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Bio-green synthesis of calcium acetate from oyster shell waste at low cost and reducing the emission of greenhouse gases


Biological wastes obtained from food, oyster shells, were recycled to calcium carbonate and then used as bio-green raw material to replace limestone/carbonate stone for calcium acetate (Ca(CH3COO)2·H2O) production. The conditions (ambient temperature occurred in an exothermic reaction, drying time, yield, and solubility) of the reaction between the bio-green CaCO3 and three different acetic (CH3COOH) concentrations (8, 10, and 12 M) were investigated. The product's maximum yield (93%) with a shorter drying time (18 h) was obtained from the reaction between the bio-green CaCO3 with 12 M acetic acid revealing a lower cost. The chemical compositions without any toxic metal impurity revealed by the X-ray fluorescence technique would be useful to suggest use in the specific application. The X-ray diffraction, Fourier Transform Infrared, and Thermogravimetric analysis data of Ca(CH3COO)2·H2O prepared by the bio-green CaCO3 obtained from oyster shell wastes in this work and those in previous works used other calcium sources were consistent. The morphologies with different sizes of the obtained Ca(CH3COO)2·H2O depend on the CH3COOH concentrations reported in this work and were different from those reported in previous works because of different calcium sources. According to the observation, it can be concluded that the low-cost and bio-green technique without the environmental effects was successfully applied to produce cheap Ca(CH3COO)2·H2O and reduce greenhouse gas emissions, which can be used in the specific industry.

1 Introduction

Commercial oysters belong to the family and principally to the genera Ostrea and Crassostrea [1]. The dominant aquaculture oyster is the Pacific (cupped) oyster, Crassostrea gigas, providing the greatest quantity of aquaculture species [1]. This species accounted for 97% of all oyster aquaculture production. Other aquaculture species are Crassostrea virginica, Crassostrea iredalei, Ostrea edulis, and Saccostrea glomerata. China, Japan, Korea, France, and the USA are the major oyster producers. Oyster is best known as live seafood, and most production is live, in-shell, or fresh chilled in half-shell [1]. Oyster, commonly consumed in both cooked and raw forms, is an excellent source of vitamin B12, vitamin A, selenium, calcium, iron, and zinc. Other value-added products such as smoked oysters and pre-cooked meals are becoming a tradition. Two oysters (28 g) provide the Reference Daily Intake of zinc and vitamin B12 [2]. However, the eatable part of the oyster had only 10–16 wt%, causing about 84–90 wt % of oyster shell waste. Table 1 shows the number of farms, area, quantity, and value of Thailand’s oyster production farms cultured from 2010 − 2021 [1]. From these data, the collected waste of oyster shells can be estimated as being over 60 kt per year.

Table 1 Number of farms, area, quantity, and value of Thailand’s oyster production farms cultured in 2010 − 2021 [3]

The increases in shellfish aquaculture, processing industries, and seafood restaurants generate huge oyster-shell waste, which is normally dumped into coastal areas. Large numbers of shell wastes i.e., oyster shells, were unduly discarded, causing serious environmental problems for water, soil, and air, resulting in negative ecosystem health, especially in Thailand. Over time, microbial decomposition appeared and progressively created many greenhouse gases (CO2, NOx, NH3, and H2S) [2], a serious problem that is of concern nowadays. Consequently, the bio-circular-green (BCG) economic model was introduced by the Ministry of Higher Education, Science, Research, and Innovation of Thailand to resolve various environmental problems [2]. The four industries, namely (i) agriculture and food, (ii) medical and wellness, (iii) bioenergy, biomaterial and biochemical, and (iv) tourism and creative economy, are focused and promoted according to the BCG model. Oyster-shell waste is one of the important problems needed to resolve, and an excellent method to resolve this issue is through recycling because the recycling technique can decrease the waste quantity and increase the valuable material, derived from the waste.

Oyster shell has a main chemical composition of 95.99% calcium carbonate (CaCO3) [3]. However, other chemical compositions were also observed as oxide forms of silicon (0.70% SiO2), magnesium (0.65% MgO), aluminum (0.42% Al2O3), strontium (0.33% SrO), phosphorous (0.20% P2O5), sodium (0.98% Na2O), and sulfur (0.72% SO3) [3]. Usually, CaCO3 naturally occurred from two sources: non-living natural sources (limestone, carbonate rock, and ores) and living sources (eggshells and all other shells), which will have different crystal structures (calcite, aragonite, and variscite), % purity, and impurity. The CaCO3 obtained from non-living natural resources has limited reserves and non-renewability. Additionally, the production process of CaCO3 powder reported the CO2 emission by IPCC.2006 found about 0.0366 kg CO2eq kg−1 [4]. The oyster shells-transferred CaCO3 powders obtained from living sources happened every day with collected huge amounts, which have non-limited reserves and renewability. The use of calcium carbonate from living sources as a substitute for non-living sources will help environmental problems, namely the disposal of eggshell waste/shells and the reduction of this material from calcium mineral resources, resulting in a positive effect on reducing greenhouse gas emissions [4]. So far, the CaCO3 powder transformed from oyster shells has been used in various industries. For instance, it has been used in the plastic industries [5]. It is an essential ingredient in paper, paints and coatings, rubbers, adhesives, and sealants [6, 7]. It is added to plastics to reduce the cost of production. Moreover, the thermal, mechanical, and rheological properties of the resulting plastics are also improved. Due to the low cost and abundance of CaCO3, it has become one of the most popular fillers for plastics. It was also used as limestone substitutes for concrete aggregates and backfills [6], as sand mixtures, cement mortars, and clinkers [7], as a calcium-enriched supplement and antimicrobial substance in the medical and pharmaceutical industries [8], as a conditioner to improve the soil quality [9], and as a calcium-mineral source for rats [8, 10]. Additionally, oyster shell powder was used as a raw agent to prepare valuable compounds such as triple superphosphate [11]. However, using oyster shells as the precursor to prepare other advanced compounds such as calcium citrate (Ca3(C6H5O7)2), calcium lactate (C6H10CaO6), calcium acetate (Ca(CH3COO)2), etc. still lacks fundamental information.

This work focuses on the generation of calcium acetate (Ca(CH3COO)2) from shell waste; it is known as lime acetate and vinegar salts, and the monohydrate (Ca(CH3COO)2·H2O) form is the most common form of calcium acetate [12]. However, another form of calcium acetate, i.e., hemihydrate form, was also synthesized by Abrahams et al. [13] and Bette et al. [14]. Bette et al. [12] also reported that calcium acetate hydrate can be synthesized by the reaction between vinegar (an aqueous solution of acetic acid, CH3COOH) and CaCO3 derived from different calcium sources such as hydrated lime (calcium hydroxide, Ca(OH)2), marble (CaCO3), limestone (CaCO3), and carbonate rocks (CaCO3) according to the Eqs. (1) and (2):

$${\mathrm{CaCO}}_3(\mathrm s)+2{\mathrm{CH}}_3\mathrm{COOH}(\mathrm{aq})\rightarrow\mathrm{Ca}({\mathrm{CH}}_3\mathrm{COO})_2\cdot{\mathrm H}_2\mathrm O(\mathrm s)+\mathrm C{\mathrm O}_2(\mathrm g)$$
$$\mathrm{Ca}(\mathrm{OH}{)}_{2}(\mathrm{s}) + 2{\mathrm{CH}}_{3}\mathrm{COOH}(\mathrm{aq})\to \mathrm{Ca}({\mathrm{CH}}_{3}\mathrm{COO}{)}_{2}\cdot {\mathrm{H}}_{2}\mathrm{O}(\mathrm{s}) + {\mathrm{H}}_{2}\mathrm{O}(\mathrm{l})$$

Calcium acetate was also synthesized by using the littleneck clam [15], black snail [16], scallop shell [17], and eggshells [18], as the starting materials. The results observed from the previous works exhibited that the physicochemical characteristics of calcium acetate depended on the starting materials and the operating conditions used in the calcium acetate preparation.

Calcium acetate was used as a promising chemical for environmental and industrial applications [14,15,16,17,18]. In the environmental field, it was employed as an alternative compound to replace corrosive deicers with environmentally unacceptable properties such as calcium chloride and sodium chloride [19]. It was employed as the adsorbent to absorb the carbon dioxide (CO2) gas, which positively results in to decrease in the greenhouse effect [20]. In food processing, calcium acetate (under the number E263) was used as the coagulant in the manufacture of tofu, as a stabilizer in food and syrups, as an anti-roping agent in bread, as a stabilizer in hard candies, and as a stabilizer and preservative in many food substances [21]. It was employed to precipitate milk fat globule membrane proteins from dairy by-products [22]. For other applications, calcium acetate was employed as the precursor to synthesize various advanced materials such as nano CaO [23], cement mortar [6, 7], calcium sulfate [24], ceramic composites [14], acetone [25], and calcium phosphate [11]. It was employed to determine the potential acidity of tropical soils and to alleviate yield loss in rice [26]. It was employed as a soil amendment, as a plant micronutrient, as foliar fertilizer, and as a soil pH adjuster [9, 17]. In addition, it was employed to treat or prevent calcium deficiency and hyperphosphatemia in patients [8].

The synthesis of calcium acetate is important for various industrial applications. In addition, the synthesis of calcium acetate from the waste can reduce the amount of waste, also increase the value of that waste, reduce the use of this raw material from ores, and reduce the emission of greenhouse gases, which is in line with the BCG strategies in Thailand [2, 4]. Consequently, this work focused on the use of oyster shells to transform CaCO3 powder and then reacted with the CH3COOH to synthesize the calcium acetate. This research presented an effective technique to generate calcium acetate by using renewable resources instead of those non-renewable resources. This designed synthesis method also helps to avoid various adverse environmental effects. The effects of the operating parameters in the calcium acetate synthesis processes such as the CH3COOH concentration were investigated. All studied parameters were carried out to achieve the optimized conditions used to prepare calcium acetate. The physicochemical properties of the synthesized calcium acetate (yield and solubility), its characteristics (X-ray fluorescence (XRF), infrared adsorption, X-ray diffraction (XRD), thermal decomposition, and morphology) as well as the production cost were also investigated and discussed.

2 Experiment

2.1 Materials and reagents

The oyster shell wastes were collected from the Sai Kaew (12.74210, 100.84089) and Bang Saen (13.28355, 100.91514) beaches, Chonburi province, Thailand. The oyster shell wastes were first rinsed with distilled water to remove dust and various impurities and then dried in sunlight for 3 h. The clean-dried oyster shells were ground and sieved using U.S. mesh number 50 (0.297 mm) (Figure S1). The obtained powder was characterized by XRF technique and found to contain 95.25% CaCO3, which was used as a starting reagent replacing minerals (limestone and carbonate rock). Additionally, the non-uniform polyhedral shapes with particle size distributions in the range of 1–6 μm were revealed by scanning electron microscope (SEM) technique (supplementary information). The CaCO3 powder obtained from oyster shells was referred to as bio-green calcium carbonate. Commercial concentrated acetic acid (99.7 wt% CH3COOH, industrial grade, Merck) was used as one of the starting reagents to synthesize the calcium acetate. CH3COOH was used without further purification and its molarity was 17.42 M. Before use, this concentrated acid was first diluted with deionized water to prepare three different concentrations of CH3COOH, namely 8, 10, and 12 M. Because the dilution processes were strong exothermic processes, the dilution solutions were therefore left to cool before further use.

2.2 Calcium acetate preparation

Calcium acetate monohydrate was prepared according to Eq. (1) and an example preparation was described in detail. Firstly, 8 M CH3COOH (50 mL, 0.40 mol) was added into a beaker containing 21.02 g (0.20 mol) of the bio-green CaCO3 (molecular weight of CaCO3: 100.09 g·mol−1). The mixture reaction was stirred at 200 rpm for 30 min under an exothermic process occurring the released carbon dioxide (CO2) gas. After that, the mixture reaction was exposed to the air atmosphere at ambient conditions (room temperature and 101 kPa) until it was completely dried, resulting in the formation of the pale cream-white Ca(CH3COO)2·H2O powder. The Ca(CH3COO)2·H2O powder synthesized by the reaction of 8 M CH3COOH was designated as CA8. For the other two products, the preparation processes were similar way but different CH3COOH concentrations of 10 and 12 M were used, and the resulting products were respectively designated as CA10 and CA12. The conditions of synthesis along with the yield, drying time, and ambient temperature were followed and recorded. The optimum parameters will be discussed.

2.3 Characterization

After three Ca(CH3COO)2·H2O samples (CA8, CA10, and CA12) were synthesized from the reaction between bio-green CaCO3 and different CH3COOH concentrations, a synchrotron rapid-scanning XRF (model 3400, Bruker) spectrophotometer was used to analyze the elemental composition and amount of trace elements in the synthesized samples. The structure and crystallinity of samples were investigated by D8 Advance XRD (Bruker) with Cu-Kα X-ray source (wavelength, λ, of 1.5406 Å). The 2θ values were scanned and collected from 5 to 60 degrees (°) with 0.01° scan step. To investigate and confirm the synthesized materials, the resulting diffraction patterns were compared and matched with the Joint Committee on Powder Diffraction Standards (JCPDS) card number (#), a crystallographic database [12,13,14]. The vibrational characteristics (chemical bonds of the molecular structures) of the synthesized compounds were recorded and analyzed by the Spectrum GX Fourier transform infrared (FTIR) spectrophotometer (Perkin Elmer) using the potassium bromide (KBr) disk technique [18]. The wavenumber ranged from 4000 to 400 cm−1 with a resolution of 2 cm−1 [17]. To prepare the FTIR sample, the synthesized Ca(CH3COO)2·H2O was homogeneously mixed with the spectroscopic-grade KBr powder using an agate mortar. Obtained finely ground was hydraulically compressed into a pellet, which was then put into a sample holder of the spectrophotometer. The Pyris Diamond thermogravimetric/differential thermal analyzer (TGA/DTA, Perkin Elmer) was used to perform the thermogravimetric (TG) and differential thermogravimetric (DTG) measurements in the N2 atmosphere from ambient temperature (30 °C) to 900 °C at a heating rate of 5 °C·min−1. Finally, the surface morphologies of the synthesized samples and raw materials (Figure S1) were observed with a SEM (LEO 1450 VP) by applying a gold-coated technique before analysis [17].

3 Results and discussion

3.1 Preparation optimization results

Three Ca(CH3COO)2·H2O samples (CA8, CA10, and CA12) were successfully prepared from the reaction between bio-green CaCO3 and three different CH3COOH concentrations (8, 10, and 12 M). Four conditions based on the synthesis of Ca(CH3COO)2·H2O by using bio-green CaCO3 and CH3COOH are ambient temperature that occurred in an exothermic reaction, drying time, yield, and solubility. The advantages of Ca(CH3COO)2·H2O production from the bio-green CaCO3 obtained from oyster shell waste precursor are able to achieve a high yield with a short drying time. These conditions point out the low production cost, which can affect the economic decision of the users.

Table 2 shows the ambient temperatures that occurred in an exothermic reaction, drying times, yields, and solubilities obtained from the Ca(CH3COO)2·H2O production using different CH3COOH concentrations. Obviously, the yields of more than 89% of all Ca(CH3COO)2·H2O samples from each acid concentration were observed, and the maximum yield of 93% was obtained from using 10 or 12 M CH3COOH. The yields of the Ca(CH3COO)2·H2O production obtained from oyster shells reported in this work are higher than those prepared from scallop shells reported in our previous work (87%) [17]. The temperature due to the exothermic reaction of the Ca(CH3COO)2·H2O synthesis was observed in the range of 37 − 58 °C, which increased with increasing CH3COOH concentration. The temperature of Ca(CH3COO)2·H2O production in this work is slightly different from those of the Ca(CH3COO)2·H2O synthesized by scallop shells in our previous work (35–64 °C) [17]. The drying time, recorded at the beginning of the reaction until the powder completely dried by itself was observed in 18–30 h, which decreased with increasing CH3COOH concentration. The drying time of the Ca(CH3COO)2·H2O production obtained from oyster shells in this work is a longer time than those of scallop shells in our previous work (10–24 h) [17]. The solubilities of all Ca(CH3COO)2·H2O samples are nearly the same value of 97%. The conditions of Ca(CH3COO)2·H2O production are caused by different calcium sources (oyster shells and scallop shells), related to the purities of calcium carbonate [17].

Table 2 The parameters of the Ca(CH3COO)2·H2O production from the reaction between the different CH3COOH concentrations and bio-green CaCO3

The optimal condition for the Ca(CH3COO)2·H2O production with short drying time and higher yields were found in the reaction between the bio-green CaCO3 and 12 M CH3COOH. The obtained conditions can be applied to an industrial scale to obtain the highest Ca(CH3COO)2·H2O yield. This Ca(CH3COO)2·H2O synthesis technique using shell waste is attractive economically and environmentally. The cost of the bio-green CaCO3 (95.3% purity) powders obtained from oyster shells determined by costs of transportation, plant, labor, water, and electricity was estimated at about 0.033 USD·kg−1. The cost of CH3COOH (industrial grade, density of 1.049 g·mL−1) used in this work is about 0.6 USD·kg−1 or 0.625 USD·L−1. In the reaction of Ca(CH3COO)2·H2O preparation (Eq. 1), 1.05 kg of the bio-green CaCO3 (0.034 USD) required 1.14 L (1.2 kg) of CH3COOH (0.72 USD). The yield of about 93% for the optimum condition would be about 1.64 kg of Ca(CH3COO)2·H2O synthesized (0.75 USD·kg−1). The overall cost of Ca(CH3COO)2·H2O with plant, labor, water, and electricity was estimated at about 0.030 USD·kg−1. Consequently, the cost for Ca(CH3COO)2·H2O product reported in this work is about 0.78 USD·kg−1. The bulk market price of Ca(CH3COO)2·H2O is about 1 USD·kg−1 so producing this compound from oyster shells would be economically profitable. For environmental issues, oyster shells are wastes from food, which are usually deposited in landfills. This food waste management will be the emissions of greenhouse gases (CO2), which were reported to be about 2.53 kg CO2eq kg−1. From Thailand, oyster shell wastes have been reported to be more than 10 kt per year, so the emissions of CO2 are estimated more than 20 kt per year (Table S1). Additionally, calcium carbonate, raw material used for Ca(CH3COO)2·H2O production at plants was normally obtained from ore sources (limestone or/and carbonate stone), in which the emission factor was about 1.067 kg CO2eq kg−1. The emissions of CO2 of the bio-green CaCO3 obtained from oyster shells determined with the used process (transportation, water, and electricity) was estimated to be 0.017 kg CO2eq kg−1 (Figure S2). Therefore, the emissions of CO2 of Ca(CH3COO)2·H2O production calculated according to Eq. (1) are found to be 2.62 and 2.22 (2.17–2.27) kg CO2 eq kg−1 for calcium carbonate obtained from ore source and oyster shell, respectively (Emission factor of acetic acid = 2.57 kg CO2eq kg−1. This result indicates that the using bio-green CaCO3 obtained from oyster shells for Ca(CH3COO)2·H2O production leads to the reduced emissions of CO2 of about 0.4 kg CO2eq kg−1 (Table S2). Moreover, it also reduces the use of calcium carbonate from rock minerals with limited reserves in Thailand and reduces greenhouse gas emissions from the production process of this chemical. From the above discussion, using the bio-green CaCO3 obtained from oyster shells for Ca(CH3COO)2·H2O production is more advantageous; low cost, recycling waste to high value compound, and reduction of greenhouse gas emissions.

3.2 XRF results

After applying the SRS 3400 XRF spectrophotometer, the XRF results of the synthesized Ca(CH3COO)2·H2O are shown in Table 3. Table 3 exhibited the chemical compositions of Ca(CH3COO)2·H2O products synthesized from different CH3COOH concentrations (8, 10, and 12 M). The purities of all synthesized Ca(CH3COO)2·H2O samples (CA8, CA10, and CA12) based on the XRF results were not significantly different. Calcium oxide (CaO) is the main chemical composition of all Ca(CH3COO)2·H2O samples with contents of around 90%. Whereas silicon oxide (SiO2) is the second chemical composition with contents of around 4%. However, some oxide forms of sodium (Na2O), magnesium (MgO), sulfur (SO3), aluminum (Al2O3), and iron (Fe2O3, ferric oxide) were observed with contents of around 1%. Other chemical compositions of less than 1% were also presented in Table 3. The essential trace elements (manganese (Mn) zinc (Zn), and copper (Cu)) in plants and animals, but not all microorganisms were observed with contents about 0.01–0.07%, which are enough when used in food and dietary supplement [8, 15, 16]. However, 0.03% of the chromium found in the CA12 sample requires caution because the amount used in food and medicine is controlled not more than 13 mg per serving [21, 22]. The accumulation of some toxic compounds in oyster shell waste may be observed, which depended on the natural source. Therefore, the elemental compositions of the synthesized compounds should be investigated. According to the XRF results obtained in this work, Ca(CH3COO)2·H2O synthesized from oyster shell waste did not contain toxic elements such as cadmium (Cd), arsenic (As), lead (Pb), etc.), pointing out the advantages of the oyster shell derived CaCO3 as an alternative starting material to synthesize Ca(CH3COO)2·H2O without toxic elements.

Table 3 Chemical compositions of Ca(CH3COO)2·H2O prepared from the reaction between the bio-green CaCO3 and 8 (CA8), 10 (CA10), and 12 (CA12) M CH3COOH

3.3 FTIR results

The Spectrum GX FTIR spectrophotometer was applied to investigate the vibrational spectroscopy of the synthesized Ca(CH3COO)2·H2O (CA8, CA10, and CA12) compounds. The FTIR spectra of all samples are very similar and are shown in Fig. 1. The vibrational broad bands in the 3690 − 2850 cm−1 region were assigned as the symmetric νs(H2C − H) and asymmetric νas(H2C − H) vibrational stretching modes of methyl (CH3) group as well as the symmetric νs(O − H) and asymmetric νas(O − H) stretching modes of water (H2O) molecule. The weak peak at 1656 cm−1 contributed to the symmetric ν3(H − O − H) bending mode of H2O [12]. Two strong adsorption peaks at 1583 and 1543 cm−1 are assigned as the asymmetric νas(C − O) stretching vibrational mode, while two intense peaks at 1452 and 1412 cm−1 are assigned as the symmetric νs(C − O) stretching vibrational mode of the C − O bond of the acetate anion (CH3COO) [13]. The out of plane ρop(CH3) stretching vibration of CH3 group was split into two peaks at 1058 and 1023 cm−1, whereas the ν(C–C) stretching vibration of CH3COO divided into three peaks at 965, 945, and 927 cm−1. Three peaks at 676, 661, and 618 cm−1 are the out-of-plane ρop(OCO) stretching vibrational modes of the O − C − O group of CH3COO. Finally, three weak peaks, that appeared in all spectra of Ca(CH3COO)2·H2O compounds at about 484, 467, and 422 cm−1 are assigned as the in-plane ρip(OCO) bending vibrational mode of the O − C − O group of CH3COO [12, 13]. These results are similar to the reported by Chung et al. [19] and Thongkam et al. [17], in a previous report of calcium acetate prepared from oyster shells and scallop shells. Therefore, the FTIR spectra of all prepared samples confirmed that the synthesized products are calcium acetate monohydrate (Ca(CH3COO)2·H2O).

Fig. 1
figure 1

Flourier transform infrared (FTIR) spectra of Ca(CH3COO)2·H2O compounds prepared from the reaction between the bio-green CaCO3 and three acetic concentrations (CA8, CA10 and CA12)

3.4 XRD results

The crystal structure of the synthesized calcium acetate compounds was characterized by the D8 Advance X-ray diffractometer. Figure 2 presented the XRD patterns of the calcium acetate samples prepared from three different CH3COOH concentrations (8, 10, and 12 M). The XRD results were compared with those of calcium acetate reported by previous reports [12,13,14,15,16]. The XRD pattern clearly showed that the product synthesized from the reaction between the bio-green CaCO3 and CH3COOH is the calcium acetate monohydrate (C4H6CaO4·H2O or Ca(CH3COO)2·H2O), which was found to match with the JCPDS card #19–0200 [27]. This JCPDS number obtained in this work is different from that of Ca(CH3COO)2·H2O prepared from scallop shell in literature (JCPDS card #19–0200) [27]. The XRD patterns of all synthesized Ca(CH3COO)2·H2O products are quite similar, pointing out that the phases of all Ca(CH3COO)2·H2O compounds prepared from different CH3COOH concentrations are different. Furthermore, all XRD patterns of the Ca(CH3COO)2·H2O compounds are in good agreement with the reference data given by Van der Sluis et al. [27] and Klop et al. [28]. The XRD results of all the prepared samples obtained are in line with the FTIR data, which confirm the identification of each Ca(CH3COO)2·H2O compound.

Fig. 2
figure 2

X-ray diffraction (XRD) patterns of Ca(CH3COO)2·H2O compounds prepared from the reaction between the bio-green CaCO3 and three acetic concentrations (CA8, CA10 and CA12)

The crystal structure of Ca(CH3COO)2·H2O was investigated and well-described by Klop et al. [28]. Ca(CH3COO)2·H2O (MW = 176 g·mol−1) crystallizes in a triclinic crystal system with the space group of P1 (space group number 1), Schoenflies symbol of C11. The crystal structure of Ca(CH3COO)2·H2O consists of infinite multiple O-bridged double-stranded Ca chains, which are cross-linked via the H bonds. The lattice parameters are a = 6.751 Å, b = 11.08 Å, and c = 11.78 Å with. The lattice angles are α = 116.50°, β = 92.41°, and γ = 97.32°. The unit cell’s volume and the number of molecules (or formula units) in the unit cell are 777.1 Å3 and 4, respectively. Whereas the density value of = 1.51 g·cm−3 [28].

3.5 TGA results

After applying the Pyris Diamond thermal analyzer, the resulting TG and DTG curves of the synthesized Ca(CH3COO)2·H2O compounds are demonstrated in Fig. 3. The curves were measured and recorded in the temperature range from 30 − 900 °C. It was observed that the TG curves of the Ca(CH3COO)2·H2O prepared from three different CH3COOH concentrations (CA8, CA10, and CA12) (Fig. 3a–c) were similar with three thermal transformation reaction regions occurring in around 70 − 190, 300 − 520, and 550 − 730 °C, with the corresponding mass-loss of around 10, 30, and 25%, respectively for all Ca(CH3COO)2·H2O samples. The total mass-loss was ~ 65%, whereas the retained mass was ~ 35%. The total mass loss and the residual mass of all prepared Ca(CH3COO)2·H2O products were different from the values obtained from the theoretical data (about 32%) and the same compound prepared by scallop shell (34%) [17]. The higher residual mass observed in this work confirms the metal impurities (Si, Na, Mg, Al, Fe, etc.) in all obtained products, which are well consistent with XRF data. The thermal decomposition reaction of Ca(CH3COO)2·H2O generated calcium oxide (CaO) as the final decomposed product with stable properties, which can be applied in various industrial fields, especially biodiesel production [23].

Fig. 3
figure 3

Thermal analytic (TG and DTG) curves of Ca(CH3COO)2·H2O compounds prepared from the reaction between the bio-green CaCO3 and three acetic concentrations (CA8, CA10 and CA12)

The DTG curves of all Ca(CH3COO)2·H2O (CA8, CA10, and CA12) samples were quite similar, and five DTG peaks at similar temperatures (108 − 117, 164 − 116, 420 − 425, 485 − 506, and 692 − 700 °C) were observed. The first TG mass-loss section corresponded to the two-first DTG peaks for all synthesized Ca(CH3COO)2·H2O compounds. The number of the two-first DTG peaks pointed out the unlike surrounding environment of the water molecules. These two-first DTG peaks were assigned as the eliminations of adsorbed water (H2Oads, first DTG peak, 108 − 117 °C) and water of the Ca(CH3COO)2·H2O crystal structure (H2Ocrys, second DTG peak, 164 − 116 °C), which called “dehydration”, resulting in the formation of calcium acetate anhydrous (Ca(CH3COO)2), which is a good agreement to the results reported by Bette et al. [12, 14]. The second TG mass-loss section was related to two individual peaks (420 − 425 and 485 − 506 °C) that occurred in DTG curves for all synthesized Ca(CH3COO)2·H2O samples. The third DTG peak (420 − 425 °C) was assigned as the decomposition of Ca(CH3COO)2, resulting in the formation of CaCO3 together with the elimination of acetone (CH3COCH3) compound (deacetonation). Whereas the fourth DTG peak (485 − 506 °C) was assigned as the decomposition of CH3COCH3, forming and eliminating the ketene (H2CCO, formal name: ethenone) (deketenation) and methane (CH4) (demethanation) [17, 25]. After that, the final TG mass-loss section with the single DTG peak (692 − 700 °C) was assigned as the decomposition of CaCO3, resulting in the formation of CaO together with the elimination of CO2 (decarbonization). From the thermal analysis (TG and DTG) results, the thermal decomposition processes of all synthesized calcium acetate samples could be written as the following equations (Eqs. (3) − (7)):

First DTG peak: Dehydration, 108 − 117 °C

$$\mathrm{Ca}({\mathrm{CH}}_{3}\mathrm{COO}{)}_{2}\cdot {\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{crys}}\cdot {\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{ads}}(\mathrm{s})\to \mathrm{Ca}({\mathrm{CH}}_{3}\mathrm{COO}{)}_{2}\cdot {\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{crys}}\left(\mathrm{s}\right)+{\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{ads}}(\mathrm{g})$$

Second DTG peak: Dehydration, 164 − 116 °C

$$\mathrm{Ca}({\mathrm{CH}}_{3}\mathrm{COO}{)}_{2}\cdot {\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{crys}}(\mathrm{s})\to \mathrm{Ca}({\mathrm{CH}}_{3}\mathrm{COO}{)}_{2}(\mathrm{s}) + {\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{crys}}(\mathrm{g})$$

Third DTG peak: Deacetonation, 420 − 425 °C

$$\mathrm{Ca}({\mathrm{CH}}_{3}\mathrm{COO}{)}_{2}(\mathrm{s})\to {\mathrm{CaCO}}_{3}(\mathrm{s}) +\mathrm{ C}{\mathrm{H}}_{3}{\mathrm{COCH}}_{3}(\mathrm{g})$$

Fourth DTG peak: Deketenation and demethanation, 485 − 506 °C

$$\mathrm{ C}{\mathrm{H}}_{3}{\mathrm{COCH}}_{3}\left(\mathrm{g}\right)\to {\mathrm{H}}_{2}\mathrm{CCO}(\mathrm{g}) +\mathrm{ C}{\mathrm{H}}_{4}(\mathrm{g})$$

Fifth DTG peak: Decarbonization, 692 − 700 °C

$${\mathrm{CaCO}}_{3}\left(\mathrm{s}\right)\to \mathrm{CaO}(\mathrm{s}) +\mathrm{ C}{\mathrm{O}}_{2}(\mathrm{g})$$

The results of TG and DTG from this work show that the thermal decomposition of the synthesized Ca(CH3COO)2·H2O from oyster shells is close to the previously reported calcium acetate monohydrate (Ca(CH3COO)2·H2O) prepared from scallop shells by Thongkam et al. [17] with the same decomposition pattern and temperature.

3.6 SEM results

After applying the LEO 1450 VP SEM, Fig. 4 illustrated the resulting SEM images of the Ca(CH3COO)2·H2O compounds synthesized from three different CH3COOH concentrations (CA8, CA10, and CA12). The SEM images of three Ca(CH3COO)2·H2O samples as presented in Fig. 4a–c illustrated similar morphologies. The SEM results showed the aggregation of the timber-like particles with many irregular particles, especially the CA12 sample. It can be noted that the distributions of the sample particle sizes increased with increasing CH3COOH concentration. The dimensions of the particles are in the range of 1 − 30 μm. The timber-like morphology for the CA8 sample occurred more than those of other Ca(CH3COO)2·H2O samples. On the other hand, the irregular particle was observed when 12 M CH3COOH was used in the Ca(CH3COO)2·H2O preparation. The morphologies of all Ca(CH3COO)2·H2O particles synthesized by bio-green CaCO3 powder obtained in this work were different in both shapes and sizes from Ca(CH3COO)2·H2O synthesized by CaCO3 powder by using the littleneck clam [15], black snail [16] and scallop [17] shells which depended on the crystal structure and particle sizes of the starting materials. These phenomena pointed out that the starting materials, used in the Ca(CH3COO)2·H2O synthesis, exert a significant impact on the Ca(CH3COO)2·H2O morphology. These results agree with the results reported by Bette et al. [12, 14].

Fig. 4
figure 4

Scanning electron microscopic (SEM) images of Ca(CH3COO)2·H2O compounds prepared from the reaction between the oyster-shell derived CaCO3 and 8 (a), 10 (b), and 12 (c) M CH3COOH

4 Conclusions

Three Ca(CH3COO)2·H2O samples were successfully synthesized from the exothermic reaction between the bio-green CaCO3 and various CH3COOH concentrations. The influences of some operating parameters, i.e., the concentrations of CH3COOH (8, 10, and 12 M), on the reaction temperature, drying time, and yield during the Ca(CH3COO)2·H2O formation were investigated. The optimum conditions focused on the maximum yield with the short drying time were obtained by using a high CH3COOH concentration. XRF results confirmed that the chemical composition and purity could indicate no toxic metal in the prepared samples, which will be useful in a specific application such as fertilizer, feed minerals, food additives, etc. FTIR and TGA results of all samples in this work are similar to those from the previous report but the obtained XRD and SEM data are different. This is possible because of raw materials sources with different purity. In conclusion, this research presents valuable information and an alternative method for Ca(CH3COO)2·H2O production for various applications by using oyster shell waste as starting materials to replace the natural ores, limited reserves. Moreover, the management of oyster shell waste in this work can help reduce emissions of greenhouse gases by recycling wastes and reducing using calcium carbonate from ore sources.

Availability of data and materials

All data generated or analyzed during this study are included in this published article.


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The authors would like to thank the Scientific Instruments Center KMITL for supporting TGA, FTIR, XRD, and SEM techniques. This work was supported by King Mongkut’s Institute of Technology Ladkrabang (2565-02-05-042).


The study was performed with financial support from King Mongkut’s Institute of Technology Ladkrabang (2565–02-05–042).

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Authors and Affiliations



Conceptualization, S.S., B.B., and P.R.; methodology, S.T., C.S., and S. M.; investigation, P.R. and C.S.; data curation, W.B., N.L., and P.R.; writing—original draft preparation, C.S., W.B., and S.S; writing—review and editing, C.S., S.S. and B.B.; visualization, W.B; supervision, S.S. and N.L.; project administration, B.B; funding acquisition, P.R. All authors read and approved the final manuscript.

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Correspondence to Banjong Boonchom or Pesak Rungrojchaipon.

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Supplementary Information

Additional file 1:

 Table S1. Calculation of carbon emissions in the production of the bio-green CaCO3 powder obtained from oyster shell wastes [1, 2]. Table S2 Calculation of carbon emissions in the production of calcium acetate from the reaction of the bio-green CaCO3 powder obtained from oyster shell wastes and various acetic acid concentrations [1, 2]. Fig. S1. SEM micrograph of the bio-green CaCO3 powder obtained from the oyster shell. Fig. S2. Process line of production for the bio-green CaCO3 powder obtained from oyster shell wastes [1].

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Seesanong, S., Seangarun, C., Boonchom, B. et al. Bio-green synthesis of calcium acetate from oyster shell waste at low cost and reducing the emission of greenhouse gases. Sustain Environ Res 33, 26 (2023).

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