In recent years, various studies have shown that the pharmaceutical compounds represent a new class of bioactive potential contaminants to the environment. The pharmaceutical compounds belong to emerging pollutants, which are not yet included in the legislation, and represent a risk to the environment because they are toxic, persistent and bio-accumulative. Lately, the levels of the pharmaceutical compounds have been determined in the effluents from wastewater plants.
Diclofenac (DCF) and carbamazepine (CBZ) are the most frequently detected pharmaceutical compounds in the effluents from wastewater plants. The DCF is a nonsteroidal anti-inflammatory drug, and CBZ is widely prescribed in the treatment of epilepsy and neuropathic pain. Several separation processes may be used to remove pharmaceutical compounds from aqueous solution; however, many of these processes have drawbacks in implementation and high operating costs. It is well documented that the activated carbon adsorption is a feasible and efficient process to remove organic compounds from water solutions. Furthermore, activated carbon has high adsorption capacity, moderate cost and availability.
The aim of this work was to study the adsorption equilibrium of the pharmaceutics DCF and CBZ in aqueous solution onto granular activated carbon (GAC) and elucidate the adsorption mechanism. Furthermore, it was analyzed the effect of solution pH, ionic strength and temperature on the adsorption capacity of GAC towards DCF and CBZ.
The DCF and CBZ used in this work are of analytical grade, and the GAC is commercially known as F-400 (Calgon Corporation). The experimental adsorption equilibrium data of DCF and CBZ on GAC were obtained in batch adsorbers. The time to attain equilibrium was 7 and 13 days for DCF and CBZ, respectively. The experimental adsorption equilibrium data for both drugs were interpreted using the isotherm models of Langmuir, Freundlich and Prausnitz-Radke. It was found that the Radke-Prausnitz isotherm best fitted to the adsorption equilibrium data of DCF and CBZ.
The CAG presented a surface area of 919 m2/g, pore volume of 0.54 cm3/g and mean pore diameter of 2.35 nm. The physicochemical characterization of the F400 indicated that the pH of the point of zero charge (pHPZC) was of 8.43, and the concentrations of total basic and total acidic sites were 0.268 and 0.053 meq/g, respectively. The predominant acidic sites were phenolic sites with a concentration of 0.017 meq/g. Thus, the nature of F400 is basic. The capacity GAC for adsorbing DCF and CBZ varied slightly with the pH of the solution. The capacity of the GAC towards DCF decreased slightly by raising the solution pH; however, the effect of pH on the capacity of the GAC towards CBZ presented an unusual behavior. The capacity for adsorbing CBZ increased, decreased, increased and then reduced by raising the solution pH from 3 to 6, 6 to 7, 7 to 9 and 9 to 11, respectively.
The adsorption mechanism of CBZ and DCF was controlled by the dispersive interactions (π-π) between the aromatic rings of the drugs and the aromatic rings of graphene sheets of GAC. In the case of DCF, the electrostatic interactions affected the adsorption capacity depending on the pH of the solution.
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