Competitive extraction of Li, Na, K, Mg and Ca ions from acidified aqueous solutions into chloroform layer containing diluted alkyl phosphates

Abstract

Alkyl phosphates were extensively used in liquid–liquid extraction of lanthanides and actinides, but to a lesser extent for alkali and alkaline earth metals. The high amount of alkyl phosphate, which is usually used in the organic layer (>40 wt%), is not favoured due to its corrosive effect and toxicity. In the present work, diluted chloroform solutions (20.0 mM) of tri-n-butyl phosphate (TBP), tris(2-ethylhexyl) phosphates (TRIS) and bis(2-ethylhexyl) phosphate (BIS) were investigated for their extraction of Li, Na, K, Mg and Ca ions. The extraction experiments were conducted on 7.0 M HNO3 aqueous solutions containing 60.0 mM of metal ions in binary (Li+ and Mg2+), ternary (Li+, Na+ and K+) and quinary (Li+, Na+, K+, Mg2+ and Ca2+) mixtures. The Li+ selectivity over Mg2+ was very high in the binary system. Remarkably, increasing HNO3 concentration in the aqueous layer had opposing effect on the extraction of Li+ (positive) and Mg2+ (negative). However, the selectivity for Li+ became less dramatic in the case of ternary and quinary system, though the selectivity varied with initial metal concentrations. The amounts of water and NO3– transferred into the organic layer demonstrated their synergistic effect on extracting metal ions. In the ternary and quinary systems, the total concentrations of metal ions in the organic layer (ranged from 49 to 85 mM) were higher than the concentration of ligand in the organic layer (20.0 mM), suggesting that metal ions may be extracted into water/ligand/NO3– aggregates in the organic layer. TBP, TRIS and BIS do not have significant difference in their extraction behaviour. The FTIR results indicated formation of P+-O-M+/M2+ in the solid TBP/metal complex.