# Molecularly Engineered Aza-Crown Ether Functionalized Sodium Alginate Aerogels for Highly Selective and Sustainable Cu2+ Removal

## TL;DR

A new type of biopolymer aerogel is developed for highly selective and sustainable removal of copper ions from water.

## Contribution

A green synthesis method creates aza-crown ether functionalized sodium alginate aerogels with precise Cu2+ selectivity and reusability.

## Key findings

- ACSA aerogels showed a Langmuir maximum adsorption capacity of 150.82 mg·g−1 for Cu2+.
- The material retained over 80% adsorption capacity after four regeneration cycles.
- Cu2+ selectivity arises from ionic radius alignment and N/O chelation within the aza-crown ether framework.

## Abstract

Developing sustainable and molecularly selective adsorbents for heavy-metal removal remains a critical challenge in water purification. Herein, we report a green molecular-engineering approach for fabricating aza-crown ether functionalized sodium alginate aerogels (ACSA) capable of highly selective Cu2+ capture. The aerogels were synthesized via saccharide-ring oxidation, Cu2+-templated self-assembly, and reductive amination, enabling the covalent integration of aza-crown ether motifs within a hierarchically porous biopolymer matrix. Structural analyses (FTIR, 13C NMR, XPS, SEM, TGA) confirmed the in situ formation of macrocyclic N/O coordination sites. Owing to their interconnected porosity and chemically stable framework, ACSA exhibited rapid sorption kinetics following a pseudo-second-order model (R2 = 0.999) and a Langmuir maximum adsorption capacity of 150.82 mg·g−1. The material displayed remarkable Cu2+ selectivity over Zn2+, Cd2+, and Ni2+, arising from the precise alignment between Cu2+ ionic radius (0.73 Å) and crown-cavity dimensions, synergistic N/O chelation, and Jahn-Teller stabilization. Over four regeneration cycles, ACSA retained more than 80% of its original adsorption capacity, confirming excellent durability and reusability. This saccharide-ring modification strategy eliminates crown-ether leaching and weak anchoring, offering a scalable and environmentally benign route to bio-based adsorbents that combine molecular recognition with structural stability for efficient Cu2+ remediation and beyond.