Cien. Inv. Agr. 36(1): 107-114. 2009

RESEARCH NOTE

 

New raw material for activated carbon. I. Methylene blue adsorption on activated carbon prepared from Khaya senegalensis fruits

 

Casmir E. Gimba¹, Odike Ocholi¹, Peter A. Egwaikhide², Turoti Muyiwa³, and Emmanuel E. Akporhonor⁴

¹Department of Chemistry, Ahmadu Bello University, Zaria , Nigeria.
²Department of Chemistry and Centre for Biomaterial Research, University of Benin, Nigeria.
³Department of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria.
⁴Department of Chemistry, Delta State University, Abraka, Nigeria.

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Abstract

Activated carbons were prepared from Khaya senegalensis fruits. Carbonization and activation temperatures were 500°C and 800°C, respectively, at residence time of 5 min for each process. Activating agents were 1M each of NaCI, KCl, CaCl₂, MgCl₂.6H₂O, Na₂CO₃ K₂CO₃ H₂SO₄, and ZnCl₂. Particle size was 125 um and adsorption studies were carried out using methylene blue in aqueous solution as adsórbate through fixed beds in micro columns. Equilibrium data were analyzed using adsorption intensity and capacity parameters from the Freundlich isotherms as well as breakthrough curves. Results were compared with performances of commercial animal charcoal and granular activated carbon. The results suggest that K senegalensis fruits are suitable raw matenals for producing activated carbons. The NaCI activated carbon has the best adsorption characteristics which are better than the commercial products .

Key words: Activation, adsorption, carbonization, isotherm Khaya senegalensis.

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Resumen

Carbón activado se preparó a partir de frutos de Khaya senegalensis. Las temperaturas de carbonización y activación fueron 500 y 800°C, respectivamente, con un tiempo de exposición de 5 min para cada proceso. Los agentes activantes fueron 1M de NaCl, KCl, CaCl₂, MgCl₂.6H₂O, Na₂CO₃ K₂CO₃ H₂SO₄, o ZnCl₂. El tamaño de partícula fue 125 um. Los estudios de adsorción se realizaron empleando azul de metileno en solución acuosa como adsorbente a través de substratos fijos en microcolumnas. La información de equilibrio se analizó empleando parámetros de intensidad y capacidad de adsorción de las isotermas de Freundlich como también las curvas de quiebre. Los resultados se compararon con el comportamiento de carbon comercial de origen animal y carbon activado granulado. Los resultados sugieren que los frutos de K senegalensis son apropiados como materia para producir carbon activado. El carbon activado con NaCl tuvo la mejor adsorción y resultó mejor que el producto comercial.

Key words: Adsorción, carbon, carbon activado, carbonización, isoterma, Khaya senegalensis.

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Introduction

It has been reponed that the global consumption of activated carbon (AC) was above 500,300 t as at 2003, and it is expected to rise annually by 7% (Maroto-Valer et al, 2001). Due to increasing market demand, cheap and readily available alternative precursors are sought while methods of production are developed. 

Khaya plants, Khaya senegalensis (=Swietenia Senegalensis Desr), is a indigenous hardwood tree of Africa belonging to the family Meliaceae that thrives well in northem Nigeria. It is relatively abundant in Ahmadu Bello University Zaria, Nigeria the locality where this study was conducted The roots wood stem bark leaves and seeds are used for varios medicinal and domestic purposes (von Maydell 1990). The fatty acid composition of the seed oil has been investigated previously, and it is composed of stearic acid, palmitic acid, oleic acids and an unidentifiable acid (Okieimen and Eromosele. 1999). However, the fruits, which are globose capsules, have not been reputedly studied like other parts of the tree. They are considered as garbage or waste which constitutes an environmental problem as much time, money and effort is spent on their disposal. A fully mature tree produces between 200 to 500 or even more fruits per year. The average diameter of a fruit is 4.5 ± 0.17 cm and dry weighs 31.88 ± 3.34 g.

Considering that other parts of the khaya plants have been employed in medicine and many other áreas, it was postulated that fruit were possible to use as an adsorbent, particularly when most of the uses of activated carbons are basically expected to be dependent on their adsorption activities. Consequently, the main objectives of this work were to investigate the adsorption characteristics of K. senegalensis fruits using methylene blue (MB) by fixed-beds in columns, and to compare the results obtained from AC's prepared with commercial products .

Materials and methods

Preparation ofthe raw material

Fruit samples were collected around the residential quarters of Ahmadu Bello University Zaria, Nigeria, between December 2004 and March 2005. Fruits were picked, washed and sundried before to broken them into small piece sizes with a wooden mortar and pestle and further pulveized into powdered form using a Siebtechnik Milling Machine and sieved into particle size of 125 µm with an Endecotts sieve.

Controls

Commercial powdered animal charcoal (Fischer Ltd. England) was purchased in Nigeria (Cardinal Chemical Supplies Company Zaria, Nigeria), and served as the first commercial control (Control 1). The Control 2 was commercial granular AC (Steve Moore Chemical Company Zaria, Nigeria). The particle size of Control 1 is less than 125 µm while particle size of Control 2 was 425 µm.

Carbonization

Raw sample was weighed (3 g) into six separate copper crucibles and introduced into a Muffle furnace at 500°C for 5 min after which they were then removed and quenched in cold water and sun dried (Gimba et al, 2001). The procedure was repeated until substantial amount of carbonized sample was obtained. This was stored in sealed plastic containers.

Characterization of the carbonized carbon

Elemental analysis. Elemental analysis of the raw and carbonized samples was done by neutron activation analysis at the reactor facility section of Centre for Energy Research and Training (CERT) (Ahmadu Bello University Zaria, Nigeria). The long regime irradiation through a thermal neutron flux was used.

Moisture and dry matter content. An amount of 1.0 g each ofthe carbonized K. senegalensis fruits samples was placed in a clean silica crucible that was previously dried in a desiccator and weighed. They were then dried in an air-circulated oven at 105°C for 3 h after which they were cooled in a desiccator and then weighed. Results were expressed as percentage ofthe initial weight carbonized samples.

Ash content. Copper crucibles were heated in a furnace at 500°C, then cooled in a desiccator and weighed. The oven dried samples from the previous section were placed in the crucible and transferred into the Muffle furnace at 900°C for 3 h. The crucibles/content were allowed to cool in a desiccator and then weighed to obtain the weight of the ash. The ash content was expresses as percentage ofthe oven dry weight.

Bulk density. The volume of distilled water displaced by a known mass ofthe carbonized sample was measured using a measuring cylinder.

Activation of carbonized carbon sample

The activation of the carbonized carbon was executed as described earlier (Gimba et al, 2001) using decimolar amounts of each of the follow-ing activating agents: NaCl (5.85 g), KCl (7.45 g), CaCl₂(11.1 g), ZnCl₂(13.6 g), K₂CO₃(13.8 g), Na₂CO₃ (10.6 g), H₂SO₄, and MgCl₂.6H₂O (20.3 g) (Table 2). Flaky pastes were made by mixing 2 g of carbonized samples with 2 mL of 1M of each of the activating agent solutions that were prepared by dissolving the indicated amount in 60 mL distilled water, transferred to a 100 mL volumetric flask, shaken and made up to the mark with the water.

The mixture was introduced into a Carbolite furnace at 800°C for 5 min after which they were quenched with cold water, washed with distilled water and allowed to dry at room temperature. These AC samples were stored in seal-able air tight polythene bags.

Preparation ofmethylene blue stock solution

The use of methylene blue (MB) to evaluate the adsorption characteristics of carbon has been employed. A mass of 0.12 g of MB (molar mass = 319.8 g) was dissolved in about 10 mL of distilled water, transferred into a 100 mL volumetric flash, shaken and made up as above.

Preparation ofcolumns using the test activated carbons and the controls

Clean columns were bedded with ground glass loaded with 60 mg of each activated carbon sample and then equilibrated with 2.0 mL of distilled water.

Test per formed

For each performed, 2.0, 4.0, 6.0 or 8.0 mL of appropriately diluted MB stock solution were passed through the equilibrated column and the absorbance of the effluent taken using a Sherwood 254 digital colorimeter at wave length (λ) = 600 nm. The latter was the λ_max previously determined in an absorbance-concentration experiment of the MB (adsórbate) solutions. The effluent concentration of the MB was obtained from a calibration curve previously constructed. Three separate determinations were made for each sample and average value obtained.

Performance evaluation

The performance (%) of each matrix was calculated using the formula below:

Amount of MB adsorbed x 100 / Total amount of MB.

Isotherms and breakthrough curves

For adsorption isotherm, x/m was plotted against C, where x was the amount of MB adsorbed in µmol, m was the mass of adsorbent and C was the amount of MB that was unadsorbed. The log₁₀ x/m was plotted against log₁₀ C to obtain the Freundlich's isotherm from where the slope (n) and intercept (k) were obtained from the computer plots. The n and k values were the parameters used in this aspect of the study. Break through curves were the plots of C_x/C_s against volume of adsorbate, where C_xwas the effluent concentration and C_swas the influent concentration.

Results and discussion

The average values of moisture and ash contení of the raw K. senegalensis fruits were 6.17 and 3.09%, respectively (Table 1). The carbon yield was 17.32%. While the moisture and ash contení are were 6.17 and 3.09%, respectively. The yield of carbon was 17.32%, greater than those reported earlier using coconut shell as raw material (Gimba et al., 2001).

The previous (Gimba et al, 2001).and present results show that the lower the moisture and ash content the greater the % yield of carbonized carbon. It is therefore most probable that fruits of K. senegalensis are a better raw material source for producing greater yield of carbon than coconut shell under similar experimental conditions. However the present results are comparable with those reported by Tsai et al. (2001) in which corn cobs were used as the precursor.

There was a change from the acidic raw material (pH = 5.18) to the basic carbonized product (pH= 9.10) and to a more basic activated product. For instance, in the KSF-NaCl-AC the pH = 9.90 (Table 1). This is in agreement with earlier work (Cheremisinoff and Ellerbusch, 1978) which showed that alkaline surfaces are charac-teristic of carbons having vegetable origins. The amount of MB adsorbed is expected to be more favorable as the carbon surfaces become more alkaline (Abram, 1973). The relatively highbulk density of 0.81 g·mL^-1 indicates longerperiodof filtration cycle (Abram, 1973).

With the exception of Br, La and Sm, the element contents were smaller in the raw materials than in the carbonized product (Table 2). The latter generally had about three times the amount of all the other elements detected in the raw fruit samples of K. senegalensis. For Na, its amount was about five times in the carbonized than in the raw. It could be that the carbonization process made the affected elements more vulnerable to detection by reducing the amount of interfering or masking agents such as the protein/volatile components of the raw fruit samples of K. senegalensis (Mozammel and Masahirom, 2002). Out of the ten elements detected by the method of analysis, K was the most abundant while Sm was the least abundant in both the raw and carbonized fruit samples. These results were preµminary studies for the characterization of the carbonized carbon from this new source of carbon that can be used for preparing activated carbon.

For the purpose of clarity of presentation of results, most of the latter are conveniently pre-sented in two groups (Figures 1,2). One group for K. senegalensis fruits carbon activated with NaCl, KCl, CaCl₂, and MgCl₂-6H₂O and the second group with Na₂CO₃, K₂CO₃, H₂SO₄ and ZnCl₂ as the activating agents.

The MB adsorption isotherms showed that the order of adsorption was the same as that of the performance in the previous section and indi-cate the validity of the performance evaluation (Figure 1,2). The adsorption isotherms of most of the KSF-AC show that the adsorption of the adsorbate (MB) was that of the C-l curves (linear curves) of the General Adsorption Isotherms (Giles et al, 1960).This means that the MB pen-etrates into the AC more thanthe solvent (water) in line with (Mozammel and Masahirom, 1978) as the adsorption is dependent on the concentra-tion of the adsórbate. The non-linear adsorption isotherm of MgO2-6H₂O-AC indicates unfavorable adsorption of the adsórbate by this matrix.

The order of n values (Table 3) of MB adsorption by the AC, in terms of the AA, were: NaCl = KCl = CaCl₂ < Na₂CO₃ = K₂CO₃ < H₂SO₄ < Control 1 < Control 2 < ZnCl₂ < MgCl₂.6H₂O. The smaller the value of n (1< n < 10) the higher the adsorption intensity (Rozada et al, 2002; Weber Jr, 1973).

The k values, which indicate the adsorption capacities of the AC, ranks thus (in terms of the AA): NaCl = KCl = CaCl₂ > Na₂CO₃ = K₂CO₃ > H₂SO₄ > Control 1 > ZnCl₂ > Control 2 > MgCl₂-6H₂O. The greater the k values the higher the adsorption capacity of an AC for the adsórbate. The previous sections on performance evaluation and adsorption isotherm show parity in the adsorption behavior of the Control 1 and some of the most effective KSF-AC carbons, for instance, NaCl, CaCl₂, KCl and Na₂CO₃-AC (Figure 1). However, the Freundlich isotherm parameters clearly showed that the newly pro-duced KSF-AC activated with NaCl or CaCl₂ or KCl have greater adsorption intensity and capacity than the commercial carbons. All the results consistently showed that MgCl₂-6H₂O is not a good activating agent for K. senegalensis fruit. This might be due to the presence of the molecules of water of crystallization which might have blocked the active sites for adsorption of the MB.

The breakthrough curves (Figures 3 and 4) show that, the control 1, NaCl-AC, CaCl₂-AC, Na₂CO₃-AC and K₂CO₃-AC gave ideal curve, KCl-AC, H₂SO₄-AC and ZnCl₂-AC gave normal curves while the Control 2 and MgCl₂-6H₂O-AC gave abnormal curves. The Control 1, NaCl-AC, CaCl₂-AC, Na₂CO₃-AC and K₂CO₃-AC curves coincided along the concentration axis up to the highest value of 8 mL of MB solution. This means that the matrices have great enduring capacities (Mozammel and Masahirom, 1978). Amongst the KSF-AC, the NaCl-AC, CaCl₂.AC, Na₂CO₃-AC and K₂CO₃-AC have the greatest adsorbing endurance, followed by KCl-AC, H₂SO₄-AC and ZnCl₂-AC and the least, MgCl₂-6H₂O-AC.

The F values calculated were not significant (p < 0.01). Therefore differences between treatments were not statistically significant (Table 4).

The results obtained in this study have unequivocally proved that K. senegalensis fruit is a potential raw material for producing activated carbon. The adsorption capacity and intensity for MB outweigh those of the commercial products employed as controls in this study. The ready avalability of the raw material and the relatively cheap activating agents, especially NaCl, suggest that the cost of production of the activated carbon will be relatively low.

 

References

Abram, J.C. 1973. The charactenstics of activated carbon. Pages 1-29. In: Proceeding of the Conferences Activated carbon in Water Treatment University of Reading, UK.

Giles, C.H., T.H. MacEwan, S.N. Nakhwa, and D. Smith. 1960. Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and measurement of specific surface areas of solids. J. Chem. Soc. 3:3973-3993.

Gimba, CE , J.Y. Olayemi, S.T. Okunnu, and JA. Kagbu. 2001. Adsorption of methylene blue by activated carbon from coconut shell. Global Journal of Pure and Applied Sciences 7:265-267.

Maroto-Valer, M., M. Zhe, L. Yinzhi, N.S. Zhang Brandon, J.M. Anderson, and H. Schobert. 2001. Enviromental benefits of producing adsorbent materials from unburned carbon. Pages 9-15. In: 4^th Int. Ash Utilization Symposium. Lexington, USA.

Mozammel, H.M., and O. Masahirom. 1978. Carbon Adsorption Handbook. Ann Arbor Science Publishers Inc, vol. 72, p. 689-690.

Mozammel, H.M., and O. Masahirom. 2002. Activated charcoal from coconut shell using ZnCl₂ activation. Biomass and Bioener. 22:397-400.

Okieimen, RE., and C.O. Eromosele, 1999. Fatty acid composition of the seed oil of Khaya senegalesis. Biosource Technology 69:279-280.

Rozada, E, LE. Calvo, A.I. Garcia, J Martín-Villarcota, and M. Otero. 2002. Dye adsorption by sewage sludge based activated carbons in batch and fixed bed systems. Bioresource Technology 87:221-230.

Tsai, W.T., C.Y. Chang, C.F Wang, S.F Chien, and HE. Sun. 2001. Preparation of activated carbons from corn cobs catalysed by potassium salts and subsequent gasification with C02. Bioresource Technology 78:203-208.

von Maydell, H-J. 1990. Trees and Shrubs of the Sahel. Their Charactenstics and Uses. Verlag Josef Margraf Scientific Books. GTZ . Germany. 525 pp.

Weber Jr, W.J. 1973.The Prediction of the performance of activated carbon for water treatment. Pages 53-71. In: Proceeding of the conference Activated carbon in Water Treatment. Water Research Association. University of Reading, UK.

 

Received 17 April 2008. Accepted 12 August 2008. 

* Corresponding author: turoti07muyiwa@yahoo.co.uk.