Efficient recovery of Cr(VI), Au(III), and Pd(II) using physically crosslinked aminoclay/polyvinyl alcohol composite microgels generated by microfluidics

Chemically crosslinked hydrogel adsorbents are typically produced using highly toxic agents for crosslinking, posing risks to both human health and the environment. In this study, we successfully demonstrated the fabrication of a physically crosslinked Fe–aminoclay (AC)/polyvinyl alcohol (PVA) composite microgel. This was achieved using a microfluidic approach to generate water-in-oil-in-water (W/O/W) double-emulsion droplets, eliminating the need for chemical crosslinkers. Compared to its chemically crosslinked counterpart, this physically crosslinked Fe–AC/PVA microgel showed superior performance in recovering the metals Cr(VI), Au(III), and Pd(II) from an aqueous solution. This enhanced capability was attributed to the abundance of free amine functional groups and a microgel structure that was conducive to metal ion adsorption. The physically crosslinked microgel demonstrated a maximum adsorption capacity of 203.7 mg/g for Cr(VI), 241.3 mg/g for Au(III), and 275.8 mg/g for Pd(II), compared to 144.3, 181.1, and 222.5 mg/g, respectively, for the chemically crosslinked microgel. The adsorption isotherms and kinetics for these metals conformed to the Sips isotherm and pseudo-second-order models, respectively, suggesting that chemical adsorption was the dominant mechanism. Furthermore, embedding magnetic nanoparticles in the Fe–AC/PVA microgel enabled straightforward separation and regeneration from an aqueous solution. The Fe–AC/PVA composite microgel also demonstrated excellent reusability, maintaining an adsorption efficiency of over 78 % across five consecutive cycles.