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Adsorption characteristics of malachite green on activated carbon derived from rice husks produced by chemical–thermal process
Bioresource Technology 96 (2005) 1578–1583
Adsorption characteristics of malachite green on activated carbon derived from rice husks produced by chemical–thermal process I.A. Rahman *, B. Saad, S. Shaidan, E.S. Sya Rizal School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 20 November 2003; received in revised form 13 December 2004; accepted 26 December 2004 Available online 16 February 2005
Abstract Phosphoric acid (H3PO4) and sodium hydroxide (NaOH) treated rice husks, followed by carbonization in a flowing nitrogen were used to study the adsorption of malachite green (MG) in aqueous solution. The effect of adsorption on contact time, concentration of MG and adsorbent dosage of the samples treated or carbonized at different temperatures were investigated. The results reveal that the optimum carbonization temperature is 500 °C in order to obtain adsorption capacity that is comparable to the commercial activated carbon for the husks treated by H3PO4. It is interesting to note that MG adsorbed preferably on carbon-rich than on silica rich-sites. It is found that the behaviour of H3PO4 treated absorbent followed both the Langmuir and Freundlich models while NaOH treated best fitted to only the Langmuir model. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Malachite green; Activated rice husks
1. Introduction Many countries have given prominence to the development of aquaculture as a long term strategy in providing sufficient source of protein to their ever growing population. The raising of large numbers of fish, prawns and others in confined space as in modern aquaculture practice necessitates the use of an extensive range of chemicals for the prevention and treatment of diseases, thus posing as a source of water pollution. The used of malachite green (triphenylmethane) in aquaculture industry has a long history, dating back to 1933, when it was first introduced. The compound, originally extensively used for the dyeing of textiles, has since found wide applications in the aquaculture industry as it is relatively cheap, effective and easy to obtain. Its use in the aquaculture practice in many countries, including *
Corresponding author. Fax: +60 4 6574854. E-mail address: [email protected] (I.A. Rahman).
0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.12.015
Malaysia has not been regulated. Many adverse effects from the consumption of the dye due to its carcinogenic, genotoxic, mutagenic and teratogenic properties in animal studies have been reported (Sandra et al., 1999). Adsorption on activated carbon has long been recognized to be one of the most effective methods for the removal of pesticides and organic compounds from aqueous solutions. Agriculture wastes such as oil palm nut shell (Chan et al., 1980), rice husks (Rahman et al., 2000), olive-waste cakes (Bcaoui et al., 2001), acorns and olive seeds (Lafi, 2001), corncobs (ElHendawy et al., 2001), coconut shells or palm seeds (Hu and Srinivasan, 2001), and guava seeds (Rahman and Saad, 2003) have been used as low-cost adsorbents. These materials were activated either by chemicals, steam, gas or their combinations. The use of adsorbents derived from rice husk to adsorb some phenolic compounds (Mbui et al., 2002) and the herbicide paraquat (Rahman et al., in press) had been reported. Relatively little work had been reported on the absorption of
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
malachite green from aqueous solution despite its widespread use. The adsorption behaviour of malachite green, formalin, chloramine-T and oxytetracycline onto coal-based activated carbon was studied by Aitcheson et al. (2000). The removal of malachite green by agriculture wastes was only reported by Guo et al. (2003) and Garg et al. (2003). In the present work, we report our evaluation on the feasibility of using rice husks pretreated with H3PO4 and NaOH for the removal of malachite green from aqueous solution. The influence of adsorbent loading and contact time were also studied.
2. Methods 2.1. Raw material Dried and fresh rice husks were used as raw materials to produce the adsorbents. The rice husks were obtained from a rice mill in the Northern part of Peninsular Malaysia. Typical composition of the rice husks used is 16% silica, 14% lignin, 36% cellulose, 22% hemicelluloses, 3% extractive materials and 9% moisture (Rahman et al., 1997). Chemicals used were sodium hydroxide (99%, Merck) and phosphoric acid (85%, Baker). Aqueous solutions of the reagents with appropriate concentrations were prepared using distilled water. Commercial activated carbon was obtained from Sigma. 2.2. Facilities Weighing of samples were performed by using an analytical balance with precision ±0.0001 g. Drying of samples was carried out in an electric oven equipped with fan (Heraeus Instrument D-6450). Carbonization was carried out in a Carbolite Furnace (CSF 1100) with facilities for nitrogen flow. The samples were milled into powder by using a ball-mill (Stoneware) with an alumina milling media. Quantification of malachite green for absorption studies was conducted using a UV–VIS spectrophotometer (Hitachi, Model U2000).
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ball-milled for 1 h into powder that passed through 75 lm mesh. The samples were referred to as PCP10 and PCP20, corresponding to the concentrations of acid used for their preparations. 2.3.2. Sodium hydroxide treated-carbonization process (NCP) About 10 g of dried rice husks were treated with 100 mL 10% sodium hydroxide at 30° and 100 °C for 1 h in a paraffin bath. The sodium hydroxide used was an equivalent amount required to remove all silica present in the husks. The husks were then filtered and washed with distilled water until the filtrate was neutral. The treated husks were dried at 85 °C in an oven and left overnight. The dried husk were ball-milled for 1 h into powder and sieved through 75 lm mesh after carbonization at 500 °C for 30 min. The samples were coded as NCP30C and NCP100C corresponding to the appropriate treatment temperature. 2.4. Determination of silica Silica and carbonaceous material content of the sample was determined by heating at 700 °C in an atmospheric furnace to constant mass. The residue and the mass losses were considered as silica and carbonaceous materials respectively. 2.5. Adsorption studies Malachite green (MG) of oxalate salt (AR grade, BDH) was used as adsorbate. An accurately weighed quantity of the salt was dissolved in distilled water to prepare 500 mg/L stock solution. A desired concentration for adsorption experiment was obtained by successive dilution. The calibration curve for adsorbate is linear from 1 to 20 mg/L (r2 = 0.9993) and the equation of the straight line is given by y = 0.1728x. One hundred millilitre of adsorbate was mixed with a quantity of adsorbent in a 250 mL flask and shaken using a mechanical shaker. It was next filtered through a filter paper, and the solution was analyzed spectrophotometerically at kmax = 617 nm at pH 5–6.
2.3. Adsorbents 2.3.1. Phosphoric acid treated-carbonization process (PCP) The dried husks were impregnated overnight in phosphoric acid with concentrations of 10% and 20% in the mass ratio of 1:10. The husks were filtered and dried at 85 °C in an oven overnight and subsequently carbonized under nitrogen atmosphere at temperatures ranging from 400 to 650 °C, for 30 min. The carbonized husks were washed with distilled water until the filtrate was about pH 6–7. The samples were then dried at 85 °C in an oven overnight and
3. Results and discussion 3.1. Characteristic of adsorbents Based on experimental observation, the reaction of phosphoric acid and organic constituents such as lignin was exothermic. Physical changes on rice husks were only observed when the husks were treated with 20% H3PO4. The husks become slightly darken and sticky and become more significant on further increase in acid concentrations. Based on the above reasons and cost
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
The adsorption behaviour of the adsorbents was studied by evaluating the fraction of MG adsorbed, fMG, calculated as fMG ¼ ðC i C f Þ=C i where Ci is the initial concentration of MG solution in contact with a certain quantity of absorbent, and Cf is the MG concentration after adsorption at a certain contact time. 3.2.1. Adsorption behaviour of PCP adsorbent A series of adsorption studies on the effect of contact time, concentration and adsorbent doses was conducted on PCP samples. The results on adsorption behaviour of PCP10 and PCP20 samples, at different carbonization temperatures as compared to commercial activated carbon (CA) are shown in Figs. 1–3. Overall, PCP20 samples reach maximum contact time faster than PCP10 at all carbonization temperatures. The order of increasing contact times for both PCP10 and PCP20 can be
Table 1 Yields and composition of PCP and NCP rice husks based absorbents Sample carbonized/ treated temperature PCP10 400 °C 500 °C 650 °C
Yield (%) 51.5 46.6 42.1
Silica (%) 40.4 47.1 68.7
Carbonaceous material (%) 59.6 52.9 31.3
PCP20 400 °C 500 °C 650 °C
53.4 47.3 47.4
46.5 48.3 75.2
53.5 51.7 24.8
NCP 30 °C 100 °C
59.2 40.0
21.2 7.6
78.8 92.4
Raw husks (no carbonization)
–
18
82
Fraction MG adsorbed (fMG)
PCP10
1.0 0.9 0.8
400°C 500°C 650°C CA
0.7 0.6 0.5 0.4 0.3 0.2 0
10
20
(a)
30
40
50
60
70
Contact time (min) 1.1
Fraction MG adsorbed (fMG)
3.2. Adsorption behaviour of adsorbents
1.1
PCP20
1.0 0.9 0.8 0.7
400°C
0.6
500°C 650°C
0.5
CA
0.4 0
10
(b)
20
30
40
50
60
70
Contact time (min)
Fig. 1. Adsorption of MG on PCP as a function of contact time, (a) PCP10 and (b) PCP20. PCP = 0.1 g/100 mL, MG = 20 mg/L.
Fraction MG adsorbed (fMG)
consideration, we choose only 10% and 20% acid for the activation process. The reaction of NaOH on rice husks were quite different to that of H3PO4. The husks remained intact when treated with 10% NaOH even at 100 °C. However, the NaOH was able to remove most of the silica, leaving behind a porous structure (Rahman et al., in press). When both samples were carbonized at 400 °C, mass loss remained constant after 30 min. Thus, it was assumed that the optimum time for carbonization was 30 min in order to eliminate most of the volatile organic constituents, leaving behind a carbonaceous materials which was considered as activated carbon. The yields ranged from 59% to 40%, depending on the treatment and carbonization temperature (Table 1).
1.1 PCP10
1.0 0.9 0.8 0.7
400°C 500°C 650°C CA
0.6 0.5 0.4 0
20
(a) Fraction MG adsorbed (fMG)
1580
60
80
100
120
Concentration (mg/L) 1.1 PCP20
1.0 0.9 0.8 0.7
400°C 500°C 650°C
0.6 0.5
CA
0.4 0
(b)
40
20
40
60
80
100
120
Concentration (mg/L)
Fig. 2. Influence of concentration on adsorption of MG on (a) PCP10 and (b) PCP20. PCP = 0.1 g/100 mL, shaking time = 30 min.
1.0 0.8 0.6 400°C 500°C 650°C CA
0.2 0.0 0.0
0.2
0.4
Fraction MG adsorbed (fMG)
(a)
0.8
1.0
1.2
1.2 PCP20
1.0 0.8 0.6 400°C 500°C 650°C CA
0.4 0.2 0.0 0.0
(b)
0.6 Adsorbent (g)
0.2
0.4
0.6
0.8
1.0
1.2
Adsorbent (g)
Fig. 3. Effect of adsorbent dose on the adsorption of MG for (a) PCP10 and (b) PCP20 samples. MG = 100 mg/L, shaking time = 30 min.
written as 500 °C > 400 °C > 650 °C. Only samples carbonized at 500 °C reached contact time that is comparable to the commercial carbon (CA) (Fig. 1). Fig. 2 shows that the fMG is 100% for CA up to 100 mg/L. The fMG decreases with increasing carbonization temperature for both PCP10 and PCP20. Again, the PCP10 and PCP20 that was carbonized at 500 °C showed an improvement in the overall uptake capacity up to 40 mg/L and 60 mg/L respectively. The effect of adsorbent doses on the fMG is shown in Fig. 3. The doses increase as carbonized temperature decreases and H3PO4 concentration increases. Only three samples were able to remove 100% of 100 mg/L MG in 100 mL solution, i.e., PCP10 (500 °C) = 0.8 g, PCP20 (500 °C) = 0.6 g, and PCP20 (400 °C) = 1.0 g, and CA required 0.5 g. The increase in adsorption capacity for the PCP10 and PCP20 carbonized at 500 °C is most likely related to the maximum surface area created and the availability of more adsorption sites for the bulky organic group. Meanwhile the decreases in adsorption in the above samples on increasing carbonization temperatures is closely attributed to the increase in silica content (Table 1). The effect is more significant when the silica content reaches >50% in the samples. The results indicate that MG adsorption is more pronounced on the carbon surface rather than on silica surface. The decreased adsorption of PCP10 and PCP20 carbonized at 400 °C was believed to be due to the fact that some pores in the
3.2.2. Adsorption behaviour of NCP adsorbent The fact that the adsorption of MG on PCP samples is preferable on the carbon site prompted further investigation. Rice husks were treated with 10% NaOH solution at 30 °C and 100 °C in order to remove silica, followed by carbonization at 500 °C. The samples was labeled as NCP30C and NCP100C, and contained 21% and 8% silica respectively for both treatment temperatures. Fig. 4 represents the adsorption capacity of MG on NCP samples at different contact times as compared to CA and raw husks that were not carbonized. It can be readily seen that fMG increases with increasing treatment temperature, although at a slower rate when compared to CA. The raw husks failed to adsorb MG to a satisfactory level due to the absence of large pores to accommodate the bulky organic group of MG. Similar trends were observed on the fMG verses
Fraction MG adsorbed (fMG)
PCP10
0.4
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carbonized samples were blocked by decomposition products of organic constituents, thus inhabiting the accessibility of active sites for the adsorption (Rahman and Saad, 2003). It is clearly shown that 500 °C is the optimum temperature to produce activated carbon from rice husks through H3PO4 treatment, and the resulting carbon showed comparable adsorption capacity to the commercial activated carbon.
1.2
1.1 1.0 0.9 0.8 0.7 0.6
Raw husks
0.5
NCP 30°C
0.4
NCP 100°C
0.3
CA
0.2 0
20
40 Contact time (min)
60
80
Fig. 4. Adsorption of MG on NCP as a function of contact time.
Fraction MG adsorbed (fMG)
Fraction MG adsorbed (fMG)
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
1.2 1.0 0.8 0.6 Raw husks
0.4
NCP 30°C NCP 100°C
0.2
CA
0.0 0
20
40
60
80
100
120
Concentration (mg/L)
Fig. 5. Influence of concentration on adsorption of MG on NCP.
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I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
treatment of wastewater from activities such as aquaculture that had been polluted with malachite green. The close proximity and abundance of source of rice husks to aquaculture sites, especially in tropical countries merit serious consideration for its implementation.
Fraction MG adsorbed (fMG)
1.2 1.0 0.8 0.6 Raw husks
0.4
4. Conclusion
NCP 30°C NCP100°C
0.2
CA
0.0 0.0
0.2
0.4
0.6 0.8 Absorbent (g)
1.0
1.2
Fig. 6. Effect of NCP dose on the adsorption of MG. MG = 100 mg/L.
concentrations and adsorbent doses (Figs. 5 and 6). The results indicate that 0.1 g NCP100C was able to adsorb up to 40 mg/L MG in 100 mL and 0.6 g/100 mL required to remove 100 mg/L. This indicates that by removing silica, adsorption sites on carbon increase and this enhanced the adsorption capacity.
Acknowledgements
3.3. Adsorption isotherms A simple and effective representation of the adsorption behaviour of both of the PCP and NCP samples is by using Langmuir and Freundlich models. When the above results are tested against these two models, PCP is well described by the Langmuir and Freundlich models while the NCP is best fitted to only the Langmuir model. Parameters for both models are presented in Table 2. On the whole, the results indicate the significance of using rice husks based activated carbon for the
Table 2 Langmuir and Freundlich parameters fitted to experimental data for malachite green, MG, adsorbed on PCP and NCP rice husks based activated carbon Sample carbonized/ treated temp.
Langmuir parameters 2
r
Xm (mg/g)
The studies show that activated carbon produced from rice husks has good adsorption capacity that is comparable to commercial activated carbon. It was found that the maximum carbonization temperature is 500 °C for effective adsorption. It is believed that the malachite green is preferably adsorbed on carbon surface as the presence of silica to a certain extent, can retard the adsorption capacity. Thus, rice husks based material can provide attractive alternative adsorbents to remove malachite green in wastewater from textile industries and aquaculture activities.
KL
Freundlich parameters r2
1/n
KF
PCP10 400 °C 500 °C 650 °C
0.9720 76.9231 0.1555 0.9881 0.4450 14.3946 0.9670 77.5194 1.4176 0.9667 0.2677 38.2296 0.9517 83.3333 0.0829 0.9936 0.5927 8.2794
PCP20 400 °C 500 °C 650 °C
0.9594 77.5194 0.4108 0.9791 0.2609 29.7167 0.9725 80.0000 0.7530 0.9637 0.3629 29.4306 0.9639 92.5920 0.1223 0.9927 0.5493 12.6970
NCP 30 °C 100 °C
0.9865 57.1430 0.8536 0.8701 0.2870 21.9837 0.9958 56.497 1.0114 0.8956 0.2146 26.9650
The authors are grateful to the Universiti Sains Malaysia for financial support to carry out this research work.
References Aitcheson, S.J., Arnett, J., Murray, K.R., Zhang, J., 2000. Removal of aquaculture therapeutants by carbon adsorption 1. Equilibrium adsorption behaviour of single compound. Aquaculture 183, 269– 284. Bcaoui, A., Yaacoubi, A., Dahbi, A., Bennouna, C., Phan Tan Luu, R., Maldonado-Hodar, F.J., Rivera-Utrilla, J., Moreno-Castilla, C., 2001. Optimization of conditions for the preparation of activated carbons from olive-waste cakes. Carbon 39, 425– 432. Chan, K.C., Goh, S.H., Toh, H.T., Tan, W.I., Tang, T.S., 1980. Activated carbon from palm (Elaeis guineensis) nut shells. Malaysian J. Sci. 6 (B), 131–137. El-Hendawy, A.N.A., Samra, S.E., Girgis, B.S., 2001. Adsorption characteristics of activated carbons obtained from corncobs. Colloids Surface A: Physicochem. Eng. Aspects 180 (3), 209– 221. Garg, V.K., Gupta, R., Yadav, A.B., Kumar, R., 2003. Dye removal from aqueous solution by adsorption on treated sawdust. Bioresource Technol. 89, 121–124. Guo, Y., Yang, S., Fu, W., Qi, J., Li, R., Wang, Z., Xu, H., 2003. Adsorption of malachite green on micro- and mesoporous rice husk-based active carbon. Dyes and Pigments 56, 219–229. Hu, Z., Srinivasan, M.P., 2001. Mesoporous high-surface-area activated carbon. Micropor. Mesopor. Mater. 34, 267–275. Lafi, W.K., 2001. Production of activated carbon from acorns and olive seeds. Biomass. Bioenerg. 20, 57–62.
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583 Mbui, D.N., Shiundu, P.M., Ndonye, R.M., Kamau, G.N., 2002. Adsorption and detection of some phenolic compounds by rice husk ash of Kenyan origin. J. Environ. Monitor 4, 978–984. Rahman, I.A., Ismail, J., Osman, H., 1997. Effect of nitric acid digestion on organic materials and silica in rice husks. J. Mater. Chem. 7 (8), 12–15. Rahman, I.A., Ismail, J., Osman, H., 2000. Studies on Zn(II) ion adsorption by rice husks digested with nitric acid. Malaysian J. Chem. 2 (1), 12–15.
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Rahman, I.A., Saad, B., 2003. Utilization of guava seeds as a source of activated carbon for removal of methylene blue from aqueous solution. Malaysian J. Chem. 5 (1), 8–14. Rahman, I.A., Sing, Y.Y., Bari, M.F., Saad, B., 2005. Adsorption of paraquat by treated and untreated rice husks studied using flow injection-spectrophotometry, in press. Sandra, J.C., Lonnie, R.B., Donna, F.K., Daniel, R.D., Louis, T., Frederick, A.B., 1999. Toxicity and metabolism of malachite green and leucomalachite green during short-term feeding to Fischer 344 rats and B6C3F1 mice. Chem-Biol. Interact. 122, 153–170.
Adsorption characteristics of malachite green on activated carbon derived from rice husks produced by chemical–thermal process I.A. Rahman *, B. Saad, S. Shaidan, E.S. Sya Rizal School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 20 November 2003; received in revised form 13 December 2004; accepted 26 December 2004 Available online 16 February 2005
Abstract Phosphoric acid (H3PO4) and sodium hydroxide (NaOH) treated rice husks, followed by carbonization in a flowing nitrogen were used to study the adsorption of malachite green (MG) in aqueous solution. The effect of adsorption on contact time, concentration of MG and adsorbent dosage of the samples treated or carbonized at different temperatures were investigated. The results reveal that the optimum carbonization temperature is 500 °C in order to obtain adsorption capacity that is comparable to the commercial activated carbon for the husks treated by H3PO4. It is interesting to note that MG adsorbed preferably on carbon-rich than on silica rich-sites. It is found that the behaviour of H3PO4 treated absorbent followed both the Langmuir and Freundlich models while NaOH treated best fitted to only the Langmuir model. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Malachite green; Activated rice husks
1. Introduction Many countries have given prominence to the development of aquaculture as a long term strategy in providing sufficient source of protein to their ever growing population. The raising of large numbers of fish, prawns and others in confined space as in modern aquaculture practice necessitates the use of an extensive range of chemicals for the prevention and treatment of diseases, thus posing as a source of water pollution. The used of malachite green (triphenylmethane) in aquaculture industry has a long history, dating back to 1933, when it was first introduced. The compound, originally extensively used for the dyeing of textiles, has since found wide applications in the aquaculture industry as it is relatively cheap, effective and easy to obtain. Its use in the aquaculture practice in many countries, including *
Corresponding author. Fax: +60 4 6574854. E-mail address: [email protected] (I.A. Rahman).
0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.12.015
Malaysia has not been regulated. Many adverse effects from the consumption of the dye due to its carcinogenic, genotoxic, mutagenic and teratogenic properties in animal studies have been reported (Sandra et al., 1999). Adsorption on activated carbon has long been recognized to be one of the most effective methods for the removal of pesticides and organic compounds from aqueous solutions. Agriculture wastes such as oil palm nut shell (Chan et al., 1980), rice husks (Rahman et al., 2000), olive-waste cakes (Bcaoui et al., 2001), acorns and olive seeds (Lafi, 2001), corncobs (ElHendawy et al., 2001), coconut shells or palm seeds (Hu and Srinivasan, 2001), and guava seeds (Rahman and Saad, 2003) have been used as low-cost adsorbents. These materials were activated either by chemicals, steam, gas or their combinations. The use of adsorbents derived from rice husk to adsorb some phenolic compounds (Mbui et al., 2002) and the herbicide paraquat (Rahman et al., in press) had been reported. Relatively little work had been reported on the absorption of
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
malachite green from aqueous solution despite its widespread use. The adsorption behaviour of malachite green, formalin, chloramine-T and oxytetracycline onto coal-based activated carbon was studied by Aitcheson et al. (2000). The removal of malachite green by agriculture wastes was only reported by Guo et al. (2003) and Garg et al. (2003). In the present work, we report our evaluation on the feasibility of using rice husks pretreated with H3PO4 and NaOH for the removal of malachite green from aqueous solution. The influence of adsorbent loading and contact time were also studied.
2. Methods 2.1. Raw material Dried and fresh rice husks were used as raw materials to produce the adsorbents. The rice husks were obtained from a rice mill in the Northern part of Peninsular Malaysia. Typical composition of the rice husks used is 16% silica, 14% lignin, 36% cellulose, 22% hemicelluloses, 3% extractive materials and 9% moisture (Rahman et al., 1997). Chemicals used were sodium hydroxide (99%, Merck) and phosphoric acid (85%, Baker). Aqueous solutions of the reagents with appropriate concentrations were prepared using distilled water. Commercial activated carbon was obtained from Sigma. 2.2. Facilities Weighing of samples were performed by using an analytical balance with precision ±0.0001 g. Drying of samples was carried out in an electric oven equipped with fan (Heraeus Instrument D-6450). Carbonization was carried out in a Carbolite Furnace (CSF 1100) with facilities for nitrogen flow. The samples were milled into powder by using a ball-mill (Stoneware) with an alumina milling media. Quantification of malachite green for absorption studies was conducted using a UV–VIS spectrophotometer (Hitachi, Model U2000).
1579
ball-milled for 1 h into powder that passed through 75 lm mesh. The samples were referred to as PCP10 and PCP20, corresponding to the concentrations of acid used for their preparations. 2.3.2. Sodium hydroxide treated-carbonization process (NCP) About 10 g of dried rice husks were treated with 100 mL 10% sodium hydroxide at 30° and 100 °C for 1 h in a paraffin bath. The sodium hydroxide used was an equivalent amount required to remove all silica present in the husks. The husks were then filtered and washed with distilled water until the filtrate was neutral. The treated husks were dried at 85 °C in an oven and left overnight. The dried husk were ball-milled for 1 h into powder and sieved through 75 lm mesh after carbonization at 500 °C for 30 min. The samples were coded as NCP30C and NCP100C corresponding to the appropriate treatment temperature. 2.4. Determination of silica Silica and carbonaceous material content of the sample was determined by heating at 700 °C in an atmospheric furnace to constant mass. The residue and the mass losses were considered as silica and carbonaceous materials respectively. 2.5. Adsorption studies Malachite green (MG) of oxalate salt (AR grade, BDH) was used as adsorbate. An accurately weighed quantity of the salt was dissolved in distilled water to prepare 500 mg/L stock solution. A desired concentration for adsorption experiment was obtained by successive dilution. The calibration curve for adsorbate is linear from 1 to 20 mg/L (r2 = 0.9993) and the equation of the straight line is given by y = 0.1728x. One hundred millilitre of adsorbate was mixed with a quantity of adsorbent in a 250 mL flask and shaken using a mechanical shaker. It was next filtered through a filter paper, and the solution was analyzed spectrophotometerically at kmax = 617 nm at pH 5–6.
2.3. Adsorbents 2.3.1. Phosphoric acid treated-carbonization process (PCP) The dried husks were impregnated overnight in phosphoric acid with concentrations of 10% and 20% in the mass ratio of 1:10. The husks were filtered and dried at 85 °C in an oven overnight and subsequently carbonized under nitrogen atmosphere at temperatures ranging from 400 to 650 °C, for 30 min. The carbonized husks were washed with distilled water until the filtrate was about pH 6–7. The samples were then dried at 85 °C in an oven overnight and
3. Results and discussion 3.1. Characteristic of adsorbents Based on experimental observation, the reaction of phosphoric acid and organic constituents such as lignin was exothermic. Physical changes on rice husks were only observed when the husks were treated with 20% H3PO4. The husks become slightly darken and sticky and become more significant on further increase in acid concentrations. Based on the above reasons and cost
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
The adsorption behaviour of the adsorbents was studied by evaluating the fraction of MG adsorbed, fMG, calculated as fMG ¼ ðC i C f Þ=C i where Ci is the initial concentration of MG solution in contact with a certain quantity of absorbent, and Cf is the MG concentration after adsorption at a certain contact time. 3.2.1. Adsorption behaviour of PCP adsorbent A series of adsorption studies on the effect of contact time, concentration and adsorbent doses was conducted on PCP samples. The results on adsorption behaviour of PCP10 and PCP20 samples, at different carbonization temperatures as compared to commercial activated carbon (CA) are shown in Figs. 1–3. Overall, PCP20 samples reach maximum contact time faster than PCP10 at all carbonization temperatures. The order of increasing contact times for both PCP10 and PCP20 can be
Table 1 Yields and composition of PCP and NCP rice husks based absorbents Sample carbonized/ treated temperature PCP10 400 °C 500 °C 650 °C
Yield (%) 51.5 46.6 42.1
Silica (%) 40.4 47.1 68.7
Carbonaceous material (%) 59.6 52.9 31.3
PCP20 400 °C 500 °C 650 °C
53.4 47.3 47.4
46.5 48.3 75.2
53.5 51.7 24.8
NCP 30 °C 100 °C
59.2 40.0
21.2 7.6
78.8 92.4
Raw husks (no carbonization)
–
18
82
Fraction MG adsorbed (fMG)
PCP10
1.0 0.9 0.8
400°C 500°C 650°C CA
0.7 0.6 0.5 0.4 0.3 0.2 0
10
20
(a)
30
40
50
60
70
Contact time (min) 1.1
Fraction MG adsorbed (fMG)
3.2. Adsorption behaviour of adsorbents
1.1
PCP20
1.0 0.9 0.8 0.7
400°C
0.6
500°C 650°C
0.5
CA
0.4 0
10
(b)
20
30
40
50
60
70
Contact time (min)
Fig. 1. Adsorption of MG on PCP as a function of contact time, (a) PCP10 and (b) PCP20. PCP = 0.1 g/100 mL, MG = 20 mg/L.
Fraction MG adsorbed (fMG)
consideration, we choose only 10% and 20% acid for the activation process. The reaction of NaOH on rice husks were quite different to that of H3PO4. The husks remained intact when treated with 10% NaOH even at 100 °C. However, the NaOH was able to remove most of the silica, leaving behind a porous structure (Rahman et al., in press). When both samples were carbonized at 400 °C, mass loss remained constant after 30 min. Thus, it was assumed that the optimum time for carbonization was 30 min in order to eliminate most of the volatile organic constituents, leaving behind a carbonaceous materials which was considered as activated carbon. The yields ranged from 59% to 40%, depending on the treatment and carbonization temperature (Table 1).
1.1 PCP10
1.0 0.9 0.8 0.7
400°C 500°C 650°C CA
0.6 0.5 0.4 0
20
(a) Fraction MG adsorbed (fMG)
1580
60
80
100
120
Concentration (mg/L) 1.1 PCP20
1.0 0.9 0.8 0.7
400°C 500°C 650°C
0.6 0.5
CA
0.4 0
(b)
40
20
40
60
80
100
120
Concentration (mg/L)
Fig. 2. Influence of concentration on adsorption of MG on (a) PCP10 and (b) PCP20. PCP = 0.1 g/100 mL, shaking time = 30 min.
1.0 0.8 0.6 400°C 500°C 650°C CA
0.2 0.0 0.0
0.2
0.4
Fraction MG adsorbed (fMG)
(a)
0.8
1.0
1.2
1.2 PCP20
1.0 0.8 0.6 400°C 500°C 650°C CA
0.4 0.2 0.0 0.0
(b)
0.6 Adsorbent (g)
0.2
0.4
0.6
0.8
1.0
1.2
Adsorbent (g)
Fig. 3. Effect of adsorbent dose on the adsorption of MG for (a) PCP10 and (b) PCP20 samples. MG = 100 mg/L, shaking time = 30 min.
written as 500 °C > 400 °C > 650 °C. Only samples carbonized at 500 °C reached contact time that is comparable to the commercial carbon (CA) (Fig. 1). Fig. 2 shows that the fMG is 100% for CA up to 100 mg/L. The fMG decreases with increasing carbonization temperature for both PCP10 and PCP20. Again, the PCP10 and PCP20 that was carbonized at 500 °C showed an improvement in the overall uptake capacity up to 40 mg/L and 60 mg/L respectively. The effect of adsorbent doses on the fMG is shown in Fig. 3. The doses increase as carbonized temperature decreases and H3PO4 concentration increases. Only three samples were able to remove 100% of 100 mg/L MG in 100 mL solution, i.e., PCP10 (500 °C) = 0.8 g, PCP20 (500 °C) = 0.6 g, and PCP20 (400 °C) = 1.0 g, and CA required 0.5 g. The increase in adsorption capacity for the PCP10 and PCP20 carbonized at 500 °C is most likely related to the maximum surface area created and the availability of more adsorption sites for the bulky organic group. Meanwhile the decreases in adsorption in the above samples on increasing carbonization temperatures is closely attributed to the increase in silica content (Table 1). The effect is more significant when the silica content reaches >50% in the samples. The results indicate that MG adsorption is more pronounced on the carbon surface rather than on silica surface. The decreased adsorption of PCP10 and PCP20 carbonized at 400 °C was believed to be due to the fact that some pores in the
3.2.2. Adsorption behaviour of NCP adsorbent The fact that the adsorption of MG on PCP samples is preferable on the carbon site prompted further investigation. Rice husks were treated with 10% NaOH solution at 30 °C and 100 °C in order to remove silica, followed by carbonization at 500 °C. The samples was labeled as NCP30C and NCP100C, and contained 21% and 8% silica respectively for both treatment temperatures. Fig. 4 represents the adsorption capacity of MG on NCP samples at different contact times as compared to CA and raw husks that were not carbonized. It can be readily seen that fMG increases with increasing treatment temperature, although at a slower rate when compared to CA. The raw husks failed to adsorb MG to a satisfactory level due to the absence of large pores to accommodate the bulky organic group of MG. Similar trends were observed on the fMG verses
Fraction MG adsorbed (fMG)
PCP10
0.4
1581
carbonized samples were blocked by decomposition products of organic constituents, thus inhabiting the accessibility of active sites for the adsorption (Rahman and Saad, 2003). It is clearly shown that 500 °C is the optimum temperature to produce activated carbon from rice husks through H3PO4 treatment, and the resulting carbon showed comparable adsorption capacity to the commercial activated carbon.
1.2
1.1 1.0 0.9 0.8 0.7 0.6
Raw husks
0.5
NCP 30°C
0.4
NCP 100°C
0.3
CA
0.2 0
20
40 Contact time (min)
60
80
Fig. 4. Adsorption of MG on NCP as a function of contact time.
Fraction MG adsorbed (fMG)
Fraction MG adsorbed (fMG)
I.A. Rahman et al. / Bioresource Technology 96 (2005) 1578–1583
1.2 1.0 0.8 0.6 Raw husks
0.4
NCP 30°C NCP 100°C
0.2
CA
0.0 0
20
40
60
80
100
120
Concentration (mg/L)
Fig. 5. Influence of concentration on adsorption of MG on NCP.
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treatment of wastewater from activities such as aquaculture that had been polluted with malachite green. The close proximity and abundance of source of rice husks to aquaculture sites, especially in tropical countries merit serious consideration for its implementation.
Fraction MG adsorbed (fMG)
1.2 1.0 0.8 0.6 Raw husks
0.4
4. Conclusion
NCP 30°C NCP100°C
0.2
CA
0.0 0.0
0.2
0.4
0.6 0.8 Absorbent (g)
1.0
1.2
Fig. 6. Effect of NCP dose on the adsorption of MG. MG = 100 mg/L.
concentrations and adsorbent doses (Figs. 5 and 6). The results indicate that 0.1 g NCP100C was able to adsorb up to 40 mg/L MG in 100 mL and 0.6 g/100 mL required to remove 100 mg/L. This indicates that by removing silica, adsorption sites on carbon increase and this enhanced the adsorption capacity.
Acknowledgements
3.3. Adsorption isotherms A simple and effective representation of the adsorption behaviour of both of the PCP and NCP samples is by using Langmuir and Freundlich models. When the above results are tested against these two models, PCP is well described by the Langmuir and Freundlich models while the NCP is best fitted to only the Langmuir model. Parameters for both models are presented in Table 2. On the whole, the results indicate the significance of using rice husks based activated carbon for the
Table 2 Langmuir and Freundlich parameters fitted to experimental data for malachite green, MG, adsorbed on PCP and NCP rice husks based activated carbon Sample carbonized/ treated temp.
Langmuir parameters 2
r
Xm (mg/g)
The studies show that activated carbon produced from rice husks has good adsorption capacity that is comparable to commercial activated carbon. It was found that the maximum carbonization temperature is 500 °C for effective adsorption. It is believed that the malachite green is preferably adsorbed on carbon surface as the presence of silica to a certain extent, can retard the adsorption capacity. Thus, rice husks based material can provide attractive alternative adsorbents to remove malachite green in wastewater from textile industries and aquaculture activities.
KL
Freundlich parameters r2
1/n
KF
PCP10 400 °C 500 °C 650 °C
0.9720 76.9231 0.1555 0.9881 0.4450 14.3946 0.9670 77.5194 1.4176 0.9667 0.2677 38.2296 0.9517 83.3333 0.0829 0.9936 0.5927 8.2794
PCP20 400 °C 500 °C 650 °C
0.9594 77.5194 0.4108 0.9791 0.2609 29.7167 0.9725 80.0000 0.7530 0.9637 0.3629 29.4306 0.9639 92.5920 0.1223 0.9927 0.5493 12.6970
NCP 30 °C 100 °C
0.9865 57.1430 0.8536 0.8701 0.2870 21.9837 0.9958 56.497 1.0114 0.8956 0.2146 26.9650
The authors are grateful to the Universiti Sains Malaysia for financial support to carry out this research work.
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