Magnesite-modified seaweed biochar-chitosan hydrogel beads for phosphate removal: adsorption mechanism and life cycle assessment.

Although seaweed waste and magnesite tailings are abundant, their co-utilization as precursors for phosphate adsorbents remains rarely explored. Herein, a high-efficiency and regenerable magnesite-modified seaweed biochar-chitosan hydrogel bead composite (Mg@SBC/CS) was developed for phosphate removal from wastewater. Under optimized conditions, Mg@SBC/CS exhibited a theoretical maximum adsorption capacity of 142.77 mg g-1, markedly higher than those of the unmodified materials, while also showing tolerance to common coexisting anions and acceptable capacity retention over five adsorption-desorption cycles. Density functional theory (DFT) calculations coupled with adsorption-energy analysis identified the NH2-functionalized site as the most favorable binding site for phosphate (Eads = - 44.6 kcal mol-1), followed by the hydroxyl group (-37.7 kcal mol-1) and the Mg-coordinated site (-18.6 kcal mol-1). Mechanistic investigations further revealed that phosphate removal proceeded through multiple pathways, including surface precipitation, coordination interactions, and electrostatic attraction. Interpretable machine-learning analysis based on tree-based models further showed that the initial phosphate concentration and contact time were the dominant variables governing adsorption performance, contributing ∼ 55% and ∼ 20%, respectively. A Python-based graphical user interface was additionally developed to facilitate experimental design and practical implementation. Life-cycle assessment indicated that the principal environmental impacts centered on marine and freshwater ecotoxicity, while carbon emissions were mainly associated with NaOH consumption and energy use. Overall, this work provides an effective waste-derived composite for phosphate removal and resource recovery, while offering mechanistic and sustainability insights for future optimization and greener fabrication.