Abstract
Catalytic ozonation processes (COPs) are an emerging technology for wastewater treatments. NaZSM-5 zeolites in three different SiO2/Al2O3 ratios (31, 45, and 120) and their metallic oxides loaded samples were compared for COP of nitrobenzene solution. NaZSM-5(120) showed high total organic carbon (TOC) removals (70.2–74.0%) by adsorption relative to NaZSM-5(45) (0.4–0.6%) at various initial pH conditions. NaZSM-5(31) was obtained by NaOH treatment of NaZSM-5(45) and displayed 20.9–23.8% of TOC removals by adsorption. In COPs, the different ZSM-5 zeolites exhibited various TOC removals and different reaction pathways. COP-NaZSM-5(120) showed high TOC removals compared to COP-NaZSM-5(45) and COP-NaZSM-5(31). The repeated uses of zeolites in COPs were performed to understand the reaction pathways and contribution of adsorption versus ozonation (i.e., catalytic oxidation and/or direct ozonation). Both adsorption and direct ozonation in COP-NaZSM-5(120) contributed TOC removal for the first use, whereas direct ozonation and •OH mediated oxidation dominated the process for eight repeated uses. Direct ozonation and •OH-mediated oxidation controlled the COP-NaZSM-5(45) process for the first and eight repeated uses. Adsorption and direct ozonation governed the COP-NaZSM-5(31) process for the first use, whereas the direct ozonation dominated it for eight repeated uses. In COPs, NaZSM-5(120) and NaZSM-5(45) showed the catalytic activity, whereas NaZSM-5(31) displayed negligible catalytic activity. The high catalytic activity of NaZSM-5(120) may be due to more Si-O bonds on zeolite surfaces. The results revealed that loading of Mg oxide on ZSM-5 zeolites can increase catalytic activity in COPs. These results show the application potential of ZSM-5 zeolites in ozonation of recalcitrant chemical wastewaters.
Highlights
The chemical industry produces large quantities of wastewater that contains various recalcitrant organic chemicals (ROCs)
Parent NaZSM-5 zeolites with the SiO2 /Al2 O3 ratios at 120, 45 and 31 were abbreviated as pNaZSM-5(120), pNaZSM-5(45), and pNaZSM-5(31), respectively
At an initial pH of 7.2 and 10.7, similar trends were observed (Figure 7b,c). These results suggest that both adsorption and ozonation contributed to total organic carbon (TOC) removal for ZSM-5s-Catalytic ozonation processes (COPs)
Summary
The chemical industry produces large quantities of wastewater that contains various recalcitrant organic chemicals (ROCs). A range of different catalysts are utilized for ROCs treatment in COPs including alumina [4], activated carbon [5], natural minerals [6,7], and metal oxides [8,9,10]. These catalysts can promote free radical (such as OH). Activated carbon can function as both an adsorbent and a catalyst for the treatment of benzenesulfonic acid and sulfanilic acid, through a complex mechanism that involves the direct ozonation, adsorption, and OH oxidation that occurs at liquid and solid phase interfaces [5]
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