Congo red dye reduction mediated by the electron (e−) transfer route of BH4 − ions using synthesized NiCo2O4/rGO hybrid nanosheets

The treatment of toxic organic pollutants is extremely important for the conservation of clean air, soil, and water. In this study, (reduced graphene oxide) NiCo2O4/ rGO hybrid nanocomposite was prepared by a facile hydrothermal technique and employed for organic dye adsorption from wastewater. The synthesized NiCo2O4/rGO hybrid nanocomposite was studied using FTIR, XRD, SEM, TEM, BET, Raman spectroscopy, and UV–visible. The physical characterizations prove the deposition of NiCo2O4 particles on the rGO surface. The transmission electron microscope image demonstrated that the NiCo2O4 particles with an average size of ∼46 nm was dispersed on the rGO surface. The obtained nanoparticles show a higher specific surface area of 56.4 m2 g−1. Adsorption dynamics as investigated by time and concentration variation show that the adsorption data follows pseudosecond order kinetics and the Langmuir isotherm model, with a maximum adsorption capacity of 106.2 mg g−1, indicating homogeneous physiochemical adsorption of CR dye on the adsorbent surface. Besides, the catalytic effectiveness of synthesized nanocomposite towards Congo red (CR) dye reduction mediated by the electron (e−) transfer route of BH4 − ions was explained in detail. The electrostatic interaction used between the NiCo2O4/rGO hybrid composite and Congo red increased the adsorption ion effectiveness of the dye sample.


Introduction
Nowadays, water contamination by the dye is one of the major threats to human health and the environment [1].Commercial dyes are normally described by their structural and color stability, high degree of aromaticity and widely conjugated chromophores.There are presently around 10,000 various types of synthetic dyes, and more than 10 × 10 5 tons of them are formed yearly all over the world, with a high percentage of them being azo dyes [2].Meanwhile, dyes are a major class of synthetic organic compounds generated from a variety of sources, such as leather, plastics, textile industry plants, paper, the food and pharmaceutical industries, others in to the ecosystem [3].Moreover, Congo Red (CR) dye, also known as Direct Red 28, is a synthetic azo dye that has been widely used in the textile industry to color garments.Mutagenicity refers to a substance's ability to cause genetic mutations, which can lead to negative health effects such as cancer.Exposure during pregnancy or to developing organisms can cause developmental defects and reproductive harm.Congo Red can irritate the skin and eyes when in touch.People who touch or come into contact with items containing Congo Red may develop skin irritation, redness, and itching.The release of Congo Red into the environment, particularly aquatic bodies, might have negative consequences [3].It may contribute to water contamination, harming aquatic ecosystems.So, the removal of these organic dyes is essential.
To solve the problem of water pollution, extensive techniques have been utilized, including Fenton, surface adsorption, chemical precipitation, flocculation, coagulation, photocatalysis, biological treatment, ozonation, membrane filtration, advanced oxidation processes, and zonal filters [4].Nevertheless, the majority of these techniques suffer from some drawbacks, such as insensitive reaction conditions, high expenditure, and phase transfer of contaminants.Therefore, there is an essential requirement for the growth of an efficient handling process for the removal of these organic dyes from the environment.To facilitate, the removal of organic contaminants from the pollutant water, the removal procedure of organic dyes by various kinds of samples has fascinated growing concentrations during the past decades.NaBH 4 is a well-known reducing agent.Removal of organic dyes by NaBH 4 in the nonexistence of an adsorbent is kinetically not easy but thermodynamically favorable [5].As a possible alternative, adsorption of organic pollutants was employed with nanoparticles such as Fe 3 O 4 , Ag and Cu nanoparticles due to their cost efficiency [6][7][8][9][10].The major complexity in their application includes nanoparticle agglomeration which can result in the deactivation of nanocatalysts and their removal from the reaction medium, therefore making revival and renewal hard.Consequently, there is a need for an increase in environmentally benevolent techniques for the synthesis of heterocomposites through the immobilization of nanoparticles on/into solid supports [11][12][13].
Among different solid support components, graphene oxide (GO) and reduced graphene oxide (rGO) are well-recognized as they offer practical benefits over other supports such as improved specific surface area, good thermal/chemical stability, good mechanical strength, high structural flexibility, ease of work-up, handling and separation of products, non-toxic nature and high adsorption efficiency [14].Moreover, rGO forms a nanocomposite sample with inorganic and polymer components due to its acquisition of many unique features, such as good thermal conductivity, more charge carriers and planar structure.Reduced graphene oxide and plenty of its composites are extensively employed in optical, optoelectronic, bioengineering, catalytic, biosensor, magnetic, electrochemical sensor and energy storage [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30].Moreover, rGO based samples act as efficient adsorbents for organic pollutants such as dyes, pesticides, and antibiotics [31][32][33].The rGO based samples were utilized as a support for many catalysts such as ZnO, Pt, Au, and Fe 3 O 4 [11].Due to their intriguing electronic conductivity, high adsorption capacity, and large surface area, many explorers also employed rGO-based components as electron acceptors and supporting matrix for catalyst particles to develop the effectiveness of the removal of dyes.Moreover, Nickel cobaltite (NiCo 2 O 4 ) has been investigated for its possible use in adsorption applications, including the removal of dyes such as Congo Red from aqueous solutions.When NiCo 2 O 4 is utilized as a composite material or in conjunction with other substances (for example, graphene oxide), synergistic effects can improve overall adsorption performance.
In this work, we have discussed the synthesis and characterization of a NiCo 2 O 4 /rGO hybrid nanocomposite with stability and good catalytic properties.Moreover, NiCo 2 O 4 nanoparticles and the high adsorption capacity of the rGO/NiCo 2 O 4 hybrid nanocomposite were prepared through a facile hydrothermal method.And nanocomposite was studied for its applicability in the removal of Congo red dye in aqueous solutions.The amount of dye reduction has been examined using UV-visible spectroscopy.

Synthesis of GO
To make graphene oxide, a modified Hummers process was used [34].In this technique, 34.5 ml of concentrated H 2 SO 4 was added to 1.5 g of graphite powder and 0.75 g of NaNO 3 , and the resulting mixture was cooled to 0 °C in an ice bath.Slowly adding 4.5 g of solid KMnO 4 while keeping the reaction temperature below 20 °C.The reaction was carried out at 35 °C for 7 h with magnetic stirring after the addition of KMnO 4 .A further 4.5 g of KMnO 4 was added, and the mixture was stirred for 12 h at 35 °C.After cooling to room temperature, the reaction mixture was poured into 200 ml of cold water and 1.5 ml of 30% H 2 O 2 .Finally, the mixture was centrifuged and rinsed multiple times with water, 30% HCl, and ethanol.The resulting product was dispersed in water, sonicated for 30 min, filtered, and then vacuum-dried overnight at 60 °C.

Synthesis of NiCo 2 O 4 /rGO hybrid nanocomposite
The preparation of the rGO/NiCo 2 O 4 hybrid sample is shown in scheme 1.A modified Hummers method was utilized to prepare graphene oxide.The NiCo 2 O 4 /rGO hybrid nanocomposite has been prepared through a hydrothermal process [35].20 mg of graphene oxide were added to 35 ml of de-ionized water by ultrasonication for 20 min.After that, 0.119 g of NiCl 2 .6H 2 O and 0.273 g of CoCl 2 .6H 2 O were dispersed in 15mL of de-ionized water.The above synthesized solution was slowly mixed into the solution of GO over 1h under strong stirring to obtain a homogeneous solution.Next, NH 4 OH (25%) was dissolved into the above synthesized sample under a constant rate of stirring to maintain the pH of the sample at ∼11.After stirring for 8h, the as-synthesized solution was transferred into a 100 ml autoclave.The reaction was carried out at 150 °C for 20 h.The resultant product was washed with ethanol, water and further dried at 70 °C in the oven for 10 h.The resultant sample was further calcined at 400 °C for 5 h under Ar atmosphere at a rate of 3 °C min −1 employing a muffle furnace, to get the final product, which is NiCo 2 O 4 /rGO hybrid nanocomposite.The pure NiCo 2 O 4 was synthesized by the same process without mixing GO. (3.5 × 10 −3 M) has dissolved, and the above solution was vigorously stirred at room temperature.UV-vis spectroscopy was used to determine the concentration of the leftover dye solution by measuring absorbance at the maximum absorption wavelength of λmax = 493 nm for CR.If the pH needs to be lowered, sulfuric acid (H 2 SO 4 ) can be added.These acids donate protons, decreasing the pH of the solution.Moreover, if the pH needs to be raised, sodium hydroxide (NaOH) can be added.These bases accept protons, increasing the pH of the solution.The following adsorption conditions were used in the experiment: an adsorbent dosage of 150 mg; solution volume of 25 ml; and a temperature of 25 °C.Equation (1) was used to get the dye adsorption percentage.
Where Ci is the initial concentration of Congo red solution (mg/l), Ce is the dye concentration at close to equilibrium, V is the volume of dye solution (L), and M is the dosage of the composite (g).

Characterization techniques
Molecular structure was examined by FTIR measurements (Thermo Nicolet FTIR −200).The x-ray diffraction (XRD) analysis of the sample was carried out on an x-ray powder diffractometer Bruker AXS D8 advanced diffractometer).Morphological structure was studied by field emission scanning electron microscopes (SEM, FEI, and Quanta 400).The microstructure and morphology were explored by TEM (JEOL JEM-3010 machine operated at 200 kV).The specific surface area has been explored employing the Brunauer-Emmet-Teller (BET) technique.Raman analysis of the obtained product was explored on (Horiba HR 800UV confocal Raman spectrometer).

The effect of different parameters on removing Congo red dye 3.2.1. Effect of initial pH
The effect of initial pH on CR adsorption has been explored over the pH range of 3-9.As depicted in figure 6(a) the effectiveness of removing CR by NiCo 2 O 4 /rGO hybrid nanocomposite decreased when the pH range increased from 3-9.This is due to the active sites of the NiCo 2 O 4 /rGO hybrid nanocomposite converting from a positive to a negative charge when the pH range increased, resulting in the presence of electrostatic repulsion between the NiCo 2 O 4 /rGO hybrid nanocomposite and CR dye as CR molecule is an intrinsically anionic dye.It was observed that the adsorption is enormous in an acidic medium.This is owed to the increases in the number

Effect of the adsorbent dose
The effect of NiCo 2 O 4 /rGO hybrid nanocomposite concentration on the adsorption of Congo red by the NiCo 2 O 4 /rGO hybrid sample is shown in figure 6(b), where we can study that increasing the concentration of the adsorbent range from 30, 60, 90, 120, 150, 180 and 210 mg L −1 improved the dye adsorption effectiveness.This may be attributed to the larger NiCo 2 O 4 /rGO hybrid nanocomposite surface area and more available adsorption active sites.The removal effectiveness of Congo red dye is not extensively affected by a further increase in adsorbent amount over 150 mg/l.This shows that a part of the NiCo 2 O 4 /rGO hybrid nanocomposite surface remains uncovered, due to the aggregation of the adsorbent particles at the higher adsorbent dose.

Effect of congo red initial concentration
As the CR concentration increases from 200, 250, and 300 mg L −1 , the percentage of CR adsorbed by NiCo 2 O 4 /rGO hybrid nanocomposite declines from 95.7%, 65%, and 46%.The removal effectiveness of CR dye decreased when dye concentration increased due to fewer surface-active sites for constant adsorbent dosage.

Adsorption study
NiCo 2 O 4 has a strong catalytic activity because of its distinct electrical structure and surface characteristics.The synergy of nickel and cobalt ions in the oxide lattice improves catalytic activity.This degradation process frequently results in the formation of reactive oxygen species on the catalyst's surface, which can oxidize and degrade dye molecules.NiCo 2 O 4 nanoparticles can also act as photocatalysts, degrading Congo Red dye in response to solar or visible light exposure.This photocatalytic activity is related to NiCo 2 O 4 's semiconductor nature, which allows for light absorption and the formation of electron-hole pairs, thereby initiating redox reactions with dye molecules.NiCo 2 O 4 catalysts are normally quite stable and can be reused several times without losing important catalytic activity.This is useful in practical applications since it eliminates the requirement for regular catalyst replacement.
Further, the catalytic activity of the as-prepared NiCo 2 O 4 /rGO hybrid nanocomposite was employed to examine its application in organic dye adsorption experiments.The catalytic activity of the NiCo 2 O 4 /rGO hybrid nanocomposite has been studied with the catalytic reduction of Congo red by NaBH 4 in water treatment.The reduction reaction can be simply examined with UV-vis spectroscopy.The figure 6(c) demonstrates that CR solution revealed the typical band at 496 nm and demonstrates the absorbance's changes calculated at different In addition, the catalytic mechanism for removal of Congo red relies on the e − move from the BH  7).These are much better than the rGO and NaBH 4 materials (removed only28% and 9%).After the completion of four cycles, there is a small decrease (∼90%) in the catalytic

Adsorption kinetics
The dye adsorption data were examined using the pseudo first-order and pseudo-second-order, which are discussed below, in order to understand the CR adsorption process on NiCo 2 O 4 /rGO hybrid nanocomposite, particularly the potentially rate-controlling stage.Lagergren proposed the pseudo-first-order kinetic model for the adsorption of solid/liquid systems, and its formula is as follows [51][52][53][54]: where q t (mg/g) and q e (mg/g) signify the quantity of dye adsorbed at any time t (min) and at the equilibrium time, respectively, and k 1 (1/min) is the rate constant of pseudo-first-order adsorption.k 1 is determined by plotting log (qe-qt) versus t.The estimated and experimental q e values, however, do not agree (table 2), and the pseudo-first-order kinetic model does not adequately represent the CR adsorption on NiCo 2 O 4 /rGO hybrid nanocomposite.The pseudo-second-order kinetic model's linear form can be written as [55,56]:  where k 2 (g/(mg min)) is the rate constant of pseudo-second order adsorption calculated using linear plots of t/ qt against t (table 2).The experimental and predicted values for q e were close; additionally, by comparing the correlation coefficients (R 2 ) values obtained for pseudo-second order and pseudo-first-order kinetics, the adsorption process can be determined to be a pseudo second-order reaction, as shown in figures 8(a) and (b).

Adsorption isotherms study
The initial concentrations of Congo red solutions were varied to study adsorption isotherms.The adsorption isotherm data of Congo red onto composites was fitted with two classic isotherm models, namely, Langmuir (equation ( 5)) and Freundlich (equation ( 6)), which are expressed by the equation: Where Ce (mg.L −1 ) represents the equilibrium concentration of CR dye, q e (mg/g) represents the equilibrium adsorption capacity, q m (mg/g) represents the maximum adsorption capacity, and b (L/mg) represents the Langmuir constant.The Freundlich adsorption isotherm model, on the other hand, posits a heterogeneous surface with adsorption sites with unequal energies.This may be represented using equation (6) as follows:

Conclusions
In summary, we studied a facile hydrothermal method for the synthesis of NiCo  Therefore, we suppose that this nanocomposite has extensive applications in catalysis and has demonstrated its suitability as a nanocomposite for the handling of polluted water with organic dyes.

Figure 5 .
Figure 5. (a) SEM image of NiCo 2 O 4 /rGO hybrid sample (b and c) TEM images of NiCo 2 O 4 /rGO hybrid sample at different magnification (d) EDS analysis of NiCo 2 O 4 /rGO hybrid sample (e) TEM image of rGO (f) TEM image of NiCo 2 O 4 .

Figure 6 .
Figure 6.Degradation of Congo red under different parameters (a) effect of pH(b) effect of adsorbent dosage (c) UV-visible spectrophotometer spectrum of Congo red dye in the presence of NiCo 2 O 4 /rGO hybrid sample with the different time intervals 0, 5,10 and 20 min (d) degradation of Congo red mechanism.

Figure 8 .
Figure 8.(a) Pseudo first order kinetics and (b) pseudo second order kinetics for CR dye adsorption over the nanocomposite (c) Langmuir and (d) Freundlich isotherm for CR adsorption over the prepared NiCo 2 O 4 /rGO hybrid sample.
[50]onor to the acceptor Congo red by adsorption of the reactant molecules on to the catalyst surface[49].The NiCo 2 O 4 /rGO hybrid nanocomposite can provide a good substrate for the e − transfer reaction from BH 4 − donor to Congo red.It recommends the catalytic adsorption of CR dye, and the catalytically active NiCo 2 O 4 /rGO hybrid composite was chosen as a model to improve the dye adsorption rate, because of its unique features, such as its reactive surface and large specific area[50].The presence of hydroxyl groups over the surface of the NiCo 2 O 4 /rGO hybrid composite interact with the CR dye through hydrogen bonding and electrostatic interactions, helping the adsorption of the CR dye.The catalytic performance of the present NiCo 2 O 4 /rGO hybrid nanocomposite is compared with that reported for other samples in table 1, demonstrating that it has a better removal performance than other composite materials for the removal of Congo red.This phenomenon is clearly clarified in figure 6(d).After 20 min at room temperature and in the presence of NaBH 4 and NiCo 2 O 4 /rGO hybrid nanocomposite materials, 95.7% of total CR has been successfully degraded from the solution (See figure

Table 1 .
Comparison of various catalysts in the reduction of Congo red.The result revealed that the NiCo 2 O 4 /rGO hybrid nanocomposite has reasonable stability in the adsorption reaction media.

Table 2 .
Kinetic model parameters for CR adsorption over NiCo 2 O 4 /rGO hybrid nanocomposite.
[56]s the linear plots of the Freundlich and Langmuir isotherms.According to table 3, the determination coefficients (R 2 ) for Langmuir were found to be 0.9982, while those for Freundlich were found to be 0.9888, demonstrating that both models can adequately represent the adsorption of CR onto NiCo 2 O 4 /rGO hybrid nanocomposite.Furthermore, the Freundlich n value is 1.460, indicating a significant affinity between the CR and the NiCo 2 O 4 /rGO hybrid nanocomposite (n > 1).According to the Langmuir isotherm, the maximal CR adsorption capacity onto NiCo 2 O 4 /rGO is 106.28 mg g −1 .Furthermore, the maximal adsorption capacity of CR onto NiCo 2 O 4 /rGO hybrid nanocomposite was compared to that of previously described adsorbents[56].This is most likely due to the fact that NiCo 2 O 4 /rGO hybrid nanocomposite can interact with CR in a variety of ways, including (i) electrostatic attraction between CR and NiCo 2 O 4 /rGO hybrid nanocomposite and (ii) conjugation between the benzene rings of NiCo 2 O 4 /rGO hybrid nanocomposite and aromatic CR. (iii) hydrogen bonding between CR and NiCo 2 O 4 /rGO hybrid nanocomposite.
F (mg.g −1 ) denotes the Freundlich constant and (1/n) denotes the adsorption extent.Figures8(c) and (d), 2 O 4 /rGO hybrid nanocomposite.The prepared NiCo 2 O 4 /rGO hybrid nanocomposite has been proven by FTIR and Raman scattering measurements.The NiCo 2 O 4 /rGO hybrid nanocomposite crystal structure and phase were investigated employing XRD and the quasi-spherical morphology was resolved by TEM analysis.The BET investigation recommended that the accumulation of rGO to NiCo 2 O 4 was the reason for the significant increase in the catalytic performance with a surface area value of 56.4 m 2 g −1 .Moreover, the NiCo 2 O 4 /rGO hybrid nanocomposite demonstrates strong catalytic activity towards Congo red dye adsorption.Enhanced adsorption activity of NiCo 2 O 4 /rGO compared to NaBH 4 was seen owing to the presence of more surface area rGO nanosheets.The important advantages of this methodology are mild reaction conditions, a short reaction time and the elimination of hazardous materials.The Langmuir isotherm model fit the adsorption equilibrium data well, but the pseudo-second-order model fit the kinetic data well.Therefore, our results prove that the NiCo 2 O 4 /rGO sample exhibits tremendous adsorption performance towards the removal of organic contaminants and can be an appropriate candidate for different eco-friendly environmental applications.