Catalytic degradation of methylene blue in aqueous solution using cobalt oxide kaolin nanocomposite

Abstract

Textile dye pollution of water is becoming a major cause of pollution in the environment and a threat to aquatic ecology. In this work, a kaolin-cobalt oxide nanocomposite was used to catalyze the degradation of methylene blue (MB) dye in an aqueous solution. The composite was prepared by oxidizing cobalt ions adsorbed onto readily available, inexpensive, and easily pretreated kaolin surfaces, forming kaolin-supported cobalt oxide nanoparticles. By using ultraviolet-visible spectroscopy, scanning electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller, Transmission emission spectroscopy, energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy, the synthesized materials’ morphology, structure, surface area, and ion interaction were all examined. The characterization results demonstrated that cobalt oxide-NPs were successfully growing on the surface of kaolin. MB dye was used as a model dye in batch tests to better understand the catalytic degradation performance of the prepared catalyst. The significance of cobalt oxide nanoparticles and the strong catalytic activity of the produced cobalt oxide nanoparticles/kaolin composite toward MB dye degradation were validated by the catalytic oxidation experiments. The degradation results showed that removing MB dye from an aqueous solution could be successfully enhanced by increasing the cobalt oxide NP content on the kaolin surface through repeated cycles. The pseudo-first-order kinetic model fits the kinetic analysis of the catalyst’s MB dye degradation. After five reuse cycles, over 96% removal efficiency persisted, indicating the remarkable resilience and reusability of the composite. To sum up, the kaolin composite supported by cobalt oxide nanoparticles was discovered to be a promising catalyst with exceptional catalytic activity to degrade a model dye, MB, concentration of 50 ppm from the aqueous solution in the presence of NaOCl by above 99% catalytic removal efficiency in 12 min at operating temperature of 45 °C with the oxidation rate constant, k_oxd, of 0.38 min^− 1.

Introduction

Environmental pollution due to industrial wastes has been increasing in the last decades [1]. Many pollutants that come from the textile, paper, leather, pharmaceutical, cosmetic, paint, plastic, food, and other industries are the main contributors to water pollution. These pollutants include organic dyes, heavy metals, chlorinated compounds, soaps, detergents, surfactants, and salts. Textile dyes contaminate water by increasing the oxygen demand for biological processes, preventing photosynthesis, stunting plant growth, penetrating the food chain, causing recalcitrance and bioaccumulation, and possibly causing toxicity, mutagenicity, and carcinogenicity. Additionally, the discharge of these compounds into water bodies may disrupt the processes of photosynthesis and oxygen exchange, severely harming aquatic life by preventing sunlight from reaching them [2]. The dye in the water stream has ordinary impacts on living things [3].

The effluents from the textile dye industry are complex, containing heavy metals, organic and inorganic salts, alkalis, and dyes. Because of their complex chemical composition, high solubility, and non-biodegradability, these pollutants are among the most challenging to treat. These organic dyes have the potential to contaminate water bodies, enter the human body through the food chain, and result in a host of hazardous conditions, including cancer, hypertension, vertigo, vomiting, gastritis, skin irritation, and permanent blindness [4].

As a result, the removal of those dyes from wastewater has received a lot of attention nowadays, as it’s crucial to do so in order to minimize their harmful impacts [5]. The three most popular methods for removing dye are chemical, physical, and biological treatment techniques [6]. The primary drawback of using biological approaches is the length of time needed for full decolorization through hydraulic retention, whereas chemical methods require large investments and expenditures. These conventional treatment techniques have limits and are unable to break down textile dyes [7]. The above-mentioned treatment techniques are not able to offer current, organized requirements for the release of effluents into natural ecosystems and drinking water [8]. Because of their strong photocatalytic activity, great solubility, and stability, transition-metal oxide nanoparticles are thought to be the most promising catalysts for cleaning wastewater in a simple, dependable, rapid, and environmentally benign manner [9–11].

Therefore, it is now essential to develop cutting-edge and reasonably priced techniques to meet stated criteria for water and wastewater as well as population needs. Due to their superior activity, abundance, ease of preparation, and ecological friendliness, the catalytic degradation of these organic pollutants (dyes) by applying nanocomposites of clay decorated with metal and metal oxide nanoparticles in a chemical method for removing dyes offers a better and promising approach in wastewater treatment.

Because of its intriguing properties and significant catalytic potential, cobalt oxide nanoparticles have previously been widely reported to be used as a catalyst for water purification, including the effective catalytic removal of organic pigments [12]. The tendency of pure nanoparticles to aggregate reduces their catalytic activity and limits their effectiveness when used alone [13]. According to several studies, growing nanoparticles on support materials can stop them from aggregating and reduce their toxicity [14, 15].

However, the synthesis procedures are intricate and time-consuming, and the majority of the support/carrier elements are costly. An economical and highly effective method to remove agglomeration is by synthesizing nanoparticles on the pores of clay compounds as the support material. The nanoparticles are anchored onto the external surfaces and confined within the interlamellar regions of the clay matrix [16]. Because of their qualities, including high surface area, excellent swelling properties, adsorption behaviors, and ion exchange capacity, clays have attracted a lot of interest in the field of material science [17]. Furthermore, it is well known that synthesizing cobalt oxide-NP on clay support can enhance the bare nanoparticles’ electrochemical signal, a crucial characteristic for the creation of sensors for many applications [18].

Due to their benefits over many other support materials, including graphene and carbon nanotubes, both economically and environmentally, kaolin clay minerals have recently garnered a lot of attention as nanoparticle supports because of their inexpensive, abundant, and environmentally friendly nature [19–21]. Kaolin is a phyllosilicate substance with a structure made of nanolayers [10]. Van der Waals interactions and electrostatic forces cause the layers that make up the main particles of kaolinite to stack together to form kaolin clay [22, 23]. Each layer consists of a tetrahedral sheet joined to a single octahedral sheet, forming one structural unit [7, 10, 24]. The layered structure of Kaolinite’s kaolinite makes it easily modifiable for the fabrication of hybrids and composites [19, 25, 26]. Specifically, its remarkable characteristics, like its great adaptability and mechanical and thermal stability, make it indispensable for serving as a support for nano catalysts [27].

A chemical oxidation method using oxidants such as potassium permanganate, hydrogen peroxide, sodium hypochlorite, calcium hypochlorite, oxygen, and chlorine is frequently used to create cobalt oxide-based composites [28, 29]. As a result, an efficient, affordable, simple, and fast one-pot solution-based technique using aqueous NaOH as a precipitating agent was used in the current study to generate the cobalt oxide NPs/kaolin composite at room temperature [30]. Cobalt chloride hexahydrate (CoCl₂.6H₂O) was used as the precursor, and kaolin served as the support material in the preparation of a heterogeneous catalyst [27]. Characterization of the resultant samples was done using spectroscopy, microscopy, and other techniques. In an aqueous solution with NaOCl present, the catalytic activity of the cobalt oxide-NPs/kaolin composite that was produced was assessed against MB dye degradation [31, 32]. Excellent catalytic activity was shown by the produced catalyst, which could be recycled for several catalytic cycles without significant loss of effectiveness [12, 33].

Materials and methods

Materials

The raw kaolin clay was mined from the Kimir Dingay (Guna Begemidir) district in the South Gondar Zone, Ethiopia [11], and was utilized as a stable substrate for the cobalt oxide NPs. The obtained clay was dried in a dry environment. Analytical-grade cobalt chloride hexahydrate (CoCl₂⸱6H₂O, Uni-Chem Chemical, 99.99%), sodium hypochlorite (NaOCl, Savgan Heights Plc, 4 wt%), methylene blue (MB, C₁₆H₁₈ClNS, Dallul Pharmaceuticals Plc), and sodium hydroxide (NaOH, Uni-Chem Chemicals, 98%) were utilized exactly as received, requiring no additional purification. All solutions were prepared using deionized water produced bya deionizer (Evoqua Water Technologies, Germany).

Synthesis of cobalt oxide nanoparticles/kaolin composites

A 0.1-gram of CoCl₂•6H₂O was weighed and then put into a beaker and dissolved with 100 mL of deionized water. One gram of beneficiated kaolin powder, prepared using the method described by Asmare et al. [26], was added to the solution and agitated on a hot plate for 1 h. Afterward, drops of 0.33 M NaOH solution were added to the mixture at 300 rpm and 47 °C until the pH of the mixture was 12, and the mixture was stirred for 1 h, and a dark green color formed. The homogenous cobalt oxide/kaolin precipitate was allowed to settle and washed three times to remove residual cobalt ions. The resulting mixture was then dried in an oven at 80 °C for overnight [10].

The dried samples were finally calcined in the furnace at 530 °C. The resulting composite was stored and named as Cobalt oxide-NPs/kaolin composite-1. For additional comparison, these steps were carried out four times with the precursor’s mass changed along the same path. To regulate the size of cobalt oxide-NPs, the mass of kaolin was fixed at 1.0 g, while different amount of the precursor CoCl₂.5H₂O (0.2, 0.3, 0.4 and 0.5 g) were used. The resulting materials were labelled as cobalt oxide-NPs/kaolin composite-2, composite-3, composite-4, and composite-5, respectively.

Characterization techniques

The synthesized nanomaterials were characterized using different techniques. X-ray diffraction (XRD) patterns were acquired using a PerkinElmer MPD X-ray diffractometer. The measurements were carried out over a 2θ range of 5°–80° at a scanning rate of 5°/min, employing Cu Kα radiation (λ = 1.542 Å) operated at 40 kV and 40 mA. Fourier transform infrared (FTIR) spectra (4000–400 cm^− 1) were recorded using a JASCO FT/IR-6600 Type A spectrometer. Ultraviolet–visible (UV–Vis) absorption spectra were recorded using a HACH DR6000 benchtop spectrophotometer. Surface morphology and elemental composition were analyzed using a SEM (JSM-7800 F) coupled with EDS. Transmission electron microscopy (TEM) was additionally utilized to further elucidate the structural characteristics using JEOL JEM 2100 F. Nitrogen adsorption–desorption analyses were carried out on a Micrometrics ASAP 2460 analyzer using the Brunauer–Emmett–Teller (BET) method at 77 K.

Catalytic performance of cobalt oxide-NPs/kaolin composites on MB dye

The catalytic activity of cobalt oxide nanoparticles-kaolin composites prepared was analyzed by determining methylene blue (MB) oxidation efficiency in an aqueous medium using sodium hypochlorite (NaOCl) as an oxidant. An aqueous solution of 50 mL containing 50 ppm of MB dye under continuous stirring at room temperature was treated with 12 mg of catalyst. To bring about the adsorption-desorption equilibrium of the dye on the composite material, the reaction mixture was stirred for two min. Thereafter, rapid addition of 300 µL of a 4% NaOCl solution was carried out into the reaction medium. To determine the degradation proficiency of the MB dye, aliquots were separated from the reaction medium and quantitatively analyzed through a UV-Vis spectrometer at regular time intervals. The instant loss of deep blue color typical of the MB dye solution was noted. To determine the concentration of the remaining MB dye solution, time-dependent absorbance in the spectral region of 500–750 nm was monitored at 664 nm through a UV–Vis spectrometer. Furthermore, key parameters such as the mass of the cobalt oxide NPs/kaolin composite and the concentration of MB dye solution were investigated in relation to their effects on the oxidation rate constants. Furthermore, the effect of depositing cobalt NPs onto the surface of kaolin on its catalytic performance was evaluated based on the degradation of MB using kaolin and its various cobalt oxide NPs/kaolin composites, namely composite-1, composite-2, composite-3, composite-4, and composite-5.

Equations (1) and (2) [34] below were used to determine the catalytic oxidation rate constant (k_ox)of the MB dye solution by fitting the data to pseudo-first-order and second-order kinetics models.

1

2

Where C₀ is an initial MB dye solution concentration, k_ox is an oxidation rate constant, and Ct is an MB dye solution concentration defined by time t at an equilibrium, At for the absorbance at a time t and A₀ for initial absorbance of MB dye. Equation (3) [35] below was used to calculate the percent removal efficiency of a catalyst.

3

Reusability test for fabricated nanocomposites

The successive catalytic activities of the cobalt oxide NPs/kaolin composites were investigated to evaluate their reusability. Firstly, 12 mg of the composite was placed into a flask containing 50 mL of MB aqueous solution (50 ppm) along with 0.3 mL of 4% NaOCl as an oxidant. The mixture was stirred for five min and then centrifuged at 3500 rpm for three min. After measuring the absorbance of the supernatant, it was discarded. The recovered nanoparticles were rinsed with distilled water, and a fresh MB solution of the same concentration was added. The subsequent steps were repeated under identical conditions, and the procedure was carried out for a total of five cycles.

Results and discussion

Beneficiated kaolin, which is a purified form of clay, was used as a support material to immobilize cobalt oxide nanoparticles directly onto its surface [36]. Successful and efficient immobilization of these nanostructures was validated by an array of physicochemical characterization techniques that play a pivotal role in determining the properties and interfacial interactions of involved materials. The resulting nanocatalytic composite of kaolin was further made use of in the oxidation degradation process of the model dye, methylene blue (MB), such that it found application in converting the dye to carbon dioxide (CO₂) and water (H₂O), as well as some additional inorganic byproducts. The catalytic activity of the composite material was evaluated completely through monitoring effectively the degradation of MB dye in the presence of sodium hypochlorite (NaOCl). Methylene blue, when present in an aqueous medium, possesses distinct ultraviolet-visible (UV–vis) absorption peaks near about 290 nm as well as 666 and 664 nm, along with a shoulder peak near about 612 nm due to electronic transitions of π → π* as well as n → π*. The progress of oxidation degradation of MB towards its discolorized product has been monitored by spectrophotometric techniques through analyzing a reduction in terms of absorbance, especially near 664 nm [37, 38].

Characterization of synthesized composite

Characteristics of cobalt oxide-NPs/kaolin composite by UV-Vis spectroscopy

The UV–visible spectrum of the nanoparticle suspension is useful information relating to the optical behavior of metal and metal oxide nanoparticles in the 200–800 nm wavelength range. Such spectra have been widely employed to characterize particles such as morphology, extent of dispersion, stability, and potential aggregation. A sharp absorption band is indicative of the formation of well-dispersed cobalt oxide nanoparticles. Raising successive load cycles shifts the absorption maximum to longer wavelengths gradually, indicating a particle size increase as more cobalt oxide is loaded. An indication of cobalt nanoparticle formation is therefore given by the position of an absorption band.

As is evident from Fig. 1, our UV and visible spectra of the synthesized sample display a sharp absorption peak found close to 335 nm. This is a close correspondence to that of an optical signature characteristic of crystalline nanostructures of CoO, implying its existence and confirming its material character. It is known from literature that a series of parameters-such as particle shape, particle size, and size of their structures-significantly influence the sharp spectral position of CoO in the UV–Vis region [39].

Fig. 1.

UV–vis absorption spectra for the identification of nanoparticle formation of the synthesized kaolin-supported cobalt oxide nanocomposites, and kaolin support material

FTIR analysis of cobalt oxide kaolin nanocomposite

The FT-IR spectra of kaolin, calcined kaolin, and the cobalt oxide/kaolin nanocomposite are shown in Fig. 2. Al–O, Al–OH, and Si–O groups are the primary absorption bands in the infrared spectrum; these groups can be found in both stretching and bending modes [10]. The bending vibrations of water molecules produce the band around 1625 cm⁻¹. Si–O and Al–O stretching vibrations are reflected by characteristic peaks at around 1090 and 1388 cm⁻¹, respectively. These are consistent with Si–O–Si symmetric stretching in disilicon oxide and quartz, respectively. The bending of hydroxyl groups linked to adsorbed water is responsible for a little absorption at about 2360 cm⁻¹ [40]. Al–Mg–OH vibrations are associated with the weak band about 800 cm⁻¹, indicating the existence of metal impurities. Asymmetric Si–O–Si stretching is seen close to 1040 cm⁻¹, whereas a peak at 535 cm⁻¹ represents Si–O–Al bending, suggesting that aluminum oxide units are connected inside the silica framework [41]. The formation of a band near 480 cm⁻¹, associated with Co–O stretching, indicates the formation of cobalt oxide nanoparticles. Calcination results in the loss of a broad band around 3420 cm⁻¹, which in untreated kaolin is associated with the stretching vibrations of –OH of Al–OH or Si–OH. The loss of hydroxyl bands is a result of the formation of crystalline phases, gehlenite, anorthite, and mullite, and structural changes due to dihydroxylation and sintering [42].

Fig. 2.

FT-IR spectra of kaolin, calcined kaolin, and the cobalt oxide/kaolin nanocomposite

XRD analysis of cobalt oxide/kaolin nanocomposite

Figure 3 depicts the X-ray diffraction (XRD) pattern of kaolin and kaolin/cobalt oxide nanocomposite that we have synthesized. The characteristic peaks of bare kaolin (BK) are observed at 2θ values of 12.06°, 20.08°, 24.62°, 35.12°, 38.38°, 55.08°, and 62.42°. These values correlate to the planes of crystalline kaolinite [43], which are denoted as (001), (110), (002), (), (122), (320), and (331). As a result of the appearance of additional diffraction peaks in the XRD pattern of the composite, it has been established that CoO nanoparticles (NPs) have been formed on the surface of the kaolin. The peaks that were detected at 35.56°, 38.78°, 58.46°, and 66.22° which correspond to the (111), (200), (220), and (311) planes of cubic cobalt oxide crystals, respectively, (ICSD: 01-075-0419) [44]. We accept that the peaks of our samples match slightly low values of CoO diffraction peaks of the standard crystal of CoO, which were recorded around 34.10°, 39.58°, 57.22°, and 68.32°, respectively. This suggests an increase in lattice constant, which can be largely due to metal substitution effects of our metal samples resulting from impurities of kaolin precursors, which were not removed properly during pretreatment process. Such effects of lattice distortion and shifting of diffraction peaks due to metal substitution caused by natural mineral precursors of transition metal oxides have already been widely reported by many researchers [44–46]. Our diffraction peaks of samples match very closely with ICSD card of CoO rather than Co₃O₄ [44]. As a result of the dehydration that occurs during the calcination process at a temperature of 500 °C, the kaolin phase transforms into amorphous metakaolin. It is established that this amorphous phase exists by the large diffraction peak that occurs in the range of 15–35° [47, 48]. Furthermore, a peak occurring at 27.51°, which has been attributed to quartz (SiO₂), is indicative of impurities in kaolin and metakaolin (JCPDS: 5-0490) [47, 49].

Fig. 3.

XRD plots for kaolin, CoO/kaolin Composite-4, and CoO/kaolin Composite-4-Reused

SEM with EDS analysis of cobalt oxide/kaolin nanocomposite

SEM displays the microstructure of the coated particle surface, the distribution of the catalyst over the substrate, the degree of homogeneity or heterogeneity, and the overall particle shape [50].

SEM coupled with EDX was used to analyze the morphological characteristics and elemental mapping of kaolin and kaolin/Cobalt oxide nanocomposite (Fig. 4A and B). The surface of kaolin alone and the nanocomposite is figured out as irregular, rough, and highly porous due to the existence of sufficient pores and grooves of distinctive sizes and shapes, which are generally accountable for the larger surface area and higher adsorption and catalytic efficacy of kaolin and composite. Figure 4B shows that the addition of Cobalt oxide -NPs on the kaolin changes its morphology and roughness compared to the raw kaolin alone (Fig. 4A). It confirms the successful deposition of Co-NPs on the surface of kaolin. Additionally, the EDX elemental mapping showed the presence of cobalt and oxygen in the pores of kaolin in addition to silicon and aluminum components of kaolin. EDS of Co shows the homogeneous distribution of cobalt oxide on kaolin surfaces in the kaolin/cobalt oxide nanocomposite.

Fig. 4.

SEM micrographs of kaolin A Kaolin/Cobalt oxide B and elemental mapping kaolin/cobalt oxide nanocomposite of aluminum (Al), Oxygen (O), Silicon (Si), Cobalt (Co)

BET and TEM analysis of cobalt oxide/kaolin nanocomposite

To investigate the specific surface area of the studied materials, BET analysis was carried out, and the adsorption-desorption graph of the raw kaolin, kaolin/cobalt oxide nanocomposite, and the composite after five reuse cycles is depicted in Fig. 5; Table 1.

Fig. 5.

A N₂ adsorption/desorption isotherm and B pore size distribution of kaolin, CoO/kaolin NC-4, and CoO/kaolin NC-4-Reused

Table 1.

Physical parameters of samples obtained by means of N₂ adsorption/desorption analysis

Material	SBET, m²/g	Pore volume, cm³/g	Pore diameter, nm
Kaolin	87.50	0.14	7.97
CoO/kaolin NC-4	60.63	0.16	12.43
CoO/kaolin NC-4-Reuse	57.25	0.19	16.77

The high surface are of kaolin (87.5 m²/g) represents its potential for supporting nanoparticles. The reduction in surface area after deposition of CoO in CoO/Kn (60.63 m²/g) is due to the growth of CoO on the pores of kaolin surface. The surface are of the composite did not show significant change after catalysis, signifying the stability of the catalyst. The BJH pore-size distribution curve reveals that the majority of pores are below 50 nm in diameter, indicating a predominantly mesoporous structure. All materials exhibit type IV isotherms with H₃-type hysteresis loops, indicating mesoporous structures (2–50 nm) typically associated with slit-like pores formed by the aggregation of nanosheets. The increase in catalytic activity i.a. due to the Co species active catalytic site.

The morphology of the synthesized composite material was further characterized by TEM. The TEM images depicted in Fig. 6A–C belong to the TEM image of bare kaolin, the composite material (CoO-kaolin and recycled composite material, respectively. The TEM images reveal the cobalt oxide nanoparticles randomly distributed within the kaolin matrix, which may have resulted in significant augmentation in the textural properties and provide the nanocomposite with a high surface area. The TEM image (Fig. 6A) shows layered and plate-like structures, which are characteristic of kaolinite clay minerals. The sheets appear transparent with smooth edges, indicating high crystallinity and relatively clean, unmodified kaolin surfaces. The layered stacking visible in some regions suggests the lamellar structure of kaolin. The TEM image (Fig. 6B) reveals a more compact and darker structure compared to pristine kaolin, suggesting successful deposition or anchoring of cobalt oxide nanoparticles on the kaolin surface. The composite shows that the particles size range 6–20 nm as shown in arrows. The regions of higher electron density (darker areas) correspond to cobalt oxide particles, which show some crystallinity, possibly due to the faceted edges. The underlying kaolin sheets remain partially visible, but the surface appears modified and covered with nanoparticulate clusters, confirming the stability of the composite. Presence of intact CoO in the reused CoO/kaolin composite (Fig. 6C) displays the structural integrity, which may influence catalytic or adsorption performance over time.

Fig. 6.

TEM micrographs of A kaolin, B CoO/kaolin NC-4, and C CoO/kaolin NC-4-Reused

Catalytic activity test

Despite being a robust oxidizing agent, an aqueous solution of sodium hypochlorite cannot efficiently oxidize MB because of the significant difference in redox potentials of the dye and NaOCl. In this work, we examined the degradation of MB by NaOCl in the presence and absence of the cobalt oxide-NPs/kaolin composite. The tests on the degradation of MB by NaOCl without the addition of the composite material revealed no discernible changes in color or λ_max intensity at 664 nm during the allotted period (Fig. 7). This indicates that MB is not significantly degraded by NaOCl alone, and its degradation in the absence of the composite occurs at a negligibly slow rate, making it difficult to observe. However, the rate of degradation was substantially improved following the addition of the catalyst, suggesting the tremendous catalytic effect of the produced cobalt oxide-NPs/kaolin composite in this process, as seen in Fig. 7. This was evident from the fading and complete disappearance of the deep blue color of MB, accomplished by a rapid decrease in the intensity of λ_max at 664 nm to nearly zero within 47 min, indicating the complete degradation of the MB solution. It was also observed that, in the absence of NaOCl, the addition of cobalt oxide-NPs/kaolin composite to the MB solution led to a decrease in the MB absorption spectra due to adsorption by the composite; however, the dye was not completely removed even after 40 min reaction time. A full disappearance of the dye was seen within about 12 min using the cobalt oxide/kn/NaOCl system, which reveals the composite’s remarkable potential for removal of this MB dye.

Fig. 7.

Successive UV–Vis absorption spectra of for degradation of aqueous methylene blue dye solution (50 mL, 50 mg/L) with 0.3 mL of NaOCl, 12 mg of kaolin, 12 mg of cobalt oxide-NPs/kaolin composite(without NaOCl), 12 mg of cobalt oxide-composite in presence of 0.3 mL of NaOCl without kaolin support, 12 mg cobalt oxide-NPs/kaolin composite in presence of 0.3 mL of NaOCl (C2) and absorption rate curves comparisons of all materials for the catalytic degradation of methylene blue dye solution in aqueous system (50 mL, 50 mg/L)

Nano composite selection

The amount of cobalt oxide NPs on the kaolin surface could alter the catalytic activity on dye degradation. The synthesis process involved numerous experiments with varying mass compositions of cobalt salt in a set amount of kaolin in order to enhance the amount of cobalt oxide NPs loading on the kaolin surface. To test their ability to catalytically degrade MB dye in the presence of NaOCl as an oxidizing agent, the materials that were obtained were chosen and given the names cobalt oxide/kaolin composite-1 (C1), cobalt oxide/kaolin composite-2 (C2), cobalt oxide/kaolin composite-3 (C3), cobalt oxide/kaolin composite-4 (C4), and cobalt oxide/kaolin composite-5 (C5).

The kinetic study on the degradation process of methylene blue (MB) showed that the reaction followed a pseudo-first-order kinetic model. The graphical plot of ln(Ct/Co) versus reaction time was found to be highly linear, with concomitant high correlation coefficient values as listed in Table 2. This significant linear correlation implies that the degradation rate of dye depends directly on the concentration of methylene blue at any given moment in time, with the catalyst and oxidant being in excess and not limiting the reaction. This kind of behavior implies that the inter-surface interactions between MB molecules and the active sites on the cobalt oxide/kaolin composites play a leading role in commanding the degradation process. In this regard, the pseudo-first-order kinetics model provides a strong framework for explanations on the reaction mechanism that lends itself to a useful comparison of the efficiencies of various composites as catalysts. Additionally, this kinetic approach allows an estimation of the rate constants that can be used for an assessment of the effect of varied nanoparticle loads as well as reaction working conditions on the total degradation performance.

Table 2.

Rate constants for the removal of methylene blue dye by different ratios of composites

MB solution (50 mL)	Catalyst	Pseudo first order		Pseudo second order
		K1 (min− 1)	R2	Rate	R2
50 mg/L	C1	0.286	0.85	4.18	0.81
	C2	0.348	0.97	6.98	0.85
	C3	0.325	0.91	5.87	0.89
	C4	0.337	0.92	6.22	0.91
	C5	0.351	0.89	8.31	0.92

Figure 8 shows UV–Vis spectra corresponding to the degradation of methylene blue (MB) dye by the utilization of the prepared composites against bare kaolin. The observation highlights that the degradation activity for the cobalt oxide/kaolin composite-4 (C4) and composite-5 (C5) was distinctly higher compared to C1, C2, and C3. Interestingly, the typical absorbance maxima corresponding to MB at 664 nm almost disappeared in the span of a single min under the action of C4 and C5, accordingly highlighting their remarkable catalytic activity. The immediate decrease in absorbance reflects that these composites provide an ample number of accessible active sites, thus enhancing effective contact between the MB molecules as well as the in-situ oxidizing species derived from sodium hypochlorite (NaOCl). It was identified that the degradation reaction proceeds according to pseudo-first-order kinetics, indicating that the reaction rate is mainly dictated by the concentration of MB under the condition where the catalyst as well as the oxidant remain in excess.

Fig. 8.

Successive UV–Vis absorption spectra for the degradation of aqueous methylene blue dye aqueous solution (50 mL,50 ppm) 12 mg of cobalt oxide-NPs/kaolin composite-1 (C1), cobalt oxide-NPs/kaolin composite-2 (C2), cobalt oxide-NPs/kaolin composite-3 (C3), cobalt oxide NPs/kaolin composite-4 (C4), cobalt oxide-NPs/kaolin composite-5 (C5) and pseudo second order graph

Under the experimental conditions, 12 mg catalyst, 50 mL MB solution (50 mg/L), and 4% NaOCl, the C4 removed 99% of MB in 6 min, and C5 achieved 99.9% in the same time frame. This side-by-side comparison reflects the pivotal role that cobalt oxide nanoparticle loading on the kaolin substrate plays adequate loading enhances catalytic efficiency by offering more active site locations, but too-high loading might not proportionally increase efficiency and can cause unjustified consumption of precursor material. That C4 and C5 performed almost equally despite C4 having slightly lower loading positions suggests that C4 strikes a valuable balance between maximizing the catalytic activity as much as possible without wastefully consuming precursor material. In addition, the enhanced performance in C4 and C5 can be attributed to the superior dispersion of the nanoparticles and stronger interaction with the kaolin substrate that stabilizes the active species and suppresses aggregation. These factors increase the effective formation of reactive oxidative species and facilitate faster degradation of MB. In consideration of efficiency as well as practicability, cobalt oxide/kaolin composite-4 was chosen as the optimal catalyst for further experiments.

From a theoretical standpoint, the catalytic degradation process proceeds through the adsorption of both dye molecules and hypochlorite ions (OCl⁻) onto the surface of the catalyst. Following adsorption, an interfacial electron transfer occurs in which OCl⁻ acts as an electron donor, while the dye molecules serve as electron acceptors-a mechanism widely reported in related catalytic systems [51, 52]. In this reaction scheme, electrons released by OCl⁻ migrate to the cobalt oxide nanoparticles, which function as an electron mediator. Subsequently, methylene blue (MB) molecules accept these electrons from the cobalt oxide sites, leading to a redox transformation of MB and the formation of its oxidized degradation products.

The catalytic activity is further enhanced by the combined effect of the support material and the cobalt oxide nanoparticles, which can promote the generation of highly reactive oxidative species. These transient species, including singlet oxygen (¹O₂) and other reactive oxygen intermediates, play a crucial role in the breakdown of the dye’s chromophoric structure and the eventual mineralization of the organic molecules [14, 53]. Based on these observations and prior mechanistic insights, the overall reaction pathway can be proposed as shown below.

Step one (I)

4

5

Step two (II)

6

Effect of operational parameters

Effect of catalyst dosage

In order to investigate the effect of the amount of catalyst loading on the oxidative degradation of MB, in this work, five reactions were performed by changing the amount of cobalt oxide /Kaolin nanocomposite from 3 to 15 mg, as shown in Fig. 9, and keeping other reaction parameters constant. It was seen that the degradation efficiency increases as the nano-catalyst loading increases from 3 to 15 mg. Therefore, the catalyst loading of 12 mg was taken to investigate the other reaction parameters. This outcome was caused by the larger catalyst dosage enlarged the number of active sites on the catalyst surface, which improved its adsorption and the decomposition rate of NaOCl. Additionally, a large number of active sites improved the adsorption efficiency of the dye and its degradation [54, 55]. An increase in catalyst dosage enhances the electron relay capacity; however, since the amount of MB is fixed at a given concentration, the oxidation rate decreases when the catalyst amount is too small. The oxidation rate constants (k_oxd) (Table 3) were calculated for each catalyst dosage using the pseudo-first-order kinetic plot shown in Fig. 9. The reaction rate increased with the increasing catalyst dosage due to enhanced electron transferability, exhibiting a good linear correlation coefficient.

Fig. 9.

Successive UV–vis absorption spectra and absorption rate curves of different catalyst mass cobalt oxide-NPs/kaolin composite (3, 5, 7, 12, and 15 mg) for the catalytic degradation of MB dye

Table 3.

Catalytic removal efficiency of the composite at different catalyst masses with a fixed MB dye concentration

MB solution	Catalyst mass	Rate constant(min− 1)	Correlation coefficient	%R	Time (min)
50 mL	5 mg	0.165	0.98	92	20
	7 mg	0.246	0.98	96.5	18
	12 mg	0.382	0.98	99.9	14
	15 mg	0.405	0.67	99.99	10

Effect of MB concentration

In this study, three aqueous MB solutions of different concentrations ranging from 25 to 50 ppm were used with a fixed concentration of the catalyst (12 mg/50 mL) and NaOCl (4%, 0.5 mL). The corresponding UV-Vis absorption spectra for the oxidative degradation of MB with the required concentrations are presented in Fig. 10. As can be seen, the time required for complete degradation increased with increasing concentration of MB. In other words, the degradation rate decreased as the MB concentration increased, supporting first-order kinetics of the degradation process. This decrease in rate can be attributed to the slowing down of the electron transfer process on the nanocomposite surface between NaOCl and the MB dye. In addition, the excess MB molecules compete with each other for the limited amount of active species generated by the kaolin-cobalt oxide system [36–39]. The pseudo-first-order kinetic plots for various MB concentrations are presented in Fig. 10, along with the corresponding oxidation rate constants (k_ox) for each MB dye concentration (Table 4). The results indicate that higher dye concentrations require longer degradation times under the same conditions. As the dye concentration increases, competition for electron capture among dye molecules also increases, while the catalytic capacity remains limited at a fixed catalyst mass. Consequently, the dye oxidation rate decreases at higher MB concentrations.

Fig. 10.

Successive UV–Vis absorption spectra of MB dye aqueous solutions at different concentrations, along with the corresponding degradation rate curves for the catalytic oxidation of each MB solution (50 mL, 25, 35, and 50 mg/L) in the presence of 0.3 mL NaOCl (0.25 M) and a fixed catalyst mass (12 mg) of the cobalt oxide NPs/kaolin composite

Table 4.

Catalytic removal efficiency of the composite with different MB dye solution concentrations with fixed catalyst mass

Catalyst amount (mg)	MB solution (50 mL)	Rate constant, kox (min− 1)	Correlation coefficient, R2	%R	Time, t (min)
10	25 mg/L	0.357	0.91	99.9	14
	35 mg/L	0.348	0.93	99.5	14
	50 mg/L	0.322	0.97	98.7	14

Effect of temperature

In order to investigate the temperature effect on the removal of MB dye, four reactions were carried out by varying the temperature from room temperature to 50 °C and keeping other reaction parameters constant. The degradation profiles of aqueous MB (50 ppm) solution over the different temperatures from room to 50 °C are shown in Fig. 11. It was observed that the oxidative degradation efficiency increased as the temperature rose from room temperature to 50 °C. The results indicate that an increase in temperature significantly reduces the time required for the reaction to reach equilibrium. The degradation efficiency of NaClO can be enhanced at higher temperatures due to the increased frequency of effective collisions among the catalyst, MB molecules, and other reactive species. In the cobalt oxide/kaolin/NaOCl system, raising the temperature may also increase the mass transfer efficiency at the solid-liquid interface, which might speed up the diffusion of MB molecules to the catalytic surface, facilitating the removal of degradation intermediates and re-exposure of active sites [40, 41]. When subjected to heat or UV radiation, hypochlorite may also produce reactive radicals, ROS, and the ROS produced during this process could further degrade, hydrolyze, or halogenate already-existing organic molecules [42, 43].

Fig. 11.

The degradation profiles of aqueous 50 mL MB (50 ppm) solution over the different temperatures from room to 50 °C, along with NaOCl (4%, 0.3 mL) solution

Proposed mechanism and possible pathway for degradation of methylene blue dye with cobalt oxide/kaolin/NaOCl system

To study the degradation mechanism of MB in the Cobalt oxide/kaolin/NaOCl system, a schematic diagram is proposed in Scheme 1. It is suggested that the support material and cobalt oxide nanoparticles act as catalysts for generations of active oxidizing species, producing highly reactive oxidizing agent radicals, particularly singlet oxygen (¹O₂), which are capable of degrading the dye molecules. The possible degradation pathways and intermediates of MB in the NaOCl system have been reported in the literature [55], indicating that demethylation, chromophore cleavage, ring opening, and eventual mineralization are the main steps involved in MB degradation. In general, NaOCl, a strong oxidizing agent, can undergo decomposition in the presence of CoO, leading to the generation of reactive oxygen species (ROS), singlet oxygen (¹O₂). The CoO nanoparticles act as a catalyst that facilitates the activation of NaOCl, enhancing its oxidizing power through the formation of these reactive species. Additionally, the porous structure of kaolin provides a high surface area that supports the catalytic activity and stability of CoO while also adsorbing organic pollutants, which can then be efficiently attacked by the generated ROS. This collective action results in an intensified photocatalytic degradation of organic contaminants, making the CoO/Kaolin composite an effective tool for water treatment applications where oxidative degradation is essential for removing pollutants

Scheme 1.

Proposed mechanism for degradation of MB dye by Cobalt oxide/kaolin/NaOCl system

The final degradation products of MB indicate its conversion into harmless CO₂ and the transformation of nitrogen and sulphur heteroatoms into inorganic ions such as ammonium, nitrate, and sulphate, respectively [47].

Reusability of cobalt oxide/kaolin nanocomposite

The reusability of the material and the release of metal ions are key indicators for evaluating its stability [64]. Although the cobalt oxide/kaolin nanocomposite exhibited excellent catalytic performance, assessing its recycling stability is essential. Therefore, the catalyst was recovered and reused to determine its effectiveness upon repeated cycles. As shown in Fig. 12, the catalytic performance of cobalt oxide/kaolin remained nearly unchanged after five consecutive cycles for MB dye degradation, indicating its high stability. Moreover, it demonstrated superior performance compared to other reported catalysts (Table 5). The slight decrease in MB conversion after five cycles could be attributed to mass loss during filtration, the reduction of active sites due to cobalt leaching, and the partial blockage of active sites by adsorbed species. This is supported by the XRD pattern of the used catalyst (Fig. 3). As shown in the figure, the diffraction peak intensities of CoO NPs after use are comparatively lower than those before use. This observation is further supported by the increased intensity of the amorphous region of calcined kaolin in the 2θ range of 15–35°.

Fig. 12.

Reusability of cobalt oxide/kaolin Nanocomposite

Table 5.

Comparison of the catalytic activity of the cobalt oxide-nps/kaolin composite with other catalysts reported in the literature for mb dye degradation

Catalyst	Catalyst dosage	MB concentration (mg/L)	Oxidizing agent	Time (min)	Temperature (°C)	Percentage removal (%)	Ref.
Ni-Fe/Al2O3	20 mg/mL	50	NaOCl	40	45	98.98	[56]
CFACoFe2O4	12 gm/100 mL	40	5 mM H2O2	60	room	99	[57]
PANI-Ag/ZnS	30 mg/L	10	3 mM H2O2	60	room	> 95	[58]
Cobalt oxide	0.5 mg/mL	12	NaOCl	25	–	> 96	[59]
Cu2O–Cu/C	0.2 mg/mL		0.03 M H2O2	40	50	> 99	[60]
CuO	10 mg/50 mL	10	12% NaOCl	30	32	95	[60]
NiO/Kaolin	20.3 mg	50 mg/L (50 mL)	4% NaOCl	6	40	> 99	[10]
Cobalt oxide/Kaolin	12 mg	50 mg/L (50 mL)	4% NaOCl	6	45	99.9	This work

Conclusions

In summary, cobalt oxide was successfully synthesized through a cost-effective in-situ growth method on the surface of locally available and inexpensive kaolin. The resulting cobalt oxide-NPs/kaolin composite exhibited excellent catalytic performances for the oxidation of the model organic dye, MB, in the presence of NaOCl, leading to its conversion into inorganic ions. The remarkable catalytic efficiency is attributed to the high loading and uniform dispersion of cobalt oxide NPs on the kaolin surface. A 12 mg dose of the cobalt oxide NPs/kaolin composite achieved over 99% degradation of 50 mL of 50 mg/L MB solution within 6 min, with a rate constant of 0.38 min^− 1. The outstanding catalytic cobalt oxide-NP promotes efficient oxidation of MB in the composite system. This simple synthesis approach can be extended to the fabrication of other nanoparticle-support composites and offers a promising route for the large-scale preparation of low-cost, high-performance catalysts for wastewater treatment. Furthermore, the prepared catalyst demonstrated excellent reusability, maintaining high activity over multiple catalytic cycles with minimal loss in performance.

Author contributions

All authors (Bamlaku Abebaw Miskir, Kedir Seid Mohammed, Zinabu Gashaw Asmare, Minaleshewa Atlabachew, Yong Liu, Tibebu Shiferaw, Belete Asefa Aragaw, and Bewketu Mehari) contributed to the study conception and design, as well as to data collection and analysis.

Funding

Not applicable.

Data availability

The data sets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Clinical trial number

Not applicable.

Competing interests

The authors declare no competing interests.