INTRODUCTION

Todays, industries are known as the main source of water contamination. Pharmaceutical industry introduces synthetic pharmaceuticals into surface and ground waters which are hazardous due to their toxicity and resistivity to biodegradation [1]. Phenazopyridine hydrochloride (PHP) as an analgesic drug is prescribed for pain relieving related to urinary tract infection or irritation [2]. Several research reported the negative effects of PHP on human health such as carcinogenicity in mice, red urine discoloration, headache, hemolytic anemia and methemoglobinemia [2-5]. In order to PHP removal, research to develop a more efficient wastewater treatment is in demand. The common purification systems are unable to fully remove or degrade persistent molecules including complex azo structures [6; 7]. Advanced Oxidation Processes (AOPs) are mainly based on generation of hydroxyl radicals (, E₀= 2.76V [8]) which have been subjected to great interest for wastewater treatment [9]. Among AOPs, Fenton reaction is common due to the low cost and high ability of  generation [10]. The main restrictions of Fenton process are strict acidic pH control, rapid consumption of Fe²⁺ rather generation and formation of ferric hydroxide sludge [11]. These restriction are addressed through immobilization of Fe²⁺ (active phase) onto porous structures (such as zeolite), which improves the feasibility of the heterogeneous Fenton reaction [11].

Electro Fenton (EF) is an improved form of Fenton reaction due to in-situ electro generation of Fenton’s reagents [12]. Furthermore, this system is eco-friendly and cost-effective with small operational footprint [13]. The EF process faces some issues like strict acidic pH control and low oxygen solubility in water which is required for electro generation of H₂O₂ [14]. Recently, a coupling strategy of AOP systems has been proposed to improve efficiency [15]. Several studies have proposed coupling of the EF with irradiation of ultrasound waves [16-18]. This can be explained by the formation of  through thermal dissociation of water molecules due to cavitation of the ultrasound waves (Eqs.2-6) [11; 19; 20]. Turbulence formation by ultrasound waves also leads to the improved mass transfer between solid-liquid phases [19].

(1)
(2)
(3)
(4)
(5)

where ))) denotes the ultrasonic waves. Oturan et al. [13] studied a hybrid system of homogenous sono electro Fenton including Fe³⁺ ions as catalyst for degradation of some organic pollutants (4,6-dinitro-o-cresol, 2,4-dichlorophenoxyacetic acid and azobenzene). The proposed system had significant improvement compared with Fenton and EF systems as result of the enhanced mass transfer, synergic generation of  and pyrolysis of pollutant molecules by cavitation. Babuponnusami et al. [21] compared Fenton, EF, photo electro Fenton and sono electro Fenton systems over homogenous Fe²⁺ catalyst for degradation of phenol. They concluded that sono electro Fenton system had better performance in compare to EF including complete phenol removal and 67.9% mineralization over 40 min. Martinez et al. [16] studied different systems for azure B dye degradation over homogenous Fe²⁺ catalyst. Their results showed that efficiency of the systems was in the following order: low-frequency sono electro Fenton > Fenton > sonication. Sahinkaya [17] reported combination of EF and sonication for degradation of Reactive Black 5 using cast iron anodes as Fe catalyst source. Optimum reaction conditions were: pH= 3, current= 250 mA, [H₂O₂] = 800 mgL^-1 and electrode distance of 2.5 cm. They concluded that efficiency improvement by applying sonication was negligible due to complex nature of textile wastewater. Hassani et al. [19] studied heterogeneous sono Fenton process using Fe₃O₄ catalyst for degradation of Basic Violet 10. Optimum conditions were pH= 3, [catalyst]= 1.5 gL^-1, [H₂O₂]= 36 mM, ultrasound power of 450 wL^-1 and [BV10]= 30 mgL^-1 which resulted in 75.94% removal over 120 min. Ma et al. [18] studied the degradation of Acid Orange 7 in aqueous solution by combination of ultrasound and visible light. They reported that the removal efficiency was increased up to 35% when the ultrasound was applied (1 MHz and 40 W). Melero et al. [22] investigated effect of ultrasound on the degradation of phenolic aqueous solution including different nanocomposite at pH of 3. They found that the degradation rate in the presence of the ultrasound was high because it hindered the agglomeration of the catalyst particles. From operational point of view, the main problem of Fenton and EF is homogeneous Fe²⁺ catalyst which cannot be easily separated for regeneration and reuse [6] which can be solved through heterogeneous electro Fenton (HEF).

It is worth noting that previous studies have investigated the hybrid system of homogenous electro Fenton with ultrasound and there is no report on combination of HEF with ultrasonic waves. For the first time, we coupled HEF with ultrasound waves to apply heterogeneous sono electro Fenton system (HSEF) for PHP removal. The main objective of the hybrid system was chemical and physical effects of ultrasound waves in favor of more  generation, better Fe²⁺ regeneration, and high rate of mass transfer and continuous cleaning of heterogeneous catalyst surfaces. Among the heterogeneous catalysts, ZSM-5 has the appropriate textural properties (high surface area and pore volume) and pore size distribution, which has been attracted more attentions for wastewater treatment. We applied the HSEF system for PHP removal using the synthesized ZSM-5 nanocatalyst including Fe species as active phase. Effect of pH, current, catalyst concentration and ultrasonic power were studied.

MATERIALS AND METHODS

Materials

Chemicals for the synthesis of the nanocatalyst were silicic acid (SiO₂.xH₂O, > 99 wt.%), sodium aluminate (NaAlO₂, Al₂O₃ wt.% = 55), iron nitrate (Fe(NO₃)₂.9H₂O, 99 wt.%), tetrapropyl ammonium bromide (TPABr, C₁₂H₂₈BrN, > 99 wt.%), ammonium nitrate (NH₄NO₃, 99 wt.%), sodium hydroxide (NaOH, 99.6 wt.%) and sulfuric acid (H₂SO₄, 98 wt.%) which were supplied from Merck (Germany). PHP was kindly donated by Shahre Daru pharmaceutical Company (Iran). General information and specifications of PHP are shown in Table 1.

Nanocatalyst preparation

The parent nanocatalyst (ZSM-5, Si/Al= 200) was synthesized using hydrothermal method at 180 °C and 48 h. The molar composition of the synthesized gel was 20 SiO₂: 0.05 Al₂O₃: 3TPABr: 1.5Na₂O: 200H₂O. Wet impregnation of the parent nanocatalyst (HZSM-5) resulted in the bimetallic nanocatalyst including 0.1 wt.% iron promoter (0.1Fe-ZSM-5). N₂ adsorption-desorption, TEM and XRD techniques were used characterized the synthesized nanocatalysts. The detailed procedure for the synthesis and characterization of the nanocatalysts can be found in literature [23].

HSEF process

Fig. 1 shows schematic of the HSEF set-up and approximate 0.1Fe-ZSM-5 nanocatalyst structure. Reaction medium included 50 mL of wastewater (10 mgL^-1 of PHP), 0.05 M of Na₂SO₄ as electrolyte and the nanocatalyst (0.2, 0.6 and 1 g L^-1 of 0.1Fe-ZSM-5). pH level of solution was adjusted by 0.1 M HCl or 0.01 M NaOH solution. The electrodes (graphite, 3 × 2 × 0.5 cm) at the edge of the 100 mL beaker provided electrical current using a DC power supply. Ambient air injection supplied the required oxygen. In each batch, the reaction medium was firstly saturated by O₂ for 5 min. Then, the electrical current was set between the electrodes and simultaneously low frequency (~20 kHz) ultrasound device (Bandelin HD 3000 Series, Germany) was turned on to generate ultrasound waves inside the reaction medium. Samples were taken at designated times and PHP concentration was determined using spectrophotometric method by UV-vis spectrophotometer (Jinan Hanon instrument Co. Ltd, China) at 430 nm. Removal efficiency was calculated using Eq.6 [24]:

(6)

where  and  are PHP concentration in initial wastewater and sample, respectively.

RESULTS AND DISCUSSION

Nanocatalyst characterization

The nanocatalysts include MFI-structure of ZSM-5 in consistent with standard ZSM-5 (JCP: 00-044-0002) (Fig. 2). The relative crystallinity is the ratio of the peak area located at 2θ= 22.5-24.5° in compare with the parent nanocatalyst (ZSM-5). 0.1Fe-ZSM-5 nanocatalyst has the lower relative crystallinity (Table 2) which is in agreement with an amorphous peak located at 2θ= 25.5-27º. This can be explained by dealumination through the impregnation process which leads to slight framework defects and shifted peak positions (Fig. 2). It is reported that the framework destruction through dealumination shifted peak position to the high 2θ values [25]. TEM images of the HZSM-5 and 0.1Fe-ZSM-5 nanocatalysts are shown in Fig. 3. It is clear that the aggregation of the nanocrystals forms the microspheres which is in consistent with literature [23; 26].

N₂ adsorption-desorption of the nanocatalysts are a combination of isotherm types I and IV (Fig. 4a). The rectangular type H4 hysteresis loops at the high relative pressure (P/P₀= 0.5-0.95) is assigned to the mesoporous structure based on the capillary condensation [27]. The mesoporous structure is created by the crystal agglomeration in the form of interparticle space. The high adsorption volume at the very low relative pressure (P/P₀= 0.1) is attributed to the microporous structures [28]. The pore size distribution of the nanocatalysts confirms the formation of mesoporous structure (Fig. 4b). The major pore diameter of the nanocatalysts is 1.70 nm. The calculated textural data show the high surface area and total pore volume (Table 2). The low BET surface area and pore volume of the bimetallic nanocatalyst results from the slightly damage or blockage of the pores. The results agree with the XRD results.

Acidimetric-alkalimetric titration [29] of the ZSM-5 and 0.1Fe-ZSM-5 nanocatalyst determines pH_pzc equal to 3.6 and 4, respectively. In fact, the introduction of Fe species through the impregnation hides some acid sites from accessible surface [30]. The surface hydroxyl groups (OH) attain positive charge at pH< pH_pzc and negative charge at pH> pH_pzc which influences adsorption capacity of the nanocatalyst at different pH levels.

Effect of operational parameters on HSEF efficiency

pH level

It is accepted that pH is the most important operational parameter in the field of AOPs especially Fenton [31; 32]. pH directly influences characteristics of both pollutant and catalyst including solubility, surface charges and ionization. It is reported that PHP has abnormal solubility over the wide range of pH [33]. Therefore, determination of the optimum pH level for the proposed HSEF system is important. Fig. 5 shows PHP removal efficiency as function of pH. At pH= 3, the HSEF system has 74% removal efficiency after 30 min of the reaction. By increasing pH to 5, PHP removal efficiency decreases to 60% due to lower oxidation ability of , ferric hydroxide formation scavenging effect of  by H⁺ (Eq.7), unstable nature and decomposition of H₂O₂ (Eq.8) [6].

(7)
(8)

PHP removal efficiency is increased at pH of 7. This phenomenon can be explained by electrostatic interactions due to surface charges of the nanocatalyst and ionization state of PHP. At pH levels higher than pH_pzc of 0.1Fe-ZSM-5 nanocatalyst (4), surface hydroxyl groups generate negative charges around the particles [34]. On the other hand, PHP is a basic drug and attains positive form at pH levels higher than its pka (5.15) [33]. Therefore, electrostatic interaction between the nanocatalyst particles and PHP molecules results in more adsorption [35]. Further increase in pH level toward alkaline conditions has insignificant improvement of PHP removal owing to unstable nature of H₂O₂ in alkaline conditions and less oxidation ability of  radical [31]. It is concluded that the HSEF system has considerable removal ability at neutral pH level which leads to less operational complicity and cost. Hence, pH= 7 is selected as the optimum pH level and further tests are conducted at this pH level.

Effect of current

The applied current between anode and cathode leads to in-situ generation of Fenton reagents (Eqs.10-12) and anodic oxidation (Eq.9) which is considered a direct cost of the HSEF system [13].

(9)
(10)
(11)
(12)

Therefore, optimization of current as an operational parameter is very important for the system optimization. Effect of current on PHP removal is studied at four levels of 0, 100, 200 and 300 mA (Fig. 6). In order to determine adsorption contribution through PHP removal in the HSEF system, adsorption test is conducted at optimum pH= 7, 0.2 gL^-1 of 0.1Fe-ZSM-5 nanocatalyst and without current between electrodes (0 mA) which results in 51% PHP removal. PHP molecules are adsorbed by cage like structure (Fig. 1) of 0.1Fe-ZSM-5 nanocatalyst through Van Der Waals, hydrophilic, hydrogen bonding or electrostatic interactions [34]. The current between electrodes (100 mA) improves PHP removal up to 90%. It is concluded that PHP removal by the HSEF system is attributed to both adsorption and oxidation mechanism [36]. The higher current (200 mA) increases amount of the available electrons for regeneration of Fe²⁺ in the reaction medium (Eq.12) and also more H₂O₂ are produced (Eq.10) which leads to generation of more  on active sites of 0.1Fe-ZSM-5 nanocatalyst. However, the very high current (300 mA) saturates active sites and excessive amounts of H₂O₂ act as scavenger for  radicals (Eq.13) [37].

(13)

Based on structural and mechanical properties of graphite electrodes, the higher currents (> 200mA) accelerate destruction of the sacrificial graphite anode in return for insignificant PHP removal improvement. From economical and operational point of view, the rapid destruction and frequent replacement of the anode is unfavorable. Therefore, 100 mA is considered as optimum level of current and further tests are conducted at this level.

Catalyst concentration

It is reported that concentration of catalyst plays a key role in efficiency of Fenton based reactions [19]. Also, amount of required catalyst should be optimized due to economic considerations. Therefore, effect of catalyst concentration in the HSEF system is studied at 3 levels (Fig. 7). 0.2 gL^-1 concentration of 0.1Fe-ZSM-5 nanocatalyst shows 90% PHP removal efficiency in the HSEF system. Increasing of the catalyst concentration (0.6 gL^-1) reduces removal efficiency. Hassani et al. [19] studied sono Fenton system for basic violet 10 removal. Their results showed that high Fe²⁺ concentration improved removal efficiency up to a certain point (76%) and then decreased as result of scavenging effect of Fe²⁺ catalyst towards  (Eq.14).

(14)

In the HSEF system, increasing of the catalyst concentration from 0.6 to 1 gL^-1 enhances PHP removal from 69 to 87%, respectively. This can be explained by the increasing of the adsorption at the high catalyst concentration which overcomes the scavenging effect of excessive Fe²⁺ sites. But this negative effect cannot be neglected as PHP removal at 1 gL^-1 which is still lower than 0.2 gL^-1. Hence, the catalyst concentration of 0.2 gL^-1 is an optimum for the HSEF system.

Ultrasound power

As discussed in Section 3.2, the ultrasonic waves facilitate mass transport and  production which suppress the electro Fenton limitations and draw backs [11]. But, utilization of the ultrasound waves requires energy and must be optimized as an operational parameter. Effect of the ultrasonic power in PHP removal is studied at 3 levels of
600, 1000 and 2000 WL^-1 (Fig. 8). Increasing of ultrasound power decreases PHP removal significantly. It is proposed that higher ultrasonic powers lead to degassing of reaction solution which results in the less  generation due to shortage of O₂ in the solution (Eqs. 10-12) [13]. However, the HSEF system including the low ultrasound power (600 WL^-1) has the higher efficiency in compare to the HEF system at the same operational conditions. Therefore, the ultrasound power equal to 600 WL^-1 is an optimum level.

Reusability

One of the main advantages of the heterogeneous catalysts is the easy and low cost recovering of the spent catalyst for the further applications in AOPs [11]. However, heterogeneous Fenton catalysts may lose catalytic activity due to different phenomena including iron leaching, pore blocking as a results of pollutant adsorption and change in nature of iron species (such as Fe(OH)O formation) [38]. After each run, the nanocatalyst was recovered from the final effluent by centrifugation. The regeneration of the nanocatalyst was at 550 ºC for 6 h (3 ºCmin^-1) to remove the adsorbed organic species from the nanocatalyst active sites. In order to evaluate stability and reusability of the regenerated 0.1Fe-ZSM-5 nanocatalyst, three consecutive HSEF cycles were carried out at the optimum operational conditions: pH=7, applied current of 100 mA, 0.1Fe-ZSM-5 nanocatalyst concentration of 0.2 gL^-1 and ultrasound power of 600 WL^-1 (Fig. 9). The results confirm that 0.1Fe-ZSM-5 nanocatalyst show insignificant reduction of the catalytic activity for PHP removal (ca. 5%) after 3 consecutive HSEF cycle. The stability of the synthesized 0.1Fe-ZSM-5 nanocatalyst depends on different factors including the effect of ultrasound waves on cleaning of the surface and pores [22], the enhanced exit of PHP molecules and oxidized intermediates from pores due to the high surface area and total pore volume [24] and the calcination of the formed Fe(OH)O during the oxidation process. The slight decrease in PHP removal may be attributed to formation of the larger iron oxide particles. These results prove the high potential of the HSEF system using the reusable 0.1Fe-ZSM-5 nanocatalyst.

CONCLUSION

In this study, the hybrid HSEF system was applied for the efficient removal of PHP using the synthesized 0.1Fe-ZSM-5 nanocatalyst. Characterization of the nanocatalyst showed the high crystallinity, high surface area and well dispersion of Fe species in the structure of the bimetallic nanocatalyst. The HSEF system successfully addressed the potential limitations and drawbacks of the homogeneous electro Fenton system owing to the physicochemical effects of the ultrasound waves including mass transfer improvement and the high rate of  production. The optimum operational conditions were obtained as pH= 7, applied current of 100 mA, 0.1Fe-ZSM-5 nanocatalyst concentration of 0.2 gL^-1 and ultrasonic power of 600 WL^-1. The developed nanocatalyst showed the high capacity of reusability which had less than 5% loss of the removal efficiency after three consecutive cycles. Consequently, the HSEF system using 0.1Fe-ZSM-5 nanocatalyst has the high potential for the efficient PHP removal from pharmaceutical wastewater including the less reaction time and energy consumption.

CONFLICTS OF INTEREST

There are no conflicts to declare.