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

• Published in: Separations Vol. 11; no. 5; p. 138
• Main Authors: Nie, Xiaobao, Li, Yinan, Wan, Junli, Ouyang, Shuai, Wang, Zhengbo, Wang, Guoqi, Jiang, Heng
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.05.2024
• Subjects: Adsorption; aggregation; Apatite; Calcium ions; CaP crystallization; Clusters; Crystallization; Dissolution; Efficiency; Electrodes; Experiments; Fines; Halides; Hydroxyapatite; ion clusters; low P-containing wastewater; Nucleation; P recovery; Recovery; Titration; Wastewater; China
• ISSN: 2297-8739; 2297-8739
• DOI: 10.3390/separations11050138

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.