Studies of the equilibrium and thermodynamics of the adsorption of Cu2+ onto as-produced and modified carbon nanotubes

This study evaluates the Cu2+ adsorption efficiency of as-produced carbon nanotubes (CNTs) and those modified by HNO3 and NaOCL The surface area, pH(pzc), pore volume, FTIR analyses, and average pore size of CNTs were determined to compare the differences between nanotubes before and after HNO3 and NaOCl modification. The HNO3 and NaOCl modifications increased the pore volume and the average pore size of CNTs; in contrast, the pHpzc was decreased. The modification processes produced some functional groups. The adsorption capacity of Cu2+ on as-produced and modified CNTs increased with the pH and temperature; however, the effects of the ionic strength on the adsorption of Cu2+ on as-produced and modified CNTs were negligible. The linear correlation coefficients of Langmuir and Freundlich isotherms were obtained and the results revealed that the Langmuir isotherm fitted the experimental results better than did the Freundlich isotherm. The adsorption capacity of Cu2+ followed the order NaOCI-modified CNTs > HNO3-modified CNTs > as-produced CNTs. Changes in the free energy of adsorption (Delta G degrees), enthalpy (Delta H degrees), and entropy (Delta S degrees) were determined. All AGO values were negative; the AHO values of as-produced, HNO3-modified, and NaOCI-modified CNTs were 10.84, 17.08, and 67.77 kJ/mol and the ASO values were 96.89, 122.88, and 319.76 J/mol K, respectively. (c) 2007 Elsevier Inc. All rights reserved.