Catalytic ozonation of three isomeric cresols in the presence of NaCl with nano-mesoporous β-molecular sieves

Abstract

Catalytic ozonation is one suitable method to efficiently remove pollutants from refractory wastewater which if treated improperly could cause the severe contamination of surface and underground water. In this study, nano-mesoporous β-molecular sieves were synthesized and employed as catalysts for ozonation. These catalysts had large specific surface area, appropriate surface acidity, high thermal stability and chemical stability. Especially, they could be used in the presence of salt which was frequently contained in industrial wastewater. We used three types of cresols (o-, m-, and p- cresol), the typical phenolic pollutants in industrial wastewater, as model target pollutants. Experimental results demonstrated that the catalytic performance of the prepared catalysts was superior to the commercial molecular sieves. High surface area as well as open porous structure of catalysts was believed to enhance the catalytic activity by surface reaction and easy access of reactants to the active sites. The influences of salt (NaCl) concentration and the aqueous pH on the catalytic ozonation of three cresols were discussed respectively in the study. Interestingly, the degradation of three cresols occurred in the order p>m>o at pH=2.0, and the similar result was obtained under alkaline condition. In addition, the conversion of three cresols was highest (more than 95%) at pH=12.0. With the rise of NaCl concentration, the removal rates of three cresols increased at first (0-0.5wt.%) and then decreased (0.5wt.% to 5wt.%). The variation of intermediates during the oxidation of cresols were monitored by GC together with in situ React IR. The degradation mechanism of three cresols was proposed, of which the possibility and feasibility were well certified by frontier molecular orbital theory and atomic charge distribution via density functional theory method.