Unravelling key factors controlling homogeneous crystallization during phosphorus recovery: From the perspective of crystallization kinetics

Phosphorus (P) recovery from wastewater is a critical process for addressing environmental concerns associated with P pollution and promoting resource sustainability through the closure of the P cycle. However, the efficiency of P recovery is often hindered by homogeneous crystallization. This study aims to elucidate the role of homogeneous crystallization in hydroxyapatite (HAP) formation and its influence on P recovery efficiency. We employed pH monitoring to assess the induction time for homogeneous crystallization, while simultaneously tracking changes in calcium and P concentrations during both formation and transformation stages. Our findings indicate that adjusting operational parameters significantly influences P removal efficiency and the induction time for phase transition in the crystallization process. Specifically, increasing the pH from 8 to 10 resulted in an enhancement of P removal efficiency, rising from 36 % to 99 %. However, this increase in pH corresponded with a substantial prolongation of the induction time for phase transition, extending from 0.53 h to 15.8 h. In contrast, raising the temperature from 15 °C to 35 °C improved P removal efficiency from 23.62 % to 56.80 % while concurrently decreasing the induction time for phase transition from 10.45 h to 1.31 h. The transformation to HAP occurs primarily through the dissolution-recrystallization of ACP. By carefully regulating operational parameters, we can control the formation and transformation of the homogeneous precipitate, thereby enhancing P recovery efficiency. This research provides valuable insights into the composition, formation, and transformation mechanisms of fine particles during P recovery, while also offering a novel perspective for assessing the risks associated with homogeneous precipitates.