Phosphorus removal from water with an iron functionalised biochar produced from sunflower seed husks: A circular economy-based solution for the agro sector

Phosphorus (P) depletion poses a growing threat to global food security. Approaches, consistent with the principes of the circular economy, in which P can be recovered and reutilised in agricultural systems are therefore urgently needed. Adsorption-based wastewater treatment offers a low-cost and straightforward method for P removal and recovery, particularly when highly efficient materials are employed. This study evaluated the production and application of iron (Fe)-functionalised biochar derived from sunflower seed husks, an abundant oil industry by-product, for phosphate (PO43−-P) removal from various aquatic matrices, including model water, multicomponent solutions, eutrophic lake water, and industrial wastewater, each containing 20 mg/L PO43−-P. Phosphate removal performance and adsorption mechanisms were investigated through batch experiments under varying pH, biochar dosage, contact time, initial PO43−-P concentration, coexisting anions, and temperature. An optimal biochar dose was 120 mg (4 g/L) achieved 90.9% PO43−-P removal after 7 h. The Langmuir (R2 = 0.97) isotherm best described adsorption equilibrium, while the Elovich model provided the best kinetic fit (R2 = 0.99), indicating a heterogeneous adsorption process. Thermodynamic analysis indicated PO43−-P adsorption was exothermic and spontaneous. The biochar was characterised using FESEM-EDX, XRD, FTIR, BET, XPS, and pHpzc. The biochar facilitated inner-sphere complexation with PO43−-P maintaining adsorption efficiency even in complex real-water systems. Furthermore, desorption with 0.1 M NaOH recovered over 96% of adsorbed PO43−-P, suggesting potential for reuse. The presence of loosely bound PO43−-P suggests potential use of the spent biochar as a fertiliser, although this requires confirmation through pot trials and toxicity assessments.