A comparison of adsorption capacity of chitosan– rhamnolipid–hydrotalcite-like compound composites synthesized by two methods towards highly efficient removal of Cu(II) ions

Copper pollution in global water resources is becoming increasingly severe, posing a serious threat to human health and aquatic ecosystems. In this paper, two chitosan (CS)–rhamnolipid (RL)–hydrotalcite-like compound (HTlc) composites, designated as CS@(RL-HTlc) and (RL-HTlc)/CS, were synthesized using two distinct methods: emulsion crosslinking–coprecipitation–ion exchange and coprecipitation–ion exchange–emulsion crosslinking, respectively, to investigate their adsorption properties for Cu(II) ions. Characterization revealed that the CS@(RL-HTlc) composite possessed a crosslinked CS core with an RL-HTlc shell structure, while the (RL-HTlc)/CS microsphere exhibited a porous structure with a rough surface. The adsorption properties of these composites for Cu(II) ions were thoroughly investigated and exhibited significantly enhanced adsorption performances due to the synergistic effect between CS and RL-HTlc. Specifically, the coordination of Cu(II) ions with N/O-containing functional groups and the isomorphous substitution of Mg(II) by Cu(II) ions within the HTlc layers. Additionally, the surface-induced precipitation of Cu(OH) 2 was identified as another adsorption mechanism for CS@(RL-HTlc). Interestingly, the (RL-HTlc)/CS composite, with a larger specific surface area and a lower isoelectric point, exhibited a higher adsorption capacity compared to CS@(RL-HTlc). Moreover, (RL-HTlc)/CS outperformed CS@(RL-HTlc) as a more suitable candidate for repeated use in wastewater treatment.