Modes of Interaction of Simazine with the Surface of Amorphous Silica in Water. Part II: Adsorption at Temperatures Higher than Ambient

Abstract

The conclusions of a previous study (S. Esposito et al. <i>J. Phys. Chem. C</i> <b>2013</b>, <i>117</i>, 11203–11210) concerning room temperature adsorption of simazine (Sim) on amorphous silica in water have been checked against a set of experiments in the range 40° to 60 °C, where equilibrium conditions are more likely to be attained. Adsorbed amount as a function of pH has a complex behavior with temperature, confirming the presence of two types of protonated adsorbed species, respectively monomeric (SimH⁺) and dimeric (Sim₂H⁺), the latter prevailing both at high temperatures and loadings. A simple model for adsorption involving proton transfer from the solid indicates that the pH value at which the uptake is maximum (pH*) is the half sum of the p<i>K</i>_a’s of both the active silanol species and the protonated entity given rise, pH* = [p<i>K</i>_a(1) + p<i>K</i>_a(2)]/2. From this, it results that (i) the dimer Sim₂ is more basic than the monomer Sim by 2 units of p<i>K</i>_a; (ii) adsorbed simazine is more basic then the molecule in solution also by ca. 2 units in p<i>K</i>_a; and (iii) the p<i>K</i>_a of the silanol species involved is probably not ca. 4 as recently proposed, but more likely ca. 7, in agreement with old classical views. From the qualitative energetic point of view, the reaction Sim­(aq) + SiOH → SiO^–···SimH⁺ is exothermic, the formation of the dimer from the monomer is endothermic (reaction SiO^–···SimH⁺ + Sim­(aq) → SiO^–···Sim₂H⁺), whereas the reaction Sim­(aq) + SiOH → SiO^–···Sim₂H⁺ is slightly exothermic. At 25 °C, the adsorbed monomer is irreversibly held, and the dimer only partially. The isotherm at 40° shows that adsorption of the dimer occurs almost reversibly, whereas equilibrium in the formation of the monomer is not completely reached. The isotherm at 60 °C shows instead that both species are formed under near-equilibrium conditions