Highly Selective Extraction and Transport through a Bulk Liquid Membrane of L-Cysteine Using [K • DC18C6]+Complexes

It is shown that dicyclohexyl-18-crown-6 (DC18C6) dissolved in chloroform can extract efficiently and selectively the anionic form of L-cysteine from aqueous solutions containing potassium ions. This ability depends on the aqueous phase pH and type of the salt used in this phase. Under optimized conditions L-cysteine can be quantitatively extracted into the organic phase. In such conditions there was no detectable extraction of amino acids tryptophan, tyrosine, leucine, methionine, arginine, asparagine, aspartic acid, glycine, serine, and threonine. The transport of L-cysteine from potassium chloride/potassium hydroxide solution across a bulk liquid membrane containing DC18C6 dissolved in chloroform has been investigated. The parameters influencing the transport efficiency such as the pH of the feed phase, the kind and concentration of the receiving phase, carrier concentration, the kind of organic diluents, and time were examined and optimized. An efficient stripping at the membrane/stripping phase interface was found using hydrochloric acid as strippant. Selectivity of the proposed system was evaluated by performing a series of competitive transport experiments on the binary mixtures containing L-cysteine with the competitor amino acids noted earlier. This investigation exhibits excellent transport selectivity of the process towards L-cysteine with respect to the examined amino acids.