reaction step, the evolution rates of both water vapor and CO2 decelerate, but that of CO2 is reaccelerated on further heating. The reacceleration behavior of CO2 evolution observed in the final stage of the first mass-loss process is explained by the initiation of the second mass-loss process, i.e., thermal decomposition of calcite. The second mass-loss process is characterized by the evolution of CO2 with a wellshaped peak in the MS ion thermogram for m/z = 44, as expected for the thermal decomposition of calcite (Figure 3b). It must be noted that a small but detectable peak for m/z = 18 is also observed, accompanied by the thermal decomposition of calcite. The source of the water vapor may be trapped water that exists at the grain boundary of calcite crystals. Similar behavior of water vapor evolution, owing to the release of trapped water during the transformation process of matrix crystals, has been reported for biomineralized and synthetic aragonite during aragonite–calcite transformation.93-94 The thermal decomposition of calcite is likely initiated in the final stage of the first mass-loss process and gradually accelerates before reaching the major stage of the second mass-loss process. Therefore, although the CO2 evolution peak is well-shaped, the second massloss process may be kinetically comprised of two reaction steps: an introductory acceleration step and a major reaction step. Changes in the XRD pattern during stepwise isothermal heating of the sample are illustrated in Figure S4 in the Supporting Information. The XRD pattern and the intensities of each diffraction peak attributed to calcite do not change within the temperature range of the first massloss process until 673 K (Figure S4a). The attenuations of the diffraction peaks commence at a temperature between 673 and 723 K, which corresponds to the final stage of the first mass-loss process. Between 1023 and 1073 K, the diffraction peaks of calcite disappear completely. The XRD 12 ACS Paragon Plus Environment

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pattern of CaO (cubic, a = 4.81059, b = 4.81059, c = 4.81059, JCPDS 050586) also appears at a temperature between 1023 and 1073 K (Figure S4b); however, it is not clearly distinguishable during the earlier stage of the thermal decomposition of calcite.

Figure 3. TG–DTA curves and MS ion thermograms (m/z = 18 and m/z = 44) of the evolved gas during sample heating in flowing He (200 cm3 min−1): (a) for the first mass-loss process from 400 K to 850 K heated at  = 20 K min−1 and (b) for the second mass-loss process from 600 K to 1100 K heated at  = 10 K min−1.

Figure 4 shows typical SEM images of the samples heated at 3 K min1 to different temperatures covering the range of the first mass-loss process in a flowing mixed gas of N2-CO2 (80% CO2). At the end of the first reaction step in the first mass-loss process (Figure 4a and b), no distinguishable textural change was observed for the inside surface of the eggshell, which exhibited the dendritically expanded fibrous protein substance (Figure 4b). In contrast, many pores appeared 13 ACS Paragon Plus Environment


on the outside surface of the eggshell (Figure 4a). Therefore, the water vapor evolution during the first reaction step is likely attributable to the release of trapped water molecules in the calcite construction of the eggshell during the modification of the structure. A distinguishable textural change on the inside surfaces was observed by the decomposition of the fibrous protein during the second reaction step comprising the simultaneous evolution of water vapor and CO2 (Figure 4d). A gradual recovery of the porous outside surfaces produced during the first reaction step was also observed in the second reaction step (Figure 4c). At the end of the third reaction step (Figure 4e and f), spherical calcite particles were observed on the outside surfaces (Figure 4e). On the inside surfaces (Figure 4f), the fibrous protein decomposed perfectly and mammillary knobs of calcite appeared. The second and third reaction steps in the first mass-loss process attributed to the thermal degradation of outer shell membrane might evolve gases other than water vapor and CO2; however, the present TG/DTA–MS measurements could not detect other gases with larger molecular masses because of the limited amount of the gases and possible condensation during transferring the gases to MS spectrometer.


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Figure 4. Typical SEM images for the sample heated to different temperatures at β = 3 K min−1 in a flowing mixed gas of N2–CO2 (80% CO2, 100 cm3 min−1): (a)–(b) 573 K, (c)–(d) 803 K, and (e)– (f) 873 K.

Figure 5 shows typical SEM images of the samples heated at  = 3 K min1 to different temperatures covering the range of the second mass-loss process in flowing N2. In the early stage of the second mass-loss process, the surface layers both on the outside and inside surfaces of the eggshell comprise submicron-sized spherical particles (Figures 5a and b), as observed at the final stage of the first mass-loss step where larger spherical particles are observed on the outside surfaces. Midway through the reaction, sintering starts in both sides (Figure 5c and d). The sintering process



on the surface layers further proceeds as the reaction advances (Figure 5e and f). The sintering of the particles on the outside and inside surface layers produces differing results: on the outside surface, cracks are formed by the sintering and the internal shell matrix is exposed to the reaction atmosphere (Figure 5e), whereas the sintering on the inside surface results in the covering of the mammillary knob surfaces (Figure 5f). At the end of the second mass-loss process, the outside surface layer is further fractured (Figure 5g), whereas on the inside surface, the sintered layer more tightly covers the mammillary knob (Figure 5h). The morphological changes on the outside and inside surface layers indicate that thermal decomposition of calcite is initiated on both the outside and inside surfaces. As the reaction advances, the reaction of the shell matrix from the outside surface side seems to become predominant because of the crack formation and fracture of the outside surface layer.


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Figure 5. Typical SEM images for the sample heated to different temperatures at β = 3 K min−1 in flowing N2 (80 cm3 min−1): (a)–(b) 923 K, (c)–(d) 958 K, (e)–(f) 983 K, (g)–(h) 1223 K.

Kinetic Behaviors Thermal degradation of the outer shell membrane. Figure 6 shows the TG–DTG curves of the first mass-loss process of ground eggshell (approximately 10.0 mg) recorded at



different  in a flowing mixed gas of N2–CO2 (80% CO2) at a rate of 100 cm3 min1, which is predominantly caused by the thermal degradation of the outer shell membrane. For the kinetic study of the first mass-loss process, the ground sample was used to increasing the repeatability of the measured TG–DTG curves and realizing systematic change in the TG–DTG curves with , after confirming the similarity of the TG–DTG curves recorded for a piece of eggshell and ground sample. Because of the high partial pressure of atmospheric CO2, the subsequent thermal decomposition of calcite is shifted to the higher temperature region so that the mass-change behavior in Figure 6 can be treated separately from the thermal decomposition of calcite. The massloss behavior is superficially recognized as comprising of three reaction steps, i.e., the introductory step at the beginning of the overall mass-loss process, the major reaction step with the largest reaction rate, and the decelerating reaction tail. In comparison with the composition of the evolved gas observed by TG/DTA–MS (Figure 3), only water vapor is detected as the evolved gas within the temperature region of the first reaction step. For the second and third reaction steps, the simultaneous evolution of water vapor and CO2 has been confirmed. From the SEM images of the outside and inside of the eggshell surfaces at the corresponding temperatures (Figure 4), the evolution of water vapor in the first reaction step is understood to have been generated by the change in the structure of the calcite layers, as observed by the pore formation on the outside surfaces. Thermal degradation of the outer shell membrane on the inside surface is observed at the corresponding temperatures of the second and third reaction steps. Therefore, the mass-loss process should be considered as consisting of three, partially overlapping, reaction steps.


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Figure 6. Changes in the TG–DTG curves for the first mass-loss process upon heating ground eggshell at different β in a flowing mixed gas of N2–CO2 (80% CO2, 100 cm3 min−1).

As a preliminary kinetic approach to the multistep reaction process, a conventional kinetic analysis that assumed a single step reaction was applied based on eq. (2).102-104

d  E   A exp   a  f   dt  RT 

(2)

where α, A, Ea, and R are the fractional reaction, Arrhenius preexponential factor, apparent activation energy, and gas constant, respectively. The function f(α) is the kinetic model function characterizing the physico-geometric reaction model for the solid-state reactions. For roughly evaluating changes in the Ea value as the overall reaction advanced, the isoconversional method in differential form, known as the Friedman method,105 was applied using the logarithmic form of the fundamental kinetic equation:

E  d  ln    ln Af    a RT  dt 

(3)

For the data point (dα/dt, T) at a selected value of α, the ln (dα/dt) versus T1 plot should produce a straight line with the slope –Ea/R. The results of the preliminary kinetic analysis using the Friedman method are shown in Figure 7. The Friedman plots at different α indicate acceptable linearity (R2 > 0.95 in 0.2 ≤ α ≤ 0.99, Figure 7a). The variation trend in Ea indicates three 19 ACS Paragon Plus Environment


distinguishable α regions that correspond to the component reaction steps (Figure 7b). The average Ea values for each reaction step were calculated to be (Ea,1, Ea,2, Ea,3) = (115.3 ± 30.2 kJ mol–1, 171.4 ± 7.9 kJ mol–1, 153.6 ± 17.0 kJ mol–1) in the ranges 0.05 ≤  ≤ 0.20, 0.20 ≤  ≤ 0.55, and 0.75 ≤  ≤ 0.95, respectively.

Figure 7. Formal kinetic analysis for the overall first mass-loss process in a flowing mixed gas of N2–CO2 (80% CO2, 100 cm3 min−1): (a) Friedman plots at different  and (b) changes in Ea value as the reaction advanced.

As the other possible preliminary approach to the partially overlapping multistep process, a mathematical peak deconvolution was trialed using the Weibull function for fitting each component DTG peak.106-109


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n d   Fi (t ) dt i 1

 a 1  Fi (t )  a0  3   a3 

with 1 a3 a3

  t  a1   a2 

 a 1    3   a3 

1 a3

    

a3 1

    t  a1 exp     a2 

 a 1    3   a3 

1 a3

a3   a3  1      a3    

(4)

where a0–a3 are the amplitude, center, width, and shape, respectively. With reference to the DTG peak shape and gaseous evolution behavior during thermal degradation of the outer shell membrane, the three partially overlapping reaction steps were assumed also for the mathematical peak deconvolution. Separated DTG curves at different β were used as the kinetic curves for each reaction step. The formal kinetic analysis based on eq. (2) was applied to the series of kinetic curves for each reaction step. The results of the kinetic analysis and subsequent formal kinetic analysis applied for the first mass-loss process are described in Section 5 (S5) in the Supporting Information. As previously mentioned for the first mass-loss process, the first reaction step that produces only water vapor differs from the subsequent reaction steps. The second and third reaction steps entail thermal degradation of the outer shell membrane, for which nearly equilibrant Ea values are estimated in Figures 7 and S7. Thus, the changes in the rate behavior from the second to third reaction steps seem to be controlled by the geometric constraint of the reaction. In this assumption, the overall reaction process can be expressed by a cumulative kinetic equation of three reaction steps without considering the mutual correlations among the reaction steps.106,110

 Ea ,i  d N    f i ( i )　with   ci Ai exp 　 dt i1 RT  

N

N

i 1

i 1

and 　  ci  1　　  ci i  

(5)

where N and c are the number of constituent reaction steps, i.e., N =3 for the first mass-loss process, and the contribution of each reaction step, respectively. The subscript i identifies the reaction step.



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For the kinetic model function f(α) in the respective reaction steps, an empirical kinetic model with three kinetic exponents SB(m, n, p), known as the Šesták−Berggren model,111,112 was applied. (6)

SB (m, n, p ) : f ( )   m (1   ) n [ ln( 1   )] p

Accordingly, a total of 18 parameters were optimized for the kinetic data at each β. The kinetic parameters estimated through preliminary kinetic approaches (as listed in Table S2 in the Supporting Information) were set in eq. (5) and an optimization run termed “kinetic deconvolution analysis” was conducted using nonlinear least squares analysis to fit the calculated data based on eq. (7) to the experimental kinetic data, so as to minimize F value.  d   d   F        j 1   dt  exp, j  dt  cal, j  M

2

(7)

where M is the number of data point in the kinetic curve. Figure 8 shows a typical result of the kinetic deconvolution analysis for the kinetic data at β = 5 K min1. Comparison of the kinetic parameters optimized among for the kinetic data at different β is the necessary to confirm the independence of the reaction steps. This is because systematic changes with β can be expected for the mutually correlated multistep reaction steps. For the first mass-loss process, a distinguishable change in the optimized kinetic parameters with β was not observed, indicating the gross kinetic behavior of each reaction step does not change within the reaction conditions applied. The kinetic parameters averaged over those determined for the kinetic data at different β are listed in Table 1. The Ea,1 value is markedly different from the Ea,2 and Ea,3 values because of the different chemical reactions that occur in a similar temperature region. Considering the comparable Ea values for the second and third reaction steps, the change in the rate behavior results from the change in the physico-geometry of the thermal degradation of the outer shell membrane as indicated by the different A and f(α). The large kinetic exponents in SB(mi, ni,


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pi), as evaluated for the first and second reaction steps, are indicative of the rate behavior being markedly different from the ideal solid-state reactions controlled by a defined physico-geometry, although the large m and p values are mutually compensated when describing the actual rate behavior. Figure 9 shows the experimental master plot of f(α) versus α for each reaction step reproduced according to the kinetic equation extrapolated to infinite temperature:102,104 d d E   exp  a   Af   d dt  RT 

 E  with    exp   a dt ,  RT  0 t

(8)

where θ is the hypothetical reaction time at infinite temperature, which is known as Ozawa’s generalized time.113,114 All the master plots indicate the characteristic shape for a diffusioncontrolled reaction. However, the master plots could not be satisfactorily fitted with any known kinetic models for the diffusion-controlled solid-sate reactions; that is illustrated in Figure 9 and exemplified by the best fitting curves using a constant rate nucleation and diffusion-controlled growth model, which is expressed by the JMA(m) model with m < 1:115-117 (9)

JMA (m) : f ( )  m(1   )[  ln( 1   )]11 / m

Figure 8. Typical results of kinetic deconvolution analysis for the first mass-loss process (β = 5 K min−1).


ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Table 1. Average kinetic parameters for each reaction step of the first mass-loss process, optimized by kinetic deconvolution analysis (0.5 ≤ β/K min−1 ≤ 10).

i

ci

Ea,i / kJ mol

1

0.07  0.01

81.4  2.9

2

0.78  0.01

3

0.16  0.01

−1

Ai / s

fi(i) = im(1i)n[ln(1i)]p

−1

m

n

p

(4.64  0.01) 

5.17  0.10

0.06  0.01

5.33  0.08

164.3  1.5

(6.64  0.01) 

10.50  0.43

1.04  0.04

11.31  0.35

161.1  9.0

(2.29  0.01) 

0.17  0.01

0.61  0.01

0.68  0.01


R2

0.988

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Figure 9. Experimental master plots for the first, second and third reaction steps of the first massloss process, as reproduced from SB(mi, ni, pi) determined by kinetic deconvolution analysis, and fitting curves using JMA(m).

Thermal decomposition of biomineralized calcite. For kinetic analysis of the thermal decomposition of the biomineralized calcite (the second mass-loss process), mass-loss curves recorded in flowing N2 (80 cm3 min−1) under (i) linear nonisothermal conditions at different β, (ii) isothermal conditions at different constant T, and (iii) controlled transformation rate conditions at different C shown in Figure 10 were used as kinetic data. It is noted that the derivative mass-loss curves recorded under linear nonisothermal conditions exhibit a shoulder before the maximum peak top. For the isothermal mass-loss data, two distinguishable peak tops are observed in the derivative mass-loss curves. The kinetic data were simultaneously analyzed by the Friedman plot. Figure 11 shows the results of the isoconversional kinetic analysis. The Friedman plots at selected α indicate satisfactory linear correlations of ln(dα/dt) versus T−1 among all the data points recorded under different temperature profiles. Nearly constant Ea values with an average of 208.7 ± 3.5 kJ mol–1



(0.1 ≤ α ≤ 0.9) were obtained during the course of reaction; however, some fluctuations in the value were distinguishable in the early stage of the reaction, α ≤ 0.2.

Figure 10. Mass-loss traces for the second mass-loss process in flowing N2 (80 cm3 min−1) under (a) linear nonisothermal conditions, (b) isothermal conditions, (c) controlled mass-loss rate


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condition (C = 7.5 μg min−1), and (d) changes in reaction temperature profile with C in the controlled mass-loss rate measurements.

Figure 11. Formal kinetic analysis for the overall second mass-loss process: (a) Friedman plots at different  and (b) changes in Ea value as the reaction advanced.

Because the shapes of derivative mass-loss curves exhibited a possible two-step reaction and the apparent Ea value fluctuated in the early part of the second mass-loss process, the overall rate data recorded under linear nonisothermal and isothermal conditions were subjected to mathematical deconvolution into two reaction steps (as was used for the first mass-loss process). From the results of this analysis, the contributions (c1, c2) of each reaction step were estimated to be (0.41  0.01, 0.59  0.01) and (0.42  0.01, 0.58  0.01) for the reaction under linear 27 ACS Paragon Plus Environment


nonisothermal and isothermal conditions, respectively. Formal kinetic analysis applied to the separated kinetic data represented the approximately constant Ea,i values during the course of each reaction step, with average values of 209.4  9.6 kJ mol−1 and 206.6  5.4 kJ mol−1 for the first and second reaction steps, respectively. Details of the kinetic results obtained through the mathematical deconvolution analysis are described in Section 6 (S6) in the Supporting Information. Kinetic deconvolution analysis for the second mass-loss process was carried out on the basis of the preliminary mathematical deconvolution analysis by assuming an independent twostep reaction process. Typical results of the kinetic deconvolution analysis for the reaction under different temperature profiles, including linear nonisothermal, isothermal, and controlled rate conditions, are shown in Figure 12. The fittings are acceptable, especially for the reaction under linear nonisothermal and isothermal conditions (R2 > 0.99). The optimized kinetic parameters for each reaction step are listed in Table 2. The values of Ea,i and Ai are comparable between the two reaction steps, irrespective of the temperature profiles of the kinetic data. The Ea,i value of approximately 210 kJ mol−1 closely corresponds to the reported Ea values for the thermal decomposition of reagent calcite under well controlled reaction conditions.19 The experimental master plots drawn using SB(mi, ni, pi) for each reaction step are shown in Figure 13, indicating markedly different rate behaviors between the reaction steps. The first reaction step exhibits linear deceleration as the reaction advances. Such first order rate behavior is understood when the reaction of surface calcite both inside and outside the eggshell was assumed as evidenced by SEM observation. The rate behavior of the second reaction step exhibits a characteristic trapezoidal shape. The second reaction step was considered to correspond to the thermal decomposition of calcite in the internal shell matrix induced by the crack formation on the outside surfaces of the eggshell. The initial acceleration component of the second reaction step is explained by the crack


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formation and the disposal of the internal shell matrix. The nearly constant reaction rate over the wide i range of the main reaction stage is attributed to the zero order rate behavior with one dimensional advancement of the reaction interface from the outside surface of the internal shell matrix to depth. The deceleration in the latter stage of the second reaction step is likely due to an increase in the diffusion path length for the removal of the product CO2 and consumption of the reactant in the shell matrix. The empirical kinetic characterization of the thermal decomposition of the calcite construction of the eggshell is thus well described in relation to the morphological changes of the eggshell construction. This also leads to the characteristic construction of the product CaO.



Figure 12. Typical results of kinetic deconvolution analysis for the second mass-loss process: (a) nonisothermal (β = 3 K min−1), (b) isothermal (T = 898 K), and (c) controlled mass-loss rate (C = 10 μg min−1) measurements.

Although the thermal decomposition of the calcite construction of the present avian eggshell is described nearly constant apparent Ea value during the course of the reaction, the overall reaction process is comprised of two kinetic steps that occur in the different reaction regions with different morphological characteristics. This type of multistep process is a typical feature of heterogeneous reaction systems, especially in solid–gas systems. The reactions in the respective reaction regions proceed through different physico-geometric mechanism and produce the CaO products with different morphological features. Many biomineralized CaCO3 have more complex constructions than the avian eggshell. Therefore, detailed understanding of the thermal decomposition kinetics and morphological features of the CaO product is essential for the most effective use of biomineralized CaCO3 as the possible resource of CO2 absorbent.


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Table 2. Average kinetic parameters for each reaction step of the second mass-loss process, as optimized by kinetic deconvolution analysis (nonisothermal: 1 ≤ β/K min−1 ≤ 10; isothermal: 888 ≤ T/K ≤ 928; controlled rate: 2.5 ≤ C/μg min−1 ≤ 20)

Condition

Nonisothermal

Isothermal

Controlled rate

i

ci

Ea,i / kJ mol

1

0.42  0.01

210.4  0.6

2

0.58  0.01

1

−1

Ai / s

fi(i) = im(1i)n[ln(1i)]p

−1

m

n

p

(6.86  0.26) 

0.06  0.16

0.96  0.07

0.05  0.15

206.8  0.2

(1.80  0.27) 

0.95  0.60 1.07  0.09

  0.54

0.42  0.01

208.2  1.7

(5.07  0.72) 

0.06  0.06

1.01  0.09

0.01  0.02

2

0.58  0.01

201.5  2.5

(9.96  3.06) 

0.00  0.36

0.71  0.16

0.41  0.36

1

0.43  0.02

207.7  3.4

(4.59  1.18) 

0.05  0.01

0.85  0.16

0.01  0.01

2

0.57  0.02

199.0  1.9

(9.88  0.09) 

0.00  0.01

0.94  0.11

0.49  0.01


R2

>0.990

> 0.996

> 0.846


Figure 13. Experimental master plots for the first and second reaction steps of the second massloss process, as reproduced from SB(mi, ni, pi) determined by kinetic deconvolution analysis.

CONCLUSIONS Upon linearly heating the avian eggshell in an inert gas, two distinguishable mass-loss processes are observed, that correspond to thermal degradation of the outer shell membrane and thermal decomposition of constructional calcite. The successive mass-loss processes occurred with approximately 3%- and 43%-mass losses. The first mass-loss process is further separated into three reaction steps based on experimental evidences of evolved gas analysis and kinetic analysis. The mass-loss process begins with the release of water vapor, accompanied by the pore and crack formations on the outside surface of the constructional calcite. The subsequent reaction steps are attributed to thermal degradation of the outer shell membrane. The second mass-loss process occurred after the completion of thermal degradation of the outer shell membrane, which corresponded with the thermal decomposition of calcite. The apparent Ea value of approximately 200–210 kJ mol−1, as determined in this study, is in accordance with previously reported Ea values for the thermal decomposition of calcite reagent under well controlled reaction conditions. However, the rate behavior of the second mass-loss process indicated a partially overlapping two32 ACS Paragon Plus Environment

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step process. The contributions of the first and second reaction steps are approximately 40% and 60%, respectively. The first reaction step occurs simultaneously on the inside and outside surfaces of the constructional calcite. The inside surfaces become covered with a sintered CaO layer, and cracks are formed on the outside surfaces exposing the internal shell matrix. The thermal decomposition of the internal shell matrix is thus activated contributing to the second reaction step in the second mass-loss process. The product CaO has a specific construction, with a smooth inside surface and a rough outside surface with aggregates of micron-sized CaO particles. The formation of constructional CaO with a specific structure is a useful information for designing thermal processing of the biomineralized CaCO3 and might also be of further merit for using the CaCO3 waste as precursor of a functional material.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ??????????. S1. Structure of Avian Eggshell (Figure S1); S2. Calibration for TG; S3. Characterization of Sample (Figure S2, Table S1, and Figure S3); S4. Thermal Behavior (Figure S4); S5. Mathematical Deconvolution and Formal Kinetic Analysis of the Thermal Degradation of Outer Shell Membrane (Figure S5, Table S2, Figure S6, Figure S7, and Figure S8); and S6. Mathematical Deconvolution and Formal Kinetic Analysis of the Thermal Decomposition of Calcite Construction (Figure S9, Table S3, Figure S10, Figure S11, Figure S12, and Figure S13).



AUTHOR INFORMATION Corresponding Author *Tel./fax: +81-82-424-7092. E-mail: [email protected] ORCID Nobuyoshi Koga: 0000-0002-1839-8163 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The present work was supported by JSPS KAKENHI Grant Numbers 17H00820 and 16K00966.

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TOC/ABSTRACT GRAPHIC

SYNOPSIS Biomineralized CaCO3 with specialized structure is a possible source of constructional CaO that might further sophisticate for sustainable applications.



Figure 1. Typical SEM images of the eggshell: (a) outside surface and (b) inside surface. 37x18mm (300 x 300 DPI)

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Figure 2. Typical TG–DTG–DTA curves for the thermal decomposition of eggshell (m0 = 4.982 mg) during heating at β = 5 K min−1 in flowing N2 (300 cm3 min−1). 49x32mm (300 x 300 DPI)



Figure 3. TG–DTA curves and MS ion thermograms (m/z = 18 and m/z = 44) of the evolved gas during sample heating in flowing He (200 cm3 min−1): (a) for the first mass-loss process from 400 K to 850 K heated at β = 20 K min−1 and (b) for the second mass-loss process from 600 K to 1100 K heated at β = 10 K min−1. 85x95mm (300 x 300 DPI)


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Figure 4. Typical SEM images for the sample heated to different temperatures at β = 3 K min−1 in a flowing mixed gas of N2–CO2 (80% CO2, 100 cm3 min−1): (a)–(b) 573 K, (c)–(d) 803 K, and (e)–(f) 873 K. 120x176mm (300 x 300 DPI)



Figure 5. Typical SEM images for the sample heated to different temperatures at β = 3 K min−1 in flowing N2 (80 cm3 min−1): (a)–(b) 923 K, (c)–(d) 958 K, (e)–(f) 983 K, (g)–(h) 1223 K. 158x305mm (300 x 300 DPI)


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Figure 6. Changes in the TG–DTG curves for the first mass-loss process upon heating ground eggshell at different β in a flowing mixed gas of N2–CO2 (80% CO2, 100 cm3 min−1). 42x23mm (300 x 300 DPI)



Figure 7. Formal kinetic analysis for the overall first mass-loss process in a flowing mixed gas of N2–CO2 (80% CO2, 100 cm3 min−1): (a) Friedman plots at different α and (b) changes in Ea value as the reaction advanced. 96x121mm (300 x 300 DPI)


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Figure 8. Typical results of kinetic deconvolution analysis for the first mass-loss process (β = 5 K min−1). 49x31mm (300 x 300 DPI)



Figure 9. Experimental master plots for the first, second and third reaction steps of the first mass-loss process, as reproduced from SB(mi, ni, pi) determined by kinetic deconvolution analysis and fitting curves using JMA(m). 50x33mm (300 x 300 DPI)


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Figure 10. Mass-loss traces for the second mass-loss process in flowing N2 (80 cm3 min−1) under (a) linear nonisothermal conditions, (b) isothermal conditions, (c) controlled mass-loss rate condition (C = 7.5 µg min−1), and (d) changes in reaction temperature profile with C in the controlled mass-loss rate measurements. 167x366mm (300 x 300 DPI)



Figure 11. Formal kinetic analysis for the overall second mass-loss process: (a) Friedman plots at different α and (b) changes in Ea value as the reaction advanced. 101x135mm (300 x 300 DPI)


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Figure 12. Typical results of kinetic deconvolution analysis for the second mass-loss process: (a) nonisothermal (β = 3 K min−1), (b) isothermal (T = 898 K), and (c) controlled mass-loss rate (C = 10 µg min−1) measurements. 126x209mm (300 x 300 DPI)



Figure 13. Experimental master plots for the first and second reaction steps of the second mass-loss process, as reproduced from SB(mi, ni, pi) determined by kinetic deconvolution analysis. 48x31mm (300 x 300 DPI)


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TOC/Abstract Graphic 47x26mm (300 x 300 DPI)


Thermal Decomposition of Biomineralized Calcium Carbonate

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