Chemical reaction-driven spreading of organic extractant on gas-water interface: Insight into controllable formation of gas bubble-supported organic extractant liquid membrane.

Extraction and recovery of low-concentration valuable metals from various complex aqueous solutions or industrial waste waters have attract extensive interests in recent years. In our previous works, we suggested a novel technique called bubbling organic liquid membrane extraction by spreading and covering an organic extractant with extremely small volume on the surface of gas bubbles to form a layer of gas bubble-supported organic liquid membrane for selective extraction and enrichment of low-concentration targets from dilute aqueous solutions. It was found that, for successfully performing the bubbling organic liquid membrane extraction, a prerequisite is how to control the formation of stable organic liquid membrane covered on the surface of gas bubbles. However, once the organic extractant starting to spread on the surface of gas bubbles, the extraction chemical reaction at the interface between organic extractant liquid membrane and rare-earth aqueous solution will happen. In the present work, the spreading behavior of organic extractant P507 on the surface of rare-earth aqueous solutions were investigated, and was compared with the behaviors on the surface of deionized water. It was revealed that the spreading of organic extractant P507 on the surface of aqueous solutions containing rare-earth ions was accelerated due to the occurrence of the chemical reactions at the gas-water interface. The difference in spreading rate of organic extractant P507 liquid droplets respectively on the surface of deionized water and on that of Er(III) aqueous solutions with the increase in P507 concentration, the saponification degrees of P507 extractant and the pre-loading amount of Er(III) in P507 extractant revealed that the chemical reaction at the interface between the spreading P507 thin liquid membrane and Er(III) aqueous solution would result in the Marangoni convection along the interface, which is favor of overcoming the resistance from the viscous force when surface tension gradient replaces the gravity as a dominant driving force for the spreading. The present work provides an experimental foundation towards understanding the effect of the interfacial chemical reaction on the spreading behavior of organic oil droplet on the gas-water interface. It is beneficial for the development of our suggested new technique of bubbling organic liquid membrane extraction, and to achieve a controllable generation of stable gas bubble-supported organic liquid membrane for performing solvent extraction at large aqueous-to-oil phase ratios.