Photocatalytic degradation of penicillin G from simulated wastewater using the UV/ZnO process: isotherm and kinetic study

Abstract

Purpose

Pharmaceutical contaminants, including antibiotics, present in the environment, especially water resources, are a main concern for human and environmental health due to their stability and non-degradability. Accordingly, the purpose of this study was to investigate the photocatalytic removal of penicillin G antibiotic from simulated wastewater using a photocatalytic process [UV/ZnO] in an isotherm and kinetic study.

Methods

In the current research, the ZnO nanoparticles [ZnO NPs] were initially characterized by scanning electron microscope [SEM] and X-ray diffraction [XRD]. Then, its efficiency was investigated in the photocatalytic degradation process of penicillin G. The evaluated parameters in the adsorption process penicillin G antibiotic were pH [1–5], penicillin G concentration [10–30 mgL⁻¹], NP dosage [0.5–4.5 gL⁻¹] and contact time [5 to 200 min]. Then, the effect of pH [3, 5, 7, 9, 11, and], penicillin G concentration [10–30 mgL⁻¹], NP dosage [0.01–1.5 gL⁻¹] and contact time [5 to 200 min] in the photocatalytic degradation (UV/ZnO) was studied. The residual penicillin G concentration was measured using a spectrophotometery at a wavelength of 283 nm.

Results

The results indicated that the penicillin G removal efficiency of photocatalytic process [UV/ZnO] using ZnO was 74.65% at the concentration of 10 mgL⁻¹, the pH value of 5, the ZnO NP dosage of 0.1 gL⁻¹ and the contact time of 180 min, as well as the kinetics of degradation followed the pseudo-first-order kinetic model.

Conclusion

It can be concluded that the use of this process is appropriate an effective for the removal of the antibiotic pollutants.

Introduction

The presence of drug residues in the environment, especially water resources, is a major environmental problem due to their stability and non-degradability [6, 7]. These medicines have been discharging to the environment continuously and without any restrictions; although they may enter into aquatic environments at low levels, its continuous availability due to cumulative effects could be a potential hazard to aquatic ecosystems and the present microorganisms [1]. These materials have typically been emitted into the environment through wastewater because of the inefficiency of conventional wastewater treatment technologies [7, 8]. Among various drug compounds, antibiotics are of particular interest because of the ability to develop antibiotic resistance in pathogenic bacteria [2]. According to the World Health Organization, Iran is considered one of the first 20 countries to consume the antibiotics in the world and has the second place in Asia, behind China, in this regard [9]. The antibiotics are man-made organic matters used since ancient times to prevent human and animal infections [3]. A large group of pharmaceuticals account for about 15% of total drug use. The wastewater containing antibiotic compounds, such as wastewater from the pharmaceutical industry, is one of the problems associated with environmental pollution. Increasing the use of antibiotics to treat humans and livestock has led to the presence of this pollutant in the water [4, 10]. The antibiotics also play a central role in environmental contamination due to their high consumption in human and veterinary medicine and may even cause bacterial drug resistance even at low concentrations [11]. The concentration of these pollutants may reach 100 mg/l in the effluent of pharmaceutical plants [5]. The standard limit of the Environmental Protection Agency is 1 mg/l for their presence in the wastewater [12]. Since penicillin G is one of the most commonly used antibiotics, it is essential to treat penicillin G-containing wastewater before discharging into the environment using an effective process to maintain and control the human and the environmental health against the harmful effects of these compounds. The penicillin G is a persistent chemical compound in the environment, whose presence in surface water and wastewater treatment plants has made concerns [13]. Different physical, chemical and biological methods are exploited to remove these contaminants from aqueous solutions [14, 15]. Some of the methods used to remove or modify antibiotics into safer structures include the processes of sonolysis [16], Photo-Fenton [17], photocatalist [18] and nanofiltration [19, 20]. Studies have shown that absorption techniques, electrocoagulation approach and membrane process can be used to remove these compounds, but these methods are not cost-effective because of low efficiency, high cost of investment, and difficult management and maintenance procedures [21]. In fact, many of the techniques mentioned, such as adsorption, only transmit pollution from one phase to another without decomposition or degradation [22]. The use of integrated processes is currently very useful because of the increased efficiency of removing organic matter and reducing related costs. Advanced oxidation processes [AOP_s] have been prominent in recent years due to their high oxidation power [23]. The AOP_s are based on the production of strong oxidizing radicals, which have a high affinity for the degradation of pollutants [24]. On the other hand, the AOP_s can often break down and decompose the antibiotics into simpler molecules and mineralize them. Ozonation is one of the methods of AOP_s Ozone alone cannot completely oxidize organic compounds and cause the generation of intermediate and toxic materials [25, 26]. Therefore, the integrated processes, such as ozone plus hydrogen peroxide [H₂O₂], ozone plus ultraviolet [UV radiation, UV plus H₂O₂, and catalytic ozonation process [COP], have been used over the past several years [27–29]. Among the various advanced oxidation processes, photocatalytic processes are one of the emerging technologies and the efficiency of these processes in the decomposition of many organic pollutants including chlorophenols, carboxylic acids and pharmaceutical compounds has been proven [30]. In a study by Emad S et al. (2010) on the degradation of amoxicillin, ampicillin and cloxacin antibiotics during UV / ZnO photocatalytic process in aqueous media, the results showed that complete degradation of amoxicillin, Ampicillin and cloxacin occurred over a period of 180 min [31]. In addition, in the study of Mohammed Shokri et al. using UV / TiO₂ process for photocatalytic degradation of penicillin G antibiotic from aqueous solutions, the results showed complete destruction of the antibiotic within 90 min [32]. The photocatalytic process mechanism involves the activation of a semiconductor by artificial light or sunlight [33]. ZnO has a broad bandwidth of 3.5 ev [34], and produces more hydroxyl radical [•OH] than TiO₂ [35]. It also has high mineralization and reaction capacity and has more adsorption sites and higher adsorption [36]. The high surface-to-volume ratio of ZnO causes higher UV absorption and high lifetime [37]. Therefore, it is widely used as a catalyst [38]. The ZnO has high optical capacity, chemical stability, availability and low cost [39, 40] and is excessively applied to degrade the organic compounds, such as ciprofloxacin, Acid Red 249, Rhodamine B, Methylene blue, Acid Red 14, as well as the antibiotics, such as metronidazole, amoxicillin, ampicillin and cloxacillin [41, 42]. Regarding the problems of antibiotics and, on the other hand, in view of the advantages of photocatalytic methods for the degradation of organic pollutants, the purpose of this study was to investigate the photocatalytic degradation of penicillin G antibiotic from simulated wastewater using UV/ZnO process.

Materials and Methods

Method of sampling and calculation of sample size

Considering the pH [3, 5, 7, 9, 11, and], dose [0.5–4.5 gL⁻¹], penicillin G concentration [10–30 mgL⁻¹] and contact time [5 to 200 min] in the adsorption process by ZnO nanoparticles, the total sample size was 224 and in the photocatalytic removal step all of the parameters were selected similar except the dose of ZnO nanoparticles in the range [0.01–1.5 gL⁻¹] Was. Because of the higher dose, it caused the opacity and the difficulty of passing the light and the number of samples was 256.

Reagents and apparatus

The penicillin G was prepared from Sigma Co. [Germany] with a molecular formula of C₁₆H₁₈N₂O₄S and a purity of 99%; its chemical characteristics are given in Table 1. Other materials used include NaOH, HCl [Merck, Germany], ZnO with a purity of over 99% [Pishgaman-e-Nanomavad-e-Iranian Co., Iran]. Figure 1 shows the chemical structure of the antibiotic studied.

Table 1.

Chemical specifications of penicillin G [42]

Formula	MolecularWeight [gr/mol]	Brand	Shape	penicillin G
C16H17N2O4S	37.356	BenPen	Crystal and white powder

Fig. 1.

molecular structure of penicillin G [43]

The crystalline structure of ZnO nanoparticles [ZnO NPs] was determined using X-ray diffraction [XRD, Pert x, Pro, Panaplytical Co.]. The scanning electron microscope [SEM, SIGMA VP-500 model, ZEISS, Germany] was utilized to study shape, mean diameter, surface details and structural analysis of ZnO NPs.

PenicillinG adsorption tests with ZnO nanoparticles

The adsorption experiments were carried out in a discontinuous absorption system using penicillin G as an absorbant and ZnO NPs as an adsorbent. The stock solution was prepared by dissolving the penicillin G in deionized water. The absorption measurements were performed by mixing different concentrations of ZnO [0.5–4.5 g/L] in beakers containing different concentrations of penicillin G [10–30 mg/L] with various pH values [3, 5, 7, 9, 11, and]. Prepared solutions were placed on a shaker at 300 rpm at different times [5–200 min]. The residual penicillin G concentration was read by the PG spectrophotometer [UV/Visible T80⁺, England] at a wavelength of 283 nm [44, 45]. First-order and second-order equations were used to study the absorption kinetics. Langmuir and Freundlich models were applied to analyze the adsorption and isotherms. The linear equations of these two models are in accordance with Eqs. 1 and 2, respectively.

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}/\left(1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}\right)$$ 1

$${\mathrm{Lnq}}_{\mathrm{e}}={\mathrm{LnK}}_{\mathrm{f}}+1/\mathrm{n}{\mathrm{LnC}}_{\mathrm{e}}$$ 2

Where, Ce and qe indicate the equilibrium concentration in the soluble [mg/l] and solid [mg/l] phases, respectively, qm is the maximum absorption capacity [mg/kg], and k, n and b are the model constants. Chemical kinetics represent the chemical reaction rates. The pseudo-first-order and pseudo-second-order kinetic models are according to Eqs. 3 and 4.

$$\mathrm{Log}\left[{\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right]={\text{Logq}}_{\mathrm{e}}-\left({\mathrm{K}}_1/2.303\right)\mathrm{t}$$ 3

$$\mathrm{t}/{\mathrm{q}}_{\mathrm{t}}=1/{\mathrm{k}}_2{{\mathrm{q}}_{\mathrm{e}}}^2+\mathrm{t}/{\mathrm{q}}_{\mathrm{e}}$$ 4

Where, q_e is equilibrium concentration of adsorbent phase [mg/g], qt shows the antibiotic concentration at the time t [mg/g] and k₁ and k₂ are the rate constants of the pseudo-first-order and pseudo-second-order kinetic equations, respectively [46].

The photocatalytic degradation of penicillin G using UV/ZnO process

At this stage of the present study, the experiments were carried out on 400-ml samples in a discontinuous system at the ambient temperature [24 ± 2 °C] using a 500-ml reactor. The evaluated parameters were the pH value [3, 5, 7, 9, 11, and], the initial penicillin G concentration [10–30 mgL-1], the NP dosage [0.5–4.5 gL-1] and the contact time [5 to 200 min] according another researches [47]. For adsorption and desorption reactions, the solution was stored in dark for 30 min before exposure to UV radiation, and the measured concentration [8.05 ± 2%] was considered as the initial concentration. The UV radiation was created by 18-W Nominal Lamp Power with a wavelength of 283 nm and a radiant intensity of 2500 μW/cm² placed inside a quartz sheath in the center of the reactor. A cooling water wall was used around the reactor to keep constant the sample temperature inside the reactor in the range of 24 ± 2 °C [Fig. 2]. The samples were tacked out at specific intervals, and the residual penicillin G concentration was determined by spectrophotometer.

Fig. 2.

Pilot schematic used in this study: 1) Heater, 2) Cooling Water inlet, 3) Quartz tube, 4) UV lamp, 5) magnet, 6) Sample collection site, 7) Outer wall, 8) Cooling water outlet

According to studies, the kinetics of decomposition are usually used to study the photocatalytic degradation of various organic compounds, especially antibiotics. The kinetics of the photocatalytic reaction of organic compounds is expressed by the Langmuir-Hinshelwood model, which is essentially related to the rate of decomposition and concentration of the organic compound, which is represented by the Eq. 5.

$$\mathrm{r}=-{\mathrm{d}}_{\mathrm{c}}/{\mathrm{d}}_{\mathrm{t}}={\mathrm{K}}_{\mathrm{r}}{\mathrm{K}}_{\mathrm{ad}}\mathrm{C}/1+{\mathrm{K}}_{\mathrm{ad}}\mathrm{C}$$ 5

Where, Kr is the reaction rate constant and Kad is the equilibrium absorption constant. The Eq. 5 can be simplified to give the pseudo-first-order kinetic model with a determined rate constant [K_obs] when the absorption is relatively low or the organic composition is low [Eq. 6].

$$\mathrm{Ln}\left[\mathrm{C}/{\mathrm{C}}_0\right]=-{\mathrm{K}}_{\mathrm{obs}}\mathrm{t}$$ 6

If the Ln [C/C₀] versus the reaction time graph is obtained as a straight line, it will show the slope of the rate constant graph [K]. The removal efficiency in each phase was evaluated in accordance with Eq. 7.

$$\%\mathrm{R}=\left[{\mathrm{C}}_0-\mathrm{Ce}/{\mathrm{C}}_0\right]\ast 100$$ 7

Where, C0 is the initial concentration and C_e is the residual concentration in mg/l. Eq. 8 was used to calculate the absorption capacity at the catalytic removal of penicillin G in the presence of ZnO.

$${\mathrm{q}}_{\mathrm{e}}=\left[\left[{\mathrm{C}}_0-\mathrm{Ce}\right]\ast \mathrm{v}\right]/\mathrm{M}$$ 8

Where, C₀ is the initial concentration, C_e is the final concentration after absorption, and qe is absorption capacity in mg/g.

Results

In this study, a solution with a specific concentration of penicillin G was made to determine the absorbance point of antibiotics. Using a spectrophotometer in the range of 200–400 nm, the peak absorption peak at 283 nm was obtained [45] Fig. 3.

Fig. 3.

Spectrum of penicillin G at wavelengths of 200 to 400 nm

Characterizations of ZnO NPs

The XRD is a technique widely used to determine the characterization of nanoparticles [48]. The XRD pattern of ZnO NPs is shown in Fig. 4. The peaks of ZnO NPs represent the hexagonal structure of these nanoparticles [49]. The XRD patterns indicate certain peaks for ZnO at 2Ø [31, 34, 36, 47, 56, 63, 68, 70], which is fitted with the data of JCPDS database.

Fig. 4.

XRD image of the ZnO nanoparticle

The FESEM method was used to assess the surface properties of nanoparticles and their morphology. The FESEM determined the shape and size of ZnO NPs. Figure 5 exhibits the FESEM image of these NPs, showing the spherical structures and the size range of 10–30 nm in the milky white color.

Fig. 5.

FESEM image of the ZnO nanoparticle

Adsorption and photocatalytic degradation of penicillin G

Absorption with ZnO NPs

The absorption tests to determine the equilibrium time of this ZnO process were performed in a discontinuous manner in conditions with different pH values [1–5], ZnO NP dosage [0.5–4.5 g/l], initial penicillin G concentration [10–30 mg/l] and contact time [5–200 min]. The results showed that the highest adsorption rate [27.97%] was at the pH value of 5 and the NP dosage of 1.5 g/l at the contact time of 120 min for the initial concentration of 20 mg/l. The lowest removal efficiency [16.22%] in the catalytic process of ZnO was at the pH value of 3. This study revealed that removal efficiency of penicillin G was decreased by increasing the pH values from 5 to 11, probably because of the anionic structure of penicillin G and the pH_ZPC of adsorbent. Studies also show that the electric charge of the absorbent surface is in equilibrium with the environment at pH = pH_ZPC; the pH_ZPC of ZnO was equal to 5.5. The adsorbent surface has a positive charge at pH values below pH_ZPC, and a negative charge at pH values higher than pH_ZPC. Given that penicillin G is soluble in water, it is considered to be a weak acidic component [pKa =2.75]. The penicillin G is an anion at pH values higher than pK_a. Therefore, the pH values higher than 5.5, due to the negatively charged of ZnO surface and the anionic nature of penicillin G, develop a repulsive electrostatic force between adsorbent and pollutant, which reduces the absorption rate [46]. In addition, the proton is released in alkaline media, and penicillin G is completely ionized to anions, which leads to a sharp decrease in the rate of absorption. The concentration of hydroxonium ions increases in solution at pH values below 5, and prevents absorption of pollutants by active sites on the adsorbent surface. Therefore, pH = 5 was selected for absorption tests in the present study. Aksu et al. reported that the optimal pH was 6 in evaluating the effect of bio-absorbent of Rhizopus arrhizus on removal of penicillin G [50]. According to the results presented in Fig. 6, increasing the absorbent dosage will increase the removal efficiency due to increased adsorbent surface and the presence of active adsorption sites on the surface of ZnO NPs. Based on the results, the absorption capacity does not change significantly with increasing the absorbent dosage. As a result, the lowest absorbent dosage with the highest removal efficiency was chosen as the optimal dose; the absorbent dosage of 1.5 g/l was selected in the next steps. The removal efficiency was reduced by increasing the initial penicillin G concentration, which is due to the occupation of the active adsorption sites of the ZnO NPs. The number of absorbent molecules increases gradually by raising the penicillin G concentration, resulting in the desorption process from the surface of the NPs. In the study of adsorption of penicillin G from aqueous solutions using modified canola carried out by Davud Blark et al., the results showed that the removal efficiency of penicillin G was increased with raising absorbent dosage and had the highest removal efficiency was obtained at the penicillin G concentration of 10 mg/L [51]. The absorption rate increases with prolonging the contact time due to the increased collision of penicillin G molecules with ZnO NPs. The absorption rate is reduced gradually by prolonging contact time until it reaches the equilibrium in contact time of 120 min due to the reduction of active adsorbent sites. In the study of the efficiency of natural LECA in the removal of tetracycline from synthetic solutions by Noori Sepehr et al., the highest removal efficiency was observed at the contact time of 180 min [52].

Fig. 6.

Effect of the concentration of ZnO nanoparticle on the removal of penicillin G [pH = 5, penicillin G initial concentration of 20 mgL-1]

In this study, Langmuir and Freundlich isotherms were investigated for ZnO after absorption of penicillin G, the results of which are presented in Table 1. The highest penicillin absorption rate [Gq_max] by ZnO NPs was calculated to be 14.99 mg/g. The results of this study showed a better agreement with Langmuir isotherm model, indicating absorption of monolayer penicillin G on ZnO NPs and homogeneity of adsorbent surface. Amir Sheikh Mohammadi et al. investigated the removal of penicillin G from aqueous solutions using chestnut shell modified with H₂SO₄. The results showed that the penicillin G adsorption on the chestnut shell was fitted to Freundlich isotherm and the absorption kinetics was fitted with the pseudo-second-order equation [53] (Table 2).

Table 2.

Results of equilibrium data fit for determination of ZnO isotherm after absorption of penicillin G

Langmuir Constants	qmax, mg/g	KL[L/mg]	R2
	14.99	0.28	0.99
Freundlich Constants	kf, mg/g	n	R2
	5.57	3.52	0.96

Chemical kinetics represents the chemical reaction rate. In this study, the adsorption rate constant of penicillin G by ZnO NPs was fitted with pseudo-first-order and pseudo-second-order kinetic models. As can be deduced from the results, the reaction of penicillin G adsorption by ZnO NPs well fitted with the pseudo-second-order model [Fig. 7]. Samadi et al. evaluated amoxicillin removal from aquatic solutions using multi-walled carbon nanotubes, and reported that the equilibrium adsorption isotherm data well fitted with Langmuir model [R² = 0.9108] and the adsorption kinetics fitted with pseudo-second-order model [54].

Fig. 7.

The kinetic curve of the pseudo second order equation for the absorption of penicillin G at different concentrations

Photocatalytic degradation of penicillin G using UV/ZnO

Effect of pH

The pH value is one of the most important parameters affecting the removal efficiency of pollutants, which affects the absorption capacity and decomposition of target compounds, the distribution of electric charge on the surface of photocatalyst and the valence band oxidation potential. Previous studies have shown that pH plays an important role in the decomposition and removal of antibiotics. In this study, the effect of pH was investigated on the penicillin G degradation process and the results showed that the lowest removal efficiency was 34.91% at pH = 11 and it was increased to 47.88% at pH = 5 [Fig. 8]. As a result, the photocatalytic degradation process of penicillin G is carried out in acidic environments with higher efficiencies.

Fig. 8.

Effect of initial pH of the solution on the removal of penicillin G in the photocatalytic degradation process UV/ZnO [penicillin G initial concentration = 20 mgL-1, ZnO doze = 0.1 gL⁻¹]

Given the pH_zpc obtained for ZnO NPs, which was 5.5, the catalyst surface was occupied by H⁺ ions at pH values lower than pH_zpc and with OH⁻ ions at pH values higher than pH_zpc. The penicillin G because of being soluble in water is a weak acidic component [pK_a = 2.75]. It is an anion at pH values more than pK_a. Therefore, the pH values higher than 5.5, due to the negatively charged of ZnO surface and the anionic nature of penicillin G, cause a repulsive electrostatic force between adsorbent and pollutant, hereby decreasing the penicillin G degradation rate. In a study titled “Visible light-driven photocatalytic decomposition of penicillin G by Ti ³⁺ self-doped TiO₂ nano-catalyst through response surface methodology”, carried out by Junjing Li et al., the results showed that the highest penicillin G removal efficiency occurred at pH = 5.5 [55].

Effect of ZnO NP dosage

In the UV/ZnO photocatalytic removal process, the effect of ZnO dosage in the range of 0.01–1.5 g/L was investigated for removal because of increasing the absorbent dosage due to turbidity created by NPs and decreasing radiation exposure to penicillin G molecules. As shown in Fig. 9, the experiments were conducted on the initial penicillin G concentration of 20 mg/L at pH = 5. The results showed that the penicillin G removal efficiency after 120 min of contact time was 34.91% and 47.88% at the doses of 0.01 and 1.5 g/l, respectively. As shown in the figure, the penicillin G removal efficiency was elevated by increasing ZnO concentration up to 0.1 g/L and then was decreased. The reason for the decrease in efficiency is related to the number of active sites on the catalyst surface and the UV light penetration in the solution, because the area of active sites is enlarged by increasing catalyst dosage, but enhancing the solution turbidity as a result of increasing the catalyst dosage can reduce the UV penetration into the solution. Consequently, the rate of antibiotic degradation was reduced. Therefore, the optimal NP dosage was selected to be 0.1 g/L for UV/Zn photocatalytic process. Emad S et al. investigated degradation of amoxicillin, ampicillin and cloxacillin antibiotics using UV/ZnO photocatalytic process in aqueous solutions. Their results showed that an optimal condition for complete degradation of antibiotics in aqueous solution with ZnO application was 0.5 g/L [31].

Fig. 9.

Effect of the concentration of ZnO nano partical on the removal of penicillin G in the photocatalytic degradation process UV/ZnO [penicillin G initial concentration = 20 mgL-1, pH = 5]

Effect of contact time and initial penicillin G concentration

The effect of initial penicillin G concentration was investigated on the efficiency of photocatalytic process [UV/ZnO] in the range of [10–30 mg/L] under optimal conditions [pH = 5, NP dosage of 0.1 g/l]. The obtained results are shown in Fig. 10. As can be seen, the removal efficiency decreases with increasing penicillin G concentration. In the initial penicillin G concentrations of 10, 15, 20, 25 and 30 mg/l at the contact time of 180 min, the removal efficiencies were equal to 74.65, 63.30, 56.34, 37.94 and 35.19. This can be justified by the fact that prolonged contact time increases the number of OH radicals and generates positively charged cavities. In addition, the removal efficiency is reduced by increasing the pollutant concentration. Because the formation of ^•OH in the photocatalytic process is constant for a constant amount of catalyst dosage, the available ^•OH is insufficient to decompose high concentrations of penicillin G. Increasing the concentration of pollutants leads to light absorption by antibiotic molecules; therefore, they are less exposed to recovered photons. In a study titled Removal of penicillin G from aqueous phase by Fe⁺³-TiO₂/UV-A process, performed by Dehghani et al., the results showed that the antibiotic removal efficiency was increased by decreasing the initial penicillin G concentration and prolonging contact time [56]. In a study titled photocatalytic degradation of metronidazole by ZnO NPs exposed by UV conducted by Farzadkia et al., the results showed that the highest removal efficiency occurred at the contact time of 180 min [57].

Fig. 10.

Effect of of penicillin G initial concentration in the photocatalytic degradation process UV/ZnO [pH = 5, ZnO doze = 0.1 gL⁻¹]

Kinetics of photocatalytic degradation of penicillin G

In order to investigate the kinetics of photocatalytic degradation of penicillin G in the UV/ZnO process, the experiments were carried out under optimal conditions [the ZnO NP concentration of 0.1 g/l, the contact time of 180 min and pH = 5]. According to studies, the pseudo-first-order model is usually used to describe the photocatalytic degradation of various organic compounds, especially the antibiotics [32]. In this study, the above model was also employed to describe the photocatalytic degradation of penicillin G. The pseudo-first-order kinetic curve for penicillin-G degradation is shown in Fig. 11 and its calculation results are presented in Table 3. As seen, the degradation rate constant [K_obs] is reduced by increasing the penicillin G concentration, so that the K_obs value was 7.9 × 10⁻³ and 0.86 × 10⁻³ min⁻¹, respectively, at C₀ = 10 mgL⁻¹ and C₀ = 30 mgL⁻¹. The results of the study of the kinetic models showed that the pseudo-first-order kinetic model is suitable for describing the reaction rate and well covers the results for all the different penicillin G concentrations, so that the values of the coefficient of determination [R²] are close to 1 and the reaction rate constant is reduced by increasing penicillin G concentration. The number of active hydroxyl radicals in the reaction was limited at high concentrations due to the increase in the concentration of intermediate products; consequently, the degradation rate constant was reduced [2]. These findings are in line with the results of the study by Khodadadi et al. regarding the photocatalytic removal of tetracycline from aqueous solutions by FeNi₃ @ SiO₂ @ TiO₂ [58].

Fig. 11.

The kinetic curve of the pseudo first order equation of degradation penicillin G at different concentrations

Table 3.

Parameters kinetic of the pseudo first order equation of degradation penicillin G

Concentration [mg/L]	Equation	K0 [min−1]	R2	t1/2 [min]
10	Y = 0.0079x + 0.1276	7.9 × 10−3	0/9193	87/72
15	Y = 0.0056x + 0.1794	5.6 × 10−3	0/8168	123/75
20	Y = 0.0038x + 0.2342	3.8 × 10−3	0/8269	182/36
25	Y = 0.0024x + 0.1445	2.4 × 10−3	0/8813	288
30	Y = 0.0023x + 0.2809	2.3 × 10−3	0/8648	301

Accordind Fig. 12, The excitation takes place in the presence UV photons, leading to the excitation of electrons from the valence band to the conduction band. The electrons and holes, further reacts with the H₂O molecules for the formation of active radicals. The recombination of the electron, holes is also happened during the photoreaction.

Fig. 12.

Schematic illustration of photocatalysis degradation of penicillin G

Furthermore when ZnO nanoparticles is illuminated with a photon energy less than 390 nm [UV light], it could generate and excite an electron from the valence band [VB] to the conduction band [CB], therefore generate positively charge hole in VB.

The excited electron promoted to the CB undergo oxidation with oxygen molecule and form superoxide anion radicals [•O₂−] and after that by protonation, it yields hydroperoxyl radical [•HO₂]. Then, these radicals combines with the trapped electrons and produce hydrogen peroxide [H₂O₂] and hydroxide radicals [•OH]. Also, the positively charge hole at VB is positive and enough powerful to generate [•OH] at the surface of ZnO.

The hydroxide radicals are active oxidizing agents which are attacks to the organic pollutants at near the surface of ZnO. They have greater potential to oxidize any form of toxic and refractory organic pollutant into harmless products such as water, carbon dioxide and etc. The degradation could be described through the following schematic [Fig. 12].

Conclusion

The present study aimed to evaluate the photocatalytic removal of penicillin G antibiotic from simulated wastewater under various variables. The results of this study showed that the ZnO nanocatalysts have an appropriate efficacy on the penicillin G degradation in the aqueous solutions. Our results demonstrated that the efficiency of photocatalytic degradation process was 76.53% under optimal conditions of pH = 5, the ZnO NP dosage of 0.1 gl⁻¹, the contact time of 120 min and the initial penicillin G concentration of 10 ppm. The penicillin G removal efficiency was enhanced by increasing the ZnO nanocatalyst dosage by 0.1 g/l and was decreased by increasing the ZnO nanocatalyst dosage from 0.1 g/l due to interference with UV penetration. The use of UV alone had no high efficiency in the removal process [32%]. The kinetics of photocatalytic degradation was fitted to the pseudo-first-order kinetic model with R² > 0.9. Therefore, due to the high efficiency of ZnO nanocatalysts in the photocatalytic process of penicillin G degradation, it can be used as a suitable catalyst for the removal and the degradation of pharmaceutical contaminants.

Compliance with ethical standards

Conflict of interest

The authors confirm no conflicts of interest associated with this publication.