The conclusions of a previous study (S. Esposito et al. J. Phys. Chem. C 2013, 117, 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 pKa’s of both the active silanol species and the protonated entity given rise, pH* = [pKa(1) + pKa(2)]/2. From this, it results that (i) the dimer Sim2 is more basic than the monomer Sim by 2 units of pKa; (ii) adsorbed simazine is more basic then the molecule in solution also by ca. 2 units in pKa; and (iii) the pKa 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 2 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.
|Titolo:||Modes of Interaction of Simazine with the Surface of Amorphous Silica in Water. Part II: Adsorption at Temperatures Higher than Ambient|
|Data di pubblicazione:||2013|
|Digital Object Identifier (DOI):||http://dx.doi.org/10.1021/jp4107487|
|Appare nelle tipologie:||1.1 Articolo in rivista|
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