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) ∼ 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 R_h 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 ^oC to ∼ 50 ^oC for 0.005 M Cu²⁺, ∼60 ^oC for 0.05 M Cu²⁺ and >> 60 ^oC 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 above a critical temperature (e.g. a life time > 6h at 45 ^oC for 0.25 M Cu²⁺) where foams rapidly collapsed for other cations (e.g. ∼2 min, ∼10 min and ∼15 min for Na⁺, Mg²⁺, Zn²⁺, respectively), showing significance for selective recovery of Cu²⁺ from competitive ions.

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 ^oC)/G’s(70 ^oC) 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 ^oC)/G’s(70 ^oC) 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.