Adsorption of sulfonamides from aqueous solution by carbon nanotubes: determination of kinetics, equilibrium and thermodynamic parameters

Adsorption by carbon nanotubes (CNTs) was used to remove sulfonamides (SAs) from aqueous solution. The target SAs were sulfamethoxazole (SMX) and sulfamethazine (SMZ). The effects of contact time, solution pH, CNTs dosage, initial SA concentration and temperature on the adsorption of SAs were determined. The pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used to simulate the kinetics, and Langmuir and Freundlich's isotherms were employed to simulate the equilibrium. The adsorption of SAs onto CNTs best-fitted pseudo- secondorder and Langmuir models. The adsorption amount of SAs increased with pH, CNTs dosage and temperature decreasing; conversely, it increased with initial SAs concentration increasing. The maximum adsorbed amounts of SMX and SMZ at pH 3 were 207.0 and 181.5 mg/g, respectively. The novel contributions of this study to the field are the evaluation of the hydrophobic and electrostatic interactions that are involved in the adsorption of SAs and the determination that the octanol-water partitioning coefficient (Kow) of SAs is important in adsorption; moreover, pH is also found to be important in hydrophobic and electrostatic interactions. The standard enthalpy change and standard entropy change of SMX were -19.7 kJ/mol and 23.7 J/mol K, respectively; and those of SMZ were -5.0 kJ/mol and 72.9 J/mol K, respectively. The adsorption of SMX and SMZ onto CNTs was a spontaneous physisorption process.