<p>This work reports a cryogenically assisted approach to modify Posidonia Oceanica sea balls using liquid nitrogen and dry CO₂, intended to enhance Methylene blue (MB) removal. Adsorption equilibrium and kinetics clarified capacities and mechanisms while Doehlert response surface optimized pH, MB concentration, and contact time. Equilibrium studies demonstrated that Langmuir isotherm provided the best fit, with maximum adsorption capacities increasing from 30.8 mg g⁻¹ at pH 3 to 62 mg g⁻¹ at pH 11, indicating monolayer adsorption. Kinetic analysis revealed that MB adsorption followed the pseudo-second-order model, suggesting that adsorption is controlled by surface interactions. Among the tested materials, N₂-cryo–modified Posidonia exhibited the best performance, achieving 98.3% removal efficiency and the fastest rate constant (k₂= 0.0295&#xa0;g mg⁻¹min⁻¹). Process optimization via Doehlert design and response surface methodology identified initial dye concentration and contact time as the most influential factors. The agreement between isotherm-derived maximum capacity and statistically optimized conditions underscores the robustness of cryogenically modified Posidonia as a low-cost adsorbent for MB removal. N₂-cryo–modified Posidonia showed a higher adsorption capacity <i>(</i>62 mg g^− 1), with regards to CO₂-cryo–modified (37.5 mg g^− 1) and raw biomass (24.3 mg g^{− 1}). In fixed-bed column experiments, cryogenically modified Posidonia maintained &gt; 80% efficiency after five regeneration cycles, with &gt; 70% regeneration achieved using only 30 mL of acetic acid–water solution. These results demonstrate that cryogenic modification offers a promising and environmentally benign strategy to enhance the adsorption performance of raw marine biomass, supporting its potential application as a sustainable bio-based adsorbent for green wastewater treatment.</p> Graphical abstract <p></p>