Abstract
Catalytic ozonation process (COP) is considered as a cost-efficient technology for the treatment of refractory chemical wastewaters. The catalyst performance plays an important role for the treatment efficiency. The present study investigated efficiencies and mechanisms of manganese (Mn)-based Y zeolites in COPs for removing nitrobenzene from water. The catalysts of Mn/NaY and Mn/USY were prepared by incipient wetness impregnation, while Mn-USY was obtained by hydrothermal synthesis. Mn-USY contained a greater ratio of Mn2+ than Mn/NaY, and Mn/USY. Mn oxides loaded on Y zeolites promoted the COP efficiencies. Mn/NaY increased total organic carbon removal in COP by 7.3% compared to NaY, while Mn/USY and Mn-USY increased 11.5 and 15.8%, respectively, relative to USY in COP. Multivalent Mn oxides (Mn2+, Mn3+, and Mn4+) were highly dispersed on the surface of NaY or USY, and function as catalytic active sites, increasing mineralization. Mn-USY showed the highest total organic carbon removal (44.3%) in COP among the three catalysts, because Mn-USY had a higher ratio of Mn2+ to the total Mn oxides on the surface than Mn/NaY and Mn/USY and the catalytic effects from intercorrelations between Mn oxides and mesoporous surface structures. The hydroxyl radicals and superoxide radicals governed oxidations in COP using Mn-USY. Nitrobenzene was oxidized to polyhydroxy phenol, polyhydroxy nitrophenol, and p-benzoquinone. The intermediates were then oxidized to small organic acids and ultimately carbon dioxide and water. This study demonstrates the potential of Y zeolites used in COP for the treatment of refractory chemical wastewaters.
Highlights
Nitrobenzene is an electrophilic and refractory environmental contaminant that contains a nitro group and a benzene ring
X-ray powder diffraction (XRD) diffraction peaks from Mn oxides were not observed for the Mn-based Y zeolites, due to surface dispersion, low concentration and/or small size
The results showed Mn-based Y zeolites had a weakmoderate stability in Catalytic ozonation process (COP) of pollutants, which warrants further investigations to improve the stability of Mn oxides on Y zeolites
Summary
Nitrobenzene is an electrophilic and refractory environmental contaminant that contains a nitro group and a benzene ring. The catalytic ozonation process (COP) is widely studied for its application toward the removal of refractory organic chemicals (ROCs) from water. This process is easy to manage, highly efficient, and safe (Chen et al, 2018). During COP experiments, an aliquot of 500 mL of nitrobenzene solution and 0.5 g of catalyst was added in the reactor at 25◦C. The concentration of Mn ion in solution was detected by ICPAES at 259.37 nm for the quantification of Mn. Single adsorption process and single ozonation process (SOP) experiments were performed with the same experimental system as COP.
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