- Email: [email protected]
Chromium adsorption on high-performance activated carbons from aqueous solution
Separation and Purification Technology 31 (2003) 13 /18 www.elsevier.com/locate/seppur
Chromium adsorption on high-performance activated carbons from aqueous solution Zhonghua Hu *, Lin Lei, Yijiu Li, Yaming Ni Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, China
Abstract The adsorption of Cr(VI) from aqueous solutions by commercial and lab-made high surface area (HSA)-activated carbons was investigated. Physiochemical factors such as equilibrium time, temperature and solution pH that affect the magnitude of Cr(VI) adsorption were studied. The HSA-activated carbons showed high performance for Cr removal, and their adsorption capacity is several times larger than that of commercial carbons. Both micropores and mesopores have important contribution on the adsorption. However, desorption is more dependent on the mesoporosity of activated carbons. Therefore, regeneration is easier for the carbon with high mesoporosity. As a result, the adsorption capacity of mesoporous carbon could be recovered over 97%, whereas 54% for highly microporous carbon. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Activated carbon; Chromium; Adsorption; Regeneration; Wastewater
1. Introduction Chromium compounds are widely used in industry, such as electroplating, metal finishing, leather tanning, pigments, etc. However, the effluent of chromium is a big environmental problem, since chromium is recognized as being essential to human health. Cr(VI) compounds are much more toxic than Cr(III) ones. Adsorption is one of the effective techniques for chromium * Corresponding author. Tel.: /86-21-6598-2594; fax: /8621-6598-6104 E-mail address: [email protected] (Z. Hu).
removal from wastewater [1 /3]. Because of their high surface area (HSA), highly porous character and relative low-cost, activated carbons have been considered as potential adsorbents. Therefore, the use of activated carbons for removing chromium from wastewater has been received a great attention for decades. However, the application in this filed is not wide as expected, because it is enslaved to the adsorption capacity, service life and regeneration of activated carbon. In this paper, we try to seek high-performance activated carbon for chromium removal. The adsorption of chromium by microporous- and mesoporous-activated carbons from aqueous
1383-5866/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 4 9 - 1
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
14
solution was studied. The influences on the adsorption of Cr(VI) were investigated in terms of equilibrium time, adsorption temperature and solution pH.
2. Experimental Three commercial activated carbons FS-100 (Flitrasorb100, Calgon), GA-3 (Jiangxing Huaiyushan Activated Carbon Co. Ltd., China) and SHT (Shanghai Cocking & Chemical Co. Ltd., China) and four laboratory-made microporous and mesoporous HSA-activated carbons from coconut shell were chosen as testing adsorbents [4,5]. The nitrogen BET surface areas and pore volumes for each of the carbons, measured using an automatic adsorption instrument (Quantachrome Corp., NOVA-1000 Gas Sorption Analyzer), were given in Table 1 (samples GA-3 and SHT were not measured). K2Cr2O7 and Cr(NO3)3 ×/ 9H2O were of analytical reagent quality unless otherwise specified. Wastewater of Cr(VI) was from Shanghai Huatong Switch Factory. Adsorption experiments were carried out using the batch equilibration technique. Initial concentrations were prepared in the range 10/200 ppm. A series of 100 ml Erlenmeyer’s flasks containing 0.05 /0.2 g carbon sample and 100 ml solution were sealed at preset temperature until equilibrium was obtained. Then the adsorbents were removed by filtration. Adsorbate concentrations were measured by a direct ultraviolet absorbance method (Shimadzu UV-365 instrument).
Table 2 Time influence on the adsorption of Cr(VI) t (day)
Amount adsorbed, q (mg/g)
1 2 3 4 5
13.7 18.5 21.9 22.1 22.4
3. Results and discussion 3.1. Adsorption time influence Table 2 lists the influence of time on Cr(VI) adsorption (GA-3 sample 0.1 g, 50 ml Cr(VI) solution of initial concentration 50 mg/l). It is clear that time has evident influence on the adsorption during the first 3 days. However, the increase in adsorption is very small beyond 3 days. Therefore, the adsorption for 5 days could be considered almost reaches the equilibrium. The adsorption rate was much slower than expected. Like organics adsorption on carbon, the adsorbate is fast adsorbed in relative large pores, and slow entries into small micropores due to the diffusion effect. The adsorption experiments in the following text were conducted for 5 days, except indicated. 3.2. Solution pH influence Fig. 1 shows the pH influence on the adsorption. The experiments were conducted using metal and carbon of 50 ppm and 0.1 g (0.05 g for CZ-130 and CK-26), respectively. The uptake of Cr(VI) in-
Table 1 Porosity parameters of activated carbons Sample
S (m2/g)
Vme (cm3/g)
Vmi (cm3/g)
Vtot (cm3/g)
˚) D (A
CZ-105 CZ-130 CK-22 CK-26 FS-100
1874 2522 2450 2424 937
0.199 0.956 0.167 0.792 0.103
0.683 0.759 1.049 0.788 0.391
0.958 1.639 1.216 1.580 0.494
20.5 26.0 19.8 26.1 21.1
S , N2-BET surface area; Vme, mesopore volume; Vmi, micropore volume; Vtot, total pore volume; D , average pore diameter.
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
15
Fig. 1. Solution pH influence on the Cr(VI) adsorption.
creases with increasing pH from 2 to 3 (except for samples FS-100 and SHT) reach a maximum, and then decreases from pH 3 to 6.5. This indicates that the solution pH affects the Cr(VI) adsorption. An acidic solution at a pH value about 3 is an optimal condition for the adsorption of Cr(VI). 3.3. Temperature influence Fig. 2 shows the temperature influence on the adsorption of Cr(VI) and Cr(III) on sample GA-3. The uptake of both Cr(VI) and Cr(III) increases with increasing temperature, indicating that higher
Fig. 2. Temperature influence on the adsorption of Cr(VI) and Cr(III).
temperature favor the adsorption. It is different from the adsorption of organics from aqueous solution, in which activated carbon adsorb more organics at lower temperature than that at higher temperature. Because most organic molecules are non- or low-polar, the uptake of these species is of physical adsorption. However, Cr(VI) and Cr(III) are ionic species possessing negative or positive charge. Therefore, their adsorption mechanism is probably different from that of organics.
3.4. Adsorption isotherms of Cr(VI) Fig. 3 shows the adsorption isotherms of Cr(VI) on activated carbons. The experiments were conducted using metal and carbon of 25/500 ppm and 0.05 g, respectively, at pH 3.2. Lab-made activated carbons are significantly superior to commercial ones. The adsorption capacity is in the order CK22 / CZ-130 / CK-26 / CZ-105 / GA-3 / FS100/SHT. It was found that the isotherms of former four samples (Fig. 3A) fit well the Freundlich model (q /Ck1/n , where q is the amount adsorbed, C the equilibrium concentration of the adsorbate, k and n are constant), indicating that the adsorption amount increases significantly with increasing Cr(VI) equilibrium concentration.
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
16
Fig. 3. Adsorption isotherms of Cr(VI) on activated carbons. Table 3 Parameters of the Freundlich equation
Table 4 Parameters of the Langmuir equation
Sample
k
1/n
R2
Sample
b
Q0
bQ0
R2
CZ-105 CZ-130 CK-22 CK-26
40.4 44.9 47.4 45.6
0.200 0.230 0.225 0.208
0.998 0.992 0.994 0.989
FS-100 GA-3 SHT
0.151 0.143 0.108
69.3 101.4 69.1
10.5 14.6 7.48
0.993 0.996 0.995
The results imply that the lab-made activated carbons have much larger adsorption capacity than commercial ones. The parameters of the Freundlich are listed in Table 3. Both parameters k and 1/n affect the adsorption isotherm. The larger the k and 1/n value, the higher the adsorption capacity in the order CK-22 /CZ-130/CK26 /CZ-105. The adsorption capacity depends on the surface chemical and physical properties of the adsorbent, in which the porosity is one of the important factors. By comparing the adsorption capacity and corresponding pore volume in Table 1, it was found that the larger the micropore volume, the larger was the capacity. However, the four samples have an average pore diameter of ˚ . Thus, it could be speculated that 19.8 /26.1 A ˚ ) and small mesopores super-micropores (15 /20 A favor the adsorption of Cr(VI). The Cr(VI) adsorption isotherms of three commercial samples fit well the Langmuir model (1/ q /(1/Q0)/1/bQ0C , where Q0 is the saturated adsorption and b the constant related to the adsorption energy), suggesting that the adsorption almost reach saturation. As seen in Fig. 3, the amount adsorbed increases slightly at high equilibrium concentration of the adsorbate. The para-
meters of the Langmuir equation are listed in Table 4. The Langmuir constant b increases in the order FS-100 /GA-3/SHT. The larger the constant b , the higher is the adsorption energy, reflected by a fast increase in adsorption at low concentrations of adsorbate. bQ0 (the product of b and Q0) is the relative affinity of the adsorbates toward the surface of the adsorbent. So the adsorption capacity Q0 and the affinity of Cr(VI) toward the carbon surface bQ0 are in the order GA-3 /FS-100 /SHT.
3.5. Test of treating wastewater The original yellow wastewater of pH 3.76 contains Cr(VI) 36.8 ppm, Cr(III) 1.8 ppm and other ions, such as Mg2, Ca2, etc. Adsorption tests were conducted using carbon dosage of 0.05 g and 100 ml solution. The pH was adjusted to 3.10. Table 5 lists the treatment results of real wastewater. As the performance for the synthesis solution, the lab-made HSA-activated carbons are superior to the commercial samples, although the wastewaster is a complicated solution. It indicates that the existence of other matters in the solution did not significantly affect the perfor-
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
17
Table 5 Results of treatment of Cr wastewater by using activated carbons Carbon sample Color
pH
CZ-130 CZ-105 CK-26 CK-22 FS-100 GA-3 SHT
4.97 0.2 4.88 1.7 4.98 0.2 5.04 0.1 4.26 11 4.59 2.2 4.36 5.8
Slight buff Buff Slight buff Slight buff Yellow Slight yellow Yellow
Efficiency
Removed Cr Original Crtotal
Cr(VI) (ppm) Cr(III) (ppm) Crtotal (ppm) Cr(III)/Crtotal ( /100%) Q (mg/g) Efficiency (%) 2.0 2.8 2.0 1.6 2.5 7.6 4.6
2.2 4.5 2.2 1.7 13.5 9.8 14.3
90.7 61.8 90.9 93.9 18.2 78.5 32.3
72.9 68.3 72.8 73.9 50.3 57.7 48.7
94.4 88.5 94.3 95.7 65.1 74.7 63.1
100%:
mance of activated carbons. The efficiency of Cr removal was as high as 95.7%. 3.6. Regeneration of spent activated carbon Carbon samples saturated in Cr(VI) solution were dried at 110 8C, then soaking in H2SO4 of 15% solution for 24 h and washed with water. Then second-time adsorption was conducted under the same condition as previous. The results were listed in Table 6. q1 and q2 are fresh and recovery adsorption capacity, respectively. The recovery of CZ-105 and CK-22 is 88.8 and 53.6%, respectively, similar to that of the conventional activated carbon of 60 /90%. However, the recovery of CZ-130 and CK-26 is high as 97.6 and 97.3%, respectively, suggesting that the desorption
Table 6 Regeneration of activated carbon-adsorbed Cr(VI) Sample
q1 (mg/g)
q2 (mg/g)
Regeneration (%)
CZ-130 CZ-105 CK-26 CK-22
170.8 135.6 164.0 182.8
166.8 120.4 159.6 98.0
97.6 88.8 97.3 53.6
of Cr(VI) is much easier from mesopores than that from micropores of the activated carbon. It implies that mesoporous HSA-activated carbons are suitable for treating Cr(VI) wastewater, because they have not only large adsorption capacity, but also high recovery. Adsorbent is one of the main costs of the adsorption process. The high recovery could reduce significantly the total cost. Although microporous HSA-activated carbon, such as CK-22, has large capacity, its recovery is low, which limits the service life.
4. Conclusion The results demonstrated that the lab-made activated carbons are superior to commercial ones for the removal of Cr(VI) from aqueous solution. It was found that the porosity has a significant influence on the Cr(VI) adsorption. HSA-activated carbons could be very effective for the chromium removal. Pore diameter is an important factor for the adsorption kinetics and desorption. High mesoporosity makes chromium desorption easier, i.e. regeneration of spent activated carbon easier. Owing to their excellent performance, mesoporous HSA-activated carbons
18
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
have good prospect in the treatment of chromium wastewater.
Acknowledgements We acknowledge the financial support provided by the Science and Technology Development Foundation of Shanghai Universities through the project 99QF07 (China).
References [1] C.P. Huang, M.H. Wu, Water Res. 11 (1977) 673. [2] D. Aggarwal, M. Goyal, R.C. Banal, Carbon 37 (1999) 1989. [3] S.B. Lalvani, T. Wiltowski, A. Hubener, A. Weston, N. Mandich, Carbon 36 (1998) 1219. [4] Z. Hu, M.P. Srinivasan, Y. Ni, Adv. Mater. 12 (2000) 62. [5] Z. Hu, M.P. Srinivasan, Y. Ni, Carbon 39 (2001) 877.
Chromium adsorption on high-performance activated carbons from aqueous solution Zhonghua Hu *, Lin Lei, Yijiu Li, Yaming Ni Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, China
Abstract The adsorption of Cr(VI) from aqueous solutions by commercial and lab-made high surface area (HSA)-activated carbons was investigated. Physiochemical factors such as equilibrium time, temperature and solution pH that affect the magnitude of Cr(VI) adsorption were studied. The HSA-activated carbons showed high performance for Cr removal, and their adsorption capacity is several times larger than that of commercial carbons. Both micropores and mesopores have important contribution on the adsorption. However, desorption is more dependent on the mesoporosity of activated carbons. Therefore, regeneration is easier for the carbon with high mesoporosity. As a result, the adsorption capacity of mesoporous carbon could be recovered over 97%, whereas 54% for highly microporous carbon. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Activated carbon; Chromium; Adsorption; Regeneration; Wastewater
1. Introduction Chromium compounds are widely used in industry, such as electroplating, metal finishing, leather tanning, pigments, etc. However, the effluent of chromium is a big environmental problem, since chromium is recognized as being essential to human health. Cr(VI) compounds are much more toxic than Cr(III) ones. Adsorption is one of the effective techniques for chromium * Corresponding author. Tel.: /86-21-6598-2594; fax: /8621-6598-6104 E-mail address: [email protected] (Z. Hu).
removal from wastewater [1 /3]. Because of their high surface area (HSA), highly porous character and relative low-cost, activated carbons have been considered as potential adsorbents. Therefore, the use of activated carbons for removing chromium from wastewater has been received a great attention for decades. However, the application in this filed is not wide as expected, because it is enslaved to the adsorption capacity, service life and regeneration of activated carbon. In this paper, we try to seek high-performance activated carbon for chromium removal. The adsorption of chromium by microporous- and mesoporous-activated carbons from aqueous
1383-5866/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 4 9 - 1
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
14
solution was studied. The influences on the adsorption of Cr(VI) were investigated in terms of equilibrium time, adsorption temperature and solution pH.
2. Experimental Three commercial activated carbons FS-100 (Flitrasorb100, Calgon), GA-3 (Jiangxing Huaiyushan Activated Carbon Co. Ltd., China) and SHT (Shanghai Cocking & Chemical Co. Ltd., China) and four laboratory-made microporous and mesoporous HSA-activated carbons from coconut shell were chosen as testing adsorbents [4,5]. The nitrogen BET surface areas and pore volumes for each of the carbons, measured using an automatic adsorption instrument (Quantachrome Corp., NOVA-1000 Gas Sorption Analyzer), were given in Table 1 (samples GA-3 and SHT were not measured). K2Cr2O7 and Cr(NO3)3 ×/ 9H2O were of analytical reagent quality unless otherwise specified. Wastewater of Cr(VI) was from Shanghai Huatong Switch Factory. Adsorption experiments were carried out using the batch equilibration technique. Initial concentrations were prepared in the range 10/200 ppm. A series of 100 ml Erlenmeyer’s flasks containing 0.05 /0.2 g carbon sample and 100 ml solution were sealed at preset temperature until equilibrium was obtained. Then the adsorbents were removed by filtration. Adsorbate concentrations were measured by a direct ultraviolet absorbance method (Shimadzu UV-365 instrument).
Table 2 Time influence on the adsorption of Cr(VI) t (day)
Amount adsorbed, q (mg/g)
1 2 3 4 5
13.7 18.5 21.9 22.1 22.4
3. Results and discussion 3.1. Adsorption time influence Table 2 lists the influence of time on Cr(VI) adsorption (GA-3 sample 0.1 g, 50 ml Cr(VI) solution of initial concentration 50 mg/l). It is clear that time has evident influence on the adsorption during the first 3 days. However, the increase in adsorption is very small beyond 3 days. Therefore, the adsorption for 5 days could be considered almost reaches the equilibrium. The adsorption rate was much slower than expected. Like organics adsorption on carbon, the adsorbate is fast adsorbed in relative large pores, and slow entries into small micropores due to the diffusion effect. The adsorption experiments in the following text were conducted for 5 days, except indicated. 3.2. Solution pH influence Fig. 1 shows the pH influence on the adsorption. The experiments were conducted using metal and carbon of 50 ppm and 0.1 g (0.05 g for CZ-130 and CK-26), respectively. The uptake of Cr(VI) in-
Table 1 Porosity parameters of activated carbons Sample
S (m2/g)
Vme (cm3/g)
Vmi (cm3/g)
Vtot (cm3/g)
˚) D (A
CZ-105 CZ-130 CK-22 CK-26 FS-100
1874 2522 2450 2424 937
0.199 0.956 0.167 0.792 0.103
0.683 0.759 1.049 0.788 0.391
0.958 1.639 1.216 1.580 0.494
20.5 26.0 19.8 26.1 21.1
S , N2-BET surface area; Vme, mesopore volume; Vmi, micropore volume; Vtot, total pore volume; D , average pore diameter.
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
15
Fig. 1. Solution pH influence on the Cr(VI) adsorption.
creases with increasing pH from 2 to 3 (except for samples FS-100 and SHT) reach a maximum, and then decreases from pH 3 to 6.5. This indicates that the solution pH affects the Cr(VI) adsorption. An acidic solution at a pH value about 3 is an optimal condition for the adsorption of Cr(VI). 3.3. Temperature influence Fig. 2 shows the temperature influence on the adsorption of Cr(VI) and Cr(III) on sample GA-3. The uptake of both Cr(VI) and Cr(III) increases with increasing temperature, indicating that higher
Fig. 2. Temperature influence on the adsorption of Cr(VI) and Cr(III).
temperature favor the adsorption. It is different from the adsorption of organics from aqueous solution, in which activated carbon adsorb more organics at lower temperature than that at higher temperature. Because most organic molecules are non- or low-polar, the uptake of these species is of physical adsorption. However, Cr(VI) and Cr(III) are ionic species possessing negative or positive charge. Therefore, their adsorption mechanism is probably different from that of organics.
3.4. Adsorption isotherms of Cr(VI) Fig. 3 shows the adsorption isotherms of Cr(VI) on activated carbons. The experiments were conducted using metal and carbon of 25/500 ppm and 0.05 g, respectively, at pH 3.2. Lab-made activated carbons are significantly superior to commercial ones. The adsorption capacity is in the order CK22 / CZ-130 / CK-26 / CZ-105 / GA-3 / FS100/SHT. It was found that the isotherms of former four samples (Fig. 3A) fit well the Freundlich model (q /Ck1/n , where q is the amount adsorbed, C the equilibrium concentration of the adsorbate, k and n are constant), indicating that the adsorption amount increases significantly with increasing Cr(VI) equilibrium concentration.
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
16
Fig. 3. Adsorption isotherms of Cr(VI) on activated carbons. Table 3 Parameters of the Freundlich equation
Table 4 Parameters of the Langmuir equation
Sample
k
1/n
R2
Sample
b
Q0
bQ0
R2
CZ-105 CZ-130 CK-22 CK-26
40.4 44.9 47.4 45.6
0.200 0.230 0.225 0.208
0.998 0.992 0.994 0.989
FS-100 GA-3 SHT
0.151 0.143 0.108
69.3 101.4 69.1
10.5 14.6 7.48
0.993 0.996 0.995
The results imply that the lab-made activated carbons have much larger adsorption capacity than commercial ones. The parameters of the Freundlich are listed in Table 3. Both parameters k and 1/n affect the adsorption isotherm. The larger the k and 1/n value, the higher the adsorption capacity in the order CK-22 /CZ-130/CK26 /CZ-105. The adsorption capacity depends on the surface chemical and physical properties of the adsorbent, in which the porosity is one of the important factors. By comparing the adsorption capacity and corresponding pore volume in Table 1, it was found that the larger the micropore volume, the larger was the capacity. However, the four samples have an average pore diameter of ˚ . Thus, it could be speculated that 19.8 /26.1 A ˚ ) and small mesopores super-micropores (15 /20 A favor the adsorption of Cr(VI). The Cr(VI) adsorption isotherms of three commercial samples fit well the Langmuir model (1/ q /(1/Q0)/1/bQ0C , where Q0 is the saturated adsorption and b the constant related to the adsorption energy), suggesting that the adsorption almost reach saturation. As seen in Fig. 3, the amount adsorbed increases slightly at high equilibrium concentration of the adsorbate. The para-
meters of the Langmuir equation are listed in Table 4. The Langmuir constant b increases in the order FS-100 /GA-3/SHT. The larger the constant b , the higher is the adsorption energy, reflected by a fast increase in adsorption at low concentrations of adsorbate. bQ0 (the product of b and Q0) is the relative affinity of the adsorbates toward the surface of the adsorbent. So the adsorption capacity Q0 and the affinity of Cr(VI) toward the carbon surface bQ0 are in the order GA-3 /FS-100 /SHT.
3.5. Test of treating wastewater The original yellow wastewater of pH 3.76 contains Cr(VI) 36.8 ppm, Cr(III) 1.8 ppm and other ions, such as Mg2, Ca2, etc. Adsorption tests were conducted using carbon dosage of 0.05 g and 100 ml solution. The pH was adjusted to 3.10. Table 5 lists the treatment results of real wastewater. As the performance for the synthesis solution, the lab-made HSA-activated carbons are superior to the commercial samples, although the wastewaster is a complicated solution. It indicates that the existence of other matters in the solution did not significantly affect the perfor-
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
17
Table 5 Results of treatment of Cr wastewater by using activated carbons Carbon sample Color
pH
CZ-130 CZ-105 CK-26 CK-22 FS-100 GA-3 SHT
4.97 0.2 4.88 1.7 4.98 0.2 5.04 0.1 4.26 11 4.59 2.2 4.36 5.8
Slight buff Buff Slight buff Slight buff Yellow Slight yellow Yellow
Efficiency
Removed Cr Original Crtotal
Cr(VI) (ppm) Cr(III) (ppm) Crtotal (ppm) Cr(III)/Crtotal ( /100%) Q (mg/g) Efficiency (%) 2.0 2.8 2.0 1.6 2.5 7.6 4.6
2.2 4.5 2.2 1.7 13.5 9.8 14.3
90.7 61.8 90.9 93.9 18.2 78.5 32.3
72.9 68.3 72.8 73.9 50.3 57.7 48.7
94.4 88.5 94.3 95.7 65.1 74.7 63.1
100%:
mance of activated carbons. The efficiency of Cr removal was as high as 95.7%. 3.6. Regeneration of spent activated carbon Carbon samples saturated in Cr(VI) solution were dried at 110 8C, then soaking in H2SO4 of 15% solution for 24 h and washed with water. Then second-time adsorption was conducted under the same condition as previous. The results were listed in Table 6. q1 and q2 are fresh and recovery adsorption capacity, respectively. The recovery of CZ-105 and CK-22 is 88.8 and 53.6%, respectively, similar to that of the conventional activated carbon of 60 /90%. However, the recovery of CZ-130 and CK-26 is high as 97.6 and 97.3%, respectively, suggesting that the desorption
Table 6 Regeneration of activated carbon-adsorbed Cr(VI) Sample
q1 (mg/g)
q2 (mg/g)
Regeneration (%)
CZ-130 CZ-105 CK-26 CK-22
170.8 135.6 164.0 182.8
166.8 120.4 159.6 98.0
97.6 88.8 97.3 53.6
of Cr(VI) is much easier from mesopores than that from micropores of the activated carbon. It implies that mesoporous HSA-activated carbons are suitable for treating Cr(VI) wastewater, because they have not only large adsorption capacity, but also high recovery. Adsorbent is one of the main costs of the adsorption process. The high recovery could reduce significantly the total cost. Although microporous HSA-activated carbon, such as CK-22, has large capacity, its recovery is low, which limits the service life.
4. Conclusion The results demonstrated that the lab-made activated carbons are superior to commercial ones for the removal of Cr(VI) from aqueous solution. It was found that the porosity has a significant influence on the Cr(VI) adsorption. HSA-activated carbons could be very effective for the chromium removal. Pore diameter is an important factor for the adsorption kinetics and desorption. High mesoporosity makes chromium desorption easier, i.e. regeneration of spent activated carbon easier. Owing to their excellent performance, mesoporous HSA-activated carbons
18
Z. Hu et al. / Separation and Purification Technology 31 (2003) 13 /18
have good prospect in the treatment of chromium wastewater.
Acknowledgements We acknowledge the financial support provided by the Science and Technology Development Foundation of Shanghai Universities through the project 99QF07 (China).
References [1] C.P. Huang, M.H. Wu, Water Res. 11 (1977) 673. [2] D. Aggarwal, M. Goyal, R.C. Banal, Carbon 37 (1999) 1989. [3] S.B. Lalvani, T. Wiltowski, A. Hubener, A. Weston, N. Mandich, Carbon 36 (1998) 1219. [4] Z. Hu, M.P. Srinivasan, Y. Ni, Adv. Mater. 12 (2000) 62. [5] Z. Hu, M.P. Srinivasan, Y. Ni, Carbon 39 (2001) 877.