INTRODUCTION

Tetracycline (TC) is one of the most medically prescribed antibiotics due to its high effectiveness in treating infections such as cholera, syphilis, and brucellosis. The poor metabolism of TC in the human body causes its excretion into the aquatic environment [1]. TC is an organic pollutant resistant to biodegradation in aqueous solutions, so conventional water and wastewater treatment processes are not able to remove it effectively [2]. In recent years, the use of advanced oxidation processes (AOPs) to remove antibiotics has been considered. Among AOPs (ozonation, Fenton process, ultraviolet, and electrochemical), the Fenton process is one of the most effective processes for removing organic pollutants [3-5]. In this process, the hydroxyl radical (HO^•) is produced by the simultaneous use of hydrogen peroxide (HP, H₂ O₂) and ferrous ions (Fe²⁺) (Eqs. (1)). Effective oxidation and removal occurs through an attack of HO^• on the organic pollutant structure [6]. To supply Fe²⁺ in the Fenton process, common iron compounds such as ferrous sulfate (FeSO₄) can be used [7]. Despite the favorable effectiveness, sludge production and high stoichiometric consumption of Fe²⁺ are the main disadvantages of the mentioned compound [8, 9]. The use of heterogeneous iron catalysts can overcome these disadvantages [10, 11].

Fe²⁺+ H₂O₂ →Fe³⁺+HO^•+OH^-                        (1)

Pyrite (FeS₂)  is an abundant heterogeneous iron catalyst in the earth's crust, which has attracted researchers due to its features such as reusability, easy access, non-toxicity, low cost, and regulating the cycle of iron ions in the solution (Eqs. (2-4)) [12, 13].

2FeS₂ + 7O₂ + 2H₂O →2Fe²⁺+4H⁺+ 4SO₄^2-     (2)

4Fe²⁺+ O₂ + 4H⁺ →4Fe³⁺+4H₂O              (3)

FeS₂ + 14Fe³⁺ + 8H₂O → 15Fe²⁺ + 2SO₄^2- + 16H⁺ (4)

In addition, it has been proven that the use of nanosizing processes to increase the surface area of the catalyst leads to the improvement of organic pollutant removal [14, 15]. According to previous studies, the use of combined AOPs has improved the efficiency of pharmaceutical removal. In this regard, the use of ultrasonic irradiation (US) in the Fenton process improves the production of HO^• (Eqs. 5). Moreover, the US causes continuous cleaning of the catalyst surface and subsequently improves its performance [16-19].

H2O2 + US → 2HO^•                                                  (5)

In the present study, the use of nano-pyrite for the effective activation of HP under ultrasound irradiation was introduced in a novel combined sono-Fenton process. According to our investigations, no study has been done on the use of nano-pyrite in sono-Fenton activation of HP for TC removal from aqueous solutions. In this way, characterization analyses of nano-catalyst were studied. The effect of operational parameters was evaluated. Furthermore, the influence of different processes on TC removal efficiency was compared. Finally, the reusability of synthesized nano-pyrite was presented.

MATERIALS AND METHODS

Chemicals

Tetracycline (C₂₂H₂₄N₂O₈) was purchased from Tolid Daru Pharmaceutical Company, Iran. Hydrogen peroxide, sodium hydroxide (NaOh), sulfuric acid (H₂SO₄), nitric acid (HNO₃), tert-butanol alcohol (C₄H₁₀O), methanol (CH₃OH), and ethanol (C₂H₆O) were supplied by Neutron Pharmachemical Company, Iran. All chemicals were used in pure form.

Preparation of nano-pyrite

Pure pyrite powder was obtained from the Faculty of Mining, University of Tehran, Iran. Pyrite powder was nanosized by a high-energy planetary ball mill method (Model Two cups, Amin Asia Company, Iran) [12]. Next, the rotation speeds of small and large Teflon cups were considered to be 300 and 160 rpm, respectively. The ratio of ball mass to powder mass was 10: 1, and pyrite nanosizing was completed in 6 h. Finally, the obtained powder was washed with distilled water to remove impurities and dried at 30 °C.

Sono-Fenton system

A glass container with a useful volume of 200 mL was applied as a sono-Fenton reactor. This reactor was equipped with a digital ultrasonic homogenizer (Model UP200St, Hielscher, Germany. Volume 2.0 L, output power 200 W, and frequency 26 kHz). The sonotrode of the sonicator was titanium (tip diameter = 7 mm).

Experimental

The TC stock solution was prepared by adding 0.05 g TC pure powder to 100 mL deionized water. Then, predetermined concentrations of TC (8.0-32 mg/L) were made from the stock solution. Sulfuric acid (1.0 M) and sodium hydroxide (1.0 M) were used to change the solution pH. The pH measurement was done through a digital pH meter (AZ 8686, Taiwan). To investigate the effect of oxidant and catalyst, HP and nano-pyrite were added to the samples in amounts of 1.0 to 4.0 mM and 0.2 to 1.0 g/L, respectively. Moreover, nano-pyrite was separated from the solution by centrifugation to perform catalyst recovery tests. All experiments were repeated three times and the average data were considered as results.

Analysis

TC concentration was measured by a UV-visible spectrophotometer at 365 nm (Model DR/5000, HACH, Germany) [20]. The measurement of aqueous Fe²⁺ was conducted by the spectrophotometric 1, 10-phenanthroline method at 510 nm.

TC removal efficiency was calculated according to the following equation:

TC removal(%)   =       ×100                      (9)                                                                                                       

Where TC₀ and TC_t represent the initial and residual TC concentrations (mg/L) in the solution, respectively.

The TC removal rate constant was calculated according to the following first-order kinetic equation:

Ln () = -kt                                                                                                                    (10)

Where k is the rate constant (min^-1) and t is the reaction time (min).

Characterization of the nano-pyrite structure was done by X-ray powder diffraction (XRD, Model PW1730, PHILIPS, Holland) analysis with copper radiation at a scan rate of 2ϴ with a range of 10–80° (λ= 1.54056 Å, V= 40 kV, and I= 30 mA). Moreover, functional groups of nano-pyrite were identified by Fourier transform infrared spectroscopy analysis (FTIR, Model AVATAR, Thermo, USA). Besides, energy-dispersive X-ray spectroscopy (EDX) and field emission scanning electron microscopy (FE-SEM, MIRA 3 TESCAN, Czech Republic) analyses were used to determine the chemical composition and morphology of nano-pyrite, respectively. Finally, Digimizer and Origin software was applied to plot the size distribution of synthesized nano-pyrite particles.

RESULTS AND DISCUSSION

Nano-pyrite characterization

The XRD patterns of pyrite and nano-pyrite are presented in Fig. 1a. Accordingly, peaks at 2 theta positions of 28.65°, 33.25°, 37.35°, 40.85°, 47.6°, 56.45°, 59.15°, 61.45° and 64.55° corresponding to the primary pyrite sample were recorded, which were completely consistent with the JPCDS no: 42-1340 crystal structure of pure pyrite [4]. Similarly, the distinct peaks appearing in the XRD diagram of the synthesized nano-pyrite confirmed the preservation of the pure pyrite crystal structure in the sample (22.95°, 28.35°, 31.25°, 32.7°, 35.35°, 36.8°, 38.95°, 40.6°, 47.05°, 53.4°, 56.3°, 58.75°, 61.45°, and 64.1°). According to the data, the crystal structure of the synthesized nano-pyrite was cubic and its nature was preserved after the nano process.

The functional groups and chemical bonds of nano-pyrite and pyrite were presented in the FTIR diagram (Fig. 1b). In this way, S-O (1141 cm^-1), O-H (3429 cm^-1 ), Fe–O–OH (1024 cm^-1), Fe–S (831 cm^-1), and Fe-SO₄ (1633 cm^-1 ) [21, 22] were observed in the pyrite sample. In addition, 660 and 623 cm^-1 absorption bands associated with the sulfur group were found in the analysis of the nano-pyrite catalyst. Besides, peak values of 1254, 2926, 1131 and 592 cm^-1 were recorded in the nano-pyrite which related to O–C, C-H, S-O, and Fe–S groups, respectively [23].

According to the FE-SEM images, the fabricated nano-pyrite had nanoscale particles with regular structure morphology (Fig. 1c). The output data from EDX analysis showed that iron and sulfur were the main constituents of the synthesized nano-catalyst (Fig. 1d). Accordingly, Fe and S were included in 47.64% and 52.36% of nano-pyrite weight, respectively. It should be noted that Fe and S constituted 28.36% and 28.77% of the weight percentage of pyrite nanoparticle samples in the previous Fenton base studies, respectively [22, 23].

Analysis of the size distribution of synthesized nano-pyrite particles by Digimizer software (Fig. 1e) showed that most of the particles were less than 80 nm in size (10-20 nm: 10%, 20-40 nm: 46%, 40-60 nm: 34%, and 60-80 nm: 10%). This confirmed the effectiveness of the mechanical method of ball milling in the catalyst nanosizing.

The effect of solution pH

To investigate the effect of solution pH, the values of 3.0, 5.0, 7.0, and 9.0 were considered (Fig. 2). TC removal efficiencies by sono-Fenton nano-pyrite/HP system under reaction conditions of 24 mg/L TC, 3.0 mM HP, 30 W US, 0.8 g/L nano-pyrite, and 20 min at pH of 3.0, 5.0, 7.0, and 9.0 were 66.8%, 54.6%, 42.4%, and 37.8%, respectively. Acidic conditions (pH=3.0) were more effective in TC removal. This phenomenon can be explained by the lower stability of HP in alkaline conditions and the higher oxidation potential of hydroxyl radical at lower pH levels [24]. Besides, the oxidation of nano-pyrite and the release of ferrous ions from it occur better in acidic conditions [25]. The result of these interactions according to Eqs. (1), (2), and (5) led to more effective production of hydroxyl radical and subsequently TC removal. In contrast, the precipitation of Fe²⁺ and the production of metal hydroxides can be the possible reason for decreasing the efficiency of TC removal at alkaline pH levels [3, 26].

The effect of TC concentration

The effect of TC concentration was tested at initial values of 8.0, 16, 24, and 32 mg/L. Within 20 min, with the increase of TC concentration from 8.0 to 32 mg/L, the removal efficiency decreased by about 10% (Fig. 3). This means that the interference of by-products in the sono-Fenton nano-pyrite/HP system at high TC concentrations decreased the efficiency. Previous studies based on AOPs also confirmed this fact [27-29].

The effect of oxidant and nano-catalyst dosages

The effect of nano-catalyst and HP dosages on TC degradation is shown in Fig. 4. The results indicated an increase in TC removal efficiency in higher doses of nano-pyrite. It seems that the possible reason for this effect is the release of more ferrous ions in high amounts of nano-pyrite and the subsequent promotion of hydroxyl radical production. Therefore, 1 g/L of nano-pyrite was selected as the optimum dose of the process with 75.3% TC removal in 20 min. Similarly, increasing the concentration of HP led to an increase in TC removal, so that the highest removal efficiency (84.6%) was obtained at 4 mM HP. The presence of more HP in the solution provided a more ideal media for the formation of HO^• by the Fenton-based processes. This is consistent with the results of our previous study [7].

The effect of US power

In the reaction conditions of pH 3.0, TC 8.0 mg/L, HP 4.0 mM, and nano-pyrite 1.0 g/L, the effect of different powers of US was tested (Fig. 5). Within 20 min, TC removal efficiencies at 10, 20, 30, and 40 W were observed to be 77.7%, 81.2%, 84.6%, and 93.1%, respectively. As proven in previous studies, the oscillation and cavitation effects of US waves promote the degradation of organic pollutants [30-32]. Furthermore, the intensity of mixing by US waves at higher powers leads to better nano-catalyst solubility and mass transfer [33]. Therefore, in this study, a US power of 40 W was recorded as the optimal value.

Comparison of TC removal in different systems

In the present study, the contribution of different components including nano-pyrite, HP, US, nano-pyrite/HP, and nano-pyrite/HP/US in TC removal under the same reaction conditions was compared (Fig. 6). At 20 min, nano-pyrite alone (1.0 g/L) removed 21.6% of TC. This amount of TC removal may be related to adsorption by nano-pyrite particles. In addition, nano-pyrite oxidation reactions (Eqs. (11-13)) produced TC oxidizing active agents such as superoxide and hydroxyl radicals.

Fe²⁺ + O₂ → Fe³⁺ + O₂^•-                                                            (11)

Fe²⁺ + O₂^•- + 2H⁺  → Fe³⁺ + H₂O₂                                  (12)

Fe²⁺ +H₂O₂ → HO^•  + Fe³⁺ + OH^-                                 (13)

In the next section, HP alone (4.0 mM) was added to the solution at the same time, which recorded 24.3% removal efficiency. Although HP removed the TC, its efficiency was not acceptable enough. The oxidant activation did not occur without nano-pyrite, and practically HP alone was not effective in TC removal. Next, the effectiveness of US power in removal efficiency was tested. Accordingly, 18.7% TC was removed by 40 W US. Since the US waves decompose HP and generate hydroxyl radical, the removal efficiency can be attributed to this reason (Eq. 5). A significant increase in TC removal efficiency (78.5%) was recorded by nano-pyrite/HP system. Obviously, with the simultaneous presence of ferrous ion and HP in the bulk solution, the Fenton reaction occurred, and the hydroxyl radical caused the effective degradation of TC (Eq. 1). The enhancement of the removal efficiency was observed in the integrated system of nano-pyrite /HP/US with 93.1% TC removal. In other words, the synergy between sono-catalysis and Fenton reactions due to the simultaneous application of nano-pyrite, HP, and US power resulted in the highest TC removal rate. In addition to the above-mentioned reactions, the application of US forces to the nano-pyrite/HP system caused high turbulence, nano-catalyst fluidization, and mass transfer [14, 15, 34]. Meanwhile, the US irradiation with its oscillating effect cleaned the ferric hydroxide deposits formed on the nano-pyrite surface and contributed to the rapid and efficient removal of TC. These statements were consistent with the observations of previous studies Diao et al.'s study [16].

The effect of radical scavengers and the determination of process mechanism

To investigate the process mechanism, tert-butanol alcohol (3.0 mM) and methanol (3.0 mM) were added to the nano-pyrite/HP/US system under optimal conditions as HO^• and SO₄^•- (sulfate radical) scavengers, respectively. The data showed that the TC removal efficiency in the presence of methanol was 54.2%, while the corresponding value in the presence of tert-butanol alcohol was 15.6% within 20 min. It can be concluded that the active oxidative species and the main agent of TC degradation in the process was HO^•. As the proposed mechanism of the process, US waves with the property of turbulence and oscillation cause the release of more ferrous ions from nano-pyrite. This phenomenon leads to more effective production of hydroxyl radical in the presence of HP, which was the main factor of TC degradation. It has also been proven in previous studies that US mechanical waves clean the surface of the catalyst and improve its performance [16]. Moreover, TC degradation rates using the kinetic model of the first-order equation for different systems were presented in Table 1. Finally, the performance of sono/Fenton processes in the presence of different iron catalysts is compared in Table 2.

Recyclability and stability of the nano-pyrite catalyst

Fig. 7 shows the stability and recyclability of synthesized nano-pyrite under optimum conditions of the Sono/Fenton process. In this way, the nano-pyrite was tested in four consecutive runs. The results showed that the TC removal efficiency from the first to the fourth run reached 93.1% to 70.3%. This means that the recyclability of nano-pyrite has been very successful. Besides, the amount of iron leached from nano-pyrite was investigated during recyclability. Accordingly, the concentration of aqueous ferrous ions reached 8.2 mg/L at the end of the fourth run. Indeed, the ferrous ion supplied from nano-pyrite was sufficient for the Fenton reaction even after four recoveries.

Determination of TC degradation intermediates

To determine the by-products of TC degradation in the proposed system, liquid chromatography-mass spectroscopy (LC-MS) device was used. Under optimal conditions, intermediates including malealdehyde, phenol, carboxylic acids, maleic acid, formic acid, and oxalic acid were identified in the solution. It is concluded that TC degradation in the nano-pyrite/HP/US system was complete due to the formation of low-weight molecules and simple organic acids.

CONCLUSIONS

The promotion of TC degradation through the combined system with the simultaneous participation of nano-pyrite and HP under ultrasound irradiation was studied. Within 20 min, the proposed sono-Fenton system degraded 93.1% of TC in an aqueous medium. In this way, except for the initial concentration of TC and pH, TC removal efficiency was increased by increasing the operating parameters (HP and catalyst dosages, US power, and time). Characterization analysis, especially Digimizer software, confirmed that the synthesized catalyst was nanosized. Accordingly, the size of all nano-pyrite particles was less than 80 nm. In summary, nano-pyrite, HP, and US waves caused synergy in promoting TC removal through their catalytic, oxidizing, and oscillatory functions, respectively.

Abbreviations

AOPs                          Advanced oxidation processes

EDX              Energy-dispersive X-ray spectroscopy

HP                                                 Hydrogen peroxide

FTIR         Fourier-transform infrared spectroscopy

LC-MS   Liquid chromatography-mass spectroscopy

SEM                            Scanning electron microscopy

TC                                                                 Tetracycline

US                                                                Ultrasound

XRD                                    X-ray powder diffraction