Enhanced agronomic performance of microbially bioactivated Ca-based phosphate precipitates recovered from metal-finishing wastewater

Phosphorus recovery from industrial wastewaters offers a promising pathway toward circular nutrient management; however, chemically precipitated phosphates typically exhibit low agronomic bioavailability. In this study, phosphate precipitates recovered from metal-finishing electropolishing wastewater via Ca-, Fe-, and Al-based chemical precipitation were microbially bioactivated using phosphate-solubilizing bacteria ( Pseudomonas stutzeri and Priestia megaterium ) and evaluated as waste-derived fertilizers in wheat pot experiments under controlled conditions. Mineralogical analyses revealed that Ca-based precipitates were predominantly low-crystalline to amorphous, whereas Al-based materials exhibited higher crystallinity. Microbial bioactivation significantly enhanced phosphorus availability and plant performance, with Ca-based precipitates showing the strongest response (up to 108% increase in P uptake and 2.3-fold biomass improvement). In contrast, Fe-based precipitates exhibited limited responsiveness. Despite high phosphorus content, trace metal concentrations (Zn, Ni, Cr) exceeded regulatory thresholds, highlighting the need for post-treatment prior to agricultural application. These findings demonstrate that microbial bioactivation can upgrade Ca-rich precipitates into functional circular fertilizers, while emphasizing the importance of material stabilization strategies for safe reuse.