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Ind. Eng. Chem. Res. 2006, 45, 2850-2855
Removal of Zinc(II) from Aqueous Solution by Purified Carbon Nanotubes: Kinetics and Equilibrium Studies Chungsying Lu,* Huantsung Chiu, and Chunti Liu Department of EnVironmental Engineering, National ChungHsing UniVersity, 250 Kuo Kuang Road, Taichung 402, Taiwan
Single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) were purified by sodium hypochlorite solutions and were employed as sorbents to study the kinetics and equilibrium of Zn2+ sorption from aqueous solution. The amount of Zn2+ sorbed onto CNTs increased with a rise in temperature. Using the same conditions, the Zn2+ sorption capacity of CNTs was much greater than that of commercially available powdered activated carbon, reflecting that SWCNTs and MWCNTs are effective sorbents. The thermodynamic analysis revealed that the sorption of Zn2+ onto CNTs is endothermic and spontaneous. The sorption/desorption study showed that the Zn2+ ions could be easily removed from the surface site of SWCNTs and MWCNTs by a 0.1 mol/L nitric acid solution and the sorption capacity was maintained after 10 cycles of the sorption/desorption process. This suggests that both CNTs can be reused through many cycles of water treatment and regeneration. 1. Introduction The presence of an excessive amount of heavy metals such as cadmium, chromium, copper, lead, mercury, nickel, and zinc in an aqueous environment may result in a major concern because of their toxicity and carcinogenicity, which may cause damage to various systems of the human body.1 As a result, more stringent requirements for the removal of heavy metals from an aqueous environment in recent years have necessitated the development of innovative, cost-effective treatment alternatives. Carbon nanotubes (CNTs) are relatively new sorbents that have been proven to possess great potential for removing many kinds of pollutants such as dioxin from air2 or lead,3 cadmium,4 fluoride,5 1,2-dichlorobenzene,6 and trihalomethanes7 from water. The comparisons of CNTs with other commercially available adsorbents made by the foregoing researchers suggest that CNTs are promising sorbents for environmental protection applications. Recently, we have demonstrated that both NaClO oxidized single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) are good Zn2+ sorbents whose sorption capacity is greater than that of powdered activated carbon (PAC).8 The kinetics and equilibrium of Zn2+ sorption from aqueous solutions by CNTs was investigated in this paper. The reversibility of Zn2+ sorption onto CNTs and PAC was also studied to evaluate repeated availability of CNTs in water treatment. 2. Materials and Methods Preparation of Purified CNTs. Commercially available MWCNTs with outer diameter (dp) < 10 nm and SWCNTs with dp < 2 nm (L-type, Nanotech Port Co., Shenzhen, China) were selected as sorbents in this study. The length of CNTs was in the range of 5-15 µm, and the amorphous carbon content in the CNTs was CdO), and hydroxyl groups (-OH), attached on the surface of NaClO oxidized CNTs.8 Similar TGA results have been reported for the pristine MWCNTs and functionalized MWCNTs.10 Zeta Potential of CNTs. Figure 2 shows the effect of pH on the zeta potential of purified SWCNTs and MWCNTs. As the pH value increased, the surface charge became more negative, probably because of the deposition of more OH- ions onto the surface of CNTs. Both purified CNTs showed negative zeta potentials for pH range tested herein, which could be explained by the presence of negative functional groups on the surface of purified CNTs.8 From an electrostatic-interaction point of view, more electrostatic attractions resulted between the metal ions and the sorbents under more negative surface-charge conditions and this leads to a faster sorption rate.
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Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 Table 2. Constants of Langmuir and Freundlich Models of Zn2+ Sorption by CNTs at Various Temperaturesa Langmuir model
Freundlich model
CNTs
temp (°C)
a
b
R2
Kf
1/n
R2
SWCNT
5 15 25 35 45 5 15 25 35 45
37.03 40.65 41.84 45.45 46.94 30.30 31.74 33.33 33.78 34.36
0.132 0.159 0.161 0.164 0.178 0.101 0.124 0.118 0.129 0.149
0.997 0.997 0.996 0.998 0.997 0.983 0.987 0.982 0.979 0.985
9.97 12.01 12.37 12.75 13.58 8.23 10.43 10.62 12.15 12.87
0.303 0.288 0.289 0.305 0.302 0.283 0.242 0.248 0.219 0.215
0.979 0.984 0.984 0.976 0.971 0.979 0.983 0.984 0.972 0.984
MWCNT
a Units: a ) mg/g; b ) L/mg; K ) (mg/g)(L/mg)1/n; n ) dimensionless; f and R ) dimensionless.
Figure 2. Effect of pH on the zeta potential of CNTs.
Table 3. Rate Constants of Zn2+ Sorption by CNTs at Various Temperaturesa C0 ) 10 mg/L
C0 ) 60 mg/L
CNTs
temp (°C)
qe
k2
qe
k2
SWCNT
5 15 25 35 45 5 15 25 35 45
14.16 15.08 15.38 15.97 16.18 12.22 14.03 14.40 15.50 15.77
0.0173 0.0176 0.0181 0.0197 0.0201 0.0107 0.0109 0.0121 0.0123 0.0127
31.54 35.97 37.17 40.00 41.66 25.77 27.93 28.16 28.90 29.94
0.0095 0.0096 0.0098 0.0105 0.0105 0.0077 0.0078 0.0083 0.0084 0.0087
MWCNT
a
Units: qe ) mg/g and k2 ) g/mg/min.
at 5, 15, 25, 35, and 45 °C are 3.34, 2.90, 2.75, 2.38, and 2.28 min for the SWCNTs and 5.04, 4.59, 4.28, 4.12, and 3.84 min for the MWCNTs. It appears that the half-sorption capacity would be reached faster at a higher temperature. The equilibrium capacities of Zn2+ at 5, 15, 25, 35, and 45 °C are 31.2, 35.6, 36.8, 39.6, and 41.4 mg/g for the SWCNTs and 25.5, 27.5, 27.8, 28.5, and 29.5 mg/g for the MWCNTs. The solution pH dropped 0.5 and 0.25 pH units during sorption of Zn2+ onto SWCNTs and MWCNTs, respectively. This could be explained by the release of H+ ions from the surface site of CNTs, where Zn2+ ions are sorbed and thus lead to a decrease in solution pH. As temperature increased from 15 to 25 °C, the equilibrium pH value slightly dropped 0.06 and 0.05 pH units for the SWCNTs and MWCNTs, respectively. This was because, as more Zn2+ ions are sorbed onto CNTs at higher temperatures, more H+ ions are desorbed from the surface site of CNTs into the solution, which therefore results in a lower equilibrium pH. The effect of pH on the Zn2+ removal by CNTs has been provided elsewhere.8 Kinetics of Sorption. To analyze the sorption rate of Zn2+ onto CNTs, the pseudo-second-order rate equation derived by Ho and McKay12 is employed, Figure 3. Effect of temperature on the sorption rate of Zn2+ by CNTs: (a) SWCNT and (b) MWCNT.
Effects of Contact Time and Temperature. Parts a and b of Figure 3 show the effect of temperature on the sorption rate of Zn2+ by SWCNTs and MWCNTs, respectively. The initial Zn2+ concentration is 60 mg/L. It is seen that, for all the experiments, the sorption of Zn2+ increased quickly with time and then slowly reached equilibrium at 60 min without regard to temperature. Similar results have been reported for the sorption of Zn2+ onto activated carbon.11 However, the times required to uptake 50% of the maximum sorption capacity (t50)
t/qt ) 1/(k2qe2) + t/qe
(2)
where qt and qe are the amounts of Zn2+ sorbed onto CNTs at time t and at equilibrium (mg/g) and k2 is the pseudo-secondorder rate constant (g/mg/min). The qe and k2 values under various conditions were calculated from the slope and the intercept, respectively, of a linear plot of t/qt versus t and are given in Table 3. The correlation coefficients (R2) are all >0.999, indicating that the kinetics of Zn2+ sorption onto CNTs follows the pseudo-second-order rate law. The sorption rate constant increased with a rise in temperature, which could be explained
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by the fact that increasing temperature results in a rise in diffusion rate of Zn2+ ions across the external boundary layer and within the pores of CNTs due to the result of decreasing solution viscosity. The calculated and experimental values of qe are very close, with the deviation ∼1%. The temperature effect on the rate constant has been found in practically all cases to be well-represented by the Arrhenius equation,13
ln k2 ) ln k0 -
Ea RT
(3)
where k0 is the frequency of sorption (g/mg/min); Ea is the activation energy of the reaction (cal/mol); R is the universal gas constant (1.987 cal/mol/K); and T is the absolute temperature. After plotting ln k2 against the reciprocal of absolute temperature, 1/T, the corresponding k0 and Ea values are determined from the intercept and the slope, respectively, of each regression line. The k0 values of SWCNTs and MWCNTs, respectively, are 6.27 × 10-2 and 4.67 × 10-2 g/mg/min at C0 ) 10 mg/L and 2.35 × 10-2 and 2.11 × 10-2 g/mg/min at C0 ) 60 mg/L. The k0 value is temperature-independent and becomes greater for the SWCNTs than for the MWCNTs. The Ea values of SWCNTs and MWCNTs, respectively, are 12.59 and 14.31 kcal/mol at C0 ) 10 mg/L and 8.87 and 9.79 kcal/mol at C0 ) 60 mg/L. The Ea value is less for the SWCNTs than for the MWCNTs. This could be explained by the fact that a SWCNT has no multiple atomic layers structure like a MWCNT, which requires more energy for the diffusion of Zn2+ ions and thus leads to a slower sorption rate. The Ea value is also less for Zn2+ sorption with a higher C0. This could be explained by the fact that the diffusion driving force would be greater at a higher C0, which causes a faster diffusion rate of Zn2+ ions across the external boundary layer and within the pores of CNTs and thus requires less energy for the progress of the sorption process. Sorption Isotherm. The Zn2+ sorption data are correlated with the isotherm models of Langmuir (eq 4) and Freundlich (eq 5),
abCe 1 + bCe
(4)
q ) KfCe1/n
(5)
q)
where Ce is the equilibrium Zn2+ concentration (mg/L); a is the maximum sorption capacity (mg/g); b is the Langmuir sorption constant (L/mg); and Kf and n are the Freundlich constants. The Langmuir and Freundlich constants were obtained from fitting the isotherm model to the sorption equilibrium data and are given in Table 2. For Zn2+ sorption onto SWCNTs, the R2 values of the Langmuir model are higher than those of the Freundlich model. For Zn2+ sorption onto MWCNTs, the R2 values of both models are very close. The constants a and Kf, which represent the Zn2+ sorption capacity, are greater for the SWCNTs than for the MWCNTs and increase with a rise in temperature, confirming the experimental results of Figure 3. The constant b which reflects the free energy of sorption (b ∝ e-∆G°/RT) presents generally the same trend. The slope 1/n, which is related to the intensity of sorption, is greater for the SWCNTs, indicating the more favorable sorption of Zn2+ onto SWCNTs. Langmuir isotherms of Zn2+ sorption onto SWCNTs and MWCNTs at various temperatures are presented in parts a and b of Figure 4, respectively. It is obvious that the Zn2+ sorption capacity of CNTs increased with a rise in temperature, and high
Figure 4. Langmuir isotherms of Zn2+ sorption by CNTs at various temperatures: (a) SWCNT and (b) MWCNT.
capacity was observed at 35 and 45 °C. The isotherm curves of 15 and 25 °C are relatively close, probably because of the experimental deviation in the determination of the amount of sorbed Zn2+. The Zn2+ sorption capacity of SWCNTs is greater than that of MWCNTs, which could be explained from the BET measurements which show that the surface area of SWCNTs (423 m2/g) available for liquid-phase mass transfer is higher than that of MWCNTs (297 m2/g). As the temperature increased from 5 to 45 °C, the maximum Zn2+ sorption capacity of SWCNTs and MWCNTs calculated by the Langmuir model increased from 37.03 to 46.94 mg/g and from 30.3 to 34.36 mg/g, respectively. These values are much greater than that of commercially available PAC (13.5 mg/g, San Ying Enterprises Co., Taipei, Taiwan) measured at 25 °C in this study. This suggests that both CNTs are efficient sorbents for the removal of Zn2+ from aqueous solution. Thermodynamic Analysis. The thermodynamic parameters, free energy change (∆G°), enthalpy change (∆H°), and entropy change (∆S°) for sorption of Zn2+ by CNTs were calculated using the following equations:14
∆G° ) -RT ln b ∆S° )
∆H° - ∆G° T
(6) (7)
The enthalpy change (∆H°) is determined from the slope of the regression line after plotting ln b against the reciprocal of absolute temperature, 1/T. The free energy change (∆G°) and
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Table 4. Thermodynamic Parameters of Zn2+ Sorption by CNTs at Various Temperatures CNTs SWCNT
MWCNT
temp (°C)
b (L/mol)
∆G° (kcal/mol)
∆H° (kcal/mol)
∆S° (cal/mol/K)
5 15 25 35 45 5 15 25 35 45
8.629 × 103 1.039 × 104 1.052 × 104 1.072 × 104 1.164 × 104 6.602 × 103 8.106 × 103 7.714 × 103 8.433 × 103 9.740 × 103
-5.006 -5.293 -5.484 -5.679 -5.916 -4.858 -5.150 -5.300 -5.532 -5.803
1.134 1.134 1.134 1.134 1.134 1.381 1.381 1.381 1.381 1.381
22.086 22.316 22.208 22.120 22.170 22.440 22.677 22.419 22.445 22.591
Figure 6. Effect of regeneration time on the Zn2+ recovery of CNTs.
Figure 5. Effect of pH of the solution on the Zn2+ recovery of CNTs.
entropy change (∆S°) are determined from eqs 6 and 7, respectively. Table 4 summarizes the values of these thermodynamic parameters. It is seen that the ∆H° values are positive, indicating the endothermic nature of the sorption process. This is supported by the increase of Zn2+ sorption onto CNTs with a rise in temperature, as shown in Figure 4. Negative values of ∆G° suggest that the sorption process is spontaneous with a high preference of Zn2+ for the CNTs,15 which becomes more negative with an increase in temperature, indicating that a better Zn2+ sorption onto CNTs is actually obtained at a higher temperature. Positive values of ∆S° reflect the affinity of the CNTs for the Zn2+ and the increase of randomness at the solid/ liquid interface during the sorption of Zn2+ onto CNTs.16 Sorption/Desorption Study. Although the CNTs show better Zn2+ sorption capacity as compared to PAC, the very high cost restricts their potential use at the present time.17 Therefore, testing the reversibility of Zn2+ sorption onto CNTs to reduce the cost for the replacement of sorbents is needed before practical use of CNTs in environmental protection applications can be realized. To evaluate the reversibility of Zn2+ sorption onto CNTs, the optimum conditions for efficient desorption of Zn2+ ions from the surface site of CNTs, such as pH value of the regeneration solution and the regeneration time, must be determined. Figure 5 shows the Zn2+ recoveries of CNTs under various pH values of the regeneration solution ranging from 1 to 5. Desorption experiments were conducted for 12 h to ensure the achievement of desorption equilibrium. The Zn2+ recovery is defined as the percentage ratio of the Zn2+ sorption capacity of the regenerated sorbents to that of the virgin sorbents. The Zn2+ recovery of SWCNTs and MWCNTs, respectively, reached 91.1 and 90.7% at a solution pH of 1 and decreased to 62.28 and
Figure 7. Zn2+ sorption capacities of CNTs and PAC under various regeneration cycles of 0, 1, 5, and 10.
43.26% at a solution pH of 5. This could be explained by the fact that the surface charge of CNTs becomes more negative with a rise in pH value of the solution, as shown in Figure 2, which causes more electrostatic attractions between the Zn2+ ions and the surface of CNTs and thus results in a lower Zn2+ recovery. Figure 6 displays the Zn2+ recoveries of CNTs under various regeneration times. The pH value of the regeneration solution was adjusted at 1. It is noted that the Zn2+ recovery of CNTs increased with regeneration time and achieved equilibrium after 4 h of regeneration, in which 91 and 90.5% Zn2+ recoveries were obtained for the SWCNTs and MWCNTs, respectively. Therefore, a solution pH of 1 and a regeneration time of 4 h were chosen in the following studies. It should be mentioned that the pH of the regeneration solution and the regeneration time used in the desorption experiment depend on the initial Zn2+ concentration of the aqueous solution. The Zn2+ sorption capacity of CNTs is greater with a higher Zn2+ concentration, as shown in Figure 4, which makes desorption of Zn2+ from the surface site of CNTs more difficult. Thus, a lower solution pH and a longer regeneration time is needed to reach the efficient desorption. Figure 7 shows the Zn2+ sorption capacities of SWCNTs, MWCNTs, and PAC, while Figure 8 displays the Zn2+ recoveries of SWCNTs, MWCNTs, and PAC under various regeneration cycles (RC) of 0, 1, 5, and 10. As the regeneration cycle
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The kinetics of the sorption process was found to follow the pseudo-second-order rate law. Results of thermodynamic analysis indicated that the sorption of Zn2+ onto CNTs is endothermic and spontaneous. The sorption/desorption study showed that CNTs could be efficiently regenerated by a 0.1 mol/L HNO3 solution and the sorption capacity was maintained after 10 cycles of the sorption/desorption process. This suggests that SWCNTs and MWCNTs can be reused through many cycles of water treatment and regeneration for the removal of Zn2+ from aqueous solution. Acknowledgment Support from the National Science Council, Taiwan, under Contract No. NSC 94-2211-E-005-038 is gratefully acknowledged. Figure 8. Zn2+ Recoveries of CNTs and PAC under various regeneration cycles of 0, 1, 5, and 10.
increased, the amount of sorbed Zn2+ and the Zn2+ recovery of SWCNTs and MWCNTs slightly decreased but those of PAC sharply decreased. This could be explained by the fact that CNTs have no porous structure like PAC, in which Zn2+ ions have to move from the inner surface to the exterior surface of the pores on PAC, which makes desorption of Zn2+ ions from the surface site of CNTs much easier. The amounts of sorbed Zn2+ under regeneration cycles of 0, 1, 5, and 10, respectively, are 34.1, 30.9, 28.8, and 27.25 mg/g for the SWCNTs; 26.8, 24.2, 22.4, and 20.59 mg/g for the MWCNTs; and 11.8, 4.8, 2.18, and 1.55 mg/g for the PAC. The Zn2+ recoveries under regeneration cycles of 1, 5, and 10, respectively, are 90.61, 84.46, and 79.91% for the SWCNTs; 90.30, 83.58, and 76.83% for the MWCNTs; and 40.68, 18.47, and 13.10% for the PAC. It is evident that the Zn2+ ions would be easily removed from the surface site of SWCNTs and MWCNTs by a 0.1 mol/L HNO3 solution and the sorption capacity is maintained after 10 cycles of the sorption/desorption process. This reflects that SWCNTs and MWCNTs can be reused through many cycles of water treatment and regeneration for Zn2+ removal from aqueous solution. The cost for the replacement of sorbents is, thus, greatly reduced. This is the key factor for whether a novel but expensive adsorbent can be accepted by the field or not. It is expected that the unit cost of CNTs can be further reduced in the future so that CNTs can possibly be cost-effective sorbents. Therefore, more studies on the removal of other environmental pollutants by CNTs are strongly recommended. 4. Conclusions The removal of Zn2+ from aqueous solution by NaClO oxidized SWCNTs and MWCNTs at 5, 15, 25, 35, and 45 °C has been investigated to evaluate the kinetics and equilibrium of the sorption process. As the temperature increased from 5 to 45 °C, the maximum Zn2+ sorption capacity of SWCNTs and MWCNTs calculated by the Langmuir model increased from 37.03 to 46.94 mg/g and from 30.3 to 34.36 mg/g, respectively. These values were much greater than that of commercially available PAC (13.5 mg/g) measured at 25 °C in this study.
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ReceiVed for reView October 30, 2005 ReVised manuscript receiVed February 10, 2006 Accepted February 13, 2006 IE051206H
