Characteristics of Chemical Modified Activated Carbons from Bamboo Scaffolding

Transcription

1 Chinese Journal of Chemical Engineering, 20(3) (2012) Characteristics of Chemical Modified Activated Carbons from Bamboo Scaffolding W.H. Cheung 1,2, S.S.Y. Lau 1, S.Y. Leung 1, A.W.M. Ip 1 and G. McKay 1, * 1 Department of Chemical and Biomolecular Engineering, the Hong Kong University of Science and Technology, Hong Kong, China 2 Scott Wilson Ltd, 38th Floor MetroPlaza Tower 1, Kwai Fong, Hong Kong, China Abstract In this study, bamboo scaffolding was used to produce activated carbon by carbonization at 600 C and 900 C with the purge of nitrogen. The 600 C char was then further modified chemically by acids and alkalis by reflux for 6 hours. The produced chars were then characterized by nitrogen adsorption isotherm, He pyncometry, ph, elemental analysis and Boehm titration. For most of the chemically modified carbons, the micropore surface areas and volumes have increased compared with the 600 C char, while the mesopore surface areas and volumes slightly decreased, which may have been due to the dissolving of some of the permeated inorganic matter and oxidizing deposited carbon that blocks the pore openings. For the acidic modified carbons, larger amounts of acidic groups were present in the carbons after being activated by phosphoric acid, phosphoric acid further treated with 2 mol L 1 nitric acid, and calcium hydroxide. Although carbon treated with 2 mol L 1 and 5 mol L 1 nitric acid also produced high acidity, the surface areas and pore volumes were relatively low, due to the destruction of pores by nitric acid oxidation. The reduction of porosity may impair the adsorption capacity. Keywords activated carbon, bamboo, surface area, chemical activation, porosity, surface functional group 1 INTRODUCTION Activated carbon is an effective adsorbent in the removal of wide variety pollutants such as dyestuff and metal ions from industrial effluents. Due to rapid industrialization throughout the world, as well as the increasing concern of public health and the presence of toxic materials such as heavy metals and dyes are increasingly of concern. Although commercial activated carbon, with high surface area, microporous character and high adsorption capacity can treat the effluent effectively, it is expensive and has relatively high operation costs. Thus, researchers are encouraged to seek effective and cheap alternate adsorbents. Wood, lignite coal, coconut shell and peat are some of the raw materials commonly used to prepare activated carbon either by physical or chemical activation. However, it is relatively expensive. Vaughan et al. [1] had studied the removal of metals including copper by untreated, water-washed, sodium hydroxide treated and acid washed corncobs. Recently, there has been a growing interest in producing activated carbons from low cost precursors such as waste materials and agriculture by-products. They are not only of reasonable hardness but also available as renewable and sustainable resources. Among many these raw materials, bamboo is a popular construction material for scaffolding in South East Asia. It is widely used in the building and construction industry for repairing, decorating, signs erecting and slope maintenance works. Over tonnes of bamboo scaffolding waste each year are dumped as construction waste. Bamboo scaffolding is economical (US$ 1.29 per 6m bamboo pole), flexible (to cut a desired length) and efficient (erecting is easy) in construction [2]. Preparing activated carbon from bamboo scaffolding not only can mitigate the burden on landfill sites, but also can minimize chemical usage as treating the raw precursor requires a large amount of chemicals and its recovery rate is relatively low. The adsorption capacity of activated carbon depends on various factors such as surface area, pore size distribution and surface functional groups on the adsorbent; polarity, solubility and molecular size of adsorbate; solution ph and the presence of other ions in solution, and so on [2]. Generally, the adsorption capacity increases with the specific surface area due to the availability of the adsorption site, while pore size and micropore distribution are closely related to the composition of the activated carbon, production stage and the frequency of regeneration [3]. In this study, activated carbon was prepared from waste bamboo scaffolding by physical activation. The produced carbon was then chemically modified by acids and alkalis. Finally, in order to investigate the sorption of activated carbons, the characterization of the modified carbon including specific surface area, pore volume and pore size distribution, helium pycnometry, ph, Boehm titration (surface functional group) and elemental analysis (CHNS) were conducted. 2 MATERIALS AND METHODS 2.1 Preparation of modified activated carbon by physical activation All chemicals used in this study were of analytical reagent grade and were utilized as received, without Received , accepted * To whom correspondence should be addressed. kemckayg@ust.hk

2 516 Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 further purification. Deionized water was used throughout the study. Raw bamboo scaffolding ( μm) was used as the starting material. At the beginning of the study, it had a ph of The 600 C bamboo char was firstly prepared by preheating the raw bamboo at 110 C for 2 hours under nitrogen in the muffle furnace by increasing the temperature at 5 C min 1 to 600 C for 4 hours. The produced char was then cooled down to room temperature. The process was repeated until the total yield of the char about 250 grams reached. The 600 C char was then washed with deionized water over a Büchner funnel to neutral ph, dried in an oven at 110 C for 24 hours and stored in a sample bottle until used. 2.2 Chemical modification of activated carbon Acidic Treatment The acid modification of 600 C bamboo char was conducted by heating the 15 g of char with 200 ml of a chemical solution (2 mol L 1 or 5 mol L 1 of hydrochloric acid, nitric acid, sulfuric acid and acetic acid; HAC2, HAC5, NAC2, NAC5, SAC2, SAC5, AAC2 and AAC5, respectively) to boiling temperature in a conical flask with a heater, followed by reflux for 6 hours with the use of a condenser. Eight modified activated carbons were produced. The produced char was then washed with deionized water until a constant ph was reached, dried in an oven at 110 C for 24 hours and stored in a sample bottles until use. The phosphorus acid modified char (PAC) was prepared by preheating the 600 C char with 85% phosphorus acid (X p = 2.5, and X p is the mass ratio of bamboo to acid) at 150 C for 2 hours, followed by purging by nitrogen in the muffle furnace with increasing the temperature by 1 C min 1 to 600 C for 4 hours. The produced char was then cooled down to room temperature. The produced char was then washed with deionized water over a Büchner funnel to neutral ph, dried in an oven at 110 C for 24 hours and stored in a sample bottle until use Basic treatment The chemical basic modification of the 600 C bamboo char was conducted using similar procedures as the acidic treatment but using the chemical solution of 2 mol L 1 or 5 mol L 1 of sodium hydroxide, calcium chloride and saturated calcium hydroxide (NaAC2, NaAC5, CaAC2, CaAC5 and SCaAC, respectively). Similarly, five modified activated carbons were produced. The produced char was then washed with deionized water until a constant ph was reached, dried in an oven at 110 C for 24 hours and stored in a sample bottles until used. 2.3 Characterization of activated carbon Determination of surface area and pore size The pore size distribution, surface area (S BET ) and pore volume (V TOT ) of the produced char were determined from the nitrogen adsorption and desorption isotherm at 196 C using a Quantachrome Autosorb 1-MP. S BET is determined by Brauner-Emmet-Teller (BET) method. Total surface areas are determined by the application of the BET equation, whereas the t-plot method was used to estimate the volumes of micropores and micropore surface area. The total pore volume (V p ) was estimated from the amount of nitrogen adsorbed at a relative pressure of P/P o = 0.97 and the average pore diameter was calculated from D p = 4V p /S BET. The BET equation can be mathematically represented by [4] p 1 ( c 1) p = + (1) o o V( p p) Vmc Vmcp where V = amount adsorbed in volume STP (cm 3 g 1 ); V m = monolayer capacity in volume STP (cm 3 g 1 ); p = equilibrium vapor pressure (Pa); p o = saturation vapor pressure (Pa); c = constant He pycnometry Since helium can enter even the smallest voids or pores which is used to measure the unknown volume, the final result is often referred to as skeletal density. The true density of the produced char was determined by a Quantachrome pentapycnometer with small sample cell size. The average helium density of each char was from 5 runs ph of carbon solution The carbon sample was firstly dried in the oven to obtain a constant mass at (105±5) C for 3 hours. The ph value of the produced char was measured after gently boiling (1.00±0.01) g of char with deionized water in a boiler flask connected with a condenser for 15 minutes. The char was then filtered with a 0.22 µm syringe filter and the carbon solution was measured with a calibrated digital ph meter at 50 C Elemental analysis (CHNS) The amount of elements (carbon, hydrogen, nitrogen and sulfur) in the produced char was determined by an Elemental Analyzer by flash combustion. The sample char was firstly dried in an oven at 110 C before the measurement was carried out. The sample char was burned at a temperature of 1000 C in flowing oxygen for C, H, N and S analysis in the analyzer. The CO 2, H 2 O, NO x and SO 2 combustion gases were passed through a reduction tube with helium as the carrier gas to convert the NO x nitrogen oxides into N 2 and bind the free oxygen [5]. The CO 2, H 2 O and SO 2 combustion gases were measured by selective IR detector. After corresponding absorption of these gases, the content of the remaining nitrogen was determined by thermal conductivity detection. The oxygen was calculated by the difference of carbon, hydrogen, nitrogen and sulfur Boehm titration The Boehm titration was performed to determine the surface functional groups of the produced char. Solutions of HCl (ca 0.05 mol L 1 ), NaOH (ca 0.05 mol L 1 ), NaHCO 3 (ca 0.05 mol L 1 ) and Na 2 CO 3 (ca

3 Chin. J. Chem. Eng., Vol. 20, No. 3, June mol L 1 ) were prepared. NaOH solution was standardized by standard KHC 8 H 4 O 4 and the HCl solution was standardized by Na 2 CO 3 solution. Both Na 2 CO 3 and NaHCO 3 were dried in an oven at 110 C for 3 hours before the standardizations were carried out. Similarly, all the produced chars were dried in the oven at 110 C for 3 hours before the Boehm titration was carried out. 50 ml of each solution was added to 0.5 g of each produced char in a 75 ml bottle and placed in a shaker for 24 hours. After shaking, the char was filtered. The 10 ml filtrate was titrated using standard NaOH for acidic solution (HCl) and standard HCl for basic solutions (NaOH, NaHCO 3 and Na 2 CO 3 ). For the titration of HCl and NaOH solution, phenolphthalein was used as an indicator while for Na 2 CO 3 and NaHCO 3, the ph meter was used; the end points were 8.3 and 4.5 respectively. The basic group content on the char was calculated from the amount of HCl that reacted in the bottle. The acidic groups were calculated using the facts that (1) NaOH neutralizes carboxylic, phenolic, and lactonic groups; (2) Na 2 CO 3 neutralizes carboxylic and lactonic groups; and (3) NaHCO 3 neutralizes only carboxylic group. 3 RESULTS AND DISCUSSION 3.1 Thermal treatment on raw bamboo 600 C char was produced through the carbonization of raw bamboo at 600 C in the muffle furnace to eliminate volatiles (carbonization) by physical activation with nitrogen purged while 900 C char was produced using a similar method at 900 C. Table 2 shows the mass yields of 600 C and 900 C char produced after carbonization of raw bamboo. About 18% (by mass) of 600 C char and 13.6% (by mass) of 900 C char were produced. Although 900 C char did not produce as much as under 600 C, it was found that the carbonization yield changed with different temperatures, i.e. the higher the temperature throughout the carbonization, the smaller the yield of carbon produced as shown in Table 1. Table 1 Yields of carbon char produced by carbonization at 600 and 900 C Sample Raw bamboo used/g Char produced/g Yield/% 600 C char C char Furthermore, after carbonizing the bamboo, the activated carbon was washed by filtration with deionized water over a Büchner funnel to neutral ph. It was observed that the color of the water after washing was dark-brown for the char produced at 600 C and was colorless for the char produced at 900 C. It is believed that there was incomplete carbonization of the bamboo at 600 C. 3.2 Characterization of activated carbon Determination of surface area and pore size Table 2 summarizes the total surface area, micropore and mesopore area, total pore volume, micropore and mesopore volume and average pore diameter. Nitrogen adsorption/desorption isotherms were used to determine standard pore structure characteristics of the carbons, such as their BET surface areas and total pore volumes. The obtained results indicated that the surfaces of the modified carbon possess greater surface area as well as pore volume as compared with the unmodified carbon. The higher the temperature operated in the carbonization of raw bamboo, the higher the total surface area and the total pore volume as shown in Table 2. However, comparing the data of 600 C and 900 C char, it is observed that both the micropore area and volume of 600 C char were greater than that of 900 C char, while the mesopore area and volume of 600 C Exp # Table 2 Physical properties of produced char and activated carbon Micropore Total surface area Micropore area Mesopore area Total pore volume /m 2 g 1 /m 2 g 1 /m 2 g 1 1 /cm 3 g Mesopore 1 volume/cm 3 g Average 1 volume/cm 3 g pore diameter/nm bam C char C char HAC HAC NAC NAC SAC AAC AAC NaAC

4 518 Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 char were smaller that that of 900 C char. All the chemical modified chars were produced by refluxing the 600 C char with a chemical solution. From Table 2, it is observed that there was an increase in the micropore area and volume while there was a decrease in that of mesopore, which may have resulted from the dissolving of some of the permeated inorganic matter and oxidizing deposited carbon that blocked pore openings, thus creating some microporosity and slightly reducing the mesoporosity. Moreover, all the total surface area, micropore and mesopore area of the chemically modified carbons significant increased compared with 600 C char, except NAC2 and NAC5. The reduction in surface and porosity may be due to the erosive action of nitric acid which caused a partial destruction of their structure He pycnometry Table 3 shows the average density and volume measured by the method of He pycnometry using the Quantachrome pentapycnometer. The results show that the average density of chemically modified carbons was similar in the range of g ml 1. Among these activated carbons, it is observed that AAC2 has the highest density followed by AAC5 and SAC2. However, NAC2 has the lowest density. Table 3 Exp # Results of helium pycnometry Average density/g ml 1 HAC HAC NAC SAC AAC AAC NaAC CaAC ph of carbon solution The ph is defined as the minus logarithm of the hydrogen ion (H + ) concentration. The dissociation constant, pk a, of water has a value of at 25 C, and is equal to [H + ][OH ]. The proton is hydrated according to Cπ + 2H 2 O Cπ H 3 O + + OH [6, 7]. Thus, for water in equilibrium with a carbon, if the water has a ph value <7, then the water is acidic from the dissociation of H + from the surface oxygen groups, such as a carboxylic group, COOH. However, for an alkaline solution, the ph has to be >7.0 [5]. However, measuring the ph with the carbon is not entirely associated with surface oxygen complexes and the understanding of surface basicity, is not straightforward. Table 4 shows the ph values of the produced carbons. The results show that the ph of raw bamboo was almost neutral (6.350). It became alkaline (8.300) after being carbonized at 600 C and slightly alkaline (7.710) at 900 C. The most acidic activated carbon Table 4 ph measurement of the produced carbons Exp# was PAC, followed by the PAC + N, HAC2 and NAC2 while the activated carbon modified by 2 mol L 1 acetic acid was the least acidic, which can be explained by the fact that acetic acid is the weakest acid among the five acids. It was expected that PAC+N would be the most acidic. The results can be attributed to the destruction of the pore structure by oxidation of nitric acid. For the basic modified activated carbon, the carbon modified with saturated calcium hydroxide was the most alkaline (7.340) while the rest of the four were slightly alkaline and had a similar ph value. However, the ph of all the basic modified activated carbons was more acidic than that of 600 C char. This may have been due to the neutralization of the acidic groups in the 600 C char Elemental analysis (CHNS analysis) Table 5 shows the result of the elemental analysis. Element nitrogen, carbon, sulfur and hydrogen were determined from the elemental analyzer by flash combustion while oxygen was determined by the difference of these four elements. Figure 1 shows that the largest amount of element inside the produced carbons was carbon, followed by oxygen, and the smallest amount was nitrogen, except raw bamboo which had about an equal amount of carbon and oxygen. By comparing the amount of oxygen inside the produced carbons and raw bamboo, it was found that there was a reduction of oxygen (~20%-30%) ph bam C char C char HAC HAC NAC NAC SAC SAC AAC AAC PAC PAC+Ca PAC+N NaAC NaAC CaAC CaAC SCaAC 7.340

5 Chin. J. Chem. Eng., Vol. 20, No. 3, June Table 5 The result of the elemental analysis Exp # N atom/% C atom/% S atom/% H atom/% O atom/% (by difference) bam C char C char HAC HAC NAC NAC SAC SAC AAC AAC PAC PAC + Ca PAC + N NaAC NaAC CaAC CaAC SCaAC Figure 1 The graph of composition of carbon, hydrogen, nitrogen and sulfur (CHNS) in produced carbon O; H; S; C; N after physical and chemical modification. On the contrary, there was an increase (~20%-40%) in carbon after modification. There was also a reduction of hydrogen except PAC. The amount of nitrogen was so small for all carbons except for those treated with nitric acid (NAC2, NAC5 and PAC + N). Also, the increase in oxygen in nitric acid modified carbons due to the strong oxidizing properties of nitric acid. Figures 2-5 show the graph of C/N, C/O, C/S and C/H ratio of the produced char, respectively. It was found that from Fig. 2, the C/N ratio was generally around 150 to 250 except NAC2 (42.159), NAC5 (36.375) and PAC+N (50.530). These three carbons had relatively low C/N ratio as they were chemically

6 520 Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 Figure 2 sample The graph of C/N ratio of produced char Figure 3 sample The graph of C/O ratio of produced char Figure 4 sample The graph of C/S ratio of produced char modified by nitric acid and the amount of nitrogen was greater than others. From Fig. 3, the C/O ratios were fluctuate among the produced chars. It was found that raw bamboo had about 1 of C/O ratio which meant that they had about equal about of carbon and oxygen in the chars. The ratio of the rest were greater than one which meant that there were more carbon then oxygen in the chars. NaAC2 and NaAC5 had the larger ratio among these carbons, followed by CaAC2, CaAC5 and SCaAC which all about 7, while the rest were ranged from 2 to 6.5.

7 Chin. J. Chem. Eng., Vol. 20, No. 3, June Figure 5 sample The graph of C/H ratio of produced char Table 6 The result of Boehm titration of produced carbon Exp # Basic groups/μeq g 1 Total acidic groups/μeq g 1 Carboxylic groups/μeq g 1 Lactonic groups/μeq g 1 Phenolic groups /μeq g 1 bam C char HAC HAC NAC NAC SAC SAC AAC AAC PAC PAC + Ca PAC + N NaAC NaAC CaAC CaAC SCaAC From Fig. 4, it was found that the C/S ratios were differently among these carbons. Both PAC + Ca and PAC + N had infinity C/S ratio as there were no sulfur in the carbons, followed by HAC2 ( ) and NaAC2 ( ). On the contrary, SAC5 ( ) had the lowest C/S ratio, followed by 600 C char ( ), SAC5 ( ) and 900 C char ( ). It was believed that sulfur was generated after the carbonization of raw bamboo at high temperatures. On the other hand, both SAC5 and SAC2 were modified by sulfuric acid which increased the sulfur content in the carbons. From Fig. 5, it was found that raw bamboo had the lowest C/H ratio and the ratios increased after carbonized at 600 C and further increased at 900 C. It was believed that most of the water was vaporized out under high temperatures. For the chemically modified carbons, most of their C/H ratios were within the range of except three carbons after modified by phosphoric acid. It was believed that there are three hydrogens in each phosphoric acid which contributed the hydrogen contents in those carbons Boehm titration Results of Boehm titration are showed in Table 6. Fig. 6 shows the amount of acidic and basic functional groups of the produced carbons. The result shows that

8 522 Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 Figure 6 The graph of amount of acidic and basic functional groups of produced char basic groups; total acidic groups Figure 7 The graph of amount of three acidic functional groups (carboxylic, lactonic and phenolic groups) of produced char carboxylic groups; lactonic groups; phenolic groups after carbonization of the raw bamboo, the acidic groups decreased significantly while the basic groups increased which could enhance the uptake of inorganic like phenols as starting materials. At the same time, however, it impaired the uptake of organics and metal ions as there was a decrease in the oxygen surface functional groups in carbon. Carbon surfaces become basic in character following an outgassing at temperatures >700 C. Subsequently, they must not be exposed to air until they have cooled to <200 C. Following this treatment, carbons exhibit basic characteristics, that is, on equilibration with water, the latter becomes basic with ph values >7.0. They also exhibit positive external surface charges and require acid titrations to restore neutrality [5]. After the acidic chemical modification of 600 C char, there were more acidic groups introduced and the amount of basic groups reduced as it was neutralized by the acids from acidic treatment. Among the 11 acidic modified carbons, PAC + N had the largest amount of acidic groups, followed by PAC and PAC + Ca while AAC2 had the smallest amount. Similarly, the acidic groups were reduced after the 600 C char was modified by alkalis. However, the basic groups did not increase except SCaAC. It was believed that the acidic groups of basic modified carbons were neutralized by the alkalis rather than having increase the basic groups on the carbons.

9 Chin. J. Chem. Eng., Vol. 20, No. 3, June Figure 7 shows the amount of carboxylic, lactonic and phenolic groups in the total acidic groups of produced carbons. As sodium hydroxide can neutralize all three groups, sodium carbonate only can neutralize the carboxylic and lactonic groups; sodium hydrogencarbonate can neutralize only the carboxylic group, and the acidity of these three groups are as follows: phenolic>lactonic>carboxylic groups. From Fig. 7, it shows that phenolic group was the major acidic group present in bamboo. After the carbonization at 600 C, the amount of phenolic group decreased significantly which contributed to the increase in the bascity of the 600 C char. All three groups increased dramatically in PAC+N, followed by PAC and PAC+N. Moreover, for the basic modified carbons, both the phenolic and lactonic groups were reduced but only a small amount of the carboxylic groups increased which contributed less acidity. For the basic reagent, only sodium hydroxide, carbonate and hydrogencarbonate were used in this investigation. However, the carbonyl groups cannot be determined by sodium ethoxide. So, the acidity of the carbons is not determined directly in this study. 4 CONCLUSIONS In this study, the bamboo scaffoldings was reused to produce activated carbon which can be used as an effective adsorbent to adsorb dyes or metal ions in effluent after physical and chemical activation. Characterizations such as BET (surface area and volume pore), He density, ph, elemental analysis (CHNS) and Boehm titration (surface functional groups) were conducted to compare whether the adsorption capacity was enhanced. For the acidic modified carbons, the carbon after being activated by phosphoric acids such as PAC, PAC+N and PAC+Ca showed comparatively larger amount of acidity groups. Although NAC2 and NAC5 also had high acidity, the surface area and pore volume were particularly low due to the destruction of pores by nitric acid oxidation. The reduction of porosity impaired the adsorption capacity. ACKNOWLEDGEMENTS One of the authors (Lau) would like to acknowledge the support of Hong Kong University of Science and Technology through the Undergraduate Research Opportunity Program. REFERENCES 1 Vaughan, T., Seo, C.W., Marshall, W.E., Removal of selected metal ions from aqueous solution using modified corncobs, Bioresour. Technol., 78, (2001). 2 Choy, K.K.H., Barford, J.P., McKay, G., Production of activated carbon from bamboo scaffolding waste-process design, evaluation and sensitivity analysis, Chem. Eng. J., 109, (2005). 3 Radovic, L.R., Moreno-Castilla, C., Rivera-Utrilla, J., Carbon materials as adsorbents in aqueous solutions, Chem. Phys. Carbon, 27, (2001). 4 Yin, C.Y., Kheireddine Aroua, M., Ashri Wan Daud, W.M., Review of modification of activated carbon for enhancing contaminant uptakes from aqueous solutioons, Separ. Purif. Tech., 52, (2007). 5 Marsh, H., Rodriguez-Reinoso, F., Activated Carbon, Elsevier, Oxford, (2006). 6 Rodrίgues-Reinoso, F., The role of carbon materials in heterogeneous catalysis, Carbon, 36, (1998). 7 Rodrίguez-Reinoso, F., Molina-Sabio, M., Textural and chemical characterization of microporous carbons, Adv. Colloid Interface Sci., 76-77, (1998).