The Degradation of Aqueous Oxytetracycline by an O₃/CaO₂ System in the Presence of ${\mathbf{HCO}}_{\mathbf{3}}^{\mathbf{-}}$: Performance, Mechanism, Degradation Pathways, and Toxicity Evaluation

Abstract

This research constructed a novel O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system to degrade antibiotic oxytetracycline (OTC) in water. The results indicated that CaO₂ and ${\mathrm{HCO}}_3^{-}$ addition could promote OTC degradation in an O₃ system. There is an optimal dosage of CaO₂ (0.05 g/L) and ${\mathrm{HCO}}_3^{-}$ (2.25 mmol/L) that promotes OTC degradation. After 30 min of treatment, approximately 91.5% of the OTC molecules were eliminated in the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. A higher O₃ concentration, alkaline condition, and lower OTC concentration were conducive to OTC decomposition. Active substances including ·OH, ¹O₂, $·{\mathrm{O}}_2^{-}$, and ·${\mathrm{HCO}}_3^{-}$ play certain roles in OTC degradation. The production of ·OH followed the order: O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ > O₃/CaO₂ > O₃. Compared to the sole O₃ system, TOC and COD were easier to remove in the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. Based on DFT and LC-MS, active species dominant in the degradation pathways of OTC were proposed. Then, an evaluation of the toxic changes in intermediates during OTC degradation was carried out. The feasibility of O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ for the treatment of other substances, such as bisphenol A, tetracycline, and actual wastewater, was investigated. Finally, the energy efficiency of the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system was calculated and compared with other mainstream processes of OTC degradation. The O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system may be considered as an efficient and economical approach for antibiotic destruction.

1. Introduction

In recent decades, antibiotics have been widely used in the pharmaceutical and animal husbandry industries. While providing substantial social benefits, they inevitably cause serious environmental pollution [1,2]. Oxytetracycline (OTC) is a kind of tetracycline antibiotic [3] with broad-spectrum bacteriostasis and a low price. As a veterinary drug, it is widely used in animal husbandry worldwide [4]. However, excessive use of OTC cannot be fully absorbed by animals, and there will be a large amount of OTC in the environment [5,6]. A residual concentration of OTC in water is still present after treatment by sewage treatment plants. A high concentration of OTC in water will slow down the development of fish [7] and inhibit photosynthesis in plants [8]. The naphthalene ring structure has antibacterial properties and chemical stability, which makes it difficult to remove OTC through traditional water treatment processes [9]. Therefore, it is imperative to seek an effective method to remove OTC in water.

Oxidation is an essential reaction from an industrial and academic point of view. In general, selecting oxidants is significant for heterogeneous reaction systems. An advanced oxidation process (AOP_S) is recognized as an effective method for treating wastewater with potential environmental risks. One of the most common active species in AOP_S is hydroxyl radicals (·OH), which have strong oxidation properties and can convert pollutants into carbon dioxide and water. A simple and commonly used substance in AOPs is O₃ [10]. O₃ is considered to be a clean oxidant in which organic pollutants are oxidized directly or indirectly with free radicals generated by O₃ to achieve the degradation of organic pollutants [11]. The direct oxidation reaction time is longer, the oxidation rate is slow, and it has a specific selectivity. Indirect oxidation refers to the solid oxidizing free radicals (mainly ·OH) generated by the catalytic O₃ reaction, which can quickly decompose most organic pollutants in water. In the indirect oxidation method, the use of a catalyst to activate O₃ increases the oxidation capacity of O₃ and promotes the degradation of organic pollutants. Recently, the couple of O₃ and other oxidation processes has become very popular, such as H₂O₂/O₃ [12], ultrasonic treatment ozonation [13], and photocatalytic ozonation [14]. Among them, the most effective method of O₃ indirect oxidation technology is the O₃/H₂O₂ method, which is also the simplest method for ·OH [15]. However, H₂O₂ is relatively dangerous, with the characteristic of being flammable and explosive [16].

So far, many studies have confirmed that a combination of several treatment technologies can improve the degradation and mineralization of organic pollutants in wastewater [17]. Some catalysts can promote the better degradation of pollutants by O₃. As a substitute for liquid hydrogen peroxide, calcium peroxide (CaO₂), a new solid oxidant, has attracted a lot of attention in the environmental field due to its remarkable advantages, such as a high efficiency, safety, stability, controllability, and low cost [18,19]. The process of releasing H₂O₂ from calcium peroxide is slow, which can be controlled by adjusting the pH, which avoids the disadvantage of wasting oxygen and the short action time caused by the rapid decomposition of liquid H₂O₂. For some pollutants (such as PPCP and PAH), the removal effect of solid calcium peroxide will exceed that of liquid H₂O₂ [20,21]. The O₃/H₂O₂ system created by O₂/CaO₂ has a low oxidation rate and poor selectivity in the absence of an activator. Sodium bicarbonate was added to the system as an auxiliary catalyst [22]. On one hand, ${\mathrm{HCO}}_3^{-}$ reacts with H₂O₂ to generate peroxymonocarbonate (${\mathrm{HCO}}_4^{-}$), which continuously activates H₂O₂ and converts it into highly active free radicals such as ·O₂⁻, singlet oxygen (¹O₂), and so on [23]. On the other hand, ${\mathrm{HCO}}_3^{-}$ is a recognized promoter of O₃ decomposition, promoting the efficient oxidation of O₃ into ·CO₃⁻ and the conversion of O₃ into indirect oxidation [24]. O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ innovatively proposes a safe and fast O₃/H₂O₂ system. Meanwhile, the addition of ${\mathrm{HCO}}_3^{-}$ could activate H₂O₂ and O₃ and significantly improve the oxidation capacity of the system [25]. As a whole, the advantages of the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system can be listed as: firstly, the synergy effect between O₃ and CaO₂ can promote the generation of ·OH. Secondly, the OH⁻ released by CaO₂ will form an alkaline environment, which will further promote the transformation of O₃ into ·OH. Thirdly, ${\mathrm{HCO}}_3^{-}$ activates O₃ radicals and catalyzes the production of ·OH radicals. Finally, the decomposition products of CaO₂/${\mathrm{HCO}}_3^{-}$ are non-toxic and harmless. However, as far as is known, there have been few reports that O₃ activates CaO₂/${\mathrm{HCO}}_3^{-}$ for antibiotic OTC degradation, and its synergistic mechanism is still unclear.

Therefore, the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system was constructed for the efficient degradation of OTC in this paper. The influence of the effect of CaO₂ dosage, ${\mathrm{HCO}}_3^{-}$ dosage, O₃ concentration, initial pH value, and OTC initial concentration on OTC degradation was investigated. The role of active substances in the OTC decomposition system was inspected via the capture agent experiment. The generated active species were characterized using chemical methods and electron spin resonance (ESR). The degradation mechanism and process were examined. The toxicity of OTC and intermediates was analyzed, as was the feasibility of the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system for the treatment of other antibiotics and actual wastewater. Finally, the energy efficiency of the system was compared with that of other technology.

2. Results and Discussion

2.1. Effect of CaO₂

Figure 1a,b reveal the changes in the OTC degradation efficiency under different CaO₂ additions. The lowercase letters in the picture represent the difference between the data, and the star symbol represents the value of lnK_obs. Experiments were carried out with fixed OTC concentrations at different CaO₂ concentrations. It was observed that the O₃/CaO₂ system had a significant effect on the degradation of OTC within 30 min. When the dosage of CaO₂ was 0.050 g/L, the degradation efficiency of OTC could reach 85.3%, which was 23.2% higher than that of the sole O₃ system. The degradation efficiency (slope of degradation curve) of OTC increased with an increase in the CaO₂ concentration, which indicates that CaO₂ is conducive to the degradation of OTC. It is worth noting that, when the dosage of CaO₂ was higher than the optimal dosage (0.050 g/L), the degradation efficiency gradually slowed down. The first-order kinetic constant in the upper right corner of Figure 1a can also reflect this trend.

CaO₂ is a good solid source of H₂O₂ [26]. It can be regularly converted into H₂O₂ and O₂ when dissolved in water (Equations (1) and (2)) [27]. The H₂O₂ it releases can reduce disproportionation and maintain the reaction for a longer time. H₂O₂ will cause O₃ (E° = 2.07 V/NHE) to decompose and transform into non-selective ·OH (E° = 2.80 V/NHE) [28]. The formation of Ca(OH)₂ will affect the pH value of the solution. The alkaline environment brought about makes H₂O₂ decompose into ${\mathrm{HO}}_2^{-}$. ${\mathrm{HO}}_2^{-}$ is converted into ·OH through a series of reactions with O₃ (Equations (3)–(13)) [29,30,31,32,33]. This ·OH is a strong oxidant, which can effectively promote the degradation of OTC. However, when the concentration of CaO₂ is too high, too much H₂O₂ will be produced [34]. Excessive H₂O₂ will decompose by itself and need to consume active free radicals such as ·OH (Equations (14)–(16)), which will adversely affect the degradation of OTC [35]. The oxidation potentials of different free radicals are displayed in Table S1.

$${\mathrm{CaO}}_2{+2\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2{+\mathrm{Ca}\left(\mathrm{OH}\right)}_2$$

(1)

$${\mathrm{H}}_2{\mathrm{O}}_2{+2\mathrm{O}}_3\to {2\mathrm{OH}+3\mathrm{O}}_2$$

(2)

$${\mathrm{OH}}^{-}{+\mathrm{O}}_3\to {\mathrm{HO}}_2^{-}{+\mathrm{O}}_2$$

(3)

$${\mathrm{HO}}_2^{-}{+\mathrm{O}}_3\to {\mathrm{HO}}_2{+\mathrm{O}}_3^{-}$$

(4)

$${\mathrm{HO}}_2^{-}\to {·\mathrm{O}}_2^{-}+·\mathrm{H}$$

(5)

$${·\mathrm{O}}_3^{-}{+\mathrm{H}}^{+}\to {·\mathrm{HO}}_3$$

(6)

$${·\mathrm{HO}}_3\to {\mathrm{O}}_2+·\mathrm{OH}$$

(7)

$${·\mathrm{O}}_3^{-}{+\mathrm{O}}_3\to {\mathrm{O}}_2{+·\mathrm{O}}_3^{-}$$

(8)

$${\mathrm{HO}}_2^{-}{+\mathrm{O}}_3\to {\mathrm{HO}}_5^{-}$$

(9)

$${\mathrm{HO}}_5^{-}\to {·\mathrm{HO}}_2{+·\mathrm{O}}_3^{-}$$

(10)

$${\mathrm{HO}}_5^{-}\to {2\mathrm{O}}_2{+\mathrm{OH}}^{-}$$

(11)

$${·\mathrm{O}}_3^{-}\to {·\mathrm{O}}^{-}{+\mathrm{O}}_2$$

(12)

$${·\mathrm{O}}^{-}{+\mathrm{H}}_2\mathrm{O}\to {·\mathrm{OH}+\mathrm{OH}}^{-}$$

(13)

$${2\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{O}}_2{+2\mathrm{H}}_2\mathrm{O}$$

(14)

$${\mathrm{H}}_2{\mathrm{O}}_2+·\mathrm{OH}\to {·\mathrm{HO}}_2{+\mathrm{H}}_2\mathrm{O}$$

(15)

$${\mathrm{HO}}_2^{-}+·\mathrm{OH}\to {·\mathrm{HO}}_2{+\mathrm{OH}}^{-}$$

(16)

2.2. Effect of ${HCO}_{\mathit{3}}^{\mathit{-}}$ Dosage

The degradation of OTC with time under the conditions of different ${\mathrm{HCO}}_3^{-}$ additions is shown in Figure 1c,d. To explore the CaO₂/${\mathrm{HCO}}_3^{-}$ combined process, with the addition of ${\mathrm{HCO}}_3^{-}$ as a variable, the experiment was carried out with the optimal CaO₂ concentration and fixed OTC concentration. It was observed that the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system had a further improvement in the degradation of OTC compared to O₃/CaO₂ within the reaction time of 30 min. When the dosage of ${\mathrm{HCO}}_3^{-}$ was 2.25 mmol/L, the degradation efficiency of OTC could reach 91.5%, which was 7.3% higher than that of the O₃/CaO₂ system and 32.1% higher than that of the sole O₃ system. The degradation efficiency of OTC increased with an increase in the ${\mathrm{HCO}}_3^{-}$ concentration. Further increasing the concentration of ${\mathrm{HCO}}_3^{-}$ based on the optimal dosage of ${\mathrm{HCO}}_3^{-}$ will inhibit the degradation of OTC. Similarly, the first-order kinetic constant in the upper right corner of Figure 1c also reflects this trend.

${\mathrm{HCO}}_3^{-}$ reacts with H⁺ in water to produce CO₂, H₂O₂ reacts with OH⁻ in water to produce OOH⁻, and CO₂ reacts with OOH⁻ to produce ${\mathrm{HCO}}_4^{-}$. The active free radicals released by ${\mathrm{HCO}}_4^{-}$ (${·\mathrm{CO}}_3^{-}$) can activate H₂O₂ to generate ·HO₂, and then react to generate ${·\mathrm{O}}_2^{-}$, ¹O₂ and other oxygen-active substances (Equations (17)–(22)) [36,37]. The consumption of ${·\mathrm{CO}}_3^{-}$ for activating H₂O₂ can be regenerated into ${·\mathrm{CO}}_3^{-}$ through intermediate ${\mathrm{HCO}}_3^{-}$ to form a cycle, which is the synergistic effect of ${\mathrm{HCO}}_3^{-}$ and CaO₂ ((Equation (19)) [23,38].

$${\mathrm{HCO}}_3^{-}{+\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{HCO}}_4^{-}{+\mathrm{H}}_2\mathrm{O}$$

(17)

$${\mathrm{HCO}}_4^{-}\to {·\mathrm{CO}}_3^{-}+·\mathrm{OH}$$

(18)

$${\mathrm{HCO}}_3^{-}+·\mathrm{OH}\to {\mathrm{CO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}$$

(19)

$${\mathrm{H}}_2{\mathrm{O}}_2{+·\mathrm{CO}}_3^{-}\to {\mathrm{HCO}}_3^{-}{+·\mathrm{HO}}_2$$

(20)

$$·{\mathrm{HO}}_2 \to {\mathrm{H}}^{+}+{·\mathrm{O}}_2^{-}$$

(21)

$${\mathrm{O}}_2^{-}+·\mathrm{OH}\to {{}^1\mathrm{O}}_2{+\mathrm{OH}}^{-}$$

(22)

The synergistic effect between ${\mathrm{HCO}}_3^{-}$ and O₃ is mainly reflected in: OH⁻ and ${·\mathrm{O}}_2^{-}$ promote the decomposition of O₃ into ${·\mathrm{O}}_3^{-}$, ·${\mathrm{O}}_3^{-}$, through a chain reaction, generates more ${·\mathrm{O}}_2^{-}$, ${·\mathrm{O}}_2^{-}$, in turn, promotes the decomposition of O₃ (Equations (23)–(31)) [39,40,41]. There is a balance point between the generation of ${·\mathrm{O}}_2^{-}$ and the decomposition of O₃, which also explains the phenomenon that the degradation efficiency of OTC decreases when the ${\mathrm{HCO}}_3^{-}$ dosing amount is higher than the optimal amount [42].

$${\mathrm{O}}_3{+·\mathrm{O}}_2^{-}\to {·\mathrm{O}}_3^{-}{+\mathrm{O}}_2$$

(23)

$${\mathrm{O}}_3+{\mathrm{OH}}^{-} \to \mathrm{H}{\mathrm{O}}_2^{-}{\mathrm{O}}_2$$

(24)

$${\mathrm{O}}_3+\mathrm{H}{\mathrm{O}}_2^{-} \to ·{\mathrm{O}}_3^{-}+·{\mathrm{HO}}_2$$

(25)

$${·\mathrm{O}}_3^{-}{+\mathrm{H}}^{+}\to {·\mathrm{HO}}_3$$

(26)

$${·\mathrm{HO}}_3\to {·\mathrm{OH}+\mathrm{O}}_2$$

(27)

$${\mathrm{O}}_3+·\mathrm{OH}\to {·\mathrm{HO}}_2+{\mathrm{O}}_2$$

(28)

$${\mathrm{H}}_2{\mathrm{O}}_2+·\mathrm{OH}\to {·\mathrm{HO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(29)

$${\mathrm{HO}}_{2^{-}}+·\mathrm{OH}\to {·\mathrm{O}}_2^{-}{+\mathrm{H}}_2\mathrm{O}$$

(30)

$$·\mathrm{H}{\mathrm{O}}_2 \to ·{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}$$

(31)

2.3. Effect of O₃ Dosage

The degradation of OTC under the conditions of different O₃ concentrations is presented in Figure 1e,f. As the amount of O₃ increased from 0.125 g/h to 0.75 g/h, the OTC degradation efficiency increased significantly from 91.5% to 97.2%, and the reaction time when the OTC degradation efficiency reached 90% was shortened from 30 min to 5 min. The reasons were as follows: firstly, the larger O₃ flux increased the contact area between the gas phase and the liquid phase, and the number of effective molecular collisions increased, which improved the reaction rate. Secondly, according to Henry’s Law, the more significant the O₃ concentration, that is, the increase in X_Z per unit volume, the more increased the pressure of the O₃ gas, thus promoting the mass transfer rate of O₃ between the two phases [43]. Third, the O₃ concentration will affect the concentrations of H₂O₂ and ·OH in the reaction system. A higher concentration will promote the generation of more active free radicals and affect the degradation effect [44]. However, a high-concentration O₃ treatment process means a high cost, which will affect the economy of this process.

2.4. Effect pH Value

The degradation of OTC with time under the conditions of different initial pHs is shown in Figure 2a. With an increase in the initial pH of the solution, the environment changed from acidic to neutral and then to alkaline. The OTC degradation efficiency was enhanced from 71.4% to 94.4%. This phenomenon was mainly due to the direct oxidation of O₃ molecules with pollutants in an acidic environment. In an alkaline environment, the concentration of dissolved O₃ decreases [45], forcing O₃ molecules to be activated into active free radicals such as ·OH and ${·\mathrm{O}}_2^{-}$ (Equations (24)–(32)) to react with OTC [46]. Because the indirect reaction is dominant, the oxidation potential of ·OH is higher than that of O₃ molecules, and the oxidation ability is stronger, which is more conducive to the degradation of OTC. On the other hand, an alkaline environment is more conducive to the release of H₂O₂ from CaO₂ and the self-decomposition of H₂O₂, which will produce more active free radicals and promote the degradation of pollutants [47]. Similarly, the O₃ treatment process under alkaline conditions also means an increase in cost, and the natural water environment is mainly neutral or weakly alkaline, so neutral environmental conditions will continue to be used in subsequent studies.

2.5. Effect OTC Concentration

The degradation of OTC with time under the conditions of different OTC concentrations is shown in Figure 2c. As the initial OTC concentration was raised from 40 mg/L to 200 mg/L, the OTC degradation efficiency decreased significantly, the degradation efficiency decreased from 94.4% to 60.6%, and the kinetic constant decreased from 0.114 min⁻¹ to 0.031 min⁻¹ (inset Figure 2c). It is unreasonable to judge the OTC degradation only by the initial OTC concentration. We calculated the total removal of the four initial OTC concentrations in Figure 2c, which were 15.1 mg, 29.0 mg, 45.8 mg, and 48.5 mg, respectively. Comparing the kinetic constant and total removal amount of OTC, with an increase in the concentration, the kinetic constant decreased and the total removal amount increased. Therefore, it is speculated that the initial OTC treatment concentration is 40 mg/L, which is close to the limit of the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. As shown in Figure 1b,d,e and Figure 2b,d, good linear correlations between the ln k and different parameters were obtained. Therefore, the establishment of an O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system can degrade OTC effectively.

2.6. Active Substance Analysis

2.6.1. Role of ·OH, ¹O₂, ${·\mathrm{O}}_2^{-}$, and ${·\mathrm{CO}}_3^{-}$

Methanol is an efficient ·OH-trapping agent, which was used to capture ·OH in the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. As shown in Figure 3a, it can be seen that the degradation efficiency of OTC decreased with increasing amounts of methanol. When the methanol concentration changed from 0 mmol/L to 10 mmol/L, the degradation efficiency of OTC decreased from 94.0% to 66.9%. Meanwhile, the kinetic constant declined from 0.114 min⁻¹ to 0.039 min⁻¹ (Figure S1a). These results indicate that ·OH played an essential role in the degradation of OTC. ${·\mathrm{O}}_2^{-}$ can react to generate ¹O₂ (Equations (32)–(34)), which has a strong oxidation potential. 1,4-Diazabicyclooctane triethylenediamine (DABCO) is a highly efficient ¹O₂-trapping agent that can capture the ¹O₂ existing in the solution. Therefore, this section intends to use DABCO to study the role of ¹O₂ in the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system.

$${\mathrm{O}}_2^{-}+·\mathrm{OH}\to {{}^1\mathrm{O}}_2{+\mathrm{OH}}^{-}$$

(32)

$${·\mathrm{O}}_2^{-}{+\mathrm{HO}}_2+{\mathrm{H}}^{+}\to {{}^1\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(33)

$${·\mathrm{HO}}_2+{\mathrm{HO}}_2\to {{}^1\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(34)

Figure 3b shows the effect of various DABCO dosages on OTC elimination. Higher DABCO dosages will inhibit the degradation effect more obviously. When the addition of DABCO changed from 0 mmol/L to 10 mmol/L, the degradation efficiency of OTC decreased from 94.0% to 79.6%, with the kinetic constant varying from 0.114 min⁻¹ to 0.060 min⁻¹ (Figure S1b). DABCO has a significant inhibitory effect on OTC elimination. It is further inferred that ¹O₂ is actively involved in OTC degradation.

P-benzoquinone is a highly efficient ${·\mathrm{O}}_2^{-}$-trapping agent which can capture the ${·\mathrm{O}}_2^{-}$ present in a solution. After adding the catalyst CaO₂ to the solution, CaO₂ and H₂O will produce a large quantity of ${·\mathrm{O}}_2^{-}$ (Equation (6)), which has a significant effect on OTC elimination. Figure 3c shows the effect of p-benzoquinone addition on OTC elimination. The degradation efficiency of OTC decreased with the p-benzoquinone addition. When the addition of p-benzoquinone was 0 mmol/L 0.5 mmol/L, 1.0 mmol/L, and 1.5 mmol/L, the degradation efficiency of OTC decreased from 94.0% to 53.2%, and the corresponding kinetic constant varied from 0.114 min⁻¹ to 0.059 min⁻¹, 0.039 min⁻¹, and 0.033 min⁻¹ (Figure S1c). This result showed that ${·\mathrm{O}}_2^{-}$ played a key role in the degradation of OTC.

Indole is an efficient ${·\mathrm{CO}}_3^{-}$ capture agent, which can capture the ${\mathrm{HCO}}_3^{-}$ existing in a solution. After adding the catalyst NaHCO₃ to the solution, NaHCO₃ and H₂O will produce part of ${·\mathrm{CO}}_3^{-}$ (Equation (19)). When the indole concentration changed from 0 mmol/L to 0.2 mmol/L, the degradation efficiency of OTC decreased from 94.0% to 59.5% (Figure 3d), and the corresponding kinetic constant varied from 0.114 min⁻¹ to 0.033 min⁻¹ (Figure S1d). This result shows that ${·\mathrm{CO}}_3^{-}$ is very involved in the degradation of OTC. The reaction rates of p-benzoquinone with ·OH and ·O₂⁻ were 1.2 × 10⁹ and 8.3 × 10⁸ M⁻¹ s⁻¹, respectively, which are comparable. The reaction rate of indole with ·OH was 7.9 × 10⁻¹⁴ M⁻¹s⁻¹, which can be ignored. Because of their low reaction rates, P-benzoquinone and Indole can be regarded as not reacting with ·OH [48,49]. As shown in Figure S2, compared with different active substances, ·O₂ made the greatest contribution to the degradation process of OTC, with 41.2% of the OTC being degraded by ·O₂.

2.6.2. Formation of Active Species

To further explore the synergetic mechanism, the formation of ·OH under different systems was compared. The results are depicted in Figure 4a. Compared to the sole O₃ system, the ·OH concentration was seriously increased when CaO₂ was added. These phenomena verified that O₃ could react with the H₂O₂ released by CaO₂ and promote the formation of ·OH [50]. As desired, the ·OH concentration was further enhanced when ${\mathrm{HCO}}_3^{-}$ was added. The production of ·OH followed this law: O₃/CaO₂/NaHCO₃ > O₃/CaO₂ > O₃. Therefore, it can be explained that the higher degradation efficiency may have come from the higher production of ·OH. It is worth noting that the generation of ·OH in the O₃/CaO₂ system and the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system showed a trend of first rising, then decreasing, and lastly increasing over time. It was speculated that the conversion of H₂O₂ into ·OH was violent, and a large amount of ·OH was generated within 5 min. ·OH could react with H₂O₂, and a certain amount of ·OH production was consumed from 5 to 20 min. After the two reactions reached equilibrium, ·OH production increased and tended to stabilize.

ESR analyses further revealed that there were obvious signals of the ·OH peak in various systems (Figure 4b). It can be seen that the characterized peaks of ·OH followed as: O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ > O₃/CaO₂ > O₃, which illustrated that ·OH was beneficial to generate in O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. Figure 4c presents the characteristic peaks of ¹O₂ in various systems. It can be seen that ¹O₂ can be generated in O₃, O₃/CaO₂, and O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ systems. Compared to sole O₃, the characteristic peaks of ¹O₂ were enhanced with the addition of CaO₂ and ${\mathrm{HCO}}_3^{-}$, which indicates that the addition of CaO₂ and ${\mathrm{HCO}}_3^{-}$ effectively increased the production of ¹O₂.

2.7. OTC Degradation Process

2.7.1. UV-Vis Spectra

Figure S3 shows the UV-Vis spectrum results of the OTC solution in the sole O₃ system and O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. The OTC spectrum shows two absorption bands near 276 nm and 353 nm, and the absorption peak near 353 nm is the characteristic peak of OTC. The common point of the two systems is that, after 30 min of processing, the characteristic peaks of OTC were significantly reduced, and the OTC molecules were weakened. The π–π * transition of its heteroaromatic ring makes the main contribution to this [51,52]. Compared with Figure S3a, the characteristic peak of OTC in Figure S3b has a more rapid decline, which indicates that O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ had a better OTC degradation ability.

2.7.2. 3D EEMFS

To study the degradation process of OTC more deeply, 3D EEMFs were used for characterization. The emission wavelength and the excitation wavelength were all set as 200–600 nm, whose sampling intervals were 5 nm and 10 nm, respectively. Figure 5a is taken from the original OTC sample, and Figure 5b,c are OTC samples processed for 10 min, 20 min, and 30 min, respectively. In the original OTC solution, there was a prominent fluorescence peak at E_x/E_m = 300–400/400–600 nm, which is located in the humic acid-like fluorescence region [53]. With the passage of time, the characteristic peak intensity gradually weakened and the fluorescence intensity dropped from 2000.23 to 220.18, indicating that the OTC molecule was destroyed and may have been mineralized into some intermediates or decomposed into CO₂ and H₂O (Figure 5d). The 3D EEMFs results show that the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system can degrade OTC, which is consistent with the UV-Vis spectrum results.

2.7.3. Variation of TOC and COD

TOC is an important parameter for evaluating the mineralization ability of oxidation systems. Figure 6a shows the degradation of OTC in terms of TOC removal, with or without two catalysts in the O₃ oxidation system. The carbon-containing organic matter in the OTC solution decreased as the treatment time increased. After the two catalysts were added, the TOC concentration decreased. When the treatment time was 30 min, the TOC degradation efficiency could reach 14.7%. Figure 6b depicts the COD concentration in the two systems. The COD concentration of the sole O₃ system underwent a slight drop, the speed was plodding, and it tended to be stable. The O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system had a very high COD concentration at 0 min, with a significant drop and a fast speed, and a COD degradation efficiency of 48.8% was obtained within 30 min of the processing time. This O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system produced a large number of active free radicals, which actively participated in the degradation process, which supports the derivation of the mechanism in the previous chapter. The O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system has an excellent OTC degradation ability, but the degradation efficiency of TOC is far below 100%. Combining the results of the UV-Vis spectroscopy and 3D EEMFs, we speculated that a great deal of intermediates were formed during OTC degradation.

2.7.4. Variation of pH and Conductivity

Figure S4a shows the pH change over time during OTC degradation. The O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system presented a slightly alkaline environment, while the sole O₃ system presented a neutral environment. As the treatment time increased, the pH values in both systems showed a gentle downward trend. Figure S4b shows the change in conductivity during OTC degradation. The conductivity of the sole O₃ system was 21.2 μS/cm at 0 min, and after 30 min of treatment, the conductivity increased slightly to 30.3 μS/cm. The conductivity of the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system was 308 μS/cm at 0 min and dropped to 262 μS/cm after 30 min of treatment. We speculate that the addition of catalysts produced more inorganic ions, and acidic free radicals were generated during the degradation of OTC. In the process of degradation, ${\mathrm{HCO}}_3^{-}$ reacted with H₂O₂ to form ${\mathrm{HCO}}_4^{-}$ on the one hand and ·OH to form ${·\mathrm{CO}}_3^{-}$ on the other, so ${\mathrm{HCO}}_3^{-}$ itself was consumed massively. The decrease in conductivity may have been due to the consumption of ${\mathrm{HCO}}_3^{-}$.

2.8. DFT Analysis and Degradation Pathway

The Fukui functions calculated with density functional and electron orbitals provide good evidence for predicting reaction sites. The molecular structure of OTC was calculated with the Gaussian 09 program. The Fukui function of OTC mapped the electron density isosurface and the calculation results are provided in Figure 7a and Table S2. Now, it is generally accepted that, the higher the f⁺ function of the atom is, the easier will it be nucleophilically attacked; the higher the f⁻ function of the atom is, the easier will it be electrophilically attacked; and the higher the f⁰ function of the atom is, the easier will it be radically attacked. To indicate the possibility of the site being attacked clearly based on the above mechanism, electron cloud diagrams were drawn. It can be seen that the highlight is above C21 and O26, which means that these two sites would be easy to electrophilically attack. For the same reason, C4, O12, and C20 were easier to nucleophilically attack. C21 and O26 were easier to radically attack. Through a comprehensive comparison of the Fukui function table, it is inferred that OTC molecules may be decomposed into A (pathway 1) and B (pathway 2) due to electron and free radical attacks on C21 and O26.

To deeply explore the OTC degradation process in the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system, a Liquid Chromatograph Mass Spectrometer (LC-MS) (ES+, ES−) was adopted and the results are illustrated in Figure S5. According to the results of LC-MS, it can be inferred that there were eight main intermediates: A C₂₂H₂₄N₂O₁₀ (m/z = 477), B C₁₅H₁₃NO₇ (m/z = 296), C C₄H₇NO₄ (m/z = 132), D C₃H₇NO₄ (m/z = 118), E C₃H₇NO₃ (m/z = 106), F C₆H₁₀O₂ (m/z = 113), and G C₆H₁₂O (m/z = 101). The molecular structures of the eight intermediates are shown in Table S3. Combined with the Density Functional Theory (DFT) calculation and LC-MS detection results, the inferred degradation path of OTC is shown in Figure 7b. Figure 7c shows a free energy ladder diagram of these reactions. The initial energy of OTC was 0 kcal/mol, and the major degradation pathways are represented by negative energy levels. These reactions may occur, in theory, by different radicals. In the OTC molecular structure formula, weak chemical bonds between atoms are easy to destroy. C21 in pathway I is prone to enol ketone isomerization, and OTC is transformed into more stable isomers. The ketone/enol portion of C21–C25 is vulnerable to ·OH attack, and further hydroxylation by-product A is obtained. O₃ in the system is an electrophilic reagent, which tends to attack the electron-rich region in the OTC structure [54]. The double-bond structure of C21–C25 in pathway II and pathway III is broken by free radicals such as O₃, and the main product is B. Fragile C5a breaks, causing B molecules to decompose into C and F. F is further dehydrated to obtain by-product G. C is demethylated to obtain by-product D, and D is further dehydrated to obtain small molecule by-product E. These intermediates could be further oxidized to small molecular organic acids, and ultimately mineralized to CO₂ and H₂O [55,56].

2.9. Toxicity Evaluation

The U.S. Environmental Protection Agency’s toxicity evaluation software tool (US–EPA–TEST, version 5.1.2) program of this study predicts the acute and chronic toxicity of OTC and its intermediates [55,57]. The European Union criteria standard is adopted for acute toxicity, and the Chinese guidelines for the evaluation of hazardous chemicals (HJ/TI 154—2004) are adopted for chronic toxicity. The test results are shown in Table S4 and Figure 8. The daphnid LC50 of OTC is 9.34 mg/L, belonging to “toxic”. After 30 min of treatment in this system, the toxicity of intermediates was significantly reduced, and all intermediates except F were located in “harmless” or “harmful” areas. A, B, and C were located in the “Harmless” area. Therefore, it can be speculated that the toxicity of OTC will decline significantly after O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ treatment, and detoxification may be achieved.

2.10. Feasible for Treatment of Other Antibiotics and Actual Wastewater

The feasibility of O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ for the treatment of other antibiotics and actual wastewater was investigated. The degradation of bisphenol A (BPA) and tetracycline (TC) under different systems is presented in Figure 9a,b. No matter the aspect of BPA or TC, the highest degradation efficiency was obtained in the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. As a whole, it can be seen that the degradation efficiency could reach above 80%, illustrating that the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system is also adapted to other antibiotic degradations. Figure 9c depicts the degradation of OTC in various water bodies. Over time, OTC molecules are gradually degraded in different water bodies such as deionized water, lake water, river water, and tap water. The degradation effect has not undergone significant changes, indicating that the system is also suitable for the treatment of actual wastewater.

2.11. Energy Efficiency Evaluation

Energy efficiency assessment is a part that cannot be underestimated. It is closely related to whether a new system can be promoted and applied. Therefore, we calculated the energy efficiency of the sole O₃ system, O₃/CaO₂ system, and O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system in different program doses, and the results are shown in Table S5. In the sole O₃ system, the energy efficiency reached 1.55 g/kWh. When 0.050 g/L of CaO₂ was added, the energy efficiency increased to 1.82 g/kWh. On this basis, when adding 2.25 mmol/L of ${\mathrm{HCO}}_3^{-}$, the optimal energy efficiency could be increased to 1.91 g/kWh. The energy efficiency of the optimal O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system could reach 1.91 g/kWh, which is an impressive result.

Table S6 lists the comparison results of the kinetic constants and energy efficiencies of other degradation OTC processes and this process. The degradation efficiency of the Al₀-Gr-Fe₀/O₂ Fenton process for OTC was higher than that of other systems, close to 100%, but the energy efficiency was extremely low [40]. CO₃O₄/CNT catalyzed the degradation of OTC, and its degradation efficiency was close to O₃/CaO₂/${\mathrm{HCO}}_3^{-}$, but required a longer processing time and a relatively small kinetic constant [43]. Among the many degradation OTC processes, O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ is undoubtedly the most economical and has the most potential [41,42,44,45].

3. Materials and Methods

3.1. Chemicals

OTC, CaO₂, NaOH, triethylenediamine, terephthalic acid, sodium indigo disulfonate, BPA, and TMPO were all analytical grade (AR), and were bought from Aladdin Reagent (Aladdin Reagent (Shanghai) Co., Ltd., Shanghai, China). H₂SO₄ (98%) and H₃PO₄ were purchased from Sinopharm Chemical Reagent (Sinopharmaceutical Group Chemical Reagent (Shanghai) Co., Ltd., Shanghai, China). Indole, TC, and DMPO were obtained from Macklin (McLean biochemical Technology (Shanghai) Co., Ltd., Shanghai, China). NaHCO₃ of analytical grade (AR). was obtained from Lingfeng Chemical Reagent (Lingfeng Chemical Reagent (Shanghai) Co., Ltd., Shanghai, China). None of the chemicals required further purification. All of the solution was made with deionized water produced by an ultra-pure water system (Biosafer, Biosafer—10R, Saifei (Nanjing) Co., Ltd., Nanjing, China).

3.2. Experimental Process

A reactor schematic diagram is presented in Figure S6. The O₃ was generated by plasma discharge, which was named as plasma-O₃ generator (CQ, Changqing—802S). The amount of O₃ used was extracted and adjusted using an atmospheric sampler. The active species were characterized using an electron paramagnetic resonance spectrometer (BRUKER, MESRV30077, Brooke Technology (Beijing) Co., Ltd., Beijing, China). To monitor the OTC degradation process, 3 mL of the aqueous solution was taken out of the reaction mixture every 5 min, and the experiment was repeated several times to take the average value for the subsequent analysis. The best degradation conditions were as follows: the concentration of CaO₂ was 0.050 g/L, the dosage of O₃ was 0.75 g/h, the dosage of ${\mathrm{HCO}}_3^{-}$ was 2.25 mmol/L, the pH was 2.4, and the initial concentration of OTC was 40 mg/L.

3.3. Analysis

The OTC calculation and analysis process is provided in Text S1. For the detection of O₃, ·OH, and H₂O₂ in the OTC degradation process, the indigo method [58], the terephthalic acid probe method [59], and the titanyl sulfate method [60] were used, respectively. The data were analyzed and plotted with the origin (OriginLab Inc., version 9.8.0.200, Northampton, MA, USA) software.

3.4. DFT Analysis and Toxicity Evaluation

The molecular structures of the organic compounds studied were built using the Gauss View 6.0 software (Gaussian Inc., Wallingford, CT, USA). The built molecular structures were then optimized using chemical DFT methods (B3LYP/6-311++G (d, p)), employing Gaussian 16 (Gaussian Inc., Wallingford, CT, USA). Detailed information is shown in Text S2. US-EPA-TEST (US EPA, version 5.1.2, USA) was adopted to assess the toxicity of OTC and its intermediates based on the bioaccumulation factor and LC50 [57].

4. Conclusions

This study constructed O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ oxidation technology to degrade OTC wastewater. The introduction of CaO₂/${\mathrm{HCO}}_3^{-}$ significantly promoted the degradation of OTC in the O₃ process in terms of degradation efficiency and kinetic constants. However, an excessive CaO₂/${\mathrm{HCO}}_3^{-}$ dosage had an inhibitory effect on OTC degradation. The greater the O₃ flow rate, the lower the OTC concentration, and the higher the pH, which were beneficial for OTC decomposition. Active substances such as ·OH, ${·\mathrm{O}}_2^{-}$, ¹O₂, and ${·\mathrm{CO}}_3^{-}$ participated in the degradation of OTC. The enhanced degradation of OTC was mainly due to the high production of ·OH. After the treatment of the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system, TOC and COD had both declined. However, as the reaction time increased, the pH value was reduced, with or without catalysts. The conductivity was enhanced without a catalyst, while an opposite trend was observed for the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system. According to a DFT analysis, LC-MS, and related studies, the degradation pathways related to OTC degradation were proposed. After the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system treatment, the toxicity of OTC could be reduced. O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ is also feasible for the treatment of other antibiotics and actual wastewater. The energy efficiencies for OTC degradation under various systems were compared. This research showed that the O₃/CaO₂/${\mathrm{HCO}}_3^{-}$ system could be considered as an efficient and commercial oxidation process for OTC treatment.