Separations (Apr 2024)

Phosphate Recovery Mechanism from Low P-Containing Wastewaters via CaP Crystallization Using Apatite as Seed: Seed Adsorption, Surface-Induced Crystallization, or Ion Clusters Aggregation?

  • Xiaobao Nie,
  • Yinan Li,
  • Junli Wan,
  • Shuai Ouyang,
  • Zhengbo Wang,
  • Guoqi Wang,
  • Heng Jiang

DOI
https://doi.org/10.3390/separations11050138
Journal volume & issue
Vol. 11, no. 5
p. 138

Abstract

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Low P-containing wastewaters (LPWs) exhibit huge P recovery potential, considering their larger volume. P recovery via CaP crystallization using apatite as seed is documented as being potentially well suited for LPWs. However, its responsible mechanisms remain a subject for debate. Taking hydroxyapatite (HAP) as the seed of LPWs, this paper conducted HAP adsorption/dissolution experiments, titration experiments, and P recovery experiments to distinguish the primary responsible mechanism. Results showed that it was HAP dissolution, not P adsorption, that occurred when the initial P concentration was no higher than 5 mg/L, ruling out adsorption mechanism of P recovery from LPWs using HAP as the seed. Significant OH− consumption and rapid P recovery occurred simultaneously within the first 60 s in titration experiments, suggesting CaP crystallization should be responsible for P recovery. Moreover, the continuous increase in P recovery efficiency with seed dosages observed in P recovery experiments seemed to follow well the mechanism of pre-nucleation ion clusters (PNCs) aggregation. During PNCs aggregation, P aggregates with Ca2+ quickly, generating CaP PNCs; then, CaP PNCs aggregate with seed particles, followed by CaP PNCs fusion, and ultimately transform into fines attached to the seed surface. PNCs’ aggregation mechanism was further supported by a comparison of seed SEM images before and after P recovery, since denser and smaller rod-shaped fines were observed on the seed surface after P recovery. This study suggests that PNCs’ aggregation is the dominant mechanism responsible for the recovery of P from LPWs via CaP crystallization using HAP as the seed.

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