Effects of thiol functionalization of a waste-derived activated carbon on the adsorption of sulfamethoxazole from water: Kinetic, equilibrium and thermodynamic studies

An activated carbon was produced from paper mill sludge (AC-P) and functionalized with thiol groups (AC-MPTMS) for the adsorptive removal of the antibiotic sulfamethoxazole (SMX) from buffered solutions prepared in ultrapure water (pH 8) and real wastewater samples. The physicochemical properties of the two materials (AC-P and AC-MPTMS) showed differences mainly in specific surface area (SBET), in the type of oxygen functional groups and in the relative percentage of sulphur groups. The adsorption results showed a decrease in the Langmuir adsorption capacity (qm) upon an increase on temperature (15, 25 and 35 °C), varying between 113 ± 7 and 42.5 ± 0.6 mg g−1 for AC-P and between 140 ± 20 and 28.0 ± 1.5 mg g−1 for AC-MPTMS. Pseudo-second order model presented the best fit for the kinetic studies, with rate constants (k2) increasing with temperature and varying from 0.005 ± 0.002 to 0.013 ± 0.004 g mg−1 min−1 for AC-P and from 0.006 ± 0.002 to 0.03 ± 0.01 g mg−1 min−1 for AC-MPTMS. Both adsorbents showed very similar thermodynamic parameters, with the adsorption process being spontaneous (−26 kJ mol−1 ≤ ΔG° ≤ −40 kJ mol−1), endothermic (69 kJ mol−1 ≤ ΔH° ≤ 78 kJ mol−1), and entropically favorable (356 ≤ ΔS° ≤ 365 J mol−1 K−1). The performance of AC-MPTMS in the removal of SMX was tested in wastewater, where the material displayed lower SMX adsorption velocity and capacity than in buffered aqueous solution (pH 8) due to competitive matrix effects.