Effect of pH and Temperature on Adsorption of Dimethyl Phthalate on Carbon Nanotubes in Aqueous Phase

Abstract

Adsorptions of dimethyl phthalate (DMP) on carbon nanotubes (CNTs) in aqueous phase at various pH and temperatures were studied. The increase in pH results in the increase in adsorption coefficient. The adsorption is governed by the π-π electron interaction which is affected by the changes in pH of the medium. The outer diameter of the CNTs greatly influences the adsorption behavior of CNT for DMP. Under the same working temperature, the adsorption capacity of CNTs for DMP is inversely related to the average outer diameter of the CNT: single-walled SWCNT (1.4 nm)>multi-walled MWCNT10 (9.4 nm)>MWCNT30 (27.8 nm)>MWCNT40 (42.7 nm). The larger surface area of CNTs provides many active sites for adsorption of DMP molecules. The Freundlich model can describe well the adsorption isotherms of DMP on CNTs. The thermodynamic parameters of standard free energy, standard enthalpy (ΔH), and standard entropy changes are determined, showing that the adsorption of DMP on CNTs is an endothermic and spontaneous reaction. The ΔH value of 27.8 nm-sized MWCNT (22.69 kJ/mol) is higher than 1.4 nm-sized SWCNT (6.05 kJ/mol), inferring that the adsorption process becomes more endothermic with the increase in the outer diameter of CNTs.

Keywords:

• Adsorption
• Carbon nanotubes
• Dimethyl phthalate
• pH
• Thermodynamics

Acknowledgments

This work was supported in part by grants from the National Outstanding Youth Research Foundation of China (40925010), the International Joint Key Project from National Natural Science Foundation of China (40920134003), the International Joint Key Project from Chinese Ministry of Science and Technology (2011DFA00120, 2009DFA92830, and 2010DFA12780), the National Natural Science Foundation of China (41103060, 41103007, and 40873060), and the Fundamental Research Funds for the Central Universities (FRF-TP-12-005A).

Notes

^a CNT: carbon nanotube, SWCNT: single-walled CNT, and MWCNT: multiwalled CNT.

^b Provided by supplier.

^c Outer diameter measured by TEM (n = 100).

^d Specific surface area (A _surf), mesopore volume (V _meso), and micropore volume (V _micro) were calculated from the adsorption-desorption isotherm of N₂ at 77 K by the multi-point BET method.

^e Water contents were determined by drying the CNTs at 105°C for 24 h.

^f Ash contents were determined by heating CNTs at 900°C for 10 h.

^g Bulk dry weight-based elemental contents of the CNTs were determined using a Vario ELIII elemental analyzer; O contents based on the mass difference.

^h SWCNT content.

ⁱ CNT content.

Note: K _f [(mg g⁻¹)/(mg L⁻¹) ⁿ ] adsorption capacity coefficient from the Freundlich model; r is the correlation coefficient; C _s is the DMP water solubility = 5.22 g/L; K′ is the adsorption coefficient per unit surface area, K′ = K/A _surf.