Investigation of Catalytic Ozonation of Recalcitrant Organic Chemicals in Aqueous Solution over Various ZSM-5 Zeolites

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.


Introduction
The chemical industry produces large quantities of wastewater that contains various recalcitrant organic chemicals (ROCs).Catalytic ozonation processes (COPs) have been regarded as the most promising and straightforward treatments due to its high efficiency [1][2][3].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) formation from the decomposition of ozone, and further stimulate oxidation that increases the reaction rate [6,11,12].Catalysts are also beneficial for the removal of other pollutants through adsorption [7].For example, 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].Pumice-loaded silicate can increase ozone mass transfer rates in aqueous solutions, and subsequently improved •OH generation, which resulted in increased mineralization of diclofenac [13].
Use of ZSM-5 zeolite as a catalyst for ozonation processes has attracted recent attention [11,[14][15][16].High-silica ZSM-5 has high adsorption capacities for ozone dissolved in water [15].According to an investigation by Fujita et al., when high-silica ZSM-5 (SiO 2 /Al 2 O 3 = 3000) was used as a catalyst, it was effective at both concentrating ozone and other organic pollutants dissolved in water, resulting in significantly increased reaction rates [17,18].Comparative studies using different ZSM-5 zeolites with various SiO 2 /Al 2 O 3 of 1000, 900, and 25 for the treatment of ROCs, exhibited enhanced ozonation activities [11,12,19,20].High-silica ZSM-5 zeolites were found to be more efficient.The catalytic ozonation of organic pollutants on ZSM-5 zeolites results from the reaction of molecular ozone with pollutants adsorbed on the surface of zeolites [18].The present investigation describes the importance of SiO 2 /Al 2 O 3 ratios of ZSM-5 zeolites when applied to COPs.
ZSM-5 zeolites had adjustable SiO 2 /Al 2 O 3 at about 10-3000, which impact their properties and applications [21,22].Therefore, further investigation on ZSM-5 zeolite with more usual and available SiO 2 /Al 2 O 3 ranges is desirable and instructive in COP of ROCs.Our previous work found that the catalytic activity and reaction mechanism of activated carbon in the COP of refinery wastewater is masked due to the strong adsorption [23].It is a question whether the natural function of ZSM-5 zeolites in COPs might also be due to adsorption.Additionally, carbon [24], Al 2 O 3 [25], and MCM-41 [26] supporting metallic oxides have been used in COPs of ROCs.However, there were few reports on metallic oxides supported on ZSM-5 zeolites in COPs.
In this study, nitrobenzene is utilized as a model ROCs in this study.Three different SiO 2 /Al 2 O 3 ratios of NaZSM-5 zeolites (31, 45, and 120) and their metallic oxide-loaded samples were designed, characterized, and evaluated.The pathway for the removal of recalcitrant was simultaneously discussed.Additionally, the influence of the solution pH values on the efficiency of COPs of nitrobenzene in water was investigated with ZSM-5 zeolites.The surface active sites of ZSM-5 zeolites in COPs were firstly discussed.
Adsorption was saturated after 30 min using NaZSM-5(31) zeolites-SAPs and TOC removals (20.9-23.8%)were not influenced by pH (Figure 6).NaZSM-5(31) zeolites showed higher TOC removals when compared with NaZSM-5(45) zeolites in SAPs, due to high surface areas.At a pH of 3.8, the TOC removal was better for COP compared with SAP (Figure 6a).Adsorption and ozonation occurred during the catalytic ozonation treatments.However, the TOC removal in COP was less than the individual sums of SOP and SAP, due to competitive interactions between ozonation and adsorption.NaZSM-5(31) zeolite-COPs performed marginally better under acidic and neutral conditions relative to alkaline.Introduction of Mg or Zn oxides onto pNaZSM-5 (31) improved TOC removals in COPs.Mg/NaZSM-5 (31) exhibited slightly increased TOC removal when compared to Zn/NaZSM-5 (31).

Changes of Solution pH Values during SAP, SOP, and COP Treatments
Changes in the nitrobenzene solution pH values were used to determine the relationships between adsorption and ozonation.Changes in solution pH after a 50 min treatment during SOP, ZSM-5s-SAPs and ZSMs-COPs are shown in Figure 7.The SOP treatment resulted in a pH decrease from an initial 3.8 to 2.7.The ZSM-5s-SAPs treatment made the solution pH increasingly neutral (Figure 7a).The pH values after ZSM-5s-COPs treatment ranged between those values after SOP and ZSM-5s-SAPs (Figure 7a).At an initial pH of 7.2 and 10.7, similar trends were also observed (Figure 7b,c).These results suggest that both adsorption and ozonation contributed to TOC removal for ZSM-5s-COPs.These results also suggest that ozonation still occurs when using NaZSM-5(120) zeolite-COPs in spite of strong adsorption.
It was found that after repeated uses, the catalytic activity of NaZSM-5(120) zeolites and Mg/NaZSM-5 (31) is masked by adsorption until saturation.These results are in agreement with previous reports using activated carbon [23].Among the ZSM-5s, pNaZSM-5 (31) and Zn/NaZSM-5 (31) displayed very weak catalytic ozonation activity.The Mg/NaZSM-5(120)-COP had the highest TOC removal capability (40.1%) at an initial pH value of 7.2 among ZSM-5s-COPs.A decrease in TOC removal efficiency was observed with catalytic ozonation using ZSM-5s supported by Zn oxide.Decreased efficiency was not observed with ZSM-5s supported by Mg oxide.The used and unused Mg/NaZSM-5(45) and Zn/NaZSM-5(45) zeolites were further analyzed by XPS (Figure 3c,d).After repeated use, Mg oxide remained and Zn oxide was nearly removed from the surface of NaZSM-5(45), as determined by XPS results.Similar results also occurred for Mg or Zn loaded NaZSM-5(120) and NaZSM-5 (31).The decreased TOC removal with Zn loaded ZSM-5 zeolites potentially results from Zn oxide leaching from the surface of zeolites.The loading of Mg oxides on ZSM-5 zeolites effectively increases catalytic activity.

Influences of NaHCO 3 on COPs
Sodium bicarbonate was used to determine the impact of •OH for the COP treatment of nitrobenzene in solution [38].The TOC removals using NaZSM-5(120) zeolites-and NaZSM-5 (31) zeolite-COPs during the first use remained unchanged after the addition of NaHCO 3 (Figure 9a).This suggests that direct ozonation and adsorption dominate these COPs.The TOC removal by NaZSM-5(45) zeolite-COPs was significantly decreased by the addition of NaHCO 3 (Figure 9a).This suggests that under these conditions TOC is reduced by •OH mediated oxidation.However, TOC reductions still occurred in spite of NaHCO 3 introduction, which was a result of direct ozonation.These results suggest that direct ozonation and •OH mediated oxidation both occurred in NaZSM-5(45) zeolite-COPs.The TOC removals of NaZSM-5(120) zeolite-COPs after eight uses significantly decreased by the addition of NaHCO 3 (Figure 9b), suggesting that •OH-mediated oxidation occurred.These results further expand the current mechanistic understanding for the use of high silica ZSM-5 during COP.These results show that there is a direct reaction of molecular ozone with the organic compounds that are adsorbed on the surface of zeolites.Similar reactions occur for the TOC removals by NaZSM-5(45) zeolite-COPs after the addition of NaHCO 3 (Figure 9b).For pNaZSM-5(31)-COP and Zn/NaZSM-5(31)-COP after the eight uses, a small decrease of TOC removal was observed after NaHCO 3 was introduced (Figure 9b).The results suggest the direct ozonation was the dominant process for pNaZSM-5(31)-COP and Zn/NaZSM-5(31)-COP.A reduction in the TOC removal by Mg/NaZSM-5(31)-COP after the eighth use was observed by NaHCO 3 introduction.This indicates that the •OH mediated oxidation and direct ozonation both regulated the COP.
The surface active sites of NaZSM-5(120) zeolites can produce active •OHs.The unstable •OHs need to rapidly react with nitrobenzene to avoid inactivation or reactions between themselves.The powerful adsorption of NaZSM-5(120) zeolites quickly decreases the nitrobenzene concentration in solution during its first use.Lower-concentrations of nitrobenzene decreases the probability of a reaction with •OHs.The catalytic activity of NaZSM-5(120) zeolites is masked under these conditions.Direct ozonation still occurs because the dissolved ozone in solution can react with the residual nitrobenzene.With increased use, the adsorption capacity of NaZSM-5(120) zeolites is gradually reduced by saturation.The adsorption function is reduced after repeated use and the nitrobenzene concentration in solution is high.The reactions between •OHs and nitrobenzene are enhanced after repeated use.The catalytic activity of NaZSM-5(120) zeolites in COPs appears, accordingly.For NaZSM-5(45) zeolite-COPs, the surface oxidation reactions are weak due to weak adsorption.The surface active sites of NaZSM-5(45) zeolites can decompose ozone to more active •OHs, which quickly react with nitrobenzene in solution, promoting TOC removal.Similar results were found in both the initial and after repeated use due to weak adsorption of NaZSM-5(45) zeolites.For pNaZSM-5(31)-COP, the adsorption is reduced after repeated use and weal catalytic capacity results in ineffective decomposition of ozone to •OHs.The pNaZSM-5(31)-COP was dominated by the direct ozonation after repeated use.Mg oxide on the surface of NaZSM-5 (31) partially decomposes ozone to more active •OHs and promotes TOC removals during the COP.

Characterization of Catalysts
The crystals were observed by X-ray powder diffraction (XRD) with a XRD-6000 powder diffraction instrument (Shimadzu, Kyoto, Japan) with a 40.0 kV working voltage and 40.0-mA electric current.The surface area and pore volume were determined with an ASAP 2000 accelerated surface area and porosimetry system (Micromeritics, Norcross, GA, USA).The pH for point zero charge (pH pzc ) of the catalysts was determined according to the pH-drift procedure [39].The composition was determined with a ZSX-100E X-ray fluorospectrometer (XRF) (Rigaku, Tokyo, Japan).The distribution of element deposition on the surface was recorded with a PHI Quantera SXM X-ray photoelectron spectrometer (XPS) (ULVAC-PHI, Chanhassen, MN, USA).The surface morphology was examined on a Quanta 200F scanning electron microscope (SEM).The functional groups on the surface were identified with a Magna-IR 560 ESP FT-IR spectrometer (Nicolet, Madison, WI, USA).

Ozonation of Nitrobenzene Solution
Nitrobenzene is one of the ubiquitous biological ROCs found in chemical wastewaters [40][41][42][43] and is, therefore, utilized as a model ROC in this study.The aqueous nitrobenzene solution was prepared at a concentration of 100 mg L −1 in ultrapure water.The initial TOC and pH values for 100 mg L −1 of nitrobenzene solution were 61.5 mg L −1 and 7.2, respectively.
The experimental system for the COP examination is illustrated in Figure 10.The system was constructed using: a 40 L of oxygen tank (Shandong Tianhai High Pressure Container, Co., Ltd., Linyi, China), a COM-AD-02 ozone generator (Anseros Asvanced Oxidation Technologies Co., Ltd., Tübingen-Hirschau, Germany), two GM-6000-OEM ozone gas analyzers (Anseros Asvanced Oxidation Technologies Co., Ltd., Tübingen-Hirschau, Germany), a D07-7 mass flow controller (Beijng Sevenstar Flow Co., Ltd., Beijing, China), a D08-1F flow readout box (Beijng Sevenstar Flow Co., Ltd., Beijing, China), a 1000 mL quartz column reactor and an exhaust gas collector.The reactor was placed on a ZNCL-BS intelligent magnetic stirrer (Shanghai Kankun Instrument Equipment Co., Ltd., Shanghai, China) at 800 rpm to ensure the efficient mass transfer of nitrobenzene, ozone, and catalyst.During COP experimentation, an aliquot of 500 mL of nitrobenzene solution and 0.5 g of catalyst were added into the reactor at 30 • C. The gaseous ozone was then introduced through a porous diffuser at the bottom of the reactor having a flow rate of 5 mg min −1 .A 10 mL aliquot of treated solution was then extracted using a 20 mL syringe.Catalyst particles were removed before further analysis by filtration using a 0.45 µm syringe filter (Tianjin Jinteng Experimental Equipment Co., Ltd., Tianjin, China).The single adsorption process (SAP) was performed using the same experimental processes without ozone.The single ozonation process (SOP) was performed using the same experimental processes without ZSM-5 zeolites.
Oxidation mechanisms were determined through •OH quenching methods.NaHCO 3 was added into the nitrobenzene solution at a concentration of 1.0 g L −1 to scavenge free •OH prior to experimental use.Catalysis efficiencies were determined after the performance of eight replicates.After the first experimental run, the treated nitrobenzene solution containing catalyst was filtered through a FB-10T filter (Tianjin Automatic Science Instrument Co., Ltd., Tianjin, China) having a 0.45 µm microfiltration membrane (Tianjin Jinteng Experimental Equipment Co., Ltd., Tianjin China).The catalyst containing filtrate was dried at 90 • C for 12 h, and the catalyst was reused.Between all subsequent experiments, 0.5 g catalyst was maintained by supplement of new catalyst before prior to use.All SAP, SOP, and COP experiments were performed in triplicate.The experiments were carried out using varying initial pH values (adjusted with 1 N NaOH or HCl).The pH and TOC values of the nitrobenzene solutions were measured with a MP 220 pH meter (Mettler Toledo, Greisensee, Switzerland) and a TOC-L CPH CN 200 TOC analyzer (Shimadzu, Kyoto, Japan).

Conclusions
Three different SiO 2 /Al 2 O 3 ratios of NaZSM-5 zeolites (31, 45, and 120) and their metallic oxide were investigated using COP for the treatment of nitrobenzene in solution.NaZSM-5(120) exhibited high TOC removal capabilities (70.2-74.0%)by adsorption, relative to NaZSM-5(45) (0.4-0.6%) at different initial pH values.NaZSM-5 (31) was produced by treating NaZSM-5(45) with NaOH and was able to remove 20.9-23.8% of TOC by adsorption.The comparative study using NaZSM-5(120), NaZSM-5(45), and NaZSM-5 (31) in COPs for the removal of nitrobenzene from solution expands the current understanding of mechanisms that occur during COPs with ZSM-5 zeolites.High TOC removals were obtained in ZSM-5-aided COPs under acidic and neutral conditions compared with alkaline.The compared ZSM-5 zeolites had different TOC removal capabilities and also different reactions COP treatment.NaZSM-5(120) was more efficient at removing TOC when compared with NaZSM-5(45) and NaZSM-5 (31).For NaZSM-5(120), the adsorption and direct ozonation jointly contributed toward the reduction of TOC during its first use.Subsequent use proceeded to remove TOC by direct ozonation and •OH mediated oxidation.For NaZSM-5(45), direct ozonation and •OH mediated oxidation controlled the process for the first and subsequent uses.For NaZSM-5 (31), adsorption and direct ozonation occurred during its initial application and subsequent application by direct ozonation.NaZSM-5(120) and NaZSM-5(45) exhibited catalytic activity, while NaZSM-5 (31) did not.The high catalytic activity of NaZSM-5(120) may be due to more surface Si-O bonds for zeolite.The loading of Mg oxide on ZSM-5 zeolites can increase catalytic ozonation activity.These results show the potential of ZSM-5 zeolites and describe the reaction mechanisms for ozonation of recalcitrant chemical wastewaters.

Figure 10 .
Figure 10.Flow diagram of COP experimental system.