Dual interface-engineered bone char seeds for calcium phosphate crystallization

Conventional phosphate removal technologies for secondary effluent rely on external chemical agents, posing sustainability challenges. Crystallizing phosphate with the Ca²⁺ naturally present in wastewater offers a promising solution. However, nucleation requires the formation of new crystal-liquid interfaces, and the associated interfacial energy creates a high nucleation energy barrier (∆G*) that hinders this process. Here, we developed bone-derived char seeds, which decreased the effective interfacial energy for hydroxyapatite nucleation by 71.1 %, lowering the ∆G* by a factor of 41.5. This reduction achieved nucleation rates in near-neutral secondary effluent comparable to those requiring pH > 11 in conventional processes. Using only endogenous Ca²⁺ in wastewater, a bone char fixed-bed column treated over 20,000 bed volumes of actual secondary effluent while achieving a phosphate retention capacity exceeding 48.8 g P/kg of bone char-the highest reported for phosphate capture materials. This exceptional performance derives from bone char's unique structure: an inorganic carbonate apatite skeleton providing atomically matched templates for crystal nucleation, and an organic methyl-rich surface layer modulating hydrophobicity. These features respectively optimized the interfacial energies of the bone char-hydroxyapatite and bone char-liquid interfaces involved in hydroxyapatite nucleation. This dual-interface engineering effectively lowered the ∆G* and accelerated hydroxyapatite crystallization. Global Ca²⁺ distribution mapping shows that the technology has immense potential for application in approximately 74.3 % of global regions. This study charts a new trajectory for deep phosphate removal, promoting the formation of a more sustainable wastewater treatment framework.