Phosphorus (P) is an essential nutrient for crops, but its excess in discharge water harms both surface water and groundwater quality. A cost-effective and eco-friendly adsorbent is desirable to meet circular economy criteria by effectively removing P from water and being safely recycled for agricultural use. Thus, this study aims to synthesize an amine-functionalized magnetic lignin nanocomposite biosorbent by first grafting poly­(ethyleneimine) on epoxidized lignin followed by coprecipitation with iron. This biosorbent shows an adsorption performance of 43 mg g–1, which is 20 times greater than the unmodified lignin reported in a previous research study and six times more than the magnetic iron metal. A series of characterization methods confirm the chemical features and the formation of a nanostructure. The pH, coexisting anions, and salt concentrations affect the P removal efficiency. The mechanism studies show that the electrostatic interaction between NH3 + functional groups and P, surface precipitation, and ligand exchange all count for P removal, which indicates the heterogeneous adsorption of P onto the sorbent surface by both chemisorption and physisorption. The seedling study confirms that the nanocomposite after P adsorption has better root development and biomass accumulation as a recycled fertilizer.

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