Azo dyes are commonly used in textile industries; however, when these dyes cross the permissible limit set by the World Health Organization, they produce many health issues related to the brain, liver, kidneys, respiration, and sexual system. Herein, polyvinyl pyrrolidone (PVP)-supported manganese oxide (MnO2) was studied for azo dye removal from an aqueous medium. The adsorption mechanism study demonstrated that the dye adsorption by MnO2–PVP composite was not only due to the electrostatic force of attraction but also involved the ion exchange amid the hydroxide group and dye molecules. The surface area of the composite (120 m2 g−1) was larger than that of metal oxide (102 m2 g−1). The point of zero charge and surface area were improved from 5.2 and 102 to 5.6 and 120 m2 g−1, respectively. The dye removal capacity of MnO2–PVP composite was significantly higher than that of plain MnO2. The film diffusion control adsorption kinetic mechanism and the kinetic data were well fitted to the pseudo-second-order equation. Experiments were conducted as a function of initial dye concentration (5–200 mg L−1), pH (3–10), temperature (298–328 K), and adsorbent dosage (0.05–0.4 g) in batch adsorption systems. The thermodynamic investigations confirm that the dye adsorption process was endothermic in nature.

  • The polyvinyl pyrrolidone (PVP)/MnO2 composite was synthesized by the solvothermal method.

  • The adsorption capacity of PVP/MnO2 was higher than plain metal oxide.

  • Detail adsorption mechanism was studied by kinetic models.

  • PVP/MnO2 could be used as a candidate in the water filtration assembly of dyes.

  • Detail characterization of the plain and composite metal oxides was performed.

Drinking water contamination is one of the environmental problems all over the world and particularly in big cities of our country (Peshawar, Karachi, Lahore, and Faisalabad), which made the general public at risk due to intolerable levels of dyes in the water system and food webs (Parveen & Rafique 2018; Mon et al. 2019; Kabuba & Banza 2021; Khan et al. 2022; Abugu et al. 2023). Azo dyes, containing one or more azo group (N = N), are the most widely used dyes such as methyl orange and Congo red. However, when these dyes cross the permissible limit set by World Health Organization, it produces many health issues related to the brain, liver, kidneys, respiration, and sexual system (Nuengmatcha et al. 2023; Oladoye et al. 2023). Even trace amount of these dyes also shows high visibility, reducing the penetration of sunlight in water and inhibiting the oxygen content and the photosynthesis process. Furthermore, aromatic amines with azo group have been shown to be carcinogenic to animals and human beings (Aliabadi & Mahmoodi 2018; Naseem et al. 2018; Zhang et al. 2019). Therefore, these dyes must be treated before and after entering the general public drinking water.

Activated carbon is a familiar adsorbent for decontamination of drinking water from toxic organic pollutants; however, activated carbon has various limitations such as slow kinetics and lower capacity for micro-pollutants, and regeneration requires energy (Alsbaiee et al. 2016). Various traditional adsorbents can solve drawbacks up to some extent. However, controlling the adsorbent surface (point of zero charge (PZC)) and its surface area toward the high adsorption is fairly difficult due to the complex structure of dyes. For this approach, finding a highly selective sustainable adsorbent for researchers is desired.

Polyvinyl pyrrolidone (PVP) is frequently exercised as a surfactant due to nontoxicity and biocompatibility and changed the exposed crystal facets. It is frequently used as a stabilizer and controls the surface configuration. PVP efficiently enhances the percentage of exposed surface area. Our current research findings are concerned with the adsorption of carcinogenic and mutagenic azo dyes from the aqueous medium by newly prepared PVP-modified MnO2 with high efficiency. A detailed investigation of the process was carried out to find the adsorption mechanism by composite materials. Kinetic and thermodynamic investigation helps us to probe into the adsorption mechanism during the adsorption of methyl orange and Congo red.

Chemicals and reagents

All the chemicals (methyl orange, Congo red, NaOH, HCl, and NaNO3) and solvents (distilled water, ethanol, and n-hexane) were used as supplied with no additional purification. A total of 1,000 mg L−1 stock solution of dyes was prepared. Buffers, NaOH (0.1–0.5 M), and HCl (0.1–0.5 M) were employed for pH adjustment. The dye solutions ranged from 5 to 200 mgL−1 for all experiments. All chemicals utilized were research grade in purity, and distilled water was used in solution preparation in the course of the investigation.

Synthesis of composite material

The composite material was synthesized by the previously applied hydrothermal method (Li et al. 2015; Khan et al. 2022) with changes. A 0.5 M MnO2 and 0.1 M PVP were mixed in 100 ml beaker and stirred for 2 h at room temperature (298 K) until the solution became clear. The white precipitate was centrifuged, washed with DI water and ethanol, and dried at 80 °C in the oven for 6 h before further processing.

Characterization of MnO2–PVP composite

The morphologies and elemental analysis of the as-formed materials were determined by scanning electron microscope (model: JSM 5910, JEOL, Japan) and Energy Dispersive X-ray Spectrometer (EDX) (model: INCA 200, Oxford Instruments, England). The surface area was calculated by Quantachrome Nova 2200e (Quantachrome Instruments, USA). The geometry was characterized by the X-ray diffraction (XRD) model JDX-3532 (JEOL). The thermo-gravimetric analysis (TGA) was performed by a TGA analyzer (Perkin Elmer, USA; model: Pyris Diamond Series TG/DTA). Fourier-transform infrared spectroscopy (FTIR) was performed by IR (model: Shimadzu 8201 PC; Shimadzu, Japan). The PZC of materials was determined experimentally by the salt addition method.

Adsorption experimentations

The mixture of adsorbent (0.05–0.4 g) and 40 ml of dye of various concentrations (5–200 mg L−1) was placed in a shaker for various intervals of time (1–2,880 min). The pH (3–10) was adjusted, and the temperature was in the range of 298–328 K. After flirtation, the dye concentration was analyzed by Vernier Spectrovis plus fluorescence. The equilibrium concentration (Ce) of dye was calculated with the help of a calibration curve.

Characterization of MnO2 and MnO2–PVP composite

Surface area

The surface areas of MnO2 and MnO2–PVP composite were calculated by the adsorption–desorption method, and the surface area of PVP-supported MnO2 was found to be 120 m2 g−1, while for MnO2, it was 102 m2 g−1. The pore size and the surface area of plain oxide (3.162 nm and 102 m2 g−1) were lower than those of supported oxide (3.30 nm and 120 m2/g). The difference in surface areas and porosity of the two materials is due to the attachment of the organic component (PVP) to the inorganic component (MnO2) where the porosity of carbon plays a role in higher surface area and particle porosity after composite formation, and hence, the material becomes carbonaceous (Bhatnagar & Jain 2005).

Point of zero charge

The PZC is the pH at which the electrical charge on the surface is zero. Above that PZC point, the charge on the surface is negative and, below that, the surface is positively charged that influences the adsorption of anionic and cationic species. Figure 1(a) shows the PZC of MnO2 and its composite at pH 5.2 and 5.4, respectively, which is close to the value (i.e., 4.9) reported for adsorbents in soil by Miyittah et al. (2016). The small change in PZC of composite compared to MnO2 confirms the physical interaction between MnO2 and PVP polymer. Higher PZC value means the surface zero charge is shifted to a higher value and the availability of positive charged surface increased, which is more favorable for adsorption of anionic dyes like methyl orange and Congo red.
Figure 1

PZC (a), FTIR (b), TGA (c), and XRD (d) of MnO2 and MnO2–PVP composite.

Figure 1

PZC (a), FTIR (b), TGA (c), and XRD (d) of MnO2 and MnO2–PVP composite.

Close modal

FTIR analysis

The FTIR study provides information about functional groups available at the solid surface. A strong peak at 525 cm−1 in MnO2 shows the stretching vibration of the MnO bond (Figure 1(b). The absorption bands at 1,068, 1,157, and 1,351 cm−1 confirm the O-H bending vibrations connected with manganese atoms. Furthermore, the band from 3,700 to 3,443 cm−1 for metal oxide and polymer-loaded metal oxide is allocated to O-H stretching, whereas the band at 1,814–1,531 in the composite corresponds to the C-O stretching and the peak at 1,945 cm−1 is due to hydrogen bonding in composite, which proves the attachment of organic groups on the surface of composite (Saeed et al. 2021; Song et al. 2021). The contact is due to electron pair donation to cation from carbonyl oxygen, and a complex formation amid cation and nitrogen (Rose et al. 2013).

TGA analysis

The TGA is a thermal technique in which the weight loss of material is monitored at different intervals of time with a temperature from 30 to 800 °C. It tells about the thermal stability of an adsorbent. The TGA was performed at 30–800 °C. The weight loss seems to be similar in both cases (Figure 1(c)). The first stage of weight loss was the same as pure PVP and appeared below 180 °C, which was because of evaporation of physically adsorbed moisture. A second weight loss attributed to chemisorbed water and appears at 200–400 °C. The third stage weight loss at higher temperature (450–550 °C) shows the oxygen emission by the reactions (Kim & Popov 2003):
formula

XRD analysis

XRD is frequently used to define the crystalline, amorphous, or semi-crystalline nature of the materials and also provides an idea about the unit cell dimensions. The XRD spectra of metal oxide and PVP-loaded metal oxide (Figure 1(d)) confirm the poor crystallinity of both materials, and similar results were presented by Yuan et al. (2015) for the MnO2/PPy composite.

Batch adsorption study

Kinetic study of adsorption

Kinetic parameters of azo dye adsorption were investigated at pH 5 at various temperatures (298–328 K). No more adsorption was observed once the process reached adsorption equilibrium. The quantity of dye adsorbed is plotted versus time, which is shown in Figure 2(a) and 2(b), respectively. The figures show that the equilibrium was reached after 150 min for methyl orange and 50 min for Congo red. The rate of adsorption was fast initially, and the Congo red adsorption was double compared to methyl orange at a given pH due to the structure of Congo red azo dye. Furthermore, no significant removal was observed behind equilibrium time. Figures show that the adsorption increases with the temperature, which confirms the endothermic nature of the process. The high adsorption is due to the movement of molecules at high temperature (328 K) (Khan et al. 2020a).
Figure 2

Kinetics of methyl orange (a), and Congo red (b), pseudo-second-order plot for methyl orange (c), and Congo red (d) on MnO2–PVP composite at pH 5.

Figure 2

Kinetics of methyl orange (a), and Congo red (b), pseudo-second-order plot for methyl orange (c), and Congo red (d) on MnO2–PVP composite at pH 5.

Close modal

Pseudo-first-order model

The pseudo-first-order expression is presented as follows:
formula
(1)
where and (mol/g) are quantity of dye adsorbed at equilibrium and time (t) respectively, and is the rate constant. The values of and can be calculated from the graph amid (not shown). In the current work, the calculated theoretical value of from the pseudo-first-order expression is inconsistent with current experimental data (Tables 1 and 2), which confirms the nonapplicability of this model.
Table 1

Kinetic parameters for methyl orange adsorption at pH 5

Pseudo-first-order parameters
Pseudo-second-order parameters
Temperature (K)Experimental
qe×105 (mol g−1)
Theoretical
qe×105 (mol g−1)
k1 min−1R2k2 (g min−1 mol−1)Theoretical
qe×105 (mol g−1)
R2
298 4.62 3.47 0.399 0.35 0.038 5.24 0.99 
308 5.62 4.83 0.561 0.44 0.063 5.98 0.98 
318 5.48 4.47 0.738 0.61 0.077 5.74 0.99 
Pseudo-first-order parameters
Pseudo-second-order parameters
Temperature (K)Experimental
qe×105 (mol g−1)
Theoretical
qe×105 (mol g−1)
k1 min−1R2k2 (g min−1 mol−1)Theoretical
qe×105 (mol g−1)
R2
298 4.62 3.47 0.399 0.35 0.038 5.24 0.99 
308 5.62 4.83 0.561 0.44 0.063 5.98 0.98 
318 5.48 4.47 0.738 0.61 0.077 5.74 0.99 
Table 2

Kinetic parameters for Congo red adsorption at pH 5

Pseudo-first-order parameters
Pseudo-second-order parmeters
Temperature (K)Experimental
qe×105 (mol g−1)
Theoretical
qe×105 (mol g−1)
k1
min−1
R2k2 (g min−1 mol−1)Theoretical
qe×105 (mol g−1)
R2
298 2.036 1.89 0.074 0.40 0.517 2.053 0.99 
308 2.162 1.99 0.121 0.40 0.490 2.177 0.98 
318 2.491 2.03 0.125 0.15 0.504 2.505 0.99 
Pseudo-first-order parameters
Pseudo-second-order parmeters
Temperature (K)Experimental
qe×105 (mol g−1)
Theoretical
qe×105 (mol g−1)
k1
min−1
R2k2 (g min−1 mol−1)Theoretical
qe×105 (mol g−1)
R2
298 2.036 1.89 0.074 0.40 0.517 2.053 0.99 
308 2.162 1.99 0.121 0.40 0.490 2.177 0.98 
318 2.491 2.03 0.125 0.15 0.504 2.505 0.99 

Pseudo-second-order model

The pseudo-second-order equation is written in the following form:
formula
(2)
where qt and qe (mole/g) are dye adsorbed at time (t) and at equilibrium, respectively, and k2 (g/min/mol) is the rate constant. The graph of t/qt versus t gives the straight line for adsorption of dyes (Figure 2(c) and (d)). The qe experimental, qe calculated, and R2 values are near to unity (Tables 1 and 2) and verify the applicability of this model. The process is physical in nature in the case of Congo red as the k2 value decreases with an increase in temperature (298–328 K), and equilibrium is reached at low temperature (Table 2), while for methyl orange, the opposite trend is observed (Table 1). The k2 values of Congo red are greater in comparison with methyl orange and show the fast adsorption of Congo red (Khan et al. 2020a).

Adsorption mechanism

Intraparticle diffusion model

The kinetic data were used to certify the adsorption mechanism by the intraparticle diffusion model. The linear form is shown as follows:
formula
(3)
where C (mg/g) is the boundary layer thickness and Kd is diffusion constant [mg/g min0.5], and the higher value of C shows a larger boundary layer effect. Özacar & Şengil (2005) also observed such types of multilinearity plots for the sorption of complex yellow onto the pin sawdust at 298 K. They assigned the first portion to the external, while the second portion was assigned to the gradual adsorption stage, where the rate-controlling step was the intraparticle diffusion.
If the graph of qt versus t1/2 passes from origin and linear, then the intraparticle diffusion is the rate-determining step. Figure 3(a) and 3(b) shows that the deviation from origin indicates the pore diffusion model is not the rate-controlling step. The transfer rates in both initial and final stages cause this deviation. R2 values (Tables 3 and 4) for both dyes confirm the applicability of the intraparticle diffusion but, because of the intercept C, the lines did not pass through the origin, which identify the existence of boundary layer effect and explained that this model is not the rate-controlling step in overall elimination process. As the Kd values increase with an increase in temperature (298–328 K) due to a change in viscosity of the mixture at high temperature (328 K), which facilitates molecular diffusion into pores of surface (Venkatesha et al. 2012).
Table 3

Intraparticle model parameters for methyl orange at pH 5

Temperature (K)Kd×107 (mol g−1 min−0.5)R2
298 19.1 0.97 
308 21.1 0.94 
318 7.1 0.91 
Temperature (K)Kd×107 (mol g−1 min−0.5)R2
298 19.1 0.97 
308 21.1 0.94 
318 7.1 0.91 
Table 4

Intraparticle model parameters for Congo red at pH 5

Temperature (K)Kd×107 (mol g−1 min−0.5)R2
298 2.43 0.91 
308 2.06 0.97 
318 5.41 0.98 
Temperature (K)Kd×107 (mol g−1 min−0.5)R2
298 2.43 0.91 
308 2.06 0.97 
318 5.41 0.98 
Figure 3

Intraparticle diffusion plots for methyl orange (a), Congo red (b), Boyd plot for methyl orange (c), and Congo red (d) on MnO2–PVP composite at pH 5.

Figure 3

Intraparticle diffusion plots for methyl orange (a), Congo red (b), Boyd plot for methyl orange (c), and Congo red (d) on MnO2–PVP composite at pH 5.

Close modal

Boyd model

The Boyd model was used for the kinetic data to determine the real rate-determining step. The equation is given as follows.
formula
(4)
where F is fractional achievement of equilibrium at time (t), which is shown by the following relation:
formula
(5)
Equation (5) can be written in the following form:
formula
(6)
and
formula
(7)
where qe is adsorbed dye by PVP-modified MnO2 at equilibrium (mole/g), qt is adsorbed dye onto the composite at time (t), F is the fraction of dye adsorbed at any time (t), and Bt is function of F. Figure 3(c) and 3(d) shows that the plots did not go through the origin, which confirms that the adsorption is mostly controlled by external diffusion (film diffusion) (Mahmood et al. 2016).

Effect of various parameters

Effect of concentration

The initial dye concentration affects dye elimination efficiency indirectly by either decreasing or increasing the availability of binding sites on the adsorbent surface. There is an immediate relationship between the percentage removal of dyes and the initial dye concentration in adsorption systems. Generally, an increase in the initial dye concentration in the solution will cause the adsorption sites on the adsorbent surface to become saturated, which eventually leads to a decrease in the removal efficiency. The effect of initial concentration on the adsorption of methyl orange and Congo red by MnO2–PVP composite was investigated with varying solution concentrations (5–200 mg L−1) using 0.1 g adsorbent dose. It could be seen from Figure 4(a) that the percent removal of dye was decreased, while the specific dye uptake was increased with the increasing concentration. The figure shows that the driving force from the concentration gradient results in the higher dye elimination at a higher concentration. The percentage of adsorption goes down from 96.5 to 45.7% for Congo red and from 70 to 30% for methyl orange, with increasing the initial concentration of dye from 5 to 200 mg L−1. This is due to a larger mass-driving force at high concentration that helps to overcome the mass transfer control. In addition, aggregation occurred on the surface at a high concentration (Mahmood et al. 2016).
Figure 4

Initial concentration effect (a), pH (b), adsorbent amount (c), and temperature (298–328 K) (d) of methyl orange and Congo red adsorption onto MnO2–PVP composite.

Figure 4

Initial concentration effect (a), pH (b), adsorbent amount (c), and temperature (298–328 K) (d) of methyl orange and Congo red adsorption onto MnO2–PVP composite.

Close modal

These occurrences could be attributed to several factors: (a) At low initial concentration, the availability of vacant pores, and binding sites on PVP-loaded MnO2 are high. However, the fractional adsorption and mass transfer of methyl orange and Congo red become low, leading to the lower percentage eliminations of azo dyes at initial dye concentrations below 200 mg L−1. (b) As the initial dye concentration increases from 50 mg L−1, the mass transfer force of dye also increases, leading to high adsorption on available binding sites of PVP-loaded MnO2. (c) As the initial dye concentration further increases above 100 mg L−1 and particularly at 200 mg L−1, the ratio of the dye molecules to the available binding sites is at levels that do not support mass transfer. Moreover, at initial dye concentration, the mass transfer of dye molecules is higher due to the increased methyl orange and Congo red to binding sites ratio; however, the number of available binding sites on the MnO2–PVP composite will decrease and disappear as the dye molecules occupy them. This results in general lower removal percentages of methyl orange and Congo red at high initial concentrations. Similar results were reported by Guo et al. while studying regenerated cellulose/polyethyleneimine composite aerogel for efficient and selective adsorption of anionic dyes (Guo et al. 2024).

Effect of pH

The adsorption of dye by PVP-supported MnO2 is studied in pH ranges from 3 to 10 at 298 K. The adsorbent quantity and initial dye concentration were set at 0.1 g, and 50 mg/L respectively. The removal was decreased with an increase in the pH of the system (Figure 4(b). This decrease at higher pH was due to OH ions on the surface at higher pH and its competition with anionic dyes toward the surface, leading to a decrease in adsorption. On the other hand, at lower pH values, the amine groups of the dye protonated to produce a positive surface, which results repulsion between the dye and surface of the adsorbent. At lower pH values, the dominant electrostatic forces create a connection amid anionic dye and positive surface. On the other hand, the electrostatic force of repulsion leading to decreased adsorption (Arrisujaya et al. 2023; Khan et al. 2020b).

Effect of adsorbent dosage

The adsorption of azo dyes from 50 mg L−1 solution was conducted by changing the adsorbent dosage from 0.05 to 0.4 g. Figure 4(c) shows that the percent removal increases when the amount increases from 0.05 to 0.1 g and remains constant from 0.1 to 0.25 g of adsorbent. The high adsorption is due to the availability of more sites when we go from 0.05 to 0.1 g, but after 0.1 g adsorbent, the agglomeration of adsorbent particles occurred and thus lowering in the adsorption is observed. The mass of 0.1 g of adsorbent was taken as the optimized dose for further experiments. Similar results were reported by Khan et al. (2020b).

Effect of temperature

The effect of temperature on the adsorption process is conducted in a range of 298 to 328 K (Figure 4(d). The dye elimination was found to increase with the increase in temperature (298–328 K) for both methyl orange and Congo red. The methyl orange adsorption increased up to 318 K but decreased at 328 K due to a change in surface configuration at a very high temperature. However, the reaction is endothermic when the temperature increased from 298 to 318 K. The high mobility of dyes at high temperature (328 K) is accountable for high adsorption (Naeem et al. 2014). A comparison of azo dye adsorption of the reported literature with our adsorbent is shown in Table 5.

Table 5

Comparison of maximum adsorption capacity of various adsorbents for Congo red removal

Adsorbentqm (mg g−1)References
Hierarchical NiO nanosheets 151.7 Cheng et al. (2011)  
Hierarchical spindle-like γ-Al2O3 176.7 Cai et al. (2010)  
Hollow hierarchical MnO 60 Fei et al. (2008)  
NiO-SiO2 hollow microspheres 204 Lei et al. (2016)  
Hierarchical porous NiO/Al2O3 186.9 Rong et al. (2017)  
MnO2–PVP composite 311 This study 
Adsorbentqm (mg g−1)References
Hierarchical NiO nanosheets 151.7 Cheng et al. (2011)  
Hierarchical spindle-like γ-Al2O3 176.7 Cai et al. (2010)  
Hollow hierarchical MnO 60 Fei et al. (2008)  
NiO-SiO2 hollow microspheres 204 Lei et al. (2016)  
Hierarchical porous NiO/Al2O3 186.9 Rong et al. (2017)  
MnO2–PVP composite 311 This study 

Adsorption isotherms

Langmuir model

The linearized form of the conventional Langmuir adsorption model is written as follows:
formula
(8)
where (mol/L) is equilibrium concentration, (mol g−1) is maximum adsorption, X (mol g−1) is adsorbed amount, and Kb (L g −1) is binding energy constant. Xm and Kb values were calculated from the plot between versus (Figure 5(a) and 5(b)). The Xm values for both cases are close to experimental values (Table 6), which confirm the applicability Langmuir model. The Xm values of methyl orange are greater than Congo red, which is opposite to their Kb values. It is clear that although methyl orange adsorption is higher than Congo red, its binding to the surface is weaker than Congo red (Mahmood et al. 2016; Abdullah et al. 2023).
Table 6

Langmuir parameters at 298 K at pH 5

DyeXm × 105 (mol g−1)Kb (L g −1)R2
Methyl orange 8.396 13,992 0.96 
Congo red 3.396 84,646 0.977 
DyeXm × 105 (mol g−1)Kb (L g −1)R2
Methyl orange 8.396 13,992 0.96 
Congo red 3.396 84,646 0.977 
Figure 5

Langmuir isotherm for methyl orange (a), and Congo red (b), DR plot for methyl orange (c), and Congo red (d) by MnO2–PVP composite.

Figure 5

Langmuir isotherm for methyl orange (a), and Congo red (b), DR plot for methyl orange (c), and Congo red (d) by MnO2–PVP composite.

Close modal

Dubinin–Radushkevich model

The current data are introduced to the Dubinin–Radushkevich (DR) isotherm, which is shown as follows:
formula
(9)
where X is adsorbed dye at equilibrium, Xm is saturation capacity, Ce is equilibrium concentration (mol L−1), ɛ is the Polanyi potential, which is equal to RT ln(1 + 1/Ce), K is the energy constant, and R is the gas constant. A plot of ln X versus ɛ2 gives a linear relationship with high correlation coefficients (Figure 5(c) and 5(d)). Kd values obtained from DR models are applied for the calculation of mean free energy (E):
formula
(10)

Equation 10 was applied to find E. The mean adsorption energy was 1.088 for Congo red, while 1.27 (kJ mol−1) for methyl orange, which gives information regarding the mechanisms of the process. If the E values are in the range of 8–16 kJ mol−1, the adsorption process is chemical in nature, and when E is less than 8 kJ mol−1, then the process in physical in nature. Herein, in both cases, the values of E are less than 8 (Table 7), which confirms the physical adsorption (Mahmood et al. 2014).

Table 7

DR parameters for at 298 K at pH 5

DyeE (kJ mol−1)R2
Methyl orange 1.271 0.96 
Congo red 1.088 0.95 
DyeE (kJ mol−1)R2
Methyl orange 1.271 0.96 
Congo red 1.088 0.95 

Thermodynamics study

The thermodynamics factors are computed by the following relationships:
formula
(11)
formula
(12)
where Kc is the constant. The Gibbs free energy (ΔG) is calculated from Equation (13):
formula
(13)
The changes in entropy and enthalpy for dyes were calculated from the plot of lnKc versus (Figure 6(a) and 6(b), respectively). The negative values of free energy changes show the spontaneous nature of the process (Tables 8 and 9). The decrease in values of with the increased temperature (298–328 K) further demonstrates that the adsorption process is favorable at high temperature (328 K).
Table 8

Thermodynamics parameters of methyl orange

Temperature (K)ΔG (kJ mol−1)ΔH (kJ mol−1)ΔS (J mol −1 K−1)
298 −22.46 21.6 75.38 
308 −23.21 
318 −23.97 
328 −24.72 
Temperature (K)ΔG (kJ mol−1)ΔH (kJ mol−1)ΔS (J mol −1 K−1)
298 −22.46 21.6 75.38 
308 −23.21 
318 −23.97 
328 −24.72 
Table 9

Thermodynamics parameters of Congo red

Temperature (K)ΔG (kJ mol−1)ΔH (kJ mol−1)ΔS (J mol −1 K−1)
298 −0.50 21.8 66.96 
308 −1.17 
318 −1.84 
328 −2.51 
Temperature (K)ΔG (kJ mol−1)ΔH (kJ mol−1)ΔS (J mol −1 K−1)
298 −0.50 21.8 66.96 
308 −1.17 
318 −1.84 
328 −2.51 
Figure 6

ln Kc versus T−1 for methyl orange (a) and Congo red (b) at pH 5.

Figure 6

ln Kc versus T−1 for methyl orange (a) and Congo red (b) at pH 5.

Close modal

Moreover, the lower negative value of ΔG is observed for Congo red compared to methyl orange, demonstrating that Congo red adsorption is energetically less favorable. The positive values of (66.96 and 75.38 J mol −1 K−1) for Congo red and methyl orange, respectively, are due to the high randomness, which confirms a good affinity of dyes toward the surface. The positive values of also reveal that the arrangement of dye molecules on solid – solution interface turns into more randomness. The positive value confirms an endothermic process (Khan et al. 2020b).

The adsorption efficiency of the MnO2–PVP composite toward azo dyes was considerably enhanced compared to MnO2, which confirms the role of polymer in composite materials. The successful interaction of PVP with MnO2 is confirmed by the FTIR analysis and other techniques. The high value of the binding energy constant (Kb) in the case of PVP-loaded MnO2 composite as compared to MnO2 is evidence of greater affinity of the dye toward the composite adsorbent. Kinetically adsorption was followed by film diffusion and intraparticle diffusion. The thermodynamic investigations confirm that the dye adsorption process was endothermic, spontaneous, and physical in nature. The MnO2–PVP composite can be utilized as the best adsorbent for wastewater treatment through adsorption.

Afsar Khan (first and corresponding author): experiments and writing – original draft; Muhammad Arif (second corresponding author): experiments and revision; Zhengwei Han: writing and review; Yu Xie: graphs and tables and manuscript revision; Chenquan Ni: editing.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

Abdullah
N. H.
,
Nizam
W. H. A.
,
Norhisham
M. H. I.
,
Husaimi
N. A.
,
Ngadiman
N.
,
Salim
N. A. A.
,
Abu Hassan
N.
&
Muslim
N. H.
2023
Phosphorus removal from ore waste in aqueous solution with different mass of ore waste adsorbent from the Johor mine site
.
Water Practice Technology
18
(
11
),
2957
2970
.
Abugu
H. O.
,
Alum
O. L.
,
Ihedioha
J. N.
,
Ezugwu
A. L.
,
Ucheana
I. A.
,
Ali
I. J.
&
Eze
S. I.
2023
Sequestration of Pb2 + from aqueous solution using bio-based-alkaline modified sorbent from waste Irvingia gabonensis seed husk
.
Water Practice Technology
18
(
11
),
2495
2513
.
Alsbaiee
A.
,
Smith
B. J.
,
Xiao
L.
,
Ling
Y.
,
Helbling
D. E.
&
Dichtel
W. R
. 2016
Rapid removal of organic micropollutants from water by a porous β-cyclodextrin polymer
.
Nature
529
(
75
),
190
194
.
Arrisujaya
D.
,
Utami
N. S.
,
Mulyawati
T.
,
Rahmalisa
D.
,
Wati
S.
&
Hidayat
H.
2023
Chromium (VI) and lead (II) adsorption using Mangifera kemanga leaves
.
Water Practice Technology
18
(
11
),
2785
2796
.
Fei
J.
,
Cui
Y.
,
Yan
X.
,
Qi
W.
,
Yang
Y.
,
Wang
K.
,
He
Q.
&
Li
J.
2008
Controlled preparation of MnO2 hierarchical hollow nanostructures and their application in water treatment
.
Advanced Materials
20
(
3
),
452
456
.
Guo
J.
,
Zhou
S.
,
Ma
X.
,
He
S.
,
Chen
D.
,
Xie
F.
,
Wang
C.
,
Yang
H.
&
Li
W.
2024
Regenerated cellulose/polyethyleneimine composite aerogel for efficient and selective adsorption of anionic dyes
.
Separation Purification Technology
330
(
1
),
125480
.
Kabuba
J.
&
Banza
M.
2021
Modification of clinoptilolite with dialkylphosphinic acid for the selective removal of cobalt (II) and nickel (II) from hydrometallurgical effluent
.
The Canadian Journal of Chemical Engineering
99
(
2
),
168
178
.
Khan
A.
,
Naeem
A.
&
Mahmood
T.
2020b
Thermodynamic study of adsorption of methyl orange and Congo red from aqueous solutions by PVP-Functionalized ZnO
.
Russian Journal of Physical Chemistry A
94
(
8
),
1581
1586
.
Khan
A.
,
Naeem
A.
,
Mahmood
T.
,
Muhammad
N.
&
Hussain
S.
2022
Fixed-bed column adsorption of methyl orange by poly (vinyl pyrrolidone)-functionalized manganese oxide
.
Journal of Chemical Technology & Biotechnology
97
(
10
),
2898
2903
.
Kim
H.
&
Popov
B. N.
2003
Synthesis and characterization of MnO2-based mixed oxides as supercapacitors
.
Journal of the Electrochemical Society
150
(
3
),
150
156
.
Li
Y.
,
Wang
Z.
,
Huang
B.
,
Dai
Y.
,
Zhang
X.
&
Qin
X.
2015
Synthesis of BiOBr-PVP hybrids with enhanced adsorption-photocatalytic properties
.
Applied Surface Science
347
,
258
264
.
Mahmood
T.
,
Din
S.
,
Naeem
A.
,
Tasleem
S.
,
Alum
A.
&
Mustafa
S.
2014
Kinetics, equilibrium and thermodynamics studies of arsenate adsorption from aqueous solutions onto iron hydroxide
.
Journal of Industrial and Engineering Chemistry
20
(
5
),
3234
3242
.
Mahmood
T.
,
Khan
A.
,
Naeem
A.
,
Hamayun
M.
,
Muska
M.
,
Farooq
M.
&
Hussain
F.
2016
Adsorption of Ni (II) ions from aqueous solution onto a fungus Pleurotus ostreatus
.
Desalination and Water Treatment
57
(
16
),
7209
7218
.
Miyittah
M. K.
,
Tsyawo
F. W.
,
Kumah
K. K.
,
Stanley
C. D.
&
Rechcigl
J. E.
2016
Suitability of two methods for determination of point of zero charge (PZC) of adsorbents in soils
.
Communications in Soil Science and Plant Analysis
47
(
1
),
101
111
.
Mon
M.
,
Bruno
R.
,
Tiburcio
E.
,
Viciano-Chumillas
M.
,
Kalinke
L. H.
,
Ferrando-Soria
J. s.
,
Armentano
D.
&
Pardo
E.
2019
Multivariate metal–organic frameworks for the simultaneous capture of organic and inorganic contaminants from water
.
Journal of the American Chemical Society
141
(
34
),
13601
13609
.
Naeem
A.
,
Khan
A.
,
Mahmood
T.
,
Muska
M.
,
Din
S. U.
,
Khan
M. S.
,
Hamayun
M.
&
Waseem
M.
2014
Kinetics, equilibrium and thermodynamic studies of Mn (II) biosorption from aqueous solution onto Pleurotus ostreatus
.
Journal of Chemical Society of Pakistan
36
(
5
),
788
792
.
Nuengmatcha
P.
,
Kuyyogsuy
A.
,
Porrawatkul
P.
,
Pimsen
R.
,
Chanthai
S.
&
Nuengmatcha
P.
2023
Efficient degradation of dye pollutants in wastewater via photocatalysis using a magnetic zinc oxide/graphene/iron oxide-based catalyst
.
Water Science Engineering
16
(
3
),
243
251
.
Oladoye
P. O.
,
Ajiboye
T. O.
,
Wanyonyi
W. C.
,
Omotola
E. O.
&
Oladipo
M. E.
2023
Insights into remediation technology for malachite green wastewater treatment
.
Water Science Engineering
16
(
3
),
261
270
.
Özacar
M.
&
Şengil
İ. A.
2005
A kinetic study of metal complex dye sorption onto pine sawdust
.
Process Biochemistry
40
(
2
),
565
572
.
Parveen
K.
&
Rafique
U.
2018
Development of cobalt-doped alumina hybrids for adsorption of textile effluents
.
Adsorption Science & Technology
36
(
1–2
),
182
197
.
Rose
P. A.
,
Praseetha
P.
,
Bhagat
M.
,
Alexander
P.
,
Abdeen
S.
&
Chavali
M.
2013
Drug embedded PVP coated magnetic nanoparticles for targeted killing of breast cancer cells
.
Technology in Cancer Research & Treatment
12
(
5
),
463
472
.
Saeed
T.
,
Naeem
A.
,
Mahmood
T.
,
Khan
A.
,
Ahmad
Z.
,
Hamayun
M.
,
Khan
I. W.
&
Khan
N. H.
2021
Kinetic and thermodynamic studies of polyvinyl chloride composite of manganese oxide nanosheets for the efficient removal of dye from water
.
Water Science Technology
84
(
4
),
851
864
.
Venkatesha
T.
,
Viswanatha
R.
,
Nayaka
Y. A.
&
Chethana
B.
2012
Kinetics and thermodynamics of reactive and vat dyes adsorption on MgO nanoparticles
.
Chemical Engineering Journal
198
,
1
10
.
Yuan
L.
,
Wan
C.
&
Zhao
L.
2015
Facial in-situ synthesis of MnO2/PPy composite for supercapacitor
.
International Journal of Electrochemical Science
10
(
11
),
9456
9465
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).