Optimization and Mechanistic Study of Methylene Blue Adsorption by Periwinkle Shell: Effects of pH, Concentration and Contact Time.

This study investigated the potential of periwinkle shell (PS) as a low-cost biosorbent for the removal of methylene blue (MB) from aqueous solution. The objective was to evaluate its adsorption performance, kinetics, equilibrium behavior, and thermodynamic characteristics. The adsorbent was characterized using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy to determine its surface morphology and functional groups. Batch adsorption experiments were conducted using 4.0 g of PS in 100 mL of MB solution under varying conditions of initial dye concentration (25–300 mg·L⁻¹), pH (3–10), temperature (308–323 K), and contact time (5–180 min). Adsorption kinetics were analyzed using appropriate kinetic models, while equilibrium data were fitted to Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. Thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were calculated to evaluate the nature of the adsorption process. SEM analysis revealed an irregular and highly porous surface structure, and FTIR spectra confirmed the presence of oxygen-containing functional groups capable of interacting with MB molecules. Adsorption capacity increased with increasing pH and temperature, and equilibrium was attained at approximately 90 min. While equilibrium capacity increased with higher initial concentration, percentage removal decreased at elevated dye loadings due to site saturation. The pseudo-second-order kinetic model provided the best fit, indicating that chemisorption predominantly controls the adsorption rate. Among the isotherm models, the Langmuir model showed the best fit (qₘ ≈ 2.08 mg·g⁻¹; R² = 0.9753), suggesting monolayer adsorption. Thermodynamic analysis revealed negative ΔG° values, positive ΔH° (≈ +61.9 kJ·mol⁻¹), and positive ΔS° (≈ +168.6 J·K⁻¹·mol⁻¹), confirming that the adsorption process is spontaneous, endothermic, and associated with increased interfacial randomness. Periwinkle shell demonstrates potential as an economical biosorbent for methylene blue removal from aqueous media, although its adsorption capacity is relatively modest compared with activated or chemically modified materials.