Fenton-like catalytic oxidation of o-cresol wastewater by H2O2 over Fe2O3/AC catalysts

A series of high specific surface Fe2O3/AC catalysts for the Fenton-like catalytic oxidation of o-cresol wastewater was designed. The catalytic performance due to the characteristics of high specific surface area, large pore volume, high chemical stability and dispersive activity center. Under the suitable reaction conditions, the Fe2O3/AC with 3 wt.% Fe2O3 loading catalyst can run efficiently and continuously for 500 h without being deactivated, meanwhile the removal efficiency of COD can still be maintained above 59%, even the activity of the catalyst can remain above 79%. As a result, this kind of high specific surface Fe2O3/AC catalysts used in the heterogeneous Fenton-like catalytic oxidation of o-cresol wastewater has a great potential for industrial application.


Introduction
As an important organic chemical material, o-cresol is widely derived from production and discharge in the chemical fields, such as pesticides, medicine, printing and dyeing, materials, etc [1,2]. O-Cresol is a prototypical poison, which is harmful for humanity's ecological life due to the difficulty to be removed under natural conditions. O-Cresol wastewater has such characteristics as difficulty in treatment, extensive source, large pollution, strong toxicity, etc [1,2].
Thus, how to efficiently treat the o-cresol wastewater is meaningful for the current research of water pollution control. A large number of literatures have been reported for the o-cresol industrial wastewater treatment, generally including physical method, biological method, chemical method, etc [3,4]. At present, the advanced oxidation process (AOPs) for the pollution control of o-cresol wastewater has been major concerned as its large-scale, efficient, systematic application in the chemical industry. The advanced oxidation process usually refers to the catalytic oxidation, such as catalytic ozone oxidation, catalytic H 2 O 2 Fenton oxidation, electrocatalytic oxidation, photocatalytic oxidation, etc [5][6][7].
The principle of catalytic H 2 O 2 Fenton oxidation technology is that a large number of oxygen-containing radicals, such as HO , 2

·
HO , 3 · O , 2 ·ˉand OH, · produced from catalytic decomposition of H 2 O 2 in the presence of Fe 2+ oxidize and degrade organic matters [8,9]. However, catalytic H 2 O 2 Fenton oxidation process has many defects, including large iron sludge output, wastewater color reversion, harsh conditions and serious corrosion, etc. Heterogeneous catalyst is used in catalytic Fenton oxidation method. To enhance the ability of catalytic H 2 O 2 Fenton oxidation reaction to control the organic wastewater and eliminate the technical barrier of homogeneous catalytic Fenton oxidation [10][11][12][13], it is of great significance to select heterogeneous Fenton catalytic oxidation process and enhance the catalytic performance, the key step is the heterogeneous catalyst and its catalytic carriers [10][11][12][13]. Zeolite was used as catalytic carriers, but the enhancement of efficiency of degrading o-cresol is limited, the removal rate only reached 50% after 120 min [14]. In order to obtain higher degradation efficiency, it is necessary to find more effective load materials.
Activated carbon (AC) is a kind of advanced porous carbon material and it has a wide application prospect especially in catalysis, separation, biology and nano-materials. Currently, AC as catalytic carriers have been used in new research fields of catalytic reaction and water pollution control. This kind of porous material has the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. characteristics as high chemical and physical stability, high specific surface area and pore volume, complicated and variable pore structure, easy to control, etc [15][16][17][18][19]. The unique and new findings is that Fe 2 O 3 /AC catalysts for the Fenton-like catalytic oxidation of o-cresol wastewater for the AOPs process, however the application of such high-performance material to treat o-cresol wastewater by catalytic H 2 O 2 Fenton oxidation has not been reported yet, no literature have reported the new AOPs process.
Because of AC material has the excellent catalytic performance in the field of catalysis, in our work a series of heterogeneous Fenton-like Fe 2 O 3 /AC materials with high specific surface area, large pore volume, high chemical stability and dispersive activity center were prepared by impregnation method. Fenton-like catalytic process has been selected in our work for the preparation of a new type of Fe 2 O 3 /AC catalysts for the control of o-cresol wastewater. With characterization of FTIR, XRD, BET, XPS, SEM, TEM, as well as NH 3 /CO 2 -TPD on the catalytic performance, this kind of high specific surface Fe 2 O 3 /AC catalysts used in the heterogeneous Fenton-like catalytic oxidation of o-cresol wastewater has a great potential for the industrial application.

Material and characterization
O-Cresol C 7 H 8 O (analytically pure) and iron(III) nitrate nonahydrate Fe(NO 3 ) 3 ·9H 2 O (analytically pure) were produced in Chemical Reagent Co., Ltd. of Sinopharm Group; AC (Activated carbon) with high specific surface (S BET 1511 m 2 g −1 , V total 0.78 cm 3 g −1 ) was home-made in laboratory. The crystal structure was observed by an X-ray diffractometer (XRD Rigaku D/Max-1200) at the scanning range of 5°-80°and the scanning speed of 10°m in −1 . The valence state was analyzed by X-ray photoelectron spectroscopy (XPS Thermo Fisher Scientific) and performed on a KAlpha 1063 system with an Al Ka radiation. The catalyst was tested on its specific surface area and pore volume by ASAP 2400 specific surface area and micropore analyzer made by Micromeritics Company after pretreated at 350°C in vacuum for 3 h by using N 2 as adsorbate. The morphology of the samples was observed by a scanning electron microscopy (SEM, SUPRA55). The transmission electron microscopy (TEM) images were obtained using FEI Tecnai G2 F20. Acid-base properties (NH 3 /CO 2 -TPD) of catalysts were determined by ChemBET-3000 chemical adsorption analyzer.

Catalyst preparation
The mesophase carbon microspheres and the activator KOH were proportionally mixed in a solution, a small amount of C 2 H 5 OH was added as dispersant, and then they were dried at 130°C. The activated carbon semifinished product was heated to the activation temperature and kept at this temperature for a period of time in a corundum crucible protected by N 2 , cooled to room temperature, washed with water and acid, respectively, and dried at vacuum at 110°C for 24 h to obtain AC (activated carbon), which was tested on its physical structure parameters (S BET =1511 m 2 g −1 , V total =0.78 cm 3 g −1 ). Fe 2 O 3 /AC catalysts were prepared by impregnation method in such a manner that a certain mass of AC was weighed and ground, and protected by N 2 and displaced by steam at 120°C for 10 h; different weights of Fe(NO 3 ) 3 ·9H 2 O were dissolved in ultrapure water, immersed in an equal volume of solution for 24 h, and dried at 80°C for 12 h; and the dried catalyst was roasted at 350°C in nitrogen tube furnace for 5 h to obtain light yellow solid, which was pressed to form the  After stable reaction, the effluent at the outlet was sampled once every 20 h. Chemical oxygen demand (COD) is the oxygen concentration corresponding to the consumed potassium dichromate calculated according to the organic matters in wastewater oxidized by potassium dichromate. A known amount of potassium dichromate solution was added to a strongly acidic medium. The chloride ions in the solution were shielded by mercury sulfate. The organic matters in the wastewater were catalytically oxidized by silver sulfate as catalyst. The wastewater was tested on its COD by spectrophotometry and on its o-cresol concentration by 4-aminoantipyrine method [20]. Figure 2 shows the XRD and FT-IR spectra of Fe 2 O 3 /AC Fenton catalyst. It can be seen from the wide angle spectrum (10°∼65°) of XRD in figure 2(a) that all Fe 2 O 3 /AC Fenton catalysts have the characteristic peak of activated carbon, in which 26.7°is the typical peak of activated carbon [21]. Compared with the unmodified activated carbon, Fe 2 O 3 /AC Fenton catalyst has not only the characteristic peak of carbon, but also characteristic peaks of iron oxide at 35.6°(JCPDS 33-0664) [21]. With increasing Fe 2 O 3 content in the catalyst, the characteristic peaks of iron oxide become more obvious, but the peak intensity of iron oxide is weak and no main phase is clearly shown due to the decrease of Fe 2 O 3 content, further suggesting that the iron oxide has been loaded into the surface of activated carbon, consistent with the electron microscopy results. It can be seen from the FT-IR spectrum of    catalyst not only has the FT-IR characteristic peak of AC, in which the peak value at 3462-3432 cm −1 increases because of stretching vibration addition of O-H on the surface of iron oxide, but also FT-IR characteristic peak of Fe 2 O 3 , in which at 1634 cm −1 is the characteristic peak of Fe-OH, and at 740 cm −1 is the characteristic peak of Fe-O in FeOOH [22,23].

Catalyst characterization
As shown in figure 3(a), SEM image shows that the catalysts has complicated surface morphology, a typical complicated amorphous porous structures of activated carbon, which provide a large specific surface and pore volume and a large number of catalytic active sites for the catalyst; and the low magnification images of TEM (figures 2(b), (c)) show that some Fe 2 O 3 active sites uniformly disperse and others agglomerate on the AC surface due to the porous irregularity of AC surface [24]. In addition, the lattice fringes with a d-spacing of 0.27 nm can be assigned to the (110) lattice plane of Fe 2 O 3 ( figure 3(c)). Figure 4 shows the full spectra of  [25,26]. In addition, it can be seen from the Fe 2p spectra of Fe 2 O 3 (3)/AC fresh Fenton catalyst and the spent catalyst from 500 h reaction in figure 4(b) that for the fresh catalyst, main peak and satellite peak of Fe 2p 1/2 are at 724.08 eV and 732.78 eV, respectively, and of Fe 2p 3/2 are at 710.68 eV and 718.68 eV, respectively, indicating that the difference in binding energy between main peak of Fe 2p 3/2 and the satellite peak of Fe 2p 3/2 is about 8 eV and between main peaks of Fe 2p 3/2 and Fe 2p 3/2 is about 13.4 eV, which are the characteristics of the peaks of Fe 2p. The analysis of the main peak of Fe 2p 3/2 shows that Fe 3+ /Fe 2+ is 96:4, indicating that Fe 3+ valence structure of Fe 2 O 3 mainly exists in the fresh catalyst [25,26]. For the spent catalyst from 500 h reaction, main peaks of Fe 2p 1/2 and Fe 2p 3/2 are at 723.08 eV and 709.58 eV, respectively, and the difference in the binding energy between the two main peaks is about 13.4 eV, which is a characteristic of the peak of Fe 2p. The analysis of the main peaks of Fe 2p 3/2 shows that Fe 3+ /Fe 2+ is 55:45, indicating that Fe 3+ and Fe 2+ of Fe 2 O 3 mainly exist in the fresh catalyst, in 500 h reaction, some of Fe 3+ is reduced to Fe 2+ , which is stable structure [25,26]. Figure 5 shows the NH 3 -TPD and CO 2 -TPD tests of Fe 2 O 3 /AC Fenton catalyst. A mount of acid/base properties on Fe 2 O 3 /AC Fenton catalyst surface was analyzed [27]. It can be known from figure 5(a) that a large number of weak acid centers (at about 170°C) and a large number of moderate/strong acid centers (at about 370°C ) are produced on the surface of Fe 2 O 3 /AC Fenton catalyst compared with the carrier AC; the spent Fe 2 O 3 /AC Fenton catalyst from 500 h reaction has fewer weak acid centers and moderate/strong acid centers on its surface than fresh Fe 2 O 3 /AC Fenton catalyst. It can be seen from figure 5(b) that a large number of weak base centers (at about 160°C) and a small number of moderate/strong base centers (at about 340°C) are produced on the surface of Fe 2 O 3 /AC Fenton catalyst compared with the carrier AC, and the spent Fe 2 O 3 /AC Fenton catalyst from 500 h reaction has fewer weak base centers and moderate/strong base centers on its surface than fresh Fe 2 O 3 /AC Fenton catalyst. Based on the catalyst performance evaluation, it shows that the activity of the catalyst is closely related to the content of acid/base centers on the surface of the catalyst, and the main active sites of the modified Fenton catalyst are Fe 2 O 3 and Fe 3 O 4 , which effectively catalyze the H 2 O 2 oxidation of o-cresol containing wastewater.  Table 1 shows the structure and catalytic performance parameters of      Figure 7 shows the 500 h H 2 O 2 oxidation of o-cresol containing wastewater in a continuous flow reaction catalyzed by 30 g of Fe 2 O 3 (3)/AC Fenton catalyst and AC, respectively, at the initial COD of 238 mg/l in the o-cresol solution, H 2 O 2 consumption of 5 mg min −1 , the wastewater HRT of 5 min and the wastewater flow rate of 0.8 l h −1 . It can be seen from figure 7 that the Fe 2 O 3 (3)/AC Fenton catalyst has much better activity and stability than AC; after continuous catalytic oxidation reaction, the COD at the AC outlet is 180-214 mg l −1 and the COD removal is 24%-10%, and the catalyst significantly deactivates because the catalyst activity reduces by 58% to only 42% of the initial activity. Fe 2 O 3 (3)/AC Fenton catalyst has better catalytic activity and stability. After 500 h continuous catalytic oxidation reaction catalyzed by Fe 2 O 3 (3)/AC Fenton catalyst, the COD at the outlet is 53-97 mg l −1 and the COD removal is about 78%-59%, and the catalytic activity reduces only by 24% to