Tailoring pore size to enhance dissolution of layered double oxides for efficient nitrogen and phosphorus recovery via crystallization of struvite from wastewater.

Nitrogen and phosphorus recovery from wastewater to produce struvite fertilizer can simultaneously alleviate water eutrophication and phosphorus scarcity. Coupled magnesium mineral dissolution and reprecipitation has been emphasized for struvite formation by avoiding excessive Mg salts addition and pH adjustment. However, the recovery efficiency is limited by the low solubility of magnesium mineral. Herein, we capitalized on the concept of "pore-size controlled solubility (PCS)" to enhance the dissolution of MgAl-layered double oxide (MgAl-LDO), where dissolution rate is inversely related to pore size. By reducing the pore size of LDOX from 9.2 to 5.5 nm, the magnesium release from LDOX dissolution increased from 64.0 to 99.0 mg/L, accompanied by OH- release to maintain an alkaline condition. This resulted in effective recovery of NH4-N (80.54%) and PO4-P (80.57%) as high-purity struvite. Scanning electron microscope revealed that LDO acted as seeded surfaces that significantly reduced struvite nucleation barrier by controlling local supersaturation and interfacial energy. Moreover, the recovery is stable across wide pH ranges (5.0-10.0), attributable to the buffering capacity of Al in LDOX. Finally, the feasibility of NH4-N and PO4-P recovery was successfully applied to real aquaculture wastewater. This study offers an effective strategy to enhance mineral dissolution-precipitation for resource recovery.