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

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

Visible-Light Driven Oxidation of Water as Catalyzed by Co-APO‑5 in the Presence of Ru Sensitizer

Cobalt aluminum phosphate with AFI zeolitic structure (Co-APO-5) catalyzes light-driven water oxidation (WO) when both the ruthenium complex [Ru­(bpy)₃]²⁺, as photosensitizer, and persulfate species S₂O₈^2–, as sacrificial electron acceptor, are present. Measurements have been run in a flow reactor, allowing the amount of oxygen evolved to be evaluated and, in particular, the initial rate of WO reaction to be measured. The latter has been studied as a function of chemical composition, and some kinetic features have been established. Competitive reactions occur extensively, causing the oxidation of the Ru complex to the detriment of oxygen production. The initial rate of WO increases with the amount of catalyst until turbidity of the suspension sets in. A reaction order close to −1 with respect to persulfate was found, which indicates the occurrence of surface processes involving Co centers at the outer layer of Co-APO-5 particles (accessed by the bulky [Ru­(bpy)₃]²⁺ species, at variance with Co species in the core), for the adsorption onto which competition takes place between the Ru complex and persulfate species. A less pronounced negative order (ca. −0.4) for the sensitizer was also observed, for which an interpretation is proposed

Hematite Nanoparticles Larger than 90 nm Show No Sign of Toxicity in Terms of Lactate Dehydrogenase Release, Nitric Oxide Generation, Apoptosis, and Comet Assay in Murine Alveolar Macrophages and Human Lung Epithelial Cells

Three hematite samples were synthesized by precipitation from a FeCl₃ solution under controlled pH and temperature conditions in different morphology and dimensions: (i) microsized (average diameter 1.2 μm); (ii) submicrosized (250 nm); and (iii) nanosized (90 nm). To gain insight into reactions potentially occurring <i>in vivo</i> at the particle–lung interface following dust inhalation, several physicochemical features relevant to pathogenicity were measured (free radical generation in cell-free tests, metal release, and antioxidant depletion), and cellular toxicity assays on human lung epithelial cells (A549) and murine alveolar macrophages (MH-S) were carried out (LDH release, apoptosis detection, DNA damage, and nitric oxide synthesis). The decrease in particles size, from 1.2 μm to 90 nm, only caused a slight increase in structural defects (disorder of the hematite phase and the presence of surface ferrous ions) without enhancing surface reactivity or cellular responses in the concentration range between 20 and 100 μg cm^–2