Preferential separation of rhodium by molecular recognition on surface of concentrated hydrochloric acid
The kinetic inertness and easily hydrated characteristics of Rh(Ⅲ)chloric-complexing anions render it difficult to separate them preferentially from other coexisting platinum group metal ions in conventional solvent extraction.This work proposed a novel molecular recognition method based on thin-layer oil film extraction for selective and preferential separation of Rh(Ⅲ)chloric-complexing anions in concentrated hydrochloric acid using benzo-15-crown ether-5(B15C5).The effects of the flow rate of aqueous phase and thickness of oil film on the extraction rates of Rh(Ⅲ),Pd(Ⅱ)and Pt(Ⅳ),and the separation factors between them were investigated.The results indicate that,compared with the conventional dispersed oil droplet stirring extraction method,the performing thin-layer oil film extraction on the surface of concentrated hydrochloric acid can achieve a selective recognition separation sequence of Rh(Ⅲ)＞Pd(Ⅱ)＞Pt(Ⅳ).Due to the stronger hydration ability of Rh(Ⅲ)chloric-complexing anions compared with Pd(Ⅱ)and Pt(Ⅳ),the H2O molecules entering the inner coordination sphere of Rh(Ⅲ)chloric-complexing anions and the hydration shell around Rh(Ⅲ)chloric-complexing anions work together,resulting in stronger hydrogen bonding interactions between Rh(Ⅲ)chloric-complexing anions and B15C5 molecules through the H2O molecules acting as"bridges".Meanwhile,due to the lower concentration of free H3O+on the surface of concentrated hydrochloric acid,the increase in flow rate of aqueous phase beneath the oil film layer facilitates the hydration of Rh(Ⅲ)anions at the interface,while the decrease in the thickness of oil film promotes the interfacial renewal,thus enhancing the separation of Rh(Ⅲ)with Pd(Ⅱ)and Pt(Ⅳ).
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