Synergistic ozonation–ion exchange strategy for nutrient recovery from algal filtrate

Harmful algal blooms pose a growing threat to freshwater ecosystems due to nutrient over-enrichment. While mechanical separation of algal biomass is commonly employed, it often produces highly concentrated algal filtrate rich in algal organic matter (AOM), nitrogen, and phosphorus, leading to secondary pollution risks. In this study, a synergistic treatment approach combining short-duration ozonation and ion-exchange processes was investigated to effectively degrade AOM and recover nutrients from algal filtrate. Anion-exchange resins with quaternary ammonium groups, along with nanoconfined La(OH)₃-loaded resins, were utilized to achieve selective adsorption of nitrate and phosphate, respectively. Advanced spectroscopic techniques were employed to elucidate the structural transformations of AOM during ozonation. A 5-minute ozone treatment rapidly decomposed fluorophores, and the La(OH)₃-loaded resins achieved nearly 100% phosphate removal with excellent reusability with minimal decline (less than 5%) in efficiency over five adsorption–desorption cycles. Furthermore, the in situ transformation of LaPO₄ back to La(OH)₃ under alkaline conditions enabled efficient regeneration of the adsorbent. This study demonstrates a promising integrated strategy for algae-laden water treatment, offering both pollutant control and resource recovery.