Study on CeO2/CuFe2O4 Catalytic Ozonation of Resorcinol

Min SHAO Anwei WANG Hui HAN Meng SHEN Yushan WAN

Abstract In order to improve the catalytic ozonation effect of resorcinol， the spinel CuFe2O4 was modified by doped CeO2， and Xray diffraction （XRD） and scanning electron microscopy （SEM） were used for characterization analysis. The effects of composite catalyst CeO2/CuFe2O4 dosage， ozone dosage， initial pH and reaction temperature on degradation were studied， and the stability of the catalyst was tested. The results showed that the composite effect of CeO2/CuFe2O4 prepared by combustion method was good， and the catalyst presented a laminated structure， in which 30wt% CeO2/CuFe2O4 degradation effect was significant. The removal rate of resorcinol using CeO2/CuFe2O4 composite catalyst was higher than that using ozone or CuFe2O4 separately by 41.8% and 11.9%， respectively. In a reaction with resorcinol concentration of 100 mg/L， the catalyst dosage of 1.0 g/L， the ozone dosage of 2.5 mg/（L·min）， at pH=9， temperature of 20， for reaction time of 40 min， the resorcinol removal rate was 88.5%. The catalyst CeO2/CuFe2O4 still showed good degradation effect after repeatedly using for 10 times， and the dissolution rate of metal ions was lower than that of CuFe2O4.

Key words Catalytic ozonation; CeO2/CuFe2O4; Resorcinol; Stability

Phenolic organics belong to national precedencecontrolled pollutants， which are widely used in pharmaceutical， dyestuff production and petrochemical industries[1-3]. Phenolcontaining wastewater is difficult to be degraded and highly toxic[4-5]. Among them， resorcinol is the raw material and intermediate for the production of a variety of dyes and medicines， which may raise risk of cancer after longterm exposure and has been included in the list of carcinogens by WHO. The discharge of industrial wastewater containing phenol not only pollutes the environment， but also causes damage to human health[6-7]. At present， the methods to degrade phenolic compounds mainly include adsorption method[8]， chemical method[9]， biological method[10]and advanced oxidation technologies[11]. Nonhomogeneous ozone catalytic oxidation has been widely used in industrial wastewater treatment due to its good degradation efficiency， low cost and no secondary pollution.

Catalysts play an important role in the catalytic ozonation system， which promotes the decomposition of ozone to form strong oxidizing ·OH. The catalytic reaction takes place on the surface of the catalyst. The unsaturated oxygen atoms on the surface of the catalyst and metal ions are equivalent to lewis base and acid， while browst acid， lewis base and acid level are the active center of the catalyst[12-14]. Spinel ferrite is a new type of catalytic material developed in recent years. It is stable in structure and easy to recover， which has been widely used in the degradation of pollutants[15-17]owe to its strong capacity in catalyzing refractory organics in ozonated wastewater. Zhang et al.[18]prepared NiFe2O4 ferrite spinel by solgel method and found that O3/NiFe2O4 can effectively degrade dibutyl phthalate in water， with a removal rate of 92.2%， showing good catalysis， magnetism and excellent stability. Qi et al.[19]prepared magnetic spinel CuFe2O4 to catalyze the ozone degradation of acetaminophenethyl ether. It was found that the addition of CuFe2O4 can significantly improve the mineralization degree of organics and enhance the degradation effect. In this paper， roglucinol was taken as the target pollutant， and CuFe2O4 was modified by doping CeO2. The effects of catalyst dosage， ozone dosage， pH and temperature on degradation effect were studied， providing a theoretical basis for the degradation of subsequent organic pollutants.

Materials and Methods

Materials

Resorcinol was purchased from Shanghai Aladdin Biochem Technology Co.， Ltd. Copper nitrate （Cu（NO3）2·3H2O）， ferric nitrate （Fe（NO3）3·9H2O）， cerium nitrate （Ce（NO3）3·6H2O）， ammonium hydroxide （25%， w/v）， citric acid， sodium thiosulfate and potassium iodide were purchased from Nanjing Chemical Reagent Co.， Ltd. All reagents used in the experiment were of analytical reagent grade.

Experimental procedures

As shown in Fig. 1， the apparatus mainly consists of the reaction column of plexiglass， ozone generation system and exhaust gas collection and absorption system. The inner diameter of the reaction column is 60 mm， and the height is 800 mm. The aerator plate is installed at the bottom. Prior to the experiment， the reaction column was washed with ultrapure water （three times）， and preozonization （5 min of ozone） was carried out to remove the impurities remaining on the reaction column. The experimental reagents were prepared using ultrapure water. The experimental tail gas was permeated into KI solution （2%） for absorption.

In the experiment， the simulated resorcinol wastewater （100 mg/L） was used， and the volume of the reaction solution was 1 L. During the experiment， appropriate amount of sample solutions were taken at different time points， and 0.5 ml of sodium thiosulfate （0.01 mol/L） was added to terminate the reaction， and then liquid chromatographic analysis was carried out.

Preparation of catalysts

The CuFe2O4 was prepared by the combustion method. 2.42 g of copper nitrate and 8.08 g of ferric nitrate were accurately weighed， with a molar ratio of 1≥2， and were dissolved with water， respectively. 6.31 g of citric acid was weighed， then put into a fourmouth flask， and dissolved with water whiling stirring. Meanwhile， the dissolved copper nitrate and ferric nitrate solutions were slowly dripping into the flask， and then 25% ammonia water was added to adjust the solution pH to 5. Subsequently， the sample was subjected to water bath treatment at 65 for 2 h， heated to 80， and then evaporated to remove moisture. After the solution became sticky， the sample was removed to the crucible， and put into an oven for drying for 8 h at 85 to form the brown gel. After that， the gel was put in a muffle furnace （temperature 10/min）， for burning at 250 for 5-8 min before cooling and grinding into powder. After grinding， the sample was subjected to presintering treatment， and finally placed in the muffle furnace for calcination at 700 for 4 h， before resulting in final product of CuFe2O4.

The preparation of CeO2/CuFe2O4： 2.61 g of （0.006 mol） cerium nitrate and 1.51 g of citric acid， with the molar ratio of Ce（NO3）3 to citric acid of 1≥1.5， were accurately weighed and dissolved with water， respectively. The two solutions were first placed in a fourneck flask with stirring， then the pH was adjusted to 5 and subjected to water bath heating at 60 for 1 h， and finally 2.41 g of CuFe2O4 powder was added， followed by stirring for 2 h. The solution was poured into a crucible， and then put into an oven for drying at 85 for 8 h. After that， it was transferred to a muffle furnace for burning at 250 for 5-8 min. After that， the sample was subjected to presintering treatment， and then to calcination for 3 h at 650， resulting in the final product of CeO2/ CuFe2O4.

Analytical Methods

Resorcinol concentrations were analyzed by HPLC （Shimadzu， LC20A） with a C18 reversed phase column （5 m， 4.6 mm≠150 mm） and a UV detection wavelength of 289 nm. The mobile phase used for HPLC analysis was methanol/water at a ratio of 50≥50 （v/v） and the flow rate was set at 0.8 ml/min. The injection volume was 20 l， and the elution time was 8 min for all samples.

The scanning electron microscope （SEM） was performed on goldcoated samples using a scanning electron microscope （Quanta 200F， FEI Company）. Power Xray diffraction （XRD） was recorded on an Xray polycrystal diffractometer （XD6， Beijing Purkinje General Instrument Co.， Ltd）. The metal ion dissolution of catalysts was measured by inductively coupled plasma mass spectrometer.

Results and Discussion

Characterization of CeO2/CuFe2O4

Fig. 2 shows the XRD patterns of CeO2/CuFe2O4 samples with different dosage of CeO2 （mass ratio： 10%-50%）. By comparing the standard card， the CuFe2O4 diffraction peak is consistent with that of PDF340425， and the diffraction peak of CeO2 is consistent with that of PDF340394. There are no mixed peaks in the figure， indicating that the combination of CeO2/CuFe2O4 was complete and the degree of completion was high. Moreover， it can be seen that with the increase of the amount of CeO2， the intensity of the diffraction peak of CeO2 increased while the intensity of the diffraction peak of CuFe2O4 decreased.

It can be seen from Fig. 3 that after 40 min of reaction over composite catalyst with different dosages of CeO2， the removal rate of resorcinol can be ranked from large to small as： 30wt%>20wt%>10wt%>40wt%>50wt%. The catalyst containing 30wt% CeO2 had the best catalytic effect， with removal rate reaching 88.5% within 40 min. With the increase of CeO2 dosage （10wt%-30wt%）， the degradation effect of resorcinol was gradually enhanced， which is possibly due to that the doped CeO2 changed the morphology of CuFe2O4 and increased the active site on the catalyst surface， thus improving the treatment effect. When CeO2 content was 40wt% and 50wt%， the removal effect decreased with the increase of CeO2 content. In particular， when CeO2 content was 50wt%， the degradation effect was significantly reduced， and the removal rate was reduced by 14% compared with that when the CeO2 content was 30wt%. The main reason is that the CuFe2O4 decreased with the increase of CeO2， which increased the specific surface area of the catalyst， plus the active components in the catalyst were reduced， so the activity of the catalyst and the treatment efficiency were reduced eventually.

Fig. 4 shows the SEM images of CuFe2O4 and 30wt% CeO2/CuFe2O4. According to the figure， the doped CeO2 in the ferrite CuFe2O4 changes the granular CuFe2O4 into a lamellar structure， and the morphology is greatly different from that of CuFe2O4. It can be seen from Fig. 4 （a） that the CuFe2O4 sample is a granular substance， and since CuFe2O4 is a magnetic ferrite， particle agglomeration phenomenon occurs. At the same time， it can be seen from the figure that the CuFe2O4 particles are relatively uniform in shape and size， with the particle size ranging from 100 to 200 nm. It can be seen from Fig. 4 （b） that when the CeO2 dosage is 30wt%， the CeO2/CuFe2O4 morphology presents a clear layered structure， with thin and transparent layers， in obvious stratification. The preparation of CeO2/CuFe2O4 by combustion method enables the catalyst to have a large effective specific surface area， providing sufficient active sites for the catalytic reaction and improving the degradation effect.

Comparison of different catalytic systems

Fig. 5 shows that the removal rates of resorcinol increases with the increase of reaction time in the catalytic system， the degradation efficiency of resorcinol within 40 min can be ranked as O3

Effect of CeO2/CuFe2O4 and O3 dosage

In the O3+ CeO2/CuFe2O4 system， the catalytic dose is an important factor affecting the degradation of resorcinol. In the reaction system， the lack of catalyst will reduce the production of ·OH and weaken the catalytic effect， while an overlarge catalytic dose will lead to inhibition between the catalysts and waste of materials.

As shown in Fig. 6， with the increase of CeO2/CuFe2O4， the removal efficiency of resorcinol gradually increased， which is higher than removal efficiency （46.7%） in the previous section――"Comparison of different catalytic systems" using O3 alone. When the catalyst dosage was increased from 0.5 to 2.0 g/L， the removal rate of resorcinol within 40 min was increased from 80.3% to 91.2%， and the degradation effect was enhanced. This is due to that with the increase of CeO2/CuFe2O4， the ozone can be in contact with more catalysts， which increased the generation rate and utilization rate of ·OH， thus enhancing the degradation effect of resorcinol. At the same time， it was found that when the catalyst dosage was 1.0 and 2.0 g/L， the removal rate within 40 min did not differ significantly from each other， which was 88.5% and 91.2%， respectively. When the catalytic dose reached a certain value （1.0 g/L）， the continued addition of the catalyst cannot rapidly and effectively improve the removal efficiency. This is due to that the amount of ozone in the reaction system is fixed， the increase of catalytic dose cannot increase the effective contact area， and may even have an inhibitory effect on the degradation effect.

As an oxidant in the catalytic reaction， ozone is an important factor influencing the oxidation of the pollutant resorcinol and the formation of ·OH. The dosage of ozone directly determines the efficiency of pollutant treatment.

Fig. 7 shows that the removal rate of resorcinol increased with the increase of ozone dosage. When the dosage of ozone was 0.5， 1.0， 2.0 and 3.0 mg/（L·min）， the removal rate of resorcinol within 4 min was 72.2%， 82.3%， 88.5% and 90.0%， respectively. When the dosage of ozone increased from 0.5 to 2.0 mg/（L·min）， the removal rate increased by 17.8%， and the removal efficiency increased significantly， which indicates that increasing the amount of ozone can effectively improve the removal efficiency of resorcinol. When the ozone dosage was 2.0 and 3.5 mg/（L·min）， the degradation effect was good， but as the amount of ozone was further increased by 1.0 mg/（L·min）， the removal rate was only increased by 1.5%. This is due to that the amount of catalyst CeO2/CuFe2O4 in the catalytic reaction system is fixed， the utilization rate of the catalytic active site is close to the peak when the dosage is 2.0 mg/（L·min）， at this point， the additional dosage has little effect on the catalytic degradation efficiency. Therefore， the optimal dosage of CeO2/CuFe2O4 was finally set to 1.0 g/L， and that of ozone was set to 2.5 mg/（L·min）.

Effect of initial pH value and reaction temperature

Initial pH is an important condition affecting ozone oxidation， which has an influence on the existing form and transmission of ozone， and even affects the properties of pollutants and surface morphology of catalysts.

It can be seen from Fig. 8 that， with the increase of initial pH， the removal rate of resorcinol gradually increased. At initial pH=3， the removal rate of resorcinol was 64.2% within 40 min. When pH increased to 11， the removal rate increased to 26.1%. It can be seen that the degradation effect under alkaline condition was better. Although the removal rate was low when the solution was acidic （pH=3， 5）， which was still higher than 46.7% when using O3 alone. This is due to that the presence of CeO2/CuFe2O4 can enhance the degradation effect of resorcinol. When the initial pH was 9 and 11， the removal rate was 88.5% and 90.3%， respectively， and the removal efficiency increased by only 1.8%. This is mainly due to that the high initial pH value changed the surface structure and morphology of CeO2/CuFe2O4 and affected the properties of the pollutant resorcinol， so that the interaction between the active site on the catalyst surface and resorcinol was negatively affected， and thus the removal efficiency was affected. Therefore， the optimal pH value in the experiment was set to 9.

As shown in Fig. 9， the volatile rate of resorcinol increased with the reaction temperature. The volatile rate of resorcinol was 2.9% at 40， which is relatively small. With the increase of reaction temperature， the removal effect of resorcinol was enhanced gradually for both O3+CeO2/CuFe2O4 system and O3 system. At different temperatures， the removal effect of resorcinol after adding CeO2/CuFe2O4 was significantly increased， which was mainly due to that the catalyst can promote the decomposition of ozone to form ·OH， and thus oxidize and degrade pollutants in water. As the temperature increased from 10 to 40， the removal rate of resorcinol within 40 min in O3+CeO2/CuFe2O4 system increased by 12.9%， while that in O3 system increased from 39.4% to 53.1%. This may be due to the increase of reaction temperature， the reaction speed， the pollutant volatility， and the removal efficiency all increased. Meanwhile， as the temperature further increased， the increasing trend in removal efficiency of resorcinol slowed down. As the temperature increased from 20 to 40， the removal rate was increased by only 5.6%. So the final optimal reaction temperature was set to 20.

Stability of catalysts

As can be seen from Fig. 10， the removal efficiency of resorcinol decreased as the number of use of catalyst increased. The removal efficiency of resorcinol remained to be 78.6% after repeatedly using CeO2/CuFe2O4 10 times within 400 min， which was still higher than the removal rate （76.6%） when CuFe2O4 was firstly used. This indicates that the doping of CeO2 improved the catalytic property of the catalyst.

The dissolution of metal ions in the solution after using CeO2/CuFe2O4 and CuFe2O4 was tested， as shown in Fig. 11. According to the figure， the dissolution rate of copper and iron ions gradually increased with the increase of the number of use of CeO2/CuFe2O4 and CuFe2O4. After repeated use of CuFe2O4 for 400 min， the dissolution rate of copper and iron ions was 0.36  and 0.23 ， respectively; and after repeated use of CeO2/CuFe2O4 for 400 min， the dissolution rate of copper and iron ions was 0.072  and 0.039 ， with no Cerium ions being dissolved. It can be seen that after adding CeO2， the dissolution rate of metal ions in catalyst decreased， and no cerium ions were dissolved. In summary， the catalytic property and stability of 30wt% CeO2/CuFe2O4 were good.

Conclusions

（1） The composite CeO2/CuFe2O4 catalyst prepared by combustion method has better degradation on resorcinol. 30wt% CeO2/CuFe2O4 degradation of resorcinol was the best， and the removal rate reached 88.5% after 40 min of reaction. Through comparing the SEM map of the samples， the CuFe2O4 sample was granular and distributed evenly， but there existed agglomeration. The morphology of 30wt% CeO2/CuFe2O4 was in clear stratified structure， with thin and transparent layer.

（2） The optimum condition for ozonation of 100 mg/L resorcinol with CeO2/CuFe2O4 as the catalyst include catalyst dosage of 1.0 g/L， the ozone dosage of 2.5 mg/（L·min）， pH=9， temperature of 20， and the removal efficiency of resorcinol was 88.5%.

（3） The catalytic property and stability of composite catalyst were good. After repeated use of CeO2/CuFe2O4 for 10 times within 400 minutes， the removal rate of resorcinol reached 78.6%. After doping with CeO2， the dissolution quantity of metal ions in the catalyst was reduced， and no cerium ions were dissolved.

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