Fabrication of a PVA-encapsulated MCC/S–VO2 composite via melt intercalation for efficient fixed-bed adsorption of methylene blue

In this study, a novel multifunctional composite comprising polyvinyl alcohol (PVA), microcrystalline cellulose (MCC), sulfur, and vanadium dioxide (VO₂) was successfully synthesized through a solvent-free melt intercalation method. The process involved dispersing MCC/S-VO₂ material within a PVA matrix in the molten state, enabling homogeneous mixing and effective interfacial integration, and subsequently calcining the mixture at 300 °C. Calcination preserved the monoclinic phase of VO₂ and enhanced the composite's porosity through thermal decomposition of PVA and MCC, resulting in uniformly distributed active phases within black, granular adsorbents. X-ray diffraction (XRD) confirmed the sustained integrity of the crystalline monoclinic VO₂ phase. X-ray photoelectron spectroscopy (XPS) detected the elements carbon, oxygen, sulfur, and vanadium, along with carbonized polyvinyl alcohol (PVA), all exhibiting distinct spectral characteristics that point to strong interfacial interactions within the composite. Scanning and transmission electron microscopy (SEM and TEM) revealed a porous, layered architecture with VO₂ nanosheets uniformly distributed throughout the carbon-rich matrix. Continuous fixed-bed column experiments were conducted under varying operational parameters, including bed heights, initial methylene blue (MB) concentrations, and flow rates. The composite demonstrated exceptional adsorption efficiency, achieving a peak capacity of 47.7 mg g^-1 under optimal conditions (bed height: 0.5 cm, MB concentration: 20 mg L^-1, flow rate: 1 mL min^-1, column diameter: 1 cm). Breakthrough curve analysis confirmed the validity of the BDST model for performance prediction (R² > 0.9), whereas the Thomas and Yoon-Nelson models exhibited lower correlation coefficients (R² < 0.9), suggesting reduced applicability under the tested parameters. Notably, the adsorbent retained over 83.6% of its initial adsorption capacity after four regeneration cycles, underscoring its structural resilience and recyclability. The adsorption mechanism for MB onto the synthesized composite was attributed to π-π interactions, hydrogen bonding, and electron donation from the amine groups of MB to vanadium sites. This study, for the first time, demonstrates the use of solid PVA in a solvent-free melt intercalation process to fabricate polymer-inorganic hybrid adsorbents, offering a novel and eco-friendly strategy for advanced wastewater treatment.