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Adsorption of Cu(II) on oxidized multi-walled carbon nanotubes in the presence of hydroxylated and carboxylated fullerenes.

Wang J, Li Z, Li S, Qi W, Liu P, Liu F, Ye Y, Wu L, Wang L, Wu W - PLoS ONE (2013)

Bottom Line: The effect of C60(OH)n on Cu(II) adsorption of oMWCNTs was not significant at low C60(OH)n concentration, whereas a negative effect was observed at higher concentration.The adsorption of Cu(II) on oMWCNTs was enhanced with increasing pH values at pH < 5, but decreased at pH ≥ 5.The double sorption site model was applied to simulate the adsorption isotherms of Cu(II) in the presence of C60(OH)n and fitted the experimental data well.

View Article: PubMed Central - PubMed

Affiliation: Radiochemistry Laboratory, School of Nuclear Science and Technology, Lanzhou University, Lanzhou, PR China.

ABSTRACT
The adsorption of Cu(II) on oxidized multi-walled carbon nanotubes (oMWCNTs) as a function of contact time, pH, ionic strength, temperature, and hydroxylated fullerene (C60(OH)n) and carboxylated fullerene (C60(C(COOH)2)n) were studied under ambient conditions using batch techniques. The results showed that the adsorption of Cu(II) had rapidly reached equilibrium and the kinetic process was well described by a pseudo-second-order rate model. Cu(II) adsorption on oMWCNTs was dependent on pH but independent of ionic strength. Compared with the Freundlich model, the Langmuir model was more suitable for analyzing the adsorption isotherms. The thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that Cu(II) adsorption on oMWCNTs was spontaneous and endothermic. The effect of C60(OH)n on Cu(II) adsorption of oMWCNTs was not significant at low C60(OH)n concentration, whereas a negative effect was observed at higher concentration. The adsorption of Cu(II) on oMWCNTs was enhanced with increasing pH values at pH < 5, but decreased at pH ≥ 5. The presence of C60(C(COOH)2)n inhibited the adsorption of Cu(II) onto oMWCNTs at pH 4-6. The double sorption site model was applied to simulate the adsorption isotherms of Cu(II) in the presence of C60(OH)n and fitted the experimental data well.

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Effect of C60(OH)n on Cu(II) adsorption on oMWCNTs as a function of pH at different ionic strength, m/V  = 0.5 g/L, T = 25±1°C, C[Cu2+]initial  = 1.87×10−4 mol/L, (A) C[C60(OH)n] = 125 mg/L; (B) C[C60(OH)n] = 250 mg/L.
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pone-0072475-g009: Effect of C60(OH)n on Cu(II) adsorption on oMWCNTs as a function of pH at different ionic strength, m/V  = 0.5 g/L, T = 25±1°C, C[Cu2+]initial  = 1.87×10−4 mol/L, (A) C[C60(OH)n] = 125 mg/L; (B) C[C60(OH)n] = 250 mg/L.

Mentions: The pH-dependent Cu(II) adsorption onto oMWCNTs in the absence and presence of C60(OH)n is given in Figure 8. One can see that C60(OH)n has almost no effect on Cu(II) adsorption on the surface of oMWCNTs at its lower concentration (i.e. 5 mg/L), but the adsorption of Cu(II) onto oMWCNTs decreases with increasing concentration of C60(OH)n. When the concentration of C60(OH)n is 125 mg/L, the adsorption of Cu(II) onto oMWCNTs increases from approximate 26% at pH 3, to nearly 71% at pH 5.4 and decreases to 50% at pH 7.5. When adding 250 mg/L C60(OH)n, the adsorption of Cu(II) is lower than when 125 mg/L C60(OH)n is added. The adsorption of Cu(II) on oMWCNTs increases from nearly 16% at pH 3, to approximately 57% at pH 5.4 and decreases to 20% at pH 7.5. This indicates that C60(OH)n inhibits the adsorption of Cu(II) on oMWCNTs. Nonetheless, it can also be seen from Figure 8 that the adsorption of Cu(II) on the oMWCNTs still increases as pH rises to pH of about 5, but sharply decreases with pH above 5 in the presence of C60(OH)n. According to the literature, the effect of organic species on metal adsorption on oMWCNTs is mainly attributed to the complexation between organic materials, metal ions, and surface functional groups of oMWCNTs by hydrophobic interaction, electrostatic attraction or repulsion, hydrogen bonding between the −OH and the tube surface −OH or−COOH groups, and π-π interactions between the phenolics and the carbon nanotubes [20], [55]. Generally, organic materials act as a “bridge” between metal ions and oMWCNTs to enhance the ability of oMWCNTs to adsorb Cu(II) from the solution. According to this explanation, if the interaction between C60(OH)n, metal ions, and oMWCNTs occurs, the adsorption of Cu(II) would increase due to the formation of three component complexation. However, this phenomenon does not appear in Figure 8. Moreover, this deduction does not explain why the addition of Cu(II) and C60(OH)n do not affect the adsorption of Cu(II) and C60(OH)n onto oMWCNTs (Table S2 in File S1). Consequently, it is possible to exclude the potential role of C60(OH)n between Cu(II) and oMWCNTs. Two other possible reasons for the inhibition of C60(OH)n are as follows: (a) there is an interaction between C60(OH)n and Cu(II), and C60(OH)n is not adsorbed to oMWCNT surfaces, so that more Cu(II) is dissolved in solution, thus reducing the adsorption of Cu(II) on the oMWCNTs; (b) C60(OH)n is adsorbed onto oMWCNT surfaces, but it does not complex with Cu(II), because the competition of C60(OH)n and Cu(II) for adsorption sites of oMWCNTs surface, and the adsorbed Cu(II) on oMWCNTs is “squeezed” down by the adsorbed C60(OH)n, thus weakening the adsorption of Cu(II) to oMWCNTs. The first case assumes that C60(OH)n is not adsorbed on the surface of oMWCNTs, because the number of hydroxyl groups of C60(OH)n are more than that of oMWCNTs, and the complex degree of C60(OH)n with Cu(II) is greater than that of oMWCNTs with Cu(II). Therefore, the adsorbed Cu(II) on the oMWCNTs surface is in turn adsorbed to C60(OH)n. Because of the adsorption competition for the functional group of C60(OH)n and oMWCNTs, Cu(II) adsorption is reduced on oMWCNT surfaces. However, the adsorption of Cu(II) on oMWCNTs increases with increasing pH at acid pH values, and the first case cannot reasonably explain the experimental results; in addition, the TEM (Figure 1B) picture indicates that C60(OH)n is adsorbed on the surface of oMWCNTs; this is also a contradiction with the first assumption. Therefore, the first explanation can be ruled out. If C60(OH)n and Cu(II) connect together due to complexation, the changing trend of Cu(II) on oMWCNTs either rises more than without C60(OH)n or declines, but this is inconsistent with the experimental results. Therefore, the second explanation is obviously reasonable since higher concentrations of C60(OH)n does not accelerate the Cu(II) adsorption on the oMWCNTs surface. Figure 9 illustrates that the concentration of background electrolyte has no significant affect on the adsorption of Cu(II) on oMWCNTs in the presence of C60(OH)n. This indicates that C60(OH)n does not interact with Na+ in the background solution. It also verifies that the interaction between C60(OH)n and the metal ions does not occur when adding C60(OH)n. The presence of π-π stacking interactions are supportive of the sorption of aromatic C60(OH)n to the benzene rings of the MWCNT sidewalls. Besides, electron-donating (OH) constituents on C60(OH)n can enhance its adsorption rate onto oMWCNTs. The dissociation constants (pKa) of C60(OH)n (organic base) and C60(C(COOH)2)n (organic acid) are shown in Table S1 in File S1. When pH < pKa, the non-dissociated species and the dissociated species are dominated by organic acids and organic bases, respectively; meanwhile, the hydroxyl groups of oMWCNTs carry positive charges due to protonation (pH < pHpzc). The deprotonation of functional groups on oMWCNT surfaces decreases with increasing pH due to deprotonation of C60(OH)n. Thus the adsorption of Cu(II) on oMWCNTs rises slowly with increasing pH due to the competition of H+ and Cu2+ for the sorption site on oMWCNTs at pH <4.6. More and more negative C60(OH)n are adsorbed onto oMWCNTs due to its greater electron donating effect of −O− at pH >4.6, thus resulting in the enhancement of space hindrance effect of oMWCNTs. Additionally, π-π stacking interactions are stronger than chemical complexation, and the increasing adsorption of C60(OH)n on oMWCNTs also “squeezes” down Cu(II) adsorption leading to further decline. In short, the adsorption of C60(OH)n onto oMWCNTs affects the surface characteristics and sorption sites, resulting in the observed changes of Cu(II) adsorbed on oMWCNTs.


Adsorption of Cu(II) on oxidized multi-walled carbon nanotubes in the presence of hydroxylated and carboxylated fullerenes.

Wang J, Li Z, Li S, Qi W, Liu P, Liu F, Ye Y, Wu L, Wang L, Wu W - PLoS ONE (2013)

Effect of C60(OH)n on Cu(II) adsorption on oMWCNTs as a function of pH at different ionic strength, m/V  = 0.5 g/L, T = 25±1°C, C[Cu2+]initial  = 1.87×10−4 mol/L, (A) C[C60(OH)n] = 125 mg/L; (B) C[C60(OH)n] = 250 mg/L.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3756995&req=5

pone-0072475-g009: Effect of C60(OH)n on Cu(II) adsorption on oMWCNTs as a function of pH at different ionic strength, m/V  = 0.5 g/L, T = 25±1°C, C[Cu2+]initial  = 1.87×10−4 mol/L, (A) C[C60(OH)n] = 125 mg/L; (B) C[C60(OH)n] = 250 mg/L.
Mentions: The pH-dependent Cu(II) adsorption onto oMWCNTs in the absence and presence of C60(OH)n is given in Figure 8. One can see that C60(OH)n has almost no effect on Cu(II) adsorption on the surface of oMWCNTs at its lower concentration (i.e. 5 mg/L), but the adsorption of Cu(II) onto oMWCNTs decreases with increasing concentration of C60(OH)n. When the concentration of C60(OH)n is 125 mg/L, the adsorption of Cu(II) onto oMWCNTs increases from approximate 26% at pH 3, to nearly 71% at pH 5.4 and decreases to 50% at pH 7.5. When adding 250 mg/L C60(OH)n, the adsorption of Cu(II) is lower than when 125 mg/L C60(OH)n is added. The adsorption of Cu(II) on oMWCNTs increases from nearly 16% at pH 3, to approximately 57% at pH 5.4 and decreases to 20% at pH 7.5. This indicates that C60(OH)n inhibits the adsorption of Cu(II) on oMWCNTs. Nonetheless, it can also be seen from Figure 8 that the adsorption of Cu(II) on the oMWCNTs still increases as pH rises to pH of about 5, but sharply decreases with pH above 5 in the presence of C60(OH)n. According to the literature, the effect of organic species on metal adsorption on oMWCNTs is mainly attributed to the complexation between organic materials, metal ions, and surface functional groups of oMWCNTs by hydrophobic interaction, electrostatic attraction or repulsion, hydrogen bonding between the −OH and the tube surface −OH or−COOH groups, and π-π interactions between the phenolics and the carbon nanotubes [20], [55]. Generally, organic materials act as a “bridge” between metal ions and oMWCNTs to enhance the ability of oMWCNTs to adsorb Cu(II) from the solution. According to this explanation, if the interaction between C60(OH)n, metal ions, and oMWCNTs occurs, the adsorption of Cu(II) would increase due to the formation of three component complexation. However, this phenomenon does not appear in Figure 8. Moreover, this deduction does not explain why the addition of Cu(II) and C60(OH)n do not affect the adsorption of Cu(II) and C60(OH)n onto oMWCNTs (Table S2 in File S1). Consequently, it is possible to exclude the potential role of C60(OH)n between Cu(II) and oMWCNTs. Two other possible reasons for the inhibition of C60(OH)n are as follows: (a) there is an interaction between C60(OH)n and Cu(II), and C60(OH)n is not adsorbed to oMWCNT surfaces, so that more Cu(II) is dissolved in solution, thus reducing the adsorption of Cu(II) on the oMWCNTs; (b) C60(OH)n is adsorbed onto oMWCNT surfaces, but it does not complex with Cu(II), because the competition of C60(OH)n and Cu(II) for adsorption sites of oMWCNTs surface, and the adsorbed Cu(II) on oMWCNTs is “squeezed” down by the adsorbed C60(OH)n, thus weakening the adsorption of Cu(II) to oMWCNTs. The first case assumes that C60(OH)n is not adsorbed on the surface of oMWCNTs, because the number of hydroxyl groups of C60(OH)n are more than that of oMWCNTs, and the complex degree of C60(OH)n with Cu(II) is greater than that of oMWCNTs with Cu(II). Therefore, the adsorbed Cu(II) on the oMWCNTs surface is in turn adsorbed to C60(OH)n. Because of the adsorption competition for the functional group of C60(OH)n and oMWCNTs, Cu(II) adsorption is reduced on oMWCNT surfaces. However, the adsorption of Cu(II) on oMWCNTs increases with increasing pH at acid pH values, and the first case cannot reasonably explain the experimental results; in addition, the TEM (Figure 1B) picture indicates that C60(OH)n is adsorbed on the surface of oMWCNTs; this is also a contradiction with the first assumption. Therefore, the first explanation can be ruled out. If C60(OH)n and Cu(II) connect together due to complexation, the changing trend of Cu(II) on oMWCNTs either rises more than without C60(OH)n or declines, but this is inconsistent with the experimental results. Therefore, the second explanation is obviously reasonable since higher concentrations of C60(OH)n does not accelerate the Cu(II) adsorption on the oMWCNTs surface. Figure 9 illustrates that the concentration of background electrolyte has no significant affect on the adsorption of Cu(II) on oMWCNTs in the presence of C60(OH)n. This indicates that C60(OH)n does not interact with Na+ in the background solution. It also verifies that the interaction between C60(OH)n and the metal ions does not occur when adding C60(OH)n. The presence of π-π stacking interactions are supportive of the sorption of aromatic C60(OH)n to the benzene rings of the MWCNT sidewalls. Besides, electron-donating (OH) constituents on C60(OH)n can enhance its adsorption rate onto oMWCNTs. The dissociation constants (pKa) of C60(OH)n (organic base) and C60(C(COOH)2)n (organic acid) are shown in Table S1 in File S1. When pH < pKa, the non-dissociated species and the dissociated species are dominated by organic acids and organic bases, respectively; meanwhile, the hydroxyl groups of oMWCNTs carry positive charges due to protonation (pH < pHpzc). The deprotonation of functional groups on oMWCNT surfaces decreases with increasing pH due to deprotonation of C60(OH)n. Thus the adsorption of Cu(II) on oMWCNTs rises slowly with increasing pH due to the competition of H+ and Cu2+ for the sorption site on oMWCNTs at pH <4.6. More and more negative C60(OH)n are adsorbed onto oMWCNTs due to its greater electron donating effect of −O− at pH >4.6, thus resulting in the enhancement of space hindrance effect of oMWCNTs. Additionally, π-π stacking interactions are stronger than chemical complexation, and the increasing adsorption of C60(OH)n on oMWCNTs also “squeezes” down Cu(II) adsorption leading to further decline. In short, the adsorption of C60(OH)n onto oMWCNTs affects the surface characteristics and sorption sites, resulting in the observed changes of Cu(II) adsorbed on oMWCNTs.

Bottom Line: The effect of C60(OH)n on Cu(II) adsorption of oMWCNTs was not significant at low C60(OH)n concentration, whereas a negative effect was observed at higher concentration.The adsorption of Cu(II) on oMWCNTs was enhanced with increasing pH values at pH < 5, but decreased at pH ≥ 5.The double sorption site model was applied to simulate the adsorption isotherms of Cu(II) in the presence of C60(OH)n and fitted the experimental data well.

View Article: PubMed Central - PubMed

Affiliation: Radiochemistry Laboratory, School of Nuclear Science and Technology, Lanzhou University, Lanzhou, PR China.

ABSTRACT
The adsorption of Cu(II) on oxidized multi-walled carbon nanotubes (oMWCNTs) as a function of contact time, pH, ionic strength, temperature, and hydroxylated fullerene (C60(OH)n) and carboxylated fullerene (C60(C(COOH)2)n) were studied under ambient conditions using batch techniques. The results showed that the adsorption of Cu(II) had rapidly reached equilibrium and the kinetic process was well described by a pseudo-second-order rate model. Cu(II) adsorption on oMWCNTs was dependent on pH but independent of ionic strength. Compared with the Freundlich model, the Langmuir model was more suitable for analyzing the adsorption isotherms. The thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that Cu(II) adsorption on oMWCNTs was spontaneous and endothermic. The effect of C60(OH)n on Cu(II) adsorption of oMWCNTs was not significant at low C60(OH)n concentration, whereas a negative effect was observed at higher concentration. The adsorption of Cu(II) on oMWCNTs was enhanced with increasing pH values at pH < 5, but decreased at pH ≥ 5. The presence of C60(C(COOH)2)n inhibited the adsorption of Cu(II) onto oMWCNTs at pH 4-6. The double sorption site model was applied to simulate the adsorption isotherms of Cu(II) in the presence of C60(OH)n and fitted the experimental data well.

Show MeSH
Related in: MedlinePlus