Sustainable Valorization of Posidonia Waste Ash for Phosphate Removal: A Surface Complexation Approach Under Variable...

Phosphorus pollution represents a persistent and significant threat to aquatic ecosystems, particularly within the Mediterranean region, where ongoing eutrophication continues to compromise both water quality and biodiversity. Concurrently, the accumulation of Posidonia oceanica residues along coastal areas presents a biomass management challenge. This study explores the sustainable use of thermally treated Posidonia ash as a low-cost, bio-based adsorbent for phosphate removal from water. Batch experiments under varying phosphate concentrations, pH, hardness, and alkalinity revealed high removal capacities (33.5-58.7 mg/g). A novel surface complexation model (SCM) was developed and validated using spectroscopic techniques to elucidate the mechanisms of phosphate retention. The SCM outperformed conventional isotherm models by providing mechanistic insights into adsorption behavior. Phosphate adsorption was found to be pH-dependent, occurring via surface complexation to neutral and basic surface sites. The release of Ca²⁺ and Mg²⁺ ions facilitated ternary complex formation and precipitation. Under alkaline conditions, competitive adsorption between phosphate and carbonate ions was observed. This study demonstrates the dual benefit of Posidonia oceanica ash: efficient phosphate removal and its reuse as a phosphorus reservoir, offering a circular strategy for tackling nutrient pollution and promoting coastal biomass valorization.