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Original Article
:19;
2202025
doi:
10.25259/AJC_220_2025

Green NiO nanoparticles: A sustainable solution for photocatalytic treatment of industrial color effluents

, , , , ,
aDepartment of Chemistry, College of Science, King Saud University, , Riyadh, Saudi Arabia
bDepartment of Chemistry, Don Bosco Institute of Technology, Mysore Road, Bangalore, Karnataka, India
cDepartment of Chemistry, JSS Science & Technology University, Mysore, Karnataka, India

*Corresponding authors: E-mail address: sfadil@ksu.edu.sa (S. Adil); shubhapranesh@gmail.com (J. P. Shubha)

Received: , Accepted: ,
© 2026 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

In this work, NiO nano-sized photocatalyst has been effectively synthesized by a proficient and cost-effective method utilizing coconut bark extract as a novel fuel for the combustion protocol. The synthesized NiO nano-sized photocatalysts were characterized using various analytical techniques, including UV-visible absorption spectroscopy, infrared spectroscopy, and X-ray diffraction (XRD). The morphology and elemental composition of photocatalysts were analyzed by scanning electron microscope (SEM) and electron-dispersion spectroscopy. Photocatalytic activities of the synthesized material are applied to degrade ten synthetic dyes and three industrial effluent samples (EFS) collected from cotton and silk dyeing units. NiO nano-sized photocatalyst degraded 56.7% of the Amido black (AB), 96.6 % of Acid orange (AR) 10, 99.9% of AR 7, 87.7% of Alizarin red S (ARS), 99.7 % of Carmoisine (CAR), 70.8% of Metanil yellow (MY), 99% of Methyl orange (MO), 97.8% of the Methylene blue (MB), 95.5 % of Rhodamine B (RB), 99.9% of Crystal violet (CV) dyes and 62 % of EFS 1, 55% of EFS 2, and 46% of EFS 3 in 100 mins under UV light irradiation at pH 7. A plausible photocatalytic process also explains the remarkable photocatalytic efficiency of NiO nano-sized photocatalyst over dyes and EFS. The results obtained in this study are consistent with previously reported findings. For instance, NiO nanoparticles synthesized through annealing NiCl₂ demonstrated a 72.5% degradation of MB dye, whereas the current protocol achieved a significantly higher degradation efficiency of 97.8%. The photocatalyst reported herein can be a promising candidate for wastewater remediation applications.

Keywords

Anionic dyes
Cationic dyes
Green synthesis
NiO
Photocatalysis

1. Introduction

Recent studies have highlighted significant advancements in environmental and chemical research, as demonstrated by the development of novel nanomaterials for water purification [1], the identification of persistent organic pollutants in aquatic ecosystems [2], the exploration of sustainable catalytic processes [3], the application of machine learning in predicting chemical reactions [4], and the investigation of microbial degradation of environmental contaminants [5].

There are various contaminants in the water that can be classified into three categories: organic (including dyes, pharmaceuticals, and cosmetics), inorganic (such as heavy metals), and biological (including microorganisms) [6-10]. Among the organic pollutants, organic dyes are further classified into cationic and anionic dyes based on the kind of interaction with chemical components of the fabric; they are basic and acidic in nature, respectively. These organic dyes used in the textile industry are specifically viewed as significantly hazardous. They block sunlight from entering water bodies and are capable of absorbing dissolved oxygen from the environmental aquatic apparatus [11,12]. This has a negative impact on the aquatic ecosystem and may also interfere with the natural photosynthetic process [13]. As a result, untreated textile effluent seriously threatens the long-term wellness of both terrestrial and aquatic life as well as the health of the surrounding ecosystem [14]. Moreover, their high stability causes these organic contaminants to linger in the environment longer. Hence, its degradation using semiconductor-based photocatalysis is a promising solution for environmental remediation [15,16]. Numerous physicochemical and biological mechanisms have been used to break down organic dyes, and fresh approaches are currently being actively investigated [17-25].

Due to the utilization of complex oxidation techniques, hazardous dye degradation from industrial effluents has recently attracted a lot of scientific interest [26-39]. These procedures have been found to be economically viable, environmentally friendly, and effective at removing a wide range of complex colors and hazardous organic compounds from wastewater. Photocatalysts and light irradiation, such as solar or ultraviolet light, can be used to degrade these dyes maximally, and are commonly used in advanced oxidation processes. Among several catalysts, semiconductor-based photocatalysts have been extensively employed to degrade harmful organic dyes. One of the significant transition metal oxides among them is NiO, which is a p-type semiconductor with a basic cubic lattice with notable magnetic and electrical properties [40-49]. Due to its photocatalytic [50,51], antibacterial [52,53], sensing [54-56], electrical [57], and optical [58] properties, NiO is well studied. These NiO NPs have been extensively prepared using various green approaches such as using plant extracts, microorganisms, and biopolymers [59-65].

Contrarily, solution combustion is a straightforward and inexpensive technique, efficient in terms of both time and energy, and easy to use [57,58,62,63]. Additionally, it may be scaled up to create enormous quantities of nanomaterials. During combustion processes, which are self-propagating high-temperature processes, an external thermal source ignites an exothermic reaction mixture. This leads to an abrupt increase in temperature, which rapidly disperses throughout the heterogeneous mixture in a self-sustaining manner, resulting in the formation of the solid substance. Conventional combustion synthesis often uses a powder mixture as the first reaction medium; however, when a reactive solution is employed in its place, the procedure is known as solution combustion. This method often involves a self-sustaining reaction in combinations that contain multiple fuels and metal nitrites. To put it in a different context, when heated to a moderate temperature (150-200°C), liquid solutions of the proper reagents ignite themselves, resulting in a quick rise in temperature that ultimately helps in the production of fine solid products with a particular composition. This technique has been utilized to produce a range of nanomaterials using different fuels and essential components.

Cocos nucifera (coconut palm) is widely farmed for its fruit. It is the most economically significant species of the palm tree, with coconuts being one of the main tropical crops. Medium-chain fatty acids are abundant in coconut meat, which can be dried or consumed fresh. The coconut water can be consumed on its own or mixed with other liquids. Harvested coconuts also yield copra, the dried meat from which coconut oil is made. Vegetable oil is primarily obtained from coconut oil. Coconut fibers are used to manufacture ropes, mats, baskets, brushes, and brooms that are extremely saltwater resistant. Both the stem and the bark are antiseptic. It is used to treat toothaches and scabies. The coconut tree’s entire body has medicinal, domestic, and industrial applications [60].

This study, presents a straightforward solution combustion method for obtaining the green NiO nano-sized photocatalyst from aqueous coconut bark extract. The synthesized NiO nano-sized photocatalyst was used for the degradation of seven anionic dyes: (i) Amido black 10B (AB), (ii) Acid orange 10 (AO10), which is used in the production of paper, wood, ink, and biological dyes. (iii) Acid orange 7 (AO7), (iv) Alizarin red S (AZS), (v) Carmoisine (CAR), (vi) Metanil yellow (MY), and (vii) Methyl orange (MO); three cationic dyes: (viii) Methylene blue (MB), (ix) Rhodamine B (RB), and (x) Crystal violet (CV); and three effluent samples: EFS1, EFS2, and EFS3, which were collected from cotton and silk dyeing units.

2. Materials and Methods

2.1. Materials

With no further purification, raw materials of analytical grade Ni(OCOCH3)2 .4H2O (Sigma Aldrich) were purchased from a local supplier and used as is. The dyes AB (Loba Chemie, Gift sample), AO8 (Sigma Aldrich), AO10 (Sigma Aldrich), MO (SD fine chemicals), MY (Sigma Aldrich), CAR (Sigma Aldrich), AO7 (Sigma Aldrich), MB (SD fine chemicals), RB (SD fine chemicals), AZS (Sigma Aldrich), and CV (SD fine chemicals) were purchased from the local supplier. The solutions were prepared with distilled water. BOROSIL glassware was utilized throughout the study.

2.2. Collection of effluent samples

Three effluent samples (EFS) were collected from different silk and cotton dyeing units near Tiptur, Karnataka, India. The samples were filtered, kept aside for two days, and centrifuged (1200 rpm). The pH was determined using a pH meter. For the degradation studies, pH 7 was maintained by adjusting the pH of EFS, and used for degradation studies.

2.3. Preparation of coconut bark extract

Aqueous extract of the coconut bark was prepared by boiling 100 g of small pieces of coconut bark in 250 mL of distilled water until the volume reduced to 50 mL. Then, it was evaporated and dried at 80°C for four hours in a hot air oven. Then, it was stored in an airtight vial and used to prepare the desired NiO nano-sized photocatalyst.

2.4. Synthesis of NiO nano-sized photocatalyst

Stoichiometric quantities of Ni(OCOCH3)2.4H2O (0.4976g) were dissolved in 100 mg of coconut bark extract at persistent stirring for approximately 20 mins at 40°C. The obtained jelly mass was transferred to a silica crucible in the muffle furnace, maintained at 400°C. After 10 mins, a dark (greyish-black) powder was obtained, which was annealed at the same temperature for 3 h.

2.5. Experimental setup for photocatalysis

The experimental setup consisted of a glass bowl (250 mL), a UV lamp of 60 W a magnetic bit, and a magnetic stirrer (Make: Remi). The speed of the magnetic stirrer was set at 450 rpm. A 100 mL of 5, 10, 15, and 20 ppm dye solution and effluents at 5, 10, 15, and 20 mg of NiO nano-sized photocatalyst were taken for the degradation tests. For 30 mins, these solutions were kept in a dark environment to aerate. Solutions were then continuously agitated and subjected to UV light. About 3 mL of the aqueous mixture was taken out of the reaction solution and centrifuged every 30 mins. A UV-visible spectrophotometer (Labman: LMSP-UV1900) was used to measure the absorption of the clear solution after it had been removed and placed in a cuvette.

3. Results and Discussion

3.1 Characterization of NiO nano-sized photocatalyst

The synthesized NiO nanosized photocatalyst was characterized by UV-visible absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The morphology and elemental composition of the photocatalyst were examined by scanning electron microscope (SEM), Transmission electron microscope (TEM), and electron dispersion spectroscopy (EDX).

3.1.1 Powder X-ray diffraction

The powder XRD spectroscopic investigation is used to understand the NiO nano-sized photocatalyst crystal structure produced by using coconut bark extract as fuel, as shown in Figure 1. The NiO nano-sized photocatalyst exhibited a diffraction pattern of 37.23°, 43.22°, 62.88°, 75.36°, and 79.5°, which is identical to the cubic crystalline phase NiO (JCPDS file no. 4-835) with space-group Fm-3m (no. 225), a = 4.1769 Å. The average crystallite size of the as-synthesized NiO was calculated by employing the Debye-Scherrer equation and was found to be 11.02 nm.

XRD analysis of the synthesized NiO nano-sized photocatalyst.
Figure 1.
XRD analysis of the synthesized NiO nano-sized photocatalyst.

3.1.2. UV-visible spectrum analysis

The UV-Vis absorption spectrum of the produced NiO nano-sized photocatalyst has been shown in Figure 2. The synthesized sample’s absorption peaks at about 367 nm and ranges from 800 to 200 nm, suggesting that the synthesized substance is photolytically active in the UV domain [61]. The Kulbeka-Munk equation calculates the band-gap of the synthesized NiO nano-sized photocatalyst and was found to be 3.93 eV [23]. A broad spectrum of light absorption promotes effective photocatalytic breakdown and enhances performance [61,62].

(a) UV-Vis absorption and (b) band gap spectra of the prepared NiO nano-sized photocatalyst.
Figure 2.
(a) UV-Vis absorption and (b) band gap spectra of the prepared NiO nano-sized photocatalyst.

3.1.3. Fourier transform infrared spectrum analysis

For the detection of functional groups present on the photocatalyst surface, FT-IR spectroscopy was frequently used. Figure 3 displays the FTIR spectrum of NiO nano-sized photocatalyst. The absorption peak noticed below 422 cm−1 could be associated with the stretching vibrations of the (Ni–O) bond [63]. Absorbing CO2 from the byproducts of burning coconut bark extract during the manufacture of NiO nano-sized photocatalyst is the cause of other absorption peaks, such as the broad peaks 809 cm-1 (C-H out of plane deformation) and 1386 cm-1 (C-C stretching).

FTIR spectrum of prepared the NiO nano-sized photocatalyst (Peak at 422 cm-1 in the inset figure shows the stretching frequency of Ni-O bond).
Figure 3.
FTIR spectrum of prepared the NiO nano-sized photocatalyst (Peak at 422 cm-1 in the inset figure shows the stretching frequency of Ni-O bond).

3.1.4. Morphological analysis of the NiO nano-sized photocatalyst by SEM and TEM

SEM examination was used to analyze the morphological characteristics of the synthesized NiO, and the resulting findings have been shown in Figure 4. The NiO nano-sized photocatalyst comprises clusters of particles, as shown in Figure 5, which is a lower magnification SEM micrograph. The higher magnification SEM micrograph has been shown in Figure 6.

SEM of the as-fabricated NiO nano-sized photocatalystat different scales (a) 4 μm and (b) 2 μm.
Figure 4.
SEM of the as-fabricated NiO nano-sized photocatalystat different scales (a) 4 μm and (b) 2 μm.
EDAX spectrum of synthesized NiO nano-sized photocatalyst.
Figure 5.
EDAX spectrum of synthesized NiO nano-sized photocatalyst.

EDX spectroscopy is used to examine the elemental composition of produced NiO nano-sized photocatalyst (Figure 5), which revealed that the elemental composition was, as desired, in line with the stoichiometric proportions and no additional oxides of Ni that could have a synergistic effect on the efficiency of the photocatalytic reaction formed.

The TEM pictures of the NiO nano-sized photocatalyst have been displayed in Figure 6. The low- and high-magnification images in Figure 6(a–c) indicate that the spherical particles were dispersed throughout the sample and that there were a few instances of agglomerations. The particles’ diameters fell between 20 and 60 nm. The material’s crystalline shape was indicated by the chosen region electron diffraction pattern (Figure 6d), and the acquired reflection planes closely matched the data inferred from the XRD pattern.

TEM of the as-fabricated NiO nano-sized photocatalyst with (a) low magnification (200 nm), (b) high magnification showing particles (50 nm), (c) HRTEM showing lattice fringes, and (d) electron diffraction image.
Figure 6.
TEM of the as-fabricated NiO nano-sized photocatalyst with (a) low magnification (200 nm), (b) high magnification showing particles (50 nm), (c) HRTEM showing lattice fringes, and (d) electron diffraction image.

3.2. Photocatalytic activity

3.2.1 Photocatalytic efficiency measurements of NiO nano-sized photocatalyst for the photodegradation of dyes and effluent samples

The amount of hydroxyl ions significantly affects the photocatalytic efficiency of nano-photocatalysts. On the other hand, factors such as surface area, particle dimension, band gap, profile, and crystalline arrangement play important roles in semiconductor photocatalysis [48]. The light absorption causes holes and electrons to develop on the semiconductor surface; the released holes and electrons either rejoin or participate in the process. If the charge carriers can access an external surface, they will go to the area where semiconductors capture electrons and hydroxyl radical ions trap holes, forming OH- and HO2.. A very active and non-stable chemical species, hydroxyl radical ions (OH.) play a major role in the photocatalytic degradation of dyes.

Coumarin was chosen as a compound model, an easy and sensitive method for OH. detection to identify whether the OH. radicals were being created by NiO nano-sized photocatalysts. When the NiO photocatalyst generates OH. radicals, coumarin changes into 7-hydroxycoumarin, a bright material with a photoluminescent peak at 455 nm in wavelength. In the current study, 0.1 g of the NiO catalyst was added to 50 mL of 0.001 M coumarin solution that had been exposed to UV radiation. A photoluminescence instrument showed the presence of a photoluminescent peak at 455 nm (Figure 7) when 2.0 mL of the sample was injected over 10 mins. This peak indicates the formation of OH- radicals, which are crucial chemical species for degrading harmful dyes. Thus, it can be said that in all the dyes studied, the photodegradation is caused by a free radical mechanism using a NiO nano-sized photocatalyst. Scheme 1 describes a potential photocatalytic degradation mechanism.

Photoluminescence spectrum indicating production of OH• radical ions via NiO nano-sized photocatalyst.
Figure 7.
Photoluminescence spectrum indicating production of OH• radical ions via NiO nano-sized photocatalyst.
Pictorial representation of photocatalytic degradation of dye and effluent by NiO nano-sized photocatalyst.
Scheme 1.
Pictorial representation of photocatalytic degradation of dye and effluent by NiO nano-sized photocatalyst.

According to Scheme 2, the photo-decomposition of dye was accomplished over the NiO nano-sized photocatalyst.

A probable mechanism of photocatalytic dye degradation.
Scheme 2.
A probable mechanism of photocatalytic dye degradation.

The synthesized NiO nano-sized photocatalysts were active in the UV area, as seen by the UV-Vis spectra. Eg = 3.93 eV is another value determined for the band gap. The study carefully assesses the photocatalytic parameters of a light source, photocatalyst quantity, dye concentration, pH value, and exposure period. The reference contaminants for photocatalytic degradation are the cationic dyes MB, MG, RB, and CV, and the anionic dyes AB, AO10, AO7, AZS, CAR, MY, and MO.

The initial and final dye concentrations (C0 & Ct) of the solution in the photocatalytic system are calculated from the UV-Vis spectra. By using the following Eq. (1), the degradation of all the dyes studied is calculated:

(1)
d e g r a d a t i o n   e f f i c i e n c y = ( C 0 − C t ) C 0 × 100

3.2.2 Influence of light source on the photodegradation of anionic dyes, cationic dyes and effluent samples

The photocatalytic decomposition of anionic dyes AB, AO10, AO7, AZS, CAR, MY, and MO; cationic dyes MB, RB, and CV; and EFS1, EFS2, and EFS3 in presence of NiO nano-sized photocatalyst were performed in three different environments i.e. dark, UV light irradiation, and visible light irradiation by maintaining identical reaction conditions [Dye] = 5 ppm/[EFS]=5mL, [NiO] = 15 mg at pH 7. The obtained data showed that the prepared NiO was active under UV irradiation, as realized by the UV–Vis spectra. When the degradation experiments were conducted in the dark, the photo-degradation of all the dyes and EFS could be neglected. Furthermore, the experiments carried out under visible light irradiation and UV irradiation revealed that the photodecomposition of all the dyes studied under UV light irradiation is largely higher than under visible light irradiation. For UV light irradiation, the prepared NiO nano-sized photocatalyst effectively degrades 56.7% of the AB, 96.6 % of AO10, 99.9% of AO7, 87.7% of AZS, 99.7 % of CAR, 70.8% of MY, 99% of MO,97.8% of the MB, 95.5 % of RB, 99.9% of CV dyes, and EFS1 62%, EFS2 55%, & EFS3 46% in 100 mins. The photocatalytic results obtained have been plotted in Figures 8(a-c). The experiments were continued for the maximum decomposition of the dyes, which were still decomposing at the 100th minute (AB, AO10, AZS, and MY). The continued studies reveal that 99.2% of the AB dye took 180 mins, while 99.5%, 99.4%, and 99.2% degradation of AO10, AZS, and MY, respectively, took place in 120 mins, EFS1 79%, EFS2 71%, and EFS3 52% in 180 mins. The detailed studies were carried out under UV light irradiation for 100 mins.

(a) Impact of light source on anionic dyes AB10, AO10, AO7, AZS, CAR, MY, and MO with 5 ppm dye, 15 mg NiO at pH7.
Figure 8.
(a) Impact of light source on anionic dyes AB10, AO10, AO7, AZS, CAR, MY, and MO with 5 ppm dye, 15 mg NiO at pH7.
(b) Impact of light source on cationic dyes MB, RB and CV with 5 ppm dye, 15 mg NiO at pH7. (c) Impact of light source on effluent samples EFS1, EFS2 and EFS3 with 5 mL effluent sample, 15 mg NiO at pH7.
Figure 8.
(b) Impact of light source on cationic dyes MB, RB and CV with 5 ppm dye, 15 mg NiO at pH7. (c) Impact of light source on effluent samples EFS1, EFS2 and EFS3 with 5 mL effluent sample, 15 mg NiO at pH7.

3.2.3 Influence of amount of NiO nano-sized photocatalyst on the degradation of dyes and effluent samples

By doing multiple degradation experiments with varying doses of photocatalyst in the range of 5-20 mg under UV light radiation, the impact of photocatalyst dose on the photodegradation of dyes was also assessed (Figures 9a-c). This was carried out following confirmation of the light source and photocatalyst composition for the produced NiO nano-sized photocatalyst’s effective photocatalytic property. The outcomes unambiguously demonstrated that the photocatalyst dose has an important influence on the photodecomposition of dyes. It is clear that the degradation of anionic dyes AB (32.3% to 75.3%), AO10 (72.3% to 99.8%), AO7 (96.4% to 99.9%), AZS (58.8% to 87.7%), CAR (37.8% to 74.9%), MY (47.8% to 70.8%), and MO (27.4% to 99%); cationic dyes MB (37.8% to 97.8%), RB (26.2% to 98.3%), CV (32.8% to 99.9%) and EFS EFS1 (27% to 62%), EFS2 (33% to 55%) and EFS3 (27% to 46%) was accelerated by increasing the photocatalyst dose from 5-15 mg. This improvement in degradation was related to the increased number of photocatalytic active sites in the medium, which could produce more radical ions when NiO nano-sized photocatalyst was added in larger quantities. However, under the same photocatalytic circumstances, a further increase in the photocatalyst dose to 20 mg resulted in a reduction in the degradation efficacy for the AB, AO10, AO7, CAR, MO, MB, RB, CV, and EFS2. However, the degradation rates of AZS (87.7% to 90.2%), MY (70.8% to 72.8%), EFS1 (62% to 66%), and EFS3 (46% to 57%) improved. There won’t be enough room for the particles to disperse in the solution when the photocatalyst quantity surpasses a critical boundary, and because of the surface energy of the particles, they may adhere to one another and aggregate. As a result, the system’s degrading efficacy declines and the photocatalytic active sites become obscured or obstructed. To improve other parameters, 15 mg was chosen as the ideal photocatalyst amount and used in the remaining trials. Figure 9 shows a graphic representation of the data that was collected.

 (a) Effect of NiO nano-sized photocatalyst dosage on the photodegradation of anionic dyes AB10, AO10, AO7, AZS, CAR, MY, and MO with 5 ppm dye at pH 7.
Figure 9.
 (a) Effect of NiO nano-sized photocatalyst dosage on the photodegradation of anionic dyes AB10, AO10, AO7, AZS, CAR, MY, and MO with 5 ppm dye at pH 7.
(b) Effect of NiO nano-sized photocatalyst dosage on the photodegradation of cationic dyes MB, RB, and CV with 5 ppm dye at pH 7, (c) Effect of NiO nano-sized photocatalyst dosage on the photodegradation of effluents EFS1, EFS2, and EFS3 with 5 mL of effluent at pH 7.
Figure 9.
(b) Effect of NiO nano-sized photocatalyst dosage on the photodegradation of cationic dyes MB, RB, and CV with 5 ppm dye at pH 7, (c) Effect of NiO nano-sized photocatalyst dosage on the photodegradation of effluents EFS1, EFS2, and EFS3 with 5 mL of effluent at pH 7.

3.2.4 Effect of dye concentration of dye on the photodegradation of anionic dyes

By changing the dye concentration from 5 ppm to 20 ppm and the effluent sample volume from 5 mL to 20 mL under UV light irradiation while keeping the photocatalyst dose of 15 mg constant at pH 7, the effect of starting dye concentrations on the decomposition performance of the dyes and EFS was also evaluated. The resulting photocatalytic data have been shown graphically in Figures 10(a) (EFS), 10(b) (cationic dyes), and 10(c) (anionic dyes). According to the acquired data, the NiO nano-sized photocatalyst’s performance is inversely proportional to the dye concentration under similar conditions. This means that the maximum degrading efficacy was observed at the lowest dye concentration, which in the case of EFS was 5 mL and 5 ppm. The degradation activity of every dye under study progressively decreased from 56.7% to 24.4% for AB10, 96.7% to 70.4% for AO10, 99.9% to 80.3% for AO7, 87.7% to 40.4% for AZS, 99.7% to 78.6% for CAR, 70.8% to 22.7% for MY, 99% to 66.7% for MO, 97.8% to 56.8% for MB, 99.2% to 45.2%, and 99.9% to 50.3% for CV (Figures 10a and 10b). In the case of EFS studied, the degradation declined from 79% to 37%, 71% to 23%, and 52% to 17%, with increasing the effluent volume from 5 mL to 20 mL (Figure 10c). This is most likely because increasing the dye concentration lowers the formation of OH• radical ions, which are essential to the photodegradation process, and decreases light absorption on the photocatalyst surface [64]. Consequently, it is crucial to increase the amount of photocatalyst when the dye concentration increases. The dye concentration of 5 ppm produced the best degrading performance, and therefore, the same is maintained for detailed studies.

(a) Effect of dye concentration on the photodegradation of anionic dyes AB10, AO10, AO7, AZS, CAR, MY, and MO with 15 mg NiO at pH 7.
Figure 10.
(a) Effect of dye concentration on the photodegradation of anionic dyes AB10, AO10, AO7, AZS, CAR, MY, and MO with 15 mg NiO at pH 7.
(b) Effect of dye concentration on the photodegradation of cationic dyes MB, RB, and CV with 15 mg NiO at pH 7, (c) Effect of effluent volume on the photodegradation of EFS1, EFS2, and EFS3 with 15 mg NiO catalyst at pH 7.
Figure 10.
(b) Effect of dye concentration on the photodegradation of cationic dyes MB, RB, and CV with 15 mg NiO at pH 7, (c) Effect of effluent volume on the photodegradation of EFS1, EFS2, and EFS3 with 15 mg NiO catalyst at pH 7.

3.2.5 Influence of pH value on the degradation of dyes

It is well known that the pH of the solution is one of the most crucial elements in the photocatalytic breakdown of organic dyes. This is because the surface charge of the photocatalyst is influenced by the pH of the solution, and this has a significant effect on the photocatalytic efficacy. The photocatalytic efficiency, which increases the photocatalytic breakdown of MB dye in an alkaline aqueous solution by a factor of many, is frequently strongly connected with the availability of OH• radical ions in the reaction media. Figure 11 illustrates how pH affects the photo-degradation of MB dye when a NiO nano-sized photocatalyst is present. Three distinct pH values—4, 7, and 10—were also used to investigate the effect of pH on the elimination of all dyes. The findings demonstrated that increasing the pH to 10 results in more photodegradation. At lowest pH value (i.e., pH 4), the lowest degradation performance was obtained with 26.4%, 40.6%, 40.6%, 27.3%, 41.5%, 56.6%, and 27.4% for anionic dyes AB, AO10, AO7, AZS, CAR, MY, and MO, respectively, in 100 mins (Figure 11a) and 27.2%, 30.5%, and 27.7% for cationic dyes MB, RB, and CV, respectively, in 100 mins (Figure 11b). In effluents, the degradation observed is less, i.e., 21%, 13%, and 32% for EFS1, EFS2, and EFS3, respectively (Figure 11c). As expected, when the pH of the solution is increased, NiO nano-sized photocatalyst showed maximum degradation activity in 100 mins and approximately for anionic dyes 77.5% of AB, and 99.9% of AO10, AO7, CAR, and MO, 92.3% of AZS, 74.3% of MY, 98.7% of MB, 99.7% of RB, and 100% of CV, 79% of EFS1, 81% of EFS2, and 70% of EFS3 at pH 10. This is made feasible by the negative charges that a higher pH value produces on the surface. Because more OH• radical ions are deposited on the photocatalyst surface at higher pH values, the photo-degradation process continues more efficiently [65].

(a) Effect of pH on the photodegradation of anionic dyes AB, AO10, AO7, AZS, CAR, and MY, with 15 mg NiO.
Figure 11.
(a) Effect of pH on the photodegradation of anionic dyes AB, AO10, AO7, AZS, CAR, and MY, with 15 mg NiO.
(b) Effect of pH on the photodegradation of cationic dyes MB, RB, and CV, with 15 mg NiO, (c) Effect of pH on the photodegradation of effluents EFS1, EFS2, EFS3, and MO with 15 mg NiO catalyst.
Figure 11.
(b) Effect of pH on the photodegradation of cationic dyes MB, RB, and CV, with 15 mg NiO, (c) Effect of pH on the photodegradation of effluents EFS1, EFS2, EFS3, and MO with 15 mg NiO catalyst.

3.2.6 Reusability of NiO nano-sized photocatalyst

Under experimental settings akin to those in in-depth investigations, the NiO nano-sized photocatalyst is treated and repurposed for the degradation of both cationic and anionic dyes and EFS. After being calcined at 300°C, the used photocatalyst was recovered and used to degrade all dyes and EFS at the ideal conditions of 15 mg catalyst, 5 ppm dye (5 mL for EFS), and pH 7 under UV light. Figures 12(a-c) demonstrate the performance of a photocatalyst made of NiO nanoparticles. Catalytic activity is seen to drop slightly after the initial reclaim; however, when the catalyst was reactivated and reclaimed, the results showed a considerable fall in catalytic performance.

(a) Graphical illustration of NiO nano-sized photocatalyst reusability data against degradation of anionic dyes AB, AO10, AO7, AZS, CAR, and MY, with 15 mg NiO at pH7, (b) graphical illustration of the reusability data of the NiO nano-sized photocatalyst against degradation of cationic dyes MB, RB, and CV, with 15 mg NiO at pH7, (c) graphical illustration of reusability data of NiO nano-sized photocatalyst [B: Fresh; C: Reuse-1; D: Reuse-2] against degradation of EFS1, EFS2, and EFS3, with 15 mg NiO at pH7.
Figure 12.
(a) Graphical illustration of NiO nano-sized photocatalyst reusability data against degradation of anionic dyes AB, AO10, AO7, AZS, CAR, and MY, with 15 mg NiO at pH7, (b) graphical illustration of the reusability data of the NiO nano-sized photocatalyst against degradation of cationic dyes MB, RB, and CV, with 15 mg NiO at pH7, (c) graphical illustration of reusability data of NiO nano-sized photocatalyst [B: Fresh; C: Reuse-1; D: Reuse-2] against degradation of EFS1, EFS2, and EFS3, with 15 mg NiO at pH7.

4. Conclusions

NiO nano-sized photocatalyst is synthesized using coconut bark extract. The characterized photocatalyst was used to degrade various anionic, cationic dyes, and effluents from silk and cotton dyeing units under UV light. Both cationic and anionic dyes were degraded nearly 100% by NiO nano-sized photocatalyst at pH 10, and hence, photocatalytic efficiency was proved to be achieved under UV light. Even the EFS studied were degraded to a maximum extent at pH 10. Maximum degradation of AO10, AO7, CAR, MO, MB, and CV was achieved in 100 mins. However, maximum degradation of AB dye in 180 mins, AO10, AZS, and MY in 120 mins, and EFS1, EFS2, and EFS3 in 180 mins was observed. Notably, electron-hole separation produced on the catalyst’s surface by UV light was primarily responsible for the exceptional photo-removal performance of NiO nano-sized photocatalyst for the removal of dyes. For wastewater treatment and related photocatalytic applications, the inexpensive, environmentally benign, and extremely effective NiO nano-sized photocatalyst with photocatalytic capabilities can be regarded as the perfect catalyst.

Acknowledgment

The authors acknowledge the funding from the Ongoing Research Funding Program (ORF-2025-222), King Saud University, Riyadh, Saudi Arabia.

CRediT authorship contribution statement

M. R. Hatshan: Investigation, Software, Writing - original draft. N. V. Sushma: Validation, Formal analysis. J. P. Shubha: Methodology, Data curation, Writing - original draft, Supervision. Mohammad Elshahat Assal: Resources, Writing - review & editing. Mufsir Kuniyil: Writing - review & editing, Project administration. Syed Farooq Adil: Conceptualization, Funding acquisition, Supervision, Writing - original draft, Writing - review & editing.

Declaration of competing 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.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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