Sustainable biosorption of methylene blue using highly-efficient activated Mediterranean macroalgae: a comparative mechanistic study

Synthetic dyes, particularly methylene blue (MB), are persistent aquatic pollutants that are poorly removed by many conventional wastewater-treatment methods. Herein, we report the first comparative study on the use of chemically activated marine macroalgae collected from the Lebanese Eastern Mediterranean coastline as sustainable biosorbents for MB removal. The novelty of this work lies in valorizing locally available Mediterranean algal biomass, Ulva lactuca , Ulva prolifera , Bryopsis hypnoides and Corallina officinalis , and systematically comparing their adsorption performance after chemical pretreatment, with particular emphasis on CaCl 2 activation as a simple and low-cost modification strategy. The algal biomasses were pretreated with CaCl 2 , NaOH, or HCl, and batch biosorption experiments were conducted to evaluate the effects of pH, initial dye concentration, biosorbent dosage, and contact time. Adsorption behavior was analyzed using Langmuir, Freundlich, and Temkin isotherm models, while kinetic data were fitted using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. All four macroalgae exhibited efficient MB removal, with CaCl 2 -treated Ulva prolifera displaying the highest Langmuir adsorption capacity, reaching a remarkable 653 mg/g. Optimal biosorption occurred over a broad pH range, with rapid uptake during the first 30 min and near-equilibrium within 90 min. Kinetic modeling showed superior agreement with the pseudo-second-order model, indicating that surface interactions, ion exchange, and chemisorption-like processes contribute significantly to the MB uptake. This study demonstrates that CaCl 2 -activated Mediterranean macroalgae represent highly efficient, locally sourced, and low-cost biosorbents for cationic dye removal, supporting their potential integration into sustainable phycoremediation strategies for industrial wastewater treatment.