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Green Functionalization: Comparison of Different Carbonaceous Materials
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 148 (2016) 795 – 805
4th International Conference on Process Engineering and Advanced Materials
Green Functionalization: Comparison of Different Carbonaceous Materials Wan Nadatul Nadwa Wan Alia,*, Suriati Sufiana, Mohd Zamri Abdullaha a
Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Tronoh, Perak, Malaysia
Abstract This study was carried out to study the effectiveness of green functionalization on three carbonaceous adsorbents which are CNF, MWCNT and GAC. There are two methods of green functionalization used; DI water functionalization and thermal treatment. From FTIR images of H2O-functionalization, CNF and MWCNT shows some improvement where low concentration OH band’s intensity is increase. However, MWCNT is the only adsorbent that obtained additional functional group after functionalized in water bath sonicator at 55oC for 24 hours which are 2 peaks of O-H at 3849.4 and 3743.2 cm-1, and alkynyl C=C stretch at 2343.1 cm-1. However, there is no significant change in functional group attached to GAC after the sonication. After thermal treatment, the FTIR images show that low concentration OH band of all three adsorbents have reduced or completely removed. While, from FESEM images, all carbonaceous adsorbents were exposed to microwave radiation (heat no 3) for 10 minutes. Before the thermal treatment, there are a lot of bright spots can be seen indicating there are surface functional groups present on the surface of the adsorbent. Unfortunately, after being treated by microwave radiation, only a few functional groups attached to the surface can be seen. Adsorbent surface becomes smoother. All these changes will reduce the adsorption capacity of the dye. The diameter range is remained the same after the thermal treatment The diameter range for CNF and MWCNT prior to heat treatment is 52.05 – 105.2 and 12.33 – 22.16 nm respectively. After treatment, range of diameters for CNF is 54.51 – 100.1 nm, while, for MWCNT diameter is range from 13.49 – 20.37 nm. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder underresponsibility responsibility of the organizing committee of ICPEAM Peer-review of the organizing committee of ICPEAM 2016 2016. Keywords: Carbon Nanofiber; Multi-Walled Carbon Nanotubes; Granular Activated Carbon; Green Functionalization; Adsorption.
* Corresponding author. Tel.: +6013-4471758. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016
doi:10.1016/j.proeng.2016.06.566
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1. Introduction According to Hameed [1] adsorption is one of the most effective physical processes for color removal and treatment of textile effluents. This techniques are widely used to remove certain classes of chemical pollutants from water, especially chemical that arguably affected by conventional wastewater biological treatment [2]. The adsorption technique has several advantages such as high efficiency, not sensitive to toxic contaminants, simple operation, and easy recovery of adsorbent [3]. Adsorption capacity is depending on effectiveness of an adsorbent. Then, to ensure that the adsorption process achieved high effectiveness, the best of adsorbent criteria must be chosen. Nowadays, there are abundant of adsorbents available in the market. These adsorbents have been tested by other researchers in dye removal application eg carbon nanotube [4, 5], fly ash [6], bottom ash [7, 8], chitosan [9], chitin [10] and oil palm ash [11].
1.1 Carbonaceous Materials as Adsorbent The application of carbonaceous materials in the field of adsorption, separation, and catalysis is a huge success which have developed researchers’ interest to improve methods in producing such materials [12]. Carbon-based is a preferred material used as engineering adsorbents due to its physical adsorption system which produce high adsorptive capacities. Plus, the presence of oxygen atoms on their surface causing more functional groups can interact with the adsorbent [13]. Last decade, nano-structured carbonaceous materials began to received research world attention due to their extraordinary mechanical, electrical, thermal and structural properties [14]. There are factors that affecting the adsorption capacity such as surface area, pore-size distribution, pore volume and the presence of surface functional groups. Generally adsorption capacity increases with the specific surface area for the adsorption sites [15]. In order to enhance the effectiveness of the adsorbent, it can be functionalized by attaching other functional group onto the adsorbent surface. 1.2 Functionalization of Adsorbent There are several ways to modify the nature of surface functional group which can be classified into three categories which are chemical, physical and bioadsorption modification as shown in Fig. 1. Adsorption capacity can be enhanced by add other functional group to the surface of the adsorbent. It been proved by Tan, et al. [11], which stated the dye removal could improve by 24.2% when the functionalized activated carbon. Most of researchers preferred to functionalize their adsorbent by chemical modification which by contacting their adsorbent with acid or basic solution. The example of acidic functionalizing agents are hydrochloric acid (HCl) [11, 16], nitric acid (HNO3) [17, 18] and hydrogen peroxide (H2O2) [17, 19]. While, the most common basic functionalizing agent are potassium hydroxide (KOH) [20, 21] and sodium hydroxide (NaOH) [22]. Unfortunately, after the adsorbents been treated by these functionalizing agents, the adsorbents need to be wash repetitively until the pH is neutral before it can be used for adsorption purpose. It is time consuming procedures and not environmental friendly since there will be chemical waste in the effluent and a lot of water needed to wash the adsorbent. As reported by Ali [23], CNF was washed using DI water repetitively until reach pH 7 after been treated by concentrated nitric acid. 1.3 Green Functionalization of Adsorbent The word “green” means both process and materials used is environmental friendly. In this case, green functionalization can be classified under chemical modification. However, the functionalizing agent used is neither acidic nor basic phase. The functionalizing agent used is deionized (DI) water (H2O) which is in neutral phase. There is no interaction occur between adsorbent and DI water at normal condition. Therefore sonicator is needed in order to ensure there is molecules vibration during the contact time which can cause other surface functional groups to attach on the adsorbent surface. This is not a new method. This method have been introduced by Ong, et al. [24], which used DI water to
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functionalized activated carbon that derived from Durian shell for electrochemical double layer capacitor. Unfortunately, this method was not explored by other researchers. Generally, carbon is nonpolar but oxygen surface group that commonly attached on it causing it become more hydrophilic [25] . Hence, organic compound such as dye is easily attracted to carbon’s surface.
Fig. 1: Available Modification for Adsorbent [26]
Thermal treatment by using microwave also can be classified as green functionalization since the process does not cause harm to the environment. Thermal treatment is functioned to selectivity remove some of the functional group [27] and also to increase the size of pore on the carbon’s surface [5]. In general, there are two ways to implement thermal treatment which are by electrical heating and microwave. In this study, microwave is used because the integration of microwave heating has been promoting the release of pollutants, lowering the volatile adsorbed on bulk temperature and shorter processing time [28]. This may be responsible for better desorption rate and the conservation of the porous structure [29]. This treatment also responsible to open routes for adsorbate molecules penetrate into the inside space [30]. Therefore, this paper will compare the effectiveness of the green functionalization method by comparing three types of carbonaceous materials namely carbon nanofiber (CNF), multi-walled carbon nanotubes (MWCNT) and granular activated carbon (GAC). 2. Experimental 2.1 Materials Multi walled carbon nanotube (MWCNT) and herringbone CNF were bought from Carbon Nano-material Technology Co., Ltd. MWCNT has diameter and length of 20 nm and 5 µm respectively. While the diameter and length of CNF is 200 nm and 20 µm respectively. Granular activated carbon (GAC) was obtained from Sigma Aldrich. It is untreated GAC and the mesh is 4-8. The melting point of GAC is 3550 oC. 2.2 Functionalization by Deionized (DI) Water Five grams of each CNF, MWCNT and GAC were put in a different Erlenmeyer flask. Then, 200 mL of DI water was added to each flask. All flasks were immersed in the water bath sonicator (Bandelin Sonorex Digitex, DT255 with ultrasonic peak output, 640 W/per) at 55oC for 24 hours. After 24 hours, all solutions were filtered using vacuum pump and membrane filter paper. All adsorbents’ cake was then dried in the oven at 80 oC until completely dry. Functionalized adsorbents were stored in vials at room temperature.
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2.3 Thermal Treatment Thermal treatment is a further treatment to the adsorbents. The samples must first go through the functionalization by DI water. However, before the adsorbents cake was dried in the oven, the adsorbents’ cake was put in the microwave at third level of heating for 10 minutes. The adsorbents’ cake was then dried in the oven at 80 oC until completely dry. The functionalized adsorbents were stored in the vials at room temperature. In order to test the effectiveness of heat treatment, two types of sample for each adsorbent were prepared which are with and without thermal treatment. 3. Results and Discussion 3.1 FTIR Characterization This analysis was carried out to distinguish the IR peaks before and after the functionalization performed on the adsorbents. Fig. 2 shows the FTIR spectra for CNF. Fig. 2(a) shows a broad peak at 3435.1cm -1 which is refers to high concentrated O-H stretch functional group. The peak at 2346.5 and 1631 cm-1 attributed to alkynyl C=C stretch and amide C=O respectively. For peak at 1021.7cm -1, it is associated to C-N aliphatic amines while peak at 665.67 cm-1 can be assign to C=C alkenes functional group. As water is used as the functionalizing agent, the expected functional group is based on oxygen which is from OH- ion. However, by comparing fig. 2(a) and 2(b), can be observed there is no new peak appeared. Fortunately, the intensity of functional group of low concentration O-H with two peaks at 3856.2 and 3746.7cm-1 were increased. As the intensity of these peaks increase, this means there are more OH functional group attach to the surface of CNF. Conversely, after the thermal treatment, these peaks have been removed completely. As mentioned by Muller, et al. [31], both electrical heating and microwave are the effective way to remove oxygen-contained functional group from carbon surface. a)
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Fig. 2: FTIR images (a) Raw commercialized CNF; (b) H 2O-functionalized CNF without thermal treatment; (c) H 2O-functionalized CNF with thermal treatment
Meanwhile, fig. 3 shows the FTIR images of raw and H2O-functionalized MWCNT. The surface functional group that adhered on raw MWCNT and CNF are same except for alkynyl C=C group which exist on raw CNF. This is because they are from the same component but different in molecules arrangement. After being treated, there are 3 new peaks appeared which at 3849.4, 3743.2 (low concentration O-H) and 2343.1cm-1 (alkynyl C=C stretch). The result shows that an additional functional group which is alkynyl was successfully attached on surface of MWCNT. This may enhance the adsorption capacity of MWCNT. Unfortunately, Fig. 3(c) shows the intensity of low concentration OH band was reduced. The same reason for CNF can be applies for MWCNT because they have similar component with different arrangement.
a)
b)
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Fig. 3: FTIR images (a) Raw commercialized MWCNT; (b) H 2O-functionalized MWCNT without thermal treatment; (c) H 2O-functionalized MWCNT with thermal treatment
The FTIR images for GAC are quite different from CNF and MWCNT as shown in fig. 4. In fig. 4(a), the peak at 3404.3cm-1 attributed to O-H stretch. For peak at 2367 cm-1, it is associated to alkynyl C=C stretch while peak at 1572.9 cm-1 is related to the carboxylate anion stretch mode. The peak at 1151.8 is attributed to ester C-O functional group. After functionalized with DI water, the intensity of alkane group which at 2925cm -1 increased. However, alkane C-H bonds are fairly ubiquitous and therefore usually less useful in determining structure. By observing fig. 4(a) and 4(b), there is no additional functional group attached after functionalization. As compared to CNF and MWCNT, GAC is less affected after H2O functionalization. This is because nanomaterials possess a high surface area to volume ratio. Therefore, it has higher reactivity upon exposure compared to GAC[32]. Fig. 4(c) is the results for
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GAC after it being exposed to microwave radiation. However, the graph pattern is quite similar to the raw GAC FTIR image. Therefore, it can be concluded that the thermal treatment is not applicable for GAC because it will revert to original raw condition after being treated by thermal treatment. a)
b)
c)
Fig. 4: FTIR images (a) Raw commercialized GAC; (b) H 2O-functionalized GAC without thermal treatment; (c) H2O-functionalized GAC with thermal treatment
3.2 FESEM Characterization FESEM is widely used to study the surface morphology and surface characteristics of the adsorbent [33]. Fig. 5 shows the comparison of CNF before and after being treated by thermal treatment. In Fig. 5(a), it can be observed that a lot crumble-like structure attached on the CNF surface. The crumble-like structure is surface functional group that clinging on CNF surface. This condition is favourable for adsorption because it will increase the adsorption site and increase the adsorption capacity. As compared to the structure after the treatment, it shows less clinging things on the surface. This proves the continuation from FTIR result which shows the OH functional group have been completely removed.
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a)
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Fig.5: FESEM images at 10,000x magnificent (a) CNF without heat treatment; (b) CNF with heat treatment.
Fig. 6 shows the CNF’s structure images at 100,000x magnificent. From these images, it shows there is no significant different in terms of the structure size. The range diameter of CNF before and after the thermal treatment is 52.05 – 105.2 nm and 54.51 – 100.1 nm respectively. In Fig. 6(a), the bright spots are the surface functional group that attached on CNF surface. Therefore, as compared these images, after the thermal treatment, CNF have silkier look surface, while, before the thermal treatment it have a lot of attach functional group. a)
b)
Fig. 6: FESEM images at 100,000x magnificent (a) CNF without heat treatment; (b) CNF with heat treatment.
Different conditions can be observed from Fig. 7. The FESEM images of MWCNT at 10,000x magnificent shows the CNT’s structure is almost the same with and without thermal treatment. However, by applying the radiation to the adsorbent, it became less organized. As mentioned by Yin, et al. [26], the result from being exposed to microwave radiation, the surface structure of the adsorbent might fractured and become shorter. However, there is no fractured structure can be pointed out. Similar to CNF images, the bright spot in Fig 7(a) shows the present of surface functional group on MWCNT surface.
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Fig. 7: FESEM images at 10,000x magnificent (a) MWCNT without heat treatment; (b) MWCNT with heat treatment.
The FESEM images at 100,00x magnificent of MWCNT with and without thermal treatment were shown in Fig. 8. By observing Fig 8(a), the whole image is contained of bright spots. As compared to Fig 8(b), there are only a few spots of bright spot shown. Same as CNF, MWCNT also possess more surface functional group without the thermal treatment. This is proven by the result of FTIR. Meanwhile, the range diameter of MWCNT without thermal treatment is 12.33 – 22.16 nm and the range diameter after thermal treatment is 13.49 – 20.37 nm. Hence, it showed no significant difference in terms of the size of the structure for MWCNT before and after thermal treatment. However, as compared the size for CNF and MWCNT, CNF have bigger diameter because CNF is built up from a stacked up of CNT. a)
b)
Fig. 8: FESEM images at 100,000x magnificent (a) MWCNT without heat treatment; (b) MWCNT with heat treatment.
Fig. 9 shows GACs’ surface without and with heat treatment. As mentioned by Gangupomu, et al. [30], unlike MWCNT and CNF, GAC has relatively large particles with fractal-like shape. However, the effect of the treatment is same as CNF and MWCNT because they are from the same component base. Without the thermal treatment, the GAC’s surface was sputter-like and by continuing with thermal treatment, the surface functional group was reduced. The sputter-like shape would attribute to the strong interaction with adsorbate and also enhance the adsorption capacity. Even though it clearly seen in these images the different before and after the thermal treatment, but, there is no significant different in FTIR images.
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a)
b)
Fig. 9: FESEM images at 10,000x (a) GAC without heat treatment; (b) GAC with heat treatment.
4. Conclusion The effectiveness of green functionalization of CNF, MWCNT and GAC was studied. For H2O functionalization, all three carbonaceous adsorbent have been functionalized with 200 mL DI water, at 55 oC for 24 hours. However, after observing the functional group attached to the adsorbent via FTIR images, CNF and MWCNT shows some improvement where low concentration OH band’s intensity is increase. However, MWCNT is the only adsorbent that suitable with the condition since it has additional functional group after functionalization. The functional group adhered to the MWCNT surface are at 3849.4, 3743.2 (low concentration O-H) and 2343.1cm-1 (alkynyl C=C stretch). However, after thermal treatment, the FTIR images show that low concentration OH band of all three adsorbents have reduced or completely removed. The FESEM images show that after the samples were expose to microwave radiation with third level of heat for 10 minutes, the surface structure changed. All adsorbents show same reaction. This is because they are from the same component which is carbon. Before being treated by the thermal treatment, all adsorbent have a rough surface and there are crumble-like shape that clinging on the surface of CNF, MWCNT and GAC. In the FESEM images for CNF and MWCNT, there are abundant of bright spots that can observe which indicate that there are surface functional groups present on the adsorbents’ surface. Unfortunately, after being treated by microwave radiation, only a few of attached surface functional groups can be spotted. The surface of the adsorbent became more silky-look. All these changes will reduce the adsorption capacity toward dyes. The diameter range for CNF and MWCNT before the thermal treatment is 52.05 – 105.2 and 12.33 – 22.16 nm respectively. After the treatment, the diameter range for CNF is 54.51 – 100.1 nm while, for MWCNT the diameter range is 13.49 – 20.37 nm. Therefore, it can be concluded that H2O-functionalization at 55oC for 24 hours is effective for CNF and MWCNT. Meanwhile the thermal treatment is not preferable for this study because the functional group that attached on carbonaceous materials is oxygen-contained molecule but this method of thermal treatment is an effective way to remove oxygenated functionalities from carbon surface. This work may provide a new approach for the research and development of new functionalization method. Acknowledgment The authors would like to acknowledge Department of Chemical Engineering, Universiti Teknologi Petronas, Tronoh, for providing research facilities. References [1] B. H. Hameed, Ahmad, A. A. & Aziz, N., "Adsorption of reactive dye on palm-oil industry waste: Equilibrium, kinetic and thermodynamic studies. ," Desalination, vol. 247, pp. 551-560, 2009. [2] K. B. Allen S, "Decolourisation of water/wastewater using adsorption," Advances in Colloid and Interface Science, vol. 40, pp. 175-92, 2005.
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4th International Conference on Process Engineering and Advanced Materials
Green Functionalization: Comparison of Different Carbonaceous Materials Wan Nadatul Nadwa Wan Alia,*, Suriati Sufiana, Mohd Zamri Abdullaha a
Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Tronoh, Perak, Malaysia
Abstract This study was carried out to study the effectiveness of green functionalization on three carbonaceous adsorbents which are CNF, MWCNT and GAC. There are two methods of green functionalization used; DI water functionalization and thermal treatment. From FTIR images of H2O-functionalization, CNF and MWCNT shows some improvement where low concentration OH band’s intensity is increase. However, MWCNT is the only adsorbent that obtained additional functional group after functionalized in water bath sonicator at 55oC for 24 hours which are 2 peaks of O-H at 3849.4 and 3743.2 cm-1, and alkynyl C=C stretch at 2343.1 cm-1. However, there is no significant change in functional group attached to GAC after the sonication. After thermal treatment, the FTIR images show that low concentration OH band of all three adsorbents have reduced or completely removed. While, from FESEM images, all carbonaceous adsorbents were exposed to microwave radiation (heat no 3) for 10 minutes. Before the thermal treatment, there are a lot of bright spots can be seen indicating there are surface functional groups present on the surface of the adsorbent. Unfortunately, after being treated by microwave radiation, only a few functional groups attached to the surface can be seen. Adsorbent surface becomes smoother. All these changes will reduce the adsorption capacity of the dye. The diameter range is remained the same after the thermal treatment The diameter range for CNF and MWCNT prior to heat treatment is 52.05 – 105.2 and 12.33 – 22.16 nm respectively. After treatment, range of diameters for CNF is 54.51 – 100.1 nm, while, for MWCNT diameter is range from 13.49 – 20.37 nm. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder underresponsibility responsibility of the organizing committee of ICPEAM Peer-review of the organizing committee of ICPEAM 2016 2016. Keywords: Carbon Nanofiber; Multi-Walled Carbon Nanotubes; Granular Activated Carbon; Green Functionalization; Adsorption.
* Corresponding author. Tel.: +6013-4471758. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016
doi:10.1016/j.proeng.2016.06.566
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1. Introduction According to Hameed [1] adsorption is one of the most effective physical processes for color removal and treatment of textile effluents. This techniques are widely used to remove certain classes of chemical pollutants from water, especially chemical that arguably affected by conventional wastewater biological treatment [2]. The adsorption technique has several advantages such as high efficiency, not sensitive to toxic contaminants, simple operation, and easy recovery of adsorbent [3]. Adsorption capacity is depending on effectiveness of an adsorbent. Then, to ensure that the adsorption process achieved high effectiveness, the best of adsorbent criteria must be chosen. Nowadays, there are abundant of adsorbents available in the market. These adsorbents have been tested by other researchers in dye removal application eg carbon nanotube [4, 5], fly ash [6], bottom ash [7, 8], chitosan [9], chitin [10] and oil palm ash [11].
1.1 Carbonaceous Materials as Adsorbent The application of carbonaceous materials in the field of adsorption, separation, and catalysis is a huge success which have developed researchers’ interest to improve methods in producing such materials [12]. Carbon-based is a preferred material used as engineering adsorbents due to its physical adsorption system which produce high adsorptive capacities. Plus, the presence of oxygen atoms on their surface causing more functional groups can interact with the adsorbent [13]. Last decade, nano-structured carbonaceous materials began to received research world attention due to their extraordinary mechanical, electrical, thermal and structural properties [14]. There are factors that affecting the adsorption capacity such as surface area, pore-size distribution, pore volume and the presence of surface functional groups. Generally adsorption capacity increases with the specific surface area for the adsorption sites [15]. In order to enhance the effectiveness of the adsorbent, it can be functionalized by attaching other functional group onto the adsorbent surface. 1.2 Functionalization of Adsorbent There are several ways to modify the nature of surface functional group which can be classified into three categories which are chemical, physical and bioadsorption modification as shown in Fig. 1. Adsorption capacity can be enhanced by add other functional group to the surface of the adsorbent. It been proved by Tan, et al. [11], which stated the dye removal could improve by 24.2% when the functionalized activated carbon. Most of researchers preferred to functionalize their adsorbent by chemical modification which by contacting their adsorbent with acid or basic solution. The example of acidic functionalizing agents are hydrochloric acid (HCl) [11, 16], nitric acid (HNO3) [17, 18] and hydrogen peroxide (H2O2) [17, 19]. While, the most common basic functionalizing agent are potassium hydroxide (KOH) [20, 21] and sodium hydroxide (NaOH) [22]. Unfortunately, after the adsorbents been treated by these functionalizing agents, the adsorbents need to be wash repetitively until the pH is neutral before it can be used for adsorption purpose. It is time consuming procedures and not environmental friendly since there will be chemical waste in the effluent and a lot of water needed to wash the adsorbent. As reported by Ali [23], CNF was washed using DI water repetitively until reach pH 7 after been treated by concentrated nitric acid. 1.3 Green Functionalization of Adsorbent The word “green” means both process and materials used is environmental friendly. In this case, green functionalization can be classified under chemical modification. However, the functionalizing agent used is neither acidic nor basic phase. The functionalizing agent used is deionized (DI) water (H2O) which is in neutral phase. There is no interaction occur between adsorbent and DI water at normal condition. Therefore sonicator is needed in order to ensure there is molecules vibration during the contact time which can cause other surface functional groups to attach on the adsorbent surface. This is not a new method. This method have been introduced by Ong, et al. [24], which used DI water to
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functionalized activated carbon that derived from Durian shell for electrochemical double layer capacitor. Unfortunately, this method was not explored by other researchers. Generally, carbon is nonpolar but oxygen surface group that commonly attached on it causing it become more hydrophilic [25] . Hence, organic compound such as dye is easily attracted to carbon’s surface.
Fig. 1: Available Modification for Adsorbent [26]
Thermal treatment by using microwave also can be classified as green functionalization since the process does not cause harm to the environment. Thermal treatment is functioned to selectivity remove some of the functional group [27] and also to increase the size of pore on the carbon’s surface [5]. In general, there are two ways to implement thermal treatment which are by electrical heating and microwave. In this study, microwave is used because the integration of microwave heating has been promoting the release of pollutants, lowering the volatile adsorbed on bulk temperature and shorter processing time [28]. This may be responsible for better desorption rate and the conservation of the porous structure [29]. This treatment also responsible to open routes for adsorbate molecules penetrate into the inside space [30]. Therefore, this paper will compare the effectiveness of the green functionalization method by comparing three types of carbonaceous materials namely carbon nanofiber (CNF), multi-walled carbon nanotubes (MWCNT) and granular activated carbon (GAC). 2. Experimental 2.1 Materials Multi walled carbon nanotube (MWCNT) and herringbone CNF were bought from Carbon Nano-material Technology Co., Ltd. MWCNT has diameter and length of 20 nm and 5 µm respectively. While the diameter and length of CNF is 200 nm and 20 µm respectively. Granular activated carbon (GAC) was obtained from Sigma Aldrich. It is untreated GAC and the mesh is 4-8. The melting point of GAC is 3550 oC. 2.2 Functionalization by Deionized (DI) Water Five grams of each CNF, MWCNT and GAC were put in a different Erlenmeyer flask. Then, 200 mL of DI water was added to each flask. All flasks were immersed in the water bath sonicator (Bandelin Sonorex Digitex, DT255 with ultrasonic peak output, 640 W/per) at 55oC for 24 hours. After 24 hours, all solutions were filtered using vacuum pump and membrane filter paper. All adsorbents’ cake was then dried in the oven at 80 oC until completely dry. Functionalized adsorbents were stored in vials at room temperature.
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2.3 Thermal Treatment Thermal treatment is a further treatment to the adsorbents. The samples must first go through the functionalization by DI water. However, before the adsorbents cake was dried in the oven, the adsorbents’ cake was put in the microwave at third level of heating for 10 minutes. The adsorbents’ cake was then dried in the oven at 80 oC until completely dry. The functionalized adsorbents were stored in the vials at room temperature. In order to test the effectiveness of heat treatment, two types of sample for each adsorbent were prepared which are with and without thermal treatment. 3. Results and Discussion 3.1 FTIR Characterization This analysis was carried out to distinguish the IR peaks before and after the functionalization performed on the adsorbents. Fig. 2 shows the FTIR spectra for CNF. Fig. 2(a) shows a broad peak at 3435.1cm -1 which is refers to high concentrated O-H stretch functional group. The peak at 2346.5 and 1631 cm-1 attributed to alkynyl C=C stretch and amide C=O respectively. For peak at 1021.7cm -1, it is associated to C-N aliphatic amines while peak at 665.67 cm-1 can be assign to C=C alkenes functional group. As water is used as the functionalizing agent, the expected functional group is based on oxygen which is from OH- ion. However, by comparing fig. 2(a) and 2(b), can be observed there is no new peak appeared. Fortunately, the intensity of functional group of low concentration O-H with two peaks at 3856.2 and 3746.7cm-1 were increased. As the intensity of these peaks increase, this means there are more OH functional group attach to the surface of CNF. Conversely, after the thermal treatment, these peaks have been removed completely. As mentioned by Muller, et al. [31], both electrical heating and microwave are the effective way to remove oxygen-contained functional group from carbon surface. a)
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Fig. 2: FTIR images (a) Raw commercialized CNF; (b) H 2O-functionalized CNF without thermal treatment; (c) H 2O-functionalized CNF with thermal treatment
Meanwhile, fig. 3 shows the FTIR images of raw and H2O-functionalized MWCNT. The surface functional group that adhered on raw MWCNT and CNF are same except for alkynyl C=C group which exist on raw CNF. This is because they are from the same component but different in molecules arrangement. After being treated, there are 3 new peaks appeared which at 3849.4, 3743.2 (low concentration O-H) and 2343.1cm-1 (alkynyl C=C stretch). The result shows that an additional functional group which is alkynyl was successfully attached on surface of MWCNT. This may enhance the adsorption capacity of MWCNT. Unfortunately, Fig. 3(c) shows the intensity of low concentration OH band was reduced. The same reason for CNF can be applies for MWCNT because they have similar component with different arrangement.
a)
b)
c)
Fig. 3: FTIR images (a) Raw commercialized MWCNT; (b) H 2O-functionalized MWCNT without thermal treatment; (c) H 2O-functionalized MWCNT with thermal treatment
The FTIR images for GAC are quite different from CNF and MWCNT as shown in fig. 4. In fig. 4(a), the peak at 3404.3cm-1 attributed to O-H stretch. For peak at 2367 cm-1, it is associated to alkynyl C=C stretch while peak at 1572.9 cm-1 is related to the carboxylate anion stretch mode. The peak at 1151.8 is attributed to ester C-O functional group. After functionalized with DI water, the intensity of alkane group which at 2925cm -1 increased. However, alkane C-H bonds are fairly ubiquitous and therefore usually less useful in determining structure. By observing fig. 4(a) and 4(b), there is no additional functional group attached after functionalization. As compared to CNF and MWCNT, GAC is less affected after H2O functionalization. This is because nanomaterials possess a high surface area to volume ratio. Therefore, it has higher reactivity upon exposure compared to GAC[32]. Fig. 4(c) is the results for
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GAC after it being exposed to microwave radiation. However, the graph pattern is quite similar to the raw GAC FTIR image. Therefore, it can be concluded that the thermal treatment is not applicable for GAC because it will revert to original raw condition after being treated by thermal treatment. a)
b)
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Fig. 4: FTIR images (a) Raw commercialized GAC; (b) H 2O-functionalized GAC without thermal treatment; (c) H2O-functionalized GAC with thermal treatment
3.2 FESEM Characterization FESEM is widely used to study the surface morphology and surface characteristics of the adsorbent [33]. Fig. 5 shows the comparison of CNF before and after being treated by thermal treatment. In Fig. 5(a), it can be observed that a lot crumble-like structure attached on the CNF surface. The crumble-like structure is surface functional group that clinging on CNF surface. This condition is favourable for adsorption because it will increase the adsorption site and increase the adsorption capacity. As compared to the structure after the treatment, it shows less clinging things on the surface. This proves the continuation from FTIR result which shows the OH functional group have been completely removed.
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Fig.5: FESEM images at 10,000x magnificent (a) CNF without heat treatment; (b) CNF with heat treatment.
Fig. 6 shows the CNF’s structure images at 100,000x magnificent. From these images, it shows there is no significant different in terms of the structure size. The range diameter of CNF before and after the thermal treatment is 52.05 – 105.2 nm and 54.51 – 100.1 nm respectively. In Fig. 6(a), the bright spots are the surface functional group that attached on CNF surface. Therefore, as compared these images, after the thermal treatment, CNF have silkier look surface, while, before the thermal treatment it have a lot of attach functional group. a)
b)
Fig. 6: FESEM images at 100,000x magnificent (a) CNF without heat treatment; (b) CNF with heat treatment.
Different conditions can be observed from Fig. 7. The FESEM images of MWCNT at 10,000x magnificent shows the CNT’s structure is almost the same with and without thermal treatment. However, by applying the radiation to the adsorbent, it became less organized. As mentioned by Yin, et al. [26], the result from being exposed to microwave radiation, the surface structure of the adsorbent might fractured and become shorter. However, there is no fractured structure can be pointed out. Similar to CNF images, the bright spot in Fig 7(a) shows the present of surface functional group on MWCNT surface.
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Fig. 7: FESEM images at 10,000x magnificent (a) MWCNT without heat treatment; (b) MWCNT with heat treatment.
The FESEM images at 100,00x magnificent of MWCNT with and without thermal treatment were shown in Fig. 8. By observing Fig 8(a), the whole image is contained of bright spots. As compared to Fig 8(b), there are only a few spots of bright spot shown. Same as CNF, MWCNT also possess more surface functional group without the thermal treatment. This is proven by the result of FTIR. Meanwhile, the range diameter of MWCNT without thermal treatment is 12.33 – 22.16 nm and the range diameter after thermal treatment is 13.49 – 20.37 nm. Hence, it showed no significant difference in terms of the size of the structure for MWCNT before and after thermal treatment. However, as compared the size for CNF and MWCNT, CNF have bigger diameter because CNF is built up from a stacked up of CNT. a)
b)
Fig. 8: FESEM images at 100,000x magnificent (a) MWCNT without heat treatment; (b) MWCNT with heat treatment.
Fig. 9 shows GACs’ surface without and with heat treatment. As mentioned by Gangupomu, et al. [30], unlike MWCNT and CNF, GAC has relatively large particles with fractal-like shape. However, the effect of the treatment is same as CNF and MWCNT because they are from the same component base. Without the thermal treatment, the GAC’s surface was sputter-like and by continuing with thermal treatment, the surface functional group was reduced. The sputter-like shape would attribute to the strong interaction with adsorbate and also enhance the adsorption capacity. Even though it clearly seen in these images the different before and after the thermal treatment, but, there is no significant different in FTIR images.
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Fig. 9: FESEM images at 10,000x (a) GAC without heat treatment; (b) GAC with heat treatment.
4. Conclusion The effectiveness of green functionalization of CNF, MWCNT and GAC was studied. For H2O functionalization, all three carbonaceous adsorbent have been functionalized with 200 mL DI water, at 55 oC for 24 hours. However, after observing the functional group attached to the adsorbent via FTIR images, CNF and MWCNT shows some improvement where low concentration OH band’s intensity is increase. However, MWCNT is the only adsorbent that suitable with the condition since it has additional functional group after functionalization. The functional group adhered to the MWCNT surface are at 3849.4, 3743.2 (low concentration O-H) and 2343.1cm-1 (alkynyl C=C stretch). However, after thermal treatment, the FTIR images show that low concentration OH band of all three adsorbents have reduced or completely removed. The FESEM images show that after the samples were expose to microwave radiation with third level of heat for 10 minutes, the surface structure changed. All adsorbents show same reaction. This is because they are from the same component which is carbon. Before being treated by the thermal treatment, all adsorbent have a rough surface and there are crumble-like shape that clinging on the surface of CNF, MWCNT and GAC. In the FESEM images for CNF and MWCNT, there are abundant of bright spots that can observe which indicate that there are surface functional groups present on the adsorbents’ surface. Unfortunately, after being treated by microwave radiation, only a few of attached surface functional groups can be spotted. The surface of the adsorbent became more silky-look. All these changes will reduce the adsorption capacity toward dyes. The diameter range for CNF and MWCNT before the thermal treatment is 52.05 – 105.2 and 12.33 – 22.16 nm respectively. After the treatment, the diameter range for CNF is 54.51 – 100.1 nm while, for MWCNT the diameter range is 13.49 – 20.37 nm. Therefore, it can be concluded that H2O-functionalization at 55oC for 24 hours is effective for CNF and MWCNT. Meanwhile the thermal treatment is not preferable for this study because the functional group that attached on carbonaceous materials is oxygen-contained molecule but this method of thermal treatment is an effective way to remove oxygenated functionalities from carbon surface. This work may provide a new approach for the research and development of new functionalization method. Acknowledgment The authors would like to acknowledge Department of Chemical Engineering, Universiti Teknologi Petronas, Tronoh, for providing research facilities. References [1] B. H. Hameed, Ahmad, A. A. & Aziz, N., "Adsorption of reactive dye on palm-oil industry waste: Equilibrium, kinetic and thermodynamic studies. ," Desalination, vol. 247, pp. 551-560, 2009. [2] K. B. Allen S, "Decolourisation of water/wastewater using adsorption," Advances in Colloid and Interface Science, vol. 40, pp. 175-92, 2005.
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