Copolymer-MnO2 nanocomposites for the adsorptive removal of organic pollutants from water

The copolymer beads prepared by suspension polymerisation were decorated with MnO2 nanoparticles and successfully implemented for the efficient removal of toxic organic contaminants from water. Copolymer-MnO2 nanocomposite was further analysed using XRD, SEM and optical microscope. The SEM images showed the surface characteristics of MnO2 nanoparticles on copolymer beads. The efficiency of the copolymer-MnO2 nanocomposite for the removal of model pollutant methylene blue and rhodamine B is then analysed by changing the concentration of pollutant. The results obtained exhibited 18.45 mg/g for methylene blue adsorption and 3.125 mg/g for rhodamine B. The adsorption equilibrium results were fitted to Langmuir adsorption isotherm for both methylene blue and rhodamine B adsorption. The desorption studies were performed for five consecutive cycles, and material was showing good regenerating capacity towards both organic pollutants. The obtained results show that copolymer-MnO2 nanocomposite is an efficient material for the removal of organic contaminants from wastewater.


Introduction
Water pollution is a worldwide issue that affects the human population around the world. Reliably, a lot of wastewater are created by industrial, horticultural and household activities and are stored in the land or water receptors (Reddy et al. 2022) (Chowdhary et al. 2020). The polluted water can cause harmful effects on aquatic as well as terrestrial life (Nagarajan and Venkatanarasimhan 2019) (Nagarajan and Venkatanarasimhan 2020). There is an increasing number of toxic pollutants in today's environment, requiring new materials to remove them. The occurrence of toxic dyes in water is one among them, and they can slowly lead to serious health problems. Wastewater or effluents need to be treated in order to be reused or disposed without harming the environment or human health.
Organic dyes, methylene blue (MB) and rhodamine B (RhB) are widely used in the textile industry and can cause various health issues when present in water (Karthik et al. 2018). Exposure to these dyes can lead to skin issues, eye problems, etc. (Tang et al. 2018). Various techniques such as coagulation, chemical oxidation, membrane filtration, adsorption have been used for the removal of dyes from water (Crini and Lichtfouse 2019), in which adsorption obtained wide attention due to its low cost and higher efficiency (Bonilla-Petriciolet et al. 2017). Researchers prepared various adsorbent materials for the removal of such toxic dyes from water. Activated carbon (Xue et al. 2022) (Han et al. 2020), graphene oxide (Benjwal et al. 2015) (Huang et al. 2019), Fe 3 O 4 (Zhang et al. 2013) are various adsorbent materials reported for the removal of MB. Sharma et al. (Sharma et al. 2018) synthesised agrobacterium fabrum biomass along with iron oxide nanoparticles for the removal of MB, and the material was showing a good regenerating capacity for four consecutive cycles. Similarly, various adsorbent materials such as polymer (Arun Viswan and Gangadharan 2018), activated carbon (Zhang et al. 2021), silica (Joshiba et al. 2021), MOF (Liu et al. 2016), nanoparticles (Hund-Rinke andSimon 2006), nanocomposites (Skiba and Vorobyova 2020) are used for the removal of contaminants (Elias et al. 2021) from water. Polymers and nanomaterials are among the newest emerging materials used to remove contaminants from water. Nanocomposites made of polymer materials have gained wide attention for their performance and ease of separation from contaminant solutions (Beyene and Ambaye 2019). Adding nanoparticles to polymers can improve their efficiency (Khodakarami and Bagheri 2021).
MnO 2 is considered an efficient nanomaterial for wide range of applications. It is widely used in batteries (Liu et al. 2019), supercapacitors (Kubra et al. 2020), sensors (Cogal et al. 2021) and wastewater purification processes (KK and Gangadharan 2022). Different materials were modified with MnO 2 nanoparticles to improve the adsorption efficiency (Cuong et al. 2021) (Verma et al. 2021). Dong et al. (Dong et al. 2010) synthesised a MnO 2 -loaded resin for the simultaneous adsorption of lead and cadmium from the aqueous solution. Here, surface complexation plays a vital role in the adsorption of lead and cadmium on MnO 2 -loaded resin. Similarly, various materials have been prepared using MnO 2 as a component. The separation of MnO 2 nanoparticle from adsorbate solution is a challenge for researchers. To overcome this, copolymer beads can incorporate with MnO 2 material for the easy separation of material from aqueous solution as well as to improve the adsorption capacity of material towards organic pollutants (Gangadharan et al. 2013). Column studies can also be performed by using copolymer beads. Nhat Ha et al. (Nhat Ha et al. 2016) synthesised layered double hydroxide embedded polymer beads for the removal of arsenate ions via batch as well as column studies. The removal of contaminants via the column method is an advantage of using polymer beads-modified materials.
Herein, this research work focused on the synthesis, characterisation and application of a copolymer-MnO 2 nanocomposite. The as-synthesised copolymer-MnO 2 nanocomposite material was then analysed using powder XRD, SEM and optical microscope. The material is then equilibrated with organic pollutants to find the maximum adsorption capacity towards organic pollutant removal. As model pollutants, methylene blue and rhodamine B were used. Furthermore, the effect of contact time and concentration of pollutants on the material was studied. The regenerating capacity of the material was established by performing adsorption-desorption studies. The concentration studies were conducted by using a UV-Visible spectrophotometer.

Materials
Manganese sulphate, potassium permanganate, methylene blue and rhodamine B were obtained from Merk. Distilled water was used in all the experiments.

Preparation of copolymer beads
As previously reported, the preparation of copolymer beads was undertaken (Gangadharan et al. 2010). To a beaker containing 35 mL methacrylic acid, add 5.4 mL of divinylbenzene followed by addition of 0.4 g of benzoyl peroxide dissolved in 12 mL toluene. To a preheated three-neck round bottom flask containing 2% starch solution as suspension medium, the monomeric mixture was gradually added at 70 °C, while the flask was preheated. Following the addition of the monomeric mixture, the temperature was raised to 80 °C and maintained under a reflux condition for 5 h. Physically decanting the solution finally separated the opaque white spheres from the solution.

Modification of copolymer-MnO 2 nanocomposite
Two grams of synthesised copolymer beads were weighed and taken in a conical flask and treated with 200 mL of 0.1 M MnCl 2 .4H 2 O solution in a mechanical stirrer for 18 h. Afterward, it was decanted and washed with deionised water to eliminate any excess Mn 2+ ions adhering to the copolymer beads. Mn 2+ ion-loaded methacrylic acid copolymer beads were treated with 200 mL of 0.1 M KMnO 4 solution in a mechanical stirrer for 18 h. The solution was then decanted and washed with deionised water to remove loosely adhering MnO 2 from the copolymer beads.

Copolymer-MnO 2 nanocomposite characterisation
The synthesised copolymer beads and copolymer-MnO 2 nanocomposite was analysed using OLYMBUS CH20i optical microscope. The cross section of copolymer-MnO 2 nanocomposite was analysed to determine the pattern of loading on the copolymer beads. SEM analysis was carried out to know the morphology of the MnO 2 loaded on the copolymer beads. An Empyrean range instrument with a Cu Kἁ target was used for X-ray diffraction experiments in a region of 2Ɵ from 20 to 80°. UV-Visible spectroscopy was used to analyse the concentration of methylene blue (MB) and RhB in the water samples.

Adsorption studies of MB on copolymer-MnO 2 nanocomposite
Two hundred milligrams of copolymer-MnO 2 nanocomposite was taken in 5 different stoppered conical flasks to which 50 mL of freshly prepared 5, 10, 15, 20 and 25 ppm methylene blue solutions was added respectively. They were kept for shaking in a mechanical stirrer for 10 h, and the solutions were tested for adsorption of different concentrations of methylene blue on copolymer-MnO 2 nanocomposite. All these studies were carried out at neutral pH 7.

Adsorption of RhB on copolymer-MnO 2 nanocomposite
The copolymer-MnO 2 nanocomposite was then used to remove the toxic dye RhB from aqueous solutions. A batch method was used to study adsorption. In a 250-mL stoppered flask, 200 mg of the copolymer-MnO 2 nanocomposite was equilibrated with 20 mL of RhB solution with a concentration ranging from 5 to 25 mg/L. The equilibration was attained within 10 h, and the absorbance of the solution after adsorption is measured using UV-Visible spectrophotometer.

Desorption of MB from copolymer-MnO 2 nanocomposite
To examine the regenerating capacity of the copolymer-MnO 2 nanocomposite, the MB dye was desorbed from the copolymer-MnO 2 nanocomposite. Two hundred milligrams of the material was equilibrated with 50 mL of 5 mg/L of MB solution for 10 h. The MB-loaded material was then treated with 30 mL of ethanol to desorb the MB. The quantity of MB desorbed from the copolymer-MnO 2 nanocomposite was then estimated using UV-Visible spectrophotometer. The experiment was repeated five times to determine the regenerative capacity of copolymer-MnO 2 nanocomposite.

Desorption of RhB from copolymer-MnO 2 nanocomposite
Two hundred milligrams of the copolymer-MnO 2 nanocomposite was equilibrated with 20 mL of 5 mg/L of RhB solution for 10 h. Twenty millilitres of 1 M acetic acid was then added to the RhB-loaded copolymer-MnO 2 nanocomposite to desorb it. The amount of RhB absorbed onto copolymer-MnO 2 nanocomposite was then determined using a UV-visible spectrophotometer. Five cycles of the experiment were conducted to test the regenerating capacity of the material.

Scheme of work
The copolymer-MnO 2 nanocomposite was treated with MB and RhB solution. The contaminants were loaded on the beads via adsorption process. MnO 2 particles were observed both on the surface and interior core of the copolymer beads. Adsorption of MB dye onto the beads turns them green. Similarly, RhB dye was adsorbed onto the black-coloured MnO 2 copolymer beads which turned red colour. The contaminants adsorbed on the surface were desorbed using ethanol and acetic acid solution. The MnO 2 loaded on the copolymer beads were removed using HCl solution. The complete scheme is provided in Fig. 1.

Powder X-ray diffraction analysis of copolymer-MnO 2 nanocomposite
The presence of MnO 2 nanoparticles on the copolymer beads was determined using powder XRD analysis data. The peaks observed at two theta values 12.2, 36.4, 65.7 and 85.7° correspond to the MnO 2 nanoparticles on the polymer surface. The lattice planes corresponding to the given two theta values are (110), (400), (002) and (512). The powder XRD spectra were given in Fig. 2. The spectra were then compared with the standard ICDD data (02-0227).

SEM analysis of copolymer-MnO 2 nanocomposite
The analysis using scanning electron microscope was performed to study the distribution of MnO 2 surface morphology of the copolymer-MnO 2 nanocomposite. The SEM images of copolymer beads and copolymer-MnO 2 nanocomposite were given in Fig. 3. Figure 3a represents the copolymer beads without MnO 2 is spherical in shape. Figure 3b represent the MnO 2 particle on the polymer surface. The MnO 2 particle exhibited spherical shape and attached on the polymer surface. Figure 3c shows the MnO2 nanoparticles observed on the cross-sectional image of polymer beads. Nanoparticles conformed to be smaller than 100 nm in size. Figure 3d represents the EDX spectra of the copolymer-MnO 2 nanocomposite. This study showed the presence of Mn on copolymer beads.

Optical microscopic analysis of copolymer-MnO 2 nanocomposite
Copolymer beads and copolymer-MnO 2 nanocomposite were analysed using optical microscope which is given in Fig. 4. The copolymer beads prepared by suspension polymerisation were shown in Fig. 4a. The colour remained white even after exchange of Mn 2+ ions and turned into black colour after treating with KMnO 4 which is given in Fig. 4b,c respectively. On the optical microscope cross-section image of the copolymer-MnO 2 nanocomposite, it can be seen that MnO 2 has formed on the surface as well as inside,  Fig. 4d. This copolymer-MnO 2 nanocomposite was used to remove the organic pollutants from water.

Adsorption of MB and RhB on copolymer-MnO 2 nanocomposite
Contact time and adsorbate concentration influenced the adsorption of MB and RhB. Adsorption of MB on copolymer-MnO 2 nanocomposite at different time interval was studied to fix the equilibration time. Two hundred milligrams of the copolymer-MnO 2 nanocomposite was equilibrated with 50 mL of 5 mg/L MB solution. A UV-Visible spectrophotometer was used to measure the absorbance of the solution at each 1-h interval. The time studies unveil that 48% of the MB got adsorbed within 1 h of equilibration and attained equilibrium within 9 h. Ninety-eight percent of the MB was adsorbed on copolymer-MnO 2 nanocomposite after equilibration. The adsorption percentage in each time interval was graphically shown in Fig. 5a.
Likewise, the effect of contact time for RhB adsorption on copolymer-MnO 2 nanocomposite was studied. Two hundred milligrams of the copolymer-MnO 2 nanocomposite was equilibrated with 20 mL of 5 mg/L solution. The obtained data was graphically shown in Fig. 5b. The material was showing a good adsorption tendency towards RhB. Initially, 46% of the RhB was adsorbed on copolymer-MnO 2 nanocomposite within 1 h and attained 96% adsorption after 10 h of equilibration.
Batch adsorption data studies for MB were then performed to check the effect of adsorbate concentration on copolymer-MnO 2 nanocomposite. The MB solution with concentration ranging from 5 to 25 mg/L was used for the analysis. Two hundred milligrams of the adsorbent was equilibrated with 50 mL of each MB solution for 10 h of equilibration. The results obtained from above studies showed that 98% of the adsorbate adsorbed onto the material from 5 mg/L solution. Adsorption percentage was decreased to 60% for 25 mg/L solution. The adsorption data was given in Fig. 5c.
Likewise, batch adsorption studies of RhB were performed using different concentration ranging from 5 to 25 mg/L. Two hundred milligrams of the adsorbent was equilibrated with 20 mL of each RhB solution for 10 h of equilibration. The result obtained showed an adsorption efficiency of 96% for 5 mg/L RhB solution. Similarly, adsorption efficiency was 94%, 88%, 85% and 84% for 5, 10, 15, 20 and 25 mg/L respectively. The results of the adsorption data are shown in Fig. 5d.

Isotherm studies for MB and RhB adsorption on copolymer-MnO 2 nanocomposite
Adsorption isotherms have been studied to determine the type of adsorption in case of MB and RhB removal from water. The results obtained from the adsorption study were fitted to Langmuir and Freundlich isotherm models. where q e (mg/g) is the quantity of MB or RhB loaded onto the copolymer-MnO 2 nanocomposite, q m (mg/g) represents the maximum adsorption capacity of the copolymer-MnO 2 nanocomposite, C e (mg/g) represents the concentration of the MB or RhB at equilibrium and K L (L/mg) is the Langmuir constant. The values of q m and K L can be calculated from the plot 1/q e vs 1/C e . The plot showed that the correlation coefficient reached its maximum value of 0.99. This material has a maximum adsorption capacity of 18.45 mg/g. The Langmuir adsorption isotherm for MB is depicted in Fig. 6a. RhB also exhibits a maximum correlation coefficient of 0.98, and its maximum adsorption capacity is 3.125 mg/g. Figure 6c shows the isotherm plot for RhB. The data for the isotherms can be found in Table 1.
We investigated the Freundlich adsorption isotherm on adsorbent for MB and RhB to investigate their surface (1) 1 q e = 1 q m + 1 K L q m C e energy distributions and modes of multilayer adsorption. Freundlich adsorption isotherm supports adsorption processes that occur on heterogeneous surfaces. The Freundlich adsorption isotherm equation is An analysis of the Freundlich plot was performed by plotting log C e versus log q e values. The value of 1/n can be obtained from the graph. 1/n was observed between 0.2 and 0.8, indicating that the copolymer-MnO 2 nanocomposite was a good adsorbent for the removal of MB and RhB. As shown in Fig. 6b, the correlation coefficient reaches a maximum value of 0.75. The Freundlich plot for RhB is shown in Fig. 6d, and the correlation coefficient was found to be 0.91. Table 1 gives the values obtained from the graph.
The correlation coefficient values of Langmuir and Freundlich adsorption isotherm were provided in Table 1. The results showed that adsorption of dyes on copolymer-MnO 2 nanocomposite fit for the Langmuir isotherm model. In comparison with the Langmuir model, the Freundlich

Desorption of MB from copolymer-MnO 2 nanocomposite
The MB loaded on the copolymer-MnO 2 nanocomposite was then desorbed with ethanol solution. Two hundred milligrams of the material was treated with 50 mL of 5 mg/L of MB solution for 10 h. The MB-loaded material was then treated with 30 mL of ethanol to desorb the MB. The experiment was continued for another five cycles to check the regenerating capacity of the copolymer-MnO 2 nanocomposite. The obtained results showed 98% adsorption efficiency and 95% desorption efficiency on the first cycle. For the fifth cycle, the adsorption and desorption efficiency decreased to 92% for adsorption and 80% for desorption. The copolymer-MnO 2 nanocomposite was showing good regenerating capacity towards the removal of MB from water. The experimental results were graphically shown in Fig. 7a.

Desorption of RhB from copolymer-MnO 2 nanocomposite
The RhB adsorbed on the copolymer-MnO 2 nanocomposite was then desorbed using acetic acid solution. Two hundred milligrams of the material was equilibrated with 20 mL of 5 mg/L of RhB solution for 10 h. The RhB-loaded material was then treated with 1 M acetic acid to desorb the RhB. A UV-Visible spectrophotometer was then used to estimate the amount of RhB desorbed from the sample. To determine the regenerative capacity of the material, the experiment was repeated five times. The obtained results showed 96% adsorption efficiency and 95% desorption efficiency on the first cycle. It was showing 88% adsorption efficiency and 85% desorption on the 5 th cycle. The copolymer-MnO 2 nanocomposite exhibited good regenerating capacity towards the removal of RhB from water ( Table 2). The experimental results were graphically shown in Fig. 7a.

Conclusion
In this study, we prepared methacrylic acid-divinylbenzene polymer beads via suspension polymerisation decorated with MnO 2 nanoparticles and successfully implemented for the efficient removal of toxic organic contaminants from water. Copolymer-MnO 2 nanocomposite was studied for the removal MB and RhB via adsorption. Copolymer-MnO 2 nanocomposite exhibited an adsorption capacity of 18.45 mg/g and 3.125 mg/L towards MB and RhB respectively. The Langmuir adsorption model was found to be the best fit model for the adsorption of MB and RhB on copolymer-MnO 2 nanocomposite. Desorption studies showed good regenerating capacity for copolymer-MnO 2 nanocomposite in five consecutive cycles for MB and RhB. The copolymer-MnO 2 nanocomposite showed 98% adsorption efficiency and 95% desorption towards MB on the first cycle. Copolymer-MnO 2 nanocomposite showed 96% adsorption and 95% desorption efficiency towards RhB on the first cycle. To conclude, the copolymer-MnO 2 nanocomposite can be used as a good adsorbent material towards the removal of organic pollutants from water.
Author contribution AVKK-execution of work, characterisation, validation, investigation, writing-original draft. NS-execution of adsorption studies. SR-performed desorption studies and regeneration studies. DG-correction and review, editing manuscript, conceptualisation of work, result interpretation. Fig. 7 a Percentage of MB adsorbed on copolymer-MnO 2 nanocomposite and percentage of MB desorbed from copolymer-MnO 2 nanocomposite; b) percentage of RhB adsorbed on copolymer-MnO 2 nanocomposite, and percentage of RhB desorbed from copolymer-MnO 2 nanocomposite

Data availability
The datasets generated and/or analysed during the current study are not publicly available but are available from the corresponding author on reasonable request.

Declarations
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Consent for publication Not applicable.

Competing interests
The authors declare no competing interests.