Lysine-cyclodipeptide-based polyamidoamine microparticles: Balance between the efficiency of copper ion removal and degradation in water

Abstract

A key requirement for materials that adsorb pollutants in aqueous media is the balance between efficiency and biodegradation owing to rising microplastic pollution. Hyperbranched polyamidoamine-based microhydrogel particles from ethylene diamine (EDA) monomer demonstrate high absorbance activity for removing heavy metal ions, yet are vulnerable to hydrolysis. Here, we copolymerize lysine diketopiperazine (L-DKP) and EDA with N,N′-methylenebisacrylamide via a Michael addition reaction-mediated inverse suspension polymerization to obtain highly efficient Cu2+-absorbing materials with controlled degradation in aqueous media. When the L-DKP content is increased, which replaces EDA, degradation is typically prevented at the cost of absorption capacity. At optimal L-DKP content (20 mol% per fed diamine monomers), the microparticle exhibits a performance of 159 Cu2+-mg/g, which is comparable to that of the EDA-only microparticles, but with higher degradation resistance, as only 38 wt% was lost at 37 °C after two weeks. During the hydrolysis of microparticles without L-DKP, the absorbed Cu2+ ions were released, polluting the aquatic environment. In the presence of L-DKP, Cu2+ ions were significantly retained within the working time. In contrast to synthetic microbeads such as polystyrene, accidently leaked L-DKP-based microparticles decompose within six months. These results provide an industrial, environment-friendly, and long-lasting absorbent for water purification.