Effective enhancement of Cu ions adsorption on porous polyurea adsorbent by carboxylic modification of its terminal amine groups

Abstract

The pollution by metal ions has been a key issue in environmental protection. Amine-rich porous polyurea (PPU), prepared by a simple one-step precipitation copolymerization of diisocyanate and diamine, is highly selective for Cu2+ adsorption, but associated with a poor reusability. We report here a chemically modified PPU by simply turning the amine groups of PPU to aminoacetic carboxyls. The results indicate that this carboxylation is an easy and effective pathway to polymer adsorbent, with high performance for Cu2+ adsorption and reusability superior to most of the reported adsorbents. Cu2+ adsorption capacity on PPU-M, i.e. PPU after carboxylation, increased to 2.84 mmol/g, an increase by 22% in comparison with PPU. The chemical modification process was optimized. The porosity and morphology of PPU-M were characterized using N2 adsorption, mercury intrusion, and scanning electron microscopy. The high Cu2+ adsorption was interpreted through a complex coordination mechanism of Cu2+ based on XPS analysis. The high performance of reusability of PPU-M, thanks to enhanced desorption of 99.5% or more of the adsorbed Cu2+ after each recycled use, was also interpreted based on the stability constants of acid-base equilibrium of amine and carboxyl towards Cu2+ and protons. Adsorption kinetics modelling demonstrated this adsorption followed the pseudo-second-order model. The adsorption analysis by isothermal adsorption showed this adsorption was well fitted by Freundlich equation and in good agreement with the suggested adsorption mechanism.