Photoelectrocatalytic degradation of sulphonamide antibiotics in aquatic media using a novel Co-doped ZnO nanocomposite: evaluation of performance, kinetic studies

ABSTRACT It is well known that antibiotics’ residues in aquatic environments are a great threat to human health and ecology. Thus, it is vital to develop efficient strategies to overcome the mentioned threat and degrade the pointed out residues as much as possible. Herein, a novel Co-doped ZnO nanocomposite was fabricated and deposited on fluorine-doped tin oxide glass sheets (FTO) and eventually applied for the photoelectrocatalytic degradation of four sulphonamides (SAs), namely sulphacetamide (SCT), sulphathiazole (STZ), sulfamethoxazole (SMX) and sulphadiazine (SDZ) under visible light irradiation. In this regard, the effect of initial pH (3–11), applied current (0.5–1.5 mA/cm2), initial concentration of SAs (5–20 mg/L) were thoroughly investigated. The results indicated that under the optimal conditions (pH = 9, [SAs] = 5 mg/L and applied current = 1.5 mA/cm2), the degradation efficiencies obtained under a 90-min of reaction time were as follows: SCT (97.1%), SMX (95.8%), SDZ (93.2%) and STZ (91.8%). In addition, the degradation of the SAs by the applied photoelectrocatalytic process (PECP) followed the first-order kinetic model. Meanwhile, the applied PECP exhibited an excellent performance with regard to real samples. Finally, PECP is reckoned to be associated with ‘green’ technologies as electricity is involved and the used catalyst will not end up in the environment as secondary toxic materials.


Introduction
Pharmaceutically active compounds in aquatic environments are considered as an important class of contaminants of emerging concerns (CECs) due to their extensive consumption and resulting environmental pollution [1].
Sulphonamides (SAs) are bacteriostatic agents widely used in human and veterinary medicine over 50 years [2].SAs are also most frequently used due to their unique chemical properties and low production cost [3].However, they are not fully absorbed by human/ animal digestive system.In fact, they are mainly excreted in their unchanged forms (up to 75-90%) and finally released into the environment [4,5].Known as polar organic chemicals and water-soluble, SAs are highly mobile and have been detected in several aqueous media including surface water, ground water and even drinking water [5,6].
Antibiotic contamination has caused a great concern for the dissemination and development of antibiotic-resistant genes and bacteria, posing a potential threat to ecosystem and human health [7,8].It is reported that SAs residues in aquatic environments have severely affected the aquatic creatures and brought about antibiotic resistance in microorganisms [9].To date, a number of conventional water/wastewater treatment processes have been applied to remove the mentioned pollutants from water media, namely biodegradation [10], chemical oxidation [11], adsorption [12] and photo-degradation [13].
Amongst the advanced oxidation processes (AOPs), the photoelectrocatalytic process (PECP) has advantages of both photocatalysis and electrochemical methods, and it is regarded as one of the most effective processes for the degradation of antibiotics [14].Generally, PECP is based on the joint application of redox reactions and photocatalytic method and has been developed for the degradation of contaminants.The process is centred on the stimulation of a bias potential, which triggers the rapid migration of electrons produced at the thin surface of a catalytic anode towards a cathode through an external circuit.This blocks the recombination of electrons and holes leading to a high level of oxidation with regard to the degradation of organic pollutants.
As a promising semiconductor, zinc oxide (ZnO) can act as a photoanode.To achieve a high level of performance in a given PECP, a new design with regard to ZnO-based PEC cells should be made to sensitise ZnO photoanodes to near-visible light [15][16][17].In addition, to adjust the optical features of wide bandgap semiconductors (like ZnO), a proper and controlled way of doping with metal/ non-metal should be performed.Similarly, the surface modification of ZnO using various metals has also been highlighted [18].For example, deposition of a number of metal/metal oxides such as Ni, Ag, Fe, Cr, Cu, Mn, Co, Sn, and Bi on the surface of ZnO resulted in better performance [19,20].Among them, Co seems to be one of the most effective material for tuning the electronic and optical properties of ZnO through doping process [21].Most importantly, doping of Co within ZnO structure can change the band gap energy facilitating the transfer of electron and tuning the Fermi level of ZnO [22].
In the present work, Co-doped ZnO nanocomposite was deposited on FTO glass sheets and successfully applied for photoelectrocatalytic degradation of four sulphonamide antibiotics from aquatic media under visible light irradiation.Then, the influence of several parameters affecting on the degradation process such as initial pH, applied electrical current, and initial antibiotic concentration was separately optimised.To the best of our knowledge, so far this has been the first report on the photoelectrocatalytic degradation of sulphonamide antibiotics from aquatic media using the mentioned nanocomposite.

Chemicals
Four SA antibiotics (sulphacetamide (SCT), sulphathiazole (STZ), sulfamethoxazole (SMX) and sulphadiazine (SDZ)) were obtained from the Sigma-Aldrich Company.The chemical structures of the SAs are shown in Table 1 [23].Trichloroacetic acid (TCA), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium chloride (NaCl), ethanol, methanol and acetone were all purchased from the Merck Company and applied with no further purification.In addition, deionised water (DI-water) was prepared in our lab.

Synthesis of Co-doped ZnO/FTO photoelectrode
The transparent Co-doped ZnO was fabricated by radio frequency (RF) technique.Initially, the FTO glass sheets were separately and thoroughly washed with DI-water, acetone, and ethanol in an ultrasonic bath and then dried at 80°C for 30 min.The Co foil (id = 3 mm) and Zn foil (id = 76 mm) were placed on two separate sputter gun sources.The film (Co-doped ZnO) was deposited on the FTO glass sheets under a gentle stream of argon (Ar) gas whose flow rate was adjusted at 25 sccm in a chamber by a mass flow controller.The background chamber pressure set at 5 mTorr throughout the deposition procedure.The distance between the targets and substrate was set at 4 cm and the substrate temperature was maintained at 25°C.It is noted that the Co and Zn foils were sputtered using RF power (100 W, 60 min).Afterwards, the photoelectrode was put in a pure oxygen furnace at 350°C for 60 min by implementing the deposition process.Finally, it gradually cooled to the room temperature [24,25].

Characterisation
For identification of the morphology of the surface, the composition of elements and the details regarding the cross section of the synthesised photoelectrode a scanning electron microscope (SEM; TESCAN mira3, Czech Republic) was utilised which was fitted with an energy dispersive X-ray spectroscopy (EDX) running under vacuum condition.

Analytical procedures
In this study, a 1-litre glass reactor was employed to conduct experiments.In each run, the aquatic solution of the SAs along with other needed chemicals were introduced to the reactor.The reactor was fitted with two electrodes, namely titanium (dimensions: 10 × 2 cm)-serving as the cathode -and Co-doped ZnO nanocomposite already deposited on the FTO glass (dimensions: 10 × 2 cm) serving as a photo anode.Furthermore, a lowpressure Hg lamp (6 W, UV-C, 254 nm), as a light source, was located in a quartz glass cylinder in the middle of the reactor.The impacts of various factors such as applied electrical current (0.5-1.5 mA/cm 2 ), initial SAs concentration (5-20 mg/L), pH (3)(4)(5)(6)(7)(8)(9)(10)(11) and reaction time (0-90 min) on the degradation efficiency were evaluated and then the optimised conditions were established.It is noted that in each run, NaCl was added to the reactor (0.75 gr/L) to serve as an electrolyte solution.The concentration of each studied SA was measured by a high performance liquid chromatograph (HPLC, Cecil CE4100) equipped with a UV detector (CE 4900) at 270 nm.A Discovery-C18 column (250 mm×4.6 mm) was used for the separation of the SAs.The HPLC operated under a TCA /methanol mobile phase (pH 3, 20:80 (v/v)) at a flow rate of 1.0 mL/min.

Kinetics of degradation
Three equations with respect to the zero-order, first-order and second-order kinetic models were used for the description of the kinetics likely involved in the degradation of the SAs by the PECP [26].
where C 0 is the reactant (SAs) concentration at time zero, C t is the concentration of reactant at a given time t, t is the reaction time, k is the reaction rate constant.

Characteristics of Co-doped ZnO deposits on FTO as a photoelectrode
Amongst the techniques commonly used for the identification of the morphology of samples, SEM has gained a lot of attention due to provision of useful data concerning the particles growth and their shapes and sizes.The spongy image of Co-doped ZnO nanocomposite is depicted in Figure 1.Within the image, the Co particles were integrated into the structure of ZnO forming a good platform acting as a nanocatalyst.This led to the surface of the catalyst to grow further followed by an increase in its density.
The cross sectional image of the deposited ZnO on the surface of FTO glass sheet is illustrated in Figure 2(a).As shown in this figure, ZnO particles are finely distributed on the FTO glass bed layer.Figure 2(b) presents that Co-doped ZnO particles well distributed within the FTO glass sheets forming a desired thickness which is highly associated with the facile light-induced activation of the catalyst.Consequently, further electron/holes are generated giving rise to high level of the photocatalyst efficiency.
The EDX image of the photoanode used (Co-doped ZnO particles deposited on the FTO glass sheet) is shown in Figure 3.As can be clearly observed, the peaks are assigned to the elements comprising of the photoanode and no other peaks were seen as a sign of impurities.The peaks indexed to Co indicate that it was successfully integrated to the structure of the photoanode.It is noteworthy that the Si and Sn peaks must have originated from the FTO glass structure.

Effect of pH
As with any photodegradation process, pH seems to have a great impact in the performance of a PECP.Due to slightly acidic properties of the selected SAs (see Table 1), they are heavily affected on the variation of pH when it comes to the degradation.For this reason, the effect of variation of pH on the applied PECP with regard to the degradation rate of the SAs was investigated within the pH range of 3-11 under the following conditions: the SAs concentration 20 mg/L and electric current density 1 mA/cm 2 .As depicted in Figure 4, the respective degradation efficiencies for STZ, SDZ, SMX and SCT improved from 25.1, 27.3, 29.1, and 31.7% to 56.1, 58.4,61.3, and 64.2% by increasing the initial pH from 3 to 9 and afterwards almost levelled out.It can be explained by the fact that at pH = 9 almost all four target SAs are in anionic forms (see Table 1) and well triggered into the photoanode for the oxidation.This could be a major driving force for the eventual degradation of the SAs in the PECP.Furthermore, at this mentioned pH (i.e.9), hydroxyl ions can react with photogenerated holes on the surface of the photoanode nanocomposite and generate more hydroxyl radicals leading to a further enhancement in the degradation efficiency [27].Thus, pH 9 was chosen as the optimal pH in the next experiments.

Effect of applied electrical current
The effect of applied electrical current on the degradation efficiency of the SAs was evaluated in the range of 0.5 to 1.5 mA/cm 2 while the rest of conditions were kept unchanged (pH 9 and initial SAs concentration of 20 mg/L) (Figure 5). Figure 5 clearly demonstrates that the degradation efficiency enhanced from 58.2 to 77.9% (SCT), 46.9% to 71.9% (STZ), 54.3% to 75.4% (SMX), and 50.8% to 73.9% (SDZ) with an increase in the applied electrical current values from 0.5 to 1.5 mA/cm 2 .As observed, the rise of the electric current led to a substantial improvement in the degradation efficiency for all four SAs.This could be explained by the fact that by increasing the applied current the electron transfer increases leading to a further enhancement in the production of hydroxyl radicals [28,29].Additionally, an increase in the electrical current leads to the elevation of electron transfer from the surface of the catalyst to the cathode electrode through an external circuit resulting in a far less recombination of electron-holes which is equivalent to a higher level of degradation rates for the target SAs [30].In a study by Hosseini et al, the degradation of ciprofloxacin by a PECP was investigated.They reported that a rise in the applied current from 0.625 to 1.87 mA/cm 2 led to an increase in the degradation efficiency from 74.02 to 95.55% [25].

Effect of initial concentration of sulphonamides
The effect of initial concentration of the SAs (5-20 mg/L) on the applied PECP was investigated under the conditions as follows: initial pH 9 and applied electrical current of 1.5 mA/cm 2 (Figure S1).As can be deduced, the degradation of the SAs decreased from 97.1 to 77.9% (SCT), 91.8% to 71.9% (STZ), 95.8% to 75.4% (SMX), and 93.2% to 73.9% (SDZ) as the initial concentration increased from 5 to 20 mg/L.
By increasing the concentration of the studied targets, a rise will take place in the production of the intermediates formed during the degradation process.Some of the targeted molecules are adsorbed onto the catalyst surface, while the others are waiting for free space area, so that they can adsorb onto it.This reduces the number of active sites of the catalyst and decreases the rate of degradation activity.In the meantime, the intermediates generated consume the reactive radicals in the process, leading to a further fall in the degradation efficiency [26,31].

Kinetic study
The kinetic parameters in relation to the zero, first and second-order models for the degradation of the SAs at the conditions of pH 9, initial concentrations of 5, 10, and 20 mg/L and the applied current of 1.5 mA/cm 2 within the reaction time of 0-90 min are presented in Table S1.According to the results obtained, high values of correlation coefficients and lower values of Root-Mean-Squared Errors (RMSE) confirmed that the first-order model is favourable to describe the degradation data over the entire SAs concentrations.This finding is in line with the study by Hosseini et al. [32].
The results obtained demonstrates that the synergetic effect-arising from the combination of electrolysis and photolysis-highly facilitated the SAs degradation efficiency, which can be ascribed to the reduction of recombination of photogenerated electrons and holes followed by the generation of more hydroxyl radicals leading to a further during the PECP [33].
In a study by Liu et.al, the degradation of ofloxacin was examined while different treatment methods such as photoelectrocatalysis (PEC), photocatalysis (PC), electrochemistry (EC) and direct photolysis (DP) were employed.They demonstrated that the respective degradation efficiencies for EC, DP, PC and PEC were determined to be 54.6, 65.6, 75.6 and 90.1% after 120 min.They also concluded that PECP exhibited better performance in the degradation of ofloxacin [34].

Discussion of PECP degradation mechanism
To elucidate a likely mechanism with regard to the applied PECP, a few number of experiments were conducted.First, in order to investigate the main active radical species in the PECP, three commonly-used scavengers, namely ammonium oxalate (AO) as a hole (h + ) scavenger, benzoquinone (BQ) as a superoxide radical (O 2 °−) scavenger, tert-butanol (TB) as a scavenger for superoxide radicals (OH°) [35] were used.The trapping experiments involving the active species are demonstrated in Figure S2.As can be seen, the respective degradation rates in the PECP in the absence of any scavengers and the presence of AO, BQ and TB were determined to be 91.8,35.8, 79.4 and 86.2% (STZ), 93.2, 39.3, 92.6 and 88.3% (SDZ), 95.8, 42.6, 83.7 and 89.3% (SMX) and 97.1, 45.4, 88.1 and 91.4% (SCT).Clearly, the degradation of SAs by the applied PECP was dramatically hindered after the addition of AO, implying that holes (h + ) play a crucial role in the PECP.
Modification of the energy band structure of ZnO by Co-doping changes the energy levels within the band gap of ZnO.Initially, an electron/hole pair is created under the excitation of incident photons.The free holes combine with H 2 O to form 0 OHs, the non-selective strong oxidising agents.Subsequently, 0 OH and h + react with the adsorbed SAs to generate the degradation products.It is clear that the photo-induced electron-hole pairs should be separated and migrate to the active sites of Co-doped ZnO which is crucial for the SAs degradation.Besides, the doped Co ions can serve as electron traps.These trapping sites can retard the recombination of electron-hole pairs and promote the separation of photo-generated charge carriers [35,36].The likely degradation mechanism for the SAs can be summarised as follows:

Application to real samples
The applicability of the applied PECP for the degradation of the studied SAs in two different real samples, namely tap water and secondary sedimentation effluent was investigated.The characteristics of the tap water and secondary sedimentation effluent is presented in Table S2.All the samples underwent the optimum experimental conditions (pH = 9, applied current = 1.5 mA/cm 2 , SAs concentration = 5 mg/L, time = 90 min) and their corresponding degradation results were illustrated in Figure S3.As can be seen, the respective degradation rates for the PECP for DI-water, tap water and raw wastewater were determined to be 91.8,85.1 and 81.9% (STZ), 93.2, 85.8 and 83.2% (SDZ), 95.8, 87.3 and 85.3% (SMX) and 97.1, 88.3 and 87.1% (SCT).The highly likely reactions taking place between wide variety of ions and organic compounds and free radicals in the real samples could lead to the reduction of the SAs degradation performance.All things considered, the appropriate rate performances indicate that the applied PECP is highly efficient for the degradation of the studied SAs in various water samples.

Conclusion
In the current study, the photoelectrocatalytic degradation of four main sulphonamide antibiotics from aquatic media using a novel Co-doped ZnO nanocomposite was successfully developed.The respective maximum degradation rates obtained under the optimal conditions (pH 9, initial concentration of antibiotics 5 mg/L, and the applied electrical current of 1.5 mA/cm 2 ) at the reaction time of 90 min were as follows: SCT (97.1%),SMX (95.8%),SDZ (93.2%) and STZ (91.8%).The experimental results were well fitted to a firstorder kinetic model.The contribution of the PEC process in the degradation efficiency was greater than that of the electrolysis and photolysis process.The PEC process exhibited an excellent performance in the treatment of several complicated real water samples.More interestingly, compared to other methods, PECP is more associated with 'green' technologies since electricity is involved and the used catalyst will not end up in the environment as secondary toxic materials.Furthermore, the photo anode electrode can be used several times with almost no loss in the removal efficiency.

Figure 1 .
Figure 1.SEM image of Co-doped ZnO on FTO glass.

Figure 2 .
Figure 2. (a) Cross-sectional SEM image of ZnO doped on FTO glass, (b) Cross-sectional SEM image of Co-doped ZnO on FTO glass.

Figure 3 .
Figure 3. EDX spectrum of the Co-doped ZnO on FTO glass.

Table 1 .
Chemical and characteristics of the four SA antibiotics.