Alkaline desorption mechanisms and fixed-bed cyclic performance of ZnAlLa-LDHs for phosphate removal

Eutrophication is a significant challenge in global water environmental management, with excessive phosphate discharge being one of its primary causes. Layered double hydroxides (LDHs) have attracted considerable attention as efficient phosphate removal adsorbents due to their excellent adsorption performance and tunable structure, making them a key focus of research for phosphate wastewater treatment. This study investigates the alkaline desorption mechanisms and fixed-bed cyclic performance of ZnAlLa-LDHs in phosphate removal. Experimental results demonstrated that approximately 74.4% of adsorbed phosphate could be desorbed using a mixed eluent (0.08 mol/L NaOH + 0.6 mol/L NaCl) through synergistic mechanisms including dissolution, ion exchange, ligand exchange, and electrostatic repulsion; meanwhile, 83.8% of the released phosphate could be recovered from the P-rich eluate as struvite. After five consecutive adsorption-desorption cycles, the adsorption capacity decreased by approximately 30% and then levelled off at 35.2 ± 0.5 mg P/g, mainly due to Al³⁺ leaching from the LDH lattice and the progressive transformation of ZnAlLa-LDHs into Zn(OH)₂/ZnO and La(OH)₃, accompanied by the formation of amorphous and/or poorly crystalline Al-containing phases. In the single fixed-bed column, the Yan model showed excellent agreement with dynamic breakthrough curves (R² > 0.96), and model interpretation suggested that external film diffusion dominated the initial adsorption stage, whereas intraparticle diffusion rapidly became the rate-controlling step as adsorption proceeded. The three-stage fixed-bed column achieved 93.8% average phosphate removal from 2 mg P/L wastewater (effluent <0.5 mg P/L) over five cycles, though the effective treatment volume decreased by 20%. The study concludes that ZnAlLa-LDHs are efficient materials for phosphate removal, with effective alkaline desorption and fixed-bed cyclic performance. However, their long-term application is constrained by the trade-off between phosphate release and adsorbent stability under alkaline desorption, highlighting the need for more selective and less destructive desorption strategies.