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. 2024 Feb 29;9(10):11232–11242. doi: 10.1021/acsomega.3c06186

Stearate-Coated Biogenic Calcium Carbonate from Waste Seashells: A Sustainable Plastic Filler

Maria Luisa Basile †, Carla Triunfo †,‡, Stefanie Gärtner §, Simona Fermani †,∥, Davide Laurenzi ⊥, Gabriele Maoloni ⊥, Martina Mazzon #, Claudio Marzadori #, Alessio Adamiano , Michele Iafisco , Devis Montroni †, Jaime Gómez Morales ∇, Helmut Cölfen §, Giuseppe Falini †,*
PMCID: PMC10938433  PMID: 38496946

Abstract

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Waste seashells from aquaculture are a massive source of biogenic calcium carbonate (bCC) that can be a potential substitute for ground calcium carbonate and precipitated calcium carbonate. These last materials find several applications in industry after a surface coating with hydrophobic molecules, with stearate as the most used. Here, we investigate for the first time the capability of aqueous stearate dispersions to coat bCC powders from seashells of market-relevant mollusc aquaculture species, namely the oyster Crassostrea gigas, the scallop Pecten jacobaeus, and the clam Chamelea gallina. The chemical–physical features of bCC were extensively characterized by different analytical techniques. The results of stearate adsorption experiments showed that the oyster shell powder, which is the bCC with a higher content of the organic matrix, showed the highest adsorption capability (about 23 wt % compared to 10 wt % of geogenic calcite). These results agree with the mechanism proposed in the literature in which stearate adsorption mainly involves the formation of calcium stearate micelles in the dispersion before the physical adsorption. The coated bCC from oyster shells was also tested as fillers in an ethylene vinyl acetate compound used for the preparation of shoe soles. The obtained compound showed better mechanical performance than the one prepared using ground calcium. In conclusion, we can state that bCC can replace ground and precipitated calcium carbonate and has a higher stearate adsorbing capability. Moreover, they represent an environmentally friendly and sustainable source of calcium carbonate that organisms produce by high biological control over composition, polymorphism, and crystal texture. These features can be exploited for applications in fields where calcium carbonate with selected features is required.

Introduction

Calcium carbonate (CaCO3) is one of the most widely used materials, having a sale of about 150 Mton per year.1 It is commonly obtained by grinding the geogenic raw mineral, limestone, forming ground calcium carbonate mineral (GCC), or by carbonation of a calcium hydroxide slurry, generating the so-called precipitated calcium carbonate (PCC).2 GCC is classified according to the size of the grains that have the shape of irregular rhombohedra. This material is made of calcite, the most thermodynamically stable CaCO3 polymorph, and no chemical change is involved in its preparation process.3 On the other hand, the manufacture of PCC requires more stages such as calcination, hydration/slacking, carbonation, filtration, and drying. The size and shape of PCC grains can be controlled, and impurities are removed during the synthetic process. PCC is generally obtained as rhombohedral or scalenohedral calcite, but aragonite and vaterite, the kinetically favored and the metastable CaCO3 polymorph, respectively, can also be obtained.2 In PCC, a rigorous CaCO3 polymorphic selection is a difficult task, and often traces of other polymorphs coexist with the preponderant one. However, PCC particles are mostly uniform in size and regular in shape when compared to GCC. Thus, PCC is used in more advanced applications than those of GCC.4,5

CaCO3 is also the main component of mollusc shells and other skeletal structures of many calcifying invertebrates, where it usually represents more than 95 wt %.6−9 The CaCO3 formed by organisms is obtained under accurate and faithful biological control. This implies, by example, that different species are able to form structures of aragonite, calcite, or with a precise spatial distribution of both polymorphs.10 In the latter case, they are located in different layers and never mixed together.7 In biogenic CaCO3 (bCC), the crystalline texture and the presence of trace elements is species-specific.8,11 This depends on the geochemical features of the living site of the calcifying organism and can be customized.12 Thus, bCC offers controlled species- and site-specific features that are difficult to find and reproduce in GCC and PCC, respectively.

An important source of bCC is mollusc shells from aquaculture.13,14 This is a farming sector of growing economic relevance and nowadays the production of mollusc shells by aquaculture is about 15 Mton per year.15 The majority of these shells are not collected and become waste which represents a major economic and environmental issue.15−18 Their valorization by sustainable and green processes is desirable and can be a good example of circular economy.19

Presently, mollusc shell powders are used as calcium integrator in poultry’s feed, liming agents in agriculture, and inert filler in diverse composite materials.13,16,20−22 When used as fillers, the bCC powders act as inert and do not provide any additional performance advantage to the material. Two examples are their application in polymers and concretes.

CaCO3 is the most widely used inorganic filler in polymers.23 GCCs are low in price and used primarily to reduce formula costs. By contrast, PCCs are higher in price and used to modify various properties of polymer composites.2 Oyster shell powder imparted to ethylene-vinyl acetate/low-density polyethylene resulted in a material with a remarkably enhanced thermal stability and antimicrobial activity against Gram-positive and Gram-negative bacteria.24 Mussel shell CaCO3 was used as filler for a compostable matrix made of polylactic acid and poly(butylene adipate-co-terephthalate). In this case, thermal, mechanical, morphological, and physical investigations of these biocomposites showed that their performances were comparable with those produced with GCC.25

Eo and Yi prepared concrete with different oyster shell powder contents (10, 20, and 30 wt %) and found that the mechanical properties of concrete replaced by 10 wt % oyster shell powder were basically the same as that of ordinary concrete.26 However, the use of oyster shell powder as fine aggregates had a negative impact on the durability of concrete.27 It was reported that, when compared with ordinary concrete, the one enriched with oyster shell powder showed a higher water absorption.28

Hydrophobic modification of the surface of CaCO3 is an effective method to improve its applications as filler in particular in the case of mollusc shell powder.29 This surface treatment facilitates the dispersion in hydrophobic polymeric matrices, for example, preventing loss of mechanical performances, such as impact resistance. In addition, moisture pick-up by CaCO3 may pose additional problems during handling and processing.30 In concretes, it prevents water from penetrating concrete increasing corrosion resistance.31

Among a variety of CaCO3 surface coaters such as silanes, phosphates, and titanates, the most widely used are fatty acids.32 In particular, a blend of stearic acid and palmitic acid (stearin) and pure stearic acid are most commonly used for industrial applications and laboratory research, respectively. The process of stearic acid/stearate adsorption on the calcite surface can occur in “dry” conditions or by using “wet” methods.33 Generally, the industrial coating is done in water because of the low cost and the simplicity of the process.34,35 It has been suggested that in water suspension, in the presence of significant amounts of dissolved ions, the adsorption and subsequent rearrangement of fatty acid micelles is probably the controlling mechanism of coating rather than chemical surface adsorption.34−36

Here, we suppose that the stearate adsorption mechanism on bCC powders should be similar to that of GCC and PCC. This is because the process is mainly controlled by the presence of micelles of calcium stearate in aqueous dispersions.34−36 However, we also suppose that the polymorphism, the crystalline texture, and the presence of the intracrystalline organic matrix, which are a unique signature of bCC and affect the structure and release of Ca ions,37,38 may influence the adsorption capability of stearate. To test this hypothesis, we use bCC powders from waste seashells from aquaculture, namely oyster shells, scallop shells, and clam shells and compared the results with those obtained using GCC powder. Finally, the selected stearate-coated bCC sample was tested as a filler in an ethylene vinyl acetate compound used for the industrial preparation of shoe soles.

Materials and Methods

Reagents and solvents were purchased from Sigma-Aldrich and utilized without any further purification. For each experiment, daily fresh solutions were prepared. Mollusc shells of Crassostrea gigas (oyster), Pecten jacobaeus (scallops), and Chamelea gallina (clams) were purchased from F.lli Terzi (Palosco, BG, Italy). Commercial GCC and stearate-coated (about 1.5 wt %) GCC were kindly provided by Finproject S.p.A.

Preparation of bCC Powders

Mollusc shells were first cleaned with tap water to eliminate mineral debris. Then, they were treated with a 5 vol % sodium hypochlorite solution for 24 h to remove the organic residues from the surface, washed with deionized water, and air-dried. The dry shells were finally crushed by a hammer mill and sieved at 45 μm by analytical sieving to remove the particles with a higher size. The bleached materials were obtained by milling each powder (15 g) with 15 vol % sodium hypochlorite solution (100 mL) for 72 h using a tube roller and ZrO2 spheres (175 g). The suspension was then filtered under vacuum, and the powder was washed with deionized water, dried in an oven at 40 °C, and sieved at 45 μm.

Preparation of the Sodium Stearate Solution

Sodium stearate was first dissolved by adding it to water at 60 °C with a concentration of 40 or 400 mM. The mixture was stirred for about 30 min until a homogeneous dispersion was obtained.

CaCO3 Coating with Stearate

The coating was carried out by adding 2.0 g of the CaCO3 particles, biogenic or geogenic, to the sodium stearate solution (50 mL) at 60 °C. The suspension was stirred for 2 h. The coated CaCO3 was then separated by vacuum filtration, thoroughly washed with water at 60 °C, and dried in an oven at 105 °C overnight.34

Physical Characterization of the CaCO3 Materials

X-ray diffraction patterns of the samples were collected using a PanAnalytical X’Pert Pro diffractometer equipped with a multiarray X’Celerator detector using Cu Kα radiation generated at 40 kV and 40 mA (λ = 1.54056 Å). The diffraction patterns were collected in the 2θ range between 20 and 60° for the starting materials and 1.5 and 12° for the coated ones with a step size (Δ2θ) of 0.05° and a counting time of 60 s. The analysis of the diffraction pattern to obtain the polymorphic composition and crystallite dimensions was performed using Profex software.39 FTIR spectra were collected by using a Thermo Scientific Nicolet iS10 FTIR spectrometer. Disk samples for FTIR analysis were prepared by mixing a small amount (2 mg) of sample with 98 mg of dry KBr and applying a pressure of 45 tsi (620.5 MPa) to the mixture using a press. The spectra were recorded with 4 cm–1 resolution and 64 scans. Scanning electron microscopy (SEM) measurements of the samples were acquired on a Carl Zeiss Leo 1530 Gemini field emission scanning electron microscope. The microscope was used with an accelerating voltage of 5 kV. The samples were glued on carbon tape, dried under vacuum in a desiccator, and 10 nm gold-coated before their observation. The size distribution measurements were performed with a Mastersizer 2000 (Malvern Panalytical Ltd.) laser diffraction particle size analyzer by dispersing bCC or GCC particles in isopropanol and measuring the distribution in isopropanol. Average value was calculated for five measurements. The specific surface area of the samples was measured by the multiple BET method using a Gemini VII 2390 Series surface area analyzer (Micromeritics Instrument Corp.) with a nitrogen flow. Thermogravimetric analysis (TGA) was carried out under nitrogen flow at a heating rate of 10 °C min–1 by a TA SDT Q600 V 8.0 instrument. The system was pre-equilibrated at 30 °C; then, a ramp from 30 to 900 °C was performed. For the measurement, 10 mg of each sample in an alumina crucible were used, and the temperature ranges considered to estimate the organic material and stearate content were different depending on the nature of the samples and were chosen based on the derivative thermogravimetric profiles (DTG). Differential scanning calorimetry (DSC) measurements were carried out under nitrogen flow at a heating rate of 10 °C min–1 from 0 to 200 °C with a TA Q100 instrument. The analysis was performed using 5 mg of each sample in a hermetic aluminum crucible to clarify the presence of hydrated calcium stearate. The contact angle with sessile water drops (volume of 10 μL) was measured using an optical system (Drop Shape Analyzer DSA30S, Krüss GmbH). Average values were calculated for five measurements on different points on each surface. The sample was compressed into a pellet before the measurement. Quantitative color measurements were acquired by an Eoptis CLM-194 colorimeter using a D65 light source and BaSO4 as reference. The results were reported in the CIE L*a*b* system, where L* represents brightness while a* and b* represent color parameters (a* on the red-green scale and b* on the yellow-blue scale).

CaCO3 Elemental Composition

The sample was set in a Teflon holder with 0.5 mL of H2O2 (30 vol %, Carlo Erba) and 6 mL of HNO3 (65 vol %, Honeywell). The holder was set in a microwave oven (Milestone) operating as follows: 2 min at 250 W, 2 min at 400 W, 1 min at 0 W, and 3 min at 750 W. The digested sample was quantitatively collected, diluted to 10 mL with water, and filtered on paper. All the liquid samples obtained were measured 3 times, 12 s each, with 60 s of prerunning, using a Spectro Arcos-Ametek ICP-OES analyzer. The calibration curve was made by using certified standards in water.

Synthesis of the Coated bCC/Ethylene Vinyl Acetate Compounds

The stearate-coated bCC/ethylene vinyl acetate compounds were prepared according to an industrial procedure. Briefly, polymers and raw materials, in the right mass ratio, were cold introduced into a closed mixer (Comerio Ercole S.p.A.). Through friction, the mixture reached the temperature of 90 °C and then was discharged into the hopper of a granulation extruder (Filmex SAS). Successively, the granules were introduced into the hopper of an injection press (Main Group), plasticized at 95 °C, and injected into the mold at about 200 °C, where the expansion and cross-linking reactions took place for a curing time of about 7 min. The specimens were cooled in a temperature-controlled environment at 23 °C and stored at the same temperature.

Mechanical and Esthetic Characterization of Polymeric Compounds

The mechanical and esthetic characterization of the polymeric compounds were performed after a preliminary specimen preparation in which a splitting machine (Camoga C420) was used to remove the external skins and a pneumatic cutter (NOSELAB ATS 10019000) was used for the preparation of specimens having the shape requested by the ISO regulation (Figure S1). For the measurement of the tensile strength and elongation at break, a Zwick/Roell Z010 dynamometer with a 10 kN load cell and Long Stroke extensometer with a 800 mm extension was used. The test was conducted according to the ASTM D 412 technical standard with a clamp removal speed of 200 mm min–1. The esthetic features were evaluated by images obtained using an optical microscope under reflection conditions.

Results and Discussion

In this work, we have employed seashells from species that (i) have a strong relevance in aquaculture,13 (ii) are made of a single CaCO3 polymorph, and (iii) have diverse crystalline textures.15,38 The bCC used in this study are the low Mg-calcite from the oyster C. gigas, the medium Mg-calcite from the scallop P. jacobaeus, and the aragonite clam C. gallina (Figure 1).

Figure 1.

Figure 1

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Camera images of pristine shells from the oyster C. gigas (A), the scallop P. jacobaeus (B), and the clam C. gallina (C).

In analyzing their capability to be coated with stearate, the seashell powders were homogenized in grain size, performing ball milling and sieving at 45 μm. The same process was done for GCC as comparison. In addition, to evaluate the effects of the surface-exposed organic matrix, an aliquot of the bCC powder was bleached. The characteristics of the obtained powders are reported in Tables 1 and 2. The X-ray powder diffraction patterns (Figures S2 and S3) showed that in the powder from clam shells, upon the dry grinding process, a phase transition of about 10 wt % from aragonite to calcite occurred. This is in agreement with some literature data that reported that this phase transition is associated with the temperature increase produced by frictional heating of grinding40,41 due to the highest thermodynamic stability of calcite32 but disagrees with other research studies reporting the opposite.42,43 The bleaching treatment in wet conditions favored this phase transition, and the content of calcite increased to about 19 wt %. A similar behavior was reported for abalone shells.44 Interestingly, the content of intraskeletal organic material did not change in a relevant way after the bleaching process. This is due to the fact that the majority of the organic material is intracrystalline, and only the organic macromolecules on the grain surface are oxidized and removed by the bleaching process.19 The surface area of the samples did not increase in a relevant way after the bleaching process, despite the decrease of the particle size, in both the biogenic and geogenic CaCO3 samples. The latter observation suggests that the reduction in grain size is more related to the grinding associated with the bleaching process than the removal of organic macromolecules.

Table 1. Percentage CaCO3 Polymorph, Organic Matrix Content, Particle Size D90, Surface Area, and Crystallite Size of Calcite of GCC (A), Oyster Shell (B), Scallop Shell (C), and Clam Shell Powder (D) after 45 μm sievinga.

sample calcite (wt %)b organic material content (wt %)c D90 (μm) surface area (m2 g–1) d(104)d (nm)
A 99.0 0.2 19.39 2.9 79.7
B 100 1.3 32.55 3.9 103.9
C 100 0.8 29.01 4.6 84.1
D 9.5 0.6 24.41 4.0 66.5
A-b 99.0 0.2 4.34 4.0 69.4
B-b 100 1.3 6.88 5.9 209.0
C-b 100 0.7 4.60 5.2 134.6
D-b 19.8 0.5 7.72 3.5 93.4
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a

D90 indicates that up to 90 % of the total particles has a grain size smaller than the reported value. that the samples were subject to a bleaching/grinding process.

b

The complement to 100% is represented by quartz for sample A and aragonite for sample D.

c

The temperature range considered to estimate the organic material content was between 300 and 550 °C.

d

Crystallite size of calcite along the [104] direction.

Table 2. Elemental Composition of the Most Relevant Elements in Geogenic CaCO3 (A), Oyster Shell (B), Scallop Shell (C), and Clam Shell Powder (D) after 45 μm Sievinga.

<!—Col Count:7-->sample Ca (mg g–1) Fe (mg g–1) K (mg g–1) Mg (mg g–1) Na (mg g–1) Sr (mg g–1)
A 385 0.56 5.6 7.28 23.52 NDb
B 392 ND 0.56 1.68 3.36 0.56
C 379 ND ND 2.8 2.8 1.12
D 373 ND ND ND 3.36 1.68
A-b 357 0.56 0.56 6.16 1.68 ND
B-b 382 ND 1.12 1.68 3.92 0.56
C-b 395 ND 1.12 2.24 2.8 1.12
D-b 378 ND ND ND 2.8 1.68
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a

The suffix -b indicates the samples that were subject to a bleaching/grinding process.

b

The concentration was below the instrumental detection limit.

We excluded a process of dissolution and reprecipitation for GCC since the crystallite sizes (d[104]), which were determined by the analyses of the powder X-ray diffraction profile, did not change relevantly. On the contrary, the crystalline domains of mollusc shell powder increased after the bleaching process. This could be due to a recrystallization process of the external layer of the crystallites that has been shown to be formed by amorphous calcium carbonate.45,46

The results of the elemental analyses are reported in Table 2 and agree with the expected elemental composition.47 The concentration of calcium is around 40 wt %, and the content of magnesium is higher in scallop shells than in oyster shells, while strontium is present in a higher amount in the aragonite shell of the clam. The content of the other elements is quite low. After the bleaching process, only the concentration of the minor elements changed, probably due to the dissolution of some soluble salts present in traces in the pristine shells.

The morphological appearance of the obtained powder particles is shown in Figures 2 and S4. The geogenic calcite did not change the grain morphology upon the bleaching process, even if the particle size was reduced, in agreement with the grain size distribution (Figure 2A,B). Differently, the mollusc shell powders showed a change in the morphology of the particles because of the bleaching process (Figure 2C–H). This effect appears species-specific, being different in the texture, the polymorphism, and the composition among species. The oyster C. gigas shells are made of low Mg-calcite with an isomorphic substitution of magnesium to calcium of about 5 mol %.48 Both valves of the adult oyster shell are predominantly composed of foliated calcite,49 which is bound on the outermost surface by prismatic calcite and is interrupted by lenses of chalky calcite.50 The image clearly shows that the (001) surface of the laths making the foliated calcite smooth before bleaching and becomes rough and highly porous (Figure 2C,D). This provokes a slight increase of the specific surface area, which mainly also contributes to the reduction of the grain size. The shell of the scallop P. jacobaeus is composed of calcite foliated layers,49 except for a thin aragonitic prismatic pallial myostracum.51 The content of Mg in the calcite structure is of about 7 mol %.48 As observed for the shell of C. gigas, the bleaching induced an increase of the surface roughness and porosity of the laths (Figure 2E,F), which is associated with an increase of the surface area. The aragonite shell of the clam C. gallina presents in the internal layer a granular texture in which the grains do not show any apparent microstructure and a thinner external prismatic layer. These structural motifs do not change their morphology in a relevant way after the bleaching process (Figure 2G,H), except for a reduction of the grain size, which is not associated with a relevant change of the surface area.

Figure 2.

Figure 2

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SEM images of unbleached (A,C,E,G) and bleached (B,D,F,H) powders from geogenic CaCO3 (A,B), oyster shells (C,D), scallop shells (E,F), and clam shells (G,H). The insets report high-magnification images. These images are representative of the entire population of particles.

The samples were also evaluated in their color, which is an important parameter for industrial applications. White fillers are in great demand by the industry because they allow a better control of the coloring of the material given by pigments and dyes.52 The data (Table S1) show that the brightness (L*) of bleached samples is higher than that of the untreated ones. This value slightly increases for geogenic calcite (0.1%), while for oysters, scallops, and clams the increase was 2.4, 1.9, and 4.8%, respectively. In addition, the color distance from the white reference (dE*ab*) decreased moving from the unbleached to the bleached samples, it was 0.2% for the geogenic calcite, while for oysters, scallops, and clams it was 47, 14, and 93%, respectively. These improvements, which were higher for clams than for oysters and scallops, could be due to a lower efficiency of the bleaching process or a higher content of colored pigment present in clam shells.

The coating of the CaCO3 particles was performed using 40 and 400 mM sodium stearate aqueous dispersions. These concentrations are both above the critical micellar concentration of stearate, which according to the literature is from 0.45 mM (at 20 °C) to 0.7 mM (at 60 °C).53 In a similar reference experiment, Shi et al. (2010)34 used a 400 mM sodium stearate aqueous dispersion with precipitated CaCO3 crystals having a surface area of about 65 g m–2. This value is about 10 times higher than that of the CaCO3 powders used (Table 1). Therefore, we used 40 mM, and also 400 mM for comparison, sodium stearate aqueous dispersions.

The adsorbed amount of stearate was determined by the weight loss in TGA experiment (Table 3, Figures S5 and S6) in the range between about 250 and 500 °C. When 40 mM sodium stearate aqueous dispersion was used, the particles were able to adsorb slightly different amounts of stearate according to their source (Table 3), on the other hand, these differences were more marked when 400 mM stearate dispersion was used (Table 4). The biogenic calcite substrates, oyster, and scallop ground shells, adsorbed more stearate than the biogenic aragonite from clam shells and geogenic calcite. Moreover, in all of the samples, the bleached substrates adsorbed a higher amount of stearate than the untreated ones. This effect, which can be related to the increase of the specific surface area after bleaching, was more marked for the biogenic calcite particles than for the biogenic aragonite.

Table 3. Thermal and Surface Analyses of Geogenic CaCO3 (A), Oyster Shell (B), Scallop Shell (C), and Clam Shell Powder (D) Coated Using 40 mM Sodium Stearate Aqueous Dispersiona.

sample OM + St content (wt %) adsorption capacity (mg m–2) contact angle (deg) crystalline-to-smectic (°C) ΔH (J g–1)
A 4.5 15.5 139.4 125.5 2.9
B 4.7 9.0 137.7 126.1 3.0
C 4.7 9.1 133.4 125.4 2.3
D 4.4 9.9 147.5 126.2 2.5
A-b 4.7 12.3 150.1 125.4 2.5
B-b 5.2 6.9 135.0 125.2 2.6
C-b 5.4 9.5 146.3 126.1 2.1
D-b 4.4 11.6 159.0 125.5 2.7
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a

The temperature range considered to estimate the organic matrix (OM) and stearate (St) contents was selected according to the derivate profile of the thermogravimetric curve. The suffix -b indicates the samples that were subject to a bleaching/grinding process.

Table 4. Thermal and Surface Analyses of Geogenic CaCO3 (A), Oyster Shell (B), Scallop Shell (C), and Clam Shell Powder (D) Coated Using 400 mM Sodium Stearate Aqueous Dispersiona.

sample OM + St content (wt %) adsorption capacity (g m–2) contact angle (deg) crystalline-to-smectic (°C) ΔH (J g–1)
A 10.2 27.2 135.4 126.4 10.1
B 23.1 60.9 136.4 126.4 18.6
C 13.1 20.2 137.3 125.6 18.9
D 11.7 20.9 139.0 126.1 8.6
A-b 13.2 25.3 149.0 126.4 9.2
B-b 26.9 45.1 92.2 126.2 15.4
C-b 14.0 18.6 146.3 126.3 10.2
D-b 14.5 35.3 128.8 125.9 10.3
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a

The temperature range considered to estimate the organic matrix (OM) and stearate (St) content was selected according to the derivate profile of the thermogravimetric curve. The suffix -b indicates the samples that were subject to a bleaching/grinding process.

The stearate was adsorbed on the CaCO3 particles in the form of a Ca salt. These results can be explained by considering that less soluble calcium stearate forms by the exchange reaction between Na stearate and Ca ions in the water suspension.54 As a consequence, Ca ions dissolve from uncoated areas of the surface of CaCO3 particles to restore equilibrium until only calcium stearate is present. The surface area covered by one molecule of the stearate at the equilibrium is one important parameter that depends on the crystal faces exposed by the CaCO3 particles.36 Kralj et al. reported the cross-sectional area of one stearate molecule of 0.21 nm2 on rhombohedral {104} calcite planes and 0.31 nm2 for scalenohedral {211} planes in nonstoichiometric calcites.55 Also, the arrangement of the stearate molecules influences the cross-sectional area.56 Assuming a cross-sectional area of stearate of 0.2 nm2, which corresponds to the surface area for one stearate molecule in a perpendicular orientation,57 for the mean surface areas of CaCO3 particles used (5 m2 g–1) the theoretical full monolayer coverage would be about 1.2 wt % for stearate coating. At both concentrations investigated in this study, the amount of stearate adsorbed is beyond the theoretical monolayer coverage as shown in Tables 3 and 4. This indicates that the stearate on the CaCO3 particle surface exists in a multilayer form, with the number of layers increasing as the amount of stearate increases.

The adsorbed Ca stearate on the CaCO3 particle surface can be expected to have complex calorimetric properties since the process conditions used during water coating can favor the formation of its hydrate form, calcium stearate monohydrate.58 The DSC profiles (Figures S7 and S8) show, as the temperature increases, first a dehydration peak (115 °C) and then a peak corresponding to the transitions from the crystalline-to-smectic phase (130 °C), which agrees with what is reported in the literature for this material.59 The relationship between the dehydration enthalpy and the crystalline-to-smectic transition enthalpy for the Ca stearate layer adsorbed on the CaCO3 particles surface versus the adsorbed amount of stearate, determined by DSC measurement, is presented in Figure S9. Both enthalpies increase linearly as the adsorbed amount of stearate increases when 400 mM stearate dispersion was used. Because the crystalline-to-smectic peak and dehydration peaks in the DSC measurements belong to only the physically adsorbed part of Ca stearate, it means that the physically adsorbed part increases in a proportional way to the total organic amount of surfactant and that the chemisorbed part remains at a constant value for each substrate. This observation is also confirmed by the data regarding the contact angle (Tables 3 and 4) that show how this value is almost constant among all samples, with the exception of the sample of oyster powder treated with 400 mM sodium stearate aqueous dispersion. The variation among samples could be related to the surface morphology, which is known to affect the contact angle.60,61 All the samples showed a similar temperature of the crystalline-to-smectic transition, while the enthalpy associated with this process was higher for the sample prepared in the presence of 400 mM sodium stearate aqueous dispersion. This observation could be related to the size of the crystalline domains of the physically adsorbed part of Ca stearate layers,62 but this has not been investigated in the present study.

Important information comes from the evaluation of the adsorption capacity (Tables 3 and 4), that here it is defined as the mass of stearate for the surface unit of the CaCO3 particles. The data clearly show that when 40 mM stearate dispersion was used the GCC and the oyster shell powder adsorbed the highest amount of stearate for the surface unit, but when 400 mM stearate dispersion was used the oyster shell powder showed an absorption capacity higher than GCC.

The rationalization of this observation needs to recall the specific features of the bCC. Among them, the presence of the intraskeletal organic matrix, which is the highest in oyster shell powder among the samples analyzed. Its presence destabilizes the calcium carbonate structure and thus increases its solubility.63,64 Thus, according to the proposed mechanism that implies the formation of calcium stearate, a higher amount of this salt is formed during the interaction between the stearate micelles and the powder.

The structure of the adsorbed stearate was further investigated by SEM, FTIR, and X-ray powder diffraction. In Figure 3, the FTIR spectra of CaCO3 particles coated by using 400 mM dispersion of stearate are shown. The two bands at about 2919 and 2852 cm–1 are due to the antisymmetric and symmetric methyl stretching vibration modes (νaCH3 and νsCH3), respectively. The absorption bands at 1577 and 1544 cm–1 are due to the antisymmetric νaCOO– stretching vibration of carboxylate groups in unidendate and bidendate coordination with Ca ions, respectively.65 No other bands associated with carboxylate groups were present, thus only calcium stearate was formed.35 Other absorption bands are due to vibrational modes associated with calcite and aragonite or due to the alkyl chains of the stearate. No band shift was observed among samples, and only the relative intensities of the bands changed according to the composition of stearate/calcium carbonate materials. When 40 mM stearate dispersion was used, the low relative amount of adsorbed stearate (Table 3) did not allow its detection, and the FTIR spectra showed only the typical adsorption bands of calcite and aragonite (Figure S10).

Figure 3.

Figure 3

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FTIR spectra of unbleached and bleached powders of (A) geogenic CaCO3, (B) oyster shells, (C) scallop shells, and (D) clam shells coated using 400 mM stearate dispersion. (i) Unbleached CaCO3 samples. (ii) Bleached CaCO3 samples. (*) Sodium stearate spectrum.

The examination of powder X-ray diffraction patterns of the materials provided information on the crystal form of calcium carbonate (Mg-calcite or aragonite) and also on the structure of its stearate coating.

The diffraction peaks at small Bragg angles (2θ < 12°) from the coated particles using the 40 mM stearate dispersion (Figure S11) did not show diffraction peaks for the unbleached samples, which contained the lowest percentage of stearate (Table 3). The only exception was observed when oyster powder was used. In this case, a weak diffraction peak was detected at 1.98°. Using the same stearate dispersion, the associated diffraction peaks were detected in the bleached samples. These were at 2θ values of 1.98, 3.90, 5.72, 7.59, and 9.48° corresponding to the first five orders of the laminar structure of calcium stearate (46.5 Å) having extended chains oriented almost normal to the plane containing the ionic groups.66 When 400 mM stearate dispersion was used, the low-angle diffraction peaks were more intense as expected having a higher concentration of stearate (Figure 4). This effect was more evident for the bleached samples (Table 4). Also, in this set of experiments, it can be noted that the oyster shell powder was the substrate with the most intense calcium stearate diffraction peaks.

Figure 4.

Figure 4

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X-ray powder diffraction patterns of powders of (A) geogenic CaCO3, (B) oyster shells, (C) scallop shells, and (D) clam shells coated using 400 mM stearate dispersion. (i) Unbleached CaCO3 samples. (ii) Bleached CaCO3 samples. (*) Sodium stearate diffraction pattern.

The absence of diffraction peaks in the samples with the lower content of stearate could be due to the detection limit of the method but also because the chemisorbed stearate does not give diffraction peaks as previously reported.34 Thus, where diffraction peaks are detected, the majority of calcium stearate is physisorbed on the surface of stearate-chemisorbed CaCO3 particles.

The SEM images of the samples treated with 40 and 400 mM stearate dispersion are reported in Figures 5 and 6, respectively. The samples treated with 40 mM stearate dispersion (Figure 5) generally preserve all of the textural and morphological features of the samples before the coating process. This is expected since a low amount of stearate is adsorbed, no more than a few layers, on the unbleached samples. These layers do not change the overall morphology and do not induce a relevant aggregation of the particles. The higher amount of stearate adsorbed on the bleached samples does not change in a relevant way the morphology of the particles, even if the X-ray data indicate that the presence of crystalline layers is able to generate a diffraction pattern. The CaCO3 particles treated with 400 mM stearate dispersion and containing a high percentage of stearate lose many of the surface characteristics of the uncoated samples (Figure 6). Moreover, the presence of deposits of layered structures, which may embed CaCO3 particles, is observable.

Figure 5.

Figure 5

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SEM images of CaCO3 particles coated using 40 mM stearate dispersion. (A) Geogenic CaCO3, (B) bleached geogenic CaCO3, (C) oyster shell powder, (D) bleached oyster shell powder, (E) scallop shell powder, (F) bleached scallop shell powder, (G) clam shell powder, and (H) bleached clam shell powder. The insets report a low-magnification image for each sample. The images are representative of the entire population of particles.

Figure 6.

Figure 6

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SEM images of CaCO3 particles coated using 400 mM stearate dispersion. (A) Geogenic CaCO3, (B) bleached geogenic CaCO3, (C) oyster shell powder, (D) bleached oyster shell powder, (E) scallop shell powder, (F) bleached scallop shell powder, (G) clam shell powder, and (H) bleached clam shell powder. The insets report a low-magnification image of each sample. The images are representative of the entire population of particles.

According to the reported data in the aqueous coating using a hot dispersion of sodium stearate, a chemisorbed-monolayer first forms on which the precipitation of micelles generates additional physically adsorbed layers of calcium stearate. During the deposition/drying process, the aliphatic groups reorganize and orient outward of the surface, leading to a hydrophobic CaCO3 surface and a low surface energy.34,35 Moreover, the diverse textures and polymorphisms of the bCC favor the chemical–physical processes associated with the adsorption process, generating stacked multilayers,67 being this effect particularly marked for the oyster shell powder.

Some of the prepared materials were finally tested as fillers in an ethylene vinyl acetate compound for the preparation of shoe soles. The CaCO3 particles were used in a 10 wt % concentration as commonly done in the industrial process. We tested the oyster shell coated materials, since they were the substrates with the highest capability of stearate adsorption. The process was performed according to the industrial procedure, thus, giving room for future potential scale-up. In a separate set, the ethylene vinyl acetate matrices were loaded with the same mass percentage of 45 μm sieved commercial stearate-coated GCC as the control. The optical images in Figure 7 show that the foamability, the qualitative distribution of pores, of the shoe soles did not change when the commercial stearate-coated GCC was replaced with the bCC coated with different amounts of stearate. This is an important parameter that affects the overall esthetic feature of the compound68 and thus its commercialization.

Figure 7.

Figure 7

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SEM images of the CaCO3/ethylene vinyl acetate compound. (A) Compound obtained using commercial stearate-coated geogenic CaCO3 particles. (B) Compound obtained using oyster shell particles coated with 40 mM stearate dispersion. (C) Compound obtained using oyster shell particles coated with 400 mM stearate dispersion.

The data reported in Figure 8 show that the oyster powder coated with stearate improved the maximum elongation at break with respect to the control, and this effect was more marked when the sample with the lower percentage of adsorbed stearate was used. The improvement was about 55%. No relevant improvement on tensile strength, being about 10%, was observed. Both sets of data indicate that a too high amount of physiosorbed calcium stearate has a negative effect on the mechanical performances of the ethylene vinyl acetate/CaCO3 composite material. The deleterious effect of the high amount of adsorbed stearate was also reported for an ethylene vinyl acetate composite containing stearate-coated Mg(OH)2 crystals.69

Figure 8.

Figure 8

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Mechanical characterization. (A) Elongation to break and (B) tensile strength of CaCO3/ethylene vinyl acetate compounds. The control is the compound prepared using a commercial sample of stearate-oated geogenic CaCO3. Oys40 and Oys400 indicate oyster shell powders coated using 40 and 400 mM stearate dispersion, respectively.

Conclusions

This study has shown that bCC from waste seashells can be successfully coated by using an aqueous dispersion of sodium stearate. All of the experimental data indicate that the adsorption process is similar to that already reported for synthetic and geogenic calcite. Shortly, a first layer of chemisorbed stearate is followed by the deposition of several layers of physiosorbed calcium stearate. However, the peculiar texture and composition of bCC affect the adsorption capacity of the substrate, being particularly efficient in the case of oyster shell powders. The coated oyster powder has been utilized as a filler in an ethylene vinyl acetate compound and improved the mechanical properties with respect to a control using commercial stearate-coated calcium carbonate. Finally, we can conclude that this work demonstrates that it is possible to recover calcium carbonate from seashells for application as fillers in replacement for geogenic calcium carbonate, thus improving the circularity and the sustainability of the industries as in the studied case of ethylene vinyl acetate compounds. It is also important to note that the approach to recycle, preserving the peculiar features of seashells, has a wide applicability to all biominerals, expanding the potential utilization of this methodology.

Acknowledgments

G.F., C.T., S.F., D.L., G.M., J.G.M., S.G., and H.C. thank the respective national funding agency members of the call BlueBio ERANET. J.G.M. acknowledges grant no. PCI2020-112108 (MCIN/AEI/10.13039/501100011033 and “NextGenerationEU/PRTR”). G.F. and S.F. thank the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4—Call for tender no. 3138 of 16 December 2021, rectified by Decree no. 3175 of 18 December 2021 of Italian Ministry of University and Research funded by the European Union—NextGenerationEU, project code CN_00000033, Concession Decree no. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, Project title “National Biodiversity Future Center—NBFC”. G.F. and C.M. thank Dr. Andrea Simoni for the support in the ICP-OES measurements. We thank Dr. Franco Corticelli (IMM-CNR) for the help in the SEM observation.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c06186.

  • SEM images, TGA profiles, DSC profiles, and XRD patterns (PDF)

Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. M.L.B., C.T., and S.G. contributed equally.

Italian Minister of University and Research, MIUR ERA-NET Cofund on Blue Bioeconomy (BlueBio) project CASEAWA (grant agreement ERA-NET no. 817992).

The authors declare no competing financial interest.

Supplementary Material

ao3c06186_si_001.pdf (1MB, pdf)

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