Degradation of Antibiotics by Fenton-like Reaction Catalyzed by Iron Oxide

Abstract

In recent years, antibiotics have been widely used in the treatment of human and animal diseases due to their effectiveness. Most antibiotics enter the environment in the form of primitive or metabolites, making them new environmental pollutants, destroying the ecological environment, and endangering human health. The Fenton method is one of the advanced oxidation technologies including the traditional Fenton method and various Fenton methods. It has a good effect on the degradation of antibiotics in wastewater. Among them, the Fenton-like method has been widely studied by scholars because of its wide pH reaction conditions and better degradation efficiency. This article takes the Fenton-like reaction as the research object to study the catalytic and promotion effect of iron oxide as a catalyst on the degradation of antibiotics in the Fenton-like reaction. The experimental results show that the iron oxide catalytic system is much better than the traditional ferrous ion-catalyzed Fenton system to degrade antibiotics. Under certain experimental conditions, the degradation rate of ofloxacin in the Fe₃O₄-H₂O₂ reaction system for 72 hours reached 79.3%.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

1.1. Background

Water is a necessary condition for the reproduction of life on Earth. With the continuous development of society and industry, the amount of industrial wastewater discharge is increasing year by year, and more and more complex and biodegradable organic pollutants are discharged into the water through various channels. Antibiotics are one of them; because antibiotics can effectively kill some pathogens, they have been widely used in the treatment of various human and animal diseases in recent years. The presence of various antibiotics has been detected in the water environment. Although the content is low, their existence is persistent. The concentration of antibiotics in the environment will continue to increase, thereby posing a threat to the ecological environment and human safety. Fenton technology uses ferrous ions as a catalyst and hydrogen peroxide as an oxidant. Through the electron transfer between ferrous ions and hydrogen peroxide, hydrogen peroxide is activated and decomposed into ·OH. It can effectively remove organic matter, color, and odor in the leachate, reduce the toxicity of the leachate, and improve biodegradability. However, the traditional homogeneous Fenton reaction requires a higher pH value, a narrow range, and the catalyst is difficult to recycle, which will form iron sludge. In order to overcome its shortcomings, many Fenton-like reactions have been put into application research, among which the photocatalytic Fenton technology is relatively effective.

1.2. Significance

Antibiotics are recognized micropollutants in wastewater and surface water. They are particularly worrying because they can trigger an increase in drug-resistant bacteria. Therefore, there is an urgent need for new and efficient antibiotic removal technologies. It is of great significance to find an effective method to remove the antibiotics remaining in the environment. This article takes ofloxacin antibiotic as the target pollutant to study the degradation effect and degradation mechanism of iron oxide on the degradation of ofloxacin by the Fenton reaction, and provides practical guidance for the removal of antibiotics in actual sewage.

1.3. Related Work

Many scholars have conducted effective research on the effective and safe degradation of antibiotics. Biao-Jun constructed a heterogeneous Fenton-like reaction based on ferrocene and selected sulfamethazine (SMZ) as the target compound. The degradation kinetics, conversion pathways, and degradation products of SMZ in this system were studied. The results show that compared with the Fc, Fc + UV, H₂O₂, H₂O₂ + UV, and Fc + H₂O₂ systems, the Fc + H₂O₂+UV system has a higher degradation efficiency for SMZ [1]. His research may not be rigorous enough in experimental methods. Li et al. represented by sulfamethoxazole, introduced ferrocene (Fc) to establish a heterogeneous optical Fenton system for the degradation of sulfa antibiotics. The results show that the aniline moiety is the preferred reaction site for sulfamethoxazole, which has been confirmed by the formation of sulfamethoxazole hydroxylation products and dimers in the Fc-catalyzed photo-Fenton system [2]. This experiment is mainly aimed at the optimal reaction point, and the specific degradation result data are not clear. Dong H studied the degradation of rhodamine B in the F-/Cu(ii)/H₂O₂ system to evaluate the acceleration effect of F-on Cu(ii)-catalyzed Fenton-like reaction. The effects of F-/Cu²⁺ ratio, F- and Cu²⁺ concentrations, initial pH, temperature, and H₂O₂ and Rhodamine B concentrations were investigated [3]. The content of his experiment is more complicated, not too targeted, and the analysis is not logical. Meng et al. studied the heterogeneous Fenton-like degradation of phenanthrene in aqueous solution using Schwertmannite biosynthesized by Thiobacillus ferrooxidans LX5 as a catalyst. The effects of different reaction parameters such as catalyst loading, H₂O₂ concentration, initial solution pH, and inorganic anions on the degradation of phenanthrene Fenton were studied [4]. The research process is worthy of reference, but the results are still not specific enough. Alalm et al. investigated the effects of irradiation time, initial concentration, initial pH, and Fenton’s reagent dosage on the reaction. A cost estimate based on a large reactor was carried out [5]. Their experiment may have neglected the influence of other influencing factors during the experiment, so there may be some errors in the results. Fawzy et al. used to study the kinetics and mechanism of two β-lactam antibiotics with alkaline hexacyanoferrate oxide ampicillin and flucloxacillin by spectrophotometry at a fixed temperature. They found that the reaction kinetics follows the first-order dependence of the oxidant and the fractional first-order dependence of [α] and [OH]. The speckle test and Fourier transform infrared spectroscopy were used to identify the obtained oxidation products, and a possible oxidation mechanism was proposed [6]. The oxidation mechanism proposed in his experiment should theoretically achieve better degradation results, but no empirical analysis has been carried out. Zhou uses iron and ferrous iron, calcium peroxide, sodium percarbonate, and sodium persulfate to construct a Fenton-like system to remove sulfonamides. The results show that compared with other Fenton-like systems, the Fe3+/CP system has better degradation ability. According to the response surface results, the optimal conditions for SA removal in the Fe3+/CP system are obtained: [Fe3+] = 2.96 mM, [CaO2] = 2.33 mM, [pH] = 6.45 [7]. The research results are more meaningful and provide new ideas for the main research directions of this article.

1.4. Innovation

The use of advanced oxidation technology to treat antibiotic wastewater is a hot spot in the research of antibiotic degradation technology. Fenton catalysis is mostly used to remove difficult-to-degrade organic pollutants. The innovative points of the research on Fenton-like reactions in this article are the following: iron oxide is used as the catalyst of Fenton-like reaction. On the basis of the traditional Fenton reaction, the catalytic effects of different nano-iron oxides on the Fenton-like reaction are compared. The effect of small-molecule carboxylic acid on the promotion of the Fe3O4-catalyzed Fenton reaction was carried out. The results showed that its catalytic effect and its complex stability constant with iron ions are consistent.

2. Related Methods of Iron Oxide-Catalyzed Fenton Reaction to Degrade Antibiotics

2.1. Introduction to Antibiotics

Antibiotics are one of the most abused drugs in the medical industry, especially in the animal husbandry industry. Even if a small amount of antibiotics are used in the natural environment, it will increase the resistance of bacteria and even cause serious harm such as “super bacteria” [8]. The sources of surface water antibiotics include livestock and poultry breeding wastewater, medical wastewater, and sewage treatment plants. The transfer path is shown in Figure 1.

[figure(s) omitted; refer to PDF]

Antibiotics are pathogens or other metabolites of microorganisms (including bacteria, fungi, and actinomycetes) or higher animals and plants in the life process, which can inhibit the growth of living cells [9]. This article takes ofloxacin as the target pollutant to carry out Fenton-like catalytic reaction. Ofloxacin (OF) is a fluoroquinolone and is often used in diseases such as digestive tract bacterial infections. Its molecular structure is shown in Figure 2, with a carboxyl group and a piperazine ring [10].

[figure(s) omitted; refer to PDF]

2.2. Fenton Reaction

The Fenton reaction was first discovered in 1894 by Henry John Horstman Fenton (H. j. Fenton). He found that the mixture of ${\mathrm{H}}_2{\mathrm{O}}_2$ and ${\mathrm{F}\mathrm{e}}^{2+}$ can quickly oxidize malic acid and called this mixture the standard Fenton reagent. The Fenton reaction mechanism is mainly that ${\mathrm{H}}_2{\mathrm{O}}_2$ and ${\mathrm{F}\mathrm{e}}^{2+}$ generate highly oxidatively active hydroxyl radicals (·OH), which can nonselectively degrade many harmful substances into carbon dioxide and water as shown in formula (1). The Fenton oxidation process with metal ions or compounds as catalysts is considered to be a metal-catalyzed oxidation reaction, which is divided into homogeneous oxidation and heterogeneous oxidation [11]:$$\begin{array}{}\text{(1)}& {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2⟶{\mathrm{F}\mathrm{e}}^{2+}+·\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}\end{array}$$

2.2.1. Homogeneous Fenton Catalytic Reaction

The traditional homogeneous Fenton reaction mechanism is dominated by free radical mechanism. The main ${\mathrm{H}}_2{\mathrm{O}}_2$ is to adsorb to the surface of iron minerals, form complexes on the surface of iron minerals, and then initiate a series of chain reactions. The highly oxidizing $·\mathrm{O}\mathrm{H}$ generated in the reaction can oxidize most organic pollutants and toxic substances. The main steps are as follows:

  ${\mathrm{H}}_2{\mathrm{O}}_2$ and ${\mathrm{F}\mathrm{e}}^{2+}$ are complexed to form ${\mathrm{F}\mathrm{e}{\mathrm{H}}_2{\mathrm{O}}_2}^{2+}$:$$\begin{array}{}\text{(2)}& {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2⟶{\mathrm{F}\mathrm{e}{\mathrm{H}}_2{\mathrm{O}}_2}^{2+}\end{array}$$

(b) After electron transfer in the complex, ·OH is generated, during which ${\mathrm{F}\mathrm{e}}^{2+}$ is oxidized to ${\mathrm{F}\mathrm{e}}^{3+}$:$$\begin{array}{}\text{(3)}& {\mathrm{F}\mathrm{e}{\mathrm{H}}_2{\mathrm{O}}_2}^{2+}⟶{\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{O}\mathrm{H}}^{-}+·\mathrm{O}\mathrm{H}\end{array}$$

(c) ${\mathrm{F}\mathrm{e}}^{3+}$ reacts with ${\mathrm{H}}_2{\mathrm{O}}_2$ or $·{\mathrm{O}}_2\mathrm{H}$ to produce ${\mathrm{F}\mathrm{e}}^{2+}$, and the catalyst regeneration process realizes the continuous Fenton reaction:$$\begin{array}{c}\text{(4)}& {\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2⟶+{\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}^{+}+·{\mathrm{O}}_2\mathrm{H}{\mathrm{F}\mathrm{e}}^{3+}+·{\mathrm{O}}_2\mathrm{H}⟶+{\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}^{+}+{\mathrm{O}}_2\\ {\mathrm{H}}_2{\mathrm{O}}_2+·{\mathrm{O}}_2\mathrm{H}⟶{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+·\mathrm{O}\mathrm{H}\end{array}$$

(d) $·\mathrm{O}\mathrm{H}$ degrades the organic matter to be degraded. The main methods are the dehydrogenation of alkyl or hydroxyl groups as in the following formula and electron transfer as in formula (6):$$\begin{array}{}\text{(5)}& ·\mathrm{O}\mathrm{H}+\mathrm{A}-\mathrm{H}⟶\mathrm{A}·+{\mathrm{H}}_2\mathrm{O}\text{(6)}& ·\mathrm{O}\mathrm{H}+\mathrm{A}-\mathrm{H}⟶{\mathrm{A}-\mathrm{H}·}^{+}+{\mathrm{O}\mathrm{H}}^{-}\end{array}$$

The catalytic efficiency of homogeneous Fenton is not only affected by the concentration of ${\mathrm{H}}_2{\mathrm{O}}_2$ and the amount of catalyst, but also the inorganic ions in the reaction solution and the initial pH of the solution. Generally, the pH value decreases, the catalytic efficiency will increase, and the inorganic anions in the natural water will react with the highly oxidizing $·\mathrm{O}\mathrm{H}$ to generate some low-activity free radicals, which will reduce the free radical utilization rate and the oxidation capacity of the reaction system [12]. Therefore, the main disadvantage of the homogeneous Fenton reaction system is that a medium with a low pH value is required during the reaction, the catalyst cannot be recycled, and a large amount of iron sludge will be formed after the reaction [13].

2.2.2. Heterogeneous Fenton Reaction

The traditional homogeneous Fenton reaction has been widely used in the degradation of organic pollutants in environmental remediation. In order to overcome the shortcomings of the traditional homogeneous Fenton process such as the narrow pH working range and the production of a large amount of iron sludge, heterogeneous Fenton has received widespread attention. The heterogeneous Fenton system using solid catalysts such as iron-based materials can solve the above problems [14]. The reaction mechanism of heterogeneous Fenton is given as follows:

(a) The interaction of hydrogen peroxide forms a complex state (${\mathrm{H}}_2{\mathrm{O}}_2$)m on the surface of the goethite:$$\begin{array}{}\text{(7)}& {\mathrm{F}\mathrm{e}}^{3+}-\mathrm{O}\mathrm{H}+{\mathrm{H}}_2{\mathrm{O}}_2\leftrightarrow {\mathrm{H}}_2{\mathrm{O}}_2\end{array}$$

(b) With iron metal charge transfer to form transition iron complex ${\mathrm{F}\mathrm{e}}^{2+}·{\mathrm{O}}_2\mathrm{H}$,$$\begin{array}{}\text{(8)}& H_{\mathit{2}}{O}_{\mathit{2}}\mathrm{m}\leftrightarrow {Fe}^{\mathit{2}+}·{\mathrm{O}}_2\mathrm{H}+{\mathrm{H}}_2\mathrm{O}\end{array}$$

(c) The iron complex dissociates to form ${\mathrm{H}\mathrm{O}}_2·$ and ${\mathrm{F}\mathrm{e}}^{2+}$:$$\begin{array}{}\text{(9)}& {\mathrm{F}\mathrm{e}}^{2+}·{\mathrm{O}}_2\mathrm{H}⟶{\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}\mathrm{O}}_2\end{array}$$

(d) ${\mathrm{F}\mathrm{e}}^{2+}$ and ${\mathrm{H}}_2{\mathrm{O}}_2$ react to produce ${\mathrm{F}\mathrm{e}}^{3+}$ and $·\mathrm{O}\mathrm{H}$, same as (1).

Compared with the traditional homogeneous Fenton, the advantages of the heterogeneous Fenton process are as follows: first, the amount of iron slag is low, the second is a wide pH working range, and the third is that the catalyst can be reused.

2.3. Fenton-like Reaction

In order to overcome the disadvantages of the traditional Fenton method such as high consumption of ${\mathrm{H}}_2{\mathrm{O}}_2$, low ${\mathrm{F}\mathrm{e}}^{2+}$ production, high processing cost, and insufficient organic mineralization, many Fenton-like technologies such as organic complex modification-type Fenton method, and ultrasonic-type Fenton method are derived . They are all based on the traditional Fenton method and integrated into some other advanced oxidation technologies, which can achieve more effective and economical treatment effects [15]. Fenton-like technology also includes homogeneous Fenton technology and heterogeneous Fenton technology. However, the homogeneous Fenton technology is similar to the traditional Fenton technology. Because soluble metal ions are used as catalysts to carry out the degradation reaction of organic substances in the solution, some problems still exist; the degraded substances cannot be directly discharged into nature, which may easily cause secondary pollution. Therefore, the heterogeneous Fenton technology has received extensive attention from researchers [16].

The heterogeneous Fenton oxidation technology is to load ${\mathrm{F}\mathrm{e}}^{3+}$ or other metal ions instead of ${\mathrm{F}\mathrm{e}}^{2+}$ on a solid support [17]. $·\mathrm{O}\mathrm{H}$ is produced by electron transfer between ${\mathrm{H}}_2{\mathrm{O}}_2$ and the variable valence transition metal in the catalyst. The main mechanism is as follows: ${\mathrm{H}}_2{\mathrm{O}}_2$ and trivalent iron on the surface of the solid catalyst form ${{\mathrm{F}\mathrm{e}}^{3+}-\mathrm{H}}_2{\mathrm{O}}_2$, and electron transfer generates ${\mathrm{F}\mathrm{e}}^{2+}$ in the reduced state and then ${\mathrm{F}\mathrm{e}}^{2+}$ reacts with ${\mathrm{H}}_2{\mathrm{O}}_2$ to generate $·\mathrm{O}\mathrm{H}$ [18]. As shown in formulas (10) to (14), in addition to the catalyzed formation of $·\mathrm{O}\mathrm{H}$ by variable valence transition metals, the oxygen holes in the catalyst can also react with hydrogen peroxide to obtain $·\mathrm{O}\mathrm{H}$:$$\begin{array}{}\text{(10)}& {\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2⟶{{\mathrm{F}\mathrm{e}}^{3+}\_\mathrm{H}}_2{\mathrm{O}}_2\text{(11)}& {\mathrm{F}\mathrm{e}}^{3+}·{\mathrm{O}}_2\mathrm{H}/{\mathrm{O}}_2^{·-}⟶{\mathrm{O}}_2/{\mathrm{H}}^{+}\text{(12)}& {\mathrm{F}\mathrm{e}}^{2+}·\mathrm{O}\mathrm{H}⟶{\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{O}\mathrm{H}}^{-}\text{(13)}& {\mathrm{H}}_2{\mathrm{O}}_2+·\mathrm{O}\mathrm{H}⟶·{\mathrm{O}}_2\mathrm{H}+{\mathrm{H}}_2\mathrm{O}\text{(14)}& {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}^{+}/2{\mathrm{H}}^{+}+·{\mathrm{O}}_2\mathrm{H}/{\mathrm{O}}_2^{·-}⟶{\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\end{array}$$

The following introduces several commonly used Fenton-like reaction catalysis principles.

2.3.1. Photo-Fenton Reaction Based on Photocatalytic Oxidation

Photocatalytic oxidation is an advanced oxidation technology that degrades antibiotics in water. That is, under the condition of light, the molecule transitions from the steady state to the excited state by absorbing the electromagnetic radiation of a specific wavelength in the radiated light, thereby causing a chemical reaction to complete the degradation process of organic matter. Under light, the photocatalyst absorbs photons and catalyzes the generation of electrons and holes [19]. The dissolved oxygen and water molecules adsorbed on the surface of the catalyst interact with electrons and holes to form free radicals and hydroxyl free radicals. The schematic diagram is shown in Figure 3.

[figure(s) omitted; refer to PDF]

Under the irradiation of ultraviolet light, the ${\mathrm{F}\mathrm{e}}^{3+}$ in the reaction system is reduced and regenerated to ${\mathrm{F}\mathrm{e}}^{2+}$, and the newly generated ferrous ions react with hydrogen peroxide to produce hydroxyl radicals and iron ions. This cycle is repeated, and the main reactions are$$\begin{array}{c}\text{(15)}& 4\mathrm{F}\mathrm{e}{\mathrm{H}\mathrm{O}}^{2+}+\mathrm{h}\mathrm{v}⟶4{\mathrm{F}\mathrm{e}}^{2+}\mathrm{H}{\mathrm{O}}_2·+\mathrm{H}\mathrm{O}·+{\mathrm{H}}_2\mathrm{O}{\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2⟶{\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{O}\mathrm{H}}^{-}+\mathrm{H}\mathrm{O}\\ {\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2⟶{\mathrm{F}\mathrm{e}{\mathrm{H}\mathrm{O}}_2}^{2+}+{\mathrm{H}}^{+}\\ {\mathrm{F}\mathrm{e}{\mathrm{H}\mathrm{O}}_2}^{2+}⟶{\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2\mathrm{O}\end{array}$$

2.3.2. Electrochemical Catalytic Fenton Reaction (Elector-Fenton)

Electro-Fenton introduces an electrochemical method on the basis of the conventional Fenton method. The dissolved oxygen is converted into ${\mathrm{H}}_2{\mathrm{O}}_2$ through the electrochemical method, and then, the Fenton reagent is formed with the ferrous ions or ferric ions produced by electrochemical sacrifice, realizes a good cycle of ferrous ions and ferric ions, and promotes the conversion of hydrogen peroxide to hydroxyl radicals to degrade organic matter [20]. Agrochemical methods include direct oxidation on the anode surface and indirect oxidation in solution. This article mainly introduces the EF-electric Fenton reaction method, which is the cathode electric Fenton method, in which the ${\mathrm{O}}_2$ produced by the oxidation of water at the anode is reduced to hydrogen peroxide at the cathode of the electrolytic cell, and then, Fenton reacts with ferrous ions. The main reaction is given as follows.

Overall response:$$\begin{array}{}\text{(16)}& {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}⟶4\mathrm{O}\mathrm{H}\end{array}$$

Anode:$$\begin{array}{}\text{(17)}& 2{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\end{array}$$

Cathode:$$\begin{array}{c}\text{(18)}& \mathrm{F}\mathrm{e}{\mathrm{O}\mathrm{H}}^{2+}+{\mathrm{e}}^{-}⟶{\mathrm{F}\mathrm{e}}^{2+}+\mathrm{O}{\mathrm{H}}^{-}{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}⟶{\mathrm{H}}_2{\mathrm{O}}_2\end{array}$$

The method does not need to add additional hydrogen peroxide and is mainly obtained from the cathode reaction. While thoroughly degrading organic matter, the reaction process does not produce toxic substances and is more efficient in terms of organic matter removal rate and improved biodegradability. However, this method has certain limitations and requires higher acidity. At present, due to the limitation of electrode materials, there is a problem of low current efficiency and the amount of hydrogen peroxide produced may be insufficient. Therefore, the use of this method is not very widespread [21].

2.3.3. Ultrasound-Assisted Fenton Reaction (US-Fenton)

The research and application of ultrasonic radiation or ultrasonic decomposition are few. At a given ultrasonic frequency, the bubble reaches an equilibrium size within a given time [22]. In the subsequent compression cycle, the bubble bursts, providing energy for chemical and physical reactions. When combined with the Fenton reagent during ultrasonic treatment, more single-electron oxidants or free radicals will be generated in the liquid phase, and the water vapor entering the bubbles will split and chain reaction to produce hydroxyl radical OH, which in turn OH generates hydrogen peroxide:$$\begin{array}{c}\text{(19)}& {\mathrm{H}}_2\mathrm{O}+\mathrm{U}\mathrm{S}⟶\mathrm{O}\mathrm{H}+\mathrm{H}\mathrm{O}\mathrm{H}+\mathrm{O}\mathrm{H}⟶{\mathrm{H}}_2{\mathrm{O}}_2\end{array}$$

The ultrasonic Fenton method has an obvious synergistic effect and can effectively promote the decomposition of hydrogen peroxide, and the catalyst is evenly distributed. Mechanical shock can play a role in stirring and mass transfer. On the other hand, the hydrogen peroxide in Fenton’s reagent makes the solution saturated with oxygen, which is beneficial to the ultrasonic cavitation process. However, due to the low energy utilization rate of Fenton ultrasound technology, the future development direction should be to improve the energy utilization rate of ultrasound and reduce processing costs.

Through the above analysis of the basic working principles and advantages and disadvantages of the conventional Fenton method and various Fenton methods, we use Table 1 to make a comparison. Based on the comprehensive research on the advantages and disadvantages of the above several antibiotic wastewater treatment technologies, this topic chooses the lowest operating cost, convenient operation, no secondary pollution, and good wastewater treatment effect photocatalytic oxidation technology to degrade antibiotics.

Table 1

Comparison of the Fenton method and Fenton-like method.

Method	Handling cost	Operation difficulty	Handling effect
Fenton	High	Low	Good
Photo-Fenton	Lower	Higher	Better
Elector-Fenton	Higher	Higher	Better
US-Fenton	High	High	Better

2.4. Iron Oxide Catalysis Fenton

Iron oxide is the most common type of heterogeneous Fenton catalyst. Its catalytic activity is related to crystal form, iron valence, etc. The heterogeneous Fenton technology with iron oxide as a catalyst can effectively degrade the antibiotic ofloxacin. Among iron oxides, triiron tetroxide, ferric oxide, and carboxyl iron oxide are the most widely studied iron oxide catalysts. Among them, the chemical stability and thermal stability of ferric oxide are good; carboxyl iron oxide has a wide applicable pH value and generally exists in the form of goethite. Fe₃O₄ has higher catalytic activity in Fenton-like catalytic systems. The smaller the nanometer size, the higher the catalytic activity and the better the magnetic properties. Therefore, this article chooses ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$ as the Fenton-like reaction catalyst for research.

3. ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$ Heterogeneous Fenton Reaction Degradation of Antibiotics

3.1. Ofloxacin

Ofloxacin solution (30 mg/L) is prepared, weighed 0.03 g ofloxacin, dissolved in 100 mL deionized water, transferred to a 1000-mL volumetric flask, added deionized water to the mark, and placed it in a dark place for later use.

Each 2 mg/L, 5 mg/L, 10 mg/L, 20 mg/L, 30 mg/L, and 40 mg/L of loxacin solution is prepared, and the standard solution is measured with UV-Vis spectrophotometer. The ultraviolet absorbance curve of ofloxacin in the interval of 200∼400 nm is shown in Figure 4(a). The absorbance value was measured at the maximum absorption wavelength of 293 nm, and the concentration (C)-absorbance (O) standard curve of the ofloxacin antibiotic was obtained by calculation, as shown in Figure 4(b). In the Fenton-like catalytic oxidative degradation process, by measuring the absorbance value (O) of the supernatant solution without the catalyst, the concentration C of the undegraded ofloxacin in the solution is calculated by formula (20). The degradation efficiency of ofloxacin is expressed by formula (21), where is the initial concentration of ofloxacin:$$\begin{array}{}\text{(20)}& \rho =1-\frac{\mathrm{C}}{{\mathrm{C}}^0}\text{,}\text{(21)}& \mathrm{O}=0.08948\ast \mathrm{C}+0.00193\text{.}\end{array}$$

[figure(s) omitted; refer to PDF]

3.2. ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$ Catalyst Characterization

Organic carboxylic acid molecules can undergo complex reactions with metal ions, as well as metal atoms on the surface of oxide catalysts, thereby affecting the chemical bonds and physicochemical properties of the catalyst surface and the interaction between the surface metal ions and ${\mathrm{H}}_2{\mathrm{O}}_2$ and the mutual conversion between metal ions of different valences, thereby affecting the catalytic performance of the catalyst. This article selects four common organic carboxylic acids: tartaric acid (KTA), citric acid (CA), ethylenediaminetetraacetic acid (EDTA), and humic acid (HA) that can undergo complex reaction with ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$surface ${\mathrm{F}\mathrm{e}}^{3+/2+},$ and add into the Fenton reaction system. The infrared absorption spectra of ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$, ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$-KTA, ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$-CA, ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$-EDTA, and ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$-HA are shown in Figure 5.

[figure(s) omitted; refer to PDF]

The magnetization curve of magnetite is shown in Figure 6. The hysteresis loop of magnetite is long and narrow. When the magnet is placed next to the suspension, solid-liquid separation can occur quickly. It shows that the magnetism is better. When a magnetic field is applied, the catalyst can be quickly separated from the solution.

[figure(s) omitted; refer to PDF]

3.3. The Results of Factors Affecting the Degradation of the Antibiotic Ofloxacin by the Fenton-like Reaction Catalyzed by Iron Oxide

3.3.1. The Influence of the Type of Iron Oxide on the Degradation Rate of Ofloxacin

In order to investigate the degradation of ofloxacin by Fenton-like systems catalyzed by different nanometer iron oxides, the experiment was conducted at a reaction temperature of 26°C, ofloxacin initial concentration of 5 mg/L, and ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration of 25 ml/L. Under the condition of pH 4.2 citric acid buffer, 5.5 g/L of different types of nano-iron oxides was added to form different reaction systems (Fe₃O₄ + H₂O₂, α-Fe₂O₃ + H₂O₂, FeOOH + H₂O₂, and Fe²⁺⁺H₂O₂). The result of ofloxacin degradation after 72 hours of the reaction is shown in Figure 7. It can be found that the Fenton-like reaction system in which iron oxides and H₂O₂ coexist has a better degradation effect on ofloxacin than the traditional Fenton reaction system, and the catalytic activity of different iron oxides is different. The degradation rate of ofloxacin in the Fe₃O₄ + H₂O₂, α-Fe₂O₃ + H₂O₂, FeOOH + H₂O₂ reaction system reached 79.3%, 72.5%, and 66.3% after 72 hours of reaction.

[figure(s) omitted; refer to PDF]

3.3.2. The Effect of the Initial Concentration of Ofloxacin on the Degradation Rate of Ofloxacin

In the experiment, the reaction temperature is 26°C, ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration is 25 ml/L, pH value is 4.2 citric acid buffer, and 1.5 g/L Fe₃O₄ is added to the system of different initial concentrations of ofloxacin to react for 72 h. The curve of the effect of different initial concentrations of ofloxacin on the degradation rate of ofloxacin is shown in Figure 8. It can be clearly seen that as the initial concentration of ofloxacin increases, the degradation rate of ofloxacin decreases. When the concentration of ofloxacin was 2 mg/L, the degradation rate of ofloxacin in 72 h was 98.9%. When the initial concentration of ofloxacin was increased to 5 mg/L and 10 mg/L, the degradation rate of ofloxacin in 72 h was 95.7% and 92.4%, respectively. However, when the initial concentration of ofloxacin was increased to 50 mg/L, the degradation rate of ofloxacin was only 70.3% in 72 h. Overall, for low concentrations of ofloxacin, the catalyst dosage conditions determined in this experiment can have a better degradation effect.

[figure(s) omitted; refer to PDF]

3.3.3. The Effect of Initial pH on the Degradation Rate of Ofloxacin

Supposing the dosage of magnetite is 1.5 g/L, the reaction temperature is 26°C, the initial concentration of ofloxacin is 2 mg/L, 5 mg/L, and 10 mg/L, and the ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration is 25 ml/L. The pH of the reaction solution was adjusted to 3.5, 4.7, 6.2, 7.3, 9.2, and 10.8 and the degradation rate of ofloxacin for 72 hours of reaction is shown in Table 2. From the comparative analysis of the data in Table 2, it can be seen that when the initial pH value of the catalytic reaction system increases from 3.5 to 10.8, the degradation rate of ofloxacin in 72-h reaction decreases from 99.12% to 51.67%.

Table 2

The effect of initial pH on the degradation rate of ofloxacin.

Concentration pH value (mg/L)	3.5 (%)	4.7 (%)	6.2 (%)	7.3 (%)	9.2 (%)	10.8 (%)
2	99.12	87.64	82.57	77.93	68.74	62.25
5	95.43	84.79	81.54	74.22	61.77	57.43
10	89.74	81.39	78.85	70.75	58.83	51.67

3.3.4. The Effect of ${\mathrm{H}}_2{\mathrm{O}}_2$ Concentration on the Degradation Rate of Ofloxacin

Fixed reaction conditions: Fe₃O₄ dosage is 1 g/L, pH is 4.2, ofloxacin concentration is 5 mg/L, reaction temperature is 26°C, and hydrogen peroxide concentration is set to 10 ml/L, 25 ml/L, 35 ml/L, and 45 ml/L. The effect of the concentration of H₂O₂ on the degradation rate was investigated. The result of the 72-h reaction is shown in Figure 9. The concentration of H₂O₂ will affect the production of reactive substances and the degradation efficiency of ofloxacin, from 10 ml/L to 45 ml/L. As the concentration of H₂O₂ increases, the degradation effect of the catalyst on ofloxacin gradually increases. When the concentration of H₂O₂ is in a low concentration range, as the concentration of hydrogen peroxide increases, the probability of contact between H₂O₂ and iron ions on the surface of iron oxide increases, which can produce more ·OH, which can increase the degradation rate of ofloxacin. However, when the concentration of H₂O₂ reaches a certain range, the degradation rate does not continue to increase, possibly because too much H₂O₂ cannot be effectively catalyzed and decomposed.

[figure(s) omitted; refer to PDF]

3.3.5. The Impact of ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$ Input on the Degradation Rate of Ofloxacin

The experimental reaction temperature was 26°C, the initial concentration of ofloxacin was 2 mg/L, the ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration was 25 ml/L, and the pH value was 4.2. Under different dosages of Fe₃O₄, after 72 hours of reaction, the degradation rate of ofloxacin over time is shown in Figure 10. It can be seen from Figure 10 that when the dosage of Fe₃O₄ is increased from 0.7 g/L to 1.5 g/L, the degradation rate of ofloxacin is increased, and the degradation rate of ofloxacin is 98.2% after 72 hours. When the amount of Fe₃O₄ added continues to increase, its degradation rate begins to decrease. This indicates that increasing the dosage of magnetite within a certain concentration range will increase the Fenton-like catalytic activity, but when the dosage exceeds the optimal dosage, the degradation rate will decrease.

[figure(s) omitted; refer to PDF]

3.3.6. The Effect of Small Molecule Carboxylic Acid on Promoting ${\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$-Catalyzed Fenton Reaction

The molecular structure and molecular weight of carboxylic acids are different, so the complexing ability with iron ions is also different. In this section of the experiment, the four small-molecule organic carboxylic acids (KTA, CA, EDTA, HA) selected in this article have good complexing capabilities for iron ions. Table 3 shows the complex constants of each carboxylic acid with Fe²⁺ and Fe³⁺ under the same conditions. Generally speaking, the larger the complex constant, the easier it is to form a complex. Table 3 shows the complex constants of each carboxylic acid and Fe²⁺/Fe³⁺ under the same conditions.

Table 3

Complex stability constants of Fe²⁺/Fe³⁺ with various carboxylic acids.

Type	Fe2+	Fe3+
KTA	—	7.49
CA	15.5	25.0
EDTA	14.83	24.23
HA	6.89	8.53

The experimental conditions were set as the reaction temperature of 26°C, pH value of 5.6, ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration of 25 ml/L, ofloxacin concentration of 15 mg/L, Fe₃O₄ concentration of 1.5 g/L, and reaction time of 120 min. Under the same experimental conditions, the promotion of the degradation of ofloxacin antibiotics by the Fe₃O₄ + H₂O₂ reaction system of four small-molecule organic carboxylic acids at the same concentration (all 0.5 mMol) was compared. The results are shown in Figure 11. It can be observed that several small-molecule carboxylic acids have a promoting effect on the reaction system. Under the same conditions, ethylenediaminetetraacetic acid (EDTA) has the most obvious promotion effect, followed by citric acid (CA), and then tartaric acid (KTA) and humic acid (HA). Compared with the reaction system without adding small-molecule carboxylic acid, adding ethylenediaminetetraacetic acid (EDTA), after 120 minutes of reaction, increases the degradation rate of ofloxacin by 34.1%.

[figure(s) omitted; refer to PDF]

4. Discussion

The main research content of this article is to use iron oxide as a catalyst to form a Fenton-like reaction to degrade antibiotics. The Fenton reaction is an advanced oxidation technology. The article systematically introduced the source and harm of antibiotics, the traditional Fenton reaction of homogeneous Fenton catalysis and heterogeneous Fenton catalysis, common Fenton-like technology (including photocatalytic oxidation Fenton, electrochemical catalysis Fenton, ultrasonic-assisted Fenton), and the advantages and disadvantages of various iron oxides as heterogeneous Fenton catalysts. The summary shows that the traditional Fenton reaction degrades ofloxacin with high oxidation efficiency, but the pH range of the reaction system is relatively narrow, the reaction will produce iron sludge, and the amount of hydrogen peroxide is high. So replacing ferrous ions with iron oxides can overcome the shortcomings of the traditional Fenton reaction. Among iron oxides, α-Fe₂O₃ and Fe₃O₄ have better catalytic performance. In the experimental part of this article, the factors affecting the degradation of ofloxacin antibiotics catalyzed by the Fenton-like reaction of iron oxide were studied. Firstly, the promotion effect of different nano-iron oxide catalysis on the degradation of antibiotics in Fenton-like reactions was investigated. Under the same experimental conditions, it is concluded that the degradation rate of the Fe₃O₄-H₂O₂ reaction system is better. Then, other factors affecting the degradation of antibiotics in the Fenton-like reaction catalyzed by iron oxides were studied; for example, the initial concentration of the target reactant, the initial pH value of the reaction system, the H₂O₂ concentration, the amount of Fe₃O₄, and the type of small-molecule carboxylic acid affect the promotion of Fe₃O₄ catalyzed Fenton reactions. The results show that with the increase in the initial concentration of ofloxacin, the degradation rate of ofloxacin decreases; acidic conditions can help the occurrence of catalytic reactions and increase the degradation rate.

5. Conclusions

The main research content of this article is to use iron oxide as a catalyst to form a Fenton-like reaction to degrade antibiotics. After sorting out the research content of this article and overall analysis, it is found that there are still some shortcomings in the content of the article and actual research: the analysis and summary of the characteristics and functions of various iron oxide catalysis are not in place; In the experiment, the discussion on the pH value is not compared with the traditional Fenton reaction. Does the Fenton-like reaction broaden the pH reaction range while ensuring the degradation efficiency. The types of pollutants in the actual wastewater are very many and complex. The paper adopted in this article is still biased towards the pure water system, which is still somewhat different from the actual wastewater. For the degradation mechanism and methods of antibiotics, continuous experiments are still needed to explore more effective degradation methods.

Acknowledgments

This work was supported by the Natural Science Foundation of Hebei Province (B2016202290).

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