Investigation on the feasibility of recycled polyvinylidene difluoride polymer from used membranes for removal of methylene blue: experimental and DFT studies

In the past few decades, membranes have been used excessively for water and wastewater treatment, leading to the increased disposal of waste membranes (Ezugbe et al. 2020). At the current rate, the disposal of membrane modules exhibits significant and escalating detrimental impacts, leading to the need to limit the direct disposal of these modules. Currently, used membranes are disposed of in landfills or incinerated. However, these methods are not completely dependable because of the environmental impacts. The discarded membranes can be recycled and reused for different separation processes with less demanding specifications with some post-processing. Several methods have been employed to recycle membranes, such as chemical cleaning (Yi et al. 2017), physical cleaning and backwashing (Park et al. 2018), and direct recycling of the various module components (Lawler et al. 2012). Paula et al. (2017) examined the technological viability of desalination membrane recycling by chemical oxidation. The end-of-life membranes deteriorate due to fouling, biofouling, etc. The polymer recovered from the membranes can be used as adsorbents to remove hazardous contaminants from wastewater. However, very few studies are available where polymers recovered from waste membranes have been used for adsorption (Zwain et al. 2014).

Synthetic dyes are found in huge quantities in textile and dye processing industrial effluent, posing a significant environmental threat. Many industries produce these dyes, including textiles, printing, paper, and plastic (Ngulube et al. 2017; Yadav et al. 2021a, 2021b, 2021c). Methylene blue (MB) is a cationic dye found in considerable quantities in textile effluent released into the environment without the significant treatment of its release into the surrounding ecosystem (Yadav et al. 2021a, 2021b, 2021c). At present, biological, physical, and chemical methods are employed for textile dye removal from wastewater (Yaseen & Scholz 2019). However, there are many shortcomings associated with these traditional technologies for dye removal, including chemical release, secondary sludge, high initial cost, and low decolourization efficiency (Piaskowski et al. 2018). Adsorption is one of the most economical, effective, and simple processes for the decolourization of textile water. Furthermore, it shows a wide range of materials such as nanomaterials, carbonaceous materials, bioadsorbents, etc. (Tran et al. 2019).

In recent years, polymeric adsorbents have become popular as an alternative to traditional adsorbents such as clays and activated carbon due to their tunable physicochemical characteristics, structural variability, reusability, and selectivity (Mok et al. 2020; Yadav & Sinha 2021). Zheng et al. (2018) studied the feasibility of polymer-functionalized magnetic nanoparticles for MB removal. Fe₃O₄-incorporated carboxyl functionalized nanoporous polymer showed high performance for MB dye adsorption from wastewater (Su et al. 2018). Carbon nanotubes-based polymer nanocomposites were found to be suitable for the adsorption of MB from the aqueous solution (Gan et al. 2020). Several researchers have used waste material to remove dyes from textile wastewater. For instance, Kiran et al. (2020) used agro-industrial waste (palm date stones) to adsorb basic violet 3 and basic red 2 from textile wastewater with high removal efficiency (77%- basic red 2, 93% basic violet 3). Wong et al. (2020) used coffee waste modified with polyethyleneimine to adsorb dye from textile wastewater with an adsorption capacity of 34.36 mg g⁻¹ (Congo red) and 77.52 mg g⁻¹ (reactive black 5). Temesgen et al. (2018) used activated orange and banana peel to eliminate reactive red dye from textile industry wastewater with very high removal efficiency (>89%). Vecino et al. (2015) synthesized biocomposites from vineyard waste entrapped in calcium alginate hydrogel beads to eliminate dye from wastewater and achieved 74.6% dye removal efficiency. Polyvinyledienefluoride (PVDF) and its co-polymer have been widely used to treat textile wastewater. Zhang et al. (2019) fabricated a PVDF/GO/ZnO composite to remove MB dye through photocatalytic degradation, achieving a removal efficiency of 86.84%.

A careful literature survey shows that PVDF effectively adsorbs MB dye from an aqueous stream. Hence, in this study, PVDF polymer from a recycled membrane was used as an adsorbent for removing MB dye from textile wastewater. The PVDF beads obtained from recycled membranes were characterized using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR) for morphological and structural analysis. A parametric study was performed to study the effect of varying adsorbent dose, initial concentration, contact time, and pH on the beads’ adsorption capacity and removal efficiency. Moreover, density functional theory (DFT) calculations, adsorption isotherm, and kinetics were studied to identify the adsorption process' mechanism.

The recycled PVDF was obtained from used membranes. Dimethylformamide (DMF) solvent (purity ∼99.5%) was purchased from Spectrochem Pvt. Ltd Mumbai, India. MB dye (purity ∼82%) was procured from NICE chemicals Pvt. Ltd, India. Deionized (DI) water (∼18 M Ω) was obtained from the Millipore Q BIOCEL unit, Millipore.

The recycled membrane was washed thoroughly, and the PVDF (16 g) was delaminated from the fabric and dissolved in DMF solvent (100 ml) for 6 h at 60 °C under constant stirring. The PVDF solution (16 wt.%) was taken in a clean syringe and added dropwise to a beaker filled with DI water (coagulation bath). The recycled PVDF polymer precipitated in the form of spherical beads. The beads were kept in the coagulation bath overnight to complete the phase inversion process.

The morphological characteristics of the adsorbent composite beads were studied from SEM (JEOL JSM 7100F). The adsorbent IR spectrography was performed using the Perkin Elmer FTIR spectrometer (USA). The thermogravimetric analysis (TGA) was accomplished with a thermogravimetric analyzer (Mettler Toledo) with nitrogen flow at a heating rate of 10° C min⁻¹. The measurement of MB dye concentrations was assessed using a UV-Vis spectrophotometer by constructing a calibration curve, and for pH monitoring, a Eutech PC2700 multiparameter device (Shimadzu, Japan) was used. The characteristic absorbance of MB dye at 663 nm was chosen to study the decolourization during the adsorption.

Figure 1 illustrates the surface and cross-sectional morphology of recycled PVDF beads. The surface morphology of the beads was rough and porous. The recycled PVDF beads’ internal porous and spongy cross-section morphology was attributed to the solvent and non-solvent exchange during the phase inversion process (Zahirifar et al. 2019). The recycled PVDF beads’ rough and porous morphology will provide a larger surface area and an adsorption process with more active sites.

TGA was conducted to study the thermal stability of the synthesized recycled PVDF beads (Figure 2(a)). Till 427 °C, there was a loss of ∼3% mass, attributed to the evaporation of humidity entrapped in the beads. When the temperature reached 427 °C, the residual mass declined rapidly to 34%, attributed to the thermal degradation of the PVDF polymer's −(C2H2F2)n− units. Hence, it was concluded that the synthesized beads were stable up to 427 °C, favouring non-isothermal adsorption.

Figure 2(b) shows the XRD spectra of the recycled PVDF beads. The sharp peaks in XRD spectra at 2θ = 17.75° and 22.80° corresponded to (100) and (110) α-crystal diffractions recycled PVDF, respectively (Janakiraman et al. 2016). The sharp peaks at 2θ = 26.07° (022) were attributed to the γ- phase of the recycled PVDF phase. The peak at 2θ = 20.03° was due to the combined β (110) and γ (101) phase (Martins et al. 2014; Esterly & Love 2004).

Figure 3 shows the FT-IR spectra of the recycled PVDF beads. The characteristic absorption bands at 1,400 and 1,180 cm⁻¹ were due to C–H bending and C–F stretching, respectively. The absorbance band at 877 cm⁻¹ was attributed to C–H wagging, while the absorbance band at 840 cm⁻¹ was due to C–F bending (Daems et al. 2018; Yadav et al. 2021a, 2021b, 2021c). The absorbance bands at 839 and 870 cm⁻¹ were attributed to the amorphous phase and at 1276 cm⁻¹ to β-phase vibration (Zahirifar et al. 2019). The FT-IR analysis of the recycled PVDF beads confirmed no impurity in the recycled PVDF beads.

The batch adsorption experiments were performed by varying one parameter at a time while the other parameters were kept constant. The effect of process parameters has been discussed in the subsequent sub-sections.

To remove MB dye with recycled PVDF beads, the influence of the adsorbent dose (0.2–3.0 g L⁻¹) was examined (Figure 4). The removal efficiency of MB dye increased from 25% to 99.10%, with an increase in adsorbent dose from 0.2 to 3 g L⁻¹. The increment in the removal efficiency was attributed to the increased availability of adsorption sites (Patel & Yadav 2022). After 2.5 g L⁻¹ adsorbent dose, the removal efficiency reached the maximum value and remained constant on further increasing the dose. Although more active sites were available, no dye molecule was available in the solution to bind with the active adsorbent site due. The adsorption capacity of recycled PVDF beads decreased rapidly in the initial stages and steadily afterwards. As the number of recycled PVDF beads increased, the adsorption capacity decreased due to increased attraction forces, which led to the agglomeration of the beads (Mahmoudzadeh et al. 2013). In addition, the adsorption capacity decrement was due to increased excess active sites. The maximum adsorption capacity of active sites on recycled PVDF beads was not achieved because the concentration of the MB dye was fixed. Moreover, the adsorption capacity is inversely proportional to the adsorbent dose (Equation (1)). Similar observations are reported in other studies (Li et al. 2020; Shojaei & Esmaeili 2022).

The effect of MB dye solution pH on the adsorption capacity and removal efficiency of the recycled PVDF beads was studied by varying the pH of the solution from 2 to 12. With the increased pH of MB dye solution, the removal efficiency increased from 55% to 99.20%, with the maximum removal efficiency at pH 10 (Figure 5). In a similar trend with an increase in MB dye solution pH, the adsorption capacity increased from 4.4 to 7.94 mg g⁻¹. This increase in removal efficiency and adsorption capacity can be explained by the negative charge present on the recycled PVDF beads (Chiao et al. 2020) and cationic MB dye. The removal efficiency increment was three stages. In the first stage (pH from 2 to 4), the removal efficiency increased slowly because the surface charge of the recycled PVDF was positive. In the second stage (pH from 4 to 8), the removal efficiency increased rapidly since the surface charge of the recycled PVDF became more negative. In the third stage (pH from 8 to 12), the removal efficiency increment was almost constant, attributed to the constant surface charge of the recycled PVDF.

The effect of recycled PVDF beads' contact time on the adsorption capacity and removal efficiency for MB dye adsorption was studied with varying contact times from 5 to 240 mins. The removal efficiency and adsorption capacity increased rapidly with the contact time of the adsorbent with dye solution and after some point of time, the curve becomes a plateau (Figure 6). The increments in removal efficiency and adsorption capacity were initially because of the availability of more active adsorption sites. As the adsorption process progressed, the active number of sites decreased as the adsorbent dose was fixed. Hence the curve becomes plateau with a maximum removal efficiency of 98.4%. Similar observations are reported in other studies (Geng et al. 2018; Yadav et al. 2022a, 2022b).

The effect initial concentration of MB dye on removal efficiency and adsorption capacity was investigated by varying the initial concentration from 10 to 250 ppm. The removal efficiency decreased as the MB concentration increased (Figure 7). With increased MB dye concentration from 10 to 250 ppm, the removal efficiency reduced from 99.1% to 27.72%. This decrement in the removal efficiency was due to the increased interaction of MB dye with available active sites on recycled PVDF beads. The adsorption capacity increased from 3.96 to 27.72 mg g⁻¹ with the MB dye initial concentration increasing from 10 to 250 ppm. The increased absorption capacity was attributed to the increased MB dye concentration. The adsorption capacity rose to 27.72 mg g⁻¹ and remained constant because the adsorbent sites were fixed and the MB dye concentration increased (Liu et al. 2019; Jahan et al. 2021).

The maximum adsorption capacity of different adsorbents (mainly waste or recycled material) reported in the literature for the adsorption of the MB dye are compared with the maximum adsorption capacity of recycled PVDF beads in Table 1. The adsorption capacity of recycled PVDF beads was comparable with the reported adsorbents. This suggested the feasibility of the cost-effective recycled PVDF beads for the MB dye adsorption.

The optimization of the contact time of the adsorbent with the adsorbate molecules plays a significant role in adsorption studies to ensure complete equilibrium between the MB dye and recycled PVDF beads. The PFO and PSO models’ adsorption kinetics were investigated to propose a plausible kinetic mechanism. Table 2 depicts different parameters of the PFO and PSO adoption kinetics for adsorption of MB dye on recycled PVDF beads. The correlation coefficient (R²) for PFO and PSO models was 0.955 and 0.991, respectively. The R² value for PFO was far more unity than the PSO model, indicating the PSO non-linear second-order model fitting for the adsorption of MB dye on recycled PVDF beads. The PSO model fitting confirmed that the adsorption was chemical (Maslova et al. 2021). The value of the residual sum of the square and reduced chi-square was smaller from the PSO model than PFO. This was due to the lesser difference between the experimental and theoretical values for PSO and PFO models (Figure 8).

Two adsorption isotherms (Freundlich and Langmuir) were investigated for the adsorption of MB dye on recycled PVDF beads in the present study (Figure 9). The R², reduced chi-square, and residual sum of the square were calculated to predict the adsorption isotherm for MB adsorption on recycled PNDF beads (Table 3). The R² value was close to unity in the case for the Langmuir model (Freundlich: 0.843 and Langmuir: 0.991). The R² value was close to unity for the Langmuir isotherm model, indicating that the experimental data and predicted results obtained for the MB dye adsorption were closer. Moreover, the value of K_L was 0.767 (less than 1). If the value of K_L is between 0 and 1, the system can be considered suitable for adsorption purposes. In addition, the smaller value of the residual sum of square (6.965) makes this model applicable for the present work. The horizontal asymptote in the Langmuir isotherm indicated saturation after monolayer adsorption. The fitting Langmuir isotherm indicated the adsorption mechanism was chemical in nature of MB dye on recycled PVDF beads (Bangari et al. 2022a, 2022b).

The DFT optimized geometries of the PVDF, MB dye, and MB-PVDF clusters are shown in Figure 10. The relaxed non-planar geometries of PVDF and MB dye agreed with the literature (Bangari et al. 2022a, 2022b; Yadav et al. 2022a, 2022b). Several starting positions and orientations for the dye molecule were tested to find the most effective contact site. The N atom of MB dye was closest to the PVDF at a distance of 3.21 Å. The adsorption energy for the MB-PVDF complex was –64.7 kJ mol⁻¹ which described that the MB-PVDF system had positive interaction, supporting the experimental findings.

To determine the stability of the post adsorbed MB-PVDF complex, the vibrational frequencies of the PVDF, MB dye before and after adsorption complexes were computed (Figure 11(a)). All the PVDF and MB dye characteristic bands were present in their respective FT-IR spectra. After the interaction between PVDF and MB dye, the PVDF and MB dye bands were visible in the FT-IR of the MB-PVDF complex. As a result, the vibrational spectra indicated that the MB and PVDF interacted during adsorption. The number of electronic states per unit of energy in a material, as a function of energy, is provided by TDOS, which is critical for understanding its properties (Dindorkar et al. 2022b). The TDOS plots for PVDF, MB, and MB-PVDF clusters are shown in Figure 11(b). For the PVDF, the occupied and unoccupied molecular orbitals were widely spaced. After adsorption of MB dye on PVDF, additional energy levels appeared, which were traced to the charge transfer from the HOMO of the PVDF to the MB dye molecule. The adsorption of MB dye on PVDF resulted in a decrease in band gaps, which might be due to the emergence of new energy levels that mix the orbitals of MB dye molecules with the molecular orbitals of PVDF (Bangari et al. 2021). The TDOS plots confirmed the findings from the above sections and gave an account of the variation in the electronic density of states after the adsorption of MB dye.

The molecular orbitals (HOMO and LUMO) representation for PVDF, MB and PVDF-MB clusters are shown in Figure 12. The HOMOs and LUMOs were distributed uniformly on PVDF before adsorption. From the HOMO–LUMO, a reasonable charge distribution can be observed on the adsorbent sites on the double bond of the ethylene group. The HOMOs were concentrated on the electronegative N atoms of the MB dye molecule, while LUMOs were delocalized. After adsorption, the HOMOs and LUMOs were concentrated on MB dye molecules. This means that the MB-PVDF attained more nature of MB dye rather than PVDF. The HOMO and LUMO energies are related to chemical parameters that provide the details of the reactivity of the molecule (Dindorkar & Yadav 2022). Higher chemical stability can be attained with higher chemical hardness as they reduce the polarizability of the molecules. Therefore, as MB-PVDF's HOMO-LUMO energy gap (HLG) reduced after adsorption, chemical hardness also reduced, but there was little chemical potential change (Table 4). The HLG value of PVDF before and after adsorption was 9.42 eV and 0.50 eV, respectively. This was because of charge transfer from MB dye to PVDF during adsorption, which led to a change in the density of occupied orbitals. As discussed above that the MB-PVDF complex attained the nature of MB dye, the complex became strong electrophile after absorption.

Studying the adsorption mechanism of MB dye on recycled PVDF beads is important to gain insights into the actual process. Usually, if the PSO kinetic model describes an adsorption process better, it was inferred to be a chemisorption process. However, the DFT studies showed that the adsorption energy was −64 kJ mol⁻¹, implying the adsorption process was physical. Moreover, the shortest distance between the PVDF and MB dye molecules was >2 Å, implying that the adsorption was physical (Yadav & Dindorkar 2022a). As a result, the adsorption mechanism in this investigation could not be categorized solely as a chemical or physical response but rather as a mixed one. Other researchers have reported this observation (Zheng et al. 2018). MB dye is a cationic dye that gets adsorbed on the surface of the recycled PVDF beads by electrostatic attraction. The DFT simulations showed that the interaction of the MB dye molecule was from the electronegative N atom of the MB dye molecule. This means that electrostatic interaction occurred between the recycled PVDF beads and the positively charged quaternary ammonium groups in MB dye. The hydrogen bonding was due to the attraction between F atoms of PVDF and the amine group present in the MB dye molecule (Zheng et al. 2018). The adsorption was enhanced by the MB dye's linearity and three parallel aromatic rings, which encourage π-π interactions (Muslim et al. 2021).

In this study, we report the feasibility of the recycled PVDF for the adsorption of MB dye which mitigates two problems simultaneously: (i) disposal of used membranes and (ii) textile wastewater treatment. Batch adsorption experiments indicated that the MB dye adsorption onto the recycled PVDF beads depended on adsorbent dosage, initial concentration, contact time, and pH. The adsorption kinetics study of the MB dye adsorption followed the PSO model, indicating chemisorption. Furthermore, the adsorption isotherm study showed that the adsorption process followed the Langmuir isotherm showing the monolayer adsorption of MB dye on recycled PVDF beads. The PSO model's equilibrium adsorption capacity was 8.56 mg g⁻¹, and the experimental equilibrium adsorption capacity was 8.30 mg g⁻¹. The maximum adsorption capacity was 27.86 mg g⁻¹. The DFT study predicted the adsorption energy of –64.7 kJ mol⁻¹ for MB-PVDF, indicating the strong interaction between MB dye and PVDF. Moreover, the gap between the HOMO and LUMO of PVDF upon interaction with MB dye decreased from 9.42 eV to 0.50 eV due to the mixing of molecular orbitals. The present study reveals the recycled PVDF beads’ potential for textile wastewater dye removal at a low cost.

The CSIR-CSMCRI PRIS number for this manuscript is 109/2022. The authors are grateful for partial funding support from the Council of Scientific and Industrial Research, India (MLP-0043). The authors acknowledge AED&CIF division, CSIR-CSMCRI for providing instrumental facilities. The authors also thank Dr B. Ganguly, CSIR-CSMCRI, for his help in theoretical calculations. The comments from anonymous reviewers and the editor have greatly improved the content.

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

The authors declare there is no conflict.