Mercaptopropyl-grafted magadiite for lead and cadmium sorption and calorimetric determination

Sodium magadiite anchored with 3-mercaptopropyltrimethoxysilane was synthesized through direct functionalization. After this reaction, it was observed a decrease in crystallinity, with a slight increase in basal distance to 1.57 nm, when compared to the precursor value of 1.53 nm. This organofunctionalization was evidenced through infrared spectroscopy, indicating by the presence of C–H stretching vibration in the 2900–2800 cm−1 region and mainly from the appearance of T species at the 29Si NMR spectrum, centered at −65 and −55 ppm. The pendant chains covalently bonded to magadiite layers have the ability to sorb lead and cadmium cations, through a batchwise method at 298 ± 1 K, to give the maximum sorption capacity values of 1.54 and 2.72 mmol g−1, respectively. The energetic effects caused by both cation/sulfur basic center interactions at the solid/liquid interface were determined through calorimetric titration, to give the enthalpies −13.48 ± 0.05 and −0.83 ± 0.053 kJ mol−1 for lead and cadmium, respectively. The sulfur basic centers attached to the pendant chains reflect in spontaneous process with negative Gibbs energies of −26.8 ± 0.1 and −24.5 ± 0.1 kJ mol−1, resulting also in positive entropies of 44 ± 1 and 79 ± 1 J K−1 mol−1, respectively. This set of favorable thermodynamic data is more pronounced for the softer cation in interacting with the basic atom.