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 (Rh) ∼ 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 ∼ 40 °C to ∼ 50 °C for 0.005 M Cu2+, ∼60 °C for 0.05 M Cu2+ and ≫ 60 °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 °C for 0.25 M Cu2+) where foams rapidly collapsed for other cations (e.g. ∼ 2 min, ∼10 min and ∼ 15 min at 45 °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 °C)/G’s(70 °C) ratio ∼ 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 °C)/G’s(70 °C) ratio ∼ 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.