Adsorption of positively and negatively charged heavy metal ions from wastewater by heteroaggregates of biopolymer particles

Abstract

Environmentally benign biopolymer particles formed from natural carbohydrate polymers alginate and chitosan have shown great capacity for adsorption of heavy metal ions from wastewater. In this study, heteroaggregates of large alginate beads (∼1.8 mm) and chitosan nanoparticles (∼250 nm) have been used to adsorb molybdate and copper ions from single ion and mixed ion solutions in order to study the equilibrium adsorption properties. Three distinct heteroaggregation regimes have been studied in order to determine how the structure and composition affect adsorption capacity. Significantly enhanced adsorption was observed for heteroaggregates compared to the sum of their individual components, in both single ion and mixed ion solutions, indicating the presence of synergistic effects. The resulting adsorption capacity values for heteroaggregates are highly competitive with previous literature results and reach a maximum capacity of approximately 200–600 mg/g adsorption of either ion in a mixed ion solution, based on the adsorbent dry mass. In addition, synergistic and competitive effects were investigated in mixed ion solutions containing oppositely charged copper and molybdate ions, in an effort to mimic more realistic wastewater, and in contrast to typical competitive adsorption studies involving only like charged ions. Moreover, the observed interaction of both ions with heteroaggregates and their components suggested that opposite charge attraction is a more effective adsorption mechanism than complexation. Finally, the heteroaggregates represent a potential path forward for using nanoparticles in practical adsorption, where the large alginate beads act as supports for the nanoparticles and as effective adsorbents.