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

Industrial activities release pollutants into aquatic environments characterized by high Chemical Oxygen Demand (COD), toxic salts, and pH-altering agents that limit the usability of water resources. Methylene blue (MB)—a widely used cationic dye—poses serious environmental and health risks due to its toxicity and resistance to degradation. This study evaluated the potential of raw periwinkle shell (PS), an abundant low-cost agricultural waste, as a biosorbent for removing the cationic dye methylene blue (MB) from aqueous solutions. PS was collected, washed, oven-dried, ground, and sieved (1–2 mm) and characterized by SEM and FTIR. Batch adsorption experiments (4.0 g PS, 100 mL solution; initial MB 25–300 mg·L⁻¹; pH 3–10; T 308–323 K; contact time 5–180 min) were performed to examine the effects of pH, temperature, contact time, and initial concentration. MB uptake increased with solution pH and temperature and reached equilibrium at ≈90 min. Kinetic analysis showed the pseudo-second-order model best described the data, indicating that chemisorption predominantly controls the rate. Equilibrium data were fitted to Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms; the Langmuir model provided the best fit (q_m ≈ 2.08 mg·g⁻¹, R² = 0.9753).