Characteristics of Chemical Modified Activated Carbons from Bamboo Scaffolding

Chinese Journal of Chemical Engineering, 20(3) 515 523 (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 50000 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 2012-03-01, accepted 2012-04-23. * To whom correspondence should be addressed. E-mail: kemckayg@ust.hk

516 Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 further purification. Deionized water was used throughout the study. Raw bamboo scaffolding (700-1000 μm) was used as the starting material. At the beginning of the study, it had a ph of 6.35. 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 2.2.1 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. 2.2.2 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 2.3.1 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. 2.3.2 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. 2.3.3 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. 2.3.4 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. 2.3.5 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

Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 517 0.1 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 1552 279 17.969 900 C char 162 22 13.563 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 3.2.1 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 2.170 0.000 2.170 0.002 0.000 0.002 14.8200 600 C char 262.070 118.100 143.970 0.154 0.062 0.091 2.3420 900 C char 330.300 238.700 91.600 0.192 0.127 0.065 2.2090 HAC2 290.700 160.000 130.700 0.160 0.084 0.077 2.2090 HAC5 310.630 151.700 158.930 0.172 0.080 0.092 2.2780 NAC2 131.170 64.480 66.690 0.075 0.035 0.039 2.1700 NAC5 69.680 43.820 25.860 0.038 0.024 0.014 2.2460 SAC2 312.890 180.000 132.890 0.176 0.098 0.078 2.1540 AAC2 285.490 131.100 154.390 0.154 0.071 0.083 2.2940 AAC5 259.500 108.400 151.100 0.149 0.058 0.091 2.1650 NaAC 2 332.470 216.600 115.870 0.180 0.117 0.063 2.1410

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. 3.2.2 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 0.80-1.04 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 HAC2 0.917 HAC5 0.988 NAC5 0.891 SAC2 1.034 AAC2 0.991 AAC5 1.023 NaAC2 0.809 CaAC2 1.163 3.2.3 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 10 14 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. 3.2.4 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 6.350 600 C char 8.300 900 C char 7.710 HAC2 3.700 HAC5 2.970 NAC2 3.000 NAC5 2.980 SAC2 3.800 SAC5 3.460 AAC2 4.060 AAC5 3.910 PAC 2.600 PAC+Ca 3.250 PAC+N 2.770 NaAC2 6.510 NaAC5 6.480 CaAC2 6.020 CaAC5 6.090 SCaAC 7.340

Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 519 Table 5 The result of the elemental analysis Exp # N atom/% C atom/% S atom/% H atom/% O atom/% (by difference) bam 0.257 47.315 0.038 8.438 43.953 600 C char 0.437 83.145 0.278 2.200 13.940 900 C char 0.466 84.190 0.195 1.547 13.604 HAC2 0.380 83.320 0.013 2.166 14.122 HAC5 0.420 84.035 0.061 2.307 13.178 NAC2 1.716 72.345 0.036 1.931 23.973 NAC5 1.931 70.205 0.029 1.891 25.944 SAC2 0.415 82.725 0.204 2.366 14.291 SAC5 0.404 82.525 0.329 2.404 14.339 AAC2 0.397 83.710 0.033 2.271 13.591 AAC5 0.405 84.420 0.029 2.458 12.689 PAC 0.471 71.820 0.103 8.592 19.015 PAC + Ca 0.402 67.945 0.000 7.683 23.971 PAC + N 1.198 60.535 0.000 4.289 33.978 NaAC 2 0.399 86.400 0.016 2.560 10.626 NaAC5 0.361 86.620 0.026 2.608 10.387 CaAC2 0.389 84.925 0.035 2.353 12.299 CaAC5 0.412 84.640 0.047 2.525 12.377 SCaAC 0.399 84.730 0.042 2.552 12.279 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

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.

Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 521 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 15.464 227.766 3.491 46.821 177.455 600 C char 75.221 79.827 0.476 14.650 64.701 HAC2 18.279 81.915 20.398 20.604 40.913 HAC5 14.543 81.258 27.943 25.586 27.730 NAC2 15.949 149.953 55.492 53.245 41.216 NAC5 11.089 187.655 83.152 58.174 46.330 SAC2 38.737 62.437 25.276 4.825 32.336 SAC5 26.858 70.005 30.308 24.976 14.721 AAC2 50.072 57.208 15.389 18.105 23.713 AAC5 52.593 72.204 12.873 28.121 31.210 PAC 7.826 426.224 215.768 85.478 124.978 PAC + Ca 119.536 323.554 115.678 100.215 107.661 PAC + N 10.188 811.337 507.275 75.134 228.928 NaAC 2 64.854 42.374 10.211 14.898 17.265 NaAC5 64.906 49.902 15.252 19.873 14.778 CaAC2 57.783 42.374 10.204 14.869 17.300 CaAC5 62.553 34.874 7.702 19.912 7.259 SCaAC 83.983 37.348 2.685 17.386 17.277 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 (6409.231) and NaAC2 (5400.00). On the contrary, SAC5 (250.836) had the lowest C/S ratio, followed by 600 C char (299.083), SAC5 (406.511) and 900 C char (432.853). 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 33-39 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. 3.2.5 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

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.

Chin. J. Chem. Eng., Vol. 20, No. 3, June 2012 523 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, 133-139 (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, 147-165 (2005). 3 Radovic, L.R., Moreno-Castilla, C., Rivera-Utrilla, J., Carbon materials as adsorbents in aqueous solutions, Chem. Phys. Carbon, 27, 228-405 (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, 403-415 (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, 159-175 (1998). 7 Rodrίguez-Reinoso, F., Molina-Sabio, M., Textural and chemical characterization of microporous carbons, Adv. Colloid Interface Sci., 76-77, 271-294 (1998).