Chemical transformation and environmental fate of metal oxide nanomaterials in phosphate removal from water systems: a review.

Metal oxide nanomaterials have emerged as highly effective adsorbents for phosphate removal in water and wastewater treatment due to their large surface area, tunable surface chemistry, and strong affinity toward phosphate species. While most studies focus on adsorption performance and treatment efficiency, the chemical transformations that these nanomaterials undergo during phosphate removal remain underexplored. Exposure to phosphate-rich aqueous environments can induce surface complexation, partial phase conversion, and the formation of secondary nanomaterials with distinct chemical and structural properties. This review systematically examines the post-treatment behavior of iron, manganese, and aluminum oxide nanomaterials, emphasizing the interplay between phosphate speciation, surface hydroxyl groups, and material reactivity. Key transformation pathways, including surface passivation via metal phosphate formation, development of mixed oxide-phosphate structures, and changes in surface charge and aggregation behavior, are highlighted. The implications of these chemical modifications for environmental persistence, mobility, and potential metal release are discussed, underscoring challenges related to material regeneration, reuse, and disposal. By reframing phosphate removal as a dynamic process that alters nanomaterial chemistry, this review emphasizes the need for post-treatment characterization and transformation-aware design of nanomaterials. Understanding these chemical evolution processes not only informs safer and more sustainable water treatment practices but also guides the development of nanomaterials with optimized performance and minimal environmental impact. This review consolidates findings from the last decade, providing a comprehensive perspective on the evolving identity of metal oxide nanomaterials during phosphate remediation in aquatic systems.