Calcium-rich Sargassum derived adsorbents for phosphorus removal from synthetic urine: A step toward a circular economy

The recurrent accumulation of pelagic Sargassum spp. along the coasts of Cancún and the Riviera Maya has become a pressing environmental and socio-economic concern. In this study, we investigate a sustainable valorization route for this invasive biomass by converting it into calcium-rich adsorbents through calcination at 800 °C (designated as CSgs) for phosphorus (P) recovery from synthetic urine (705.56 mg P/L). Batch adsorption experiments were conducted with adsorbent dosages ranging from 0.5 to 15 g/L. The materials were characterized by FTIR, XRD, and SEM. CSgs demonstrated high P recovery efficiencies (up to 97 %) and an adsorption capacity of 46 mg P/g at the highest dosage. Phosphorus removal occurred via precipitation as apatite (Ca 5 (PO 4 ) 3 OH) at 15 g/L, and as brushite (CaHPO 4 •2H 2 O) at lower dosages (0.5–2.5 g/L). Langmuir isotherm modeling yielded maximum adsorption capacities ( Q max ) of 293 ± 16 mg P/g at pH 6 (brushite) and 158 ± 6 mg P/g at pH 10 (apatite). The recovered P showed high agronomic potential, with 94–97 % solubility in 2 % formic acid and 14–17 % in water. A preliminary cost analysis estimated that, under simulated urine conditions, CSgs achieved 97 % phosphorus recovery with an adsorption capacity of 46 mg/g, at a total cost of 5.99 USD per 0.5 m 3 treated, highlighting their economic feasibility for nutrient recovery applications. These findings support the development of decentralized nutrient recovery systems and promote circular economy strategies by transforming marine biomass waste into sustainable biofertilizers. • First demonstration of Ca-rich adsorbents from Sargassum for P recovery. • Achieved 97 % P removal from synthetic urine for sustainable fertilizer production. • Recovered Ca 5 (PO 4 ) 3 OH released 94 % of P under soil-like conditions. • Valorization of invasive Sargassum biomass supports circular economy strategies.