Trimetallic Fe-Zn-Mn (Oxy)Hydroxide-Enhanced Coffee Biochar for Simultaneous Phosphate and Ammonium Recovery and Recycling

Excess phosphorus (P) and nitrogen (N) in wastewater contribute to eutrophication, driving the need for low-cost and sustainable recovery technologies. This study presents a novel adsorbent synthesized from spent coffee grounds biochar (CB) chemically modified with Mn²⁺/Zn²⁺/Fe³⁺ (oxy)hydroxide nanoparticles (CB-M) for simultaneous removal of phosphate and ammonium. Batch adsorption experiments using both synthetic solution and municipal wastewater were conducted to evaluate the material's adsorption performance and practical applicability. Kinetic, isotherm, thermodynamic, and sequential extraction analyses revealed that CB-M achieved maximum phosphate adsorption capacities ranging from 42.6 to 72.0 mg PO₄^3-·g^-1 across temperatures of 20-33 °C, reducing effluent phosphate concentrations to below 0.01 mg·L^-1. Ammonium removal was moderate, with capacities ranging between 2.8 and 2.95 mg NH₄⁺·g^-1. Thermodynamic analysis indicated that phosphate adsorption was spontaneous and endothermic, dominated by inner-sphere complexation, while ammonium uptake occurred primarily through weaker, reversible ion exchange mechanisms. Sequential extraction showed over 70% of adsorbed phosphate was associated with Fe-Mn-Zn phases, indicating the potential for use as a slow-release fertilizer. The CB-M retained structural integrity and exhibited partial desorption, supporting its reusability for nutrient recovery. Compared to other biochars, CB-M demonstrated superior phosphate selectivity at a neutral-pH, avoided the use of hazardous metals, and transformed coffee waste into a multifunctional material for wastewater treatment and soil amendment. These findings underscore the potential of CB-M as a circular economy solution for nutrient recovery without introducing secondary contamination.