Adsorption of carbamazepine from water by hydrothermally and steam activated agricultural by-products: equilibrium, site energy, and thermodynamic studies

Abstract

Pollution of water sources by pharmaceuticals has attracted attention. In this work, carbamazepine (CBZ), as a model pharmaceutical pollutant, was used to investigate its characteristics of adsorption from an aqueous solution. Flax shives and oat hulls, representing cellulosic materials of agricultural by-products, were characterized and used to develop hydrochars and steam activated hydrochars adsorbents. Hydrothermal treatment and steam activation significantly enhanced the CBZ adsorption capacity. The isotherms of CBZ adsorption by the steam activated hydrochars were experimentally investigated at various temperatures (20–40 °C) and satisfactorily simulated by the Sips model. The steam activated hydrochars of flax shives and oat hulls had higher maximum CBZ adsorption capacities of 293.5 and 162.6 mg/g than the respective hydrochars (75.3 and 81.8 mg/g) and raw materials (16.5 and 15.0 mg/g). Based on the isotherm modeling results, the site energy distribution of CBZ adsorption was further determined. The weighted means of site energy obtained at 40 °C (13.3–15.0 kJ/mol) were higher than those at 30 °C (11.2–13.4 kJ/mol) and 20 °C (7.7–8.0 kJ/mol), indicating higher adsorption affinity, thus more favorable adsorption. Furthermore, the thermodynamic parameters of CBZ adsorption were determined. The results indicated that the CBZ adsorption on hydrochars and steam activated hydrochars were endothermic and spontaneous.