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Removal of Cu(II), Pb(II), and Ni(II) by Adsorption onto Activated Carbon Cloths K. Kadirvelu, C. Faur-Brasquet,* and P. Le Cloirec Ecole des Mines de Nantes, De´ partement Syste` mes Energe´ tiques et Environnement, 4 Rue Alfred Kastler, BP 20722, 44307 Nantes Cedex 03, France Received March 31, 2000. In Final Form: July 18, 2000 The adsorption of three metal ions, Cu(II), Ni(II), and Pb(II), is performed by activated carbon cloths (ACC). Two adsorbents, CS 1501 (with more than 96% of micropore volume) and RS 1301 (with 32% of mesopore volume), are studied. Batch experiments are carried out to assess kinetic and equilibrium parameters. They allow kinetic data, transfer coefficients, and maximum adsorption capacities to be computed. These parameters show the fast external film transfer of metal ions on fibers, because of their low diameter (10 µm). Intraparticular diffusion coefficients are lower than those obtained with a granular activated carbon, but maximum adsorption capacities agree with literature values for GAC. They show the dependency of adsorption on metal ion size and ACC porosity, the largest cation Pb(II) being more adsorbed by the mesoporous cloth. The pH effect is studied, and pH adsorption edges are determined. They are short, only 2 pH units, and located below the precipitation edges. A decrease of equilibrium pH with an increase of metal ion concentration, coupled with a regeneration study of saturated ACC by HCl, lead us to propose an adsorption mechanism by ion-exchange between metal cations and H+ ions at the ACC surface. Carboxylic groups seem especially involved in this mechanism, and precipitation between metal ions could happen.
Introduction Environmental contamination due to heavy metals is caused by several industries, metal plating, mining, painting, and car radiator manufacturing, and also by agricultural sources such as fertilizers and fungicidal sprays.1 The presence of the above metals in the environment is a major concern because of their toxicity and threat for human life and for the environment, especially when tolerance levels are exceeded.2 In this context, the recovery of heavy metals from wastewater has become a major topic of research in water treatment. Several methods are commonly used (chemical precipitation, membrane filtration, and ion exchange), but adsorption has been shown to be an economical alternative for removing trace metals from water.3-6 Previous research has demonstrated the ability of granular activated carbon (GAC) for metal ion adsorption.7-11 Recently, a new form of activated carbon has appeared: activated carbon cloth (ACC). We have reported its faster adsorption kinetics than GAC and its high adsorption * Corresponding author: e-mail:
[email protected]. (1) Sitting, M. In Handbook of Toxic and Hazardous Chemicals; Noyes Publications: Park Ridge, NJ, 1981. (2) World Health Organization International Standards for Drinking Water, WHO, Geneva, 1971. (3) Huang, C. P.; Ostovic, F. B. J. Environ. Engn. Div. ASCE, 1978, 104, 863-878. (4) Lai, C. H.; Lo, S. L.; Lin, C. F. Water Sci. Technol. 1994, 30, 175-182. (5) Matsumoto, M. R.; Weber, A. S.; Kyles, J. H. Chem. Eng. Commun. 1989, 86, 1-16. (6) Allen, S. J.; Brown, P. A. J. Chem. Technol. Biotechnol. 1995, 62, 17-24. (7) Khalfaoui, B.; Meniai, A. H.; Borija, R. J. Chem. Technol. Biotechnol. 1995, 64, 153-156. (8) Gabaldon, C.; Marzal, P.; Ferrer, J.; Seco, A. Water. Res. 1996, 30, 3050-3060. (9) Toles, C. A.; Marshall, W. E.; Johons, M. M. J. Chem. Technol. Biotechnol. 1998, 72, 255-263. (10) Corapcioglu, M. O.; Huang, C. P. Water. Res. 1987, 21, 10311044. (11) Seco, A.; Gabaldon, C.; Marzal, P.; Aucejo, A. J. Chem. Technol. Biotechnol. 1999, 74, 911-918.
capacities for organic micropollutants,12-17 but its use for the removal of inorganics (and especially metal ions) has not yet been studied. The aim of this work is to evaluate the feasibility of using ACC for the removal of toxic heavy metals from aqueous solution. The influence of experimental conditions such as contact time, metal ion concentration, or pH will be studied. Experimental results will be analyzed to provide an understanding of the adsorption mechanism. Experimental Section 1. Adsorbents. The activated carbon cloths used in this study are commercial products from the Actitex Co. (Levallois, France). They were compared with a granular activated carbon1,18 prepared by chemical activation (H2SO4 + NH4S2O8) of coirpith, whose particle size ranges between 2.5 and 5 mm and which is mainly microporous with a BET surface area of 592 m2 g-1. The main physicochemical characteristics of the cloths are summarized in Table 1. Specific surface areas and pore volumes were determined by nitrogen adsorption isotherms at 77 K carried out with a COULTER SA 3100 apparatus (IPSN, Fontenay aux Roses, France) and by using BET and R-plot equations. These porosity parameters were used to confirm microscopic observations carried out with a JEOL JSM 6400-F scanning electron microscope SEM (IMN, Nantes, France). Acidic and basic surface groups were determined by titration using the Boehm method.19 The pH of the point of zero charge pHPZC, i.e., the pH above which the total surface of the carbon is negatively charged, was measured by the so-called pH drift method. ACC suspension tests were performed, to study the (12) Brasquet, C.; Roussy, J.; Subrenat, E.; Le Cloirec, P. Environ. Technol. 1996, 17, 1245-1252. (13) Brasquet, C.; Roussy, J.; Subrenat, E.; Le Cloirec, P. Water. Sci. Technol. 1996, 34, 215-222. (14) Brasquet, C.; Subrenat, E.; Le Cloirec, P. Water. Sci. Technol. 1999, 39, 201-205. (15) Le Cloirec, P.; Brasquet, C.; Subrenat, E. Energy Fuels 1997, 11, 331-336. (16) Brasquet, C.; Le Cloirec, P. Langmuir 1999, 15, 5906-5912. (17) Brasquet, C.; Le Cloirec, P. Environ. Sci. Technol. 1999, 33, 4226-4231. (18) Kadirvelu, K. Ph.D. Thesis, Bharathiar University, India, 1998. (19) Boehm, H. P. High Temp., High Press. 1990, 22, 275-288.
10.1021/la0004810 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/29/2000
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Table 1. Main Characteristics of Activated Carbon Cloths commercial name company raw material activation gas Activation temp (°C) BET surface area (m2 g-1) pore vol (cm3 g-1) micropore vol (%) median micropore diam (Å) acidic surface groupsb (mequiv g-1)
Basic surface groups (mequiv g-1) pHPZC
GI GII GIII GIV
CS 1501 Actitexa rayon CO2 1200 1689 0.665 96.4 6.9 0.60 0.10 0.40 0.00 1.10 7.60
RS 1301 Actitexa rayon H2O 900 1460 0.506 68.2 7.3 0.25 0.15 0.40 0.00 0.80 9.50
a Levallois, France. b GI ) carboxyl groups, GII ) lactone groups, GIII ) hydroxyl groups, GIV ) carbonyl groups.
amphoteric behavior of ACC in water. Five hundred milligrams of ACC were stirred for 6 h in 250 mL of distilled water, whose initial pH was adjusted to between 3 and 10 with NaOH and HCl 0.1 M. 2. Adsorbates. Three metal ions were studied (Cu(II), Pb(II), and Ni(II)). Their molecular weight are 64 g mol-1 for Cu(II), 207 g mol-1 for Pb(II), and 59 g mol-1 for Ni(II). Adsorbate stock solutions, 1000 mg L-1, were prepared by dissolving commercially available metal salts in 1% HNO3 solution to prevent hydrolysis formation. Salts used were CuSO4 for Cu(II), PbCl2 for Pb(II), and NiCl2 for Ni(II) (Aldrich) with a high degree of purity, respectively higher than 98%, equal to 98%, and higher than 99.9%. The solubility of the salt was 1.25 M for CuSO4, 0.0357 M for PbCl2, and 4.85 M for NiCl2.20 The stock solutions were diluted with distilled water to obtain standard solutions with concentration ranging from 0.1 to 1.4 mmol L-1 (in mass 10-70 mg L-1). Speciation diagrams of metal ions at a concentration of 30 mg L-1 are given in the Supporting Information. 3. Analysis. Metal ions were analyzed on a Perkin-Elmer 2280 atomic absorption spectrophotometer. 4. Batch Mode Adsorption Studies. Batch adsorption studies were conducted to determine the adsorption rates and capacities. First, adsorption kinetics allowed the equilibrium time needed to reach saturation to be determined. They were carried out at 20 ( 1 °C using 250 mL of metal ion solution containing the desired concentration (10, 20, 30, and 40 mg L-1) and 500 mg of adsorbent, in 500-mL conical flasks (stirring rate 300 rpm). At predetermined time intervals, samples were separated and filtered using 0.45 µm cellulose acetate filters. Except when pH effect was studied, all experiments were carried out at initial pH of 5.0, where the adsorption is significant but below the pH where metal hydroxide precipitation occurs. Adsorption isotherms were also performed (T ) 20 ( 1 °C), with an initial metal ion concentration ranging from 10 to 70 mg L-1 (i.e., 0.1-1.4 mmol L-1), a solution volume of 250 mL, and an activated carbon weight of 500 mg. The stirring time was 12 h. The effect of pH on percent removal was studied from pH 2 to 10 using 250 mL of metal ion solutions of 20 and 40 mg L-1 with 500 mg of carbon; 0.1 M of HCl and NaOH was used to adjust the pH. 5. Batch Mode Desorption Studies. After adsorption, the metal ion loaded carbons were separated and slightly washed with distilled water to remove unadsorbed metal ions on the carbon surface. They were stirred with 250 mL of HCl of various strengths ranging from 0.00125 to 0.01 M for 6 h. Metal ions concentrations were analyzed as before.
Results and Discussion 1. Observation of ACC with a Scanning Electronic Microscope. Figures 1 and 2 compare the activated carbon fiber cross sections of CS 1501 (magnification × 5000) and RS 1301 (magnification × 10 000) samples respectively, observed with a SEM. Both fibers have
Figure 1. Observation by SEMscross section of a CS 1501 fiber.
Figure 2. Observation by SEMscross section of a RS 1301 fiber.
diameters around 10 µm, with a ribbed structure arising from the rayon precursor.21 Whereas the CS 1501 fiber surface is quite smooth with numerous ribs along the fiber axis, the RS 1301 fiber surface contains numerous holes and only three larger ribs. These micrographs confirm the microporous character of the CS 1501 sample, more than 96% of whose volume is microporous, and show on the RS 1301 sample some aligned mesopores with diameters ranging from 100 to 800 Å. Differences in porosity characteristics arise from the activation conditions. Carbon dioxide, which has a higher diffusion coefficient than steam, leads to a tight microporosity.22,23 2. Activated Carbon Cloth Suspension Tests. Figure 3 gives the results of CS 1501 and RS 1301 suspension tests. This figure shows that, for both carbons, the final pH value (pHf) is lower (respectively higher) than initial pH value (pH0) for pH0 > pHPZC (respectively pH0 < pHPZC). It shows the amphoteric character of surface oxides S-CO2, where S represents the ACC surface. For pH < pHPZC, the dominant reaction is: S-CO2 + 2H2O T S-C(OH+)2 + 2OH-. The release of hydroxyl ions induces an increase of pH and a protonated surface of ACC. For pH > pHPZC, the following reaction takes place: S-CO2 + 2H2O T S-C(O-)2 + 2H3O+. The ACC surface is deprotonated and the release of protons induces a decrease in pH. 3. Adsorption Kinetics. Figure 4 shows the effect of contact time and initial metal ion concentration on Cu(II) adsorption onto CS 1501. The equilibrium was reached
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Figure 3. Supension tests of ACC. ACC weight ) 500 mg, V ) 250 mL, 6 h stirring.
Figure 4. Adsorption kinetics of copper(II) onto CS 1501 at various concentrations. V ) 250 mL, pH ) 5.0, ACC mass ) 0.5 g, T ) 20 °C.
within 2 h for all concentrations of all three metal ions. The contact times required for all the metal ions and both carbons are thus very short. This result is interesting because equilibrium time is one of the parameters for economical wastewater treatment plant applications. According to these results, the stirring time was fixed at 12 h for the rest of the batch experiments to make sure equilibrium is reached. Kinetic plots were modeled to compute the initial adsorption kinetic coefficients γ (L mg-1 min-1),24 the external film mass transfer coefficients ksA (L s-1),25 and the intraparticular diffusion coefficients Kw (mg L-1 s-0.5)26,27 presented in Table 2. Saturation capacities qs and kinetic coefficients of a granular activated carbon18 are also given in this table. Coefficients are calculated from these equations:
γ)
(dC dt )
V tf0mC0
( )
For Cu(II) and Ni(II), initial adsorption coefficients are higher onto CS 1501 than RS 1301. In the case of Pb(II), kinetics are faster with RS 1301. These results will be explained in terms of ion molecular weight (MW) and activated carbon cloth porosity. The higher MW ion, Pb(II), is indeed better removed by the mesoporous carbon. Adsorption capacities at saturation qs confirm this kinetic result: CS 1501 is a better adsorbent than RS 1301 for Cu(II) and Ni(II). In return, capacity values are similar for both carbons in the case of Pb(II) uptake. The comparison of kinetic properties of ACC and GAC show that despite γGAC > γACC, the external mass transfer coefficients ksA of ACC are higher than those of GAC. This result may be imputed to the lower diameter of fibers (10 µm) compared with GAC (2.5-5 mm), which induces a higher external surface area and thus a faster film diffusion. Higher values of γ for GAC may then be the consequence of the faster intraparticular diffusion of metals through GAC porosity, shown by higher Kw. Furthermore, some different operating conditions for GAC and ACC, resulting in a different ratio (VC0/m), could also explain these differences. According to eq 1, for constant V and m values in the case of ACC, the initial adsorption kinetic coefficient γ increases with the initial concentration C0. The decrease of ksA values when C0 increases can be related to the concentration dependence of diffusivity. 4. Adsorption Isotherms. Adsorption isotherms of the three metal ions onto CS 1501, with adsorption capacities expressed in mg g-1 and mmol g-1, are respectively given in Figures 5 and 6. When concentration is expressed in mg L-1, the adsorption capacities (mg g-1) increase following the order: Pb(II) > Cu(II) > Ni(II). In the case where they are expressed in mmol L-1, the adsorption capacities (mmol g-1) order is the following: Cu(II) > Pb(II) > Ni(II). Allen et al.6 had the same results for the adsorption of Cu, Zn, and Cd onto granular activated carbon. This result, due to the inclusion of the metal ion molecular weight when initial concentrations units are moles per liter, will be taken into account in the rest of this study. With the purpose of a multicomponent adsorption, it would be important to express isotherm capacities on a molar basis. Two models were tested to model the adsorption isotherms: Freundlich’s equation28 and the BrunauerEmmett-Teller (BET) model29 to determine parameters which quantify the adsorption process. These models are respectively described by eqs 4 and 5. The Langmuir equation was also tested but low determination coefficients r2 demonstrated that some assumptions, like the monolayer coverage of the adsorbent surface, must not be respected. The Freundlich isotherm is represented by:
qe ) KfCe1/n
(1)
C t ) ksA -ln C0 V
(2)
m q ) Kwt1/2 V
(3)
where t is time (min), C is the metal ion concentration at time t (mg L-1), V is the solution volume (L), m is the activated carbon weight (mg), C0 is the initial concentration (mg L-1), A is the surface concerned by external mass transfer (m2), and q is the adsorption capacity at time t (mg g-1).
(4)
where Ce is the equilibrium concentration (mmol L-1), qe is the amount adsorbed at equilibrium (mmol g-1), and Kf (mmol1-1/n L1/n g-1) and 1/n are Freundlich constants incorporating all factors affecting the adsorption process. The linear form of the BET equation, which assumes a multilayer coverage, is given by
Ce (Cs - Ce)qe
)
( )
1 b - 1 Ce + bqm bqm Cs
(5)
where Ce is the equilibrium concentration (mmol L-1), qe is the amount adsorbed at equilibrium (mmol g-1), Cs is
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Table 2. Initial Adsorption Coefficients, Transfer Coefficients, and Equilibrium Capacities for Adsorption of Metal Ions onto CS 1501 and RS 1301a V (mL)
m (mg)
C0 (mg L-1)
VC0/m
γ (×105 L mg-1 min-1
ksA × 105 L s-1
Kw (mg L-1 s-0.5)
qs (mg g-1)
Cu(II) 250 mL
CS 1501 500 mg
250 mL
RS 1301 500 mg
50 mL
GAC 50 mg
Ni(II) 250 mL
CS 1501 500 mg
250 mL
RS 1301 500 mg
50 mL
GAC 20 mg
Pb(II) 250 mL
CS 1501 500 mg
250 mL
RS 1301 500 mg
50 mL
GAC 20 mg
10 20 30 40 10 20 30 40 20 30 40 50 10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40 25 50 75 100
0.005 0.01 0.015 0.02 0.005 0.01 0.015 0.02 0.02 0.03 0.04 0.05 0.005 0.01 0.015 0.02 0.005 0.01 0.015 0.02 0.025 0.05 0.075 0.1 0.005 0.01 0.015 0.02 0.005 0.01 0.015 0.02 0.063 0.125 0.188 0.250
2.31 1.64 1.56 1.28 1.65 1.20 0.98 0.94 4.10 4.00 3.60 3.24 4.09 2.82 2.29 2.07 3.04 1.70 1.96 2.40 78.1 8.28 6.14 5.55 2.27 1.74 1.81 1.19 2.50 2.33 2.11 2.06 12 10.1 9.46 7.94
85 25 22.5 17.5 22.5 12.5 9.50 8.92 7.75 7.00 5.75 4.85 12.5 12.5 8.88 8.88 8.50 7.00 6.75 6.50 3.75 4.35 2.60 2.60 158 54 38.7 22.8 49.8 21.8 18.3 15.3 10.3 7.65 6.25 na
0.382 0.417 0.560 0.615 0.188 0.293 0.346 0.435 0.503 0.699 0.852 0.967 0.118 0.171 0.204 0.252 0.083 0.109 0.169 0.269 0.164 0.373 0.418 0.560 0.422 0.504 0.695 0.708 0.246 0.365 0.498 0.553 0.714 1.269 1.713 2.086
5.0 9.2 12.3 15.3 4.5 8.0 9.5 11.5 18.4 24.9 31.8 36 2.3 3.8 4.5 5.8 1.5 2.3 3.7 5.1 20.63 39.5 45.00 64.38 5.0 10.0 14.5 17.3 5.0 9.8 14.1 17.2 60.0 120.6 167.5 210.6
a
na, not available; V, solution volume; m, activated carbon mass; C0, metal initial concentration.
Figure 5. Adsorption isotherms of metal ions onto CS 1501. Concentrations and adsorption capacities are expressed in milligrams per liter and milligrams per gram.
Figure 6. Adsorption isotherms of metal ions onto CS 1501. Concentrations and adsorption capacities are expressed in millimoles per liter and millimole per gram.
the saturation concentration, i.e., solubility (mmol L-1), qm is the maximum adsorption capacity (mmol g-1), and b is the Langmuir constant related to energy of adsorption (L mmol-1). All parameters, Kf, 1/n, qm, and b, are presented in Table 3. 1/n values lower than 1 confirm a favorable adsorption of metal ions onto the activated carbon cloths. As can be observed by comparison of modeled results presented in Table 3, the maximum adsorption capacities qm order like Cu(II) > Ni(II) > Pb(II) at the same initial pH. We can imagine that the high ionic radius of Pb(II)
(1.12 Å) compared to that of Cu(II) (0.70 Å) or Ni(II) (0.69 Å)30 induces a quick saturation of adsorption sites, because of steric overcrowding. It results in lower maximum adsorption capacities for Pb(II) than for Cu(II) and Ni(II). Previous research has reported that the preference of hydrous solids for metals has been related to the metal electronegativity.6 The reported effect is a stronger attraction for the higher electronegativity, which seems to be also observed in the case of adsorption onto activated carbon cloths if we refer to qm values and Pauling electronegativity values (2.00 for Cu(II), 1.87 for Pb(II),
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Table 3. BET and Freundlich Parameters (r2 ) Determination Coefficient) BET parameters metal ion
ACC
Cu(II)
CS 1501 RS 1301 CS 1501 RS 1301 CS 1501 RS 1301
Pb(II) Ni(II)
qm (mmol
g-1)
b (L
0.174 0.134 0.147 0.124 0.152 0.127
mol-1)
46.5 68.8 0.81 0.94 15.4 11.3
and 1.91 for Ni(II)30). Furthermore, adsorption capacities observed for both carbons are in agreement with those previously reported for other carbons, with maximum capacities slightly higher than those obtained onto GAC, which range between 0.056 and 1.02 mmol g-1 for Cu(II),10,31-33 Pb(II),10,34 and Ni(II).18,33 4. Approach of the Adsorption Mechanism. To understand the adsorption mechanism of metal ions by ACC, the effect of pH was studied. First, the final values of pH at adsorption equilibrium were measured, for an initial pH of 5.00, for each isotherm. Results of measurements showed that for both ACC and for all initial metal concentrations, the final pH was higher than initial pH with a value which decreased when the initial metal concentration increased. The decrease of pH when initial metal concentration increases may be due to a release of H+ ions and may indicate an adsorption mechanism by ion-exchange. Despite this release of H+ ions, final pH value is higher than initial pH (5.00) due to the release of hydroxyl ions by ACC as was shown by suspension tests. The ion exchange mechanism between H+ ions at the ACC surface and metal ions may happen following these reactions:
2 S-COH + M+ T S-(CO)2M+ + 2H+
(6)
S-COH22+ + M+ T S-COM2+ + 2H+
(7)
Freundlich parameters r2 0.987 0.999 0.958 0.967 0.933 0.986
(20) Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press Inc.: Boca Raton, FL, 1996. (21) Lord, E. J. Text. Inst. 1995, 46, 191-213. (22) Alcaniz-Monge, J.; Cazorla-Amoros, D.; Linares-Solano, D.; Yoshida, S.; Oya, A. Carbon 1994, 32, 1277-1283. (23) Molina-Sabio, M.; Gonzales, M. T.; Rodriguez-Reinoso, F.; Sepulveda-Escribano, A. Carbon 1996, 34, 505-509. (24) Baudu, M.; Le Cloirec, P.; Martin, G. Water Sci. Technol. 1991, 23, 1659-1665. (25) Spahn, H.; Schlu¨nder, U. Chem. Eng. Sci. 1975, 30, 529-537. (26) McKay, G.; Bino, M. J. Water Res. 1988, 22, 279-288. (27) Morris, J. C.; Weber, W. J. Advances in Water Pollution Research; Proceedings of the 1st International Conference; Pergamon Press: New York, 1962; Vol. 2, pp 231-266. (28) Freundlich, H.; Heller, W. J. Am. Chem. Soc. 1939, 61, 2228. (29) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (30) McQuarrie, Rock, General Chemistry, 3rd ed.; DeBoeck University, 1991. (31) Ferro-Garcia, M. A.; Rivera-Utrilla, J.; Rodriguez-Gordillo, J.; Bautista-Toledo, I. Carbon 1989, 26, 363-373. (32) Namasivayam, C.; Kadirvelu, K. Chemosphere 1997, 34, 377399. (33) Seco, A.; Marzal, P.; Ferrer, J. J. Chem. Technol. Biotechnol. 1989, 68, 23-30. (34) Kadirvelu, K.; Navasivayam, C. Environ. Technol. in press.
L 1/n g-1)
0.201 0.175 0.125 0.171 0.187 0.098
1/n
r2
0.225 0.228 0.559 0.331 0.311 0.655
0.901 0.846 0.985 0.972 0.979 0.961
Figure 7. Influence of initial pH values on adsorption onto CS 1501 and hydroxide precipitation of Cu(II). V ) 250 mL, ACC weight ) 500 mg, C0 ) 20 and 40 mg L-1. Table 4: pH Adsorption and Precipitation Edges metal ion
ACC
Cu(II)
CS 1501 RS 1301 CS 1501 RS 1301 CS 1501 RS 1301
Ni(II) Pb(II)
The final pH after Ni(II) adsorption was higher than that obtained after Cu(II) or Pb(II) adsorption. It could be explained on the basis of adsorption capacity and equilibrium pH. One conventional method of removing metals from aqueous solution is the precipitation of metal hydroxides using an alkali. This method has, however, some demerits in that the complete removal of metals is not possible due to the solubility product of metal hydroxide. The effect of
Kf
(mmol1-1/n
pH adsorption edge pH precipitation edge 2.50-5.00 2.00-6.00 4.00-9.00 5.00-9.00 3.00-4.00 3.00-4.00
6.00-7.00 8.00-9.00 7.00-8.00
initial pH value was then studied, to determine at which pH the hydroxide precipitation of metal ions occurs. Figure 7 presents, for two initial concentrations, 20 and 40 mg L-1, the influence of initial pH on the adsorption and precipitation mechanism, in the case of Cu(II) adsorption onto CS 1501. The number of pH units over which the fraction adsorbed increases from 10 to 80% is defined as the pH adsorption edge. In Figure 7, the pH adsorption edge is 2.5-4.5 for an initial concentration of 20 mg L-1 and 3-5 for 40 mg L-1. pH adsorption edges are given for both ACC and all metal ions in Table 4; they are very short, which shows the strong dependence of adsorption on pH. This study of initial pH influence leads to the following conclusions: 1. When adsorption occurs at pH 5.00, for both ACC and all metal ions, it occurs below the pH of precipitation and this is the only mechanism of removing metal ions. In return, at the above precipitation pH edges (Table 4), due to the incomplete removal of metals by precipitation, metals were removed by both adsorption and precipitation. 2. The increase in metal ion removal as pH increases confirms the adsorption mechanism by ion-exchange proposed above, because of a decrease in electrostatic repulsion between cations and the positively charged surface of ACC. As pH increases, there is a decrease in competition between H+ ions in solution and metal cations at the surface adsorption sites. Maximum adsorption occurred in a pH range 4.00-6.00 for Cu(II) and Pb(II), which may be due to their partial hydrolysis resulting in the formation of hydrolyzed species [CuOH]+, [PbOH]+, Cu(OH)2, and Pb(OH)2 (see species diagrams in Supporting
Removal of Cu(II), Pb(II), and Ni(II)
Information). In the case of Ni(II), the maximum range was shifted to pH 7.00-9.00, which might be due to formation of [NiOH]+ and Ni(OH)2 at these pHs. Low solubilities of hydrolysis metal ion species may be another reason for maximal adsorption. 3. At constant pH, the removal efficiencies of metal ions are affected by an increase in the initial metal concentration. pH curves are shifted to alkaline regions, as has been previously reported by several authors.10,11,32,35 At low metal concentration, the metal adsorption involves the higher energy surface sites. As the metal ion concentration increases, the higher energy surface sites are saturated and adsorption begins on the lower energy surface sites, resulting in a decrease of the adsorption efficiency.36,37 Desorption studies were carried out to confirm the adsorption mechanism proposed above and to recover precious metals from wastewater and adsorbent. Their aim was to desorb metal ions from the metal loaded carbons with various molarities of HCl. For both carbons and three metal ions, the quantitative percent of recovery of metal ions (from 80 to 100% for HCl concentrations ranging from 0.005 to 0.05 M) demonstrates that regeneration of carbons is possible. Furthermore, this recovery is another evidence that ion exchange must be involved in the adsorption mechanism. The proposed adsorption mechanism is also confirmed by adsorption isotherms results. The comparison of maximal adsorption capacities qm of the BET model, expressed in milliequivalents per gram (two times the qm expressed in millimoles per gram), and of acidic surface groups shows that for the RS 1301 sample, all adsorption capacities are of the same order as the concentration of carboxylic groups (group I) and for the microporous cloth, CS 1501, adsorption capacities qm are higher than those obtained with RS 1301 for a given metal ion but always lower than the concentration of carboxylic groups (0.60 meq g-1). The result may be explained in terms of pore accessibility. In the case of CS 1501, some micropore entrances may be blocked by hydrolyzed metal species which are larger than the metal ions. Accordingly, surface groups located in micropores are no longer accessible and all surface groups are not used for adsorption, inducing a qm value inferior to the surface group concentration. In return, reactions between metal ions may occur on surface groups, inducing a multilayer adsorption which explains the agreement of BET model to experimental data. In the case of RS 1301, which contains more mesopores, accessibility to micro- and mesopores is not blocked by (35) Reed, B. E.; Nonavinakere, S. K. Sep. Sci. Technol. 1992, 27, 1985-2000. (36) Reed, B. E.; Matsumoto, M. R. J. Environ. Eng. Div., ASCE, 1993, 119, 332-348. (37) Marzal, P.; Seco, A.; Gabaldon, C.; Ferrer J. J. Chem. Technol. Biotechnol. 1996, 66, 279-285.
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hydrolyzed species and all carboxylic surface groups are used for adsorption of metal ions. Conclusion The objective of this work was to study the dependence of adsorption on adsorbent and adsorbate characteristics, in the case of removal of three metal ions (Cu(II), Ni(II), and Pb(II)) onto two activated carbon cloths. A comparison was made with results obtained with granular activated carbon. The adsorption kinetic data showed the relationship between the metal ion size (ionic radius) and the ACC porosity. For example, the high MW metal ion (Pb(II)) is adsorbed more by the mesoporous ACC. They demonstrated also the fast film transfer diffusion in the case of fibers, due to their low diameter compared with a granular activated carbon. However, global kinetics were faster with the GAC because of a faster intraparticular diffusion. All kinetic data may be useful for environmental technologists in designing heavy metal containing wastewaters. In batch mode studies, the adsorption capacities of metal ions were demonstrated to be dependent on the solution pH, initial metal ion concentration, and carbon dosage. The adsorption followed both BET and Freundlich isotherm models. BET modeling showed the following order for maximum adsorption capacities (molar basis): Cu(II) > Ni(II) > Pb(II), with values ranging between 0.124 and 0.174 mmol g-1. The pH effect on metal ion removal was high, with short pH adsorption edges (two pH units) located below precipitation pH. All experimental results allow proposal of an adsorption mechanism by ion-exchange between metal cations and H+ ions at the ACC surface. These reactions induce a release of H+ ions in solution and thus a decrease of pH. Carboxylic groups seem especially involved into the adsorption process, and in the case of the microporous cloth, all surface groups are not saturated because of a blocking of micropore entry by hydrolyzed species of metal ions. Regeneration of ACC saturated by metal ions was also studied using HCl. It was possible and confirmed that ion-exchange seems to be an important process in the adsorption of metal ions by activated carbon cloths. Acknowledgment. We are grateful to Region Pays de la Loire, France, for financial support of this work. Supporting Information Available: Speciation diagrams of copper, lead, and nickel. This material is available free of charge via the Internet at http://pubs.acs.org. LA0004810
