Effect of composition ratio on chitosan-gum Arabic copolymers for methylene blue dye adsorption: comparative evaluation using a Box–Behnken model

The discharge of synthetic dyes into aquatic environments remains a significant environmental concern because of their colour persistence, potential toxicity, and negative effects on water quality. In this study, three chitosan-gum Arabic materials with different composition ratios (Copolymer A, B, and C) were prepared under mild conditions and comparatively evaluated for methylene blue adsorption. The materials were characterised using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray diffraction (XRD). FTIR spectra showed shifts and intensity changes in hydroxyl, amide, and carboxylate-related bands, suggesting intermolecular interactions between chitosan and gum Arabic. SEM images indicated composition-dependent surface texture, while XRD patterns showed changes in structural ordering among the prepared materials. Adsorption of methylene blue was investigated using a Box-Behnken response surface design with four variables: adsorbent dosage (0.2–1.0 g), pH (3–13), contact time (30–240 min), and temperature (25–60 °C). The statistical models showed different levels of adequacy. For Copolymer A, the significant lack-of-fit and negative predicted R² indicated limited predictive reliability, and the model was therefore interpreted cautiously. Copolymer B followed a linear trend in which contact time was the most influential factor, while Copolymer C gave the most acceptable model response, with a significant pH-dosage interaction and non-significant lack-of-fit. Experimental validation gave removal efficiencies of 44.3%, 38.8%, and 59.8% for Copolymers A, B, and C, respectively. Although the removal efficiencies were moderate compared with highly modified adsorbents reported in the literature, the results demonstrate that composition ratio influences the structural features and adsorption response of chitosan-gum Arabic systems. The prepared biopolymer materials therefore provide a low-cost and environmentally benign platform for further development in dye-removal applications.