Marine waste-derived hydrochar beads for circular phosphate recovery: site energy distribution, molecular interactions and life cycle assessment

Sustainable management of phosphate-laden wastewater requires functional materials that pair efficient removal with practical recyclability. Here, a magnesite-modified seaweed hydrochar–chitosan composite hydrogel bead (Mg@SHC/CS) was prepared by combining hydrothermal carbonization of marine biomass waste with chitosan-mediated gelation, converting two low-cost feedstocks into a shaped and recoverable adsorbent. The resulting beads exhibited a hierarchically porous structure, rapid phosphate uptake under near-neutral conditions, good tolerance toward common competing anions, and stable regeneration, retaining 87.03% of the initial adsorption capacity after five adsorption–desorption cycles. Equilibrium data were best described by the Sips model, with the maximum adsorption capacity increasing from 73.41 to 85.35 mg P/g as the temperature rose from 298 to 318 K. Site energy distribution (SED) analysis confirmed preferential occupation of high-energy sites at low coverage, in line with energetically heterogeneous binding under a finite-capacity constraint. Spectroscopic characterization combined with density functional theory (DFT) and independent gradient model based on Hirshfeld partition (IGMH) calculations revealed that phosphate immobilization was governed mainly by Mg–O centers, carboxyl groups, and amino/ammonium functionalities through inner-sphere complexation, ligand exchange, hydrogen bonding, and electrostatic interactions. Cradle-to-gate life cycle assessment identified chitosan and Mg@SHC precursor preparation as the principal environmental hotspots, pointing to priorities for greener process design. These results establish Mg@SHC/CS as an effective, recyclable adsorbent for phosphate recovery and provide mechanistic and preparation-stage sustainability insights for the design of waste-derived water treatment materials.