Room temperature fabrication of cobalt mullite for the snappy adsorption of cationic and anionic dyes

Cobalt mullite adsorbent for the robust adsorption performance toward Victoria Blue (VB) and Metanil Yellow (MY) is fabricated by the sol–gel method at room temperature using dipropylamine as a structure-directing agent. The synthesized adsorbent is characterized by XRD, FT-IR, and HRTEM. From these analyses, it is found that dipropylamine binds with the alumina and cobalt oxide, which makes it into tetrahedral to octahedral form. This interaction causes the formation of cobalt mullite. It is observed that trigonal alumina and orthorhombic cobalt mullite are interlinked to form a hybrid network. The special feature of adopting this adsorbent for the adsorption of VB and MY is that it has a large amount of Brønsted acid site because of the octahedral coordination of Al and Co. The large availability of acid sites in the framework and hybridization of two different network systems favors robust adsorption. The rate of adsorption (K2 = 0.00402 g/mg.min for VB and K2 = 0.004 g/mg.min for MY) and adsorption capacity (Qe = 102.041 mg/g for VB and Qe = 19.0406 mg/g for MY) are greater for VB than MY. This may be due to the more steric factor involved in MY than VB. Thermodynamic parameter indicated that the adsorption of VB and MY is spontaneous, endothermic, and increased randomness in the adsorbent-adsorbate interface. The results from the enthalpy value (ΔH° = 65.43 kJ/mol for VB and ΔH° = 44.729 kJ/mol for MY) revealed that the chemisorption is involved in the adsorption process.


Introduction
Typically, organic hazardous dyes are manufactured in developing nations generating variety of industrial waste products from the textile, paper, rubber, plastics, leather, cosmetics, pharmaceutical, and food sectors. However, synthetic dyes are widely utilized in the pharmaceutical and textile sectors, producing colored effluent (Giovannetti et al. 2015;Bedin et al. 2018;Panão et al. 2019;Huang et al. 2019). Approximately 10 to 15% of all dyestuffs used in dyeing operations are lost to wastewater directly (Parshetti et al. 2012). Such industrial wastes are the primary cause of soil and water contamination. Actually, the release of wastewater creates an ecotoxic hazard and poses a possible risk to bioaccumulation (Ahmad et al. 2020;Sharavanan et al. 2020). As a consequence, one of the most serious problems in the current world is the removal of organic dyes from wastewater without generating secondary waste.
Several techniques have been developed to remove colors from industrial wastewater. The traditional method for treating wastewater includes adsorption, chemical oxidation, membrane filtration, biological treatment, and electrochemical treatment (Ejder-Korucu et al. 2015;Awual et al. 2020). Organic dyestuffs are decolored by the chemical oxidation process utilizing Fenton's reagent (Mohamed et al. 2018(Mohamed et al. , 2019. But the disadvantage of this technique is the creation of a large amount of ferrous Responsible Editor: Tito Roberto Cadaval Jr sludge. Biological treatments of dye-contaminated wastewater are comparatively unsuccessful due to the inability to break down large molecular weight of dye molecules by microorganisms (Awual et al. 2013;Awual and Ismael 2014;Masi et al. 2019). Membrane filtration has been found to be an excellent method for cleaning wastewater by continuously concentrating and separating dye species from effluent (Astuti et al. 2019;Md. Munjur et al. 2020). However, it has a high capital cost and the membranes are prone to clogging. Catalytic electrooxidation is ineffective for entirely mineralizing dye species (Farhana et al. 2014;Hossain et al. 2020). On the other hand, the adsorption technique is recognized to have a particular advantage: it may totally remove color molecules without leaving any pieces of dye molecules in the effluent. Additionally, the technology based on the adsorption process would be remarkably considered as the environmentally benign and economically cost-effective approach for the treatment of wastewater (Ali 2012;Alam et al. 2019aAlam et al. , b, 2020Astuti et al. 2019).
Several reports on the removal of VB and MY by various adsorbents have been published. Quick adsorption of dye molecules by the adsorbent, on the other hand, is extremely important. For instance, removal of VB by incense stick ash (Jain et al. 2020), zinc oxide nanoparticles (Kataria et al. 2016), cyclodextrin-grafted thiacalix (Chen et al. 2016), amino-functionalized graphene (Guo et al. 2013), and rice husk with bovine serum albumin (BSA) (Zein et al. 2020) took more than 90 min to attain equilibrium. In the case of removal of MY by bottom ash (Malviya et al. 2020) and hexadecyltrimethylammonium bromide, surfactant supported silica (Nagappan et al. 2019) required 4 and 6 h, respectively, to reach the saturation point.
As per the literature review, many adsorbents took longer time for adsorption. Hence, to be the best adsorbent, the adsorbent should be able to adsorb quickly with higher adsorption capacity. This can be achieved by creating more acid/active sites in the framework of the adsorbent. In this view, aluminosilicate is one of the adsorbents for the removal of dye molecules (El-Safty et al. 2011;Martínez-Zapata et al. 2011;Boukoussa et al. 2017;Abdelrahman 2018;Rashwan et al. 2019), which bears acid/active sites in their tetrahedral framework. But it is not sufficient to reduce the equilibrium adsorption time. Hence, the main objective of this study is to prepare mullite structure (octahedral framework) in cobalt aluminosilicate to further enhance the active/acid sites. This can enhance the adsorption efficiency with shorter time period. According to literature reports, mullite was synthesized above 1000 °C (Ismail et al. 1986;Treadwell et al. 1996;Komarneni et al. 2006;Zhang et al. 2010;Induja and Sebastian 2017). Here, we synthesized cobalt mullite at room temperature using dipropylamine as the structure directing agent by adopting simple sol-gel method without autoclave. To our knowledge, there is no literature report regarding cobalt mullite for the removal of organic dyes within short time period.

Synthesis of cobalt aluminosilicate
Cobalt aluminosilicate is successfully synthesized by employing the facile sol-gel method under room temperature and atmospheric pressure without using an autoclave. First, dissolve the sodium metasilicate (12.206 g) in 100 ml of double-distilled water and the aluminum chloride (13.33 g) in 60 ml of doubledistilled water. The mixture is then combined and magnetically stirred at room temperature for an hour. Then, 13.7 ml of dipropylamine is added dropwise to the mixture, and the mixture is kept stirring for 2 h. After stirring the mixture for an hour, 4.76 g of cobalt chloride hexahydrate that has been dissolved in 20 ml of water is added to the content. The pH of the solution is adjusted to 7. The gel must crystallized for two days after it has been prepared. The wet nano-gel is dried for 12 h at 200 °C in an oven. The material is ground to a fine powder and calcined at 400 °C for five hours to remove the organic moieties present in the material. Cobalt aluminosilicate's end product has the following molar composition: 1.0 AlCl 3 :1.0 SiO 2 :1.0 DPA: 0.02 CoCl 2 :8.33 H 2 O.

Adsorption experiment
The removal of Victoria Blue (a cationic dye) and Metanil Yellow (an anionic dye) from cobalt aluminosilicate was investigated through an adsorption study. The experiment was performed to evaluate the kinetics, isotherm, and thermodynamics as a function of contact time, initial dye concentration, and temperature. 250 ml reagent bottle, which was mounted on the orbital shaker and contains adequate amounts of adsorbent and adsorbate, was used for the experiment. To regulate the temperature for the thermodynamic study, a temperature-controlled magnetic stirrer was used. After reaching equilibrium, the final dye concentration was measured using a UV-visible spectrophotometer that had been calibrated with VB at 615 nm and MY at 435 nm. The following equations are used to calculate the amount of dye adsorbed on the adsorbent Eq. (1) and the percentage of removal Eq. (2), where W is the weight of the adsorbent (g), C o is the initial dye concentration (mg/l), C e is the final dye concentration (mg/l), and V is the volume of the adsorbate (l). The molecular structures of VB and MY are shown in Fig. 1.

Physico-chemical characterizations
Fourier transform infrared (FTIR) analysis of the synthesized materials using the KBr pellet method was performed using a Nicolet iS5 FTIR (ThermoFisher) model spectrophotometer. This procedure involves mixing 150-200 mg of KBr powder with about 15 mg of finely powdered material. At a pressure of 5 tonnes, the combined material is pressed to form a glassy pellet. After that, the sample pellet is put in the sample holder of the FTIR instrument for analysis. Utilizing Cu-Kα radiation (λ = 1.5406 Å) over a range of 5-90°, the XRD pattern of the synthesized material was captured on a Bruker D8 Advance Powder X-Ray Diffractometer. On JEOL JEM 2100, HRTEM was evaluated with a lattice resolution of 0.14 nm and a point-to-point resolution of 0.19 nm with an accelerating voltage of 200 kV.

Error analysis
Recently, several mathematical error functions have been examined and confronted (Foo and Hameed 2010). For kinetics and isotherm, the results of error analysis are evaluated using reduced Chi-square analysis, residual root mean square error (RMSE) and average relative error (ARE) (Subramanyam and Das 2009;Chan et al. 2012). The mathematical expressions for the above said error analysis are given below where, q e(exp) and q e(calc) are experimental and calculated equilibrium adsorption capacities and n is number of observations in the experimental data.

X-ray diffraction
The wide-angle X-ray diffraction (WAXRD) of cobalt aluminosilicate has been carried out (Fig. 2). The low intensity peaks are suggested that there is a disordered mesoporous structural arrangement in the material framework (Abdelrahman 2018). The XRD pattern of cobalt aluminosilicate exhibits the reflections due to the orthorhombic Co-mullite phase (COD card no. 96-901-0160), trigonal α-Al 2 O 3 (COD card no. 96-900-9680), cubic Co 2.62 O 4 (COD card no. 96-152-8447), and orthorhombic mullite (COD card no. 96-900-9680). Among these, Co-mullite is more prominent Structures of (a) Victoria Blue and (b) Metanil Yellow than the other peaks suggesting that Co-mullite formed in the framework. The formation of different phases in the synthesized materials revealed that the material is polymorphic material. The average crystallite size is calculated by using Scherrer equation which is found to be 43.71 nm. The crystal structure is obtained from the VESTA software using the unit cell parameters (Momma and Izumi 2011), which is shown in Fig. 3. Typically, the structure of mullite that is characterized by the chain edges is connected to AlO 6 octahedral which are cross-linked to Al and Si tetrahedron (Schneider et al. 2008(Schneider et al. , 2015. The crystal structure reflected that aluminum and cobalt are in octahedral and tetrahedral coordination, and silicon is in tetrahedral coordination. Octahedral and tetrahedral aluminum and cobalt are cross-linked together to form hybrid network. Octahedral coordination of aluminum and cobalt could be due to the interaction of dipropylamine with the respective species. Octahedrally coordinated aluminum and cobalt will provoke more acidity in the material (Gopal and Chellapandian 2023). This confirmed the presence of mullite in the framework.

FTIR
FTIR analysis of as-prepared and calcinated cobalt mullite is shown in Fig. 4. The hydroxyl group vibration of Al-OH, Si-OH, Co-OH, and NH 4 + -OH − groups causes the broadening of band at 3443 cm −1 (Dufau et al. 2001). Hence, these peaks intensities are not completely reduced after the calcination. Smaller peaks at 2954 cm −1 , 2797 cm −1 , and 2367 cm −1 are attributed to the asymmetrical stretching of -CH 3 group of dipropylamine, asymmetrical stretching -CH 2 group, and symmetrical stretching of -CH 2 group, respectively. -OH bending mode of the silanol group produces a sharp band at 1637 cm −1 (Rajesh et al. 2002). Overtones at 1389 cm −1 and 1376 cm −1 arise from the symmetric stretching of the  (Kosari et al. 2020). These peaks appeared even after calcination which suggested that ammonium ions are binded inside the pores or channels of the framework (Mali et al. 2002). Asymmetric stretching vibration of Si-O-Si bond produces a distinctive peak at 1017 cm −1 and 978 cm −1 (Nampi et al. 2010). Peak at 619 cm −1 owing to the vibration of cubic Co 3 O 4 (Girardon et al. 2005;Esposito et al. 2007). Additionally, this band is designated for Si-O-Co network of Co-O stretching (Yin et al. 2009). Band situated at 554 cm −1 corresponds to Co-O stretching. Peaks in the range of 495 cm −1 to 463 cm −1 are because of the octahedral moiety of aluminum species. These characteristic peaks confirmed the existence of the mullite phase and the incorporation of cobalt in the framework of the synthesized material.

HRTEM
The representation of HRTEM micrographs of Co-mullite are presented in Fig (b), it is observed that Co-mullite creates large flattened rod-shaped crystals surrounded by the agglomeration of aluminosilicate. Figure 5(c) shows that the d-spacing of the Co-mullite lattice fringes is in good agreement with the XRD results. This clearly revealed the existence of Co-mullite in the framework. Furthermore, some of the cobalt species can also be seen on the surface of the crystals and agglomerates in the form of dark spots (Grzybek et al. 2022). Figure 6 represents the UV-visible spectrum of cobalt mullite which provides information on the oxidation state of cobalt ion. The color of the solid cobalt mullite turned from pink to blue upon calcination. The blue colored calcinated cobalt mullite exhibits a band at 233.06 nm because of the octahedral Co 3+ species whereas the bands in the region of 540 nm to 626 nm are due to Co 2+ in tetrahedral coordination (Caetano et al. 2006). This proved the hybridization of octahedral and tetrahedral framework of cobalt mullite. These findings suggested that tetrahedral Co 2+ is more dominant than octahedral Co 3+ . This may be owing to the possibility of the interaction of cobalt with moisture present in the air. This results in lack of an inversion center in the former's symmetry. Hence, the contribution of octahedral Co 3+ is masked (El Nahrawy et al. 2020).

Effect of time and kinetics
The removal of the large amount of the target pollutant in the shortest period of time is defined as a key objective of Basically, the rate of adsorption intensity increases steadily over time. Considering Fig. 7(a), cobalt mullite removes VB more quickly than MY. 10 min for VB and 25 min for MY are required to reach equilibrium. The immediate VB adsorption may be caused by the octahedral and tetrahedral coordination of aluminum and cobalt, which will increase the amount of Brønsted and Lewis acid/active sites in the framework. These sites are cause for the rapid uptake of dye molecules. In the same way, adsorption capacity also increases steadily over time Fig. 7(b). This is owing to availability of large active/acid sites on the adsorbent initially. After reaching the equilibrium, the active/acid sites of the adsorbent get saturated, and this leads to desorption of dye from the adsorbent is in dynamic equilibrium. Hence, removal percentage of dye molecules and adsorption capacity of the adsorbent increases over time (Crini and Badot 2008). To determine the rate of adsorption, two kinetic models are investigated, namely, pseudo-first order kinetics (Eq. 6) and pseudo-second order kinetics (Eq. 7).
where Q e represents the amount of adsorbed dye at equilibrium (mg/g), Q t represents the amount of adsorbed dye at time t in minutes (mg/g), K 1 represents the rate constant of pseudo-first order kinetics (1/min), and K 2 represents the rate constant of pseudo-second order kinetics.
Understanding the nature of the interaction between the adsorbent and adsorbate is very much essential to comprehend the adsorption kinetics (Deniz and Karaman 2011;Rehman et al. 2012). Adsorption kinetic plots for the dyes VB and MY are represented in Fig. 8. The fitting results for VB and MY adsorption behavior are presented in Table 1. The close relationship between the Q e (exp) and Q e (calc), higher R 2 value and smaller value of error values (Table S1) show that pseudo-second order kinetics can be applied more effectively for these 2 dyes, supporting the dominance of chemisorption (Ho and McKay 1999). Table 2 reveals that the present material has excellent adsorption performance compared to other literature reports. This is only because of the existence of the cobalt mullite phase in the synthesized Cobalt mullite. This phase formation is mainly due to the interaction of dipropylamine with the cobalt aluminosilicate framework. This leads to the conversion of tetrahedral cobalt and alumina into octahedral coordination. Hence, the adsorption capacity of cobalt mullite is higher for the adsorption of VB and MY with short equilibrium time.

Effect of pH
To study the effect of pH, the pH of the dye solution is adjusted to 2 to 11 using 0.1 N HCl and 0.1 N NaOH. The pH of the solution controls the intensity of electrostatic charges associated with the ionized dye molecules. This fluctuates the removal percentage and adsorption capacity (Önal et al. 2006). Figure 9(a) and (b) displays the influence of pH for the adsorption of VB and MY. Figure 9(a) and (b) clearly represents the low pH is favorable for anionic dye and high pH is favorable for cationic dye. This change  (Zein et al. 2020) in adsorption ability depends on the pH of the dye solution where H + /OH − is the potential determining ions (Calvete et al. 2009;Demirbas 2009;Khaled et al. 2009;Ponnusami et al. 2009;Deniz and Karaman 2011;Salleh et al. 2011;Yan et al. 2013). This resulted in protonation and deprotonation of the adsorbent surface by altering the pH from low to high (Karmaker et al. 2015). This causes electrostatic attraction or repulsion between the adsorbent-adsorbate interfaces. The optimum pH for the adsorption of VB and MY is 8 and 7, respectively.

Effect of initial dye concentration and isotherm
The percentage of organic dyes removed from the adsorbent depends critically on initial dye concentration. The initial dye concentration has a significant impact on the removal percentage (Eren and Acar 2006). The removal percentage decreases as the initial dye concentration rises, as shown in this Fig. 10(a). The adsorbent surface is being completely occupied at higher adsorbate dosages and thus the removal efficiency decreases (Salleh et al. 2011). On contrary, the adsorption capacity of cobalt mullite for VB and MY increases with increasing initial dye concentration Fig. 10(b). This is attributed to the mass transfer driving force gets increased at higher dye concentration (Bulut and Aydin 2006). For VB and MY, the optimum dye concentrations are 200 mg/L and 100 mg/L, respectively (Fig. 10).
In order to understand the interaction between the adsorbent-adsorbate interface and its maximum monolayer adsorption capacity is also investigated using the Langmuir isotherm (Eq. 8) and Freundlich isotherm (Eq. 9).
where C e represents the equilibrium residual dye concentration, Q e represents the dye adsorbed amount at equilibrium (in mg/g), Q max represents the maximum monolayer adsorption capacity (in mg/g), K L and K F denote the Langmuir and Freundlich rate constants, and n is the adsorption intensity. Table 3 contains a summary of all the outcomes for these models.
By fitting the experimental data with adsorption isotherm models, it is possible to comprehend how the adsorbate and adsorbent interact during isothermal adsorption (Fytianos et al. 2000). The results from Table 3 and Fig. 11 indicated that the adsorption of VB and MY by cobalt mullite obeyed Langmuir adsorption isotherm with higher correlation coefficient and smaller error values (Table S2). This suggested that the adsorption is chemisorption. Q max value presented in the Table 3 suggested that the higher adsorption efficiency of VB toward cobalt mullite than MY. This may be because of the steric hindrance around the amine group of MY restricts the interaction between the MY and acid/ active sites of cobalt mullite. Total adsorption capacity is calculated by the product of Q max and n. It is found that the total adsorption capacity for VB is higher than MY. This is because of the hybridization of octahedral and tetrahedral cobalt and aluminum.

Effect of temperature and thermodynamics
Temperature is a crucial physicochemical parameter in determining whether an adsorption process is endothermic or exothermic. Figure 12 shows the adsorption of VB and MY by cobalt mullite at various temperatures. From the Fig. 12(a) and (b), it is clear that increasing the temperature increases the removal percentage and adsorption capacity for both VB and MY. This is owing to the increased mobility of dye molecules on the active sites of cobalt mullite as the temperature increases (Senthilkumaar et al. 2006;Ma et al. 2014). This indicated that the adsorption is an endothermic process (Senthilkumaar et al. 2006).
In order to find out the spontaneity of the adsorption process, thermodynamic parameters are investigated. Using the following equations, enthalpy change (ΔH°), Gibbs free energy (ΔG°), and entropy (ΔS°) are calculated.
(10)  Equating (10) and (12), we get where ΔG° is change in Gibbs free energy, ΔH° is enthalpy change, ΔS° is entropy change, K d is distribution constant, R is gas constant in (J/mol K), and T is the absolute temperature in Kelvin. A plot of 1/T vs. ln K d , we get a slope (ΔH°/ RT) and intercept (ΔS°/R) (Fig. 13). The results from the Table 4 revealed that the adsorption process is endothermic since the enthalpy change values for both dyes are positive. In addition to that, enthalpy values (> 40 kJ/mol) suggested that the chemisorption is involved in the removal of VB and MY. The negative value of change in Gibbs free energy and positive value of entropy change shows that the process is spontaneous and an increased level of randomness occurred during adsorption.

Effect of adsorbent dosage
Effect of adsorbent dosage is one of the important parameter in adsorption process because it gives an idea of adsorption of organic dye with minimal adsorbent dosage from economic point of view (Salleh et al. 2011). Figure 14 (a) presents the effect of adsorbent dosage for the adsorption of VB and MY where the removal efficiency increases with increase of adsorbent dosage. This is owing to the large availability of active sites on increasing the adsorbent dosage (Ofomaja 2008). On the other hand, the adsorption capacity decreases with increasing adsorbent dosage ( Fig. 14(b)). This is due to the unsaturation of active/acid sites on the adsorbent surface. The optimum adsorbent dosage for the adsorption of VB and MY is 0.1 g and 0.5 g, respectively.

Regeneration and reusability studies
Desorption, regeneration, and reusability of adsorbent are very much significant since it makes the adsorbent promising and cost-effective (Obeid et al. 2013;Zhang et al. 2018;Md. Munjur et al. 2020). Hence, in this study, the dye molecule is desorbed using ethanol at various concentrations. Figure 15(a) shows the desorbing efficiency is increased by increasing the ethanol concentration. This suggested that the ethanol has a greater tendency to desorb the adsorbate molecule from the aqueous phase. After desorption, the adsorbent is washed with double distilled water three times to remove the ethanol adhered on the surface of the adsorbent. Then, it is regenerated at 300 °C for 30 min in a muffle furnace. The regenerated Cobalt mullite is then tested for the reusability study. This process is conducted for 5 successive cycles. Figure 15(b) clearly represents that the removal efficiency of both dyes gradually decreased upon number of cycles (90% to 60% for VB and 81% to 9% for MY). Therefore, it is concluded that cobalt mullite is reusable for 5 times for the adsorption of VB and 4 times for the adsorption of MY.

Plausible mechanism for the adsorption of VB and MY
The interaction of the adsorbent and dye molecule depends on the characteristics of the adsorbent and the nature of the dye molecule (Yeamin et al. 2021). Based on the results obtained from the kinetics, isotherm, and thermodynamics, the mechanism of interaction between the adsorbent and adsorbate is proposed. The presence of mullite phase in the synthesized cobalt mullite indicated that the chain edges are connected to AlO 6 octahedra running parallel to the crystallographic c-axis (Fig. 3). These rigid chains are joined together by CoO 6 octahedra and (Al,Si)O 4 tetrahedra (Schneider et al. 2008). Robust adsorption of VB and MY is mainly due to the mullite phase present in the prepared cobalt mullite. Existence of mullite phase in the synthesized cobalt mullite is due to the interaction of dipropylamine with the material. This causes the conversion of tetrahedral coordination of cobalt and aluminum to octahedral coordination. These tetrahedral and octahedral aluminum and cobalt are responsible for the faster adsorption of VB and MY with higher adsorption capacity. The plausible mechanism of interaction between cobalt mullite and both dye molecules is demonstrated in Scheme 1 and Scheme 2. The results indicated that the adsorption capacity of VB is greater than MY. This variation in adsorption capacity is practically acceptable since the extent of adsorption also depends on the number of active sites in the adsorbent and the number of electron-donating sites in the adsorbate molecule without steric hindrance since steric hindrance inhibiting the possible interactions with the adsorbent. Furthermore, compared to MY, VB has more interaction sites (3 amine positions with lone pairs of electrons) for adsorption to occur. But Metanil Yellow has only one effective interaction site (SO 3 − ) and other amine sites are hindered by benzene ring. Hence, adsorption capacity for VB is higher than MY with a faster adsorption rate (0.00402 g/mg.min for VB and 0.004 g/mg.min for MY).
Enthalpy value (65.43 kJ/mol for VB and 44.73 kJ/mol for MY) determined from a thermodynamic study revealed that chemisorption is involved in the adsorption for both dyes. Lewis acid site interaction, Brønsted acid site interaction, and ionic bonding are taken part in the sequestration of VB and MY. These interactions take place by the abstraction of the electron from the nitrogen and oxygen present in the VB and MY, respectively. Tetrahedral aluminum and octahedral aluminum and cobalt cause charge imbalance which is neutralized by protons. These protons act as the Brønsted acid site which can able to abstract the electrons in the respective dye molecules. Lewis acid site interactions and ionic bonding also take part in adsorption. Moreover, octahedral sites have more adsorption sites to interact than tetrahedral sites.

Conclusion
The present work demonstrated the rapid adsorption of VB and MY over cobalt mullite. This material is synthesized by the facile sol-gel method at room temperature. Adsorbent is characterized by XRD, FT-IR, and HRTEM, and it is perceived that dipropylamine coordinated with Al 3+ and Co 3+ to form an octahedral cobalt mullite. It is found that orthorhombic (cobalt mullite) and trigonal (alumina) crystal systems are cross-linked to form hybrid network. It may enhance the adsorbent activity. The adsorption of VB and MY by cobalt mullite followed pseudo-second order kinetics with high rate constant (K 2 = 0.00402 g/ mg.min for VB and K 2 = 0.004 g/mg.min for MY) and Langmuir isotherm. The monolayer adsorption capacity obtained from isotherm is higher for VB (101.01 mg/g) than MY (11.92 mg/g). This is owing to the steric hindrance around the amine group of MY by benzene rings. But VB has less steric hindrance around the amine group. So VB interaction with the adsorbent is more compared to MY. Moreover, the positive enthalpy values indicating the process are endothermic and also the values are greater than 40 kJ/mol. These findings evidenced that the chemisorption is involved in the removal of VB and MY by cobalt mullite. The chemical interactions are occurred via the Brønsted and Lewis acid sites of the material to the lone pair electrons present in the dye molecules. In addition to that, ionic bonding also assists the adsorption. The main advantages of this work are room temperature synthesis of cobalt mullite, eco-friendly, low-cost, and highly efficient for adsorption of dye molecules within short period of time.