Role of Silica on Clay-Catalyzed Ozonation for Total Mineralization of Bisphenol-A.

Catalytic ozonation for the total mineralization of bisphenol-A (BPA) from aqueous solution was investigated in the presence of various silica-based catalysts such as mesoporous silica, acid-activated bentonite (HMt) and montmorillonite-rich materials (Mt) ion-exchanged with Na+ and Fe2+ cations (NaMt and Fe(II)Mt). The effects of the catalyst surface were studied by correlating the hydrophilic character and catalyst dispersion in the aqueous media to the silica content and BPA conversion. To the best of our knowledge, this approach has barely been tackled so far. Acid-activated and iron-free clay catalysts produced complete BPA degradation in short ozonation times. The catalytic activity was found to strongly depend on the hydrophilic character, which, in turn, depends on the Si content. Catalyst interactions with water and BPA appear to promote hydrophobic adsorption in high Si catalysts. These findings are of great importance because they allow tailoring silica-containing catalyst properties for specific features of the waters to be treated.

Clay-Catalyzed Ozonation of Organic Pollutants in Water and Toxicity on Lemna minor: Effects of Molecular Structure and Interactions.

The use of clays as adsorbents and catalysts in the ozonation of organic pollutants (Atrazine, bis-Phenol A, Diazinon, and Diclofenac sodium) allowed simulating their natural oxidative degradation in clay soils and to evaluate the ecotoxicity of mixtures partially oxidized on the species Lemna minor, a biodiversity representative of plants in the aquatic environment. Kinetic data showed that the adsorption of organic pollutants on clay particles obeys the pseudo-second-order model, while the adsorption isotherms satisfactorily fit the Langmuir model. Adsorption reduces the dispersion of the organic pollutant in the environment and prolongs its persistence and its natural degradation probability. Measurements of the Zeta potential and particle size as a function of pH demonstrate that the catalytic activity of clay depends on its cation, its silica/alumina ratio, and therefore on its permanent and temporary ion exchange capacities. These factors seem to govern its delamination and dispersion in aqueous media, its hydrophilic-hydrophobic character, and its porosity. Tests conducted on Lemna minor in contact with ozonation mixtures revealed that the toxicity could be due to pH decrease and to the toxicity of the intermediates yielded. Ecotoxicity would depend on the structure of the organic molecules, the chemical composition of the clay surface and ozonation time, which determines the oxidation progress. These results are of great importance for further research because they allow concluding that the negative impact of the persistence of an organic molecule in clay-containing media depends on the type and composition of the very clay mineral.

Insight into natural medium remediation through ecotoxicity correlation with clay catalyst selectivity in organic molecule ozonation.

The oxidative degradation of diazinon (DAZ) and diclofenac sodium (DCF) in aqueous media was comparatively investigated and correlated with the mortality of Artemia salina in the presence of clay catalysts. For this purpose, montmorillonites (Mt) exchanged with Na+ and Fe2+ cations (NaMt and Fe(II)Mt), acid activated bentonites and hydrotalcite were used as clay catalysts. Surface interaction and adsorption on the clay surface were found to govern the catalyst dispersion in aqueous media and both activity and selectivity in ozonation. These catalysts' features were correlated with the ecotoxicity of ozonised reaction mixtures as expressed in terms of mortality rates of Artemia salina. DAZ and DCF display specific intrinsic ecotoxicity that evolves differently during ozonation according to the catalyst. The ecotoxicity was found to strongly depend on the distribution of the ozonation intermediates, which, in turn, was narrowly correlated with the acid-base properties of the catalyst surface. These valuable findings allow the prediction of the behaviour of the clay-containing media in natural remediation.