pH Induced Fabrication of Kaolinite-Chitosan Biocomposite

. Functionalization of indigenous materials improves inherent physicochemical properties that depend mainly on their fabrication techniques. Here, pH triggered biocomposites using different proportions of kaolinite and chitosan were fabricated. It was revealed that the biocomposites were formed in 1M acetic acid and stabilized after dropwise addition of the mixture of kaolinite and chitosan solution in 3M NaOH. Binding of kaolinite and chitosan at their interface through functional groups was studied using Fourier transform infrared (FT-IR) spectroscopy and dynamic light scattering (DLS). The average particle size of the biocomposite in aqueous system having 80% w/w kaolinite and 20% w/w chitosan was determined to be 400.8 nm. Crystallinity disappearance of chitosan in the biocomposite, as shown in x-ray diffraction (XRD) spectrum, supports the wrapping of kaolinite with soft and flexible chitosan. Differential scanning calorimetry (DSC) showed the thermal stability of the biocomposites and it was found that the biocomposite fabricated from 50% w/w kaolinite and 50% w/w chitosan was stabled up to 318 o C. Morphological studies were carried out using scanning electron microscopy (SEM), where a progressive tendency towards granular morphology was evidenced with increase in kaolinite content. These functionalized materials in bionanocomposite structure would play a vital role in advanced research in analytical and environmental science


Introduction
Composites, fabricated from petroleum-based synthetic polymers, adversely affect the Earth's ecosystem due to their non-biodegradable nature. There are also limited disposal methods for the persistent plastic wastes which are generated after their respective applications. As a result it is an urgent need to develop eco-friendly green composite materials of natural origin because they don't involve toxic or noxious chemicals for manufacture. At the same time they will be naturally converted into degradation products [1]. Biocomposites add new dimensions to the existing properties of biopolymer and form an interdisciplinary arena combining biology, chemistry and material science together [2]. Biocomposites from waste and naturally occurring materials are promising candidates for numerous industrial applications. In this regard, biopolymer chitosan is an excellent choice because it possesses some special characteristics e.g. it is cheap, available, biocompatible, biodegradable, hydrophilic, non-toxic, it easily undergoes chemical modification, it has good adhesion, ion-exchange and adsorpion properties etc. [3]. However, due to some drawbacks the industrial applications of chitosan are limited. The limitations are: it has a tendency to gel formation and it becomes soft in aqueous media due to hydrophilicity, it has a low specific surface area, it has a low specific gravity, it swells and floats in water, it has weak mechanical properties, higher solubility in acidic media etc. [4][5][6]. Although, various chitosan based composites have been developed in recent years to overcome these drawbacks, greater attention has been paid to the immobilization of chitosan on clay minerals [7]. Clay minerals can significantly improve the properties of chitosan by providing excellent surface interaction due to its smaller particle size, higher surface area, aspect ratio, better dispersion properties etc. [1]. Besides clay minerals possess some excellent properties such as good biocompatibility, non-toxicity and great prospects for controlled release which make grounds for their applications in food, medicine, pharmacy, cosmetics etc. [8]. However, to the best of our knowledge, little attention has been paid to the kaolinite based chitosan biocomposite with a fewer number of scientific publications. Therefore, we were interested in developing an eco-friendly kaolinite-chitosan green biocomposite material.

Vision of the Study
The major objectives of this study are: preparing kaolinite-chitosan bicomposites using acetic acid without any cross-linking agent, thoroughly investigating the effect of solution pH on the composite fabrication process, preparing composite via simple dispersion method and characterizing the prepared composite.

Materials
Kaolinite clay and waste prawn shells were collected from local source of Bangladesh. Purified sodium hydroxide pellets and hydrochloric acid (35%) were supplied by Active Fine Chemicals Limited, Dhaka, Bangladesh. Anhydrous glacial acetic acid (100%) was purchased from Merck KGaA (64271 Darmstadt, Germany).

Preparation of chitosan from waste prawn shells
A four steps process was followed for the preparation of chitosan from waste prawn shells. The process comprised of washing, deproteination, demineralization and deacetylation. The product chitosan was tested by its solubility in 1% (v/v) acetic acid [35].

Purification and activation of kaolinite
The raw clay was first mixed with distilled water and the suspension was passed through 325 mesh screen (45 µ) to separate larger particles. The underflow was filtered and the cake was dried in the oven at 60°C for 10 hours. The dried cake was then ground with a mortar and pestle. The purified clay was treated with 2M hydrochloric acid at room temperature of 25°C for 48 hours at 150 rpm using a mechanical shaker to remove the free quartz. Higher content of quartz may decrease the 2 ILCPA Volume 68 performance of the composites. The clay was separated by centrifugation and washed several times to remove residual acid. The acid activated kaolinite was dried in the oven at 60°C for 10 hours.

Preparation of Biocomposites
Chitosan flakes were dissolved in 100 mL 1M acetic acid and kaolinite was then added to the chitosan solution. The resulting mixture was agitated by a sonicator for 30 minutes. The mixture was then sprayed drop wise into a neutralization solution of 15% NaOH and 95% ethanol in a volume ratio of 4:1. The formed composite beads were washed several times and dried in the oven at 60°C for 10 hours. The beads, after drying, were ground to reduce their particle size [16]. A simplified scheme along with core-shell structure of biocomposite is clearly depicted in Fig. 1.

Results and Discussion
Various parameters may have direct impact on the clay/polymer interactions such as size and shape of clay particles, its surface charge, concentration of clay and polymer, molecular weight of polymers, hydrolyzing groups of polymers, temperature, pH etc. [36]. Considering the above facts we have closely monitored the effect of pH on the composite fabrication process.

International Letters of Chemistry, Physics and Astronomy Vol. 68
Solution pH played a vital role in the fabrication of the composites. Suspension of kaolinite in chitosan solution was centrifuged at 3000 rpm for 10 minutes. We closely monitored that kaolinite particles were separated due to the centrifugal motion. So, the interaction of kaolinite with chitosan was not stable in acidic media. Dropwise addition of the suspension in alkaline media resulted in bead formation of the composites. The rapid neutralization of the acetic acid helped the retention of the spherical shape of the beads [26]. The beads were stable in nature and kaolinite lost its individual identity due to bead formation. We further centrifuged the mixture containing the beads. This time kaolinite couldn't separate itself from the beads. Therefore, this study confirmed the pH dependent nature of the biocomposite fabrication process.

Fourier transform infrared spectroscopy (FT-IR)
In the spectrum of chitosan, as shown in Fig. 2, the absorption band at 1635.64 cm -1 is assigned to the in-plane N-H bending vibration of the polysaccharide [37]. The peak at 1631.78 cm -1 in case of kaolinite is assigned to the H-O-H bending vibration due to adsorbed water [38]. The spectrum of composite showed the combination of characteristic absorption bands due to kaolinite and chitosan. From the figure it is clear that the absorption band of chitosan at 1635.64 cm -1 was shifted to 1624.06 cm -1 in composite. This shift occurred due to the electrostatic interaction between the protonated amine groups (NH 3 + ) of chitosan and negatively charged sites of kaolinite. In order to make it confirm whether it was actually a composite or just a clay-chitosan physical mixture, 100 mg of kaolinite and 100 mg of chitosan was ground using mortar-pestle. FT-IR spectrum of finely ground kaolinite-chitosan physical mixture was taken. No significant shift in the absorption band (1633.71 cm -1 ) was found for the physical mixture which confirmed that the composite prepared by dispersion method was actually a composite, not a physical mixture.

X-ray Diffraction (XRD)
XRD patterns of raw and acid treated clay showed significant differences as shown in Fig. 3(A). The raw clay exhibited well-defined reflections at 2θ around 12.365 (d = 7.152 Å) and 24.932° (d = 3.5684 Å). These two peaks are assigned as characteristic peaks of kaolinite [32]. Clay also showed reflection at higher 2θ values from different crystallographic planes. The figure also revealed the presence of free quartz phases. Among various quartz phases the most prominent one was obtained at 2θ = 26.68° (d = 3.3385 Å) [39].

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ILCPA Volume 68 From the figure it is evident that the quartz phase was highly affected by acid treatment because the intensity of quartz phase was reduced to a significant extent [40]. Although the kaolinite phases were not strongly affected by acid treatment, the lowering of the intensity of the peaks was attributed to the structural disorder [41]. Therefore, it can be concluded that quartz can't be completely eliminated by 2M HCl, but its amount can be lowered to a significant extent. It can also be said that it would not be logical to use higher concentration of HCl because highly concentrated acid will cause structural disorder of kaolinite. The XRD pattern of chitosan, as depicted in Fig.  3(B), showed broad diffraction peaks at 2θ around 9.63 and 20.53 corresponding to the typical fingerprints of semi-crystalline chitosan [42]. These two peaks attribute a high degree of crystallinity to chitosan and are assigned to the crystal I and crystal II in chitosan structure [36,43]. From the figure, it is clear that the crystallinity of chitosan was disappeared in composite as two sharp peaks of chitosan at 2θ = 9.63 and 2θ = 20.53 are absent in composite.

Differential scanning calorimetry (DSC)
As shown in Fig. 4, there was a clear distinction in the DSC thermograms of chitosan, kaolinite and composite. Chitosan showed the exothermic degradation peak between 303.77-304.28 o C whereas composite showed the degradation peak between 318.51-319.39 o C i.e. the degradation of chitosan was delayed in the composite due to its chemical attachment with kaolinite. Therefore, the thermal stability of chitosan was improved due to composite formation. The enhanced thermal stability of the biocomposite is attributed to the changes in dynamics of molecular motion introduced by the addition of clay minerals [44]. Clay platelets create a tortuous pathway for the volatile decomposition products out of the biocomposite bulk thereby increasing the effective path length for diffusion. As a result the rate of diffusion is reduced [45].

Dynamic light scattering (DLS)
The hydrodynamic diameter of kaolinite was found to be 222.2 nm as indicated in Fig. 1(A). Since DLS measure the hydrodynamic diameter, the actual particle size was smaller than 222.2 nm. Therefore, from the DLS study it can be confirmed that kaolinite utilized in this study is a nanoscale material. The hydrodynamic diameter of the composite was found to be 400.8 nm. The enhancement of hydrodynamic diameter of composite was due to the attachment of chitosan polymer with kaolinite particles in the form of composite. So there was some molecular interaction at the bio-nano surface of the composites. Therefore, we can conclude that the prepared composite was actually a bio-nanocomposite. The enhancement of hydrodynamic diameter from kaolinite to composite clearly indicates that hard kaolinite was wrapped with soft chitosan. This type of wrapping results in core-shell structure of the biocomposite having kaolinite in the core and chitosan as shell (Fig. 1). Fig. 1(B) indicates that the morphological features of the biocomposite are clearly distinct from that of kaolinite. Kaolinite showed fragmented particle nature whereas a uniform granular morphology developed in case of the biocomposite. This uniform granular morphology is highly desirable for its industrial applications as the uniform and spherical surface will certainly show better properties than the kaolinite or chitosan alone.

Conclusions
In this present study, biocomposites from waste and naturally occurring indigenous materials were successfully prepared and characterized. The incorporation of kaolinite improved the thermal properties of chitosan and the crystallinity of chitosan disappeared in biocomposites. Kaolinte contains a net negative charge on its surface and chitosan shows polycationic nature in acidic media. As a result there is a versatile charge distribution on the surface of the bicomposites. The smaller particle size, larger surface area, numerous active sites on the surface, availability of raw materials and cost-effective nature of preparation have made these biocomposites potential candidate for numerous industrial applications in leather, textile, food, pharmaceuticals etc.