Kinetic study of the adsorption of nitroimidazole antibiotics on activated carbons in aqueous phase

Abstract

The adsorption kinetics of four nitroimidazoles, Dimetridazole (DMZ), Metronidazole (MNZ), Ronidazole (RNZ) and Tinidazole (TNZ), were studied on three activated carbons: two commercial carbons from Sorbo-Norit (S) and Merck (M) and a third prepared by chemical activation of petroleum coke (C). Experimental data of the corresponding adsorption kinetics were analyzed by applying pseudo-first and pseudo-second-order models and a general diffusion model. Application of pseudo-first and pseudo-second-order kinetic models verified the following: (i) The kinetic model used that better predicts the adsorption rates depends of both the adsorbent and adsorbate studied. (ii) Nitroimidazole adsorption rate decreases in the order MNZ > DMZ > RNZ > TNZ; therefore, in the case of MNZ, molecular size does not appear to be a determining factor in the process. (iii) Nitroimidazole adsorption rate on carbons increases in the order C < S < M, which is related to the increase in carbon hydrophobicity. Hence, in general, hydrophobic interactions appear to govern the kinetics of the adsorption process. Finally, a general diffusion model was applied that combines external mass transport and intraparticle diffusion, achieving an adequate fit to the experimental data. There are notable differences among the diffusivity values for the different nitroimidazoles that do not appear to be exclusively related to carbon textural parameters or adsorbate size. Therefore, adsorbent and adsorbate chemical characteristics are highly important to establish the adsorption mechanism of nitroimidazoles on activated carbons.