Engineered MIL-100(Fe)/GO-alginate hydrogel beads: A sustainable platform for efficient methylene blue removal in...

Efficient removal of cationic organic dyes such as methylene blue (MB) from wastewater is essential to mitigate aquatic toxicity, protect ecosystems, and safeguard public health. Despite the exceptional adsorption potential of metal-organic frameworks (MOFs), their practical deployment remains severely constrained by poor mechanical stability, handling difficulties in powder form, challenging recoverability, and limited performance in complex matrices containing competing co-pollutants. To overcome these critical barriers, this work introduces a sustainable, mechanically robust hydrogel bead platform (MIL-100/GO@SA) fabricated by integrating a MIL-100(Fe)/graphene oxide (GO) composite into a sodium alginate (SA) matrix through a simple, green droplet-gelation process using only water as solvent and Ca²⁺ as crosslinker. This ternary design synergistically combines the porosity and abundant Fe(III)/carboxylate active sites of MIL-100(Fe), the π-electron-rich surface and mechanical reinforcement provided by GO, and the ion-exchange capacity and structural integrity of alginate, resulting in a negatively charged three-dimensional network. The multifunctional architecture enables highly efficient and selective MB removal through cooperative mechanisms, electrostatic attraction, π-π stacking, hydrogen bonding, and cation exchange, while demonstrating pronounced preference for cationic MB over anionic co-pollutants (methyl orange and indigo carmine) in binary mixtures, with negligible interference from competitors. This selectivity, driven predominantly by electrostatic interactions, highlights the material's potential for targeted remediation of cationic dyes in realistic wastewater scenarios. Furthermore, the beads exhibit excellent structural stability and retain high removal efficiency (>90%) over 13 regeneration cycles. Also, translation to continuous operation was confirmed through rapid small-scale fixed-bed column studies, which exhibited a high breakthrough capacity (1275 mL), excellent hydraulic performance, and stable dynamic adsorption behavior. Thus, this study presents a straightforward, scalable, and environmentally benign strategy to transform powdered MOFs into durable, recyclable macroscopic adsorbents, offering a practical and effective solution for cationic dye removal in both batch and continuous-flow wastewater treatment systems.