UiO-66(Ti)-Fe3O4-WO3 photocatalyst for efficient ammonia degradation from wastewater into continuous flow-loop thin film slurry flat-plate photoreactor

Journal Pre-proof UiO-66(Ti)-Fe3 O4 -WO3 Photocatalyst for Efficient Ammonia Degradation from Wastewater into Continuous Flow-Loop thin Film Slurry Flat-Plate Photoreactor M. Bahmani, K. Dashtian, D. Mowla, F. Esmaeilzadeh, M. Ghaedi

PII:

S0304-3894(20)30348-4

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122360

Reference:

HAZMAT 122360

To appear in:

Journal of Hazardous Materials

Received Date:

10 October 2019

Revised Date:

18 January 2020

Accepted Date:

19 February 2020

Please cite this article as: Bahmani M, Dashtian K, Mowla D, Esmaeilzadeh F, Ghaedi M, UiO-66(Ti)-Fe3 O4 -WO3 Photocatalyst for Efficient Ammonia Degradation from Wastewater into Continuous Flow-Loop thin Film Slurry Flat-Plate Photoreactor, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122360

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UiO-66(Ti)-Fe3O4-WO3 Photocatalyst for Efficient Ammonia Degradation from Wastewater into Continuous Flow-Loop thin Film Slurry Flat-Plate Photoreactor

M. Bahmania,b, K. Dashtianc, D. Mowlaa,b,d,* , F. Esmaeilzadeha,b,d, M. Ghaedic,*

aChemical

Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran

bEnvironmental

Research Center in Petroleum and Petrochemical Industries, School of Chemical and Petroleum

cChemistry

Department, Yasouj University, Yasouj 75918-74831, Iran

Enhanced Oil and Gas Recovery Institute, Advanced Research Group for Gas Condensate Recovery, School of Chemical

*

[email protected] (Dariush Mowla) [email protected] (Mehrorang Ghaedi)

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*

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and Petroleum Engineering, Shiraz University, Shiraz 71348-51154, Iran

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Graphical abstract

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Engineering, Shiraz University, Shiraz, Iran

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Highlights

Preparation of UiO-66(Ti/Zr) supported Fe3O4-WO3 heterojunction as a novel regenerable photocatalyst.



Design of a continuous flow-loop thin film slurry flat- plate photoreactor



Excellent response of photocatalyst toward visible- light illumination



Suggestion of possible enhanced photocatalytic mechanism of the prepared heterojunction.

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

Abstract

This work presents the characterization of novel synthesized UiO-66(Ti)-Fe3O4-WO3 magnetic photocatalyst and investigates their photocatalytic activity for the photodegradation of ammonia in a designed continuous flow-loop thin-film slurry flat-plate photoreactor (TFSR). Excellent ammonia degradation efficiency was achieved in the presence of the synthesized catalyst at ambient conditions using no more reactive oxidant

species. Several independent variables involving catalyst mass, flowrate, pH, irradiation time and initial ammonia concentration as well as corresponding experiments were analyzed and design using the central composite design (CCD). The influence and significance of each parameter and their binary interactions were then evaluated by applying the analysis of variance. The ammonia degradation efficiency of 91.80% with the desirability of 0.903 were obtained at optimum values of operational parameters including 550 mL/min,10, 0.125 g/L, 60 min and 30 mg/L for solution flowrate, pH, catalyst mass, irradiation time and initial ammonia concentration, respectively. Furthermore, the liquid phase products of ammonia degradation such as nitrate and

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nitrite ions were completely removed, and purified water was produced using the combination of reverse O



2



was the

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osmosis process and mixed resins beds. The photocatalyst mechanism study revealed that

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predominant reactive oxygen species in the ammonia photodegradation.

Keywords: UiO-66(Ti)-Fe3O4-WO3, Ammonia degradation, Blue light irradiation, Central Composite Design,

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Reactive oxygen species (ROS)

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1. Introduction

Ammonia in wastewater is mostly generated from industries such as chemical, petrochemical, pharmaceutical, fertilizer, food and catalyst manufacturing and causes environmental problems including the enhancement of

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dissolved oxygen consumption, water eutrophication and harmful effects on human and marine living organisms’ health [1-3]. The treatment of ammonia in wastewater has been received much attention in the environmental studies field. To this end, the advanced oxidation process (AOP) is one of the treatment methods, which has been accepted as an effective approach besides other conventional water purification techniques

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such as biological treatment, physiochemical, air stripping and ion exchange [4, 5]. The photocatalytic process has been adopted as a practical and efficient method which is considered in AOPs category. The photocatalytic degradation process is based on producing powerful and efficient oxidizing agents such hydroxyl and superoxide free radicals (





OH



& O 2 ) and composed of semiconductors as well as a light illumination source

for a degradation process [6-8]. In the photocatalytic systems, the light irradiation sources have an enormous impact on catalyst activation and energy consumption of process. In this regard, UV lights were the main irradiation source for the conventional photoreactors while they are so expensive and have several essential

drawbacks including producing a large amount of heating energy, toxic by-products, insufficient mechanical resistance and lifetime [9]. Besides the aforementioned problems, the conventional photocatalytic reactors suffer from an inappropriate light distribution at all parts of the reactor surface and adequate mass transfer limitation between catalyst particles and available reactants (i.e., pollutants) in the reactor volume that restrict them for industrial-scale purposes [10, 11]. Hence, the researchers have devoted lots of their attention for designing novel photoreactors equipped to a visible light source such as light-emitting diodes (LEDs) that an essential issue to overcome the reported problems [12, 13]. The thin-film slurry flat- plate photoreactor (TFSR)

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is an efficient strategy to overcome the defects of the conventional photoreactors and also to eliminate the high amount of electrical energy consumption as well as uncertainty of penetration depth at all parts of the reactor

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volume, existing in some other novel photoreactors such as a rotating packed-bed reactor [14, 15]. TFSR was designed as a catalytic photoreactor with a simple geometry, uniform thin-film at the entire surface of the

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reactor and the flow-circulation mode for scaling-up at industrial applications, ensuring the depth of light

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penetration and external mass transfer improvement purposes, respectively. The designed equipment has been equipped with stripes LEDs at the top of the reactor surface with low distance to the solution (catalyst+

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wastewater) to compensate the non-uniform light distribution with the confidence of an effective penetration depth of the light in all parts of solution body to provide a minimum amount of necessary light for the catalyst activation [16]. TFSR could be efficient for removing or reducing the external mass transfer while the synthesis

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of particles in the micro or smaller size could decrease the internal mass transfer in the photocatalytic water purification [17, 18]. Therefore, great efforts have been made to prepare a photocatalyst with desirable properties including high mechanical stability, low rate of electron-hole recombination, facile separation from the purified water, capable of the activation with a visible light (i.e., short band gap) and a crystalline structure

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[19, 20]. Metal-organic frameworks (MOFs) have been known as a material category with multi-dimensional networks and composed of clusters (metal ions) coordinated to organic ligands with tremendous remarkable properties including large surface area, adjustable pore size and crystalline network [21, 22]. The superiority of utilizing MOF as a supporter for other semiconductor photocatalysts was widely reported in the published literature such as the capability of MOF matrix to control particles size of photocatalysts, generating more active sites for the contact time improvement between reactive species and particles sites owning to their high surface area and MOFs with high porous building blocks, comforting the separation of charge carriers by

providing more directions for the photoexcited electron transmission [23-26]. MOF could be an excellent option for compositing with other conventional visible-light photocatalysts such as WO3, BiVO4, Fe3O4, CuWO4 due to the aforementioned reasons [27, 28]. Amongst, great attention has been focused on the WO3 as a photocatalyst due to its exciting properties such as powerful hole oxidizing performance, facile synthesis method, nontoxicity and an appropriate band gap (2.4-2.7 eV) for more solar light absorption toward other similar catalysts [29, 30]. Based on the novel green chemistry and also environmentally friendly process, developing recyclable and reusable photocatalysts is an essential issue in the preparation of novel visible-light

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heterojunctions [31, 32]. In this case, Fe3O4 is an ideal candidate with strong magnetite properties, capable for

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more hydroxyl radicals (  O H ) production, low cost and a green material [33, 34]. Therefore, this study presents a novel hybrid photocatalyst, namely UiO-66(Ti)-Fe3O4-WO3, which synthesized by an ultrasound-assisted

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hydrothermal method. The ternary recyclable photocatalyst with enhanced visible-light (blue LEDs) absorption, high stability and a lower band gap concerning each photocatalyst was then applied for the

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photodegradation of ammonia in a thin-film slurry flat- plate photoreactor. The characterization of the prepared photocatalyst was performed using several analytical techniques. To save the time and cost, analyzing and

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optimizing the operational parameters were evaluated by using the central composite design (CCD) as well as a desirability function (DF), respectively. The liquid phase products of the ammonia photodegradation were analyzed, and an approach was suggested for complete treatment of the ammonia in wastewater beside the

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TFSR. The reusability as well as the stability experiments of the novel synthesized photocatalyst in ammonia degradation was fulfilled in several cycles. Finally, an indirect method was applied to evaluate the types of reactive oxygen species (ROS) during the photocatalytic degradation process with the aid of various

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scavengers.

2.Experimental

Vendor information for all used instruments and chemicals contain company and country name were reported in the supplementary information.

2.1.

UiO-66(Ti)-Fe3O4-WO3 preparation

UiO-66 was synthesized via the procedure described in the literature [35]. In a typical synthesis, 0.5 g of ZrOCl2.8H2O and 0.25 g of 1,4-benzenedicarboxylic acid were dissolved in 70 mL DMF under ultrasonication. Then, 0.5 mL HCl was added to the mentioned solution, which was then kept in a 100 mL Teflon-lined pressure vessel for 24 h at 120 °C. After cooling to room temperature, the precipitating solid was purified with ethanol for several times, followed by drying under vacuum for 24 h at 100 °C. The UiO-66(Ti) nanocomposites were prepared via a modified post grafting method as follow: a proper amount of Ti-(OBu)4 was blended with the as-prepared UiO-66 in toluene, and then the mixture was heated at 100 °C for 24 h under N2 atmosphere.

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Finally, the obtained sample known as UiO-66(Ti) was washed with toluene for several times and dried in an oven at 50 °C.

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The UiO-66(Ti) supported Fe3O4 was synthesized by the hydrothermal technique as follow: 0.5 g of UiO66(Ti) was dissolved in 100 mL of EG solution via 60 min sonication. Then, 1.2 g of iron (III) chloride, 2.0 g

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of sodium acetate and 10 mL of ethylenediamine were dispersed in the above solution under sonication. The

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obtained homogenous yellow solution was transferred to a Teflon-lined autoclave and heated at 200 °C for 8.0 h and in the later stage was cooled to ambient temperature. The UiO-66(Ti)-Fe3O4 was collected using an

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external magnetic field, washed several times with ethanol and double distilled water respectively, and then dried in an oven at 50 °C for 12 h.

The hybridized UiO-66(Ti)-Fe3O4-WO3 composites were prepared by a facile hydrothermal method. Initially,

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0.75 g of sodium tungsten dehydrate (Na2WO4.2H2O) was dissolved in 100 mL HCl solution at room temperature under the continuous and vigorous stirring to get a yellowish precipitate of tungstic acid (H2WO4) by adjusting the pH value to 2. After 1 h, 1.0 g UiO-66(Ti)-Fe3O4 was added to the above solution under sonication condition. The final solution was then poured into a Teflon-lined stainless steel autoclave, sealed

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and maintained at 150 °C for 24 h and then cooled to room temperature. Finally, the resulting product as UiO66(Ti)-Fe3O4-WO3 was filtered and washed several times with distilled water and ethanol to remove any possible ionic remnants and subsequently was dried at 60 °C for 12 h.

2.3.

Experimental setup

A thin-film slurry flat-plate photoreactor was developed and fabricated based on flow-circulation mode and applied for the ammonia photodegradation process as shown in Fig. 1. Among the advantages of the designed

TFSR, the uniform light distribution with the identical permeation depth at all parts of the reactor surface and mass transfer coefficient improvement due to thin-film forming and continuous flow-loop mode, are noteworthy, respectively. Designed TFSR consists of transparent flat-plate surface, a peristaltic pump (Shenchen, China), a mechanical mixer, an aeration pump, stripes of blue LEDs (SMD 5050 flexible stripe, 14.4 W/m, 12 v), a reservoir tank (with volume of 2.5 L), a flowmeter (Yuyao Yinhuan, China), a sampling valve, mechanical structure with adjustable standing and a control panel. The main section of the designed apparatus was the reactor (length (L)= 45 cm, width (W)=22.5 cm, height (H)=5cm and thickness= 8 mm)

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which was made of transparent glass. To form a uniform film of the solution as well as more contact time between the catalyst and light source, nanosize roughness was made in the entire reactor surface. Furthermore,

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the photoreactor was equipped to an inlet pond (L= 5 cm, W= 22.5 cm, H= 4 cm) with a longitudinally narrow slit of 2 mm height from the reactor surface and an inclined outlet pond (L= 22.5 cm, W=2.5 cm and H=5cm)

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to improve liquid distribution and better solution discharge at the end of the reactor, respectively. The reactor

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was mounted on the mechanical structure with a small slope to the horizontal axis for spontaneous movement of the fluid with the help of gravity force. The blue LEDs stripes were embedded onto the special holder and

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positioned on the top of the reactor surface as a light source. To ensure a uniform light-distribution and an efficient permeation depth at all parts of the reactor, the LEDs stripes were situated in a small distance with

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each other and at the height of 5 cm to the reactor surface.

2.4. Photodegradation procedure

The photocatalytic performance of UiO-66(Ti)-Fe3O4-WO3 as a novel ternary visible-light photocatalyst for ammonia degradation was studied in a TFSR. 36 experiments were conducted according to ½ fraction CCD

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matrix. A certain amount of ammonium chloride was added to deionized water and rigorously mixed by a stirrer, and the pH was then adjusted using 1 M HCl and/or NaOH solutions. Subsequently, the specified amount of catalyst was poured to the solution and completely mixed by a mechanical stirrer. The well-dispersed slurry solution was fed to the storage tank. Afterward, the solution was flowed into the inlet pond of the reactor by a peristaltic pump. The solution was uniformly dispersed under the blue LEDs illumination and was left the reactor via the outlet inclined pond after the photodegradation process. Finally, the effluent solution was flowed to the storage tank and was aerated during the experiments. The aforementioned procedure was continued as a

cycle for all experiments. Fifteen minutes was considered as adsorption–desorption equilibrium before each experiment. At the end of each experiment, 10 mL of solution was taken out from the sampling valve, and the amount of ammonia concentration was then determined using the Nessler method along with a UV-visible spectrophotometer [36]. Ultimately, the ammonia photodegradation percentage (R%) was calculated similarly to our previous studies [14-17]. Furthermore, the concentrations of the generated nitrite and nitrate in the liquid phase after the ammonia

and ultraviolet spectrophotometric screening methods, respectively [37].

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3. Results and discussion

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degradation was obtained using the ferrous sulfate (Hach reagent, powder pillows, in the range of 2- 250 mg/L)

3.1. Characterization of UiO-66(Ti)-Fe3O4-WO3

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The XRD patterns of as-prepared UiO-66 (Fig. 2) displayed a similar simulated XRD pattern reported in the

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literature [35]. The UiO-66(Ti) possesses similar XRD patterns as UiO-66, which points out that the structure of UiO-66 was not changed with the introduction of Ti. However, no new diffraction peaks pertained to Ti

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could be considered. Meanwhile, the diffraction peak at 2θ around 7.37º and 8.57º were shifted to 7.43º and 8.63º respectively, in the UiO-66(Ti). These issues suggested that Ti is not simply grafted or engaged in the UiO-66 framework, while Ti might cooperate with Zr to the formation of oxo-bridged hetero-Zr–Ti clusters,

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which behave as the skeleton of the UiO-66(Ti) framework. Particularly, after adding Ti, some of the characteristic peaks allocated to UiO-66 disappeared; confirming the loss of its well-ordered porous structure due to the partial substitution of larger Zr in Zr–O oxo-clusters, and the imbalance atom size of Zr and Ti would not lead to the homogenous porous structures. XRD pattern of UiO-66(Zr/Ti)-Fe3O4 has in six additional

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characteristic peaks of assign to Fe3O4 at 2θ= 25.71º, 35.56º, 43.36º, 50.24º, 56.67º and 62.62º belong to (220), (311), (400), (422), (511) and (440) planes (matched with JCPDS No. 75-0033) and confirm preparation of Fe3O4-NPs and has good agreement with literature [38]. Whereas in the case of WO3 (Fig. 3) all diffraction peaks correspond to the monoclinic phase of WO3 (JCPDS No. No.43-1035) which shows successfully preparation of the intended photocatalyst [39].

The morphologies of UiO-66(Ti)-Fe3O4-WO3 can be observed via FESEM images presented in Fig. 3. For comparison, FESEM images of pure UiO-66(Ti) and UiO-66(Ti)-Fe3O4 were also included. From Fig. 3a and b, it can be seen that UiO-66(Ti) sample exhibit a plate-like and different nanosizes crystals stacking layers with smooth structures. As shown in UiO-66(Ti)-Fe3O4 FESEM images (Fig. 3c and d), Fe3O4 displays aggregated stacking small spherical-like morphology due to aggregation and gathering of the magnetite Fe3O4 particles which they were well dispersed on the surface of UiO-66(Ti), resulting in the formation of a heterostructure. Moreover, the surface of particles is less smooth than that of the pure UiO-66(Ti), which may

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be due to the presence of Fe3O4 nanoparticles coating on the UiO-66(Ti) plates. Finally, UiO-66(Ti)-Fe3O4WO3 displays aggregated small spherical-like morphology due to aggregation and gathering of the WO3 smooth

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particles in which they were well dispersed on the surface of UiO-66(Ti)-Fe3O4. In this case, WO3 and Fe3O4 nanoparticles with size of tens of nanometers can be seen on the surface of UiO-66 (Ti) octahedrons, which

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can be more clearly seen from TEM image (Fig. 4). In the TEM image, the particles with the highest

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accumulation due to the magnetite force are Fe3O4 and the particles with the lowest concentration are WO3. The EDS mapping analysis of as prepared UiO-66-Fe3O4-WO3 indicates the homogeneous distribution of W,

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Fe, and O element in the surface region, and the strong signals of Zr(Ti), O and C are found in the support area, revealing that the Fe3O4-WO3 is uniformly distributed over UiO-66-(Ti) (Fig. 5). Photoluminescence (PL) spectra have been applied to reveal the charge carrier trapping, migration and

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recombination processes of the as prepared photocatalysts since PL emission arises from the recombination of free carriers [40]. It is well certified that the higher PL intensity indicates the fast recombination of the charge carriers, resulting in lower photocatalytic activity. As shown in Fig. 6, pure UiO-66 sample has a main emission peak at about 470 nm due to the band gap recombination of electron–hole pairs. After the Ti and Fe3O4 were introduced, the heterostructured sample showed lower PL emission intensity compared with that of pure UiO-

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66 due to better photogenerated electron−hole pair separation efficiency. The much lower peak intensity implied that the cooperative effects between UiO-66-MOF, Ti, Fe3O4 and WO3 contributed to decreasing the recombination of electron-hole pairs effectively and enhancing the charge separation efficiency. Moreover, the PL intensity of UiO-66(Ti)-Fe3O4-WO3 sample is the lowest, demonstrating that it achieves a fastest separation efficiency of photogenerated electron−hole pairs, which is in agreement with the photocatalysis performance.

Fig. 7a shows the N2 adsorption isotherms of as-prepared samples, and revealed that specific surface area of UiO-66 (Ti)-Fe3O4-WO3 is 290.4 m2/g with a micorpore volume of 0.25 cm3/g, which are lower than those of unmodified UiO-66 (Ti) support (385.5 m2/g with a micorpore volume of 0.17 cm3/g). The decrease in the surface area and pore volume for UiO-66 (Ti), may be attributed to the partial occupation of the UiO-66 (Ti) cavities by the deposited Fe3O4-WO3 nanoparticles.

Fig. 7b demonstrates the magnetization curves of UiO-66(Ti)-Fe3O4 and UiO-66(Ti)-Fe3O4-WO3

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nanocomposites. For UiO-66(Ti)-Fe3O4 nanoparticles, the saturation magnetization is observed to be 44.2 emu/g at 15000Oe, which is due to the disorder of the atomic magnetic moments or the spin glass effect on the

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surface of the nanoparticles. The saturated magnetization of UiO-66(Ti)-Fe3O4-WO3 at 8000 Oe is equal to 18.00 emu/g. In brief, UiO-66(Ti)-Fe3O4-WO3 magnetization is observed to be lower than that of UiO-66(Ti)-

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Fe3O4, which can be explained in terms of the fact that UiO-66(Ti)-Fe3O4 surfaces are covered by WO3 particles

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which are not magnetically active. In these cases, using an external magnetic field for UiO-66(Ti)-Fe3O4-WO3

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magnetization, particles were properly separated.

To estimate the optical absorption properties, band gap and band alignments of the UiO-66(Ti)-MOF, UiO66(Ti)-Fe3O4 and UiO-66(Ti)-Fe3O4-WO3, DRS analysis were examined (Fig. 8a). The UiO-66(Ti)-Fe3O4 and

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UiO-66(Ti)-Fe3O4-WO3 samples displayed strong absorption in almost all regions, which can be ascribed to its small bandgap. The as-prepared UiO-66(Ti)-Fe3O4 displayed stronger absorption as compared to others, corresponding to the existence of low band-gap Fe3O4nanoparticles, while due to the its fast electron-hole recombination rate cannot be as efficient photocatalyst. Hence, the UiO-66(Ti)-Fe3O4-WO3 nanocomposite

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with middle band gap would show more enhanced photoactivity than others. For certifying Tauc plots were drowned and above results and the exact value of bandgap were calculated (Fig. 8b) and subsequently CB and VB for Fe3O4 and WO3 were calculated according to the our previous reported equations which their alignments well shown in mechanism section [17, 41].

The optical properties of pure UiO-66, pure UiO-66(Ti), pure Fe3O4 and pure WO3 samples obtained in same reaction condition from above mention procedure ware collected and revealed that WO3 and Fe3O4 exhibit

strong absorption at λ>400 nm, which is assigned to their intrinsic absorption which show significantly enhanced absorption in the whole visible light region, as well as indicates that photocatalysis could cover the UV and visible light range in these samples (Fig. 9a). For certifying Tauc plots were drowned and above results and the exact value of bandgap for pure samples were calculated (Fig. 9b). The Mott-Schottky curves of pure UiO-66, pure UiO-66(Ti), pure Fe3O4 and pure WO3 in 0.5 mol/L Na2SO4 solution were measured in a standard three-electrode electrochemical workstation. The three-electrode system consists of Pt film, FTO conductive glass (coated with 1 cm2 of UiO-66, pure UiO-66(Ti), pure Fe3O4 and pure

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WO3 films), and Ag/AgCl electrode. The applied voltage to the FTO working electrode for photocurrent was 0 V, and the applied potential for Mott-Schottky curves ranged from -0.6 V to 0.6 V, and the scan frequency was

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10 kHz. The flat band potential (EFB) of UiO-66, pure UiO-66(Ti), pure Fe3O4 and pure WO3 can be obtained using Mott-Schottky analysis according to our previous report [41].The EFB value can be roughly obtained by

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extending the linear section to 1/C2 =0 (See Fig. 9c). The EFB values of pure UiO-66, pure UiO-66(Ti), pure

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Fe3O4 and pure WO3 are -0.47, -0.20, -0.24 and +0.63 V vs NHE, respectively. In addition, the positive slope of linear part indicates that all samples exhibit n-type semiconductor properties. The ECB values are

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approximately equal to EFB for n-type semiconductors, and the relevant parameters are summarized in Table 1.

3.2. CCD analysis (different models fitting with ANOVA analysis)

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The ANOVA analysis has been examined for various common applied models including linear, cubic, twofactor interaction and quadratic model. The significance of the model parameters including individual or their interaction could be confirmed based on the following criteria: p-value<0.05 and lack of fit>0.05 for each factor in the model. Based on the ANOVA analysis, the quadratic model was accepted as the best-fitted model to the

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obtained experimental data of ammonia photodegradation percentage with appropriate quality and reliable aforementioned statistical criteria (Table S3). The corresponding F-value and lack-of-fit P-values of the ammonia degradation model were found to be 229.50 and 0.4333, respectively which were proved the significance of the applied model. The developed quadratic model with significant parameters for the ammonia degradation could be presented in the form of a second-order polynomial equation as:

PD %

A m m o n ia

 1 .7 8 X 2 X

3

  8 6 .9 0  3 .0 3 X 1  6 .2 2 X  1 .0 7 X 2 X

4

 2 .1 9 X 2 X

5

2

 1 0 .3 7 X

 2 .0 7 X 3 X

4

3

 1 0 .0 6 X

 1 .7 3 X 3 X

4

5

 8 .5 4 X

5

 1 .0 3 X 1 X

 0 .8 8 X 4 X

2

 1 .5 6 X 1 X

 6 .5 8 X 1  6 .3 7 X 2

5

2 2

3

 1 .0 8 X 1 X

 3 .5 3 X

2 3

4

 1 .3 2 X 1 X

 2 .7 5 X

2 4

5

 1 .1 9 X

2 5

(3)

3.3. Optimization of the photocatalytic process Statistica software (version. 10) was applied for the optimization of efficient independent variables in the

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ammonia photodegradation process using the desirability function (Fig. S1). The range of DF variation was between 0 to 1 for undesirable and very desirable cases, respectively. The maximum degradation percentage

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was occurred at a desirability value of 0.903. The maximum ammonia degradation percentage of 91.80% was achieved at optimum conditions including flowrate of 550 ml/min, pH level of 10, UiO-66(Ti)-Fe3O4-WO3

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mass of 0.125 g, irradiation time of 60 min and the initial ammonia concentration of 30 mg/L. For checking the validity of the predicted values, the ammonia degradation experiment at optimum conditions was fulfilled

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three times where the average of degradation percentage for three replicates was around 91.04%, which showed

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the high accuracy prediction of the desirability function.

3.4. 3D-surface plots of the ammonia photodegradation

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After an accurate model was developed to interpret the mutual interactions between independent variables and responses, three dimensional (3D) surface Figures were selected to explain the variation effect of two process parameters in the range obtained from CCD on the photocatalytic degradation of ammonia at constant values of other parameters (Fig. 10). In all experiments, a constant aeration rate was fed to the solution tank to provide the required oxygen to increase the photocatalytic degradation efficiency. In this case, the effect of solution

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flowrate and ammonia concentration as significant factors were assessed on the ammonia degradation percentage as shown in Fig. 10a. Flowrate is a remarkable variable that could have either a positive or negative effect on the photodegradation process. At low flowrates, the catalyst distribution and uniform thin-film formation in all parts of the reactor surface was insufficient while the residence time of catalyst in the reactor would increase and as a result, more light exposure of catalyst for activation purpose occurred. At high flowrates, the convective mass transfer coefficient between the catalyst particles and ammonia solution was

substantially enhanced while the contact time of catalyst and light illumination reduced, which caused the photocatalytic degradation of ammonia to reduce. Moreover, high flowrates can reduce the degradation efficiency due to interfacial area reduction between the catalyst and ammonia as pollutant [42, 43]. Furthermore, increasing the solution flowrate up to 600 ml/min causes the ammonia photodegradation process to improve, while more flowrate increment leads to less degradation efficiency. Ammonia concentration is another factor, which was explained in Fig. 10a. These tests highlighted that lower ammonia degradation efficiency was attained at higher pollutant dosage, which can be attributed to the reduction in the number of

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available active sites of the catalyst per each ammonia molecule and inadequate light distribution, causing less photodegradation percentage [44]. The influence of catalyst mass variation on the ammonia degradation was

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assessed in Fig. 10b. As it is obvious, increasing photocatalyst mass results in the generation of more free radicals and provides more active sites for the generation of powerful oxidants including hydroxide and

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peroxide radicals, which increase the photodegradation process. The pH and blue LED light illumination time

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were other operational parameters, which were evaluated within the ranges of 5-13 and 20-100 min, respectively, in the 3D-surface plots (Fig. 10c). The finding confirms that pH enhancement from acidic to

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basic conditions leads to more degradation percentage of ammonia. The maximum ammonia degradation efficiency was obtained at pH level of 11. Further increase in pH level (i.e., more than 11) results in lower photodegradation efficiency. The proved reasons for these results are not yet wholly mentioned in the previous

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investigations. However, it may be due to the fact that attractive forces and electrostatic interactions can be predominant factors at basic condition (pH level of lower than 11), whereas partial dissolution of photocatalyst or strongly reduction inactive sites was important factor at pH level of more than 11, leading to a higher and or a lower ammonia degradation percentage, respectively [1, 45]. As seen in Fig. 10c, high degradation

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percentage was achieved as the exposure time increases due to more contact time between the catalyst and ammonia molecules that subsequently generate more free radicals. 3.5. Liquid phase products and completely treatment study The photocatalytic process as one of the advanced oxidation process converts the harmful and dangerous pollutants to safe or less dangerous products [4, 46]. The ammonia photodegradation under blue LED irradiation leads to produce the nitrate and nitrite ions in the liquid phase [47]. As shown, the amount of generated nitrate and nitrite ions and also ammonia degradation in the solution was plotted against time (See

Fig. 11). As the photoreaction of ammonia on the UiO-66(Ti)-Fe3O4-WO3 catalyst was proceeded, the amount of produced nitrate and nitrite ions in the solution increased, as it was anticipated. Finding an applicable and appropriate method in terms of energy consumption and costly to treat the wastewater and to produce purified water for different goals has received much attention in the last decades due to the existence of a universal drought crisis. In this case, complete removal of ammonia degradation products was studied. Furthermore, nitrite levels have always been higher than produced nitrate during the ammonia photodegradation process in various reaction times. The amount of total generated ions in the solution was measured using conductivity

of

parameter. The conductivity measurement after the photodegradation ammonia in the presence of the UiO66(Ti)-Fe3O4-WO3 catalyst resulted in 305 µs/cm for a solution with an initial ammonia concentration of 30

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mg/lit containing 5.09 and 6.71 mg/L nitrate and nitrite ions, respectively. In order to treat the solution including ions, a reverse osmosis process in series with the mixed cationic and anionic resins was applied (See

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Fig. 12). As seen, 2 liters of solution were fed to the system. The conductivity of the solution containing nitrate

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and nitrite ions was then measured at the end of the process. Based on the obtained results from the existing stream of the system, the amount of nitrate and nitrite ions were negligible, and the ammonia has been

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photodegraded more than 90%, which shows a nearly complete treatment of the studied wastewater.

3.6. Proposed photocatalytic mechanism of the prepared catalyst

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In order to study the photocatalytic mechanism, two well-known approaches including electron spinning resonance as a spectroscopic and direct method besides using chemical scavengers namely indirect method are applicable to assess the generated reactive oxygen species (ROS) during the photodegradation process [48, 49]. Hereon, various ROS scavengers involving benzoquinone (BQ), sodium oxalate (Na2C2O4), ascorbic acid 





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(ASA) and dimethyl sulfoxide (DMSO) was applied to evaluate the O 2 , h , H 2 O 2 and O H  , respectively, in the ammonia photodegradation process [50-53]. Different scavengers were dissolved in ammonia solution with the dosage of 0.1 mM for BQ and 5 mM for the other used scavengers before the addition of UiO-66(Ti)-Fe3O4WO3 catalyst at the constant level of other parameters similar to the previous experiments. The photodegradation efficiency of ammonia after 150 min reaction was indicated in Fig 13. As seen in Fig. 13, the inhibitory behaviors of each of the radical scavengers toward ammonia degradation in the presence of UiO66(Ti)-Fe3O4-WO3 catalysts were different. It was found that a remarkable reduction in ammonia degradation

efficiency was observed as BQ and Na2C2O4 were added to ammonia solution as superoxide and hole scavengers, respectively. However, the addition of other radical scavengers showed no significant effect on the 



quenching of the efficient ROSs, highlighting that O 2 is the predominate ROS in the photocatalytic 



degradation of ammonia. The source of O 2 generation in the photocatalytic process is attributed to the reduction reaction of diatomic molecules, which provides through aeration or H2O oxidation with produced h



. Therefore, in the proposed mechanism (Fig. 14) the light is absorbed on the WO3, Fe3O4 and UiO-66(Zr/Ti)

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species surface, while the Fe3O4 due to the its lower band gap is more active. Then, the valence band electrons (e-) readily excited to the conduction band (CB), and then corresponding holes (h+) formed in the valence band

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(VB) of Fe3O4. The potentials of both CB and VB of Fe3O4 are higher than that of WO3 and UiO-66(Zr/Ti). Therefore, under LED light irradiation, the photogenerated holes in WO3 can be easily transferred into the VB

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of UiO-66(Zr/Ti), as well as the photoinduced electrons of Fe3O4 and WO3 conduction band could be transfer into conduction and valence band of UiO-66(Ti), respectively, which promotes the separation of electron–hole

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pairs and reduces the probability of electron–hole recombination. In composite photocatalyst, the WO3 electrons surface could facilitate their participation in a multiple electron reduction of oxygen (●O2). The H2O

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reacts with electrons to produce active ●OH, which will further oxidize the ammonia molecules which are adsorbed on UiO-66(Zr/Ti) surface. On the other hand, some of the photo-induced holes on the VB of UiO-

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66(Zr/Ti) can be trapped by OH- to further produce ●OH, which is a strong oxidant to the partial or complete degradation of ammonia. In addition, the electrons in the conduction band of UiO-66(Zr/Ti), which are excited by the electrons of its valence band and flowing out of Fe3O4, can react with ammonia and reduce it to N2 and H2O. As can be seen, the ammonia reduction potential is -0.05 eV (See Fig. 14).

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3.7. Kinetic study of UiO-66(Ti)-Fe3O4-WO3 catalyst Kinetic study plays a significant role in the design and optimization of the photodegradation process to explain the performed reactions on the catalyst surface [54, 55]. An acceptable kinetic model can improve the photocatalytic process by the minimization of dark reactor volume. The pseudo-first order LangmuirHinshelwood (L-H) kinetics model is a common model to describe the photocatalytic process of several pollutants such as ammonia by considering the physical adsorption of pollutant molecules on catalyst surface

for van der Waals interactions with the surface as reported in our previous publications [56-58]. After linearization of the L-H kinetic model and its plotting as a function of time at different ammonia concentrations, a linear equation with acceptable statistical parameter (R2> 0.99) was obtained, which proves the capability of the applied model to explain the ammonia degradation experimental data (Fig. S2a). Furthermore, the reaction mechanism at solid-liquid interface corresponding to photocatalysts and ammonia solution can well describe by using the L-H kinetic model and surface coverage factor. In this case, the inverse of the reaction rate was plotted against the inverse of initial ammonia concentration, and the L-H kinetic model accuracy was checked

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as seen in Fig. S2b. Based on the applied concentration range and the linear equation, the L-H kinetic model is suitable to explain the catalytic degradation kinetic of the ammonia. The equation constant values were

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achieved from the intercept and slope of the obtained linear equation. More details of the used equation can be

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found in our previous studies [17, 59, 60].

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3.8. Comparison of different treatment processes and applied catalysts efficiencies for ammonia polluted water The ammonia polluted water treatment was carried out utilizing different treatment processes including

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photolysis (LED irradiation without catalyst), adsorption (catalyst without LED irradiation), batch photocatalysis and continuous photocatalysis (Both of them using LED irradiation with catalyst) in the presence of pure UiO-66(Ti)-MOF and UiO-66(Ti)-Fe3O4-WO3 catalysts, to assess the contribution of each process

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efficiency (See Fig. 15) in the achieved optimum conditions. The obtained results showed that the ammonia degradation

percentages

were

p h o t o c a t a ly s t T F S R  p h o t o c a t a ly s t B a t c h

higher

for

the

photocatalyst

treatment

process

(

) with respect to photolysis and adsorption processes in the presence of

both applied catalysts. The low efficiency of the photolysis and adsorption treatment processes toward the

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photocatalysis processes (batch or continuous mode) can be attributed to the absence of the produced oxidant agents such as hydroxide and superoxide free radicals. To show the priority of the novel ternary catalysts (UiO-66(Ti)-Fe3O4-WO3) toward other pristine and binary samples, the photocatalytic performance of them was examined at the obtained optimum conditions. As seen in Fig. 16, the UiO-66(Ti)-Fe3O4-WO3 catalyst indicate higher photoactivity for ammonia degradation under blue-LED illumination.

3.9. Stability and reusability of the UiO-66(Ti)-Fe3O4-WO3 catalyst The stability and reusability of the novel ternary UiO-66(Ti)-Fe3O4-WO3 photocatalyst under blue LEDs illumination, as an essential criterion for industrial applications, were assessed by repeating the experiments for several times at constant values of all operational parameters (see Fig. 17). As can be seen, no significant change was observed in the photodegradation efficiency of the synthesized catalyst toward ammonia after six cycles. After each cycle, the UiO-66(Ti)-Fe3O4-WO3 catalyst was washed (using ethanol and distilled water) and heated in an oven at 70 ᵒC until the moisture was completely removed. Subsequently, the dried catalyst

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was introduced to the next photodegradation cycle. As shown, the photodegradation of the ammonia had nearly constant rate after each injection of the UiO-66(Ti)-Fe3O4-WO3 catalyst, which indicates the high stability as

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well as appropriate reusability of the ternary photocatalyst. Furthermore, in each cycle at the start period of the reaction, the photodecomposition rate of ammonia is much more than that at the end of the photodegradation

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-p

process (higher and sharper slope was observed at the beginning with respect to the end of reaction).

4. Conclusions

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The new UiO-66(Ti)-Fe3O4-WO3 composite with enhanced visible light absorption property was successfully prepared and well-characterized using XRD, VSM, EDS, TEM, Mapping, BET, FESEM, DRS and PL analyses. The prepared sample indicated remarkable catalytic activity toward ammonia degradation under blue

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LEDs light illumination in the TFSR as an efficient designed photoreactor. Furthermore, the observed increase in the photocatalytic activity of the UiO-66(Ti)-Fe3O4-WO3 heterojunction was attributed to the enhancement of photo-excited electron-hole pairs besides the appropriate charge careers separation of the structure of the synthesized catalyst. CCD was applied to design the experiments at a suitable level of operation factors for

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saving in time and costs. The optimum conditions were found to be at 550 ml/min,10, 0.125 g, 60 min and 30 mg/L corresponding to the flowrate, pH, UiO-66(Ti)-Fe3O4-WO3 mass, irradiation time and initial ammonia concentration, respectively. At the achieved conditions, the ammonia photodegradation percentage was found to be 91.80% with the desirability of 0.903. The RO system in series of the mixed resins bed was successfully applied after the photocatalytic degradation process to remove the generated liquid phase products including nitrate and nitrite ions, which leads to complete treatment of the ammonia polluted water. Furthermore, kinetics study experiments were carried out and an acceptable ammonia degradation rate constant of 0.0228 min-1 was

achieved. In the ammonia photodegradation process, different types of ROSs such as were examined and

O

 2



O





2

,h



, H 2O

2

and

OH



was found to be the most effective one. Finally, the high stability and appropriate

reusability of the synthesized heterojunction were proved by repeating the photodegradation process for six periods of experiments at constant levels of all factors.

AUTHORSHIP STATEMENT: Author’s name K. dashtian, D. Mowla, M. Bahmani

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D. Mowla, M. Ghaedi M. Bahmani F. Esmaeilzadeh K. dashtian M. Bahmani Kheibar dashtian

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Substantial contribution to acquisition of data Writing - review & editing Substantial contribution to analysis and interpretation of data Drafting the article Final approval of the version to be published

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Component of the research Substantial contribution to idea conception and design Supervision conceptualization writing - review & editing

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Acknowledgement

The authors would like to thank the Environmental Research Center in Petroleum and Petrochemical Industries of Shiraz University, Chemistry Department of Yasouj University and Shiraz Petrochemical Complex for their

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technical supports.

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Microchemical Journal, 145 (2019) 996-1002.

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Fig. 1. Schematic diagram of slurry flat-plate photoreactor

(140)

(020)

(022) (220)

(200)

(002)

UiO-66(Ti)-Fe3O4-WO3 WO3: JCPDS No. 43-1035

UiO-66(Ti)-Fe3O4

(440)

(511)

(422)

(400)

Intensity (a. u.)

(220)

(311)

Fe3O4: CPDS No. 75-0033

7.43 As prepared UiO-66(Ti)

5

6 7 8 9 2 Theta (Degree)

of

As prepared UiO-66

8.63

As prepared UiO-66(Ti)

10

ro

7.37

-p

8.57

5

10

15

20

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As prepared UiO-66

25

30

35

40

45

2 Theta (Degree)

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Fig. 2. XRD pattern of the prepared samples

50

Simulated UiO-66

55

60

65

of ro -p re lP ur na Jo Fig. 3. FESEM images of the prepared samples including UiO-66(Ti) (a, b), UiO-66(Ti)-Fe3O4 (c, d) and UiO66(Ti)-Fe3O4-WO3 (e, f) in different magnifications

Jo

ur na

lP

re

of

-p

ro

Fig. 4. TEM images of the UiO-66 (Ti)-Fe3O4-WO3 sample

of ro -p

Jo

ur na

lP

re

Fig. 5. The EDS mapping analysis of as prepared UiO-66-Fe3O4-WO3

12

UiO-66 UiO-66 (Ti) UiO-66 (Ti)-Fe3O4 UiO-66 (Ti)-Fe3O4-WO3

8

6

of

PL intensity (a. u.)

10

4

ro

2

350

400

450

500

550

-p

0 600

Jo

ur na

lP

Fig. 6. Pl spectra of the prepared samples

re

Wavelength (nm)

650

700

400

60

(b) Ads UiO-66 (Ti)

350

Des UiO-66 (Ti)

300

Magnetization (emu)

Ads UiO-66 (Ti)-Fe3O4-WO3 Des UiO-66 (Ti)-Fe3O4-WO3

250 200 150 100 50

40

20

0

-20

-40

UiO-66 (Ti)-Fe3O4

of

Quantity Adsorbed (cm3/g STP)

(a)

UiO-66 (Ti)-Fe3O4-WO3

0 -60 0.0

0.2

0.4

0.6

0.8

1.0

-15000 -10000 -5000

ro

P/P0

0

5000 10000 15000

Magnetic Filed (Oe)

-p

Fig. 7. N2 adsorption-desorption of UiO-66 (Ti) and UiO-66 (Ti)-Fe3O4-WO3 (a) and VSM analysis of UiO-66

Jo

ur na

lP

re

(Ti)-Fe3O4 and UiO-66 (Ti)-Fe3O4-WO3 (b)

0.8 UiO-66(Ti) UiO-66(Ti)-Fe3O4

(a) 0.7

(b)

UiO-66(Ti)-Fe3O4-WO3

0.5

(h)1/2

0.4 0.3 0.2

UiO-66(Ti) UiO-66(Ti)-Fe3O4

0.1 200

300

400

500

600

700

1.5

2.0

Wavelength (nm)

2.5

3.0

3.5

-p re lP ur na

4.0

h (eV)

Fig. 8. DRS spectra (a) and Tauc plots (b) of the prepared samples

Jo

of

UiO-66(Ti)-Fe3O4-WO3

ro

Absorbance

0.6

4.5

5.0

UiO-66

UiO-66(Ti)

UiO-66(Ti)

Fe3O4

Fe3O4

0.4 0.2

(c)

4 -2 cm F

UiO-66

WO3

4 3

UiO-66(Ti) WO3 Fe3O4

10

WO3

0.6

(b)

1/C2 (10

Intensity (a. u.)

0.8

5

UiO-66

(h)(eV)1/2

(a)

0.0

2 1 0

200

300

400

500

600

700

Wavelength (nm)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-0.6 -0.4 -0.2

0.0

0.2

0.4

0.6

Potential (V, vs. NHE)

h(eV)

of

Fig. 9. DRS spectra (a), Tauc plots (b) and mott schottky plots (c) of the pure UiO-66, pure UiO-66(Ti), pure

Jo

ur na

lP

re

-p

ro

Fe3O4 and pure WO3 samples

of

ro

Fig. 10. 3D-surface plots of the influence of operational parameters on the ammonia degradation at optimum conditions (flowrate: 550 ml/min, pH: 10, Catalyst mass: 0.125 g, irradiation time: 60 min and ammonia

Jo

ur na

lP

re

-p

concentration: 30 mg/L)

9

Ammonia Nitrite Nitrate

5 7 6

20 5 15

4

4

Nitrite (mg/L)

Ammonia (mg/L)

25

8

3

2

of

10

3

Nitrate (mg/L)

30

6

2

ro

1

5

1

0

0

0

10

20

30

40

Time (min)

50

60

-p

-10

0

70

re

Fig. 11. The liquid phase products of the ammonia photodegradation at optimum conditions (flowrate: 550

Jo

ur na

lP

ml/min, pH: 10, Catalyst mass: 0.125 g, irradiation time: 60 min and ammonia concentration: 30 mg/L)

RO Nitrate & Nitrite polluted water Mixed Bed Resine

Reservoir

Purified Water

Pump

ro

of

Control Valve

Jo

ur na

lP

re

-p

Fig. 12. The schematic diagram of the combination of mixed resin bed and RO system

100

91.8 82

80

60

51 38

of

40

20

ro

Ammonia degradation (%)

85

No scavenger

BQ

Na2C2O4

-p

0 ASA

DMSO

re

Fig. 13. Reactive species trapping experiments for ternary UiO-66(Ti)-Fe3O4-WO3 heterostructure at optimum

Jo

ur na

lP

conditions

of ro re

-p Jo

ur na

heterojunction photocatalyst

lP

Fig. 14. Proposed mechanism for charge transferring and ammonia degradation over the prepared

80

Photocatalyst (TFSR) Photocatalyst (Batch) Photolysis Adsorption

60

of

40

ro

20

0 UiO-66 (Zr/Ti)

-p

Photocatalytic degradation (%)

100

WO3/Fe3O4/UiO-66 (Zr/Ti)

re

Fig. 15 The removal efficiency of the various process types in the presence of pure UiO-66(Ti/Zr)-MOF and

Jo

ur na

lP

UiO-66(Ti)-Fe3O4-WO3 catalysts on the ammonia degradation at obtained optimum conditions

80

Fe3O4 WO3 UiO-66(Ti) WO3-Fe3O4 UiO-66(Ti)-WO3 UiO-66(Ti)-Fe3O4 UiO-66(Ti)-Fe3O4-WO3

60

ro

of

40

20

-p

Photocatalytic degradation (%)

100

0

re

Applied Catalyst

Fig.

16.

lP

The photocatalytic performance of various catalyst for ammonia degradation at optimum conditions (flowrate:

Jo

ur na

550 ml/min, pH: 10, Catalyst mass: 0.125 g, irradiation time: 60 min and ammonia concentration: 30 mg/L)

Cycle 6

Cycle 5

Cycle 4

Cycle 3

Cycle 2

Cycle 1

1.0

0.6

of

C/C0

0.8

ro

0.4

0.0 60

120

180

240

re

0

-p

0.2

300

360

lP

Time (min)

Fig. 17. The cycling degradation efficiency of ammonia for stability test of the UiO-66(Ti)-Fe3O4-WO3 catalyst

Jo

ur na

under blue LEDs illumination at optimum conditions

Table 1 Electronic properties of pure UiO-66, pure UiO-66(Ti), pure Fe3O4, pure WO3, UiO-66(Ti)/Fe3O4 and UiO-66(Ti)/Fe3O4/WO3 Band

Flat

gap (eV)

(eV)

UiO-66

3.35

UiO-66(Ti)

band

ECB (eV)

-0.47

+ 2.70

-0.47

3.00

-0.12

+2.80

-0.20

Fe3O4

1.20

-0.10

+1.44

-0.24

WO3

2.20

0.74

+2.82

+0.62

UiO-66(Ti)-Fe3O4

1.60

-0.25

+1.85

2.20

-0.21

+2.41

UiO-66(Ti)-

Jo

ur na

lP

re

-p

Fe3O4-WO3

of

EVB (eV)

ro

Sample

-0.25 -0.21