Efficient photodegradation of orange II dye by nickel oxide nanoparticles and nanoclay supported nickel oxide nanocomposite

The Nickel oxide nanoparticles (NiO NPs) and Nickel oxide/Nanoclay nanocomposite (NiO/Nc NC) were synthesized by chemical reduction method and used for the photodegradation of Orange II dyes in aqueous solution. The synthesized samples were characterized through SEM, EDX, FTIR and TGA. The SEM analysis presented that the NiO NPs are spherical with irregular shapes and sizes. The size of particles is in the range of 100–400 nm. It was also observed that the NiO NPs are deposited on the surface of Nanoclay. The EDX study confirmed the formation of metal NPs and verify and purity. The FT-IR spectra of NiO and NiO/Nc NC presented peaks at 432 cm−1 and 504 cm−1, which correspond to NiO while peak at 1090–940 cm−1 is due to Si–O stretching vibration of orthosilicate anions and Si–O–Al groups. The photodegradation study illustrated that the NiO NPs and NiO/Nc NC degraded about 93 and 96% Orange II in aqueous medium, respectively within 20 min. The effect of various parameters like initial concentration of dyes, pH, recycled catalysts, and catalyst dosage were also studied.


Introduction
Dyes have wide range of applications in various industries like textile, paper and pulp, food and, dye industries. It is reported that the textile industries consume about 10×10 3 various dyes and pigments and worldwide more than 0.7 million tons of dyes are synthesized annually (Naderpour et al. 2012;Khan et al. 2022). The discharge of effluents from these industries causes considerable water pollution. It is predicted that about 1-15% of dyes are released during the processing or manufacturing operations into wastewater stream, which are toxic, carcinogenic, and non-biodegradable due to their large size and complex structures (Zhu et al. 2009;Saeed et al. 2015). Water contamination by these coloring materials is a serious life-threatening and harmful to the surrounding environment, aquatic life, and particularly to human beings (Khan et al. 2020d). Such types of organic coloring compounds can be removed by different methods such as precipitation, adsorption, ozonation, flocculation, reverse osmosis, and ultrafiltration (Santhanalakshmi and Komalavalli 2012;Khan et al. 2020b). These conventional treatment methods were found ineffective in handling wastewater containing synthetic dyes. Each conventional method of treatment has its disadvantages, which limit their practical employing not only in terms of cost but also in terms of feasibility, efficiency, reliability, sludge production, practicability, environmental impact, operation difficulty, pre-treatment requirements, and the generation of toxic byproducts (Khan et al. 2020d). Recently, semiconductor photocatalysis attained great attention due to its advantages over traditional techniques because of their efficiency during the photodegradation of organic compounds into simple and nontoxic species like CO 2 and H 2 O (Solomon et al. 2012). During the photocatalytic process, the semiconducting materials / nanoparticles absorb photon equal to or more than energy gap, which produces the positive holes and electrons. The positive holes can react with water molecules and generate hydroxyl radicals ( • OH). While the electrons are trapped by O 2 molecules and as a result, the formation of reactive superoxide radical ( • O 2 ) occurs. Both • OH and • O 2 − radicals are highly reactive and cause degradation of organic dyes Gao et al. 2013). Several metals and metal oxides NPs like Silver (Sinha et al. 2014), ZnO (Yu et al. 2004 (Khan et al. 2020a), etc. were used as photocatalysts for the photodegradation of organic pollutants.
In the present study, NiO NPs and NiO/Nc NC were synthesized by chemical reduction method, where Nanoclay is used as a supporting material for NiO NPs. Ni was selected due to its excellent electronic, optical, magnetic, chemical, and catalytic properties (Barakat et al. 2013;Hashemi et al. 2017). NiO NPs were reported for synthesis of organic compound (Sachdeva et al. 2013;Nasseri et al. 2015), catalytic transfer hydrogenation (He et al. 2018), water splitting (Wang et al. 2017), photocatalytic degradation of dyes (Jayakumar et al. 2017;Akbari et al. 2020), etc. The NiO NPs and NiO/Nc NC were then used as a photocatalyst for the photodegradation of Orange II (O II) dye in aqueous medium under UV-light irradiation. O II is monoazo anionic and acidic textile dye and its chemical name is 4-(2-hydroxy-1 naphthyl) azobenzene sulfonic acid. Their stability is valuable in the textile manufacturing industry but makes its removal difficult, such as in wastewater treatment plants, where it does not undergo biological degradation. It shows resistance to degradation through light, common acids or bases, and the action of O 2 (Khan et al. 2020a). The effect of irradiation time, initial concentration of dye, pH of the medium, catalysts sustainability, and catalyst dosage were evaluated. The photodegradation of Orange II dye was monitored by using UV-Vis spectrophotometer.

Materials
Nickel chloride hexahydrate was purchased from BDH Company (England). The analytical grade sodium hydroxide and hydrochloric acid were purchased from Riedel-de Haen Company. Orange II dye was purchased from Sigma-Aldrich.

Preparation of NiO nanoparticles and NiO/nanoclay nanocomposite
About 11.9 g of nickel chloride hexahydrate was dissolved in 250 mL double distilled water and then NaOH (2 M) was added to it dropwise until the pH became basic. After basification of the solution, 0.5 g of Nanoclay were added and dispersed in the basified solution through sonication. The obtained mixture was continuously stirred and heated at 80 °C for 3 h. The NiO/Nc NC were cooled, filtered, and washed with double distilled water until the particles became neutral. The NiO/Nc NC was dried in an oven for overnight and stored for further use. Similarly, NiO NPs were also synthesized through the same protocol without adding nanoclay.

Photodegradation of orange II dye
The NiO NPs and NiO/Nc NC were used as photocatalysts for the photodegradation of orange II (O II). In degradation reaction, 10 mL OII (90 ppm) dye solution and known amount of photocatalyst was taken in 25 mL beaker and placed in dark for 20 min in order to achieve adsorption-desorption equilibrium. The mixture was kept under the UV light for specified time. After completion of degradation reaction time, the photocatalyst was separated from the reaction mixture via centrifugation (time = 10 min, rpm = 1200). The photodegradation study was carried out by UV/VIS spectrophotometer and the percent degradation of O II in aqueous media was calculated by the following equation: where "A o " represents the initial absorbance and "A" represents the dye absorbance after UV irradiation.

Characterization
The morphological study of NiO NPs and NiO/Nc NC was carried out by JEOL, JSM-5910 SEM. The EDX analyses of NiO NPs and NiO/Nc NC were performed on EDX (Model INCA 200/Oxford Instruments, UK, company oxford). The FT-IR study was performed by FT-IR spectrometer (Perkin Elmer, serial number 95120). The TGA analysis was carried out through Perkin Elmer USA in inert atmosphere. The photodegradation study of OII was monitored through UV-VIS spectrophotometer (UV-1800, Shimadzu, Japan). Figure 1 shows the morphological study of NiO NPs and NiO/Nc NC. Figure 1a shows the morphology of NiO NPs. The NPs are found to be spherical with irregular shape and the size are in nano range. The NPs are mostly found in agglomerated and bulky forms. Figure 1b illustrates the

SEM analysis of NiO NPs and NiO/Nc NC
SEM image of NiO/Nc NC. The image clearly shows that the NiO NPs are spherical and present both in dispersed and aggregated form on the surface of the nanoclay. The images confirmed that the average particle size of NiO/Nc NC is below 100 nm.    Fig. 4b. The material loses weight quite gradually at 120 °C because of the vaporization of residual water loss. After this stage, an abrupt decomposition occurs under these conditions all the organic fraction present in the nanoclay is destroyed.

Photodegradation of orange II Dye using NiO NPs and NiO/Nc NC
The NiO NPs and NiO/Nc NC were utilized for the photodegradation of O II dye under UV-light irradiation. Figure 5a, b shows the UV/VIS spectra of O II dye before and after photodegradation in the presence NiO NPs and NiO/Nc NC respectively. The degradation of O II dye was calculated by the relative intensity of the UV/ VIS spectra that gave highest absorbance peak at 482 nm. Figure shows that the degradation of O II dye gradually increased with increasing the irradiation time. Figure 5c demonstrates the %degradation of O II dye photodegraded by NiO NPs and NiO/Nc NC. The results presented that the NiO NPs and NiO/Nc NC degraded about 59 and 70% of O II dye within 5 min, which increased up to 93 and 96%, respectively as the irradiation time increased to 20 min. Figure 6a, b shows the digital photograph of O II dye before and after degradation reaction under UV-light photodegraded by NiO NPs and NiO/Nc NC respectively, as a function

Sustainability of catalyst
The photodegradation of O II was also carried out by recycled catalysts in order to check their sustainability. The NiO and NiO/Nc NC catalysts were recovered from the reaction mixture, washed thoroughly with double distilled water, and dried overnight before the next run. Figure 7a,  In the semiconductor photocatalysis, the photon having greater or equal energy to the energy of bandgap of photocatalyst is required. As the light energy falls on the NPs, the electron (e − ) of the valence band (VB) are excited to conduction band (CB) and create a positive charged holes (h + ) in the valence band. The conduction band electron (e − ) react with O 2 species as an electron acceptor and result in the formation of strong oxidizing agent superoxide anion radical ( . O 2 − ). Similarly, the h + of the VB reacts with H 2 O molecule and generates hydroxyl radical ( . OH). Both radical are liable for the degradation of dyes. These steps are summarized in the following reaction equations and can be easily understood from Fig. 8. (1) NPs → NPs e − + h +

Effect of photo catalyst dosage
The effect of amount of photocatalyst on the photodegradation of OII dye was also checked by adding various dosage of catalyst (0.01, 0.015, 0.02, 0.025, 0.03 g) applying constant irradiation time (5 min) and initial dye concentration (90 ppm). Figure 9a, b demonstrate the UV/Vis spectra of OII dye using different dosages of NiO NPs and NiO/Nc NC, respectively. Figure 9c represents %degradation of O II dye employing different dosages of NiO and NiO/Nc NC. The results revealed that 0.01 g of NiO NPs and NiO/Nanoclay NPs degraded about 34% and 36%, respectively. Increasing the photocatalyst dosage causes an increase in %degradation and it was observed that 0.03 g of NiO NPs and NiO/Nc NC degraded 76% and 90%, respectively. It was observed that with an increase in catalyst amount, the rate of degradation of dye was improved due to the increase in the number of active sites for dye degradation reaction (Reutergårdh and Iangphasuk 1997).

Effect of initial dye concentration
The effect of initial dye concentration on dye degradation was evaluated by degrading O II dye at various initial The decrease in photodegradation of dye with increasing initial dye concentration might be due to the color intensity which decreases light penetration and results in generation of less number of radicals (Sauer et al. 2002).

Effect of pH of the medium
Different industries like cosmetics, pharmaceuticals, paper, paint, etc. discharge their effluents at various pH and thus pH of the medium is very important parameter and is necessary to investigate the effect of pH on photodegradation of dyes (Siddique et al. 2014;Rache et al. 2014). The effect of pH of the medium was evaluated by degrading O II dye in different pH medium (2, 4, 6, 8 and 10) keeping dye concentration (90 ppm), irradiation time (5 min) and photocatalyst dosage (0.02 g) constant. Figure 11 represents  with increasing irradiation time, photocatalysts dosage and decreases with increasing initial dye concentration and pH of the medium.
Author contributions All the authors contributed equally to the compilation of the manuscript.
Funding Not applicable.

Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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