Ind. Eng. Chem. Res. 2002, 41, 5085-5093
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Uptake of Heavy Metals in Batch Systems by Sulfurized Steam Activated Carbon Prepared from Sugarcane Bagasse Pith K. Anoop Krishnan and T. S. Anirudhan* Department of Chemistry, University of Kerala, Kariavattom, Thiruvananthapuram 695 581, India
In this work, sorption of Pb(II), Hg(II), Cd(II), and Co(II) on sulfurized steam activated carbon (SSAC) has been studied by using a batch technique. The SSAC has been prepared from sugarcane bagasse pith by single-step steam pyrolysis in the presence of SO2 and H2S at 400 °C. The adsorption of metal ions on SSAC has been found to be time-, concentration-, pH-, and temperature-dependent. The kinetic data obtained at different temperatures have been analyzed using a pseudo-second-order equation. Kinetic and thermodynamic parameters have been determined based on the rate constants using Arrhenius and Eyring equations. The adsorption of heavy metal ions from aqueous solutions increased with increasing pH, and maximum removal [99.2% for Pb(II), 97.2% for Hg(II), 93.1% for Cd(II), and 81.9% for Co(II)] was observed in the pH range of 4.0-8.0 with an initial concentration of 100 mg/L. The selectivity order of the adsorbent is Pb(II) > Hg(II) > Cd(II) > Co(II). The H-type adsorption isotherm obtained for the adsorbent indicated a favorable process. The applicability of the Langmuir and Freundlich adsorption isotherm models has been tested. The SSAC had maximum adsorption capacities (evaluated from fits of the Langmuir isotherm to batch adsorption data for a contact time of 4 h at 30 °C) for Pb(II), Hg(II), Cd(II), and Co(II) of 200.00, 188.68, 153.85, and 128.70 mg/g, respectively. The competitive adsorption capacities of the SSAC for all metal ions were found to be lower than noncompetitive conditions. Heavy metal adsorption from synthetic wastewaters was also studied to demonstrate its efficiency in removing metals from wastewaters containing other cations and anions. Metal ions, which are bounded to the SSAC, could be stripped by acidic solutions (0.2 M HCl) so that SSAC can be recycled. Surface modification of activated carbon using steam pyrolysis in the presence of SO2 and H2S greatly enhanced metal removal and resulted in a product with possible commercial potential for wastewater treatment. Introduction The treatment of heavy-metal pollutants from aqueous systems is one of the most important environmental issues facing every country today. Activated carbon adsorption has emerged as one of the most effective technologies for removing heavy metals, present in trace amounts, from water and wastewaters.1 The use of activated carbons prepared from cheaper and easily available sources such as rice husk, groundnut husk, sawdust, coconut husk, and walnut shell for the removal of heavy metals from aqueous solutions has been reported in the literature.2-4 Sugarcane bagasse pith is another commonly available and inexpensive byproduct of the sugar industry. Currently this is being used as a filler in building materials, and no proper methodology of treatment or application of this waste has been worked out. Recently, several attempts have been made to convert the bagasse pith into suitable adsorbents for the removal of heavy metals from aqueous solutions.5,6 Because heavy metals show a high affinity toward sulfur groups (Pearson’s rule), a method of improving the adsorption efficiency of adsorbent materials could be based on immobilizing sulfur onto the adsorbent surface. In a previous paper,7 it was found that the replacement of oxygen by sulfur on the surface of activated carbon shows an increase in the adsorption efficiency of the material against heavy metal in aqueous solutions. Extensive studies8,9 were carried out for the formation of CdS complexes by heating carbon with different sulfurizing agents. Activated carbons are usu-
ally prepared by heating precursors at higher temperatures followed by activation either physically or chemically. Gergova et al.10 have successfully used a singlestep steam pyrolysis activation to make carbons from a variety of materials. This method has many advantages over the other methods used for the production of activation carbons and was described in the literature.11 Earlier workers11-13 in this direction produced highquality activated carbons for water purification. Cost effectiveness, availability, and adsorption properties are the main criteria for choosing an adsorbent to remove heavy metals from aqueous solutions. Taking these criteria into consideration, the present study is aimed at studying the adsorption capacities of the sulfurized activated carbon produced from sugar industry waste, bagasse pith, using single-step steam pyrolysis in the presence of SO2 and H2S for heavy-metal removal from water and wastewaters. Experimental Section Preparation and Characterization of Adsorbents. The starting material for the preparation of activated carbon was the bagasse pith, a sugar industry waste product, obtained from a local sugar industry. Bagasse pith was treated with dilute HCl (0.2 M) to remove the adhering materials and washed with distilled water and then dried. Carbonization of bagasse pith at 200 °C for 2 h (C-200) was done in a Matri made (India) furnace. About 100 g of bagasse pith was placed in a purpose-made graphite tube and positioned at the center of the furnace. The temperature of the sample
10.1021/ie0110181 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/04/2002
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in the furnace was monitored. The steam activated carbon (SAC) was prepared using the method described in the literature.13 Steam produced by a steam generator entered in the graphite tube at the rate of approximately 3 mL/min. The sample was then heated at a fixed rate of approximately 10 °C/min to 400 °C and held at this temperature for 2 h. The product (SAC) was treated with a 1 M HCl solution to remove the ash content and then washed with distilled water. The material was dried at 100 °C and sieved to -80 + 230 mesh size. The SO2 and H2S gases were used as sulfurizing agents to introduce the sulfur on the carbon surface and were produced by the reactions between NaHSO3 and H2SO4 and between HCl and FeS, respectively. Sulfurized steam activated carbon (SSAC) was prepared by steam activation of C-200 at 400 °C in the presence of SO2 and H2S. About 5 g of C-200 was placed in a graphite tube and flushed with a flow of steam (3 mL/ min) in the presence of a stream first of SO2 (10 mL/ min) for 90 min and then of H2S (15 mL/min) for 30 min from gas generators at a heating rate of 10 °C/min from 30 to 400 °C. After the furnace was allowed to cool, the product (SSAC) was washed with distilled water and dried at 100 °C. The dried material was sieved to separate the particles of -80 + 230 mesh size. The amount of sulfur attached to the carbon surface was estimated by using the Eschika method (standard analysis). This deals with the transformation into BaSO4 of the sulfur present in the carbon by treatment with a Eschika mixture (Na2CO3 and MgO) at 800 °C and then successively with Br2 and BaCl2 solution.14 The maximum amount of sulfur retained on the carbon surface was found to be 8.9 wt %. The introduction of different proportions of sulfur on the carbon surface was achieved by controlling the time required for flushing the SO2 and H2S gases into the graphite tube. However, the flow rate of steam, SO2, and H2S, heating rate, and total reaction time were the same as specified earlier. To retain the required amount of sulfur on the carbon surface, the following flow times for SO2 and H2S were used during the treatment process. (1) 0.8% sulfur content: flow time, 10 min for SO2 and 110 min for H2S. (2) 1.5% sulfur content: flow time, 15 min for SO2 and 105 min for H2S. (3) 2.3% sulfur content: flow time, 30 min for SO2 and 90 min for H2S. (4) 4.0% sulfur content: flow time, 45 min for SO2 and 75 min for H2S. (5) 6.5% sulfur content: flow time, 60 min for SO2 and 60 min for H2S. (6) 7.3% sulfur content: flow time, 75 min for SO2 and 45 min for H2S. (7) 8.9% sulfur content: flow time, 90 min for SO2 and 30 min for H2S. The Fourier transform infrared (FTIR) spectra of the SAC and SSAC were measured on a Bruker IFS 66V FTIR spectrophotometer. The surface area of the SAC and SSAC was measured by means of a Quantasorb surface area analyzer (model 05/7) using a BrunauerEmmett-Teller nitrogen adsorption technique. Porosity measurements were done using a micrometric mercury intrusion porosimeter (model 9310). The pH at the zero point of charge, pHzpc, is defined as the pH of the suspension at which the surface charge density σ0 ) 0. Potentiometric titration15 was carried out to determine
σ0 as a function of pH. Titrations were made in 0.01 and 0.1 M NaNO3 solutions. A total of 1 g of carbon was suspended in a 100 mL NaNO3 solution (0.01 and 0.1 M), and the titration was performed at 30 °C, by successive 20.0-100.0 µL increments of 0.1 M HNO3 or NaOH. Following each addition of titrant, the pH of the suspension was measured after 3-5 min, when the emf drift was usually less than 0.3 mV/min, using a pH meter. σ0 (C/cm2) was calculated from the titration data using the following equation:
σ0 ) F(CA - CB + [OH-] - [H+])/A
(1)
where F is Faraday’s constant (C/equiv), A is the total surface area in the suspension (cm2/L), and CA and CB are the concentrations of acid and base after each addition during titration (equiv/L). [H+] and [OH-] are the equilibrium concentrations of H+ and OH- bound to the suspension surface (equiv/cm2). The total number of acidic groups of the activated carbon was determined by a conductometric titration method described in an earlier publication.16 In the present study, 1.0 g of the activated carbon was treated with 250 mL of a NaOH (0.01 mol/L) solution and stirred at 30 °C. After reaching equilibrium, the supernatant solution was titrated with a standard solution of HNO3 (0.1 mol/L). The conductivity of the solution was measured using a Systronics (India) made conductivity meter (model 304). The cation-exchange capacity (CEC) of the activated carbon was determined using a procedure described by Helferrich17 with Na+ as the exchanging ions for H. The apparent density of the activated carbons was also determined using a pycnometric method using nitrobenzene as the displacing liquid. Adsorption and Desorption Experiments. All divalent metal cation stock solutions (of Pb, Hg, Cd, and Co) were prepared from the corresponding chloride salts (Fluka, Switzerland) at a concentration of 1000 mg/L. Working solutions were prepared by diluting a different volume of the stock solution to achieve the desired concentration. In the batch kinetic study, 0.1 g of SSAC was placed in a flask containing 50 mL of a metal solution of desired concentration. The pH of the solution was fixed at 6.0. The flasks were continuously shaken in a temperature-controlled flask shaker at 30 °C and 200 rpm. At the end of the predetermined time intervals, the supernatant liquids were filtered and the metal concentrations were determined. Pb(II), Cd(II), and Co(II) were estimated using an atomic absorption spectrophotometer (Perkin-Elmer model 2100). Hg(II) was determined spectrophotometrically using the eosin method.18 The equilibrium time was determined as the contact time required for the concentration of metal in the solution to reach equilibrium. The amount of metal adsorbed by the solid (q) was calculated from the difference between the metal remaining (Ce) and that initially present (C0) using the following equation:
q ) (C0 - Ce)V/m
(2)
where V is the volume of the solution (L) and m is the mass of the adsorbent (g). The effect of the initial concentration (varied between 25 and 1000 mg/L) was also studied in order to determine the effect of the parameter on the adsorption of metal from the solution. During these tests the contact time (4 h), the amount of adsorbent (0.1 g), the shaking speed (200 rpm), the pH (6.0), and the temperature (30 °C) were kept
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constant. The adsorption rate study was conducted at four different temperatures ranging between 30 and 60 °C using a 250 mg/L metal solution. The experimental procedure was exactly similar to that which was followed during the kinetic study at 30 °C. Experiments were conducted to determine the pH range at which the maximum adsorption of metal would take place on the SSAC. To a series of 100 mL flasks, each containing 50 mL of a 100 mg/L metal solution, the initial pH was adjusted to values ranging from 2.0 to 8.0 by 1 N NaOH and/or HCl, and then 0.1 g of SSAC was added. The samples were shaken for 4 h at 30 °C. The equilibrium pH was recorded, and aliquots of the suspension were filtered to collect supernatant and analyzed for metals. Adsorption isotherm experiments were also performed by agitating 0.1 g of SSAC with 50 mL of the varying concentrations of metal from 50 to 1000 mg/L at 30 °C. The pH of the solution was adjusted to an optimum pH (6.0) obtained from the earlier study. After the established contact time (4 h) was reached, the suspension was filtered, and supernatant was analyzed for metal concentration. To find the competitiveness of metal adsorption on SSAC, batch experiments were conducted using the solution containing all of the metal ions. A total of 50 mL of a solution containing 50 mg/L each of Pb(II), Hg(II), Cd(II), and Co(II) was shaken (200 rpm) with 0.1 g of SSAC for 4 h at pH 6.0 and 30 °C. The sample was removed and filtered. The concentrations of four metal ions in the supernatant were measured concurrently by the method as previously described. The effect of the adsorbent concentration on the removal of metals from synthetic wastewaters was also studied. Batch experiments were conducted by equilibrating an accurately weighed amount of adsorbent (varying between 1.0 and 10.0 g/L) with 50 mL of synthetic wastewaters [composition (mg/L): Na, 25; K, 25; Mg, 10; Ca, 10; NH4, 10; NO2, 50; SO4, 40; NO3, 31; CH3-COO, 33; Cl, 3583] containing 50 mg/L each of metal ions at pH 6.0 and 30 °C. After attainment of equilibrium, the amount of metal adsorbed on the solid was calculated as described earlier. Similar batch experiments were also conducted using a commercial activated carbon (CAC) for comparison. For this, CAC obtained from E. Merck India Ltd. was used. No pretreatment was given to the CAC, and it was used as received in the experiments. The surface and physical properties of the CAC are as follows: surface area, 284.0 m2/g; porosity, 0.43 mL/g; density, 0.92 g/mL; total acid groups, 2.1 mequiv/g; cartion exchange capacity, 1.02 mequiv/g; pHzpc, 5.8. Desorption studies were carried out as follows. About 0.1 g of SSAC was added to each flask containing 50 mL of a 100 mg/L metal solution. The pH of the solution was fixed at 6.0. The flasks were shaken for 4 h at 30 °C and 200 rpm. After equilibration, the phases were separated. The filtrate was analyzed for metal to ascertain the amount adsorbed. The amount of metal adsorbed on SSAC in a series of experiments under identical conditions was fairly constant: in most of the cases about 49.5, 48.6, 45.0, and 40.5 mg of Pb(II), Hg(II), Cd(II), and Co(II), respectively, per gram of adsorbent were observed. The metal-laden SSAC was washed with distilled water to remove any unadsorbed metal. After washing, the adsorbent was dried at 70 °C. The spent adsorbent (0.1 g) was then placed in a flask containing 50 mL of 0.2 M desorbing reagents. The
Table 1. FTIR Data of Sulfur-Free and Sulfurized Activated Carbons band position (cm-1) SAC
SSAC
possible assignments
3764 2925 2854 1724 1600 1357
3762 2925 2854 1730 1608 1360 1165 1111 460
O-H stretching of the hydroxyl group CdCsH stretching CdCsH stretching CdO stretching of the -COOH group CdO stretching of the carbonyl group C-H deforming CdS stretching SdO stretching S-S stretching
Table 2. Surface and Physical Properties of the SAC and SSAC value no.
parameter
1 2 3 4 5 6 7 8
apparent density (g/mL) CEC (mequiv/g) surface area (m2/g) pHzpc porosity (mL/g) sulfur content (wt %) total acid groups (mequiv/g) particle size (mesh size)
SAC 1.10 0.79 536.5 5.6 0.52 2.3 -80 + 230
SSAC 1.39 1.32 500.5 4.3 0.43 8.9 2.9 -80 + 230
desorbing reagents such as NaCl, NaNO3, Na2SO4, HCl, HNO3, H2SO4, and NaCl + HCl were used to desorb the adsorbed metal. The flasks were shaken for 6 h in a shaker at 30 °C and 200 rpm. At the end of equilibration, the suspensions were filtered and the extractant filtrate was analyzed for metal. Comparison of this value with the loss observed in the initial sorption step was used to compute the percentage recovery values. Results and Discussion Adsorbent Characterization. The possible assignments of the observed peaks in the FTIR spectra of SAC and SSAC are furnished in Table 1. The IR spectra of SAC and SSAC show the appearance of bands at 1600 and 1608 cm-1, respectively, corresponding to the conjugated hydrogen-bonded carbonyl groups on both carbons. For SSAC, new bands appeared around 1165, 1111, and 460 cm-1, which are the characteristic bands of CdS, SdO, and S-S stretching vibrations, respectively, due to surface sulfur groups bonded to activated carbon.19 This confirms the modification of the carbon surface with sulfur groups. The surface and physical properties of the SAC and SSAC (SSAC with 8.9% sulfur content) are given in Table 2. The point of intersection of the σ0 vs pH curves (Figure 1) showed that the pHzpc’s of SAC and SSAC were found to be 5.6 and 4.3, respectively. The decrease in pHzpc after sulfurization indicated that the surface of SAC has been modified after incorporating sulfur groups and the surface becomes more negative, which facilitates the electrostatic interaction with metal cations. On the other hand, the low pHzpc shown by SSAC indicates that it has more acidic functional groups than SAC or, in other words, it is less basic, which is advantageous to the removal of metal cations. The total number of acid groups on SAC and SSAC obtained from the conductometric titration method is on the order of acid groups of acidic carbons. The values of the surface area and porosity of the SSAC are slightly lower than those observed in SAC.
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Figure 1. Effect of the pH on the surface charge density of SSAC.
Figure 3. Effect of the contact time on the adsorption of different metal ions on SSAC. Table 3. Effect of the Initial Concentration for the Adsorption of Metal Ions onto SSAC amount adsorbed
Figure 2. Effect of the sulfur content in the SSAC on the adsorption of different metals.
Effect of the Sulfur Content of the Adsorbent on Metal Adsorption. The dependence of metal ion on the sulfur content in the SSAC is shown in Figure 2. The concentration of sulfur on the carbon surface varied from 0.8 to 8.9%. From an initial concentration of 100 mg/L, the removal of metals by adsorption increases from 37.55 mg/g (75.1%) to 49.60 mg/g (99.2%) for Pb(II), from 35.68 mg/g (71.4%) to 48.62 mg/g (97.2%) for Hg(II), from 31.85 mg/g (63.7%) to 46.55 mg/g (93.1%) for Cd(II), and from 28.55 mg/g (57.1%) to 40.95 (81.9%) for Co(II) by increasing the sulfur content from 0.8 to 8.9%. A significant feature observed from the experiment is the minimum adsorption capacity of metals, i.e., 35.65 mg/g (71.3%), 33.20 mg/g (66.4%), 29.65 mg/g (59.3%), and 25.15 mg/g (50.3%) for Pb(II), Hg(II), Cd(II), and Co(II), respectively, in the sulfur-free SAC. Effect of the Contact Time and Initial Concentration. The effect of the contact time on the adsorption of metal ions by SSAC is shown in Figure 3. The results show that increasing the contact time increased the adsorption of metal ions, and it remained constant after the equilibration time of 4 h. Consequently, the contact time was set to 4 h in each experiment. The amount of heavy metal removed (mg/g) by SSAC from an initial concentration of 100 mg/L after a 4 h equilibration time was Pb(II) (49.60 mg/g) > Hg(II) (48.61 mg/g) > Cd(II) (46.55 mg/g) > Co(II) (40.95 mg/g). The dependence of the processes of Pb(II), Hg(II), Cd(II), and Co(II) removal from different initial concen-
initial concn (mg/L)
mg/g
%
mg/g
25 50 100 150 250 400 600 800 1000
12.49 24.93 49.35 68.54 96.78 134.01 167.92 184.21 198.11
99.9 99.7 98.7 91.4 77.4 67.0 55.9 46.1 39.6
12.49 24.81 48.61 66.00 93.75 127.54 157.08 170.76 183.84
Pb(II)
Hg(II)
Cd(II) %
mg/g
Co(II) %
mg/g
99.9 12.46 99.7 11.70 99.2 24.38 97.5 21.39 97.2 45.06 90.1 40.50 88.0 57.94 77.3 53.48 75.0 79.44 63.6 74.51 63.8 96.38 48.2 95.28 52.4 122.03 40.7 107.40 42.7 138.97 34.7 121.60 36.8 151.51 30.3 132.50
% 93.6 85.6 81.0 71.3 59.6 47.6 35.8 30.4 26.5
trations (25-1000 mg/L) by SSAC is illustrated in Table 3. At low concentrations (below 25 mg/L), the adsorption of metal ions was 98-100%. This suggested that SSAC could remove most of the metals from water if their concentrations were below 25 mg/L. The examination of the data also reveals that at a fixed adsorbent dose the amount adsorbed increased with the concentration of the solution, but the percentage adsorption decreased. It can be noted from the table that the removal of metal ions is highly concentration-dependent. At lower concentrations of metal ions, the number of metal ions available in the solution is less as compared to the available sites on the adsorbent. However, at higher concentrations the available sites for adsorption become fewer and the percentage removal of metal ions depends on the initial concentration. Adsorption Kinetics. The metal ions become adsorbed onto the carbon surface, that is, onto a complexforming group on the carbon surface (mostly onto sulfur groups and surface hydroxy groups). Hence, the mechanism of metal removal is thought, basically, to be complexation and ion exchange. Earlier workers20 have also proposed a similar mechanism for divalent metal ion sorption onto the carbon surface. According to the concept of surface complex formation, the adsorption reaction between a metal and solid can be best described by the following general equations:
2S h + M2+ T MS2
(3)
SH + M2+ T MS2 + 2H+
(4)
or
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where S h and SH are polar sites on the carbon surface. M2+ and MS2 are the concentrations of metal in solution and on the solid surface, respectively. Here the surface hydroxyl groups, i.e., S h and SH, are Lewis bases, whereas metal ions are Lewis acids. The variation of the metal adsorption rate before reaching equilibrium can be described using a pseudo-second-order reaction kinetics21
d(S)t ) k2[(S)0 - (S)t]2 dt
(5)
d(HS)t ) k2[(HS)0 - (HS)t]2 dt
(6)
where (S)0 and (HS)0 are the number of equilibrium sites available on the carbon surface and (S)t and (HS)t are the number of active sites occupied on the carbon surface at time t. k2 is the pseudo-second-order rate constant. The rate of the reaction may be dependent on the amount of metal ions adsorbed at time t (qt) and the metal ions adsorbed at equilibrium (qe). The kinetic rate equation can be rewritten as
dqt ) k2(qe - qt)2 dt
(7)
Separating the variables in eq 7 and integrating for the boundary conditions give
∫qq)0)q (q t
t
dqt
e
e
- qt)
2
) k2
∫0tdt
1 1 ) + k2t qe - qt qe
(8) (9)
The above equation can be rearranged to obtain a linear form
t 1 1 ) + t 2 qt k q qe
(10)
2 e
The product k2qe2 is actually the initial sorption rate represented as h ) k2qe2. It is noted that k2 and qe in eq 10 can be obtained from the intercept and slope of the plot of t/qt versus t. The plots of t/qt versus t were found to be straight lines (Figure 4) for all metals at different temperatures, suggesting the applicability of pseudo-second-order kinetics to the present systems. Table 4 lists the values of k2 and qe. The perusal of data in Table 4 for an equilibrium time indicates that the metal ion adsorbed qe is higher for Pb(II) followed by Hg(II), Cd(II), and Co(II). In all cases the values of k2 were found to increase with an increase of the temperature, indicating that the adsorption process was endothermic. The findings clearly indicate that the high temperatures favor the metal adsorption on SSAC. This may be due to the desolvation of adsorbing species and the increase in the active surface centers available for adsorption. The influence of temperature on the adsorption rate can be examined thermodynamically by using an Arrhenius equation (ln k2 ) ln k0 - Ea/RT). The Arrhenius plots of ln k2 versus 1/T were found to be linear (figure not shown). The values of the energy of activation, Ea, calculated from the slope of the plots are given in Table 5. The energies of activation show an
Figure 4. Pseudo-second-order kinetic plots for the adsorption of different metals on SSAC.
increase in the magnitude with increasing ionic size that causes more steric hindrance to its entry into the solid phase and thus a need for more energy to its mobility through the pores of carbon particles. In addition to determining Ea values, one can calculate the enthalpy (∆Hq), entropy (∆Sq), and free energy (∆Gq) of activation for metal adsorption kinetics by using the following Eyring equation:
ln(k2/T) ) [ln(kb/h) + (∆Sq/R)] - ∆Hq/RT (11)
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Table 4. Kinetic Parameters for the Adsorption of Metal Ions onto SSAC Pb(II)
Hg(II)
Cd(II)
Co(II)
temp (°C)
k2 (g/mg‚min)
qe (mg/g)
r2
k2 (g/mg‚min)
qe (mg/g)
r2
k2 (g/mg‚min)
qe (mg/g)
r2
k2 (g/mg‚min)
qe (mg/g)
r2
30 40 50 60
2.48 × 10-3 3.95 × 10-3 4.87 × 10-3 6.11 × 10-3
96.78 99.98 109.11 116.21
0.997 0.999 0.999 0.999
2.38 × 10-3 2.94 × 10-3 3.94 × 10-3 5.22 × 10-3
93.46 95.24 107.53 114.94
0.998 0.998 0.996 0.997
2.34 × 10-3 2.80 × 10-3 3.58 × 10-3 4.46 × 10-3
79.37 86.21 93.46 101.01
0.997 0.997 0.995 0.991
1.87 × 10-3 2.05 × 10-3 2.52 × 10-3 3.21 × 10-3
71.43 76.89 83.45 89.67
0.986 0.990 0.990 0.991
Table 5. Reaction Parameters Derived from Arrhenius and Eyring Equations for the Adsorption of Metals onto SSAC metal
Ea (kJ/mol)
∆Hq (kJ/mol)
∆Sq (J/mol)
∆Gq at 30 °C (kJ/mol)
Pb(II) Hg(II) Cd(II) Co(II)
24.56 22.17 18.26 15.24
21.92 19.59 16.51 12.60
-222.05 -230.86 -248.96 -256.05
45.36 50.36 58.93 64.98
where k2 is the rate constant, kb is the Boltzmann constant, h is Plank’s constant, and R is the gas constant. The values of ∆Gq can be determined as ∆Gq ) ∆Hq - T∆Sq. The plots of ln(k2/T) versus 1/T for all metals were found to be linear (figure not shown). The values of ∆Hq and ∆Sq were obtained from the slope and intercept of the plots and are shown in Table 5. The positive values for ∆Hq suggest that the adsorption process is endothermic, meaning it consumes energy. The magnitude of ∆Hq suggests a weak type of bonding. The magnitude and sign of ∆Sq give an indication of whether the adsorption reaction is an associative or dissociative mechanism.22 The large negative values for ∆Sq suggest that metal sorption on the carbon surface is an associative mechanism. This also indicates a greater order of reaction during the adsorption of metals onto SSAC and also reflects the affinity of the adsorbent material for metal ions. The positive values of ∆Gq suggest that the sorption reactions require energy to convert reactants into products. The ∆Gq values determine the rate of the adsorption, i.e., rate increases as ∆Gq decreases, and the reaction proceeds only when the energy requirement is fulfilled. The results from Table 4 show that the SSAC-Pb(II) system has the highest k2 (2.48 × 10-3 g/mg‚min at 30 °C) and the SSAC-Co(II) system has the lowest k2 (1.87 × 10-3 g/mg‚min at 30 °C) for the metals selected in this study. The results in Table 5 illustrate this trend for ∆Gq in which the SSACPb(II) system has the lowest ∆Gq value (45.36 kJ/mol) compared with the largest ∆Gq value for the SSACCo(II) system (64.98 kJ/mol), showing that the higher k2 corresponds to a lower ∆Gq for the SSAC-Pb(II) system than for Co(II). Effect of the pH on Metal Adsorption. The pH was a predominant factor affecting the removal of metal ions under the conditions employed (Figure 5). From the figure it is clear that SSAC is effective for the quantitative removal of metal ions over the pH range of 4.08.0. Below and above this pH range, adsorption is minimum. At very high pH, the precipitation of metal hydroxides was encountered. The maximum adsorptions of 98.9, 97.4, 92.8, and 81.0% for Pb(II), Hg(II), Cd(II), and Co(II), respectively, by SSAC took place at pH 6.0 and at an initial concentration of 100 mg/L. Adsorption of metal ions from solution onto the solid phase can occur with the formation of a surface complex between functional groups of adsorbent and metal. The pHzpc of SSAC was found to be 4.3. Above this pH, the surface charge of the carbon is negative, and below this pH, the
Figure 5. Effect of the pH on the adsorption of different metals on SSAC.
surface charge is positive. Perusal of the literature on a metal speciation diagram23 shows that in the presence of Cl- the dominant metallic species at pH > 6.0 is M(OH)2 and those at pH < 6.0 is M2+ and M(OH)+. The increase in metal adsorption above 6.0 may be due to the retention of M(OH)2 into micropores of carbon particles. The maximum adsorption efficiency in the pH range of 4.0-6.0 may be due to the interaction of M2+ or M(OH)+ ions with sulfur present in SSAC. According to the Pearson theory,24 during an acid-base reaction, hard acid prefers to coordinate with hard base and soft acid to soft base. Positively charged metallic species are soft acids and, as a rule, interactions of M2+ or M(OH)2+ with sulfur groups (soft bases) are likely favored at the pH range of 4.0-6.0. The positively charged species present in the solution may also interact with other functional groups such as -COOH or C-OH on the carbon surface, which enhances the adsorption at this pH range. Adsorption Isotherm. The adsorption isotherms of metals studied are presented in Figure 6. The equilibrium adsorption isotherms are of fundamental importance in the design of adsorption systems. According to the slope of the initial portion of the curves, the adsorption isotherms may be classified as H-type of the Gile’s classification.25 The H-type isotherms are the most common and correspond to the high affinity of the adsorbate for the adsorbent, and because of that, there is no competition from the solvent for adsorption sites. On the other hand, the sorption isotherm in Figure 6 tends to define a plateau; therefore, it seems reasonable to suppose that the formation of a complete monolayer of metal ions covering the adsorbent surface occurs (isotherms belonging to subgroups of Gile’s classification). The adsorption isotherms for the adsorbed metal ions on SSAC can be analyzed by both the Langmuir and Freundlich adsorption isotherms. The main assumption
Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 5091
∆g (%) ) 100 ×
Figure 6. Plots of qe versus Ce for the adsorption of different metals on SSAC. Lines are Langmuir and Freundlich model curves. Table 6. Langmuir and Freundlich Constants for the Adsorption of Metals onto SSAC Langmuir Q° metal (mg/g) Pb(II) Hg(II) Cd(II) Co(II)
200.00 188.68 153.85 128.70
b (L/mg) 3.110 × 10-2 2.812 × 10-2 1.836 × 10-2 1.712 × 10-2
Freundlich r2
∆g (%)
KF
1/n
0.987 0.989 0.980 0.998
5.02 4.86 6.70 7.40
39.83 33.14 22.84 12.29
0.247 0.267 0.282 0.337
r2
∆g (%)
0.992 6.78 0.992 6.39 0.994 9.16 0.965 12.25
of the Langmuir method is that adsorption occurs uniformly on the active part of the surface, and when a molecule is adsorbed on a site, the latter does not have any effect upon other incident molecules. The linear representation of the Langmuir isotherm can be expressed as
Ce Ce 1 + ) qe Q°b Q°
(12)
where Ce is the concentration of the metal in the equilibrium solution, qe is the amount of metal adsorbed per gram of the adsorbent, Q° is the maximum amount of metal that can be adsorbed in a monolayer, and b is a constant related to the energy of adsorption. The Freundlich isotherm is empirical and used for heterogeneous surface energies in which the energy term, b, in the Langmuir equation varies as a function of the surface coverage, qe, due to variation in the heat of adsorption. The linear representation of the Freundlich adsorption equation is
log qe ) log KF +
1 log Ce n
(13)
where KF and 1/n are the Freundlich constants related to adsorptive capacity and intensity of adsorption, respectively. The Langmuir (Q° and b) and Freundlich (KF and 1/n) constants were calculated from the plots of Ce/qe versus Ce and log qe versus log Ce, respectively, using a linear least-squares fitting. The values of the constants of these models, together with correlation coefficients (r2) and normalized standard deviation (∆g), are given in Table 6. The values of ∆g are calculated using the following equation:
exp 2 - qcal ∑[(qexp t t )/qt ]
x
N-1
(14)
where the superscripts exp and cal are the experimental and calculated values and N is the number of measurements. Figure 6 typically illustrates the comparison between the calculated and observed results for the adsorption of metals on SSAC. The values of ∆g were compared to determine the appropriate type of isotherm model for metal adsorption. The ∆g values for the fits of all of the metals to the Langmuir isotherm were much lower than the values for the Freundlich isotherm, as shown in Table 6. Hence, the Langmuir model agrees well with these data, assuming that all of the adsorption sites have equal energy, while surface characterization of the carbon clearly suggests a range of site energies. Earlier studies26 in this direction have clearly demonstrated that the Langmuir equation gives adequate results in many cases where surface heterogeneity is known to be present. The amount adsorbed from an aqueous solution for a given equilibrium concentration showed an increase in the order Pb(II) > Hg(II) > Cd(II) > Co(II). The variation in the order of the adsorption capacity is in agreement with the order of variation of the ionic size of metal ions. The values of ionic radii are 1.32 Å for Pb(II), 1.12 Å for Hg(II), 1.03 Å for Cd(II), and 0.82 Å for Co(II). The higher the ionic radii (or smaller the hydrated ionic radii), the greater its affinity to active groups of the adsorbent. This suggests that the energy required for the dehydration of the metal ions, so that they could occupy a site in the adsorbent, plays an important role in determining the selectivity series for the metal ions. The values of 0.1 < 1/n < 1 show favorable adsorption of metal ions into SSAC. The lower fractional values of 1/n (0.247-0.337) signify that weak adsorptive forces are operative on the surface of the adsorbent. The isothermal data can be used to calculate the ultimate adsorption capacity of the adsorbent by substituting the required equilibrium concentration into the Freundlich equation. For an equilibrium concentration of 1 mg/L, each gram of SSAC can remove 39.83, 33.14, 22.84, and 12.29 mg/g of Pb(II), Hg(II), Cd(II), and Co(II), respectively, at 30 °C. Different sorbents with a wide range of adsorption capacities for heavy metals have been used. For comparison, the adsorption capacities of other adsorbents reported in the literature are given in Table 7 along with that for the adsorbent of the present study. The adsorption capacities achieved in this study were higher than the values reported. Competition among Cations. Adsorption from a mixture of metals has also been studied. It was observed that the sorption of these metals on SSAC was found as follows: Pb(II), 19.08 mg/g (76.3%); Hg(II), 17.80 mg/g (71.2%); Cd(II), 16.42 mg/g (65.7%); Co(II), 14.77 mg/g (59.1%). It should be noted that the competitive adsorption capacities of SSAC for all metal ions were lower than the noncompetitive conditions. It was observed that, among the cations used, sorption of Pb(II) was the highest, followed by Hg(II), Cd(II), and Co(II). This order of affinity [Pb(II) > Hg(II) > Cd(II) > Co(II)] is the same with noncompetitive conditions. For ions of the same valency, the sorbent prefers the metal with the higher atomic number.
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Table 7. Comparison of Langmuir Constants (Q° and b) with Other Adsorbents adsorbent
Q° (mg/g)
b (L/mg)
ref
Pb(II) modified groundnut husk kaolinitic clay tea leaves SSAC
39.33 4.76 150.16 200.00
peanut hull carbon photo film waste sludge 2-mercaptobenzothiazole-clay SSAC
109.57 11.72 2.69 188.68
0.0506 1.0500 0.0090 0.0311
Okieimen et al.27 Orumwense 28 Singh et al.29 present work
0.2045 0.0983 0.0281
Namasivayam and Periyasamy30 Selvaraj et al.31 Filho et al.32 present work
0.0267 0.3781 0.2310 0.0543 0.0184
Okieimen et al.27 Levya-Ramos et al.33 Namasivayam and Ranganathan34 Sun and Shi35 present work
0.0171
Choi and Nho36 present work
Hg(II)
Cd(II) modified groundnut husk activated carbon Fe(III)/Cr(III) hydroxide sunflower stalks SSAC
42.71 8.21 40.49 42.18 153.85
styrene-g-polyethylene SSAC
24.98 128.70
Co(II)
Table 8. Extraction of Metals from Spent SSAC
Figure 7. Effect of the SSAC concentration on the removal of metals from synthetic wastewaters.
Test with Synthetic Wastewater and Effect of the Adsorbent Concentrations. Synthetic wastewater samples (composition given in the Experimental Section) were also treated with SSAC to demonstrate its adsorption potential and utility in removing metals from wastewater in the presence of other ions. The effect of the adsorbent dose on metal removal by SSAC is shown in Figure 7. In all cases, the percent removal increased with increasing adsorbent concentration. This may be attributed to the greater availability of adsorption sites or surface area at higher concentrations of the sorbent. The results of the experiments show that a minimum adsorbent dosage of 25 mg in 50 mL of wastewater (1 g/L) is sufficient for the removal of 70.2, 67.9, 51.3, and 43.5% of Pb(II), Hg(II), Cd(II), and Co(II), respectively. The complete removal of Pb(II), Hg(II), Cd(II), and Co(II) from 50 mL of the sample was achieved by 150, 175, 200, and 300 mg adsorbent doses, respectively, which is in good agreement with that obtained from the batch experiments mentioned above. This demonstrates that SSAC can be successfully used for the removal of metal ions from wastewater. Desorption Studies. Attempts have been made to recover the adsorbed metal as well as regenerate the adsorbent. Studies in this direction have been done with different reagents as extractants. The results of the experiments are shown in Table 8. The relatively inexpensive HCl desorbed all of the sorbed metals from
desorption (%)
extractant (0.2 M)
Pb(II)
Hg(II)
Cd(II)
Co(II)
NaNO3 NaCl Na2SO4 HCl HNO3 H2SO4 NaCl + HCl
10.5 22.9 20.4 96.7 48.5 43.0 68.0
15.2 18.5 24.7 94.0 44.4 41.6 65.5
24.7 14.6 28.8 94.6 71.4 65.5 82.6
18.0 9.4 24.0 87.7 66.0 60.1 68.4
carbon. The H+ ions from HCl easily displaces metal ions bonded to the adsorbent during the desorption stage. Efficiencies of 96.7% Pb(II), 99.0% Hg(II), 99.6% Cd(II), and 81.7% Co(II) were obtained by using a 0.2 M HCl solution. The results show that the spent SSAC can be effectively regenerated for further use by 0.2 M HCl. Cost Estimation. The cheapest variety of activated carbon in India costs nearly $1100/ton for laboratorygrade activated carbon and $750/ton for commercialgrade carbon. Bagasse pith, however, the raw material used in our experiments, is much cheaper and is available almost free of cost. After considering the cost of handling charges and steam activation in the presence of SO2 and H2S gases, the cost of SSAC is $70/ton, which will be very low as compared with activated carbons. The adsorption capacities of SSAC and commercially available carbon (E. Merck, India) were determined using 50 mL solutions of 50 mg/L metal concentration with 0.1 g of carbon at pH 6.0. The amounts of metal ions adsorbed per unit weight of E. Merck carbon for Pb(II), Hg(II), Cd(II), and Co(II) were 22.13 mg/g (88.5%), 20.47 mg/g (81.9%), 19.64 mg/g (78.6%), and 17.75 mg/g (71.0%), respectively. The corresponding values for SSAC were 24.93 mg/g (99.7%), 24.82 mg/g (99.3%), 24.30 mg/g (97.2%), and 21.30 mg/g (85.2%). The data clearly show that SSAC exhibited a higher adsorption potential when compared with CACs. Conclusions We have produced a novel SSAC from sugarcane bagasse pith and demonstrated its potential application for the wastewater treatment as a low-cost adsorbent. SSAC has been shown to exhibit a high capacity for heavy metals with the sorption sequence Pb(II) >
Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 5093
Hg(II) > Cd(II) > Co(II) in accordance with the order of increasing ionic radii. The removal efficiency decreases with an increase in the concentration. A pseudosecond-order rate equation has been used to describe the kinetics of sorption of metals on SSAC at different temperatures. The adsorption is found to be endothermic. The values of Ea, ∆Hq, ∆Sq, and ∆Gq for the adsorption of metals have been calculated to predict the nature of adsorption. Adsorption of metals on SSAC is pH-dependent, and a maximum adsorption percentage occurs in the pH range of 4.0-8.0. The Langmuir isotherm equation has been successfully used to describe the equilibrium data. The process of metal removal from synthetic wastewaters was examined. Spent adsorbent can be regenerated using 0.2 M HCl. Additional research is warranted to evaluate the adsorption potential of this adsorbent with respect to various real industrial wastewaters. Literature Cited (1) Liu, M.; Zhang, H.; Zhang, X.; Deng, Y.; Liu, W.; Zhan, H. Removal and recovery of chromium(III) from aqueous solutions by a spheroidal cellulose adsorbent. Water Environ. Res. 2001, 73, 322-328. (2) Raji, C.; Manju, G. N.; Anirudhan, T. S. Removal of heavy metal ions from water using saw dust-based activated carbon. Indian J. Eng. Mater. Sci. 1997, 4, 254-260. (3) Manju, G. N.; Raji, C.; Anirudhan, T. S. Evaluation of coconut husk carbon for the removal of arsenic from water. Water Res. 1998, 32, 3062-3070. (4) Kein, J. W.; John, M. H.; Kein, D. S.; John, M. N.; Kwon, F. S. Production of granular activated carbon from waste walnut shell and its adsorption characteristics for Cu2+ ions. J. Hazard. Mater. 2001, B85, 301-315. (5) Laszlo, J. A.; Dintzis, F. R. Crop residues as ion exchange materials. Treatment of soybean hull and sugar beet fibre (pulp) with epichlorohydrin to improve cation-exchange capacity and physical stability. J. Appl. Polym. Sci. 1994, 52, 531-538. (6) Gupta, V. K.; Mohan, D.; Sharma, S. Removal of lead from wastewater using bagasse fly ash- a sugar industry waste material. Sep. Sci. Technol. 1998, 33, 1331-1343. (7) Valenzuela-Calahorro, C.; Macias-Garcia, A.; Bernalte Garcia, A.; Gomez-Serrano, V. Study of sulfur introduction in activated carbon. Carbon 1990, 28, 325-331. (8) Macias-Garcia, A.; Valenzuela-Calahorro, C.; Gomez-Serrano, V.; Espinosa-Mansilla, A. Adsorption of Pb2+ by heat-treated and sulphurized activated carbon. Carbon 1993, 31, 1249-1255. (9) Gomez-Serrano, V.; Macias-Garcia, A.; Espinosa-Mansilla, A.; Valenzuela-Calahorro, C. Adsorption of mercury, cadmium and lead from aqueous solution on heat treated and sulphurized activated carbon. Water Res. 1998, 32, 1-4. (10) Gergova, K.; Petrov, N.; Eser, S. Adsorption properties and microstructure of activated carbons produced from agricultural byproducts by steam pyrolysis. Carbon 1994, 32, 693-702. (11) Warhurst, A. M.; McConnachie, G. L.; Pollard, S. J. T. Characterization and applications of activated carbons produced from Moringa Oleifera seed husks by single-step steam pyrolysis. Water Res. 1997, 31, 759-766. (12) Sankaran, N. B.; Anirudhan, T. S. Adsorption dynamics of phenol on activated carbon produced from Salvinia Molesta Mitchell by single-step steam pyrolysis. Indian J. Eng. Mater. Sci. 1999, 6, 229-236. (13) Vinod, V. P.; Anirudhan, T. S. Effect of experimental variables on phenol adsorption on activated carbon prepared from coconut husk by single-step steam pyrolysis: Mass transfer process and equilibrium studies. J. Sci. Ind. Res. 2002, 61, 128-138. (14) Rump, H. H.; Krist, F. Laboratory Manual for the Examination of Water, Wastewater and Soil; VCH: New York, 1992.
(15) Schwarz, J. A.; Driscoll, C. T.; Bhanot, B. K. The zero point of charge of silica-alumina oxide suspensions. J. Colloid Interface Sci. 1984, 97, 55-61. (16) James, R.; Parks, G. A. In Surface and Colloid Science; Matpec, P. T., Ed.; Plenum Press: New York, 1982. (17) Helferrich, F. Ion Exchange; McGraw-Hill: New York, 1962. (18) Jayateerth, M. R. Spectrophotometric determination of trace amounts of mercury with phenanthroline and eosin. Analyst 1984, 109, 1579-1583. (19) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Complexes; Wiley: New York, 1988. (20) Corapcioglu, M. O.; Huang, C. P. The adsorption of heavy metals onto hydrous activated carbon. Water Res. 1987, 21, 10311044. (21) Ho, Y. S.; McKay, G. The kinetics of sorption of divalent metal ions into sphagnum moss peat. Water Res. 2000, 34, 735742. (22) Scheckel, K. G.; Sparks. D. C. Temperature effects on nickel sorption kinetics at the mineral-water interface. Soil Sci. Soc. Am. J. 2001, 65, 719-728. (23) Shubha, K. P.; Raji, C.; Anirudhan, T. S. Immobilization of heavy metals from aqueous solutions using polyacrylamide grafted hydrous tin(IV) oxide gel having carboxylate functional groups. Water Res. 2001, 35, 300-310. (24) Pearson, R. G. Absolute electronegativity and hardness: Application. Inorg. Chem. 1988, 27, 135-139. (25) Giles, C. H.; McEvan, T. H.; Nakhwa, S. N.; Smith, D. Studies in adsorption. Part II. A system of classification of solution adsorption isotherms. J. Chem. Soc. 1960, 4, 3973-3993. (26) Tiwari, R. K.; Ghosh, S. K.; Rupainwar, D. C.; Sharma, Y. C. Managing aqueous solutions rich in Mn(III): an inexpensive technique. Colloids Surf. 1993, 70, 131-137 (27) Okieimen, F. E.; Okundia, E. U.; Ogbeifun, D. E. Sorption of cadmium and lead ions on modified groundnut (Arachis hypogea) husks. J. Chem. Technol. Biotechnol. 1991, 51, 97-103. (28) Orumwense, F. F. O. Removal of lead from water by adsorption on kaolinitic clay. J. Chem. Technol. Biotechnol. 1996, 65, 363-369. (29) Singh, D. K.; Tiwari, D. P.; Saksena, D. N. Removal of lead from aqueous solution by chemically treated used tea leaves. Indian J. Environ. Health 1993, 35, 169-177. (30) Namasivayam, C.; Periasamy, K. Bicarbonate-treated peanut hull carbon for mercury(II) removal from aqueous solution. Water Res. 1993, 27, 1663-1668. (31) Selvaraj, K.; Chandramohan, V.; Pattabhi, S. Removal of Hg(II) from aqueous solution and chlor-alkali industry wastewater using photo film waste sludge. J. Sci. Ind. Res. 1998, 57, 271278. (32) Filho, N. C. D.; Polito, W. L.; Gushikem, Y. Sorption and preconcentration of some heavy metals by 2-mercaptobenzothiazole-clay. Talanta 1995, 42, 1031-1036. (33) Leyva-Ramos, R.; Rangel-Mendez, J. R.; Mendoza-Barron, J.; Fuentes-Rubio, L.; Guerrero-Coronado, R. M. Adsorption of cadmium from aqueous solution onto activated carbon. Water Sci. Technol. 1997, 35, 205-211. (34) Namasivayam, C.; Ranganathan, K. Removal of Cd(II) from wastewater by adsorption on “waste” Fe(III)/Cr(III) hydroxide. Water Res. 1995, 29, 1737-1744. (35) Sun, G.; Shi, W. Sunflower stalks as adsorbents for the removal of metal ions from wastewater. Ind. Eng. Chem. Res. 1998, 37, 1324-1328. (36) Choi, S. H.; Nho, Y. C. Adsorption of Co2+ by styrene-gpolyethylene membrane bearing sulphonic acid groups modified by radiation-induced graft copolymerisation. J. Appl. Polym. Sci. 1999, 71, 2227-2235.
Received for review December 14, 2001 Revised manuscript received July 19, 2002 Accepted July 26, 2002 IE0110181
