Shear Laminar Flow-Induced Controllable Preferential Separation of Rh(III) Chloric-Complexing Anions Using Microfluidic Extraction

The kinetic inertness and easily hydrated characteristics of the Rh(III) anion render it difficult to accomplish preferential separation of Rh(III) from other coexisting platinum group metal ions using conventional turbulent flow extraction (TFE) based on dispersed oil droplets. In this work, a laminar-flow microfluidic extraction (LME) was suggested using 4-tert-butyl-1-(ethoxycarbonylmethoxy) thiacalix[4] arene (tBuTCA) as the extractant for selective separation of Rh(III) prior to Pd(II) and Pt(IV) anions in concentrated hydrochloric acid. Notably, a separation order of Rh(III) > Pd(II) > Pt(IV) in the LME process was achieved, while the order in TFE was Pd(II) > Rh(III) > Pt(IV). Experimental results indicated that an enhanced "water bridge" interaction induced by shear laminar flow in microchannel occurs through the hydrogen bond coordination between the oxygen atoms in tBuTCA and the hydrogen atoms in the H2O molecules around hydrated Rh(III), Pd(II), and Pt(IV) anions. Due to the stronger hydration ability of RhCl63– compared to PdCl42– and PtCl62–, the interaction between tBuTCA and the H2O molecules in the hydration shell of Rh(III) anions acting as a "bridge" drives the preferential migration of Rh(III) toward the interface. However, during the process of conventional TFE, the shear turbulence flow around dispersed oil droplets contributes to a stronger stripping of the H2O molecules in the hydration shell around those anions. The dissociation of Pd–Cl bonds promotes the preferential extraction of PdCl42– by tBuTCA molecules following a substitution mechanism. This novel insight lays a foundation for achieving preferential extraction and separation of Rh(III) anions, which is hard to be done in a conventional TFE process. The LME also provides a new approach to achieve controllable separation order of Rh(III), Pd(II), and Pt(IV) chloric-complexing anions, with the potential to develop a shear flow-induced switch molecular recognition based on weak interaction-dominated host–guest chemistry.