Activated eco-waste of Posidonia oceanica rhizome as a potential adsorbent of methylene blue from saline water

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 ( X 2 ) 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 ( R 2 = 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. Graphical Abstract