Enhancement of photocatalytic activity of Ba-doped CoO for degradation of Emamectin benzoate in aqueous solution

The present study was focused on the preparation of cobalt oxide (CoO) and barium-doped cobalt oxide (Ba-doped CoO) by following the co-precipitation method for the degradation of Emamectin benzoate pesticide in the aqueous medium. The prepared catalysts were characterized using SEM, EDX, and XRD to confirm the formation of catalysts and to observe the variation in the composition of catalysts during the degradation study. It can be suggested from the results of SEM, EDX, XRD, and FTIR analyses that Ba atom has successfully incorporated in the crystalline structure of CoO. The degradation of Emamectin benzoate pesticide was studied under the influence of different factors like solution pH, the dose of catalyst, contact time, temperature, and initial concentration of pesticide. It was observed that solution pH affects the degradation of the pesticide, and maximum degradation (23% and 54%) was found at pH 5.0 and 6.0 using CoO and Ba-doped CoO, respectively. The degradation of pesticides was found to be increased continuously (27–35% in case of CoO while 47–58% in case Ba-doped CoO) with the time of contact. However, the degradation was found to be decreased (23–3% in case of CoO while 47–44% in case Ba-doped CoO) with an increase in temperature. Likewise, in the beginning, degradation was observed to be increased up to some extent with the dose of catalyst and initial concentration of pesticide but started to decrease with further augmentation in the dose of catalyst and initial concentration of pesticide. It may be concluded from this study that doping of Ba considerably enhanced the photocatalytic ability of CoO for Emamectin benzoate pesticide.


Introduction
Pesticides are abundantly employed in modern agricultural process to improve the quality and quantity of the crops and to protect the crops from disease outbreaks and insect infestation (Duan et al., 2018;Wang et al., 2017).Numerous environmental factors cause degradation and dissipation of the sprayed pesticides such as light, microbes, and pH, and hence, the presence of pesticide residues in agricultural products is low and does not cause serious adverse effects (Buerge et al., 2019;González-Curbelo et al., 2017;Zhang et al., 2019).However, their constant and intensive applications necessarily lead to a potential threat to human health and the ecosystem (Calatayud-Vernich et al., 2016;Fevery et al., 2016).Long-term exposure to pesticides results from potential health problems such as immune suppression, Vol:.( 1234567890) neurotoxicity, and reproductive toxicity (Bano & Mohanty, 2020;De Gavelle et al., 2016;Taghizadeh et al., 2019).
Emamectin benzoate belongs to that group of insecticides that activates chloride channels in the nervous system of insects.It is used to control insect pests of ash trees such as emerald ash borer in agriculture and forestry (Cook et al., 2004;Fanigliulo & Sacchetti, 2008;Grosman et al., 2009).Its forestry use is the main source of contamination of groundwater and soil.Its application for the treatment of sea lice causes the contamination of seawater at concentrations that are dangerous for aquatic organisms (Armstrong et al., 2000;Ramstad et al., 2002).It is also recognized for affecting non-target organisms, and its presence in the environment can reduce marine ecosystem processes like nutrient cycling (Tariq et al., 2014).
Therefore, it is of fundamental importance to eliminate Emamectin benzoate from natural water resources and industrial effluents.In the last few decades, several conventional methods have been investigated for elimination of pesticides from wastewater such as chemical oxidation, ion exchange, adsorption, solvent extraction, and reverse osmosis.However, these methods have several disadvantages including partial elimination of contaminants, formation of toxic sludge, and other secondary wastes which need additional purification resulting in increased entire cost and time duration (Thilagavathi et al., 2021).
In this context, it is desirable to investigate new technologies which support the easy elimination of bio-recalcitrant organic compounds such as pesticides.Photocatalytic degradation is one of the most promising technologies for the treatment of wastewater containing pesticides.In this process, the semiconductor material is excited by an energy source having energy greater than the band gap of the semiconductor catalyst (Al-Musawi et al., 2022a;Amarzadeh et al., 2022;Moghaddam et al., 2022).The electron-hole pairs are generated after excitation which either may be recombined or may be reacting with the target substance.The target substance may be an electron donor such as a hydroxide ion or an electron acceptor such as molecular oxygen (Bruno & Antoninho, 2019;Nasseh et al., 2019Nasseh et al., , 2021a)).During photocatalytic degradation, highly reactive species like superoxide and hydroxyl radicals are yielded which attack the organic pollutants and leads help in the purification of water (Al-Musawi et al., 2022b;Chuanxi et al., 2017;Nasseh et al., 2021b).
Considerable efforts have been fascinated to increase the photocatalytic activities of the catalysts like the rate of electron-hole pairs induced reduction-oxidation reaction and, the rate of electron and hole recombination (Nasseh et al., 2022;Rahimi et al., 2022).It is generally acknowledged that the structure and optical properties of the photocatalysts greatly affect the surface charge transfer and electron and hole recombination (Al-Musawi et al., 2022c;Nasseh et al., 2021c).Hence, to increase the photocatalytic activities of the catalysts, there should be a reduction in the rate of recombination of the electrons and holes and an increase in the surface charge transfer.In the past, different strategies have been adopted to enhance the surface charge transfer and to reduce electron-hole recombination such as doping of other metals (Mandal et al., 2021;Iqbal et al., 2021;Dhivya et al., 2022), synthesis of nanocomposite and hetero-structures, and dye sensitization (Iqbal et al., 2021;Saleh & Djaja, 2014).
The present study was focused on the synthesis of CoO and Ba-doped CoO by co-precipitation method and characterized by SEM, EDX, XRD, and FTIR techniques.The synthesized materials were employed as photocatalysts for the degradation of Emamectin benzoate pesticide under sunlight irradiation.The photocatalytic degradation study was carried out under the influence of initial solution pH, contact time, initial pesticide concentration dose of catalyst, and temperature.

Chemicals and reagents
In the present research study, all the chemicals and reagents were of analytical grade and were used without any further purification.In this study, cobalt sulfate, barium chloride, potassium hydroxide, methanol, sodium hydroxide, acetic acid, phosphoric acid, boric acid, hydrochloric acid, and ammonia were used for their respective purposes.

Preparation of standard solution of emamectin benzoate
The standard stock solution of Emamectin benzoate of 1000 ppm was prepared by taking 0.526 mL commercially available (19 g/L) in a 100-mL volumetric flask and diluting up to mark with distilled water.
Vol.: (0123456789) Working standard solutions of different concentrations were prepared in 100-mL volumetric flasks from this stock solution for further research work.The structural formula of Emamectin benzoate is given below.

Preparation of CoO and Ba-doped CoO
In this study, CoO nanoparticles were synthesized by the co-precipitation method which is one of the most successful methods.The pure CoO and Ba-doped CoO nanoparticles were synthesized by the reaction of Co 2+ , Ba 2+ , and OH in methanol.A solution of cobalt sulfate (20.1 mmol) was prepared by dissolving 3.0 g of cobalt sulfate in 100 mL methanol in a beaker (solution A).Another solution of cobalt sulfate (20.1 mmol) and barium chloride (2.4 mmol) was prepared in 100 mL methanol by dissolving 3.0 g of cobalt sulfate and 0.4 g of barium chloride in a beaker (solution B).Similarly, a third solution of KOH (140 mmol) was prepared by dissolving 7.84 g in 100 mL methanol (solution C).Solution C was added to both solutions A and B with constant stirring at 52 °C for 2 h.These solutions were cooled to room temperature and allowed to age for 2 days.The dark brown precipitates so formed were separated by

Structure of Emamectin benzoate
a filtration process and washed with distilled water numerous times.Finally, the precipitates were dried in air and nano-crystalline powders of CoO and Badoped CoO were obtained and stored in air-tight bottles for further study (Nair et al., 2011).

Analytical techniques for characterization
The crystallography of the CoO and Ba-doped CoO was studied by using X-ray diffraction (D8 Advance, Bruker).The surface morphology of the CoO and Ba-doped CoO was studied with the help of a scanning electron microscope (SEM-Model-JSM-5910, Japan JEOL).The elemental composition of CoO and Ba-doped CoO was determined by energy-dispersive X-ray (EDX-INCA 200 Oxford Instruments, UK).To determine the weight of the materials, analytical balance (PA413 OHAUS Corporation, USA) was employed, and pH measurement was performed with a pH meter (WTW-Inolab pH7110).The CoO and Ba-doped CoO catalysts were dried employing an electrical oven (Memmert Celsius, 2005).The concentrations of Emamectin benzoate pesticide were determined by UV-Vis double-beam spectrophotometer (C-7200S, Peak Instruments Inc. USA).

Degradation experiment
To investigate the degradation ability of the CoO and Ba-doped CoO as photocatalysts, the degradation of Emamectin benzoate pesticide was conducted in aqueous media.The degradation experiments were performed by transferring 10 mL of Emamectin benzoate in the concentration range of 2 to 20 mg L −1 in a conical flask containing the desirable amount of photocatalysts (0.01-0.1 g), and the pH was adjusted in the range of 3 to 12 with Britton Robinson buffer solution.The mixture was agitated for different time intervals in the range of 10 to 120 min at different temperatures (30, 40, 50, 60, 70, and 80 °C).The mixture was filtered, and the concentration of nondegraded Emamectin benzoate was determined at 206 nm with the help of a double-beam UV/visible spectrophotometer.
The percent degradation of Emamectin benzoate was determined by using the following formula: Herein, C o is the initial concentration of Emamectin benzoate and C t is the concentration of Emamectin benzoate at time t (min).

Characterizations of CoO and Ba-doped CoO
The synthesized CoO and Ba-doped CoO were characterized by using various techniques before and after the degradation of Emamectin benzoate.The scanning electron microscope (SEM-Model-JSM-5910, Japan JEOL) was used to study the morphology of CoO and Ba-doped CoO, and energy-dispersive X-ray (EDX-INCA 200 Oxford Instruments UK) was utilized for the elemental analysis.

Characterization
The synthesized CoO and Ba-doped CoO were characterized by using different techniques before and after the degradation study.The details of the techniques are given below.

SEM analysis
The SEM analysis of CoO and Ba-doped CoO was accomplished to observe variation in surface morphology before and after doping and degradation of Emamectin benzoate.It may be depicted from the SEM photograph as shown in Fig. 1a that pure CoO consists of inhomogeneous size distribution and spherical aggregated nanoparticles with distinctive boundaries.The aggregation of these nanoparticles is the result of van der Waal forces among the particles (Bagade et al., 2023;Sadia et al., 2021).After doping with barium, the particle size of the nanoparticles was found to be larger to some degrees as compared to pure CoO as shown in Fig. 1b.This may be due to the distribution of Ba atom in the crystalline structure of CoO (Sarteep et al., 2016).It also seemed from Fig. 1c and d, after degradation of Emamectin benzoate, that nanoparticles of CoO and Ba-doped CoO are aggregated more at different points which may be due to the presence of other elements present in the pesticide.

EDX analysis
The elemental composition of the CoO and Ba-CoO was studied by energy-dispersive X-ray analysis before and after doping and photocatalytic degradation of Emamectin benzoate.The EDX results are summarized in Table 1 which indicates that undoped CoO consists of Co, O, Cl, and K.However, Ba was found in the CoO after the doping process which suggests that Ba is successfully doped in the CoO (Ahmad et al., 2021;Saroj et al., 2020).Likewise, additional elements were detected and the quantities of some existing metals were increased after photocatalytic degradation of Emamectin benzoate which suggests degradation of Emamectin benzoate such as C and O.

XRD analysis
The variation in phase structure and composition of the CoO and Ba-doped CoO before and after the degradation of Emamectin benzoate was investigated by using XRD analysis (Mayoufi et al., 2014).It can be illustrated from Fig. 2 that major diffraction peaks of the material seen at 28°, 40°, and 50° should be indexed to ( 110  been reported in previous investigations (Ahmad et al., 2019).After degradation of the Emamectin benzoate, however, the XRD spectrum has become complicated and all the peaks disappeared which suggests that particles of the CoO and Ba-doped CoO are oxidized in the Emamectin benzoate solution (Rizo et al., 2020).

FTIR analysis
The FTIR analysis was carried out in order to probe the functional groups of CoO and Ba-doped CoO.
Figure 3 depicts the FTIR spectra of CoO and Badoped CoO before and after the degradation study of Emamectin benzoate.The major peaks were found at 3381, 2914, 1409, 995, and 662 cm −1 .The broad band at 3381 cm −1 may be attributed to stretching mode of ─OH group and 2914 cm −1 was corresponding to C─H stretching vibration.Similarly, the absorption bands at 1409 and 992 cm −1 can be assigned to C══O stretching vibration (Saiganesh et al., 2021).The prominent peak at about 662 cm −1 can be attributed to the absorption of O─Co─O stretching signifying the formation Co─O bond in the synthesized materials.This figure also illustrates the FTIR spectra of Ba-doped CoO nanoparticles which consists of the same peaks but slightly shifted to 650 cm −1 .The shifting of the peak confirms the doping of barium atom, while the decrease in peak's intensity indicates that doping is low and it does not bother the skeletal structure of CoO to a large extent (Atul et al., 2019).Moreover, the shifting of peaks and variations in their intensities are also an indication of the degradation of pesticide after the degradation study.

Effect of pH
It was found that the electrical charge property of photocatalysts and substrate plays a vital role in photocatalysis (Bahrudin, 2022).Therefore, catalytic degradation of Emamectin benzoate was investigated under the influence of pH by varying the pH of the solution from 3 to 12 with Britton Robinson buffer solution.It may be seen from Fig. 4 that the percent degradation of the Emamectin benzoate was not appreciably influenced by the pH of the solution.
However, an infinitesimal increase in the degradation of Emamectin benzoate was observed at pH 5.0 and pH 6.0 in the case of CoO and Ba-doped CoO, respectively.Therefore, further study was performed on these two pH values.Moreover, this figure also illustrates that degradation of the Emamectin benzoate was increased after doping of the barium in CoO.

Effect of catalytic dosage
The catalytic dose was found to an important experimental parameter for preventing the excessive use of catalysts and for validation of the scope of catalysts in industries.Therefore, the dependency of catalytic dose on the degradation of Emamectin benzoate was studied by varying the amount of CoO and Ba-doped CoO in the range of 0.01 to 0.06 g, while the other parameters were kept constant.It may be illustrated from Fig. 5 that degradation increases initially with augmentation of the catalytic amount up to 0.02 g and decreases after the threshold dose.The enhancement of the degradation in the initial steps is due to various factors like more active sites are available when the dose of photocatalyst is increased which leads to more adsorption of pollutants on the surface of the photocatalyst.Secondly, when the dose of the catalyst is increased, more photons will be absorbed which leads to the generation of more charge carriers.Thirdly, more free radicals are formed as a result of a high dose in the solution which may start the fast degradation rate (Selvam et al., 2013).Therefore, the degradation of Emamectin benzoate pesticide was increased when the dose of catalyst was increased in the initial stages.On the other, degradation efficiency decreases as the dose of the photocatalyst exceeds the optimum dose.This result may be explained that a large amount of the catalyst particles aggregates in the solution which leads to a decrease in the available surface area.Secondly, the absorption capacity of the catalyst is reduced due to the turbidity of the solution at the high dose of the catalyst (Vignesh et al., 2014).Hence, to obtain maximum degradation of Emamectin benzoate, further study was performed at the optimum dosage of the catalyst, i.e., 0.02 g.

Effect of contact time
It has been ascertained that photocatalytic degradation is substantially affected by contact time.
Therefore, the degradation of Emamectin benzoate was studied by varying the contact time from 10 to 120 min with a fixed initial concentration of pesticide (6.66 µg mL −1 ), catalytic dose (0.02 g), and optimum pH of the solution.It can be seen from Fig. 6 that degradation of Emamectin benzoate was found to be increasing gradually with increased contact time.This result may be due to the decrease in the concentration of pesticides and augmentation in the formation of free radicals.Likewise, the surface of the catalyst is unoccupied in the initial stage and available for assimilation of pesticide molecules leading to the rise of degradation efficiency.However, with time, a decrease in the escalation of degradation efficiency was observed due to a decrease in the available sites on the surface of the catalyst (Yeganeh et al., 2022).

Effect of temperature
The temperature of the photocatalytic reaction may influence the efficiencies of the catalysts and the reaction system.In most cases, the rate of reaction is enhanced with an increase in temperature, but in some cases, the reaction rate shows an opposite trend at high temperatures (Zare et al., 2021).Therefore, to observe the effect of temperature on the degradation of Emamectin benzoate, the temperature was varied from 30 to 60 °C with the fixed dose of the catalysts (0.02 g), initial concentration of Emamectin benzoate (6.6 µg mL −1 ), and optimum pH of the solution.The mixture was agitated for 60 min in an orbital shaker and filtered the mixture.The concentration of the Emamectin benzoate was determined by double-beam UV-Vis spectrometer, and the result is illustrated in Fig. 7.It may be seen from the figure that degradation of the Emamectin benzoate increases with temperature up to 40 °C in the case of Ba-doped CoO as a catalyst.It may be due to the increase in kinetic energy of the pesticide molecules which in turn enhanced the mobility of pesticide molecules and increased their interaction with light at high temperatures.However, after 40 °C, a decrease in degradation was observed which may be ascribed to the decreasing of the sorptive forces between the pesticide molecules and the active sites of the catalyst (Ahmed et al., 2020), while in the case of the CoO as a catalyst, the degradation of Emamectin benzoate was continuously decreased with an increase in temperature.Perhaps, this might be attributed to the increasing rate of recombination of the electron-holes pair which leads to desorption of the absorbed molecules of pesticide (Malik et al., 2016).Therefore, further study was performed at the above optimum values.

Effect of initial concentration of pesticide
To explore the effect of the initial concentration of Emamectin benzoate on the degradation efficiency, 0.02 g of CoO and Ba-doped CoO and 30 mL of Emamectin benzoate solution with initial concentrations in the range of 9-10 µg mL −1 were agitated for 60 min of contact time.As illustrated in Fig. 8, it was apparent that initially, the degradation of Emamectin benzoate increases up to a certain concentration and then decreases non-uniformly.At a low initial concentration of pesticide, the adsorption capacity of the photocatalyst is high because of more available active sites on the surface of CoO and Ba-doped CoO photocatalysts for the adsorption of pesticide molecules.However, when the initial concentration of pesticide was increased further, the active sites on the surface were occupied leading to the inhibition of the photocatalytic reaction.Therefore, the degradation of Emamectin benzoate was observed to decrease with further initial concentration (Liu et al., 2019).The high amount of the initial pesticide concentration may also cause the absorption of light, and the photons have access to reach the surface of the photocatalyst which ultimately causes the reduction the photocatalytic degradation (Alkaykh et al., 2020).

Kinetics of photocatalytic degradation
The photocatalytic degradation data were fitted into Langmuir-Hinshelwood kinetic model to determine the rate of degradation of Emamectin benzoate.This model can be represented by the following equation: Herein, C o denotes the initial concentration (mg L −1 ) of Emamectin benzoate and C represents the concentration (mg L −1 ) of Emamectin benzoate after degradation at time t, while k 1 (min −1 ) is the rate constant of Langmuir-Hinshelwood kinetic model and was obtained from the slope of plot ln(Co/C) versus irradiation time (t) as shown in Fig. 9.The values of rate constant of photocatalytic degradation of Emamectin benzoate were found to be 2 × 10 −3 and 1 × 10 −3 min ×1 for CoO and Ba-doped CoO photocatalysts, respectively.The outcomes of the kinetics of photocatalytic degradation indicate that Ba-doped CoO has better degradation ability as compared to CoO photocatalyst which is consistent with other doped photocatalysts as reported in the literature (Wongrerkdee et al., 2023).

Conclusions
In the current study, CoO and Ba-doped CoO were fabricated successfully by the co-precipitation method, and the photocatalytic degradation abilities of Emamectin benzoate were studied in aqueous media.Analytical techniques such as SEM, EDX, and XRD confirmed that barium atoms were doped in the CoO lattice.The initial solution pH effect indicates that pH 5 and 6 are the optimum pH for the degradation of pesticide in the case of CoO and Ba-doped CoO, respectively.The photocatalytic capabilities of CoO and Ba-doped CoO were decreased with temperature and enhanced with contact time.Furthermore, photocatalytic degradation was increased up to a certain value with the initial concentration of pesticide and catalyst dose and then declined with a further increase in the initial concentration of pesticide in the dose of catalyst.It can therefore be concluded that photocatalytic degradation efficiency was enhanced by doping of barium atoms in the CoO lattice.
Fig. 1 SEM images of CoO a and Ba-doped CoO b before c and after d degradation

Fig. 2
Fig. 2 XRD analysis of CoO and Ba-doped CoO before and after degradation

Fig. 4
Fig. 4 Effect of pH on degradation of Emamectin benzoate

Fig. 5 Fig
Fig. 5 Effect of dose of catalyst on the degradation of Emamectin benzoate

Fig. 7
Fig. 7 Effect of temperature on degradation of Emamectin benzoate Fig. 8 Effect of initial concentration of Emamectin benzoate on the degradation

Table 1
EDX analysis of CoO and Ba-doped CoO before and after degradation Vol:. (1234567890)