Modeling adsorption rate of Trimethoprim, tetracycline and chlorphenamine from aqueous solutions onto natural bentonite clay. Elucidating mass transfer mechanisms

In this work, the External Mass Transport Model (EMTM), Pore Volume Diffusion Model (PVDM) and Pore Volume and Surface Diffusion Model (PVSDM) were implemented to analyze the adsorption kinetics of trimethoprim (TMP), tetracycline (TC) and chlorphenamine (CPA) in aqueous solution on a natural bentonite clay (Bent). Experimental adsorption kinetics data were obtained at different experimental conditions of initial concentration and stirring speed. The findings indicated that the Bent capacity for adsorbing TMP, TC and CPA was 596.9, 493.6 and 160.9 mmol/kg, respectively. The external mass transfer does not control the overall adsorption rate because the EMTM model predicted that the adsorption rate is much faster than the experimental one. The stirring speed does not affect speeds above 450 RPM, indicating that intraparticle diffusion governs the overall adsorption rate. The PVSDM model revealed that surface diffusion and pore volume diffusion participate in the adsorption kinetics of TMP and TC, while the PVDM model demonstrated that the adsorption rate of CPA is governed by pore volume diffusion. The dependence of the mass transport parameters on the uptake adsorbed at equilibrium (qe) was studied, and it was noticed that the surface diffusion coefficient (De,s) increased with the values of qe of TMP and TC due to the interaction magnitude between the adsorbed solute and the active sites of the adsorbent. The effective pore diffusion coefficient (De,p) of CPA did not vary with the uptake adsorbed of CPA, and it was possible to estimate an experimental tortuosity factor of Bent, which was 9.64.