An effective strategy to construct pH-responsive microemulsion for cobalt recovery and its responsive mechanisms

Microemulsions for heavy metal separation show advantages of fast separation rate, high separation efficiency and extraction capacity. However, it remains challenging in destabilizing microemulsions for back-extraction due to their thermodynamic stability. This study describes an effective strategy to construct pH-responsive microemulsion by adding N,N-dimethylethanolamine (DMEA) during back-extraction to avoid the negative impact of responsive materials on the extraction, meanwhile the back-extraction can be realized through a mild pH stimulus compared to traditional strong acid demulsifier. Cetyltrimethylammonium bromide (CTAB)-stabilized microemulsion was formulated and optimized for the selective Co(II) extraction reaching 95.4 %, while extraction efficiency of Ni(II) kept low at 10.5 %. The back-extraction efficiency of Co(II) reached 99.53 %. The responsive mechanism was studied by measuring the dynamic interfacial tension and interfacial dilatational rheology of CTAB solution in heptane under the effects of NaCl, DMEA and pH. By pH stimulus, DMEA protonated and increased the ionic strength, consequently inducing a “salting-out” effect and deactivating CTAB at oil-water interface, ultimately destabilizing the microemulsion. This study proposes a new approach for constructing responsive microemulsions for recovering heavy metals and also provides useful information on its responsive mechanisms by establishing a relationship between interfacial property and macroscopic stability of microemulsions. [Display omitted] •pH-responsive microemulsion was used for the first time in metal recovery.•CTAB-stabilized microemulsion was optimized for the selective Co(II) extraction.•pH-responsiveness was realized by introducing DMEA after metal extraction.•The formulated microemulsion had excellent selectivity of Co(II) over coexisting ions.•The responsiveness is due to the salting-out effect induced by the protonated DMEA.