The ultrastructure and physicochemical and thermal properties of Palm Kernel Shells (PKS) in comparison with Coconut Kernel Shells (CKS) were investigated herein. Powder samples were prepared and characterized using Surface Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Chemical and elemental constituents, as well as thermal performance were assessed by Van Soest Method, TEM/EDXA and SEM/EDS techniques. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were also performed for thermal characterization. SEM/EDS and TEM/EDXA revealed that most of the PKS and CKS materials are composed of particles with irregular morphology; these are mainly amorphous phases of carbon/oxygen with small amounts of K, Ca and Mg. The DSC data permitted to derive the materials’ thermal transition phases and the relevant characteristic temperatures and physical properties. Thermal Transition phases of PKS observed herein are consistent with the chemical composition obtained and are similar to those of CKS. Nonetheless, TGA/DTG showed that the combustion characteristics of PKS are higher than those of CKS. Taken together, our results reveal that PKS have nanopores and can be efficiently used for 3D printing and membrane filtration applications. Moreover, the chemical constituents found in PKS samples are in agreement with those reported in the literature for material structural applications and thus, present potential use of PKS in these applications.
Ensuring economic sustainability in developing countries has led to harnessing biomass materials having potential value added applications [
There has been a long-standing interest to using PKS in various engineering applications. Herawan et al. [
Considering the increasing interest of microwave pyrolysis [
Sabzoi et al. [
Due to its higher content of silica (SiO2) and higher powder fineness, PKS Ash (PKSA) was recently used as binder and coarse aggregate in geopolymer concrete and concrete formulation [
However, despite the high growing interest on the PKS, its physicochemical and thermal characterization as an engineering raw material has received limited attention over the past decades, while the production for this agroindustrial waste has increased worldwide. Multiple other PKS value added and engineering applications could therefore be further explored, such as energy production, ceramics [
PKS Tenera waste was obtained from Mbambou Palm Oil Mills of Socapalm Land Development Authority, Dizangué Sub-Division in the littoral region of Cameroon, while PKS Dura waste was collected from the Institute of Agricultural Research for Development (IRAD) of Mbongo. The shells were washed using a sodium hydroxide solution, rinsed by demineralised water and dried in an oven at 70˚C during 48 h, prior to analyses. Whereas, CKS waste was collected from local coconut commercial garbage zones at Missole and underwent the same preparation. The shells were grounded and sieved. Powders obtained are from different sizes varying from 0.04 mm to 0.5 mm, in addition weighted.
The well-established pycnometers measuring technique was used. The density of PKS is given by:
( d PKS ) E = m 1 − m 2 ( m 3 − m 1 ) ( m 4 − m 2 ) d s (1)
where ds is the density of solvent, m1 represents the mass of the empty pycnometer, m2 the mass of the pycnometer filled with PKS powder, m3 the mass of the pycnometer filled with the immersion fluid, and m4 the mass of the pycnometer filled with the immersion fluid and PKS powder.
The value obtained was compared to that of the simple Rule of Mixture (ROM) density produced as:
( d PKS ) R O M ≈ 1 ( w L d L + w H C d H C + w C d C ) (2)
where, wL, wC and wHC stand for mass fractions of Lignin, Cellulose and Hemicelluloses, respectively and are to be obtained herein by wet chemistry analysis; dL, dC and dHC are used for densities of Lignin, Cellulose and Hemicelluloses respectively, and values of these chemicals were obtained from the literature [
For SEM observations, the PKS/CKS powders were spread onto an aluminum stub covered with a conductive carbon tape, such that the powder was evenly distributed on the surface of the carbon tape. The samples were coated with a mixture of gold and palladium by a sputter coater (Polaron SC 7640). All powders were brown. PKS sample was the roughest, and CKS was the finest by naked eyes.
A JEOL JSM-6900 Low Vacuum SEM was used to observe the surface morphologies of the products. It is a high-performance scanning electron microscope for fast characterization and imaging of fine structures on both small and large samples. Energy Dispersive Spectroscopy (EDS) for qualitative analysis of the sample elements was performed by using the Integrated Microanalyzer for images and X-rays (IMIX) system.
Likewise, for TEM/EDS, the as-received PKS/CKS powders were crushed with an agate mortar and pestle under distilled water to reduce the particle size. TEM specimens were made with the ground powder as follows: powder was sonicated for 5 minutes in distilled water then dropped onto a lacey carbon coated grid.
For the ultrastructure characterization, a JEOL TEM, with a JSX-1000S Fluorescence Spectrometer X-ray analyzer was used. It is also an elemental analysis tool, capable of identifying the elements in areas less than 0.5 µm in diameter from carbon to uranium.
The PKS (Tenera and Dura) powders were used in this study. Cellulose, hemicelluloses and lignin contents were firstly investigated, as lignocellulosic raw resources main structural chemical components. The well-known and widely accepted Van Soest Method (NDF and ADF analyses) [
Samples for DSC analysis were prepared by adding the desired amount of the as-received PKS (Tenera and Dura) and CKS powders (ca. 8 mg) to a tared DSC pan. After recording the mass, the pan and lid were crimped and the sample sealed within the space. Hermetically sealing (crimping) the pan is done to reduce the temperature gradient within the pan. For TG analysis, the alumina (Al2O3) sample cups are tarred in the instrument and the desired sample is added to the sample cup. Since the sensitivity of the TGA instrument is on the microgram scale, sample sizes need only be ca. 1 mg or less. The furnace is closed and the software logs the sample mass as time/temperature increase. Thermal characteristics were evaluated using DSC measurements via a TA instruments Q200 V24.11 Build 124 model DSC. Measurements were performed in the temperature regime of ambient to 500˚C under a nitrogen atmosphere (flow rate of 50 mL・min−1) using the forced air cooling accessory as the cooling unit. The cell is calibrated with an Indium standard reference material (melting point 156.6˚C and an enthalpy of 28.71 J・g−1) prior to samples being run. A second temperature reference is also used at 327.5˚C. In addition, TGA measurements were conducted on a TA instruments SDT-Q600 V20.9 Build 20 model TG. Measurements were performed in the temperature regime of ambient to 600˚C under argon (flow rate of 50 mL・min−1). The TG SDT-Q600 is controlled by proprietary thermal software, and has auto-sampler accessories for unattended operation. The mass of the specimens was monitored as a function of temperature or time. The TGA and DTGA served to determine the residue, moisture and volatiles [
The endotherm of melting corresponds to the portion of the DSC curve that is far from the baseline, and, later, returns to it. The melting temperature, Tonset, is defined by the extrapolated beginning of the curve, being defined by the point of intersection of the tangent with the point of maximum slope, on the principal side of the peak with the base line extrapolated. The melting temperature value Tm from the endothermic and the corresponding heat flow latent (calorific value) Hm are readily obtained on the DSC thermogram. From the DSC thermogram glass transition temperature, Tg, can be derived. For, the glass transition is a second order transition. Tg is manifested by a sudden increase in the base line, indicating an increase in the heat capacity of the polymer probably after water and volatiles total evaporation and decomposition.
The thermal step-height of the heat capacity ∆Cp of the samples is to be evaluated at glass transition temperature. The heat capacity Cp is defined by:
Δ C p = Δ d H d T = Δ d H d t d t d T (3)
where, represents the temperature scan rate. The difference in the heat capacity of the sample and the reference is defined as in Equations (4) and (5):
Δ C p = C psample − C preference (4)
C psample = Δ d H d t d t d T + C preference (5)
where, Δ d H d t is the shift in the baseline of the thermogram. Equation (5) is readily derived from Equation (3). Hence, at glass transition, Cp is given by:
C psample = Δ d H d t d t d T + C preference (6)
The thermal conductivity, k and thermal diffusivity coefficient a, are given by
k C p . d = a (7)
where, d is the density of the tested material. The value of k obtained by means of direct ROM from the known k-values (kC = 0.40, kL = 0.35 − 0.279, kHC = 0.38 W・m−1・K−1) [
Morphologically, CKS appears similar to PKS. CKS particles have very clear plant cell/tissue structures.
EDS data of CKS plant cell/tissue show a composition similar to those of PKS, containing C, O and silicon (Figures 3(i)-(l)). Occasionally, some rock-like particles were observed and EDS analysis showed that they mainly contain silicon. Al, K, Ca, Fe, Mg, C and O were also detected in the rock-like particles (
Representative micrographs are shown in
To sum up, most of the PKS material is composed of microporous particles with very irregular morphology. These particles are mainly amorphous phases of carbon/oxygen with small amounts of Ca and K. Secondary phases, mainly silicon oxides, could be contaminants, some of which may be sand. Also detected are various other materials in small amounts such as graphite.
The majority phase and an X-ray spectrum of a CKS particle are shown in
The measured density was found to be (dPKS)E = 1.502 g・cm−3 (sd = 0.107) for PKS Dura consistent withthatreported in references [
Analysis N˚ | Chemical compounds (w%) | |||
---|---|---|---|---|
wL | wC | wHC | wMMn/wMMa | |
PKS1 | 44.20 | 35.20 | 18.79/13.82 | 1.81/6.79 |
PKS2 | 43.76 | 34.04 | 20.77/15.41 | 1.43/6.78 |
Sample | Temperatures (˚C)/Enthalpies (J/g) | Step-Height at Tg/Thermal Conductivity | |||
---|---|---|---|---|---|
Tg | Tm,HC/∆Hm,HC | Tm,C/∆Hm,C | Tm,L/∆Hm,L | ∆Cp (J∙g−1∙˚C−1)/k (mW∙m∙−1K−1) | |
PKS | 241.66 | 264.88/48.85 | 343.77/28.33 | 423.58/27.33 | 0.106/306 |
CKS | 246.39 | 274.28/62.09 | 357.3/23.47 | 423.23/37.74 | 0.086/- |
PKS [ | - | 295/- | 330/- | 515/- | - |
Material | Chemical compounds (w%) | |||
---|---|---|---|---|
wL | wC + wHC | Ash | volatiles | |
PKS | 39.67 | 53.80 | 2.021 | 4.509 - 9.63 |
CKS | 26.30 | 52.85 | 11.091 | 6.53 - 8.70 |
CKS [ | 29.35 | 62.76 | 0.68 | 7.073 |
Palm Kernel Shells (PKS) have been assessed as a potential source of raw materials for various engineering applications in this paper. First, the ultrastructure of PKS has been revealed using SEM/EDS and TEM/EDXA techniques. Micrographs observations show that the PKS microstructure is not homogeneous as commonly expected. Morphologically, PKS appears similar to CKS. Indeed, PKS particles have very clear plant cell/tissue structures, with micro/nanopores that make this material a suitable candidate for membrane filtration, for instance. From the spectrum of EDS, the sample contains carbon and oxygen identified as main elements, and silicon in a small proportion. Some rock-like particles were occasionally observed and EDS analysis showed they mainly contain silicon. However, Al, K, Ca, Fe, Mg, C and O were also detected in the rock-like particles. Again, X-ray microanalysis performed on TEM sample nano-locations is consistent with that performed on SEM sample micro-locations. The detection of Ca, P, Mg and O, justifies the empirical use of PKS in sanitation and purification. Moreover, wet chemistry by means of Van Soest (NDF and ADF) has enabled the quantification of the amount of hemicelluloses, cellulose and lignin as major chemicals, with higher lignin content. Additionally, the relevant thermal characteristics were studied using DSC and TGA. No thermal reversibility was observed above 300˚C. Thermal Transition phases as observed from DSC and TG/DTG thermograms confirm the composite nature of PKS/CKS with 3 major thermal events that are related to melts of main PKS and CKS chemicals, precisely hemicelluloses, cellulose and lignin respectively. Thermal characteristics and phase transition temperatures of PKS/CKS that were assessed in this study are consistent with their chemical composition. Nevertheless the necessity arises here to clarify events priority during biomass thermal degradation. The enthalpy of PKS/CKS microparticles reached 66.25/62.09 J・g−1. Therefore, these materials could be used as a wall material of phase change materials. Further investigations could also focus on the reinforcement of polymer ceramics with PKS/CKS, as a novel use for 3D printing biomaterials and membrane filtration materials.
We acknowledge Dr. Craig Bennett and Dr. Haixin Zhang for their support in insightful microstructural analyses of the samples, and Professor Awitor and Thibault Peudon for their valuable support during laboratory chemical analyses carried out at the IUT, the University of Clermont-Ferrand II (France). We are also grateful to Dr. Ngando, Senior Researcher and Director of IRAD Mbongo for providing us with the PKS Dura materials. This work has been supported by the NSCC’s General Research Fund 2018/2019, and the organizations (CFI, AIF and other partners) funding the Facilities for Materials Characterization, managed by the Clean Technologies Research Institute at Acadia University.
The authors declare no conflicts of interest regarding the publication of this paper.
Ntenga, R., Mfoumou, E., Béakou, A., Tango, M., Kamga, J. and Ahmed, A. (2018) Insight on the Ultrastructure, Physicochemical, Thermal Characteristics and Applications of Palm Kernel Shells. Materials Sciences and Applications, 9, 790-811. https://doi.org/10.4236/msa.2018.910057