Kinetic and equilibrium studies for the adsorption of acid nucleic bases onto K10 montmorillonite

Abstract The kinetic of the adsorption of two nucleic acid bases, namely adenine (A) and cytosine (C), on K10 montmorillonite (MMT) clay has been investigated over a wide range of nitrogenous bases concentrations, by using the spectrophotometric method. The measurements have been performed at T = 25.0 °C and pH = 4.0 where adenine and cytosine exist both in their protonated and neutral forms. Preliminary to the kinetic data treatment, the adsorption isotherms have been gathered and, then, analyzed by using the Freundlich and the Dubinin–Radushkevich model. The obtained results allow to draw the conclusion that physisorption of adenine and cytosine on MMT occurs and the process is mainly governed by weak interactions. Different kinetics models have been applied to analyze the kinetic data and the discrimination has been performed by means of various statistics. It has been found that for both A and C the adsorption kinetics follow a double exponential model (DEM), corresponding to two parallel steps involving the cationic and the neutral species which interact with different sites of the clay surface, i.e., the interlayer region and the faces of the lamella. The kinetic constant values associated to the proposed pathways are independent of the nitrogenous bases' concentrations and the nucleic acid bases' nature. The mechanism, proposed in the light of the equilibrium data, has been corroborated by performing kinetic measurements at pH = 2.0 and pH = 6.0 where A and C exist only in their protonated form and neutral form, respectively.