Pharmaceutical pollutants threaten the health of humans, animals, and plants. Toxicity, non-biodegradability, and drug resistance have led to the use term “pseudo stable pollutants” for these compounds [1, 2]. Pharmaceutical compounds could create potential hazards for aquatic and terrestrial organisms by entering the environment and natural resources as well as direct contact with humans. Therefore, the need to develop suitable, inexpensive, fast, and recyclable wastewater treatment technologies is essential. One of these drug combinations is Phenazopyridine Hydrochloride (PHP) as an important aromatic compound. PHP is an analgesic drug and treats symptoms such as pain and burning in the urine, feeling of reflux, or urine rejection due to stimulation of the mucous membrane of the lower urinary tract . Exerted PHP into aquatic resources has harmful effects on the liver and other organs. Also, signs of cancer in mice were observed due to the prolonged use of PHP-contaminated water sources .
Among studied methods for wastewater treatment, advanced oxidation processes (AOPs) such as electro-Fenton (EF) have attracted great interest [5, 6]. EF is more suitable for treating wastewater containing pharmaceutical compounds owing to the great potential to complete pollutant elimination [7, 8]. EF processes is an indirect electrochemical method  in which free hydroxyl radicals () are produced in-situ by means of electrical power in a mildly acidic environment. The need for transporting no hazardous materials such as H2O2, no expensive equipment, and compact set-up are considered as the advantages of the EF process . However, strict pH control for inhibiting iron sludge formation, loss of Fenton’s reagent, and inability to recover aqueous Fe2+ as a catalyst is considered economic drawbacks that prevent practical scale-up of the homogeneous EF process . The drawbacks could be potentially addressed by the utilization of a heterogeneous catalyst, such as iron minerals like pyrite, magnetite, hematite, and iron incorporated porous materials including clays, zeolites, and MOFs . Metal-Organic Frameworks (MOFs) have regular structures that result from the bonding of minerals and organics . The unique properties of these compounds such as high free surface, high thermal and mechanical strength, low density, and high porosity  have been widely considered by researchers in order to remove organic pollutants like coloring agents and pharmaceutical contaminants [15, 16].
Imidazolate zeolite frameworks (ZIF) are hybrid porous compounds that consist of four-faced units. These compounds are a bunch of MOFs in which the metal ions are zinc or cobalt cations and the connectors of the metal ions are imidazolate. Imidazole is an aromatic heterocyclic compound and a diazole group that converts to imidazolate by losing a proton in the presence of a strong basic . The heterogeneous ZIF catalyst shows better thermal and hydrothermal stability than other types of MOFs . Thi et al.  reported that iron loading on the ZIF-8 increased the adsorption capacity compared with the pure ZIF-8. Fe-ZIF-8 also showed faster kinetics than ZIF-8, while the presence of Fe2+ played a vital role in the effective elimination of Rubidium. The results showed that the maximum adsorption capacity of the Fe-ZIF-8 for the removal of colored compounds was 193 .96 mg g-1, which was 1.4 times more than the ZIF-8. Pan et al.  applied ZIF-67 to eliminate phenol and achieved the maximum phenol removal capacity of 378.8 mg g-1. Wu et.al  applied ZIF-8 to remove arsenate. The results showed that zeolite ZIF-8 was a porous adsorbent with an excellent performance in removing arsenate and had a maximum adsorption capacity of 12.287 mg g-1. Several studies have reported PHP removal using AOPs. Khataee et al.  reported that plasma-modified clinoptilolite (PMC) performance was more than natural clinoptilolite (NC) in optimum operational conditions, including pH=5, PMC concentration of 2 g L-1, ultrasonic power of 300 W, and 10 ppm PHP in heterogeneous sono-Fenton-like processes. PMC showed 90.14 % removal efficiency of PHP in 20 min. Eskandarloo et al.  studied the photocatalytic process using Ag/SiO2–TiO2 nanoparticles and reported that PHP removal of 97.14 %. Abbasi et al.  studied HKUST-1 MOF for adsorption of PHP. Eskandarloo et al.  concluded that Sm-doped ZnO had higher PHP removal (59%) than pure ZnO (51%) in PHP degradation.
To the best of our knowledge, the utilization of Fe-doped ZIF-8 as heterogeneous nanocatalyst in the EF process has not been reported yet. In this research, we report the synthesis, characterization, and catalytic performance of the Fe-ZIF-8 nanocatalyst in the HEF process for the PHP removal. The effect of operating parameters of the HEF process, including wastewater pH, applied current, and nanocatalyst concentration was investigated.
Zinc nitrate hexahydrate (Zn(NO3)2.6H2O), 2-methylimidazole (Hmim, C4H6N2), ammonia (NH3), dimethylformamide (DMF, C3H7NO), sulfuric acid (H2SO4), sodium hydroxide (NaOH), iron chloride (FeCl3.6H2O), sodium sulfate (Na2SO4), and sodium borohydride (NaBH4) were delivered by Merck Company (Germany). PHP as a model organic contaminant was kindly provided by the Shahre Daru pharmaceutical Company (Iran). Table 1 shows the general specification of PHP.
The thermal solvent method was applied for the ZIF-8 synthesis. At the first step of the ZIF-8 synthesis procedure, 7.96 g of zinc nitrate hexahydrate was dissolved in 74.9 mL of ammonia. Then, 4.4 g of Hmim was dissolved in 80 mL of DMF. The resulted solutions were mixed and then stirred at room temperature for 2 h. The final solution had the molar ratio composition of Zn2+: Hmim: DMF: NH3=1: 2: 41: 30. In the end, the final mixture was centrifuged (5000 rpm, 30 min) and the recovered solid was washed with methanol three times. The obtained solid (ZIF-8) was dried at 110 °C for 12 h to remove any remaining methanol.
The Fe species were impregnated on the ZIF-8 nanocatalyst through a rotary evaporator. The process involved four steps at several temperatures (65-75 ºC) and pressures (200-300 mmHg) for 120 min. The corresponding intervals were 5 ºC, 50 mmHg, and 30 min. Subsequently, the drying and calcination were at 105 °C and 530 °C overnight, respectively. The reduction process was applied using NaBH4 and NaOH solution (NaBH4/NaOH=1.5 w/w), which was added to the impregnated powder and mixed for 10 min. Finally, the mixture was centrifuged and dried at 110ºC overnight in order to obtain Fe-ZIF-8 nanocatalyst.
The detail of characterization methods was provided in the supplementary information.
To measure the PHP removal, the experiments were carried out in a 100 ml bubble reactor. The graphite electrodes (3×2×0.5 cm) were attached to a DC power supply and immersed partially in the synthetic PHP wastewater solution, including 50 ml of PHP (10ppm), 0.05 M of Na2SO4 as an electrolyte, and the specified amount of the Fe-ZIF-8 nanocatalyst. The pH of the solution was adjusted using a 0.01 M solution of H2SO4 or NaOH. Before connecting the electrical current between electrodes, the solution was saturated with air for 5 min. During the experiments, aliquot samples were taken every 15 min. After centrifuging the samples, the concentration of PHP was calculated according to Eq.(1) using a UV-Vis spectrophotometer (i3, Jinan Hanon Instrument Co. Ltd, China) at 431 nm wavelength.
where C0 is the initial PHP concentration and Ct is the concentration of PHP in the taken samples.
RESULTS AND DISCUSSIONS
The index peaks of ZIF-8 (Fig. 1) appear at 29.7, 10.32, 12.65, 16.50, and 18.18°, which are related to the (011 ), (022), (112), (022), (013), (222), (114), (233), (134), (044), (244) and (235) plates, respectively . The XRD pattern of the synthesized sample is in agreement with the reference XRD pattern of ZIF-8  that confirms the synthesis of ZIF-8. The compatible pattern of the Fe-ZIF-8 and ZIF-8 samples represents a lack of significant framework destruction through the impregnation process. The XRD peaks are intensified after modification owing to the removal of some impurities from the parent structure, which is in line with the literature . There are no peaks related to Fe3O4 (JCPDS 19-629) in the Fe-ZIF-8 pattern, indicating a very strong interaction in the metal solution and uniform dispersion of Fe species on the ZIF-8 structure.
The specific surface of the nanocatalysts depends on various parameters, including the raw materials, molar composition, and synthesis conditions. Fig. 2a shows the nitrogen adsorption-desorption isotherm for the synthesized ZIF-8 and Fe-ZIF-8 nanocatalysts. The isotherm of the nanocatalysts is Langmuir type I. Table 2 shows the calculated data for the samples. The synthesized ZIF-8 has a high surface area (1335 m2g-1) and a pore volume (0.64 cm3g-1), which are decreased for the Fe-ZIF-8 nanocatalyst. Furthermore, the impregnation increases the mean pore size distribution by the creation of some mesopores (Fig. 2b). This phenomenon could be explained by the slight framework destruction through the reduction step of the impregnation process . Morabito et al.  found that ZIF-8 included a flexible framework due to the dissociation and association of linkers in an aqueous solution.
The FTIR spectrum of the ZIF-8 and Fe-ZIF-8 samples is shown in Fig. 3. The original core groups were identified correctly in the ZIF-8 sample , indicating that the ZIF-8 sample was properly synthesized and also support the XRD results. The peaks at 2929, 3130, and 3500 cm-1 are assigned to the aliphatic and aromatic C–H stretching vibration of the imidazole, and the stretching vibration of –OH, respectively . The peaks at 1200, 1600, 1674 cm-1 are attributed to the stretching vibration of C–N, C=N, and C=C. It is worth noting that the modification of ZIF-8 particles leads to the decrease in intensity of absorbing bands and elimination of the peak related to C=C. The peaks at 1350–1500 cm-1 have resulted from the entire ring stretching. The peaks around 900-1330 cm-1 and 1440 cm-1 are related to the imidazole ring. The peak at 462 cm-1 is assigned to Zn–N stretch mode . The absorbing band at 628 cm-1 is attributed to Fe-N bands, confirming Fe incorporation in the synthesized adsorbent .
SEM and TEM
Fig. 4 shows that the ZIF-8 surface morphology consists of spherical particles with aggregation. It is clear that the surface morphology does not change significantly during the impregnation process. Furthermore, TEM images support the FE-SEM results and represent the formation of nanostructure with nano-scale particle size. The results are inconsistent with the literature .
The different nature of iron species is detectable from the UV-vis spectrum. The peak in the 200-300 nm characterizes isolated Fe with tetrahedral or higher coordination. FexOy iron oxide oligomers are detected in the range of 300-400 nm. The presence of any peaks higher than 400 nm is usually attributed to large Fe2O3 particles. According to the UV-vis spectrum for the Fe-ZIF-8 nanocatalyst (Fig. 5), a sharp peak is detected in the range of 300-400 nm. Therefore, iron species in the Fe-ZIF-8 nanocatalyst are in the form of small and uniformly distributed iron oxide clusters (Fe3O4 or γ-Fe2O3), which is consistent with the XRD results.
Fig. 6 shows the performance of the Fe-ZIF-8 in the HEF system in comparison with anodic oxidation (AO), PHP adsorption on the Fe-ZIF-8 nanocatalyst, and the blank HEF process with no amount of the nanocatalyst at reaction conditions of pH=7, 25ºC, and 100mA. Adsorption of PHP molecules on the outer and inner surface of the Fe-ZIF-8 nanoparticles can be through hydrogen bonding, Van der Waals, electrostatic, and π-π interactions. The low PHP removal by the adsorption process confirms the significant role of the developed nanocatalyst. The results show that the parent ZIF-8 nanocatalyst is not active in the HEF process. Fe loading on the ZIF-8 structure increases the performance of the nanocatalyst in the HEF process. The HEF process using the Fe-ZIF-8 nanocatalyst results in 99% PHP removal. This phenomenon can be explained by anodic oxidation at the surface of anode (M) (Eq.2)  and Fenton reaction by electro-generated Fenton’s reagent (Eq. 3) .
PHP removal mainly results from the reaction of the PHP molecules with radicals, which are produced through the Fenton reaction. In-situ electro-generated H2O2 on the surface of graphite cathode (Eq. 4) diffuses into the bulk of the reaction medium as well as the structure of the Fe-ZIF-8 nanocatalyst.
The active iron species of the Fe-ZIF-8 nanocatalyst, in the form of nano-sized iron oxide clusters (Fe3O4 or γ-Fe2O3) according to the DRS results , react with H2O2 and produce highly oxidative (Ev:2.80 eV) radicals. The formed iron species include cubic inverse spinel structures such as magnetite that contains Fe cations with tetrahedral and octahedral sites. It is reported that octahedral sites include Fe2+ and tetrahedral sites include both Fe2+ and Fe3+ , resulting in oxidation and reduction ability of Fe species [37, 38]. Therefore, it is expected that the Fe-ZIF-8 nanocatalyst is regenerated electrically in the reaction solution through the HEF system (Eq.5) .
However, Fe2+ consumption (Eq. 3) occurs at a higher rate than its regeneration (Eq. 5) which is in line with the high PHP removal at the beginning.
It is stated in the literature that the kinetics of the HEF process and activation of H2O2 onto heterogeneous reactive iron oxide sites are very complex. Wang et al.  proposed that the HEF kinetics simply fits the well-known Haber-Weiss mechanism as follows:
Effect of pH
The pH parameter controls the amount of radical hydroxyl production and the concentration of ionic ferrous in the solution. Hence, pH is one of the most important parameters in the HEF process. When the pH increases, the precipitation of Fe3+ ions increases, preventing the formation of Fe2+ ions so the reaction efficiency drops. H2O2 is also unstable at pH= 5 and rapidly converts to hydrogen and oxygen so the high alkalinity reduces H2O2 stability and iron ion deposition. At pH< pHpzc, the proton on the surface leads to a positive charge. But at pH>pHpzc, the proton deficiencies result in a negative charge. In contrast, the existence of protons on the surface of the acidic materials neutralizes the compound at pH<pHpKa, but it is negatively charged by the loss of proton at pH>pHpKa. The basic materials are neutralized by taking proton at a pHpHpKa.
By comparing the catalytic performance of the Fe-ZIF-8 nanocatalyst at different pH (Fig. 7), it is clear that pH=3 results in the highest PHP removal. At pH=7, a sharp reduction of oxidation ability and conversion of H2O2 to water hinder the HEF efficiency. However, the enhanced adsorption of the PHP molecules onto the Fe-ZIF-8 particles as a result of electrostatic forces between the positively charged Fe-ZIF-8 particles (pHPZC=8) and the negatively charged PHP molecules (pKa=5.15), improves the efficiency of the HEF system. Since operation at neutral pH is conducted with the addition of no acid and alkali compounds. Therefore, pH=7 is selected as the optimum pH for the other experiments, which can be the best pH both economically and environmentally. Furthermore, this result also proposes that the main drawback of Fenton and Fenton-like reactions (strict acidic pH control) can be addressed through the utilization of the Fe-ZIF-8 nanocatalyst at neutral pH in the HEF system.
Effect of nanocatalyst concentration
The nanocatalyst concentration is a very important parameter for controlling the removal efficiency in the Fenton and Fenton-like reactions. Increasing the concentration of the nanocatalyst increases the rate of radical hydroxyl production as well as the removal efficiency removal of PHP. The excessive nanocatalyst concentration raises the probability of the nanocatalyst present on the surface of the graphite cathode and reduces the volume of the cathode pores as well as the production of H2O2 [40, 41]. Furthermore, the high concentration of iron has a negative effect on the process performance due to the side reaction (Eq. 11).
Fig. 8 shows that the PHP removal efficiency reduces from 99% to 94% in 60 min by increasing the Fe-ZIF-8 nanocatalyst from 0.2 to 0.6 gL-1. However, the increase of the nanocatalyst concentration from 0.6 to 1 gL-1 does not change the efficiency (94%). Therefore, the nanocatalyst concentration of 0.2 gL-1 is the optimum concentration.
Effect of applied current
We conducted the HEF experiments with three different applied current levels. Fig. 9 shows that the increase of the current from 0.1 to 0.2 A leads to a loss of efficiency owing to the production of additional H2O2, which reacts with radical hydroxyl and leads to its destruction. The high current promotes the side reactions: i) excessive H2O2 generation, ii) transformation radical hydroxyl into the weaker oxidative (HO2•), iii) HO2• reaction with OH• (Eqs. 12-13) [42, 43]. Bagheri et al.  applied different current for the purification of formaldehyde (0.85 mA.Cm-2 to 17 mA.cm-2) and found the current of 8.5mA.cm-2 as the optimum. The process efficiency is improved by the excessive current (0.3A) owing to the increase of the reaction between the electrodes and the rate of iron ion production on the anode but destroy the cathode. Therefore, the current of 0.1A is selected as the optimum value, resulting in the high and durable performance of the process.
Effect of PHP concentration
Fig. 10 shows that the high concentration of PHP leads to low removal efficiency. This result can be clarified by the constant amount of active hydroxyl radicals, hindering the degradation of pollutants within the excess amount of PHP. The high concentration of PHP would consume more radicals, so the effect would be decreased with the high initial concentration of PHP. Saeid et al.  investigated the effect of PHP concentration in the range of 20-50 mgL-1 in the UV/H2O2 process. The results showed that the process efficiency decreased with the increase of PHP concentration. Furthermore, the non-specific oxidation of major intermediates with radicals may result in the side reactions and the competitive consumption of radicals .
Pseudo-First-Order kinetic model well describes the PHP degradation data. The kinetic is according to Eq. (14 ).
where C and C0 (mg L-1) are the residual and initial PHP concentrations, respectively. k (min-1) is the constant of Pseudo-First-Order kinetic and t (min) is the reaction time. Fig. 11 shows the kinetic of PHP degradation at optimum operating conditions. The results confirm that the proposed kinetic model is in line with the experimental data.
Fig. 12 represents the application of the Fe-ZIF-8 nanocatalyst for the PHP removal in the sequence runs. The spent nanocatalyst was washed with methanol to remove the adsorbed organic molecules and dried at 105ºC overnight. The regenerated nanocatalyst was evaluated at the optimum operating conditions: pH=7 applied current of 0.1A, the nanocatalyst concentration of 0.2 gL-1, and PHP concentration of 10 ppm. The results prove the high reusability of the Fe-ZIF-8 nanocatalyst in the sequence runs. The stability of the nanocatalyst depends on diverse factors, including the exit of pollutant molecules and oxidized intermediates from pores that are accelerated by the high surface area and total pore volume.
Table 3 compares the PHP removal over the developed Fe-ZIF-8 nanocatalyst with the literature. It is clear that the high performance and applicable operating conditions, including neutral pH and low catalyst concentration, result from the appropriate physicochemical properties of the Fe-ZIF-8 nanocatalyst.
The ZIF-8 nanocatalyst was synthesized using the thermal solvent method and doped with an iron promoter through the multi-step impregnation technique. The characterization showed the high surface area and uniform dispersion of Fe species on the ZIF-8 nanocatalyst. The synthesized Fe-ZIF-8 nanocatalyst was applied for the PHP removal in the HEF process at different operating conditions. The results showed that the highest PHP removal efficiency (99%) was obtained at a pH of 7, nanocatalyst concentration of 0.2 gL-1, and applied current of 0.1 A. Furthermore, the nanocatalyst showed high reusability due to its proper properties. The results confirmed the high potential of the ZIF-8 nanocatalyst for wastewater treatment applications.
The authors also wish to acknowledge the Sahand University of Technology for its support of this study.
CONFLICTS OF INTEREST
The authors declare there are no conflicts of interest.