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Environ Eng Res > Volume 25(3); 2020 > Article
Zouanti, Bezzina, and Dhib: Experimental study of degradation and biodegradability of oxytetracycline antibiotic in aqueous solution using Fenton process


The degradation of aqueous oxytetracycline (OTC) from an aqueous solution antibiotic using H2O2/Fe2+ process was studied in one 1 L batch chemical reactor. The extent of OTC degradation (20 mg/L) was investigated from a known initial pH solution, temperature and the type of catalyst (Fe2+, Fe3+) and for various initial concentrations of OTC, H2O2 and Fe2+. The degradation efficiency achieved was found to be very important (90.82% and 90.63%) at initial pH solution of 3 and 4, respectively. However, the type of catalyst and the reaction temperature had a slight impact on the final degradation of OTC. The results showed that the OTC removal increased with increasing initial H2O2 concentration in the range of 70 to 150 mg/L and initial Fe2+ concentrations in the range of 2 to 5 mg/L. The highest degradation efficiency obtained at ambient temperature was 90.95% with initial concentration of OTC of 10 mg/L, H2O2 = 150 mg/L and Fe2+ = 5 mg/L. Moreover, biodegradability improved from 0.04 to 0.36 and chemical oxygen demand degradation was 78.35% after 60 min of treatment. This study proved that Fenton process can be used for pretreatment of wastewater contaminated by OTC before a biological treatment.

1. Introduction

Effluents of waste products coming from pharmaceutical industry enter environmental ecosystems through various channels such as pharmaceutical equipments, wastewater treatment plants and medical center wastes [1,2]. During the last few decades, the production and consumption of pharmaceutical products have drastically increased due to the considerable usage of medical products by the fast growing world population. Today, large quantities of medical products are produced each year for human and animal healthcare [3]. As a result, huge quantities of wastewaters are proportionally generated in pharmaceutical industries and medical centers. These relevant wastes are characterized by a high biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids and toxicity. Besides, the effluents contain important amounts of toxic and persistent compounds such as organic solvents, catalyst, reactants and raw materials [47].
Antibiotics are among the most widely used drugs for the treatment of diseases. In particular tetracyclines (TCs) stand out as one of the most important antibiotic groups in terms of high production and worldwide distribution [8]. TCs are broad-spectrum of bacteriostatic antibiotics that are effective in treating infections caused by certain strains of Gram-negative and Gram-positive, which explains their wide use in the treatment of infectious diseases for humans and animals [9]. In fact, the most extensively used TCs are oxytetracycline (OTC), TC, chlortetracycline (CTC), and doxycycline (DC). TC is frequently used in aquaculture and veterinary [10], whereas OTC and CTC are widely administrated as growth promoters. Therefore, it is possible to find these antibiotics discharged into the environment. For instance, the concentration of TCs detected in surface water is in the range of 0.11 to 4.20 μg/L and in wastewater treatment plant (WWTP) effluents with concentrations of 46–1,300 ng/L for TC, 270–970 ng/L for CTC, and 240 ng/L for OTC [11]. Kolar et al. [12] found that OTC was moderately toxic to organisms of activated sludge (EC5017.9 mg/L). Therefore, it is necessary to develop appropriate techniques for the removal of these pollutants from contaminated wastewaters for preserving the environment.
Biological processes are quite economical for the treatment of wastewaters and they are widely studied. But, they have been proven to be ineffective in the removal of persistent or non-biodegradable compounds [13]. For instance, biological treatment processes are not successful in effectivelly removing TCs antibiotics [14]. Alternatively, physico-chemical techniques such as reverse osmosis, adsorption, ultrafiltration and coagulation are also used for the treatment of wastewater, however these treatments transfer only the pollution from one phase to another, which subsequently requires further treatment [13, 15]. Therefore, it is necessary to develop more effective treatment methods such as chemical and photochemical degradation to remove completely contaminants.
For instance, advanced oxidation processes (AOPs) are proven to be very effective techniques for the degradation of a wide range of refractory pollutants in aqueous solution such as polymers [16], phenol [17], pesticides and dyes [18, 19]. These techniques are based on the generation of highly reactive radicals, such as hydroxyl radicals (OH) and hydroperoxyl radicals (HO2) which can react effectively with organic pollutants and reduce intermediate reaction species to CO2 and H2O without producing harmful by-products or sludges that must require further processing [3, 20].
Several researchers have studied the degradation of OTC (Fig. S1) by different AOPs methods such as ultraviolet (UV)/H2O2 [19, 21, 22], photo-Fenton [23], gamma radiation [11], anodic oxidation [24], pulse radiolysis [25], Ozone process [26] and photocatalysis UV/TiO2 [27]. However, few studies focus on the mineralization and biodegradability of antibiotics using Fenton process.
In the present work, the degradation of OTC solution is investigated using Fenton process in a batch chemical reactor. The main aim of this study is to analyze how the degradation process of OTC can be affected by initial concentrations of OTC, H2O2, Fe2+, and pH as well as temperature and catalyst type. The mineralization and the biodegradability of antibiotic were also investigated.

2. Experimental

2.1. Reagents

OTC hydrochloride (> 95%) was purchased from Sigma-Aldrich and was used without further purification. 50% (w/w) of hydrogen peroxide (H2O2) was purchased from Merck (Germany) and was used by diluting it to 35% (w/w). Analytical grade ferrous sulfate heptahydrate (FeSO4. 7H2O) was purchased from Biochem (France). Ferric sulfate (Fe2(SO4)3) with purity exceeding 90% was obtained from Prolabo (France). The pH of the solution was adjusted at the beginning with either dilute H2SO4 (0.5 N) and NaOH (0.1 N).

2.2. Experimental Setup and Analysis

All experiments were carried out in a double jacket 1 L stirred batch reactor as shown in Fig. 1. The reactor was covered with aluminum paper to prevent the degradation of OTC by the photo- Fenton system. The reactor was connected to a thermostatic bath to maintain a constant temperature of 25°C. The reactor was also equipped with a magnetic stirrer to ensure a homogeneous reaction medium and a uniform concentration distribution in the reactor. A thermometer was placed in the reactor for temperature measurement and a port for sampling. The pH level was adjusted using a pH-meter (HANNA pH 209) at the onset of each experiment and no further pH adjustment was made during the reaction. The description and operating conditions of the reactor are given in Table 1.
For each experiment, OTC solution was prepared by dissolving 10 mg of OTC in a 500 mL flask containing distilled water. The mixture was stirred at 600 rpm for 20 min to make a homogenous stable solution. The desired concentration of Fe2+ was added to the solution of OTC. Specific amounts of H2SO4 or NaOH were injected for pH solution adjustment. The desired amount of H2O2 was poured into the reactor and the temperature kept constant at 25°C.
Samples were taken from the reactor at different time intervals and were immediately analyzed after being filtered in micropore disk (PES D.25 mm) of porosity of 0.45 μm. The degradation of OTC was evaluated upon measuring the absorbance of the treated solution with a spectrophotometer UV-VIS (SELECTA UV-2005). The maximum absorption wavelength is 353 nm. A calibration curve relating the absorption signal to OTC concentration was established. The UV absorption spectrum of OTC is provided in Fig. S2. The percentage of OTC removal was calculated as follows:
Where C0 is the initial concentration of OTC and Ct is the concentration of OTC at reaction time. COD was analyzed according to the method detailed by Thomas and Mazas [28], using a dichromate solution as oxidizer in a strong acid medium. Two (2 mL) of the test solution was mixed with dichromate reagent and digested for 2 h at 150°C. The absorbance for the color change of dichromate solution at λ = 440 mm was determined with a UV-VIS spectrophotometer. If the sample contained hydrogen peroxide (H2O2), to reduce interference in COD determination pH was increased to above 10 to decompose hydrogen peroxide to oxygen and water [29]. A cooled incubator (VELP Scientifica, FOC 120I) with BOD sensor was used to measure the value of BOD for five days (BOD5).

2.3. Reaction Mechanism

The mechanism of the OTC degradation in the H2O2/Fe2+ and H2O2/Fe3+ process is based on the generation of OH upon decomposition of H2O2 in presence of Fe2+ or Fe3+. These OH are powerful oxidants, nonselective and highly reactive with organic matter. OTC reacts instantly with OH and produces intermediate products which sequentially react with the OH radicals and undergo a series of oxidation reactions up to total mineralization of the OTC into H2O and CO2 (Eq. (2)). OTC can also react with HO2 and generate other byproducts as given in Eq. (3).
OTC+OH Products CO2+H2O
OTC+HO2Products CO2+H2O

3. Results and Discussion

Using the experimental conditions in Table 1, several experimental tests were conducted to investigate the effect of reaction temperature and of initial concentration of the reactants on the reactor performance to degrade the organic compound considered.

3.1. Role of •OH Radicals in OTC Degradation

First, the results shown in Fig. 2 are very interesting and clearly exhibit the impact of H2O2 and Fe2+ in the degradation of OTC when they are used separately or combined. The contribution of Fe2+ in the generation of OH radicals leads to an oxidation of 90.67% of OTC compared to 12.22% when H2O2 is alone. Hence, this result highlights the significant role of OH radicals in the elimination of OTC in H2O2/Fe2+ process. The degradation rate of OTC was much higher in H2O2/Fe2+ system than in H2O2 alone, which is probably due to the production of substantial amount of highly reactive radicals (OH) that are mainly can disintegrate molecules of OTC antibiotic as OH have a high oxidation potential (E0 = 2.8 V).

3.2. Effect of Initial H2O2 Concentration

Hydrogen peroxide dosage is an important factor to evaluate the performance of Fenton process. Thus, the concentration of H2O2 was varied from 70 mg/L to 200 mg/L for an invariant initial concentration of OTC and ferrous ion of 20 mg/L and 5 mg/L, respectively. The lowest concentration of H2O2 (70 mg/L) that was required to start the OTC degradation was based on the necessary stoichiometric ratios of the reaction to completely oxidize 20 mg/L of OTC according to the reaction below:
Plots in Fig. 3(a) describe the degradation of OTC at various initial concentrations of H2O2 and show that the rate of degradation of OTC increases with higher concentration of H2O2. In the presence of Fe2+, hydrogen peroxide decomposes much more effectively and produces more free radicals which can immediately react with OTC. It is obvious that with higher H2O2 concentration, there is a higher chance to get more OH in the reacting system.
However, no enhancement was observed when the initial concentration of H2O2 exceeded 150 mg/L beyond a reaction time of 40 min. This result is quite interesting. It is due to an excess of hydrogen peroxide in the solution which leads to a scavenging phenomenon on OH as shown in Eq. (5).
This reaction produces more HO2 that have an oxidation potential E0 of 1.7 V. It is therefore less reactive than OH radicals and results in lowering the consumption rate of OTC. The highest consumption of H2O2 occurred during the first 30 min of degradation for all tests with different H2O2 initial concentrations. Elsewhere, Subramonian et al. [30] observed that a drop of COD removal occurred during the treatment of pulp and paper mill effluent when higher H2O2 was used due to the scavenging effect.

3.3. Effect of Initial Catalyst (Fe2+) Concentration

Further, the presence of the catalyst in Fenton process for the generation of OH plays an important role as shown in the reaction below:
In this work, the impact of the catalyst Fe2+ was studied over the range of 2 to 10 mg/L. In fact, Fe2+ favors the generation of more OH radicals which therefore improves the degradation of OTC, which is confirmed by plots shown in Fig. 3(b) for higher concentration of Fe2+. However, beyond 30 min of treatment, the degradation rate of OTC was lowered for Fe2+ initial concentration of 10 mg/L and the highest degradation efficiency achieved was 90.82% at Fe2+ = 5 mg/L. In fact, the reduction of OTC removal during the increase of the concentration of Fe2+ from 5 mg/L can be explained by the setting up of a parasitic reaction consuming the OH (Eq. (7)). The rate of this reaction becomes important and will compete with the degradation of the OTC. On the other hand, the Fe3+ ions formed at higher concentrations can react with hydrogen peroxide (Eq. (8)), and consequently reduce the degradation efficiency. Similar observations have been reported by some publications at higher concentration of Fe2+ [29,31,32].

3.4. Effect of Initial OTC Concentration

The dependence of OTC degradation on its initial concentration has been investigated as well over an interval between 10 and 40 mg/L with an optimum H2O2 concentration of 150 mg/L and Fe2+ concentration of 5 mg/L.
The results in Fig. 3(c) show that the OTC degradation rate decays starting from high initial OTC concentrations. For instance, with initial OTC concentrations of 10, 20, 30 and 40 mg/L, the removal efficiency achieved after 60 min were 90.95, 90.82, 88.47 and 85.27%, respectively. Keeping the operating conditions invariant, the OTC degradation rate decreased much faster as the initial OTC concentration is augmented. However, high OTC concentrations resulted in more intermediate products which compete with OTC to react with OH and can also become limiting reagents. Consequently, a reduction of the rate constants was obtained.
Moreover, the reduction in degradation yield for an initial concentration of OTC can be related to a similar amount of OH radicals formed in all solutions considered. These results are in accordance with the data reported by Chekir et al. [33], who studied the degradation of spiramycine antibiotic by UV/TiO2 under solar light. The degradation efficiency obtained in this study is quite similar to that of Chekir et al. [33]. In another study, Hoseini et al. [34] investigated the degradation of TC antibiotic by means of sonocatalytic system for initial concentrations of 25, 50, 75 and 100 mg/L and the removal rates reported were 70.1, 65.3, 58.3 and 50%, respectively.

3.5. Effect of Temperature

Three experimental tests were performed at temperatures of 25, 40 and 50°C to evaluate the reaction thermal effect on OTC degradation. Plots in Fig. 4 demonstrate that there is hardly no degradation of OTC when no H2O2/Fe2+ was utilized. Thermal energy (40 and 50°C) alone did not break OTC molecules. The tests show that OTC degradation is only effective when hydrogen peroxide H2O2 is mingled with ferrous iron Fe2+.
Besides, plots in Fig. 4 show also that the degradation rate of OTC is favored by increasing the temperature. In the first stage, the degradation efficiency after 20 min of reaction increases from 76 to 89% for a temperature of 25 to 50°C. The oxidation reaction of the OTC by the OH is therefore favored by an addition of thermal energy. In fact, the solution temperature influences the electron transfer and the mass transfer rates, which in turn influences the rate of generation of OH and their reactivity with OTC. In the second stage (up to 20 min), the degradation rate slowed down due to a lower production rate of Fe2+ from Fe3+ [35] and the degradation efficiency slightly improved. The final degradation efficiency achieved after 60 min were 90.82, 92.23 and 92.88 % for the temperature of 25, 40 and 50°C, respectively. However, because of the rise in operating cost for a low gain achieved by increasing the temperature from 25 to 50°C, ambient temperature seems the best setting to apply to an OTC system. Similar observations were reported by Aygun et al. [36] in the first and the second stage of treatment using Fenton process.
The apparent rate constants (kapp) of the degradation of OTC obtained in the first OTC degradation stage at 25, 40 and 50°C were 0.068, 0.145 and 0.213 min−1, respectively. Accordingly, the apparent activation energy (Eapp) of the OTC degradation reaction in a Fenton process was determined using the Arrhenius equation as follows:
ln   kapp=ln   A-(EappRT)
Where A is the frequency factor, Eapp is the apparent activation energy, R is the gas constant and T is the temperature. Fig. S3 shows a linear plot of ln kapp versus 1/T with a determination coefficient greater than 0.99. The apparent activation energy Eapp, obtained from the Arrhenius plot (Fig. S3) is 36.58 kJ/mol. This result indicates that the degradation of an aqueous solution of OTC by Fenton process requires low activation energy and therefore it can be easily achieved. In this context, the results reported by Karatas et al. [37] are in accordance with the data obtained in this study.

3.6. Effect of Initial pH Solution

Several AOPs studies [16] which were conducted on wastewater treatment concluded that the determination of pH of a solution being treated is a key variable to investigate. In particular, the degradation of organic compounds by Fenton process is generally carried out in an acidic pH medium to preserve the aqueous Fe2+ ions in the solution.
In fact, solutions with initial pH between 1 and 7 were made to investigate the acidity effect on OTC degradation in a Fenton process. Each solution consists of OTC, H2O2 and Fe2+ which initial respective concentrations are 20 mg/L, 150 mg/L and 5 mg/L.
Plots in Fig. 5 show that for solutions with a pH values from 1 to 7, the OTC degradation yield in the Fenton process are 68.72, 77.37, 90.82, 90.63, 80.75, 68.62 and 46.26%, respectively. The experimental tests demonstrate that the OTC degradation rate increases for solutions which pH is less below a threshold of about 3.5 which can characterized as an optimum pH for solutions to be treated in the Fenton process. For pH values below 3, the reaction of hydrogen peroxide with Fe2+ is seriously affected causing the reduction in hydroxyl radicals production. At pH 1 and 2, the low degradation may be due to the •OH scavenging of H+ ions (Eq. (10)) [38]. Moreover, the H2O2 gets solvated in the presence of high concentration of H+ ion to form stable oxonium ion H3O2+. An oxonium ion makes hydrogen peroxide electrophilic to enhance its stability and presumably to reduce substantially the reactivity with ferrous ion [39]. On the other hand, the drop in OTC degradation efficiency at pH > 4 may be due to the precipitation of dissolved iron in presence of high concentration of OH ions (Eq. (11) and (12)), and a decrease of H2O2 decomposition into hydroxyl radicals. In addition, another study [38] reported that higher pH in solutions caused the oxidation potential of hydroxyl radical to decrease.

3.7. Effect of Type of Catalyst (Fe2+, Fe3+)

Fig. 6 shows a comparison between two systems of degradation, Fenton system (H2O2/Fe2+) and Fenton-like system (H2O2/Fe3+) that were operated using identical operating conditions (OTC = 20 mg/L, H2O2 = 150 mg/L, Fe2+ = Fe3+ = 5 mg/L). The degradation yield achieved was 90.82% and 88.20% in Fenton and Fenton-like systems, respectively. But, in the first 30 min, the degradation rate in the Fenton process was much faster than that in the Fenton-like process. This can be related to OTC oxidation having much more affinity for OH radicals in the Fenton process according to Eq. (3) which has a rate constant of 70 Lmol−1s−1. However, the oxidation in the Fenton-like process is much more favored with HO2 which has less power of oxidation than OH radicals. Moreover, the rate constant (0.02 Lmol−1s−1) of production of HO2 is very slow as defined in Eq. (8).
From 30 min onwards to the reaction end, the rate of OTC degradation is almost the same in both processes. Similar observations were reported by Wang et al. [40] who studied the decolourisation of wastewater from an azo dye in Fenton and Fenton-like processes.

3.8. Mineralization and Biodegradability of Treated OTC Solution

To evaluate the effectiveness of mineralization of OTC using Fenton process, COD was investigated at optimal operating conditions found for OTC degradation. The effect of initial antibiotic concentration on the COD reduction was studied by considering initial OTC concentration of 20, 40, 60 and 80 mg/L which correspond to initial COD of 31.70, 63.25, 90.56 and 125.29 mg/L, respectively. The percent of COD reduction achieved after 60 min of treatment were found to be 78.35, 70.13, 63.80 and 51.27% (figure not shown). These results show that COD removal decreases with higher initial COD, which can be explained by the increasing of byproducts that slow down the mineralization. In the other hand, the effect of initial pH solution was also examined in the range of 2 to 5 for initial OTC concentration of 20 mg/L. The highest CODs removal percent obtained were 78.35 and 76.62% for initial pH of 3 and 4, respectively. As plotted in Fig. S4, the COD removal decreases progressively when the initial pH is higher than 3, this is may be due to a lower dissolved iron that can be responsible for the decomposition of H2O2 into hydroxyl radicals. These results confirm an optimum pH of 3 for the Fenton process.
As shown in Fig. 7, the rate of COD reduction was quite fast in the first 20 min (65.37% of COD removed) and then slows down gradually until the end of the reaction. This trend may be due to a rapid consumption of Fe2+ at the reaction beginning and the generation of large amount of hydroxyl radicals which are mainly responsible on disintegrating the organic compound. Then, the OTC degradation rate remained slow because of a decrease in hydroxyl radicals and the mineralization was achieved with other species radicals such as the hydroperoxyl radical which has a low oxidation power than hydroxyl radical. Fig. 7 also shows that the ratio of BOD5/COD increased significantly from 0.04 to 0.36. This increment demonstrates clearly that the treated OTC product has a higher biodegradability compared to OTC before mineralization. The ratio of biodegradability BOD5/COD is 0.36 and it is considered adequate for biological treatment as wastewater is considered biodegradable if this ratio is 0.4 [41].

4. Conclusions

Oxidation and molecular degradation of OTC were investigated under different operating conditions in Fenton and Fenton-like processes for 60 min of reaction. An OTC degradation efficiency of 12.22% was obtained when H2O2 was used alone in the Fenton process. However, the degradation rate of OTC in the Fenton process (H2O2/Fe2+) and Fenton-like process (H2O2/Fe3+) was much higher, with a removal efficiency of 90.82% and 88.20%, respectively. An increase of OTC initial concentration lowered the degradation efficiency and an excess of H2O2 can also impede removal efficiency of OTC. Besides, the initial pH is an important parameter in the degradation of OTC. A high degradation rate was obtained for a pH of 3 confirming the optimum pH value for Fenton process. The highest value of 90.95% removal efficiency was achieved in the Fenton process operated with a ratio H2O2/OTC of 15, Fe2+ concentration of 5 mg/L and pH value of 3 at an ambient temperature of 25°C. Under the same operating conditions, the highest percent of COD removal achieved was 78.35%. A remarkable enhancement in OTC biodegradability from 0.04 to 0.36 was also achieved in 60 min reaction time. This study indicated that Fenton process can be used for pretreatment of wastewater contaminated by OTC before a biological treatment.

Supplementary Information


The authors would like to thank the Ministry of Higher Education in Algeria for the financial support.



Apparent activation energy (kJ/mol)


Apparent rate constants (min-1)


Frequency factor


Oxidation potential (V)


OTC antibiotic


Hydrogen potential of the solution


Gas constant (8,314 J mol-1 K-1)


Temperature of the solution (°C)


Tetracycline antibiotics group


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Fig. 1
Schematic diagram of the H2O2/Fe2+ system. Legend: 1. Chemical reactor, 2. Cooling or heating water outlet, 3. Cooling or heating water inlet, 4. Thermometer, 5. Syringe, 6. Circulating water bath, 7. Magnetic bar, 8. Reaction medium, 9. Magnetic stirrer.
Fig. 2
Degradation of OTC by H2O2 or Fe2+ alone and H2O2/Fe2+ system, with [OTC]0 = 20 mg/L, [H2O2]0 = 200 mg/L, [Fe2+]0 = 5 mg/L, pH = 3, T =25°C.
Fig. 3
Effect of different operating conditions on OTC degradation (a: initial H2O2 concentration with [OTC]0 = 20 mg/L, [Fe2+]0 = 5 mg/L, b: initial Fe2+ concentration with [OTC]0 = 20 mg/L, [H2O2]0 = 200 mg/L, c: initial OTC concentration with [H2O2]0 = 150 mg/L, [Fe2+]0 = 5 mg/L). All tests were carried out at pH = 3 and T = 25°C.
Fig. 4
OTC degradation versus time for different reaction temperatures, with [OTC]0 = 20 mg/L, [H2O2]0 = 150 mg/L, [Fe2+]0 = 5 mg/L, pH = 3.
Fig. 5
OTC degradation efficiency versus initial pH solution, with [OTC]0 = 20 mg/L, [H2O2]0 = 150 mg/L, [Fe2+]0 = 5 mg/L, T = 25°C.
Fig. 6
Performance of Fenton and Fenton-like processes for OTC degradation, with [OTC]0 = 20 mg/L, [H2O2]0 = 150 mg/L, [Fe2+]0 = 5 mg/L, [Fe3+]0 = 5 mg/L, pH = 3, T = 25°C.
Fig. 7
Mineralization and biodegradability of treated OTC solution with [OTC]0 = 20 mg/L, [H2O2]0 = 150 mg/L, [Fe2+]0 = 5 mg/L, pH = 3, T = 25°C.
Table 1
Reactor Operating Conditions
Item Experimental operating range
Reactor volume 1 L
Temperature 25–50°C
pH 3–7
[OTC]0 10–40 mg/L
[H2O2]0 70–200 mg/L
[Fe2+]0 2–10 mg/L
Treated volume 500 mL
Agitation speed 600 rpm
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