Background
Tetracycline (TC) is extensively used for the prevention and treatment of infectious diseases in human and veterinary medicine and as feed additives for promote growth in agriculture [1, 2]. Because of their extensive usage, their strongly hydrophilic feature, low volatility [2] and relatively long half-life [3], TC antibiotic has been frequently detected in different environmental matrices: surface waters (0.07-1.34 μg/L) [4], soils (86.2-198.7 μg/kg) [5], liquid manures (0.05-5.36 μg/kg) [5] and in 90 % of farm lagoon samples (>3 μg/L) [6]. In addition to environmental contamination, the occurrence of TC in the aquatic environments would also increase antibiotic resistance genes [7]. However, due to the antibacterial nature of TC, they cannot effectively be removed by conventional biological processes [8]. In wastewater treatment plants, the TC removal efficiency varied in the range of 12 % to 80 % [9, 10]. For example, concentrations of TC residues have been detected in values of 0.97 to 2.37 μg/L in the final effluent from wastewater treatment plants [11]. Hence, the effort to develop new processes to minimize the tetracycline residues discharges into the environment is become essential. Physicochemical processes such as membrane filtration and adsorption using activated carbon have been used to removal of TC. These processes are not efficient enough, transfer the pollutant from one phase to another [12, 13]. Advanced oxidation processes (AOPs) such as (O3/H2O2, US/O3, UV/O3, UV/H2O2, H2O2/Fe2+, US-TiO2 and UV-TiO2) have been proposed as very effective alternatives to degrade tetracycline antibiotics. The primary of AOPs is production of hydroxyl radical in water, a much powerful oxidant in the degradation of a wide range of organic pollutants [12–15]. Recently, the application of sulfate radical-based advanced oxidation processes (SR-AOPs) to oxidation of biorefractory organics have attracted great interest [16, 17]. Persulfate (PS, S2O82−) is a powerful and stable oxidizing agent (E0 = 2.01 V vs. NHE), which has high aqueous solubility and high stability at room temperature as compared to hydrogen peroxide (H2O2, E0 = 1.77 V vs. NHE) [18, 19].
Sulfate radicles could be produced through the activation of persulfate (PS, S2O82−) with ultraviolet [20], heat [21, 22], microwave [23], sonolysis [24], base [25], granular activated carbon [26], quinones [27], phenols [28], soil minerals [29], radiolysis [30] and transition metals [31, 32]. Sulfate radicals are more effective than hydroxyl radical in the oxidation of organic contaminants. They have higher redox potentials, longer half-life and higher selectivity in the oxidation of organic contaminants (SO4-•, E0 = 2.5-3.1, half-life = 30–40 μs) than hydroxyl radical (HO•, E0 = 1.89–2.72 V, half-life = 10−3 μs) [33–39]. Hence, the organic pollutants could be oxidized entirely by SO4-•, especially benzene derivatives compounds [18]. Generally, sulfate radical reacts with organic contaminants predominantly through selective electron transfer, while hydroxyl radical mainly reacts through hydrogen abstraction and addition. Therefore, the possibility of sulfate radical scavenging by nontarget compounds is lower than hydroxyl radical [39–42].
Sonochemical treatment is an emerging and efficient process that applied pyrolytic cleavages to degradation of organic compounds [42, 43]. This process is a cleaner and safe technique compared with UV, ozonation, and has the ability of operation under ambient conditions [43, 44]. However, combination of ultrasound with various processes has been detected as an economical and successful alternative for the degradation and mineralization of some recalcitrant organic compounds in aqueous solution [42]. The combination of ultrasound and persulfate (US/S2O82−) has been effective for the degradation of compounds such as; methyl tert-butyl ether (MTBE) [45], nitric oxide [18], 1,4-dioxane [46], arsenic(III) [44], amoxicillin [47], tetracycline [48] and dinitrotoluenes [24]. In aqueous solutions, acoustic cavitation leading to produce plasma in water and free radicals and other reactive species such as HO• and H• radicals due to the thermal degradation of water according to Reaction (1) and (2). The HO• and H• radicals can also react with PS to production of more reactive SO4-• radicals according to Reactions (3) to (7) [42, 44, 49, 50].1 H2O→))))H2Oplasma2 H2O→))))HO•+H•
Where “))))))” refers to ultrasonication.
In the presence of S2O82−:3 S2O8−2+→))))2SO4−•4 SO4−•+H2O→SO42−+HO•+H+5 S2O8−2+HO•→HSO4−+SO4−•+12O26 S2O8−2+H•→HSO4−+SO4−•7 S2O8−2+→pyrolysis2SO4−•
In aqueous solution, Hydroxyl radicals may be produced via the degradation of persulfate and/or ultrasonic irradiation. Ultrasonic irradiation could also lead to cavitation through the formation, growth and collapse of tiny gas bubbles in the water [51]. Moreover, during US irradiation, the collapse of cavitation bubbles leads to higher temperatures and pressures that produces free radicals and other reactive species and would also increase the number of collisions between free radicals and contaminants [42, 44, 49, 50].
The specific objectives of this study were to optimize the TC degradation in aqueous solution using US/S2O82− process. Response surface methodology (RSM) is a reliable statistical technique for developing, improving and optimizing processes and can be used to assess the relative significance of several affecting factors with the least experiments [52–54]. Therefore, an experimental design methodology using RSM and CCD was used to evaluate the effect of operational parameters such as initial TC concentration, initial S2O82− concentration, initial pH and reaction time on the sonochemical degradation of tetracycline. In addition, the effect of temperature, degradation kinetics, mineralization, changes of ultraviolet Visible (UV–Vis) spectra and the proposed degradation pathway of TC by the US/S2O82− process were investigated. This study as part of a PhD dissertation of the first author was performed at Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences in 2015.