Abstract

In this study, the difference in oxidative capacity for removing antibiotics and the mechanism between the Cu(II)/peroxymonosulfate (PMS)/UV and Cu(II)/persulfate (PDS)/UV systems were compared under various conditions. The optimal Cu(II) concentration in the Cu(II)/PMS/UV system was 30 μM, and in the Cu(II)/PDS/UV system was 50 μM. With the PMS or PDS concentration increasing, higher tetracycline (TC) degradation in these two systems occurred. Investigation on the mechanism revealed that •OH was the primary radical in the Cu(II)/PMS/UV system, while SO4• was the primary radical in the Cu(II)/PDS/UV system where •OH also played an important role. In these two systems, it was observed that Cu(I) was generated by PMS or PDS activated via UV illumination; however, oxygen alone could not promote TC removal. The degradation of TC was increased with the increasing pH level. In addition, TC degradation in the Cu(II)/PMS/UV system followed the pseudo-first-order kinetics model during the entire reaction period. It was found that the TC degradation kinetics in the Cu(II)/PDS/UV system can be divided into two parts (0 to 7 min and 10 to 50 min) and these two parts had good agreement with the pseudo-first-order kinetics model, respectively.

INTRODUCTION

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).

Sulfate radical (SO4•), generated from the activation of peroxymonosulfate (PMS) by UV, transition metals and heat (Anipsitakis & Dionysiou 2004; Cui et al. 2016; Jorge et al. 2017; Yang et al. 2017), have been shown to be effective in treating pharmaceuticals due to the high redox potential (2.5–3.1 V), compared with hydroxyl radical (•OH, 1.9–2.7 V) (Buxton et al. 1988; Yang et al. 2014; Zhang et al. 2015; Ye et al. 2017). Many studies found that the combination of ultraviolet and transition metals like Fe(II) and Co(II) can strongly enhance the oxidation capacity of a single system (Khan et al. 2016; Jorge et al. 2017). For instance, Fe(II) activates PMS to produce Fe(III) at first, and then Fe(II) regenerated via the reaction between Fe(III) and UV, indicating that UV promotes the transformation of the iron-redox cycle. Meanwhile, SO4• is produced and aims to remove the organic compounds. The reaction is grown in Equations (1) and (2) as follows (Khan et al. 2016): 
formula
(1)
 
formula
(2)

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 (ksp) 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.

Similar to PMS, peroxydisulfate (PDS) can also be activated by UV to generate SO4•. Zhang's study showed that the AOP's performance in removing pharmaceuticals was higher with PDS/UV than H2O2/UV (Zhang et al. 2015). In addition, the sulfate radical was produced through combining Fe(II) with PDS according to Equation (3). 
formula
(3)

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.

MATERIALS AND METHODS

Materials

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).

Experimental procedure

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).

Analysis methods

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).

RESULTS AND DISCUSSION

Oxidative degradation of TC

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.

Figure 1

Oxidative degradation of TC by PMS (top) and PDS (bottom) activated. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 1

Oxidative degradation of TC by PMS (top) and PDS (bottom) activated. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Effect of Cu(II) concentration

As shown in Figure 1, TC degradation in the PMS/UV and PDS/UV systems with Cu(II) were both higher than that without Cu(II), indicating that the Cu(II) concentration may be one of key factors to influence the oxidative capacity of such systems. As shown in Figure 2, the degradation of TC was 84.0%, 86.9%, 95.4%, 94.7% and 92.8% at 50 min in the Cu(II)/PMS/UV system (top), when the Cu(II) concentration was 5, 15, 30, 50 and 100 μM, respectively. It was found that TC degradation increased with the increased Cu(II) concentration, but TC degradation was decreased slightly when the Cu(II) concentration exceeded 30 μM. Thus, the optimal Cu(II) concentration was 30 μM. Likewise, as shown in Figure 2, the TC degradation followed the same rule in the Cu(II)/PDS/UV system (bottom), where the optimal Cu(II) concentration was 50 μM. The enhanced degradation rate with higher Cu(II) concentration for these two systems is likely attributed to the higher amount of Cu(I) and radicals. The phenomenon of much higher Cu(II) concentration may be because excessive Cu(I) reacted with radicals, following Equations (4) and (5) (Buxton et al. 1988; Liang & Su 2009). 
formula
(4)
 
formula
(5)
Figure 2

Effect of Cu(II) concentration in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. Experimental conditions: [Cu(II)]0 = 5, 15, 30, 50, 100 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 2

Effect of Cu(II) concentration in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. Experimental conditions: [Cu(II)]0 = 5, 15, 30, 50, 100 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Effect of PMS and PDS concentration

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.

Figure 3

Effect of oxidative agents concentration in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.25, 0.75, 1.00, 1.25 and 1.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 3

Effect of oxidative agents concentration in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.25, 0.75, 1.00, 1.25 and 1.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

The reaction mechanism of the Cu(II)/UV system with PMS and PDS

Quenching studies for radical identification

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).

Besides, from the results of Figure 4(a), •OH may play an important role in these two systems. In order to further determine •OH, coumarin was chosen to capture •OH to generate the fluorescent 7-hydroxycoumarin (7OHC), following Equation (7) (Takuya et al. 2011). However, coumarin cannot capture •OH absolutely, so it is a semi-quantitative method to estimate the generation of •OH through the concentration of 7OHC (Zhou et al. 2019). As shown in Figure 4(b), the concentration of 7OHC in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems at 50 min were 10.9 and 9.45 μM, respectively, indicating that •OH was favorable for TC degradation. 
formula
(6)
 
formula
(7)
Figure 4

(a) Effect of the addition of different radical scavengers in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. (b) The concentration of 7OHC. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TBA]0 = 50 mM; [EA]0 = 50 mM; [coumarin]0 = 2 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 4

(a) Effect of the addition of different radical scavengers in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. (b) The concentration of 7OHC. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TBA]0 = 50 mM; [EA]0 = 50 mM; [coumarin]0 = 2 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

The generation of Cu(I)

It has been proposed that the production of free radicals catalyzed by the copper-redox cycle could strongly enhance the oxidative capacity of the PMS/UV or PDS/UV systems, as shown in Figure 1. Therefore, the generation of Cu(I) may be a key factor to influence the oxidative capacity. The concentrations of Cu(I) in the Cu(II)/PMS/UV, Cu(II)/PDS/UV and Cu(II)/UV systems were detected in order to examine the generation of Cu(I) without the addition of the target compound. As shown in Figure 5, the concentration of Cu(I) in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems were similar and higher than that in the Cu(II)/UV system, indicating that UV can reduce Cu(II) to Cu(I) at first, and then PMS and PDS can subsequently react with Cu(I) to generate radicals, while Cu(I) is oxidized to Cu(II). Because of this, the copper-redox cycle is formed. According to Equations (1) and (3), the reactions of these two systems can be described by Equations (8)–(10): 
formula
(8)
 
formula
(9)
 
formula
(10)
Figure 5

The concentrations of Cu(I) in the Cu(II)/PMS/UV, Cu(II)/PDS/UV and Cu(II)/UV systems. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 5

The concentrations of Cu(I) in the Cu(II)/PMS/UV, Cu(II)/PDS/UV and Cu(II)/UV systems. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

The role of oxygen

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).

Figure 6

(a) The role of oxygen in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. (b) The reaction mechanism of the Cu(II)/UV system with PMS and PDS. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; O2 flow rate = 0.5 L•min−1; N2 flow rate = 0.5 L•min−1; reaction time = 50 min.

Figure 6

(a) The role of oxygen in the Cu(II)/PMS/UV (top) and Cu(II)/PDS/UV (bottom) systems. (b) The reaction mechanism of the Cu(II)/UV system with PMS and PDS. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; O2 flow rate = 0.5 L•min−1; N2 flow rate = 0.5 L•min−1; reaction time = 50 min.

Effect of pH

In order to further investigate the reaction mechanism of the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems, the degradation of TC was examined at different initial pH values ranging from 2.0 to 6.0. As shown in Figure 7, TC degradation was increased with the increasing pH level in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems, which may be related to the species of the free radicals. According to the results in the section, Quenching studies for radical identification, compared with the Cu(II)/PMS/UV system, TC degradation was higher than that in the Cu(II)/PDS/UV system, indicating that •OH was more effective than SO4• in removing TC. Many previous studies showed that SO4• could be further converted to •OH at high pH, following Equation (11) (Buxton et al. 1988; Guan et al. 2011). Thus, more TC was removed at pH 5 and 6 due to the great amount of •OH formed in these two systems. Ji et al. (2016) found that deprotonated, non-dissociated TC was more reactive for the generation of SO4• because TC has three pKa (pKa1 = 3.3, pKa2 = 7.7 and pKa3 = 9.7) and PDS was activated by TC itself. It can be noted that TC degradation at pH 3.0 in the Cu(II)/PMS/UV was higher than that at pH 4.0, which may be likely that pH 3.0 was close to pKa1 and the form of TC at pH 3.0 in the system would generate considerable SO4•. 
formula
(11)
Figure 7

Effect of different pH in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems at 50 min. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 2, 3, 4, 5 and 6; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 7

Effect of different pH in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems at 50 min. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 9 mg/L; pH = 2, 3, 4, 5 and 6; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Degradation kinetics

To investigate the degradation kinetics of TC in the Cu(II)/PMS/UV and Cu(II)/PDS/UV systems, different concentrations of TC ranging from 3 to 15 mg/L, were introduced. As shown in Figure 8(a) and 8(b), the TC degradation in these two systems was decreased with the increasing TC concentration, in accordance with the general rule. As shown in Figure 8(a), the degradation of TC during a 50 min period in the Cu(II)/PMS/UV system can be described by the pseudo-first-order kinetics model, and the kinetics equation is grown in Equation (12). The rate constant (k) and correlation coefficient (R2) in the Cu(II)/PMS/UV system are summarized in Table 1. Moreover, as shown in Figure 8(b), according to reaction time, the TC degradation kinetics in the Cu(II)/PDS/UV system can be divided into two parts: one from 0 to 7 min and the other from 10 to 50 min. These two parts were in good agreement with the pseudo-first-order kinetics model, respectively. R2 and k values in the Cu(II)/PDS/UV system are summarized in Table 2, where the rate constants during 0 to 7 min were higher than that during 10 to 50 min. In addition, compared with the Cu(II)/PDS/UV system, k in the Cu(II)/PMS/UV system was much higher, indicating that TC was removed more quickly in the Cu(II)/PMS/UV system. These results are consistent with the phenomenon in Figures 1 and 4. 
formula
(12)
Table 1

Pseudo-first-order rate constants for TC degradation in the Cu(II)/PMS/UV system

[TC], mg/L 12 15 
k, min−1 0.206 0.098 0.050 0.038 
R2 0.996 0.993 0.993 0.994 
[TC], mg/L 12 15 
k, min−1 0.206 0.098 0.050 0.038 
R2 0.996 0.993 0.993 0.994 

Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = 0.50 mM; [TC]0 = 3, 6, 12 and 15 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Table 2

Pseudo-first-order rate constants for TC degradation in the Cu(II)/PDS/UV system

[TC], mg/L 0–7 min
 
10–50 min
 
12 15 12 15 
k, min−1 0.181 0.086 0.031 0013 0.034 0.017 0.017 0.016 
R2 0.971 0.998 0.975 0.948 0.974 0.934 0.968 0.981 
[TC], mg/L 0–7 min
 
10–50 min
 
12 15 12 15 
k, min−1 0.181 0.086 0.031 0013 0.034 0.017 0.017 0.016 
R2 0.971 0.998 0.975 0.948 0.974 0.934 0.968 0.981 

Experimental conditions: [Cu(II)]0 = 20 μM; [PDS]0 = 0.50 mM; [TC]0 = 3, 6, 12 and 15 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 8

(a) Effect of TC concentration and linear fit of rate constants from kinetic models for TC degradation in the Cu(II)/PMS/UV system. (b) Effect of TC concentration and linear fit of rate constants from kinetic models for TC degradation in the Cu(II)/PDS/UV system. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 3, 6, 12 and 15 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

Figure 8

(a) Effect of TC concentration and linear fit of rate constants from kinetic models for TC degradation in the Cu(II)/PMS/UV system. (b) Effect of TC concentration and linear fit of rate constants from kinetic models for TC degradation in the Cu(II)/PDS/UV system. Experimental conditions: [Cu(II)]0 = 20 μM; [PMS]0 = [PDS]0 = 0.50 mM; [TC]0 = 3, 6, 12 and 15 mg/L; pH = 3.5; T0 = 25 °C; power of UV light = 25 W; reaction time = 50 min.

CONCLUSIONS

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.

ACKNOWLEDGEMENTS

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

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