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Dead leaves of seagrass Posidonia oceanica were activated by using one mol L −1 acetic acid and used as an eco-adsorbent for the removal of methylene blue (MB) and Pb 2+ from aqueous solutions. The seagrass was characterized by chemical and physical measurements that confirmed the acid-activation of seagrass. The favourable conditions for MB and Pb 2+ adsorption onto the activated seagrass (SG a ) were determined to be a pH range of 2–12 and ≥6, an adsorbent dosage of 3.0 and 0.5 g L −1 , respectively, and a shaking time of 30 min, which are suitable for a wide range of wastewaters. The equilibrium data were analysed using the Langmuir, Freundlich and Dubinin-Raduskavich-Kaganer (DRK) adsorption isotherm models. The Freundlich and DRK models best describe the adsorption processes of MB and Pb 2+ , on SG a with capacities of 2681.9 and 631.13 mg g −1 , respectively. The adsorption isotherm fitting and thermodynamic studies suggest that the adsorption mechanism of MB may combine electrostatic and physical multilayer adsorption processes, in which MB may be present as monomers as well as dimers and trimers which were confirmed from UV spectroscopy whereas Pb 2+ is chemically adsorbed onto SG a . The pseudo-2 nd -order kinetic model was utilized to investigate the kinetics of adsorption processes. The removal process was successfully applied for MB-spiked brackish waste water from Manzala Lake, Egypt, with removal efficiencies of 91.5–99.9%.

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