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A comparison of the sorbtive characteristics of leaf mould and activated carbon columns for the removal of hexavalent chromium
ProcessBiochemist~ Vol. 31, No. 3, pp. 213-218, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-9592/96 S15.00 + 0.00 ELSEVIER
0032-9592(95)00049-6
A Comparison of the Sorbtive Characteristics of Leaf Mould and Activated Carbon Columns for the Removal of Hexavalent Chromium D. C. Sharma & C. E Forster* School of Civil Engineering,BirminghamUniversity,Edgbaston, BirminghamB 15 2TT, UK (Received 9 January 1995; accepted 3 June 1995)
Previous batch studies have shown that leaf mould is a potentially useful biosorbent for the treatment of wastewaters contaminated with hexavalent chromium. This paper examines its use for continuous adsorption in columns and compares the results with a parallel study using an activated carbon column. Both columns were operated at a pH of 2"5 and a flow-rate of 74 ml/min. The results show that, although the Cr(VI) adsorption capacities of the leaf mould were only 25"9 mg/g compared with a value of 75"6 mg/g for the activated carbon, the leaf mould caused little or no reduction and produced an effluent with very low concentration of trivalent chromium. The activated carbon, on the other hand, had a high reducing action. The data were also tested against the Bed-DepthService-Time model and it was found that, although the compliance was nonlinear, the model was appropriate for design purposes.
INTRODUCTION
energy and chemical requirement and removal efficiencies are not always good.1 Adsorption is a process which has been examined as an alternative technology and activated carbon is usually considered to be the adsorbent against which others are assessed, z,3 However, the costs of the activated carbon can be high and in recent years there have been many studies into the acceptability and applicability of low-cost adsorbents. In the main, the materials considered for this role have been either wastes or naturally occurring and readily available. They have included lignin, 4 tea leaves, 5 coconut husk fibre and palm pressed leaves 6 and peat. 7 Leaf mould has also been examined. 8 Much of this work has been done as batch studies and it is necessary to examine any potential adsorbent in continuous operation
Chromium is one of the so-called toxic heavy metals ~ which can be present in industrial wastewaters. It is an element which can exist in several oxidation states, the most common are the trivalent and hexavalent forms, with the latter being the more toxic. Numerous methods exist for removing heavy metals from aqueous solutions and the most common technique for treating chromium-rich wastewaters is to effect a reduction of the hexavalent form using, for example, sulphur dioxide, so that the chromium can be removed as Cr(III) by precipitation at p H > 8.0. However, this type of technology has a high *To whom correspondence should be addressed. 213
D. C. Sharma, C. F. Forster
214
before its potential for wastewater treatment can be properly assessed. This paper, therefore, reports the use of leaf mould for the continuous removal of hexavalent chromium by column adsorption and compares it with a parallel study using an activated carbon column.
Adsorption columns The colunms used in this study are shown schematically in Fig. 1. They had an internal diameter of 5 cm, an overall height of 1 m and were constructed so that samples could be withdrawn at several points (10, 22 and 35 cm). The work was done in a constant temperature room (25°C). Two columns were used, one packed with leaf mould (220 g), the other with activated carbon (360 g). In both cases, the packed bed height was 35 cm. A layer of gravel (2 cm) at the bottom of each column acted as a support for the absorbent and a second layer (5 cm) at the top of the beds prevented any loss of material from the system. The feed was prepared in a storage tank (1000 litre) with an initial chromium concentration (Co) of 100 mg/litre and a pH (pHin) of 2.5. This was pumped continuously to the constanthead tank which was used to feed the columns using a peristaltic pump (Watson Marlow, 601). The flow-rate through both the columns was maintained at 75 ml/min. Samples were taken at each of the sampling points on the columns, filtered and analysed for pH, hexavalent and total chromium. The effluents from the columns were combined and passed through a polishing unit (1.5 m x 10 cm diameter) packed with activated carbon and peat. This was done to prevent any chromium being discharged into the sewerage system.
MATERIALS AND METHODS Materials The activated carbon (Filtrasorb-400, Chemviron Ltd) had a mean particle diameter of 0.9-1.1 mm, a specific surface of 1050-1200 mm2/g and an effective pore size of 5.5-6.5 A. The leaf mould was obtained from a deciduous woodland area near Birmingham. 8 Prior to use, it was dried (105°C) and sieved (BS410/43, mesh no. 14). All the chemicals were analytical grade. Potassium dichromate was used as the source of hexavalent chromium and pH adjustments were made with concentrated sulphuric acid. Hexavalent chromium was measured colorimetrically using a 1,5diphenyl carbazide method. 9 Total chromium concentrations were measured by oxidizing any trivalent chromium with potassium permanganate and then remeasuring the hexavalent chromium concentration. The difference between the two values was taken as the trivalent chromium concentration.
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Removal of hexavalent chromium by leaf mould RESULTS AND DISCUSSION Activated carbon The breakthrough curves for Cr(VI) at the three bed heights (H1--10; H2 = 22; H3--35 cm) are given in Fig. 2 and show that the overall system was capable of treating some 400 litres before any breakthrough occurred. The data also show that there was some initial leakage of chromium ions (3 mg/litre) through the 10 cm bed indicating that, at the flow-rates used, the bed height should be greater than 10 cm if a complete removal of chromium ions is required. Bowers & Huang ~° have also reported that there can be an initial leakage of Cr(VI) with activated carbon and have suggested that this is due to pH values being greater than 4 at the onset of operation. They also suggested that this problem could be overcome by pre-treating the column with an acid (pH 2.5) wash. Two mechanisms have been postulated for the removal of hexavalent chromium by activated carbon;
• adsorption within the micropores in the carbon;= • reduction of Cr(VI) to Cr(III). 12 The reduction process is the more dominant at pH values below 4. Figure 3 shows the concentrations of trivalent chromium in the column effluent at the three bed heights. These results clearly confirm that a significant amount of reduction does occur within the bed. However, the data also
show that, at each height, there is a peak value and that trivalent chromium ion concentrations do ultimately decrease. This suggests that there is a finite amount of material in or on the activated carbon which is capable of reducing the hexavalent chromium. This concept is supported by the fact that the peak values occurred after different time intervals. Taken together, the results mean that, although there was an immediate breakthrough of total chromium (Fig. 4), there was an appreciable removal overall. The treatment
50-
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Fig. 3. Concentrations of trivalent chromium in the activated carbon column effluent.
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Fig. 2. Breakthrough curves for the activated carbon/ hexavalent chromium system.
Fig. 4. Breakthrough curves for the activated carbon/total chromium system.
216
D. C. Sharma, C. F. Forster
capacities, calculated from the breakthrough curves and based on both adsorption (Xads) and reduction (Xred), were 75.6 and 63.5 mg Cr(VI)/g carbon, respectively. This indicates that the two processes were of approximately equal significance. Such a high level of Cr(VI) reduction is possibly due to the high flow-rate being used. Earlier studies using peat columns have shown that increasing the flow-rate does enhance the reduction process, 13 with a (ot=Xred/Xads) increasing from 0.053 to 0.116 when the flow-rate was increased from 60 to 120 ml/min.
Leaf mould The breakthrough curves for the leaf mould/ hexavalent chromium column are presented in Fig. 5 and show, as was found in the batch tests, 8 that leaf mould is not as good an adsorbent as activated carbon. Breakthrough occurred immediately and 50% exhaustion occurred after the passage of about 60 litres. An examination of the concentrations of trivalent chromium (Fig. 6) show that the reduction process, which was so significant in the activated carbon column, did not occur to any appreciable extent, the maximum concentration being only 9.5 mg/litre. This implies that the leaf mould surface contains little or no material which is capable of being oxidized. The adsorption capacity, calculated from the breakthrough curves, was 25-9 mg/g and the avalue was 0.186.
BDST equation The Bed Depth Service Time (BDST) model, which is a modified form of the Bohart and Adams model, 6,14,1s is widely used to compare adsorption columns. Thus, the service time of a column (t) is given by: t = n ( N o / C o U) - ( 1/kCo) In [( Co/Ct)- 1 ],
where t is service time to breakthrough (h), No is adsorption capacity (mg solute/litre adsorbent); CO is initial solute concentration (mg/litre); C t is effluent solute concentration (mg/litre); U is linear flow-rate (m/h); H is depth of adsorbent bed (m); k is adsorption rate constant (litre/mgxh). At 50% breakthrough, (Co/Ct) = 2 and the logarithmic term in the equation is reduced to zero. The equation can then be expressed as; t = 50 = n ( N o / C OU). Thus, a 50% breakthrough curve (time/bed depth) ought to produce a straight line passing through the origin. As such, this can be used to test the applicability of the BDST model:, ~3 Breakthrough times for 50% adsorption for activated carbon and leaf mould are shown in Fig. 7 in relation to bed depth and the regression constants for these relationships are given in Table 1. These results show that the 50% breakthrough lines do not pass through the origin and that, therefore, the adsorption of Cr(VI) ions does not conform to the BDST model. This nonconformity has been reported previously for peat/
90-
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Fig. 5. Breakthrough curves for the leaf mould/hexavalent chromium system.
Fig. 6. Concentrationsof trivalent chromium in the leaf mould columneffluent.
Removal of hexavalent chromium by leaf mould hexavalent chromium 13 and for the adsorption of Cr(VI) ions by coconut husk fibres and palm pressed leaves.6 The BDST model has also been used in its more complete form to examine adsorption with the reciprocal of the slope of the bed height/time line being used to define the rate of bed exhaustion. ~5 The results of regression analysis for 30 and 70% adsorption are given in Table 1 and Fig. 7. These show that, used in this way, the BDST model does provide a realistic description of the adsorption of Cr(VI) onto activated carbon, leaf mould and peat and that,
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217
therefore, the model and the appropriate regression constants can be used for the design of largescale columns.
Comparisons with other studies In order to assess the viability of a treatment process based on adsorption, the column capacities of the test adsorbents must be compared with those of other adsorbents tested under similar conditions (Table 2). Using a similar activated carbon, Filtrasorb-400 (Calgon type), Kim & Zoltek 2 reported that the optimum removal occurred when the proton:Cr(VI) was 1:1. They also reported a column capacity of 125 mg/g using an initial Cr(VI) concentration of 104 mg/litre and a pH of 2.7. On the other hand, Bowers & Huang ~° concluded that the maximum adsorption on Filtrasorb-400 occurred at a pH of 2.5. They also argued that the 1:1 requirement for optimum uptake was not a general condition and was only applicable at pH values greater than 4-0, which is not a reducing condition. At a low-rate of 44 rnl/ min, the maximum adsorption reported by these workers was 70 mg/g. Column tests using an activated carbon based on coconut shell as the adsorbent, ~6 showed that the overall removal capacity, at a pH of 2.5 and an initial concentration of 50 mg Cr(VI)/litre, was 20 mg/g. Table 2 also gives the column capacities for peat which were derived in an earlier study by Sharma & Forster? 3 At first sight, an examination of Table 2 would
Table 1. Regression constants for the BDST relationships
Breakthrough
(%)
30 50 70
Activated carbon
Leaf mould
Peat"
Slope
Intercept
Slope
Intercept
Slope
Intercept
0"884 2"968 4"176
1"3 1"96 3"66
0"15
-0"712
0"225
- 1"25
"At pH 2.5 and flow-rate = 60 ml/min. Taken from Sharma & Forster) 3 Table 2. A comparison of column capacities
Source Kim & Zoltek 2 Bowers & Huang "~ Alaerts et a1.16 Sharma & Forster t3 This study
Adsorbent Activated carbon Activated carbon Coconut shell carbon Peat (pH 2; 80 ml/min) (pH 2"5; 120 ml/min) Leaf mould Activated carbon
Column capacity lmg Cr(Vl)/gI 123 70 20 65"9 38"2 25.9 75.6
218
D. C. Sharma, C. F. Forster
suggest that leaf mould was not very efficient as an adsorbent. However, it is comparable with the activated carbon produced from coconut shells and with peat being used at high flow-rates. Furthermore, leaf mould does have the advantage of producing an effluent which contains low concentrations of trivalent chromium and, therefore, no secondary treatment should be required. This is not the case with adsorbents which possess highly active reducing groups.
CONCLUSIONS The results show that the uptake of Cr(VI) by activated carbon is a rapid process whereas natural biosorbents, such as leaf mould and moss peat, adsorb these ions more slowly. This is mainly due to the micropores in the carbon. The activated carbon also contains reducing components which are highly active. By comparison, at pH 2.5, any organic material in natural biosorbents is less active. This means that the effluent produced from a biosorbent column will contain a lower concentration of trivalent chromium than that produced from an activated carbon column. Because of this it must be said that there is a very real role for the use of biosorbents in the treatment of wastewaters contaminated with hexavalent chromium. The adsorption data also show that the BDST model can be used for scale-up design.
REFERENCES 1. Davila, J. S., Matos, C. M. & Cavalcanti, M. R., Heavy metals removal from wastewater by using activated peat.
WaterSci. Technol., 26 (1992) 2309-12. 2. Kim, I. J. & Zoitek, J. C., Chromium removal with activated carbon. Progr. Water Technol., 9 (1977) 143-55. 3. Huang, C. P. & Bowers, A. R., The use of activated carbon for Cr(VI) removal. Progr. Water Technol., 10 (1978) 45-64. 4. Srivastava, S. K., Singh, A. K. & Sharma, A., Studies on the uptake of lead and zinc by lignin obtained from black liquor -- a paper industry waste material. Environ. Technol., 15 (1994) 353-61. 5. Tan, T. W. & Khan, A. R. M., Removal of lead, cadmium and zinc by waste tea leaves. Environ. Technol. Lett., 9 (1988) 1223-32. 6. Tan, T. W., Ooi, S. T. & Lee, C. K., Removal of Cr(VI) from solution by coconut husk and palm pressed fibres. Environ. Technol., 14 (1993) 277-82. 7. Allen, S. J., Murray, M., Brown, P. & Flynn, O., Peat as an adsorbent for dyestuffs and metals in wastewater. Resources, Conservation Recycling, 11 (1994)25-39. 8. Sharma, D. C. & Forster, C. F., The treatment of chromium wastewaters using the sorptive potential of leaf mould. Biores. Technol., 49 (1994) 31-40. 9. Anon, Standard Methods for the Examination of Water and Wastewater, 16th Edn. APHA, AWWA, WPCF, Washington DC, USA. 10. Bowers, A. R. & Huang, C. P., Activated carbon processes for the treatment of Cr(VI)-containing industrial wastewaters. Water Sci. Technol., 13 ( 1981) 629-50. 11. Huang, C. P. & Blankenship, D. W., The removal of mercury(II) from dilute solutions by activated carbon. Water Res., 18 (1984) 37-46. 12. Huang, C. P. & Wu, M. H., The removal of Cr(VI) from dilute aqueous solutions by activated carbon. Water Res., 11 (1977)673-9. 13. Sharma, D. C. & Forster, C. F., Column studies into the adsorption of chromium(VI) using sphagnum moss peat. Biores. Technol., 52 (1995) 2449-54. 14. Bohart, G. S. & Adams, E. Q., Some aspects of the behaviour of charcoal with respect to chlorine. J. Am. Chem. Soc., 42 (1920) 523-9. 15. Muraleedharan, T. R., Ligy, P., lyengar, L. & Venkobacher, C., Application studies of biosorption for monozite processing industry effluents. Biores. Technol., 49 (1994) 179-86. 16. Alaerts, G. J., Jitjaturunt, V. & Kelderman, P., Use of coconut shell based activated carbon for chromium (VI) removal. WaterSci. Technol., 21 (1989) 1701-4.
0032-9592(95)00049-6
A Comparison of the Sorbtive Characteristics of Leaf Mould and Activated Carbon Columns for the Removal of Hexavalent Chromium D. C. Sharma & C. E Forster* School of Civil Engineering,BirminghamUniversity,Edgbaston, BirminghamB 15 2TT, UK (Received 9 January 1995; accepted 3 June 1995)
Previous batch studies have shown that leaf mould is a potentially useful biosorbent for the treatment of wastewaters contaminated with hexavalent chromium. This paper examines its use for continuous adsorption in columns and compares the results with a parallel study using an activated carbon column. Both columns were operated at a pH of 2"5 and a flow-rate of 74 ml/min. The results show that, although the Cr(VI) adsorption capacities of the leaf mould were only 25"9 mg/g compared with a value of 75"6 mg/g for the activated carbon, the leaf mould caused little or no reduction and produced an effluent with very low concentration of trivalent chromium. The activated carbon, on the other hand, had a high reducing action. The data were also tested against the Bed-DepthService-Time model and it was found that, although the compliance was nonlinear, the model was appropriate for design purposes.
INTRODUCTION
energy and chemical requirement and removal efficiencies are not always good.1 Adsorption is a process which has been examined as an alternative technology and activated carbon is usually considered to be the adsorbent against which others are assessed, z,3 However, the costs of the activated carbon can be high and in recent years there have been many studies into the acceptability and applicability of low-cost adsorbents. In the main, the materials considered for this role have been either wastes or naturally occurring and readily available. They have included lignin, 4 tea leaves, 5 coconut husk fibre and palm pressed leaves 6 and peat. 7 Leaf mould has also been examined. 8 Much of this work has been done as batch studies and it is necessary to examine any potential adsorbent in continuous operation
Chromium is one of the so-called toxic heavy metals ~ which can be present in industrial wastewaters. It is an element which can exist in several oxidation states, the most common are the trivalent and hexavalent forms, with the latter being the more toxic. Numerous methods exist for removing heavy metals from aqueous solutions and the most common technique for treating chromium-rich wastewaters is to effect a reduction of the hexavalent form using, for example, sulphur dioxide, so that the chromium can be removed as Cr(III) by precipitation at p H > 8.0. However, this type of technology has a high *To whom correspondence should be addressed. 213
D. C. Sharma, C. F. Forster
214
before its potential for wastewater treatment can be properly assessed. This paper, therefore, reports the use of leaf mould for the continuous removal of hexavalent chromium by column adsorption and compares it with a parallel study using an activated carbon column.
Adsorption columns The colunms used in this study are shown schematically in Fig. 1. They had an internal diameter of 5 cm, an overall height of 1 m and were constructed so that samples could be withdrawn at several points (10, 22 and 35 cm). The work was done in a constant temperature room (25°C). Two columns were used, one packed with leaf mould (220 g), the other with activated carbon (360 g). In both cases, the packed bed height was 35 cm. A layer of gravel (2 cm) at the bottom of each column acted as a support for the absorbent and a second layer (5 cm) at the top of the beds prevented any loss of material from the system. The feed was prepared in a storage tank (1000 litre) with an initial chromium concentration (Co) of 100 mg/litre and a pH (pHin) of 2.5. This was pumped continuously to the constanthead tank which was used to feed the columns using a peristaltic pump (Watson Marlow, 601). The flow-rate through both the columns was maintained at 75 ml/min. Samples were taken at each of the sampling points on the columns, filtered and analysed for pH, hexavalent and total chromium. The effluents from the columns were combined and passed through a polishing unit (1.5 m x 10 cm diameter) packed with activated carbon and peat. This was done to prevent any chromium being discharged into the sewerage system.
MATERIALS AND METHODS Materials The activated carbon (Filtrasorb-400, Chemviron Ltd) had a mean particle diameter of 0.9-1.1 mm, a specific surface of 1050-1200 mm2/g and an effective pore size of 5.5-6.5 A. The leaf mould was obtained from a deciduous woodland area near Birmingham. 8 Prior to use, it was dried (105°C) and sieved (BS410/43, mesh no. 14). All the chemicals were analytical grade. Potassium dichromate was used as the source of hexavalent chromium and pH adjustments were made with concentrated sulphuric acid. Hexavalent chromium was measured colorimetrically using a 1,5diphenyl carbazide method. 9 Total chromium concentrations were measured by oxidizing any trivalent chromium with potassium permanganate and then remeasuring the hexavalent chromium concentration. The difference between the two values was taken as the trivalent chromium concentration.
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Schematic diagram of the column system.
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Removal of hexavalent chromium by leaf mould RESULTS AND DISCUSSION Activated carbon The breakthrough curves for Cr(VI) at the three bed heights (H1--10; H2 = 22; H3--35 cm) are given in Fig. 2 and show that the overall system was capable of treating some 400 litres before any breakthrough occurred. The data also show that there was some initial leakage of chromium ions (3 mg/litre) through the 10 cm bed indicating that, at the flow-rates used, the bed height should be greater than 10 cm if a complete removal of chromium ions is required. Bowers & Huang ~° have also reported that there can be an initial leakage of Cr(VI) with activated carbon and have suggested that this is due to pH values being greater than 4 at the onset of operation. They also suggested that this problem could be overcome by pre-treating the column with an acid (pH 2.5) wash. Two mechanisms have been postulated for the removal of hexavalent chromium by activated carbon;
• adsorption within the micropores in the carbon;= • reduction of Cr(VI) to Cr(III). 12 The reduction process is the more dominant at pH values below 4. Figure 3 shows the concentrations of trivalent chromium in the column effluent at the three bed heights. These results clearly confirm that a significant amount of reduction does occur within the bed. However, the data also
show that, at each height, there is a peak value and that trivalent chromium ion concentrations do ultimately decrease. This suggests that there is a finite amount of material in or on the activated carbon which is capable of reducing the hexavalent chromium. This concept is supported by the fact that the peak values occurred after different time intervals. Taken together, the results mean that, although there was an immediate breakthrough of total chromium (Fig. 4), there was an appreciable removal overall. The treatment
50-
Flow - 75ml/min Adsorbent = 360g nH,.j - 2.s
/
, ~
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40-
g ~
30-
~
20-
o
10-
1~o
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Fig. 3. Concentrations of trivalent chromium in the activated carbon column effluent.
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x 40-
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10.
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~'~_ 100
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400
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=
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200
300
400
500
, 600
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Fig. 2. Breakthrough curves for the activated carbon/ hexavalent chromium system.
Fig. 4. Breakthrough curves for the activated carbon/total chromium system.
216
D. C. Sharma, C. F. Forster
capacities, calculated from the breakthrough curves and based on both adsorption (Xads) and reduction (Xred), were 75.6 and 63.5 mg Cr(VI)/g carbon, respectively. This indicates that the two processes were of approximately equal significance. Such a high level of Cr(VI) reduction is possibly due to the high flow-rate being used. Earlier studies using peat columns have shown that increasing the flow-rate does enhance the reduction process, 13 with a (ot=Xred/Xads) increasing from 0.053 to 0.116 when the flow-rate was increased from 60 to 120 ml/min.
Leaf mould The breakthrough curves for the leaf mould/ hexavalent chromium column are presented in Fig. 5 and show, as was found in the batch tests, 8 that leaf mould is not as good an adsorbent as activated carbon. Breakthrough occurred immediately and 50% exhaustion occurred after the passage of about 60 litres. An examination of the concentrations of trivalent chromium (Fig. 6) show that the reduction process, which was so significant in the activated carbon column, did not occur to any appreciable extent, the maximum concentration being only 9.5 mg/litre. This implies that the leaf mould surface contains little or no material which is capable of being oxidized. The adsorption capacity, calculated from the breakthrough curves, was 25-9 mg/g and the avalue was 0.186.
BDST equation The Bed Depth Service Time (BDST) model, which is a modified form of the Bohart and Adams model, 6,14,1s is widely used to compare adsorption columns. Thus, the service time of a column (t) is given by: t = n ( N o / C o U) - ( 1/kCo) In [( Co/Ct)- 1 ],
where t is service time to breakthrough (h), No is adsorption capacity (mg solute/litre adsorbent); CO is initial solute concentration (mg/litre); C t is effluent solute concentration (mg/litre); U is linear flow-rate (m/h); H is depth of adsorbent bed (m); k is adsorption rate constant (litre/mgxh). At 50% breakthrough, (Co/Ct) = 2 and the logarithmic term in the equation is reduced to zero. The equation can then be expressed as; t = 50 = n ( N o / C OU). Thus, a 50% breakthrough curve (time/bed depth) ought to produce a straight line passing through the origin. As such, this can be used to test the applicability of the BDST model:, ~3 Breakthrough times for 50% adsorption for activated carbon and leaf mould are shown in Fig. 7 in relation to bed depth and the regression constants for these relationships are given in Table 1. These results show that the 50% breakthrough lines do not pass through the origin and that, therefore, the adsorption of Cr(VI) ions does not conform to the BDST model. This nonconformity has been reported previously for peat/
90-
10 / 91
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Flow - 75ml/min Adsorbent = 220g
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Flow - 75ml/min Adsorbent - 220g PHadj - 2.5 Co - 100mg/I
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120
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Volume Treated (I)
Fig. 5. Breakthrough curves for the leaf mould/hexavalent chromium system.
Fig. 6. Concentrationsof trivalent chromium in the leaf mould columneffluent.
Removal of hexavalent chromium by leaf mould hexavalent chromium 13 and for the adsorption of Cr(VI) ions by coconut husk fibres and palm pressed leaves.6 The BDST model has also been used in its more complete form to examine adsorption with the reciprocal of the slope of the bed height/time line being used to define the rate of bed exhaustion. ~5 The results of regression analysis for 30 and 70% adsorption are given in Table 1 and Fig. 7. These show that, used in this way, the BDST model does provide a realistic description of the adsorption of Cr(VI) onto activated carbon, leaf mould and peat and that,
140-
120-
100 -
t .8_
80-
60-
40-
20-
& 0~
lo
;s
2'0
Leaf Mould 50%
2's
~
_. I
3's
Bed Height (cm)
Fig. 7.
BDST curves at various breakthrough levels.
217
therefore, the model and the appropriate regression constants can be used for the design of largescale columns.
Comparisons with other studies In order to assess the viability of a treatment process based on adsorption, the column capacities of the test adsorbents must be compared with those of other adsorbents tested under similar conditions (Table 2). Using a similar activated carbon, Filtrasorb-400 (Calgon type), Kim & Zoltek 2 reported that the optimum removal occurred when the proton:Cr(VI) was 1:1. They also reported a column capacity of 125 mg/g using an initial Cr(VI) concentration of 104 mg/litre and a pH of 2.7. On the other hand, Bowers & Huang ~° concluded that the maximum adsorption on Filtrasorb-400 occurred at a pH of 2.5. They also argued that the 1:1 requirement for optimum uptake was not a general condition and was only applicable at pH values greater than 4-0, which is not a reducing condition. At a low-rate of 44 rnl/ min, the maximum adsorption reported by these workers was 70 mg/g. Column tests using an activated carbon based on coconut shell as the adsorbent, ~6 showed that the overall removal capacity, at a pH of 2.5 and an initial concentration of 50 mg Cr(VI)/litre, was 20 mg/g. Table 2 also gives the column capacities for peat which were derived in an earlier study by Sharma & Forster? 3 At first sight, an examination of Table 2 would
Table 1. Regression constants for the BDST relationships
Breakthrough
(%)
30 50 70
Activated carbon
Leaf mould
Peat"
Slope
Intercept
Slope
Intercept
Slope
Intercept
0"884 2"968 4"176
1"3 1"96 3"66
0"15
-0"712
0"225
- 1"25
"At pH 2.5 and flow-rate = 60 ml/min. Taken from Sharma & Forster) 3 Table 2. A comparison of column capacities
Source Kim & Zoltek 2 Bowers & Huang "~ Alaerts et a1.16 Sharma & Forster t3 This study
Adsorbent Activated carbon Activated carbon Coconut shell carbon Peat (pH 2; 80 ml/min) (pH 2"5; 120 ml/min) Leaf mould Activated carbon
Column capacity lmg Cr(Vl)/gI 123 70 20 65"9 38"2 25.9 75.6
218
D. C. Sharma, C. F. Forster
suggest that leaf mould was not very efficient as an adsorbent. However, it is comparable with the activated carbon produced from coconut shells and with peat being used at high flow-rates. Furthermore, leaf mould does have the advantage of producing an effluent which contains low concentrations of trivalent chromium and, therefore, no secondary treatment should be required. This is not the case with adsorbents which possess highly active reducing groups.
CONCLUSIONS The results show that the uptake of Cr(VI) by activated carbon is a rapid process whereas natural biosorbents, such as leaf mould and moss peat, adsorb these ions more slowly. This is mainly due to the micropores in the carbon. The activated carbon also contains reducing components which are highly active. By comparison, at pH 2.5, any organic material in natural biosorbents is less active. This means that the effluent produced from a biosorbent column will contain a lower concentration of trivalent chromium than that produced from an activated carbon column. Because of this it must be said that there is a very real role for the use of biosorbents in the treatment of wastewaters contaminated with hexavalent chromium. The adsorption data also show that the BDST model can be used for scale-up design.
REFERENCES 1. Davila, J. S., Matos, C. M. & Cavalcanti, M. R., Heavy metals removal from wastewater by using activated peat.
WaterSci. Technol., 26 (1992) 2309-12. 2. Kim, I. J. & Zoitek, J. C., Chromium removal with activated carbon. Progr. Water Technol., 9 (1977) 143-55. 3. Huang, C. P. & Bowers, A. R., The use of activated carbon for Cr(VI) removal. Progr. Water Technol., 10 (1978) 45-64. 4. Srivastava, S. K., Singh, A. K. & Sharma, A., Studies on the uptake of lead and zinc by lignin obtained from black liquor -- a paper industry waste material. Environ. Technol., 15 (1994) 353-61. 5. Tan, T. W. & Khan, A. R. M., Removal of lead, cadmium and zinc by waste tea leaves. Environ. Technol. Lett., 9 (1988) 1223-32. 6. Tan, T. W., Ooi, S. T. & Lee, C. K., Removal of Cr(VI) from solution by coconut husk and palm pressed fibres. Environ. Technol., 14 (1993) 277-82. 7. Allen, S. J., Murray, M., Brown, P. & Flynn, O., Peat as an adsorbent for dyestuffs and metals in wastewater. Resources, Conservation Recycling, 11 (1994)25-39. 8. Sharma, D. C. & Forster, C. F., The treatment of chromium wastewaters using the sorptive potential of leaf mould. Biores. Technol., 49 (1994) 31-40. 9. Anon, Standard Methods for the Examination of Water and Wastewater, 16th Edn. APHA, AWWA, WPCF, Washington DC, USA. 10. Bowers, A. R. & Huang, C. P., Activated carbon processes for the treatment of Cr(VI)-containing industrial wastewaters. Water Sci. Technol., 13 ( 1981) 629-50. 11. Huang, C. P. & Blankenship, D. W., The removal of mercury(II) from dilute solutions by activated carbon. Water Res., 18 (1984) 37-46. 12. Huang, C. P. & Wu, M. H., The removal of Cr(VI) from dilute aqueous solutions by activated carbon. Water Res., 11 (1977)673-9. 13. Sharma, D. C. & Forster, C. F., Column studies into the adsorption of chromium(VI) using sphagnum moss peat. Biores. Technol., 52 (1995) 2449-54. 14. Bohart, G. S. & Adams, E. Q., Some aspects of the behaviour of charcoal with respect to chlorine. J. Am. Chem. Soc., 42 (1920) 523-9. 15. Muraleedharan, T. R., Ligy, P., lyengar, L. & Venkobacher, C., Application studies of biosorption for monozite processing industry effluents. Biores. Technol., 49 (1994) 179-86. 16. Alaerts, G. J., Jitjaturunt, V. & Kelderman, P., Use of coconut shell based activated carbon for chromium (VI) removal. WaterSci. Technol., 21 (1989) 1701-4.