Efficient degradation of tetracycline by ultraviolet-based activation of peroxymonosulfate and persulfate

There is increasing attention to the presence of pharmaceuticals in the aquatic environment, due to the great negative impact on human health and the ecosystem (Kuster & Adler 2014; Natija et al. 2017). Among various pharmaceutics, tetracycline (TC), a common type of antibiotics, is widely used to treat human and animal diseases and in the animal husbandry and fisheries fields (Shi et al. 2017; Zhou et al. 2017). However, TC is difficult to be metabolized completely by humans and animals and finds its way into the water system (Natija et al. 2017; Shi et al. 2017). Traditional methods such as electrochemistry and adsorption can remove TC (Oturan et al. 2013; Zhang et al. 2011; Wu et al. 2016). Additionally, ultraviolet (UV)-based advanced oxidation processes (AOPs) have been shown to effectively degrade pharmaceuticals. TC photolytic degradation has also been reported in recent years, attracting more and more attention because of the high efficiency in removing refractory compounds (Zhang & Li 2014; Ai et al. 2015; Sun et al. 2016; Xiao et al. 2017).

Although iron is the most common transition metal in wastewater treatment, there are some drawbacks in iron-based AOPs. The most obvious one is the limitation of the pH level due to the precipitation of Fe²⁺/Fe³⁺ (Khan et al. 2016). Other metals like copper have drawn wide concern recently in overcoming this shortcoming, because the solubility product (k_sp) of Cu(OH)₂ (1.6 × 10⁻¹⁹) is much higher than Fe(OH)₃ (2.5 × 10⁻³⁵) (Meighan et al. 2008; Kang et al. 2014). It shows that copper is not easy to precipitate at neutral or alkaline conditions compared with iron. Moreover, Verma et al. (2016) reported that the addition of Fe³⁺ in a PMS/UV system was found to be more efficient in the degradation of anatoxin-a. Furthermore, cuprous copper (Cu(I)) is considered as a promising catalyst and has a high potential to activate the other oxidation agents like oxygen (O₂), hydrogen peroxide (H₂O₂) and PMS, but it is easily oxidized to cupric copper (Cu(II)) (Yuan et al. 2012, 2013). Thus, copper is seldom used in wastewater treatment.

Most previous studies used reducing agents like hydroxyl amine (HA) to reduce the transition metals (Lee et al. 2016). Based on Equation (2), it has been verified that UV is also capable of reducing high valence transition metal ions to low valence transition metal ions (Bedoui et al. 2011; Liu et al. 2016a, 2016b). As oxidation agents and transition metals simultaneously exist, on the one hand, the addition of UV directly activates oxidation agents like PMS and PDS to produce free radicals (Lou et al. 2016; Liu et al. 2016a, 2016b). On the other hand, UV continuously reduces transition metals that are oxidized by oxidation agents, forming a metal cycle with high and low valence ions (Wu et al. 1999; Bedoui et al. 2011). Thus, it is reasonable to consider that Cu(II) may be reduced to Cu(I) via UV first, and then Cu(I) activates PMS or PDS to promote the production of radicals like SO₄⁻• and •OH, which further enhance the oxidative ability of the Cu(II)/PMS or Cu(II)/PDS systems.

This study compares and evaluates the efficiency of removing the pharmaceuticals TC in Cu(II)/PMS/UV and Cu(II)/PDS/UV systems. The objects of this study are: (1) to compare the oxidative capacity for removing TC in different systems; (2) to examine the effect of Cu(II) concentration, PMS and PDS concentration in the two systems; (3) to determine the mechanism of radicals generation and investigate the species of radicals in different systems.

Potassium peroxymonosulfate (PMS) and copper sulfate pentahydrate (CuSO₄·5H₂O, ≥99.0%) were supplied by Sigma-Aldrich. Tetracycline (TC) was bought from Aladdin. Sodium persulfate (PDS, ≥99.0%), neocuproine hemihydrate (NCP, ≥98%), sulfuric acid (H₂SO₄, 75%), sodium hydroxide (NaOH, ≥96%) and coumarin (≥98%) were supplied by Sinopharm Chemical Reagent Co., Ltd. Tert-butyl alcohol (TBA, ≥99.5%) and ethyl alcohol (EA, 75%) were bought from Kelong chemical reagent factory. All of the reagents above were of analytic purity. Pure oxygen (O₂, ≥99.2%) and pure nitrogen (N₂, ≥99.9%) were stored in special high-pressure gas cylinders. Methanol and ammonium acetate were chromatographically pure and were bought from Sinopharm Chemical Reagent Co., Ltd (China).

All experiments were performed in a 500 mL tailor-made columniform glass container with a quartz tube of 35 mm diameter. A 25 W low pressure (LP) ultraviolet lamp peaking at 254 nm was placed in the quartz tube. During the experiments, samples in the container were surrounded and cooled by condensate water in order to keep the temperature of the samples constant at the room temperature. Cu(II) (CuSO₄·5H₂O) and TC at the desired concentrations were spiked into 500 mL pure water. The initial pH was adjusted by 0.5 M H₂SO₄ or 1.0 M NaOH. Oxygen or nitrogen was bubbled into the samples at 1.0 L•min⁻¹ for 20 min before the experiments, and was then set at 0.5 L•min⁻¹ during the experiments. Each run was started by PMS or PDS addition and UV light emition. TBA and EA, the scavengers of •OH and SO₄⁻•, were added to the samples immediately after UV emition in the quenching experiments, respectively. Samples with coumarin were quenched by NaNO₂ after being withdrawn and then were detected by high performance liquid chromatography (HPLC).

The concentration of TC was detected at 357 nm by a UV-vis spectrometer (MAPADA, UV-1800) (Shi et al. 2017). The concentration of Cu(I) was spectrophotometrically measured by the neocuproine method (American Public Health Association 2005). In addition, the concentration of dissolved oxygen (DO) was determined by a dissolved oxygen meter (JPB-607A). A pH meter (pHs-25) was used to detect the pH level and the dates changed less than ±0.1. Each experiment was replaced three times at the same conditions and the standard deviation was less than 2%.

The product of coumarin and •OH, 7-hydroxycoumarin (7OHC), was determined by HPLC (Waters e2695, USA) equipped with a reverse-phase C18 column (4.6 × 150 mm). The mobile phases were methanol and 0.1% ammonium acetate (50:50, v/v). The analysis conditions were as follows: the excitation wavelength: 346 nm; the determination wavelength: 456 nm; the column temperature: 30 °C; the injection speed: 1 mL/min (Louit et al. 2005; Takuya et al. 2011; Zhou et al. 2019).

To investigate the efficiency of different systems, experiments for removing TC were examined at pH 3.5 in the PMS, PDS, UV, Cu(II), Cu(II)/UV, PMS/UV, PDS/UV, Cu(II)/PMS/UV and Cu(II)/PDS/UV systems, respectively. As shown in Figure 1, TC removal was 49.3%, 71.8% and 93.1% in the PMS, PMS/UV and Cu(II)/PMS/UV systems (top). Through these results, it was found that the addition of UV (PMS/UV) increased TC degradation of the PMS system more than 20%. Besides, TC degradation of the Cu(II)/PMS/UV system was increased by over 20% than that in the PMS/UV system, indicating that the addition of Cu(II) and UV could strongly enhance the oxidative capacity of the PMS system. The degradation of TC in the PDS, PDS/UV and Cu(II)/PDS/UV systems (bottom) were 17.1%, 40.7% and 67.1%, which was similar with the former results of PMS. Moreover, single Cu(II), single UV and Cu(II)/UV systems could not remove TC during the 50 min reaction period.

To find the effect of PMS and PDS concentration on TC degradation, different PMS and PDS concentrations were examined in their own systems. As shown in Figure 3, TC removal was 85.0%, 92.1%, 96.6%, 95.2% and 96.8% during 50 min in the Cu(II)/PMS/UV system (top). Clearly, TC degradation was increased with the increasing PMS concentration, which may be because higher PMS concentration could be activated to generate more radicals by UV and Cu(I) at the same time. Based on the above, more free radicals such as SO₄⁻• and •OH could remove more TC due to its strong oxidative capacity. Similar to PMS, when the PDS concentration was increased from 0.25 mM to 1.50 mM, TC degradation in the Cu(II)/PDS/UV system (bottom) was increased as well. In general, TC was removed more completely in the Cu(II)/PMS/UV system compared with the Cu(II)/PDS/UV system at the same PMS or PDS concentration.

Based on the results above, it is established that the oxidative capacity of the Cu(II)/PMS/UV system is higher than that of Cu(II)/PDS/UV system at the same conditions. One of reasons may be related to the different species of the free radicals that exist in these two systems (Liang & Su 2009; Verma et al. 2016). In order to identify the species of radicals in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems, experiments involving the addition of TBA and EA were undertaken. TBA, an effective scavenger for •OH, has a higher reaction rate with •OH (k = 6.0 × 10⁸ M⁻¹ s⁻¹) (Buxton et al. 1988) and slightly lower reaction rate with SO₄⁻• (k = 8.0 × 10⁵ M⁻¹ s⁻¹) (Neta et al. 1988). In contrast, EA is effective for both •OH and SO₄⁻•, whose reaction rates are 1.2 × 10⁹–2.8 × 10⁹ M⁻¹ s⁻¹ and 1.6 × 10⁷–7.7 × 10⁷ M⁻¹ s⁻¹, respectively (Buxton et al. 1988; Neta et al. 1988). As shown in Figure 4(a), TC degradation was greatly decreased from 93.1% to 28.8% and 23.8% in the Cu(II)/PMS/UV system with TBA and EA (top), respectively, which indicated that •OH was the primary free radical in the Cu(II)/PMS/UV system. TC removal was 54.0% and 27.8% in the Cu(II)/PDS/UV system with TBA and EA compared with 67.1% in the Cu(II)/PDS/UV system. This result showed that a great amount of SO₄⁻• and a small amount of •OH exist in the Cu(II)/PDS/UV system, following Equation (6) (He et al. 2013).

Many studies have shown that oxygen can be activated by UV and transition metals to generate H₂O₂, which is further decomposed to produce •OH (Wen et al. 2014; Lee et al. 2016). In order to examine the role of oxygen in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems, O₂ or N₂ was continuously bubbled into these two systems. As shown in Figure 6(a), compared with the Cu(II)/PMS/UV (or Cu(II)/PDS/UV) and Cu(II)/PMS/UV/N₂ (or Cu(II)/PDS/UV/N₂) systems, it was observed that the addition of O₂ could enhance the oxidative capacity and increase TC degradation, which was consistent with the former studies. Nevertheless, the degradation of TC was almost the same in the Cu(II)/PMS/UV, Cu(II)/PMS/UV/O₂ and Cu(II)/PMS/UV/N₂ systems, indicating that PMS rather than O₂ is the primary oxidizing agent in the Cu(II)/PMS/UV system. The same results were found in the Cu(II)/PDS/UV system as well. Based on the description above, the reaction mechanism of the Cu(II)/UV system with PMS and PDS can be shown in Figure 6(b).

This study investigated TC degradation with Cu(II) and UV in order to compare with the activation of PMS and PDS. The results showed that TC degradation was much higher in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems via comparison with other different systems. The TC degradation was increased with the increasing Cu(II) concentration, but a much higher Cu(II) concentration may result in more Cu(I) being generated, which could quench the radicals and inhibit TC removal. It was observed that the optimal Cu(II) concentration of the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems were 30 μM and 50 μM, respectively. With the PMS or PDS concentration increasing in their own systems, the TC degradation was increased due to more radicals being generated during the same time period. Quenching studies indicated that •OH was primary free radicals in the Cu(II)/PMS/UV system. However, SO₄⁻• was the main free radical in the Cu(II)/PDS/UV system where •OH also played an important role. The degradation of TC was increased with increasing pH level. Besides, the degradation of TC in these two systems can be described by the pseudo-first-order kinetics model.

This work was supported by the National Natural Science Foundation of China (No. 51508353, 51741807, 51878357).