Studies of the equilibrium and thermodynamics of the adsorption of Cu(2+) onto as-produced and modified carbon nanotubes.

This study evaluates the Cu(2+) adsorption efficiency of as-produced carbon nanotubes (CNTs) and those modified by HNO(3) 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 HNO(3) and NaOCl modification. The HNO(3) and NaOCl modifications increased the pore volume and the average pore size of CNTs; in contrast, the pH(pzc) was decreased. The modification processes produced some functional groups. The adsorption capacity of Cu(2+) on as-produced and modified CNTs increased with the pH and temperature; however, the effects of the ionic strength on the adsorption of Cu(2+) 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 Cu(2+) followed the order NaOCl-modified CNTs > HNO(3)-modified CNTs > as-produced CNTs. Changes in the free energy of adsorption (DeltaG(o)), enthalpy (DeltaH(o)), and entropy (DeltaS(o)) were determined. All DeltaG(o) values were negative; the DeltaH(o) values of as-produced, HNO(3)-modified, and NaOCl-modified CNTs were 10.84, 17.08, and 67.77 kJ/mol and the DeltaS(o) values were 96.89, 122.88, and 319.76 J/mol K, respectively.