Synthesis of carbon xerogels and their application in adsorption studies of caffeine and diclofenac as emerging contaminants

• Synthesis of carbon xerogels and chemical and thermal treatments were carried out.
• Textural, morphological and chemical characterization of the materials was developed.
• Batch adsorption tests of caffeine and diclofenac were performed.
• Adsorption kinetic and equilibrium data were obtained using several models.
• Adsorption mechanism of contaminants, based on chemical affinities, was elucidated.

This work involves the application of carbon xerogels in the removal of two emerging contaminants, caffeine and diclofenac, from aqueous solutions. Textural characterization of the carbon xerogels prepared by polycondensation of resorcinol with formaldehyde (with a molar ratio of 1:2) has been investigated using N2 adsorption–desorption at −196 °C. Chemical surface groups were analyzed by FTIR spectroscopy, elemental microanalysis and determination of isoelectric point. The equilibrium tests were carried out using different weights of adsorbent and the experimental data were best correlated by Sips isotherm equation. The kinetic experimental data were described using pseudo-first and pseudo-second order kinetic models, being well described by a pseudo-second order model. The maximum adsorption capacity was observed for adsorption of caffeine onto a xerogel treated with urea solution (182.5 mg g−1), due to the presence of Lewis bases on its surface, which increase the adsorption affinity for organic compounds. On the other hand, the higher extent of diclofenac adsorption was obtained with a carbon xerogel treated with sulfuric acid (80.0 mg g−1), mainly due to electronic interactions. Comparing these results with other data reported in the literature, the carbon xerogels employed in our study were found to exhibit comparable adsorption capacities and higher kinetic properties.