Novel Nano-Fe2O3-Co3O4 Modified Dolomite and Its Use as Highly Efficient Catalyst in the Ozonation of Ammonium Solution

Institute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay, Ha Noi, Vietnam Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay, Ha Noi, Vietnam Faculty of Chemical Technology, Hanoi University of Industry, 298 Minh Khai, BacTuLiem, Ha Noi 100000, Vietnam NTT Hi-Tech Institute, Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City, Vietnam


Introduction
Agricultural and industrial effluents containing ammonia (NH 4 + -N) discharged from production operation, dairy processor, and fertilizer plants could reduce water quality and cause a danger to human health [1]. Methods for removal of NH 4 OH solution such as biological nitrification processes, ion exchange, membrane separation, chlorination, and catalytic degradation have often been used [2][3][4][5][6]. However, ion exchange and membrane separation methods require expen-sive operating costs and can cause secondary pollution, while biological methods require a long treatment time and strict operation control [3][4][5]. An alternative approach is the catalytic ozonation. Liu et al. investigated the employing of MgO as an effective catalyst for the catalytic ozonation of NH 4 + solution. This MgO catalyst exhibited high NH 4 OH conversion (95%) but the N 2 selectivity was very low; nitrites and nitrates were the main products [6]. Ichikawa [7]. Chen et al. 2018 reported the synthesis of MgO-Co 3 O 4 (molar ratio 8 : 2) and claimed that an ammonia nitrogen removal and gaseous nitrogen selectivity reached to the value of 85.2% and 44.8%, respectively [8].
Dolomite is a mineral clay composed of calcium magnesium carbonate CaMg(CO 3 ) 2 [9]. However, raw dolomite cannot be used as adsorbent and catalyst due to its low surface and high impurities. Therefore, the activation or modification of dolomite is necessary to expand its application. Chaudhary and Prasad reported that thermally activated dolomite could enhance its surface area, pore size distribution, and pore volume, consequently increasing fluoride removal from aqueous solution [10]. Modified dolomite can be used as an efficient adsorbent for removal of arsenic and heavy metals and dyes from aqueous solution [11]. Thermally activated dolomite can be used as an efficient catalyst for biodiesel production and decomposition of pentachlorophenol [12,13] Activated dolomite was treated with a solution of 10 M KOH for 24 h at 80°C. The solid product was separated from the mixture by centrifuge and then was washed several times with distilled water until a pH of 7 and dried overnight at 80°C in a furnace.

2.2.2.
Modification of Activated Dolomite with Fe 2 O 3 . The incorporation of Fe ions into activated dolomite framework (denoted as Fe 2 O 3 /dolomite) was carried out by using an "atomic implantation" method. Atomic implantation has occurred in a tubular quartz reactor with two compartments, separated by a membrane of quartz fibers. 0.483 g of FeCl 3 ·6H 2 O and 0.9 g of activated dolomite are introduced into each compartment. Tubular quartz reactor is placed in a tubular furnace and heated up at 500°C with a heating rate of 20°C/min. At this temperature, FeCl 3 is evaporated and with N 2 flow (60 mL/min) went through the membrane to the compartment containing the activated dolomite (Scheme 1). At this stage, FeCl 3 is dissociated into Fe 3+ , Cl -, and Fe 3+ ions incorporated into an activated dolomite matrix. After one hour of heating at 500°C, the reactor is cooled down to the room temperature. The scheme of modifying dolomite by the atomic implantation method is illustrated as below. . Solution 2 was dropwise added to solution 1, and then, solution 3 was dropwise added to the above mixture solution 1 + 2 and adjusted pH of 10 by adding 2 M NaOH solution. The mixture is further stirred and aged at 80°C for 24 h. The solid product is separated by centrifuge filtration and washed with distilled water until pH of 7. The product is dried at 90°C and calcined at 450°C for 3 h and then cooled down to room temperature.

2.3.
Characterization. The X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker, Germany) using CuK α as radiation source, λ = 0:154 06 nm, and a range of 2θ = 10°-80°. The morphology of samples was examined by scanning electron microscopy on JEOL JSM 6500F. The FT-IR measurement was performed on a Jasco 4700 spectrometer. The surface area of samples was determined by BET measurements on Tristar-3000 instruments using nitrogen adsorbate. EDS of samples were measured on a JEOL JED-2300 spectrometer. Temperatureprogrammed reduction of hydrogen (H 2 -TPR) was performed on an Autochem II 29020 instrument (micromeritics) with a thermal conductor detector (TCD).  Journal of Nanomaterials mixture of reaction product. NO 3 and NO 2 concentration was determined by using the spectrometric methods, which complies to ISO (ISO 7890-3:2006 and ISO 6777-1984). The solution was transferred to the cuvette of the spectrophotometer (UV-Vis Lambda 25, Germany) and measured under UV light at a wavelength of 425 nm [14][15][16].
The conversion and selectivity are calculated as follows:

Results and Discussion
3.1. X-Ray Diffraction. In X-ray diffraction pattern (XRD) of raw dolomite (Figure 1 3.1.1. Fourier-Transform Infrared Spectroscopy. As observed in Figure 2(a), in the FTIR spectrum of dolomite (raw ores) appeared the bands at 720; 873; 1429; 1797; 2516; and 2868 cm -1 which are assigned to the occurrence of fluctuations in dolomite structure [21,22]. In the FTIR spectrum of activated dolomite (Figure 2(b)) appeared the band at 3448 cm -1 which was assigned to the vibration of OH groups [23]. In addition, the peak at 428 cm -1 is attributed to the stretching of Mg-O bonds [24]. In the FTIR spectrum of 3.1.2. Energy-Dispersive X-Ray Spectroscopy Analysis. From Figure 3 and   Figure 5. N 2 adsorption-desorption isotherms of all dolomite samples look like a type IV according to the IUPAC classification [29]   Journal of Nanomaterials (S BET ), pore diameter (D BJH ), and pore volume (V pore ) are listed in Table 2.
As seen in Table 2, S BET , V pore , and D BJH of activated dolomite were much higher than those of raw dolomite. This indicated the efficiency of activation, making dolomite structure more porous and consequently increasing the surface area, pore volume, and pore diameter.    (Figure 6(a)) appeared a peak at ca. 706°C which was assigned to the reduction of MgO to Mg [5]. In the H 2 -TPR profile of Fe 2 O 3 /dolomite (Figure 6(b)) appeared peaks at 318°C, 408°C, 550°C, and 689°C which are characteristic for the reduction Fe 2 O 3 to Fe 3 O 4 , reduction of Fe 3 O 4 to FeO, and the reduction of MgO to Mg, respectively [5,30]. H 2 -TPR profile of Fe 2 O 3 -Co 3 O 4 modified dolomite ( Figure 6(c)) shows peaks at 328°C, 420°C, 548°C, and 688°C which is attributed to the reduction of Co 3+ to Co 2+ , Co 2+ to Co o , Fe 3 O 4 to FeO, and MgO to Mg [30][31][32]. From the obtained results, it can be concluded that the Fe 2 O 3 and Fe 2 O 3 -Co 3 O 4 modified dolomite exhibits much higher reduction ability as compared to that of activated dolomite. Thus, much lower reduction temperature of modified dolomites (320-420°C) than that of activated dolomite (680-730°C) was noted. The Fe 2 O 3 -Co 3 O 4 /dolomite exhibited the highest peak intensity at 318-340°C, indicating a larger amount of H 2 reduction as compared to that of Fe 2 O 3 modified dolomite. This lowest reduction temperature of Fe 2 O 3 -Co 3 O 4 modified dolomite is due to the fact that M-O strength of this catalyst is weak. The high reduction ability of this catalyst played a decisive role in the improvement of N 2 selectivity, by enhancing the reduction of NO 3 to N 2 .
3.1.6. Catalytic Ozonation of NH 4 + Solution. NH 4 + conversion to NO 3 and N 2 overactivated and modified dolomites is presented in Figure 7. As seen in Figure 7,  selectivity of 22.32%. The result obtained in this work is better than that reported in the literature. Thus, Chen et al. [8] have shown the N 2 selectivity of 44.8% over MgO-Co 3 O 4 catalyst in the catalytic ozonation of ammonium solution. Liu et al. [7] have shown the conversion of 83.2% and gaseous nitrogen selectivity of 51.8% on SrO-Al 2 O 3 catalyst in the catalytic ozonation of ammonium solution. The mechanism of catalytic ozonation of NH 4 + solution is involved with the following process. The ozonation reaction was conducted at pH of 9, and ammonium exists as NH 3 molecule, (Equation (3)). At pH of 9, the ozonation catalyzes the formation of HO 2 · and OH radical, according to Equations (4) and (5) [8].
The decomposition of NH 3 in water through ozonation with a metal oxide catalyst is performed by hydroxyl radicals  Journal of Nanomaterials (Equation (6)). In addition, the hydroxyl radical oxidizes NH 3 in the solution to intermediate product nitrite, which rapidly oxidizes to nitrate (Equation (7)) and (Equation (8)) [33]. NH 3 in solution can be oxidized to nitrate nitrogen by hydroxyl radical (Equation (9)).
In summary, the reactions occurring during ammonium decomposition by catalytic ozonation are as follows:   According to the mentioned reactions, in order to obtain high N 2 selectivity, reactions (11) and (12) [4,7]. To investigate the stability of the catalysts, we carry out three cycles of catalytic ozonation of NH 4 + solution over Fe 2 O 3 -Co 3 O 4 /dolomite, and the result is presented in Figure 8.
As observed in Figure 8, NH 4 + conversion over Fe 2 O 3 -Co 3 O 4 /dolomite after 3 cycles was slightly decreased, less than 5% as compared to that of the fresh catalyst. N 2 selectivity over Fe 2 O 3 -Co 3 O 4 /dolomite after 3 cycles of reaction decreased only ca. 7-8% as compared to that of the fresh catalyst, while NO 3 selectivity slightly increased (ca. 5%). From the obtained result, it could confirm that the Fe 2 O 3 -Co 3 O 4/ dolomite has high activity stability and it can be reused.
To check the change of structure and morphology of the Fe 2 O 3 -Co 3 O 4 /dolomite after 3 cycles of reaction, XRD pattern and SEM image of the spent catalyst were performed.
From XRD patterns (Figures 9(a) and 9(b)) and SEM images (Figures 9(c) and 9(d)), it can be seen that the structure and morphology of the spent catalyst were maintained; no change of metal oxide phase as well as agglomerating of metal oxide particles were noted. Based on the above results, the high catalytic activity and stability of the Fe 2 O 3 -Co 3 O 4 /dolomite catalyst could be proved.

Conclusions
From the obtained results, some conclusions could be drawn.
It was successful to modify dolomite with Fe 2 O 3 by using the "atomic implantation" method. By this method, nano-Fe 2 O 3 particles with small size and high dispersion on dolomite surface were achieved.
It was also successful to modify   Journal of Nanomaterials role in the improvement of N 2 selectivity by the reduction of NO 3 to N 2 gas.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.