Reactive dye adsorption and recovery using a PVA/TiO2 film containing Saccharum spontaneum flower-fibre cellulose: kinetic model discrimination beyond pseudo-second-order

Reactive dyes are difficult to remove from textile effluent, and adsorbents made from discarded plant fibres offer a low-cost option. Cellulose microcrystals were isolated from the flower fibre ofSaccharum spontaneum (catkin) and incorporated with TiO2 nanoparticles into a citric acid-crosslinked poly(vinyl alcohol) film prepared by solution casting. The film was characterized by FTIR, XRD, particle size analysis, FE-SEM with EDX, DSC, swelling, tensile and color measurements. In batch tests with the anionic reactive dye Avitera Light Red SE, 0.5 g of film in 50 mL removed 37 to 41 % of the dye at 20 to 60 mg L−1 after 170 min. The kinetic and equilibrium data were analyzed by nonlinear model discrimination over eleven rate laws and three isotherms, with leave-oneout validation and propagation of calibration uncertainty. Fast initial uptake followed by slow intraparticle diffusion described the kinetics best (Akaike weight 0.77), and pseudo-second-order kinetics received little support once the definitional origin point was excluded from the fit. Equilibrium uptake followed a near-linear Freundlich isotherm (1/n = 1.21, 95 % profile interval 1.06 to 1.38) with no saturation in the tested range. A preregistered simulation of 95,000 synthetic kinetic experiments showed that the eight-point sampling design identifies single-rate laws reliably but cannot fix the form of a slow second phase, and that a physics-informed neural network did not improve on the classical description at this data density. Adsorbed dye was released by heating the film in water at 80 °C, and the film could be reused.