Introduction

Nowadays, many kinds of dyes are being used in many fields including textile, tannery, electroplating, food processing, and papermaking. At the same time, a large amount of highly colored effluents are released into the environment, probably containing more than 100,000 kinds of commercial dyes and over 7 million tons annually [1–5], which will cause serious environmental problems. Among dyes, azo dyes are mostly used, which contain an azo group (–N=N–) as part of the structure. Although azo dyes are not strongly toxic, they can cause serious damage because these dyes prevent sunlight and oxygen penetration and then have a derogatory effect on photosynthetic activity in aquatic systems [6]. The azo dyes are very stable against heat, light, and many chemicals. Therefore, it is very difficult to remove these dyes from wastewater by sunniness, chemical, and biological degradation methods. Methyl orange (MO) is a kind of azo dye which has been widely used in many fields. Removal of MO at a low cost is still a significant challenge.

Various conventional methods have been developed to treat wastewater including coagulation–flocculation [7], membrane separation [8], photocatalytic degradation [9], and adsorption [10–17]. Among the traditional wastewater treatment process techniques, adsorption technique has been considered to be a convenient and effective wastewater treatment technique due to simplicity, low cost, and insensitivity to toxic substances [18]. To date, various adsorbents, such as nanoporous silica, hematite, MnO–Fe2O3, CuFe2O4, Fe2O3–SiO2, mesoporous TiO2, and activated carbon and/or mesoporous carbon, have been investigated to remove azo dyes from wastewater. Activated carbon and/or mesoporous carbon have large specific surface area, so they are efficient adsorbents for organic compounds. However, activated carbons and mesoporous carbon cannot be reused, resulting in high cost, and limited its large-scale application.

A practical adsorbent must meet the following conditions: (1) high adsorption capacity; (2) adsorbent is easily separated from the solution after adsorption; (3) adsorbent can be repeatedly used after simple processing. Obviously, magnetic materials could satisfy the requirements above because they have high adsorption capacity, and can be easily separated from treated water under an external magnetic field and repeatedly used after simple processing. Spinel ferrites (MFe2O4, M = transition metal and/or alkaline earth metal) behave with ferromagnetic property at room temperature. So, they have been widely used as the magnetic carrier in adsorbent to realize magnetic separation [19, 20]. To date, various spinel ferrites have used as adsorbents to treat wastewater containing dyes, including ZnFe2O4 [21], CuFe2O4 [22], NiFe2O4 [23], MnFe2O4 [24], CoFe2O4 [25], ZnCr2O4 [26], and Fe3O4 [27]. However, the adsorption capacity of ferrites is far lower than that of the activated carbon in recent reports due to small specific surface area of ferrites. Therefore, synthesis of ferrites with high surface area is the key to improve the adsorption capacity of ferrites. It was reported that ferrite with 3D porous structure could have high efficient adsorption property due to its large specific surface area [23].

MnCo2O4 is a very important mixed metal oxide with an inverse spinel structure and magnetic property. The quality of MnCo2O4 powders strongly depends on the synthesis method and conditions, which determine particle size, morphology, specific surface area, lattice parameters, stoichiometry, and average Mn valence of MnCo2O4 associated with adsorption, catalytic, and electrochemical performances [28–30]. Preparations of high-quality samples with unique three-dimensional structure and/or large surface area have generally been considered to be very helpful to improve adsorption, catalytic, and electrochemical performances of MnCo2O4. To the best of our knowledge, adsorption performances of hierarchical rambutan-like MnCo2O4 have rarely been reported in previous studies.

In this paper, we report a facile and scalable approach to fabricate hierarchical rambutan-like MnCo2O4 with large specific surface area. It is worth mentioning that the unique architecture is obtained via a self-assembly process without any additional templates. Investigations on adsorption properties of rambutan-like MnCo2O4 as adsorbent for methyl orange were performed. The results show that the rambutan-like MnCo2O4 is easily separated from the solution after adsorption under an external magnetic field. More important, the adsorption capacity of methyl orange (185.14 mg g−1) on this rambutan-like MnCo2O4 is very high. It is expected to be a promising reusable adsorbent. The adsorption kinetics of methyl orange on rambutan-like MnCo2O4 was interpreted by the pseudo-first- and second-order rate equations [23, 31, 32]. The adsorption activation energy E a and mechanisms of methyl orange on the rambutan-like MnCo2O4 are discussed in this paper for the first time.

Experimental

Reagent and apparatus

All chemicals used are of reagent-grade purity (purity > 99.9 %). The thermogravimetry (TG) measurement was conducted using a Netzsch STA 409 PC/PG thermogravimetric analyzer under continuous flow of O2 (30 mL min−1). The sample mass was approximately 13 mg. X-ray powder diffraction (XRD) was performed using a Rigaku D/Max 2500 V diffractometer equipped with a graphite monochromator and a Cu target. The radiation applied was Cu Kα (λ = 0.15406 nm), operated at 40 kV and 50 mA. The XRD scans were conducted from 5° to 70° in 2θ, with a step size of 0.02°. The morphologies of the synthesis products were observed using a S-3400 scanning electron microscope (SEM). The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics Tristar 3020 analyzer. The IR spectra of the rambutan-like MnCo2O4 before and after adsorption of MO were recorded on a Nexus 470 Fourier transform IR (FT-IR) instrument. Zeta potential was analyzed by DLS (ELS-Z, Photal).

Preparation of MnCo2O4

The rambutan-like MnCo2O4 precursor was prepared by a co-precipitation method in water–ethanol solution at room temperature [33]. In a typical synthesis, 60 mL of 0.1 M MnSO4 aqueous solution, 120 mL of 0.1 M CoSO4 aqueous solution, and 180 mL of ethanol (C2H5OH) were mixed for 5 min at room temperature at first. After that, 185 mL of 0.1 M Na2C2O4 solution was quickly poured into the above solution under magnetic stirring. The reaction mixture was kept at room temperature for 2.5 h. The pink precipitate was collected by filtration, washed with deionized water, and dried at 60 °C for 6 h. The resulting precursor was determined to be MnCo2(C2O4)3·5.54H2O. Finally, with a slow heating rate of 1 °C min−1, the precursor was further treated at 400, 450, and 550 °C for 10 min in air to produce the rambutan-like MnCo2O4 microspheres, respectively.

Adsorption kinetic experiments

Adsorption experiments were carried out in a 200-mL beaker containing 6 mg MnCo2O4 powders and 100 mL methyl orange solutions. The beaker was placed in a thermostat oil bath at a stirring speed of 300 rpm and desired temperature (±1 °C). The pH value of methyl orange solution was adjusted with sulfuric acid solution. The effect of initial concentration on methyl orange adsorption kinetics was studied by varying initial methyl orange concentration from 4.5 to 12 mg L−1  (4.5, 6, and 12 mg L−1) at temperature of 284 K. While to study the effect of temperature on methyl orange adsorption kinetics, experiments were performed at temperature of 284, 314, and 324 K with 6 mg L−1 methyl orange initial concentration. The contact time intervals (5, 10, 15, 20, and 30 min) were adopted to obtain adsorption kinetic data. The supernatant liquid in the beaker was separated from adsorbent by filtration, and the filtrate was collected for analysis. Methyl orange concentration in this study was measured based on spectrophotometry. The adsorption amount of methyl orange on rambutan-like MnCo2O4 at time t, q t (mg g−1), was calculated using the following formula (Eq. 1):

$$ q_{\text t} = \frac{{(C_{0} - C_{\text t} ) \times V}}{m} $$
(1)

where C o (mg L−1) and C t (mg L−1) are the initial and the current methyl orange concentration at time t, respectively, V (mL) is the volume of the adsorption solution, and m (mg) is the mass of rambutan-like MnCo2O4.

Method of determining adsorption activation energy and adsorption kinetic models

Kinetics of adsorption process was interpreted by the pseudo-first- and second-order rate equations, which have been widely used for adsorption study of pollutants from wastewater in recent years [23, 34, 35].

The pseudo-first-order equation can be expressed as Eq. (2):

$$ \frac{{{\text{d}}q_{\text t} }}{{{\text{d}}t}} = k_{\text p1} (q_{\text e} - q_{\text t} ) $$
(2)

Equation (2) can be rewritten into Eq. (3):

$$ \log (q_{\text e} - q_{\text t} ) = \log q_{\text e} - \frac{{k_{\text p1} }}{2.303} \times t $$
(3)

The dependence of log(q e − q t) on t must lead to a straight line. Thus, rate constant of the adsorption process (k p1, 1/min) can be obtained from linear slope [−k p1/2.303, Eq. (3)].

The pseudo-second-order equation can be expressed as follows:

$$ \frac{{{\text{d}}q_{\text t} }}{{{\text{d}}t}} = k_{\text p2} (q_{\text e} - q_{\text t} )^{2} $$
(4)

where k p2 (g mg−1 min−1) is the pseudo-second-order rate constant of adsorption process [36]. The integral form of Eq. (4) with boundary conditions t = 0 to t = t, and q t = 0 to q t = q can be expressed as Eq. (5):

$$ \frac{1}{{(q_{\text e} - q_{\text t} )}} = \frac{1}{{q_{\text e} }} + k_{\text p2} \times t $$
(5)

By a series of transforms, thus Eq. (5) can be rewritten as Eq. (6):

$$ \frac{1}{{q_{\text t} }} = \frac{1}{{q_{\text e} }} + \frac{1}{{k_{\text p2} q_{\text e}^{2} }} \times \frac{1}{t} $$
(6)

Dependence of 1/q t on 1/t should be a straight line if the adsorption kinetics follows the pseudo-second-order rate equation. Thus, the rate constant of the adsorption process (k p2) can be obtained from linear slope [\( 1/k_{\text{p2}} q_{\text{e}}^{ 2} \), Eq. (6)]. Equations (3) and (6) will be used to fit the adsorption kinetic data of methyl orange on rambutan-like MnCo2O4.

The adsorption activation energy (E a) can be calculated using Arrhenius equation [Eq. (7)]:

$$ \ln k = - \frac{{E_{\text{a}} }}{RT} + \ln A $$
(7)

where k is the rate constant of the adsorption process, E a is the activation energy (kJ mol−1), A is the pre-exponential factor, R is the gas constant (8.314 × 10−3 kJ mol−1 K−1), and T is adsorption temperature (K). The dependence of ln k on 1/T must lead to a straight line. Thus, the adsorption activation energy E a can be obtained from linear slope [–E a/R, Eq. (7)].

Entropy (ΔS #), enthalpy (ΔH #), and Gibbs energy (free enthalpy) (ΔG #) of the adsorption process were calculated using Eyring equations [37, 38]:

$$ \Delta S^{\# } = R\left[ {\ln \left( {\frac{{hA_{\upalpha } }}{{k_{\text B} T_{\upalpha } }} - 1} \right)} \right] $$
(8)
$$ \Delta H^{\# } = E_{{{\text{a}},\alpha }} - RT_{\alpha } $$
(9)
$$ \Delta G^{\# } = \Delta H^{\# } - T_{\alpha } \Delta S^{\# } $$
(10)

Results and discussion

Composition analysis of the precursor

0.0295 g precursor sample was dissolved in 10 mL 50 vol% HCl solution and then diluted to 100.00 mL with deionized water. Manganese (Mn) and cobalt (Co) in the solution were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin Elmer Optima 5300 DV). The results showed that the Mn and Co mass percentages were 10.20 and 22.00 %, respectively. In other words, molar ratio of Mn:Co in the precursor was 1.000:2.01, which was close to the value of the pre-design and synthesis.

TG/DTG analysis of the precursor

Figure 1 shows the TG/DTG curves of the precursor at a heating rate of 10 °C min−1. The TG/DTG curves show that the thermal process of MnCo2(C2O4)3·5.54H2O below 600 °C occurred in two well-defined steps. The first step started at about 50 °C and ended at 175 °C, which can be attributed to the dehydration of the 5.54 waters from MnCo2(C2O4)3·5.54H2O (mass loss: observed, 18.32 %; theoretical, 18.60 %). The second thermal process step started at 175 °C and ended at 307 °C, attributed to the reaction of MnCo2(C2O4)3 with 2O2 into MnCo2O4 and the six CO2 molecules (mass loss: observed, 37.56 %; theoretical, 37.28 %).

Fig. 1
figure 1

TG/DTG curves of MnCo2(C2O4)3·5.54H2O at a heating rate of 10 °C min−1 in O2

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X-ray powder diffraction and particle distribution characterization

Figure 2 shows the XRD patterns of the samples calcined at different temperatures. All the diffraction peaks in the pattern can be indexed to cubic MnCo2O4 with space group Fd-3 m (227) (PDF card No. 23-1237). No impurity peaks are detected, revealing the high purity of the as-synthesized samples.

Fig. 2
figure 2

XRD patterns of MnCo2O4 at different temperatures for 10 min

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The crystallite diameter of MnCo2O4 was estimated using the following Scherrer formula [39, 40]:

$$ D = K\lambda /(\beta \cos \theta ) $$
(11)

where D is the crystallite diameter, K = 0.89 (the Scherrer constant), λ = 0.15406 nm (wavelength of the X-ray used), β is the width of line at the half-maximum intensity, and θ is the corresponding angle. The crystallite sizes of MnCo2O4 from calcining the precursor at 400, 450, and 550 °C are 9.5, 9.6, and 12.6 nm, respectively. The crystallinity of MnCo2O4 can be calculated by MDI Jade 5.0 software. The crystallinity of MnCo2O4 obtained at 400, 450, and 550 °C was approximately 100 %.

Figure 3 shows SEM images of the MnCo2O4 synthesized at 400, 450, and 550 °C, respectively. It is found that all the as-synthesized MnCo2O4 samples exhibit uniform rambutan-like microspheres with a size of about 4.3 µm, which are composed of many end-connected nanorods. In Fig. 3a, b, the diameter of nanorods in the sample obtained at 400 and 450 °C is about 80 nm. By contrast, after being calcined at 550 °C, the nanorods become thicker and shorter but the hierarchical structure is able to be well preserved without obvious deformation (Fig. 3c).

Fig. 3
figure 3

SEM images of the calcined samples at different temperatures for 10 min: a 400 °C, b 450 °C, and c 550 °C

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The nitrogen adsorption–desorption isotherms of the samples calcined at different temperatures (400, 450, and 550 °C) are shown in Fig. 4. All these isotherms exhibit a typical type IV characteristic with an obvious hysteresis loop, indicating the mesoporous structures in the samples [41]. Owing to the nanorod subunits and the presence of the void spaces between them, the measured values of the BET surface areas reach 155, 126, and 43 m2 g−1 for 400, 450, and 550 °C, respectively, which are much higher than those of MnCo2O4 microspheres and MnCo2O4 nanowires [42, 43]. The specific surface area is highly dependent on the calcination temperature. With the increase in the calcination temperature, the aggregation of primary nanoparticles also increases, resulting in a significant decrease in specific surface area. The facile fabrication process and a unique three-dimensional structure will make the as-synthesized samples possible to be applied in adsorption and other relevant fields.

Fig. 4
figure 4

Nitrogen adsorption and desorption isotherms of rambutan-like MnCo2O4

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For sample synthesized by co-precipitation method in this study, rambutan-like MnCo2O4 nanocrystals showed superparamagnetic behavior. Figure 5 shows that the rambutan-like MnCo2O4 powder can be easily separated from the aqueous solution after adsorption with a permanent magnet.

Fig. 5
figure 5

Photograph of magnetic separation of rambutan-like MnCo2O4 obtained at 400 °C from the aqueous solution with a permanent magnet

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IR spectroscopic analyses of the rambutan-like MnCo2O4 microspheres before and after adsorption of MO

The FT-IR spectra of MO and the rambutan-like MnCo2O4 microspheres before and after adsorption are shown in Fig. 6. Bands at 3354 and 2930–2900 cm−1 were attributed to the stretching vibration of –OH and the stretching vibration or the asymmetric stretching vibration of –CH3, respectively. The bands located at the wavenumber ranges of 1600–1400 and 910–720 cm−1 were assigned to the C=C stretching vibration and the C–H out-of-plane bending vibration of the phenyl. The characteristic peak appeared at 1450 and 1418 cm−1 assigned to the azo group. The bands at 1369, 1315, and 1114 cm−1 were due to the C–SO2–O stretching vibration, the S=O stretching vibration, and the S–O– stretching vibration of the –SO3Na group, respectively [44]. After the rambutan-like MnCo2O4 microspheres adsorbed MO, the band at about 648 cm−1 (Co–O, typical of spinel oxide [45, 46]) shifted toward high wavenumber, and the bands at 1642 and 1074 cm−1 became strong, implying that an interaction exists between rambutan-like MnCo2O4 and MO. All changes in the bands before and after adsorption are consisted with the main bonds of MO. Similar phenomenon was also observed for the adsorption of methyl blue (MB) on the porous NiFe2O4 [23] and porous MnFe2O4 [24].

Fig. 6
figure 6

FT-IR spectra of MO and the rambutan-like MnCo2O4 microspheres before and after adsorption of MO

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Adsorption kinetics of the calcined samples for methyl orange

Effect of pH values on the MnCo2O4 sorbing methyl orange

The effect of pH values on the rambutan-like MnCo2O4 sorbing methyl orange is shown in Fig. 7. The pH values have a marked effect on the adsorption degree of methyl orange in the pH range of 2–6 (Fig. 7a); the adsorption capacities of methyl orange increase with the decrease in pH values and the increase in sample calcination temperature (Fig. 7b). When solution pH value is 2.09, methyl orange concentration is 6 mg L−1, and the adsorption percentage of methyl orange on the rambutan-like MnCo2O4 powder obtained at 400 °C is 93.2 % within 15 min, which shows that rambutan-like MnCo2O4 powder has a quick adsorption process for methyl orange.

Fig. 7
figure 7

Effect of pH values on the rambutan-like MnCo2O4 sorbing methyl orange: methyl orange concentration, 6 mg L−1; contact time, 15 min

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Figure 8 shows the zeta potentials of the dispersed MnCo2O4 particles obtained at 400 °C under different aqueous pH values. With increasing aqueous pH, the zeta potential of the MnCo2O4 decreases from positive to negative values. The pHPZC value of MnCo2O4 particles is approximately 5.3. MnCo2O4 particles showed a negative charge surface (–11.75 mV) at the original pH (~7).

Fig. 8
figure 8

Change in zeta potential of MnCo2O4 particles with change in solution pH

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Adsorption kinetics with different initial methyl orange concentration

Dependence of methyl orange adsorption amount on contact time is shown in Fig. 9a. The adsorption amount q t increases fast within initial 7 min, and then, its growth slows down. That is, the adsorption process of methyl orange on rambutan-like MnCo2O4 can be divided into rapid increase stage and the slow increase stage. The latter is also called near-equilibrium stage [31]. The rapid increase stage possibly derived from the electrostatic force between methyl orange and active adsorption sites on the surface of rambutan-like MnCo2O4. The near-equilibrium stage is attributed to the decrease in electrostatic attraction after the rambutan-like MnCo2O4 being saturated by methyl orange. It can also be seen that the equilibrium adsorption capacity (q e) of methyl orange on rambutan-like MnCo2O4 increases with increasing initial methyl orange concentration. Similar phenomenon was also observed by Vadivelan [47] for methylene blue onto rice husk.

Fig. 9
figure 9

Adsorption kinetics of methyl orange on rambutan-like MnCo2O4 under different initial methyl orange concentration: a experimental plots, b pseudo-first-order kinetic plots, and c pseudo-second-order kinetic plots (conditions: pH = 2.09; temperature = 284 K; sorbent dose = 60 mg L−1)

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Compared to other spinel oxides (Table 1), maximum adsorption capacity q t of MO on the rambutan-like MnCo2O4 microspheres in this work is very high. Therefore, it can be concluded that the rambutan-like MnCo2O4 microspheres is a fine adsorbent with both good adsorption capacities and good magnetic separations.

Table 1 Maximum dyes adsorption capacities of some spinel oxide adsorbents
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The experimental data are analyzed using the pseudo-first- and second-order adsorption kinetic models. The results are shown in Fig. 9b, c. The calculated values of the two kinetic models are shown in Table 2. The correlation coefficient of the pseudo-second-order equation is greater than that of the pseudo-first-order. Besides, experimental equilibrium adsorption amounts (q e, Exp) are close to the theoretical values (q e, Cal) calculated from the pseudo-second-order equation (Table 2). However, q e, Exp has large deviation from that of the pseudo-first-order rate equation. These results show that the adsorption mechanism of methyl orange on rambutan-like MnCo2O4 most matches the pseudo-second-order kinetic models.

Table 2 Parameters of the pseudo-first- and second-order kinetic models for methyl orange adsorption on MnCo2O4 under different initial methyl orange concentration
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Adsorption kinetics at different temperature

The adsorption kinetics of methyl orange on rambutan-like MnCo2O4 at 284, 314, and 324 K are plotted in Fig. 10. From Fig. 10a, the adsorption amount of methyl orange increases with the increase in contact time, and a large fraction of methyl orange is removed within the first 5 min; the equilibrium time is about 20 min; the equilibrium adsorption amount of methyl orange is slightly improved with the increase in adsorption temperature, implying that adsorption of methyl orange on rambutan-like MnCo2O4 is an endothermic and rapid process. The adsorption kinetic curves at different temperature are analyzed with the pseudo-first- and second-order adsorption rate equations. The results are presented in Table 3. Figure 10b shows the dependence of log(q e–q t) on contact time; it can be seen that the pseudo-first-order equation is not applicable within the entire contact time of methyl orange adsorption on rambutan-like MnCo2O4. Figure 10c shows that the experimental plots agree well with the pseudo-second-order kinetic equation, of which the correlation coefficient is much greater than that of the pseudo-first-order kinetic equation. From Table 3, it can also be seen that the pseudo-second-order rate constant k p2 increases with the increase in adsorption temperature from 284 to 324 K, indicating that the increase in temperature can accelerate the adsorption of methyl orange on rambutan-like MnCo2O4.

Fig. 10
figure 10

Adsorption kinetics of methyl orange on rambutan-like MnCo2O4 at different temperature: a experimental plots, b pseudo-first-order kinetic plots, and c pseudo-second-order plots (conditions: pH = 2.09; initial methyl orange concentration = 6 mgL−1; sorbent dose = 60 mg L−1)

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Table 3 Parameters of the pseudo-first- and second-order kinetic models for methyl orange adsorption on MnCo2O4 at different temperature
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Adsorption activation energy E a

The pseudo-second-order rate constant k p2, R, and T are placed into Eq. (7) at first, and then, the dependence of ln k p2 on 1/T is plotted. Thus, adsorption activation energy E a can be obtained from linear slope (k = –E a/R). Figure 11 shows the dependence of ln k p2 on 1/T. From linear slope, the adsorption activation energy E a is 5.71 ± 0.056 kJ mol−1.

Fig. 11
figure 11

Arrhenius plots for the adsorption of methyl orange on rambutan-like MnCo2O4

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Entropy (ΔS #), enthalpy (ΔH #), and Gibbs energy (free enthalpy) (ΔG #) of the adsorption process were calculated using Eyring equations. The results show that ΔS # and ΔH # of adsorption process are 77.60 J mol−1 K−1 and 9.30 kJ mol−1, respectively. ΔG # values of the adsorption process at 284, 314, and 324 K are −12.74, −15.07, and −15.84 kJ mol−1, respectively. The positive value of ΔH # indicates that the adsorption of methyl orange on rambutan-like MnCo2O4 is an endothermic process. So, the increase in temperature can increase the adsorption amount of methyl orange on rambutan-like MnCo2O4 (Fig. 10a). Usually, adsorption enthalpies ranging from 2.1 to 20.9 kJ mol−1 correspond to physical adsorption [48, 49]. So, the adsorption of methyl orange on rambutan-like MnCo2O4 is mechanism of physical adsorption. Based on the zeta potential analysis of the dispersed MnCo2O4 particles, MnCo2O4 particles with positive surface charge adsorb methyl orange with negative charge via electrostatic attraction. A positive ΔS value (77.60 J mol−1 K−1) suggests an increase in the randomness at the adsorbent–solution interface during the adsorption process.

Conclusions

In summary, we have developed a facile and scalable co-precipitation method to fabricate the rambutan-like MnCo2O4 for the first time. The ethanol plays a key role in the formation of 3D self-assembly structure. After calcined at 400 °C in air for 10 min at a heating rate of 1 °C min−1, the precursor MnCo2(C2O4)3·5.54H2O was transformed into the rambutan-like MnCo2O4, which has the large specific surface area (155 m2 g−1) and excellent adsorption ability for methyl orange. The adsorption capacity of methyl orange on rambutan-like MnCo2O4 is 185.14 mg g−1 under the conditions of initial MO concentration 12 mg L−1 and pH 2.09, which is much higher than that of other spinel oxides for other dyes adsorption. Adsorption kinetics of methyl orange on rambutan-like MnCo2O4 follows the pseudo-second-order kinetic model. The calculated values of adsorption activation energy E a and enthalpy ΔH # are 5.71 ± 0.056 and 9.30 kJ mol−1, respectively. Methyl orange adsorption on rambutan-like MnCo2O4 is a physical adsorption process. The rambutan-like MnCo2O4 with large specific surface area also will be an excellent adsorbent for other azo dyes.