Cu2+ tunable temperature-responsive Pickering foams stabilized by poly (N-isopropylacrylamide-co-vinyl imidazole) microgel: Significance for Cu2+ recovery via flotation

Froth flotation has been a key chemical process extensively used in the recovery of heavy metal ions (e.g. Cu2+) from contaminated water, while often criticized by a secondary pollution of the added collectors, and a less selectivity to specific ions since the ion removal by particles is independent from the post-step of particle flotation. To facilitate the industrial operation, a smart flotation with foams responsively stabilized after selective adsorption of target ions from competitive ions are highly desired. In the current study, smart foams stabilized by Cu2+ responsive microgels were presented as a proof-of-concept. Firstly, a Cu2+-responsive thermo-sensitive poly (N-Isopropylacrylamide-co-Vinyl imidazole) (PNV) microgel with a hydrodynamic radius (R-h) similar to 334 nm and a fuzziness 37.2 nm was synthesized. Cu2+-imidazole complexation was demonstrated to enhance the microgel swelling with a softer and more homogenous microstructure, having the Rh increased by 30-50 nm for 0.005 M to 0.25 M Cu2+ and a significant volume phase transition temperature (VPTT) shift from similar to 40 degrees C to similar to 50 degrees C for 0.005 M Cu2+, similar to 60 degrees C for 0.05 M Cu2+ and >> 60 degrees C for 0.25 M Cu2+. Secondly, temperature responsive foams with a ultra-stability below VPTT of the microgel and a rapid collapse above the VPTT were readily produced based on PNV microgels. Cu2+ complexation enabled a modulation of temperature responsiveness of the foams, able to maintain a good foam stability in high temperatures (e. g. a life time > 6 h at 45 similar to C for 0.25 M Cu2+) where foams rapidly collapsed for other cations (e.g. similar to 2 min, similar to 10 min and similar to 15 min at 45 similar to C for Na+, Mg2+, Zn2+, respectively), showing significance for selective recovery of Cu2+ from competitive ions via flotation. Furthermore, an interfacial study at air-water interface revealed a better surface activity of Cu2+ -complexed PNV microgel with a less temperature dependence. An abrupt reduction of interfacial rheology around the VPTT with a G's(25 degrees C)/G's(70 degrees C) ratio similar to 300.1 observed was believed to be the main reason of the responsive foam destabilization for PNV2, which was avoidable by entanglements between Cu2+-complexed PNV microgels, giving a G's(25 degrees C)/G's(70 degrees C) ratio similar to 1.68. Ultimately, the good Cu2+ selectivity of PNV2 microgel was well demonstrated in mixed solutions of competitive ions Na+, Mg2+, Zn2+ in terms of Cu2+ sorption, particle swelling and foam stabillity. This study demonstrated a responsive Pickering foam stabilized by a Cu2+ tunable microgel, laying foundations to develop microgel-based flotation as a smart and green technology for Cu2+ recovery.

Automated summary

In this study, smart foams stabilized by Cu2+ responsive microgels were presented as a proof-of-concept and demonstrated their potential for selective recovery of Cu2+ from competitive ions. The research laid foundations for the development of microgel-based flotation as a smart and green technology for Cu2+ recovery.