Simultaneously enhanced ELM selectivity and stability by difunctional additives for batch and continuous separation of Cd(II)/Cu(II)

Abstract

Low selectivity and weak stability constantly restrict the industrial application of emulsion liquid membrane (ELM) for metal ions separation. This study proposed an improved ELM system, in which selectivity and stability were simultaneously enhanced for separation of Cd(II)/Cu(II) by adding citric acid and its salt as retardant and buffer difunctional additives. Initially, the separation coefficient (βCd/Cu) in the blank experiment was only 1.15 in 15min, which indicated that P507 exhibited a poor selectivity as a carrier. After adding citrate additives, the βCd/Cu of the improved ELM system increased to more than 395.19 for two reasons. Firstly, the selectivity was enhanced by the selective chelation of citrate radicals on the Cu(II) ions, which were trapped in feed solution. Conversely, the Cd(II) ions were preferentially extracted by the P507 and then stripped into internal phase. Secondly, the stability was also improved, because the pH variation in the feed solution caused by the instability of membrane breakage was effectively controlled by the citrate additives as a pH buffer. Important variables in the citrate-enhanced ELM system including total citrate concentration, P507 concentration, dosage ratio of H3Cit to Na3Cit, H2SO4 concentration, and metal ion concentration were studied, respectively. The scale-up continuous experiment was executed in a pilot-scale modified rotating disc contactor with βCd/Cu of 859.61, which indicated that the citrate-enhanced ELM has considerable industrial application potential for efficient and steady separation of Cd(II)/Cu(II).