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%.

A new activated adsorbent was produced from the debris of Posidonia oceanica rhizomes (POR). POR were activated in acetic acid and utilized as an eco-adsorbent for the removal of cationic dye methylene blue (MB) from saline solutions. The purified Posidonia oceanica rhizomes (PPOR) and its activated form (APOR) were characterized by elemental analysis, pH-metric titration, Fourier transformer infrared (FTIR), and surface area measurements, which inferred a remarkable activation of APOR. An enhancement in the free acidic sites was confirmed. The adsorption data obtained were analyzed using Langmuir, Freundlich, Temkin, Dubinin-Kaganer-Raduskavich (DKR), and Redlich and Peterson (RP) isotherm models. The obtained data from these isotherm models were tested using some error functions (residual root mean squares error (RMSE), sum square error (SSE), and chi-square test (X2) function). Temkin isotherm model was the best isotherm fits the experimental data of APOR. Kinetic data were evaluated by pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion models. The adsorption rate was found to follow PSO model with a good correlation (R2 = 0.999–1). A suggested, endothermic, multilayer, combined electrostatic and physical adsorption mechanism may be responsible for the removal of MB from water utilizing APOR. Adsorption is anticipated to start with chemisorption on active functional groups of adsorbent’s surface followed by physisorption of the subsequent layers through adsorbate–adsorbate interaction. The removal process was successfully applied for MB-spiked saline and brackish water with removal efficiencies of 51.7–97.2%. The results revealed that activated Posidonia oceanica rhizomes is a promising adsorbent for the removal of the methylene blue dye from real saline and brackish water with high removal efficiencies.

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