Bio-based cellulose membrane from Phlomis tuberosa L. for methylene blue dye removal from wastewater and molecular interaction mechanisms

Abstract

The rapid rise in the use of synthetic dyes, particularly cationic dyes such as methylene blue (MB), has raised serious environmental and public health concerns. Effective removal of these pollutants from wastewater requires adsorbents that are both efficient and sustainable, positioning bio-based cellulose membranes as a promising alternative. In this study, cellulose was extracted from the stems of Phlomis tuberosa L. (P. tuberosa), an indigenous plant of Kazakhstan, and subsequently fabricated with polyvinylidene fluoride (PVDF). The incorporation of PVDF enhanced the mechanical strength, thermal stability, and structural integrity of the membranes, while also improving their reusability and operational durability without compromising adsorption performance. Through physicochemical analysis such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX), and Transmission electron microscopy (TEM), cellulose isolation and its fabrication with PVDF membrane was successfully confirmed. Prominent functional groups corresponding to O–H, C = O, C–H, and N-H, were detected by FTIR which confirms effective fractionation of lignocellulosic biomass components. SEM and EDX images showed well-defined morphology with less agglomeration while XRD analysis indicated enhanced crystallinity at 22.35°, supporting the stability of the synthesized membranes. From adsorption tests, maximum adsorption capacity of 94 mg/g was obtained from the optimized C/PVDF_M (90:10) membrane for MB adsorption at pH 7 within 100 min proving the best choice amongst the synthesized membranes. The pseudo-second-order kinetics and Langmuir isotherm model best described the adsorption mechanism while molecular dynamics (MD) simulations and density functional theory (DFT) calculations elucidated the role of electrostatic interactions, robust hydrogen bonding, and van der Waals forces in facilitating MB adsorption onto the membrane surface. These findings confirm that cellulose membranes derived from P. tuberosa are a renewable and high-performance material for the removal of cationic dyes from wastewater.