Simultaneous removal of Ni(II) and Cr(VI) from aqueous solution by froth flotation using PNIPAM-CS intelligent nano-hydrogels as collector

Abstract

In this work, the feasibility of froth flotation for simultaneous removing Ni(II) and Cr(VI) from aqueous solution by using thermo-responsive intelligent nano-hydrogels consisting of poly(N-isopropylacrylamide) (PNIPAM) and chitosan (CS) as the collector was investigated. PNIPAM-CS nano-hydrogels were synthesized by free radical precipitation polymerization and they were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and laser nano-particle size tester. Experimental results indicated that CS content had a significant impact on the phase transition behaviors of PNIPAM-CS nano-hydrogels. The suitable mass ratio of PNIPAM and CS was 2:1. The suitable pH of 6 for performing adsorption experiments was determined according to the species distribution of Ni(II) and Cr(VI) at different pH values. The adsorption kinetic data of PNIPAM-CS nano-hydrogels towards Ni(II) and Cr(VI) followed the pseudo-second-order model and their adsorption behavior were fitted well to Freundlich equation. Based on X-ray photoelectron spectroscopy (XPS) analysis, the driving forces for adsorption of Ni(II) and Cr(VI) by PNIPAM-CS nano-hydrogels mainly involved electrostatic adsorption, ion exchange and complexation. Finally, froth flotation was performed under the conditions of air flow rate 30 mL/min, cocamidopropyl betaine (CAB) concentration 70 mg/L, temperature 313 K and PNIPAM-CS nano-hydrogels concentration 6.0 g/L. The removal percentages of Ni(II) and Cr(VI) were 86.1% and 91.5%, respectively.