Mechanism and performances of methyl orange and Congo red adsorption by MnO₂–PVP composite Open Access

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.

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.

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.

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.

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 (C_e) of dye was calculated with the help of a calibration curve.

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).

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).

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.

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).

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).

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).

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.

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).

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 (K_b) 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.