Electrochemical Phosphorus Mobilization from Municipal Biosolids: Colloid-Facilitated Nano-Hydroxyapatite Synthesis

Abstract Phosphorus scarcity threatens global food security as finite phosphate rock reserves decline. Municipal biosolids contain substantial phosphorus reserves that predominantly exist as particulate and organic compounds inaccessible to standard precipitation methods. This work presents a novel colloid-facilitated approach that recovers nano-Hydroxyapatite (nano-HAp) from biosolids through low-voltage electrochemical pretreatment followed by ambient calcium oxide (CaO) addition. Alkaline electrolysis (±2 V) converts biosolid aggregates into colloids that simultaneously release orthophosphate (PO43–; reported as P–PO43–, the phosphorus contained in orthophosphate), achieving 413 ppm, a 63% increase over chemical treatment, and are hypothesized to serve as heterogeneous nucleation sites for nano-HAp formation. This electrochemical conditioning enables nano-HAp formation (SEM-measured median feature size of 71.7 nm) at a Ca/P molar ratio of 3 without an additional pH-adjustment step before precipitation, achieving 86% recovery of the PO43– present in the supernatant within 30 min, corresponding to 25.3% of total phosphorus in the raw biosolids being recovered in the final product. Unlike chemical treatment, which yielded large agglomerates, SEM observations of the electrochemically derived product indicate the presence of nanoscale structural features within agglomerated particles. Calcination at 700 °C increased the phosphorus content from 5.71 to 12.13 wt %, with nanoscale structural features still observed by SEM within agglomerated particles. This integrated electrochemical solubilization and calcium-induced recovery process establishes a promising pathway for converting phosphorus-rich waste streams into value-added nano-structured fertilizers while advancing circular economy principles in wastewater treatment.