Enhanced Phosphate Sequestration by Alginate-based Aerogel Granules Functionalized with Nanoscale Zerovalent Iron

Polymeric aerogels, with their versatile physicochemical properties and capacity for functionalization, are innovative materials being increasingly explored for water treatment applications. In this study, novel millimetric sized alginate-based aerogel granules functionalized with nZVI (nanoscale zero-valent iron) were developed and evaluated for their phosphate sequestration performance. Efficient phosphate removal from water is critical as excessive levels of phosphates can lead to eutrophication and negatively impact water quality. nZVI-aerogel granules exhibited significant enhancements in phosphate removal efficiencies (up to 97%) compared to non-functionalized bare-aerogel granules (15%). Average Langmuir removal capacities of 77 mg-PO₄³⁻/g were observed consistently for nZVI-aerogel granules across a broad pH range from 3 to 7, which further increased under alkaline conditions reaching up to 180 mg-PO₄³⁻/g at pH 11. Kinetic studies were well described by the pseudo first-order kinetic model in the pH 3–7 range, with rates declining from 0.11 h⁻¹ to 0.07 h⁻¹ as pH increased. In contrast, mixed kinetic trends were observed in alkaline pH with rapid phosphate removal followed by a short-term desorption. Solution pH measurements, and analysis of nZVI-aerogel granule surface chemistry and morphology post batch experiments revealed the involvement of multiple sequestration mechanisms including electrostatic adsorption, ion exchange, and surface precipitation. nZVI-aerogel granule morphology remained stable under all tested conditions (except at pH 11) suggesting their strong potential for facilitating efficient post-treatment separation and recovery.