PREPARATION AND CHARACTERIZATION OF ACTIVATED CARBON FROM WASTE SAWDUST FROM SAW MILL

Activated carbon was prepared in laboratory using waste sawdust powder of Shorea robusta (Sal) from saw mill by chemically activated Na2CO3 followed by carbonization technique. Thus prepared activated carbon was characterized by Iodine Number (IN) and Methylene Blue Number (MBN). Iodine number was found to be 534.6 mg/g whereas MBN was found to be 196.08 mg/g which indicated the presence of micropores as well as mesopores in the prepared material. This was also exposed by SEM image. Specific surface area was measured by BET method and was found to be 10.01 m 2 /g. Then phase state of samples was determined by X-ray diffraction (XRD), which indicated the amorphous nature of the prepared material. The development of surface functionality due to activation was examined by Fourier Transform Infrared Spectroscopy (FTIR) which showed the presence of oxygenated functional groups such as ether, diketone, lactone, phenol on the material. These functional groups are considered to be electrochemically active and contribute to the capacitance of electrical double layer capacitance (EDLC). Further, electrochemical characterization of prepared material was carried out by conductivity test and cyclic voltammetry. The cyclic voltametric curve showed symmetric rectangular shape indicating electrical double layer capacitive behavior.


INTRODUCTION
The preparation of activated carbon from woodbased industrial residue like sawdust from sawmill is one of the most environmental friendly solutions of transforming negative-valued wastes to valuable materials.Wood is a natural fiber raw material with varying properties in morphology and chemical composition.The major chemical constituents of wood are cellulose, hemicelluloses and lignin (Chowdhury et al., 2012).Such lingo-cellulosic carbonaceous material can be used for the preparation of activated carbon (Beguin et al., 2010).Activated carbons are unique and versatile in their performance.They have major industrial significance.
Activated carbon materials characterized by high specific surface area and tunable porosity find utility in many vital technologies namely energy storage, energy conversion, sensors, environmental protection, production of fine and bulk chemicals, and catalysis (Nahil et al., 2012).The choice of carbon as the material is due to its unique properties of electrical conductivity as well as structural diversity.Different literatures showed that the activated carbon could be used as electrode materials for electric double layer capacitors (EDLCs), Li-ion batteries and fuel cells (Beguin et al., 2010;Frackowick et al., 2007;Frackowick et al., 2001;Becker et al., 1957, Endo et al., 2001).Preparation of highly porous carbons may be a challenging art and attention has been given for activation (physical and chemical), carbonization and templeting methods using zeolite and mesoporous silica (Pandolfo et al., 2006;Sharma et al., 2010;Wang et al., 2012;Simon et al., 2008;Zhang et al., 2009).Each approach has its own advantages for the formation of carbons with controlled pore texture and /or improved surface area, which are of great importance and considered to be the key factors in optimizing the performance in their applications.For energy storage system having high energy density and excellent rate performance is desirable.This study presents preparation of activated carbon from locally available material.Sawdust from Shorea robusta (Sal), a solid residue generated in the timber industry has been selected to prepare activated carbon.The prepared material was subjected for electrochemical characterization.

Materials
All the chemicals and reagents used were of analytical grade and procured from Merck Company.All the experiments were carried out in distilled water.The sawdust of Shorea robusta species has been collected from local saw mill.

Methods Preparation of sawdust powder
The collected waste wood from saw mill was sundried for few days and followed by preliminary stages such as crushing, grinding and sieving through 150 µm sized sieve.Thus obtained sawdust powder was used as a precursor for the preparation of activated carbon.

Preparation of activated carbon by carbonization method
The fine powdered precursor was preheated for two hours and was impregnated with Na 2 CO 3 .The ratio of precursor to activating agent was 1:1 (wt:wt).They were left for 24 hours for proper soaking.After soaking, the precursors were dried in an oven at 110 0 C. The dried precursors were carbonized in tube furnace at 400 0 C for 3 hours in an inert atmosphere of nitrogen.Continuous flow of pure nitrogen was used to create an inert atmosphere.The carbonized samples were cooled to room temperature maintaining inert atmosphere of nitrogen.The cooled samples were washed with distilled water for several times till they became neutral.Finally the washed sample was dried in an oven at 110 0 C and stored in air tight bottles.Table 1 show the sample name and preparation condition.

Sample characterization The Proximate analysis
The proximate analysis of the precursor was conducted according to ASTM D121 (ASTM 2009) and the results are expressed in terms of moisture, volatile matter, fixed carbon, and ash contents.

Determination of Iodine Number (I N )
Iodine number (I N ) is a relative indicator of porosity in an activated carbon.It is the measure of micro pores up to 2 nm size found in activated carbon.It is determined according to the ASTM D4607-94 method (ASTM 1999).0.1 g of WAC-Na 2 CO 3 was separately taken in different conical flasks and added 5 mL of 5% HCl, boiled and cooled.To this solution, 10 mL of 0.1N Iodine solution was added.The contents were vigorously shaken for about 4 minutes and filtered.Then 10 mL filtrate was titrated against standard (0.1N) hypo solution using starch as an indicator.The concentration of iodine adsorbed by WAC-Na 2 CO 3 was calculated by using following equation.

Surface area determination by BET:
The specific surface area was measured from nitrogen adsorption-desorption isotherms at 77 K data using Brunauer, Emmett and Teller (BET).

Electrochemical performance: Preparation of electrode
In order to prepare electrode, 0.3 g of graphite powder, 0.3 g of paraffin wax, 0.4 g of WAC-Na 2 CO 3 was mixed homogenously and heated for few minutes.Then the electrode material was formed which was poured and packed tightly to about 2 cm at the bottom of 3 mL plastic syringe.Then the electrical contact was made by keeping a copper wire.This electrode has been used as working electrode.

Preparation of Electrochemical cells
Three electrode cell system has been applied.Here, as prepared electrode was used as the working electrode, saturated calomel electrode has been used as reference electrode, whereas platinum electrode was used as counter electrode.The electrolyte used here was 0.5 M H 2 SO 4 .

Cyclic Voltammetry (CV)
Electrochemical performance of WAC-Na 2 CO 3 was evaluated by Cyclic Voltammetry (CV).The CV measurements were obtained in Potentiostat/Galvanostat by using Polarization Monitor Software interfaced with IBM computer in the potential range of -0.5 to 1 volt.The cyclic stability of prepared ACs was obtained at scan rate of 100 mV/S for 100 cycles.Capacitance values were estimated from CV.

RESULTS AND DISCUSSION Proximate Analysis of Precursor
The results obtained from proximate analysis of precursor are given in Table 2.It revealed that the moisture and ash content of the precursor was 8.56% and 1.07% respectively.This result also showed that the precursor has 31% of volatile matter, low ash and moisture content.The low ash content in the sample indicates that the precursor contains low inorganic matter.

Iodine Number and Methylene Blue Number
Iodine number of prepared WAC-Na 2 CO 3 was found to be 534.6 indicating the presence of micropores in the sample.Similarly, methylene blue number was found to be 196.08 mg/g indicating presence of mesopores.

Scanning Electron Microscopy (SEM)
Scanning Electron Microscopic (SEM) image of sample WAC-Na 2 CO 3 is presented in figure 1.

Fig. 1. SEM image of WAC-Na 2 CO 3
In SEM image of WAC-Na 2 CO 3 , one can observe mesopores along with micropores.Similarly, the cylindrical micro and mesoporous channel could also be seen.Besides this, bigger sized macropores are also obvious on the surface which is anticipated to be responsible for low surface area of this sample.The breakdown of wall of micro and mesopores may have been occurred due to continuous reaction of excess activating agent.

X-ray Diffraction (XRD)
X-ray diffraction analysis of the sample was carried out in order to determine the degree of crystallinity or amorphous nature of the activated carbons.The X-ray diffraction spectra WAC-Na 2 CO 3 is presented in figure 2.

Fig. 2. XRD pattern of WAC-Na 2 CO 3
As can be seen in figure 2, the XRD pattern of the sample exhibit diffraction peaks at around 2θ = 25 degree.In the same way, WAC-Na 2 CO 3 exhibited another weak peak at around 2θ = 43 degree.The diffraction peaks at around 2θ = 25 degree having 002 planes and 2θ = 43 degree having [101] plane assigned for hexagonal graphitic carbon (Liu et al., 2009).The XRD pattern indicates that the laboratory prepared activated carbons are amorphous, similar to commercial activated carbon and poorly graphitized.

Fourier Transform Infrared Spectroscopy (FTIR)
In figure 3, the spectra clearly indicate the creation of surface functional groups on the surface of sample.It may be due to an effect of activating agent used during carbonization.

Fig. 3. FTIR Spectra of WAC-Na 2 CO 3
The small peak at around 2353cm -1 in FTIR spectra is recognized for the presence of C-N group.The band at 1680 cm -1 was likely to be a combination of the carboxyl C=O stretching of non aromatic carboxylic acid and lactone structure.Similarly, the next band is observed at the region around 1576 cm -1 which may be assigned to a combination of C=C stretching vibration of the aromatic ring structures and conjugated systems such as diketone, ketoester and quinine (1550-1680 cm -1 ).The band at 1300-1000 cm -1 may be probably due to the C-O-C lactone structure, the stretching C-O vibrations of phenol structures and ethers, and the bending O-H modes of phenol structures.Similarly, the band at 1700 cm -1 is attributed due to the presence of C=O stretching (Joshi et al., 2014).

BET Surface Area
BET surface area WAC-Na 2 CO 3 was found to be 10.45m 2 /g, which is considered to be small.It may be due to break down of walls of micro as well as mesopores and certainly due to the formation of macropores.

Cyclic Voltammetry
Figure 4 shows the cyclic voltamogram of WAC-Na 2 CO 3 at the scan rate of 100 mVs -1 for 100 cycles measured in 0.5 M H 2 SO 4 aqueous electrolyte solution within a potential range of -0.5 to 1 volt against saturated calomel as reference electrode.In figure 4, one can see the current density of 1 st cycle was 0.21 A/g.But there was slight decrease in current density after 10 cycles.

Fig. 4. Cyclic Voltamogram of WAC-Na 2 CO 3
However, the shape of CV was found to be rectangular indicating electrical double layer capacitance (EDLC) behavior.In the same way, CV shape is mirror symmetric even at high scan rate indicating the high reversibility of the sample.However, the specific capacitance was found to be 7.2 Fg -1 .This small capacitance value may be due to small surface area as indicated by BET surface area and development of macropores which was also perceived from SEM images.The low specific conductance value 3.2×10 -8 S/cm also supports for the obtaining of less capacitance value.Low specific conductance means high resistance.It was probably due to presence of wax as a binder during electrode preparation.

CONCLUSION
It has been concluded that activated carbon can be prepared by using sawdust of Shorea robusta (Sal), by carbonization in a tube furnace at 400 o C, for 3 hours, in an inert atmosphere of nitrogen.
According to proximate analysis, the moisture content, ash content and total carbon were found to be 8.56%, 1.07% and 48.0%respectively.This indicates that the sawdust precursor is suitable for preparing activated carbon.From the SEM image, it has been concluded that the porous surface formation was a consequence of chemical activation.The FTIR spectra confirmed that sample contained oxygenated functional groups such as diketone, lactone and ether.The XRD pattern at around 2θ = 25 degree having 002 planes and 2θ = 43 degree having [101] plane assigned for poorly graphitic carbon, however broad peak showed amorphous nature.The electrodes prepared from activated carbon showed a small current response during cyclic voltammetry measurement.Similarly, the specific capacitance was found to be low.It might be due to high resistance because specific conductivity value was also found to be low.These values are quite reasonable as the specific surface area was found to be small.

Table 2 : Proximate analysis of sawdust of Shorea robusta (Sal)
The abundance of macropores may be another reason for low capacitance.However, rectangular shape of CV curve showed electrical double layer capacitive behavior.Conversely, it needs further enhancement of surface area for significant improvement in capacitive behavior.ACKNOWLEDGEMENT DS is thankful to Institute of Science and Technology, Tribhuvan University to conduct Ph.D. research in Central Department of Chemistry, Kirtipur.Global Research Laboratory, Sun Moon University is highly acknowledged for recording SEM, XRD and FTIR.