Coupling the Batch Adsorption Enrichment using Ternary LDHs with Struvite Crystallization in a Fluidized Bed Reactor for High-efficiency Phosphorus Recovery from Wastewater

Adsorption represents a well-documented, effective and reliable means for phosphorus (P) recovery from waste streams, and is often followed by chemical precipitation for attaining value-added fertilizers or feedstocks. To assess the feasibility of recovering P from water by combining batch adsorption enrichment with struvite crystallization, we prepared four ternary layered double hydroxides (LDHs) with P-preferring elements (i.e., zirconium (Zr) or lanthanum (La)) via a facile coprecipitation method, and then evaluated their performance in capturing P from water, particularly in enriching P from a low-level P solution. We find that P adsorption on all ternary LDHs is pH-dependent and ionic strength-independent, showing a maximum adsorption efficiency at pH ~5 regardless of the ionic strength. Besides, all ternary LDHs demonstrate remarkably high P adsorption capacities, i.e., 842.2, 958.8, 499.6 and 1029.3 mg P g−1 under a certain condition for ZnFeZr, ZnFeLa, ZnAlZr and ZnAlLa, respectively, outperforming other LDHs reported so far. Microstructural analyses show that all ternary LDHs have high stability against the acidic or basic solution, and that the P uptake mechanisms are attributable to anion exchange between P and intercalated nitrate ions, complexation at both the edge of LDHs and the surface of metal (hydr)oxides co-occurred, and electrostatic attraction. Results of recycling tests indicate that all ternary LDHs present good enrichment for P, with enrichment factors above 2.6 after only five adsorption-desorption cycles. In addition, more than 96% of the phosphorus in the P-enriched eluates can be efficiently reclaimed via struvite crystallization in a fluidized bed reactor at an Mg:N:P ratio of 2:5:1 in the feed solution. These findings demonstrate the feasibility of combining adsorption enrichment with struvite crystallization for P recovery.