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Cockle (Anadara granosa) shells of CaCO3 (aragonite phase) were subjected to heat treatment at various temperatures (200-1000°C) under atmospheric conditions for 5 h. X-ray diffraction patterns showed that the aragonite phase was completely transformed into calcite at 400°C and subsequently decomposed to calcium oxide (CaO) at 800°C. The BET specific surface area decreased as the aragonite phase transformed to calcite but increased when the calcite phase transformed to CaO. The surface morphology showed layers, agglomerated round-shape particles and lumps with stick-like surfaces in the aragonite, calcite and CaO samples, respectively. The resulting calcite phase was found to be suitable for use as a precursor for the formation of CaO-CNT nanocomposites (CCN).
Calcium carbonate, CaCO3, commonly found in rocks (Reddy, 2007) occurs naturally as the main component of crustacean shells. Due to its varied polymorph properties, the mineral is widely used in diverse applications such as an active ingredient in agricultural lime (West and McBride, 2005), as a heterogeneous catalyst for biodiesel production (Nakatani et al., 2008; Boey et al., 2009), as a raw material for hydroxypatite synthesis in bone implants (Hu and Ben-Nissan, 2001) and as a biosorption material to isolate toxic heavy metal ions (Kim, 2004), etc.
Research on the utilisation of CaCO3 from seashells has been of recent interest, for example in the utilisation of the mineral from crab shells as a catalyst (Boey et al., 2009) and a biosorption material (Kim, 2004). Apart from high-priced crab and molluscs such as oyster, cockle is inexpensive and consumed as part of the daily diet. In Malaysia, cockle contributes to about 14% of marine aquaculture production (Department of Fisheries Malaysia, 2009). Therefore, there is an abundant supply of blood cockle shells. In addition, the shells do not have any other important uses and are normally discarded as waste. Due to the abundance of these seashells, they have a high potential to be utilised as a cheap source of CaCO3 for industrial applications.
In this work, the shells of Malaysian blood cockle, Anadara granosa, were used as the starting material. The present study was undertaken to evaluate the physico-chemical and phase transformation of the minerals of Anadara granosa shells by heat treatment at various temperatures, and to subsequently use the resulting material for the formation of calcium oxide-carbon nanotube (CaO-CNT) nanocomposites (CCN). CaO associated with carbonaceous material has been of recent interest in applications such as transesterification catalysts (Wei et al., 2007; Wan and Hameed, 2011) and sorbents for sulphur dioxide (Marcias-Pérez et. al., 2008). The utilisation of the economical resources of waste biominerals from blood cockle shells is expected to promote the commercial application of the resulting nanocomposites.
Materials and Methods
Blood cockle shells, obtained from a local market, were cleaned and air dried for a few days. Twenty grams of cleaned shells were manually ground and milled for 3 h and labelled as CS. Other samples were prepared by heating the shells in a Vulcan muffle furnace at 200, 400, 600, 800 and 1000°C, and were labelled as NC2, NC4, NC6, NC8 and NC10, respectively. The samples were cooled to room temperature, manually ground first and milled using a planetary mill (Pulverisette 6) for 3 h. The resulting material was stored in a sample bottle for further use and characterisation.
Powder X-ray diffraction (PXRD) patterns of the samples were recorded on a Shimadzu XRD-6000 powder diffractometer using CuKα radiation (λ = 0.15406 Å) at 40 kV, 30mA at 4° min−1. Thermogravimetry and differential thermogravimetry (TGA/DTG) analyses were performed using a Perkin Elmer (TGA-7 series) with heating rate of 10°C min−1 between 30-1000oC. The BET surface area of the samples was determined by a Quantachrome AS1Win surface area and pore size analyser using a nitrogen gas adsorption-desorption technique at 77 K together with the BET equation. A field emission scanning electron microscope (FEI Nova Nanosem 230) was used to study the surface morphology of the samples. To examine the cross-sectional image of the samples, a transmission electron microscope (Hitachi H7100 TEM) was used.
The cockle shells heated at 600°C (NC6) were used for CCN nanocomposite preparation, using catalytic decomposition of hexane. A metal salt, Fe(NO3)3.9H2O (HmbG), was dissolved in distilled water and impregnated onto the NC6 sample by direct addition into the solution. The total loading of Fe, to be used as a catalyst for CNT preparation, was about 5 wt%. The resulting mixture was continuously stirred and heated to evaporate the solvent. The resulting material was dried at 60°C overnight and kept in a sample bottle. The decomposition of hexane was carried out in a horizontal furnace at 850°C. Approximately 1 g of the prepared NC6-Fe was placed in a quartz boat and inserted into a quartz tube under nitrogen flow at 100 mL/min and heated at 850°C for 1 h, followed by a flow of hexane at 0.06 mL/min for 1 h. The product was cooled under nitrogen flow and stored in a sample bottle for further use and characterisation.
Results and Discussion
Phase transformation on heating at various temperatures
3.1.1 X-ray diffraction
In Fig. 1, the PXRD pattern of the as-prepared blood cockle shell powder (CS) shows it was of aragonite phase (JCPDS Card No. 5-0453). The PXRD pattern of NC2 is generally similar to CS, i.e. the phase remained in the aragonitic phase but with the addition of a peak indicating the calcite phase at 2θ = 29.73°. The phase fully changed from aragonite to calcite on heating at 400-600°C. At 400°C (NC4), the aragonite phase was fully transformed into pure calcite phase (JCPDS Card No. 83-0578) with the strongest reflections at 2θ = 29.84°, 47.91° and 48.94°. The PXRD patterns of NC6 show the same calcitic phase, similar to NC4 but with more incisive, higher and sharper reflections, which indicate a well-crystallised material. Increasing the temperature from 800-1000°C resulted in the formation of three strong reflections at 2θ = 37.68°, 54.18° and 32.52° which were identified as calcium oxide, CaO (JCPDS Card No. 37-1497). At this temperature, the calcite was completely decomposed to calcium oxide and no calcite phase trace could be detected. The phase transformation of aragonite to calcite, and calcite to CaO is clearly shown in the PXRD data, i.e. at 400 and 800°C, respectively.
3.1.2 Thermal Analysis
The TGA/DTG analysis shows the calcium carbonate of the shells, both in the aragonite and calcite phases, was stable up to 600oC and then decomposed in one step, as shown in Fig. 2. A mass loss of about 40-43% was observed with endothermic behaviour from 600-800oC, indicating a significant decomposition of CaCO3 to CaO with the removal of CO2. However, about 22% mass losses with endothermic behaviour were observed for NC8 in the temperature range from 315-450oC. This could have resulted from the removal of water molecules from the mineral lattice as the CaO obtained at this temperature was less stable and tended to unite with water molecules upon storage, resulting in Ca(OH)2 formation. The NC10 sample was more stable than the NC8 sample and no significant mass loss could be observed. The high heating temperature provided a more perfect crystal lattice of CaO with denser structural formation of the CaO (Singh and Singh, 2007). These properties have been attributed to the lower free energy of the crystal surface, which therefore lowered the hydration activity of the CaO. Detailed analyses of thermal decomposition of all samples are given in Table 1.
3.1.3 Surface morphology
Fig. 3 shows the surface morphology of the cockle shells, before and after heat treatment. A complex crossed-lamellar structure was observed for the preheated sample showing a layered structure of the crystal growth of CaCO3. The layer thickness was approximately 100-200 nm. The scissure observed from the sample image could be due to sample preparation such as cleaning and shell cutting and natural stress during the growth procedure (Barthelat et al., 2009). The calcite sample NC6 showed continuous appearance of particles and round, amorphous agglomerated particles with poorly defined shapes. The size appeared to be homogeneously dispersed at around 1-2 µm. NC10, which was the calcium oxide sample, showed well-formed lumps on the surface which were not smooth, stick-like in shape, and with a stick thickness around 50-200 nm at higher magnification.
3.1.4 Surface properties
The nitrogen gas adsorption-desorption isotherms and BJH pore size distributions (PSDs) are illustrated in Fig. 4. All the samples indicated Type IV isotherms with a hysteresis loop, which was indicative of mesoporous material (Wang et al., 2005). The PSDs show that the pores of the aragonitic samples, PNC and NC2, were mainly centred at 32 nm. However the PSDs of the calcite sample appeared to be smaller and were mainly centred at around 19 and 17 nm for NC4 and NC6, respectively. The PSDs of the NC8 were most similar to the calcite sample, at 17 nm. However, upon heating at 1000°C, the highest distribution value increased slightly to 21 nm. The BET specific surface areas and the volume and radius of the pore of the samples prepared at different heating temperatures are summarised in Table 2.
CaO formed at 1000°C showed the highest BET specific surface area, followed by the aragonite and calcite samples. The plot of BET specific surface area against temperature is given in Fig. 4. The surface area decreased when the sample was heated at 200oC but increased on further heating at 400-1000oC. This was due to the phase transformations from aragonite to calcite and finally to CaO with the release of CO2 gas from CaCO3 at high temperatures. Released carbon dioxide resulted in the formation of pores in the material, thus a higher surface area was observed.
3.2 Formation of CaO-CNT nanocomposites
The PXRD patterns (Fig. 5) show low intensity CNT-related peaks. In order to confirm the presence of CNTs, treatment with 65% HNO3 was performed to remove the CaO phase and revealed the presence of CNTs. As a result, two distinctive graphite peaks were observed at 26.1° and 44° (Lee et al., 2005) as shown in the figure with no trace of CaO.
Fig. 6 shows the FESEM and TEM micrographs of the as-synthesised CaO-CNT nanocomposites after treatment with 65% HNO3. As shown in the figure, the CNTs can be seen as a bunch of rope-like structures. By using TEM, traces of CaO can still be observed as lumps attached to the CNTs even after the acid treatment process.
This study shows that the dominant mineral of the cockle (Anadara granosa) shells is calcium carbonate crystallised as a single mineral phase, aragonite. The mineral undergoes two transformations, into the calcitic phase at 400-600°C and in to CaO at 800-1000°C. The calcite phase shows amorphous, round-shaped agglomerated particles while CaO is observed as lumps on the surfaces of stick-like shapes. The resulting CaO-CNT nanocomposites synthesised in this work are expected to enhance the use of this abundant and cheap waste mineral resource, particularly from blood cockle shells, and will subsequently promote the commercial application of these nanocomposites.
The authors wish to acknowledge the research facilities and financial support provided through Research University Grant Scheme (RUGS) [05-01-09-0809RU Vote: 91814] and Special Graduate Research Allowance (SGRA) for SAZ under the same vote number from Universiti Putra Malaysia, UPM.