Study on the mechanism of degradation of tetracycline hydrochloride by microwave-activated sodium persulfate

In the past few decades, antibiotics have been commonly used as human medicines, veterinary medicines, aquaculture, and planting (Ahmed et al. 2015, 2017). However, due to the abuse of antibiotics, incomplete metabolism in humans or animals and unsuitable treatment processes, antibiotics are frequently detected in surface water, groundwater, and sediment (Kim et al. 2017; Binh et al. 2018; Chen et al. 2018). Antibiotics are proven to induce the production of resistant pathogens, lead to pollution of drinking water and food, and threaten human health and environmental balance by gene transformation (Christou et al. 2017; Marti et al. 2018).

Tetracycline hydrochloride (TCH) is one of the most frequently used antibiotics owing to its low cost, ease of use, and relatively minor side effects. It is also the focus of scientific research (Oseghe & Ofomaja 2018; Pang et al. 2018; Xu et al. 2018; Liang et al. 2019). Several technologies have been used to remove TCH, such as photocatalysis (Yu et al. 2014; Pang et al. 2018), adsorption (Mahamallik et al. 2015; Ali et al. 2018), sonochemical degradation (Safari et al. 2015), and UV-Fenton (Yu et al. 2019) and so on. However, these techniques take too long to remove TCH. Thus it is imperative to develop low cost, efficient, and environmentally friendly technology for TCH degradation.

Advanced oxidation processes (AOPs) are a family of technologies based on the production of very reactive species, and they offer a promising alternative for wastewater treatment (Yin et al. 2018). Recently, AOPs have been widely studied as versatile systems attempting to degrade antibiotics, such as biochar-induced activation of persulfate degradation of sulfamethoxazole (Kemmou et al. 2018), degradation of antibiotic ampicillin using the combined electrochemical oxidation–sodium persulfate (SPS) process (Frontistis et al. 2018), and microwave irradiation activation of peroxymonosulfate degradation of organic contaminants (Qi et al. 2017). Most of the AOPs used are based on the hydroxyl radical (·OH, E^o between +1.8 and +2.7 V vs., half-life: 20 ns), which can destroy a wide range of organic compounds quickly and non-selectively, although the instability of ·OH limits its application (Oncu et al. 2015).

Activated decomposition of the persulfate anion (⁠⁠) produces a powerful oxidant known as the sulfate-free radical (⁠⁠, E^o between +2.5 and +3.1 V vs., half-life: 30–40 μs). Previous studies used ultrasound activation of persulfate, with 88.51% of TCH removal achieved under optimum operational conditions after 120 min (Nasseri et al. 2017). The removal of TCH in aqueous solution reached 81.1% within 240 min by electrolysis with persulfate oxidation (Liu et al. 2018). These activation methods are too inefficient.

Microwave (MW) heating has the ability to increase reaction rates by providing selective heating. Compared to conventional heating, MW induces SPS to produce more ⁠, selectively heats certain compounds, and makes treatment reactions more controllable (Ravera et al. 2010; Homem et al. 2013). Nowadays, many researchers have studied the application of AOPs and MW technology applied to antibiotics in wastewater (Qi et al. 2014, 2017). To the best of our knowledge, there has been no systematic research on the use of MW to activate SPS directly for the degradation of TCH.

In this study, we sought to evaluate the technical feasibility of TCH removal by MW/SPS in aqueous solution. This paper aims: (1) to explore the MW/SPS process for the degradation of TCH; (2) to investigate the influences of several important parameters, including MW power, initial SPS and TCH concentrations, and initial solution pH, on the degradation efficiency of TCH; (3) to identify the dominant reactive oxygen species involved in the MW/SPS process; and (4) to propose a possible mechanism for TCH degradation.

TCH (C₂₂H₂₄N₂O₈·HCl, 98%) was purchased from Beijing Bailingwei Technology Co., Ltd (China). The chemical structure of TCH (molecular weight: 444 g/mol) containing several carbonyls, amino, and hydroxyl functional groups, is shown in Figure 1. Sodium persulfate (Na₂S₂O₈, 98.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Methanol (CH₃OH, 99.7%) and tert-butyl alcohol (TBA, (CH₃)₃COH, HPLC grade, 98.0%) used as radical scavengers, were purchased from Saimofeishier Technology Co., Ltd (Shanghai, China) and Tianjin Yongsheng Fine Chemicals Co., Ltd (Tianjin, China) respectively. All other chemicals used, such as hydrochloric acid and sodium hydroxide, were of analytical grade and purchased from Beijing Chemical Works (Beijing, China). The pH of the solution was adjusted using a PhS-25 meter (Shanghai Jingmi Company, China) with 1 mol L⁻¹ NaOH and/or 1 mol L⁻¹ H₂SO₄. All solutions were prepared with Milli-Q water (>18 mΩ cm) purified with a deionizing system.

A stock solution of TCH (60 mg/L) was prepared with ultrapure water before each batch experiment and stirred overnight at room temperature until the antibiotic had completely dissolved. The initial pH of all solutions was unadjusted. The initial pH of the TCH solution was 6.5. A microwave digestion system (Ao Pule Company, China) was used to irradiate the TCH (Supplementary Material, Figure S1). Degradation reactions were carried out in the digestion tank vessel. The addition of 10 mL of TCH solution (20 mg/L) with appropriate concentrations of SPS were held in a 100 mL microwave digestion tank vessel with the sleeve for the microwave treatment. After a period of microwave irradiation, the reaction solution of TCH was collected with injection syringes, filtered through a 0.22 μm organic membrane, and then its absorbance was measured. In this study, all the experimental tests were undertaken in duplicate to ensure reproducibility.

To analyze the intermediates of TCH, a Thermo TSQ triple series quadrupole mass spectrometer (Thermo Scientific, USA) system was used for high performance liquid chromatography-mass spectrometry (HPLC-MS).

To investigate the efficiency of different systems, experiments for removing TCH were carried out under optimal conditions using microwave (MW) alone, conventional heat (CH), conventional heat/sodium persulfate (CH/SPS), and microwave/sodium persulfate (MW/SPS) (Figure 3(a)). The effect of the MW alone on the degradation of TCH was unremarkable, as the removal of TCH was only 10.3%. However, 99.4% of TCH was degraded within 5 min in the MW/SPS process. The removal efficiency of TCH within 5 min using CH alone was 7.5%. Only 67.9% of TCH was degraded in 5 min with the addition of persulfate in the CH/SPS process. The corresponding pseudo-first-order kinetic plots for the degradation of TCH are given in Figure 3(b). It can be seen that the sequence of TCH degradation in the different processes was MW/SPS (K_obs = 1.049) > CH/SPS (K_obs = 0.225) > MW (K_obs = 0.017) > CH (K_obs = 0.004). The TCH removal rates in the MW/SPS system under different process conditions are listed in Table S1 (Supplementary Material). The results indicate that SPS can be activated by MW to generate and ·OH in the MW/SPS process. The phenomenon is similar to previous studies (Lee et al. 2009). Rapidly increasing the temperature enhanced TCH degradation significantly, so MW is an attractive method to activate persulfate and thus to produce the sulfate free radical which is a powerful oxidant that can degrade TCH. Moreover, this phenomenon is typical for many thermally activated persulfate oxidation reactions, where more free radicals are generated at higher temperatures, resulting in faster organic contaminant degradation (Qi et al. 2014; Ji et al. 2015). This MW/SPS process would also be useful to treat organic compounds of medical wastewater. Therefore, SPS is an indispensable constituent of the MW/SPS process, playing a significant role in TCH degradation.

MW power was an important parameter for the experiments. The results shown in Figure 4(a) indicate that TCH removal efficiency gradually increased by increasing the MW power from 500 to 700 W. The TCH degradation efficiency was only 71.4% at 500 W within 1 min, whereas it increased noticeably to 72.8% at 600 W and then reached 85.3% at higher power values (700 W). Furthermore, the K_obs of TCH removal increased obviously from 1.049 min⁻¹ to 1.117 min⁻¹ when the MW power was increased from 500 W to 700 W (Figure 4(b)). These results may be explained by the fact that increasing MW power leads to the rapid elevation of the reaction temperature (Falciglia et al. 2018).

As shown in Figure 5, it is apparent that under MW/SPS with increasing TCH concentration, from 10 mg/L to 30 mg/L, TCH removal efficiency decreased. When the concentration of TCH increased, the K_obs decreased from 2.901 min⁻¹ to 0.271 min⁻¹. At the TCH concentration of 20 mg/L, the TCH degradation efficiencies reached 99.3% in 5 min. The degradation of TCH was >99.0% after 4 min with lower TCH concentrations of 10 mg/L and 20 mg/L, respectively. This indicated that the MW/SPS process has an excellent TCH degradation performance at low concentration levels. This phenomenon can probably be explained by the fact that the amount of generated at a given SPS dose was identical, so was not enough to degrade high concentrations of TCH.

When the concentration of SPS reached 6 mM, the TCH degradation efficiency was 99.3% at 5 min. This indicates that a moderate persulfate dose is an essential condition for the complete degradation of TCH. However, not to waste resources, we choose the 6 mM concentration of SPS for this study.

and ·OH produced in the SPS activation process can effectively degrade TCH in solution. In this study, to identify the radicals and their contribution to treatment efficacy in the MW/SPS process, radical scavenger experiments were conducted using methanol and TBA. TBA acts as a scavenger of ·OH, and methanol acts as a scavenger of both and ·OH. As shown in Figure 8(a), in the absence of radical scavengers, the removal efficiency of TCH was 99.3%. The addition of methanol and TBA resulted in a 10.9% and 3.0% drop of TCH removal efficiency respectively. The corresponding pseudo-first-order kinetic plots for the degradation of TCH are given in Figure 8(b). It can be seen that the sequence of scavengers in suppressing the effect was TBA (K_obs = 0.152) > methanol (K_obs = 0.128) corresponding to > ·OH in the MW/SPS system. The results demonstrate that the inhibition effect of methanol was greater than that of TBA. The results also indicated that was the predominant radical and that ·OH played a less important role in the MW/SPS process.

To elucidate the possible mechanism of TCH degradation in the MW/SPS system, HPLC–MS was used to identify polar intermediates with a higher molecular weight. Figure 9 shows the mass spectra of TCH before and after 5 min MW/SPS treatment.

The accurate mass spectrum of TCH showed a protonated molecular ion [M+H]⁺ at m/z 443, which is attributed to the parent compound of TCH with the formula C₂₂H₂₄N₂O₈ (M = 444). In addition, according to the present results and literature data, the oxidation pathways of TCH included the following main processes: dehydroxylation, deamination, the methyl group, ring-opening reaction, followed by degradation to low-molecular-weight organic compounds (Wang et al. 2018). After MW/SPS treatment, the peak of m/z = 443 disappeared, and many peaks (such as m/z = 380, 334, 261, 239, 161, 113, 97) appeared along with the degradation of TCH. Combining the relevant literature, the free radicals (i.e. and ·OH) created in the MW/SPS system attack the double bonds, an aromatic ring, and an amino group in the degradation process of TCH (Nasseri et al. 2017; Zhang et al. 2018). As shown in Figure 10, most of the TCH was degraded to many kinds of intermediates after 5 min, and some of those intermediates would be degraded and eventually form CO₂, H₂O, and other inorganic substances. Intermediates include the detachments of acylamino (m/z = 380), hydroxyl group (m/z = 334, 261, 97), methyl group (m/z = 334) or methylene (m/z = 261). Then, as the reaction continued, more such groups detached from the TCH molecules, and then the cyclic hydrocarbon structure was opened (m/z = 261, 239). Finally, all the intermediates generated would be degraded to smaller molecules.

This work supplies a simple and rapid method for persulfate activation to generate in aqueous solution using microwave energy. Compared with conventional heating, microwave heating accelerated the degradation rates. The TCH degradation efficiency reached 99.3% in a short period (5 min) with 20 mg/L TCH, which indicates that the proposed MW/SPS system is efficient and very suitable for TCH wastewater treatment compared to other technologies. The degradation of TCH in this system can be described by the pseudo-first-order kinetics model. Quenching studies indicated that the was the primary free radical in the MW/SPS system. Using HPLC–MS analysis, we speculated on the degradation intermediates and possible degradation pathways of TCH.

There are no conflicts of interest to declare.

This work was supported by the research and development and demonstration of integrated treatment technology for water environmental pollution in river basins (Major science and technology projects of Jilin province, No. 20200503003SF).

The data used to support the findings of this study are available from the corresponding author on request.