Boron removal by means of adsorption processes with magnesium oxide — Modelization and mechanism

The boron adsorption process, in borate ion form, onto magnesium oxide was described in terms of mathematical equations and, taking into account the knowledge acquired by means of experimental data, the most probable mechanism of boron removal process was proposed. The modelization has been carried out by establishing Langmuir adsorption isotherms and kinetic law. Upward convex curves are observed indicating that the process is advantageous towards boron adsorption. Moreover, the adjustment of experimental data to their corresponding functions leads to highly satisfactory correlations for all the studied conditions and pseudo-second-order kinetic model was observed. As for the mechanism of the adsorption process, a superficial process of chemosorption comprising three stages or reactions is proposed: 1) the hydration reaction of magnesium oxide as it is in contact with the aqueous solution to yield a magnesium hydroxide gel over the surface of which the active points for adsorption are located, 2) the alkalisation reaction of the solution due to the acid-base reaction between magnesium oxide and water and 3) the stereospecific chemical reaction between borate ions and active centres. This mechanism develops with time to reach stabilisation and leads to the formation of an adsorption monomolecular layer that is typical in chemosorption processes, as well as in systems following Langmuir adsorption isotherms and pseudo-second-order kinetics.