Systematic assessment of visible light driven photocatalysts for the removal of cefixime in aqueous solution sonophotocatalytically

ABSTRACT A comparative study of Cu-impregnated ZnO and Ni-impregnated ZnO for the sonophotocatalytic degradation of cefixime in aqueous solution under visible light irradiation was carried out. Sonophotocatalysts were prepared by using a wet impregnation method and characterised using different spectroscopic techniques. Sonophotocatalytic efficiency of degradation is superior to photolysis, sonolysis, photocatalysis and sonocatalysis. Different operational parameters like pH, catalyst dosage, different enhancers, scavenger’s effect, initial concentration of drug, reusability of catalyst and catalyst settling time were optimised. Optimum time of 30 min was found for the 45.7% degradation of cefixime using Cu-ZnO and 42.2% degradation using Ni-ZnO photocatalysts at pH 10. The degradation of cefixime increased from 75.3% to 100% with 5 mmol of hydrogen peroxide using 0.1 g/ L of Cu-ZnO and 99.8% using 0.15 g/L of Ni-ZnO, while other oxidising agents were found less effective using (Cu–ZnO-Ni-ZnO) photocatalyst. A very little decrease in degradation was observed with different types of scavengers. Catalyst reusability was checked up to three cycles and good results up to three cycles were achieved and followed pseudo-first-order kinetic model. Furthermore, the sonophotocatalytic activity was tested for real pharmaceutical wastewater resulting in 91 and 89% removal of total organic carbon (TOC) using (Cu-ZnO-Ni-ZnO), respectively. The proposed method was successfully applied on commercial and synthetic sample with satisfactory results.


Introduction
Organic pollutants are toxic chemicals that adversely affect the human health and the surrounding environment [1].Among them, some persist for long period of time in the environment and can accumulate and pass-through food chain from one specie to another [2].These pollutants constitute a diverse group of organic substances such as pharmaceutical waste products [3][4][5].Pharmaceutical industries also rapidly expanded in the country.Drugs of all type enter the water supply when they are improperly disposed [6].According to U.S. geological survey, antidepressants, antibiotics, diabetics & heart medications, codeine and many other pharmaceutical products are all showing up in the water supplies [7,8].The occurrence of pharmaceutical products in the environment causes aquatic toxicity, resistant development in pathogenic bacteria, genotoxicity and endocrine disruption [9,10].Cefixime was selected as a subject.Cefixime is a thirdgeneration cephalosporin and is active against gram positive and gram-negative bacteria.Cefixime is an antibiotic used for several bacterial infections like otitis media, strep throat, pneumonia, urinary tract infections and Lyme disease.This drug is poorly soluble in water and unstable and a low bioavailability of about 40-50% can be adsorbing from gastrointestinal track.These pharmaceutical wastes are actually non-biodegradable and their presence even in low concentration results in the formation of antibiotic resistance bacteria [3,4,[11][12][13].Presence of even trace quantities of pharmaceutical products is undesirable [14][15][16][17].There are several techniques which are common for pollutants removal from wastewater like filtration, chemical oxidation, coagulation-flocculation, microbial degradation, photocatalysis and adsorption [18][19][20].These techniques are effective and quick but are not generally employed due to the associated high chemical and operational costs as well as complex sludge generation and the pollutants are only transferred from one phase to another.In Pakistan, biological treatment is one of the most commonly used methods but due to low biodegradability of many pollutants and chemicals used in various antibiotic processes; the biological treatment alone is not a very good option These processes are facing difficulties in decomposing and mineralising the complex structure of the pollutants [21].
Many advanced processes are used for the removal of pollutants from water samples like electro-Fenton process [22], sono electro-Fenton [23,24], photolysis [25], sonocatalysis [26] and photocatalysis [27,28] but we were interested to use a synergistic effect of photocatalysis and sonolysis, i.e. sonophotocatalysis.In sonophotocatalysis, the combination of these advanced oxidation process speeds up the degradation of organic pollutants in a little time with great efficiency.In photocatalysis, when the photons fall upon the semiconductor photocatalyst it generates a pair of electrons and holes.Electrons in a valence band and holes in a conduction band react with water molecules and sometime oxygen from air and generate free radicals.These generated free radicals are very reactive, and they directly attack on organic molecules and sometime holes also react with organic molecule and degrade it.While in sonocatalysis a production of large number of free radicals occurs by ultrasonic waves, these ultrasonic waves produce cavitation bubbles, when these bubbles collapse, they release high energy which leads to the formation of an electron-hole pair in a semiconductor catalyst which generates even more free radicals OH˙, which is an extraordinary reactive species that attacks pollutants' organic matter in water and leads to its degradation.
The presence of photon, ultrasonic waves and a semiconductor catalyst results in increased degradation of organic pollutants [28][29][30].In the present work, we prepared a metal-impregnated photocatalysts for the degradation of emerging pollutant.ZnO was impregnated with Nickel and Copper.ZnO exhibits broadband energy limitations, low light absorption capacity and rapid recombination rates of electrons and photoinduced holes that prevent the practical use of this technology [31][32][33][34].Nickel and copper were selected because impregnation of these metal ions shifts the band gap of the ZnO from UV to visible region and to enhance photodegradation.Cu was reported to involve in photo-Fenton effect and electron trapping to increase the photoactivity of ZnO and widely used due to its abundant availability, good selectivity, simple process for semiconductor layer formation, low cost and acceptable photocatalytic performance [35][36][37][38].Prepared photocatalysts contains a very low percentage of metal ions i.e. 3% of copper-and nickel-impregnated ZnO, which is considered less toxic to the environment as compared to emerging pollutants selected for degradation.The fruit of proposed method is that we can reuse the prepared photocatalyst many times, even a very trace quantity of photocatalyst can do photocatalysis [39,40].The traces of photocatalysts will be used as a support for the preparation of new visible light photocatalysts.Novelty of the present work is the use of the prepared photocatalyst (Cu-ZnO-Ni-ZnO) for the degradation of Cefixime.The two systems, sonophotocatalysis when combined, it produces an integrated synergistic effect which accelerates the catalytic degradation of organic compounds.

Material and methods
All chemicals used were of analytical purity and used without further purification.From BDH Laboratory Supplies, Poole, BH151TD, England, zinc oxide (ZnO) and hydrated copper chloride (CuCl 2 .2H 2 O) and nickel nitrate (Ni (NO 3 ) 2 ) were purchased.Standard drug references were provided by Cirin Pharmaceutical (Pvt) Ltd., Hatter, Pakistan.Commercial formulations of Cefixime (molecular formula = C 16 H 15 N 5 O 7 S 2 , molecular weight = 453.4g/ mol, λ max = 285 nm) magnetic caps 400 mg, S J & G Fazal Elahi (Pvt) Ltd.Darmstadt.The pH of the antibiotic was adjusted using Britton Robinson's buffer.

Instrumentation
UV/vis spectrophotometer of matched 1 cm quartz cells (Model SP-3000 plus, Optima, Japan), was used for absorbance measurement at maximum wavelength of cefixime (285 nm).Kum Sung Ultrasonic bath with 40 KH frequencies was used as Ultrasonic radiation source.For a visible source, 100 W of Tungsten filament lamp was used for photocatalytic degradation of emerging pollutant.
Surface morphology of ZnO and impregnated Cu-ZnO and Ni-ZnO was analysed by scanning electron microscopy (SEM) using JSM5910 (JEOL, Japan).Samples for SEM analysis were prepared by coating the samples with a thin layer using double carbon tape on aluminium tabs.The surface area was determined by a surface area analyser (Quanta chrome, Nova Station, A) with nitrogen adsorption-desorption isotherms.The samples were aerated before analysis at 100°C for 2 h using a high vacuum line to remove any moisture or adsorbed gases from the surface and pores of the catalyst.Brunauer-Emmett-Teller (BET) method was used to calculate the surface area of the sample.Phase analysis was performed with an X-ray diffractometer (JEOL model JDX-9 C, Japan) at room temperature, using monochromatic Cu-Kα radiation (λ = 1.5418Å) at 40 KV and 30 mA in the range 2θ of 10-80° with 1.03° per minute.The measurement of contents of TOC were analysed using TOC-VCPH analyser (Shimadzu Co., Japan).Chemical Bonding in a photocatalysts were determined by Fourier transformed infrared spectrometer FTIR analysis was performed using Shimadzu QATR 10, IR affinity-1S.

Photocatalyst preparation
Wet impregnation method was used to prepared metals impregnated catalysts over zinc oxide (ZnO) support following the procedure reported in our former paper [20,41].3% Metal salt solutions were prepared and added drop wise into the slurry of 97% ZnO and stirred for 1 h at 60°C at 900 rpm.The catalyst was dried in an oven for 12 hours and calcined in a furnace at 500°C for 4 hours.The calcined material was crushed and passed by mesh size of 150 µm, same procedure was applied for the preparation of Ni-ZnO.The dried sample was ground to powder and screened to a particle size ≤150 µmphotocatalyst that has excitation energy in a visible EMR region were prepared.Impregnation of metals like Cu and Ni with ZnO shifted band gap to visible region.In order to shift ZnO band gap energy to visible region research study was carried out in different conditions which confirmed the conversion of energy gap to visible region (Figure 1) shows that ZnO in presence of UV light show decrease in absorbance, while in case of visible light no decrease in absorbance take place because ZnO band gap is in UV region.(Cu-ZnO, Ni-ZnO) shows a little decrease in absorbance in presence of UV light, it must be because of ZnO while in presence of visible light it shows a maximum decrease in absorbance, which confirms the shift in a band gap from UV to Visible region.

Sonophotocatalytic study
In a typical experiment, in 100 mL flask, a 50 µg/mL of cefixime with optimised photocatalyst weight (ZnO, Cu-ZnO and Ni-ZnO) was taken in a photoreactor.The suspension in the photoreactor was placed in the dark for 30 min to ensure the establishment of adsorption-desorption equilibrium of the cefixime on the surface of ZnO (Cu-ZnO-Ni-ZnO) and then placed under tungsten filament lamp in sonicator for 1 h at optimised pH (pH 10), catalyst weight (0.1 g/ L of Cu-ZnO and 0.15 g/L of Ni-ZnO) and oxidising agent (5 mmol of H 2 O 2 ).At optimised time, 5 mL was taken from this solution and diluted up to 25 mL (10 µg/mL) with distilled water.The solution was centrifuged for 30-45 min for the sake of obtaining clear solution.Absorbance was noted at maximum wavelength of cefixime (λ = 285) [42].Degradation was calculated using equation 1 [41] where C o and C f is the initial and final concentration in mg L −1 of drug solutions after irradiation at time t.Triplicate analysis was carried out for all the experiments.The degradation conditions were investigated by varying different parameters under sonophotocatalytic degradation.For catalyst reusability, the catalyst was washed with distilled water many times and dried in an oven at 110 °C and used again for the degradation of emerging pollutant using the same procedure.TOC analyses were carried out for the real pharmaceutical wastewater before and after sonophotocatalytic reaction to evaluate the feasibility of this process in the industry and % removal of TOC was determined by using equation (2) [43].
where TOCC o and TOCC f are the initial and final concentration in mg L −1 of drug solutions after irradiation at time t.

Catalyst characterisation
The prepared catalysts were characterised using SEM, FTIR, EDX, XRD, TOC and surface area analyser.SEM analysis was used to confirm the impregnation of metal ion on ZnO support as well as morphology of photocatalysts.From SEM images, the impregnation of metal ion on ZnO was confirmed (Figure 2 b, c).The morphology of ZnO, Cu-ZnO, Ni-ZnO, reused Cu-ZnO, Ni-ZnO and reused after treatment Cu-ZnO, Ni-ZnO are given in Figure 2.
The EDX spectra for ZnO, Cu-ZnO, Ni-ZnO, reused Cu-ZnO, reused Ni-ZnO and reused after treatment Cu-ZnO and Ni-ZnO are given in Figure 3.The impregnation of copper and nickel on ZnO was confirmed from EDX spectra in Figure 3b, while Figure 3c and 3e shows the negligible change in concentration of copper and nickel after three times repeatedly used.FTIR analysis is used to identify the chemical bonding and purity of a material.Interatomic vibrations causes the metal oxides with generally absorption band below 1500 cm −1 .The FTIR spectra for ZnO and (Cu-ZnO-Ni-ZnO) are given in Figure S1a and  b.FTIR spectra shows peaks below 1500 cm −1 , which is a figure print region, and confirms the presence of non-metals.Above 1500 cm −1 , the presence of no peaks confirms the absence of any functional group.In addition, the sharp peak below 1500 cm −1 is also a characteristic peak of the Zn-O strain vibration.The peak at 845 cm −1 in ZnO is attributed to the Zn-O bond which is shifted to 847 cm −1 in Cu-ZnO, confirming the impregnation of ZnO by copper.given in (Figure 4b).Height, area and thickness of the peak were calculated using a Scherer's equation for ZnO, Cu-ZnO and recycled Cu-ZnO.The thicknesses of the of ZnO ranges from 17.8 to 44.8 nm, 20.6 to 61.9 nm of Cu-ZnO and 12.2 to 35.5 nm recycled Cu-ZnO and for Ni-ZnO it was found to be in the range of 18.2 to 37.4, 19.8 to 38.1 nm of recycled Ni-ZnO and 20.2 to 41.9 nm for recycled Ni-ZnO after catalytic treatment.By applying Bragg's law, particle to particle distance (d) was found to be in the range of 0.271 to 0.63 nm for ZnO, 0.25 to 0.63 nm for Cu-ZnO and recycled Cu-ZnO, 0.271 to 0.63 nm Ni-ZnO.Similar results were also reported in our previous research work used for the degradation of different textile dyes [44].
Surface area was determined using BET method and it was found 182.274 m 2 g -1 for ZnO, and 185.736 m 2 g -1 for Cu-ZnO and for Ni-ZnO it was 192.88 m 2 g -1 (Table 1.).

Photocatalytic activity
The shift in band gap of ZnO (Cu-ZnO and Ni-ZnO) confirms the impregnation of metal ions.As the metal ions i.e. copper and nickel shift the band gap energy of ZnO from UV (ZnO = 3.3 eV) to visible region (Cu-ZnO = 2.6 ev) (Ni-ZnO = 2.4 eV) (Figure 5).The preliminary study was carried out using solutions of cefixime under different conditions to find out its reactivity such as photolysis, photocatalysis, sonolysis, sonocatalysis and sonophotocatalysis for 1 h.2.6% adsorption was observed using Cu-ZnO.Minimal drug degradation was observed for photolysis and sonolysis, which was increased to 6.4% using the photocatalytic procedure and to 7.1% for the sonocatalytic procedure, which further increased to 9.42% using the using the sonophotocatalytic procedure (Figure 6).The pH was optimised in the range of 2-10 using Britton Robinson buffer and pH 10 was found as the ideal pH for degradation of cefixime.Using Cu-ZnO as photocatalyst 45.7% degradation was observed for cefixime while it was 42.2% by using Ni-ZnO as a photocatalyst (Figure 7).At alkaline pH, cefixime will be negatively charge due to deprotonation of acidic groups (-COOH), making it strong nucleophile which will be very reactive towards the electrophilic attack of the OH.radicals.In alkaline solutions, the OH. is also formed from the hydroxide ions.At acidic pH, the decrease in degradation of cefixime may be due to decrease in the concentration of OH radicals as well as the amine groups are protonated making the molecule electrophilic, therefore electrostatic repulsion occurs between the cefixime and photocatalyst and this catalytic repulsion decreases the rate of degradation of cefixime at acidic pH. the decrease in degradation may also be due to agglomeration of (Cu-ZnO-Ni-ZnO) catalyst at acidic pH, which results in decrease in surface area and in turn decreases the photon absorption.For further studies, basic pH is used as optimised pH for the degradation of Cefixime [41].Furthermore, the point of zero charge for Cu-ZnO was 5 and for Ni-ZnO was 4. The surface of sonophotocatalyst is negatively charge at higher pH and positively charge at lower pH, resulting in decreasing degradation at acidic medium.

Time study
The effect of time on the sonophotocatalytic degradation of cefixime using (Cu-ZnO, Ni-ZnO) as photocatalyst was studied in the range of 10-30 min.A steady increase in degradation was observed up to 30 min using Cu-ZnO and Ni-ZnO.No increase in degradation was observed beyond 30 min (Figure 8).For subsequent studies, 30 min was selected as optimised timing.

Catalyst dosage effect
For the improvement in cost efficiency of the proposed method, the effect of catalyst dosage on the degradation of cefixime was studied.The weight of catalyst was varied in the range of 0.01-0.3g/L at pH 10 using 10 mg/L drug concentration (Figure 9).It was observed that with increasing weight of catalyst (0.01 g/L to 0.1 g/L) using Cu-ZnO and (0.01 g/L to 0.15 g/L) using Ni-ZnO there was an increase in % degradation of cefixime.Maximum degradation of 75.3% was achieved with 0.1 g/ L of Cu-ZnO and 0.15 g/L of Ni-ZnO.Further increase in the weight of the catalyst leads to a decrease in the % degradation of the cefixime.This may be due to the aggregation of the catalyst particles which results in a decrease in the active sites of the photocatalyst for the generation of hydroxyl radicals.In addition, the excessive amount of catalyst causes light scattering and prevents penetration of radiation by decreasing the transparency of the sample, which leads to a decrease in the degradation process.

Enhancer's effect
Several oxidants, such as hydrogen peroxide, sodium perchlorate and potassium peroxydisulfate, have been studied as enhancers of the sonophotocatalytic degradation of cefixime using (Cu-ZnO, Ni-ZnO).The reactions of hydroxyl radicals are given in the following equations (Eq.3-9) and the mechanism has also been published by many researchers [45][46][47][48][49][50][51].The effect of sodium perchlorate on the degradation of antibiotics is due to the capture of the electron generated in the conduction band of the photocatalyst [Eq.10] [49,50].
Potassium peroxydisulphate generates sulphate radicals, which react with water and form OH. Radicals which react with antibiotic and cause degradation [Eq.11-14] [49,50].Using (Cu-ZnO, Ni-ZnO) as photocatalyst at optimum conditions (pH 10, catalyst dose for Cu-ZnO = 0.1 g/L, Ni-ZnO = 0.15 g/L and the initial drug concentration 10 mg/L for 30 min), the concentration of each activator ranged from 3 mmol to 8 mmol.The degradation of cefixime increased from 75.3% to 100% with 5 mmol of hydrogen peroxide and to 77% with 8 mmol of sodium perchlorate and 8 mmol of potassium peroxydisulfate, respectively, using Cu-ZnO as photocatalyst.Whereas for Ni-ZnO catalyst degradation increased from 75.3% to 99.8%, 82.8% and 79.5% with 5 mmol of hydrogen peroxide, 8 mmol of sodium perchlorate and 8 mmol of potassium peroxydisulfate, respectively (Figure 10).Among the enhancers, hydrogen peroxide was found the most effect with both catalysts.The confirmation of mineralisation of cefixime was carried out using TOC analysis.The experimental conditions to performed TOC were 10 mg/L cefixime using 5 mmol of hydrogen peroxide at pH 10 for time of 2 h and catalyst dosage of 0.1 g/ L of Cu-ZnO and 0.15 g/L of Ni-ZnO, under tungsten filament lamp (100 W).The % removal of TOC before and after the degradation was calculated using equation ( 2) and it was found as 91% and 89% removal using (Cu-ZnO-Ni-ZnO), which confirms the free radical mechanism of reaction also.

Effect of initial drug concentration
The effect of the initial concentration of each drug was investigated in sonophotcatalytic degradation in the range of 10 mg/L to 100 mg/L.It was observed that for 10 mg/mL of cefixime at optimised conditions (pH 10, for Cu-ZnO 0.1 g/L, for Ni-ZnO 0.15 g/L, 5 mmol hydrogen peroxide) 100% degradation of cefixime was obtained using Cu-ZnO as photocatalyst while 99.8% degradation using Ni-ZnO.The percent degradation of the drug decreased with increasing initial concentration of the drug with both catalysts (Figure 12).With increase in initial concentration of cefixime, decrease in concentration of OH˙radicals occurred that may be because of increase in initial concentration of drug, more molecules of drug are adsorbed on the surface of catalyst (Cu-ZnO, Ni-ZnO).As the intensity of light and catalyst are constant and at that time light penetration is less because of the intensity of the  solution and path length of the photons entering the solution is decreased, resulting in fewer photons reach the catalyst surface.Hence the production of OH˙ radicals are reduced.Therefore, the drug initial concentration also affects the degradation [53].

Catalyst reusability
To assess the practical importance of reusing the prepared photocatalysts, the sonopotocatalytic activity of photocatalysts (Cu-ZnO, Ni-ZnO) was carried out for the degradation of cefixime.The dried catalysts are recovered and washed with various polar organic solvents (ethanol, methanol, acetone and acetonitrile) then with distilled water.The effect of the solvents used to wash the catalyst was the same due to the solubility of the adsorbed drug in organic solvents.Therefore, ethanol was chosen because of its cost, availability and safety.After drying, the catalyst was again used with a new drug solution under optimised degradation conditions.In the case of Cu-ZnO, the degradation decreased from 100% to 66.8% for the third use with increasing degradation time, while in the case of Ni-ZnO, the degradation efficiency increased from 99.8% to 64.9% for the third time of reuse (Figure 13).

Sample application
The proposed sonophotocatalytic method was applied to the sample of synthetic and industrial cefixime.The sonophotocatalytic degradation of the samples was carried out under optimised conditions and % degradation was calculated.It was observed that the synthetic cefixime sample was degraded 89.6% using Cu-ZnO and 91.8% using Ni-ZnO as a photocatalyst with 60 min reaction time and industrial waste containing sample degraded 67% using Cu-ZnO and 70% using Ni-ZnO with 60 min reaction time (Figure 14).

Kinetic models
Different kinetic models were applied for the determination of rate constants on the sonophotocatalytic degradation of cefixime.
A linear plot of log(q e-q t ) against t was plotted for the pseudo first order kinetic equation in order to determine the rate constant and degradation efficiency using (Cu-ZnO, Ni-ZnO) as sonophotocatalyst (Figure S2) and pseudo second order kinetic model for (Cu-ZnO, Ni-ZnO) (Figure S3).And the rate of degradation of Cefixime using Ni-ZnO and Cu-ZnO was found the same (Table S1).

Mechanism of degradation and comparison with other methods
The possible degradation mechanism (Figure S4) shows that when catalyst exposed to visible light, an electron hole pair is formed under photocatalytic degradation based on equations (2-8), afterwards the free hydroxyl radicals are generated and those free hydroxyl radicals attack on different positions of cefixime and convert cefixime to intermediates which are highly unstable [54, [55][56][57][58].During ultrasonic cavitation effect, sonocatalytic mechanism takes place through three routes including sonoluminescence, 'hot spot', and oxygen atom escape.Sonoluminescence consists of light emitting (200−700 nm) with a relatively high intensity which is created by ultrasonic radiations.The light energy from sonoluminescence is greater than the band gap of selected photocatalysts could induce electrons for generating electron/hole pair, which is reactive to form free radicals.Decomposition of water molecules by heat energy of ultrasonic cavitation effect under 'hot spot' generated free radicals.These intermediates are further attacked by free radicals and acoustic cavitation energy from sonicator in a form of ultrasonic radiations and convert it into H 2 O, CO 2 and different mineral acids.Furthermore, the graphical representation of degradation by visible light induced photocatalysts are given in (Table S2) (Figure S5).

Conclusion
A comparative study of Cu-impregnated ZnO and Ni-impregnated ZnO for the sonophotocatalytic degradation of cefixime in aqueous solution under the irradiation of visible light was performed.Using 0.1 g/L of Cu-ZnO for 10 mg/mL of drug at pH 10100% degradation of cefixime was obtained while 99.8% degradation of cefixime achieved using 0.15 g/L of Ni-ZnO.The anions (chlorides, sulphates, and carbonates) reduce the effectiveness of the degradation of cefixime.Scavenger's study shows that the decreasing efficiency of chlorides was greater than that of carbonates and sulphates.Chloride anions reduce the efficiency of degradation up to 77.6%, carbonates up to 80.5% and sulphates 79.8%.The results showed that both catalysts followed pseudo first order kinetic model, recycling capacity of both the photocatalysts were excellent.The method allowed analysing of the pharmaceutical effluents containing µg/mL levels of the studied compound.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 6 .
Figure 6.Degradation of cefixime using different procedures at different time (min).

Table 1 .
Surface area, pore volume and pore size of ZnO (Cu-ZnO and Ni-ZnO).