Transport of lipophilic ligands through permeation liquid membrane in relation to natural water analysis

Trace metal ions can exist in many chemical forms, such as free metal, labile and liposoluble metal complexes in natural waters. Although, free metal ions are well known to be bioavailable, liposoluble species may also be potentially bioavailable and toxic. For understanding the metal uptake and toxicity to microorganisms, analytical tools are required to measure the various metal species under natural water conditions. Permeation liquid membrane (PLM) devices, comprising of 1, 10 dicecyl diaza 18 crown ether 6 and lauric acid in phenylhexane–toluene mixture, were demonstrated to be well suited for the trace metal speciation, in particular, for the free Cu(II), Pb(II) and Cd(II) concentration measurements in natural waters. In the presence of both free metal ions and lipophilic complexes, both these species will be transported across the membrane. In order to evaluate the contribution of the lipophilic species in the measured flux, the concentration of the lipophilic species must be determined. In this paper, transport characteristics of 8-hydroxyquinoline (HQ, also known as oxine) through a flat sheet PLM comprising a mixture of toluene/phenylhexane has been studied to obtain information on the permeability. HQ was chosen as model organic compound because its metal complexations are well known and it forms neutral lipophilic complex with Cu(II). Copper-oxine is used as a pesticide and may be present in natural waters. Important parameters such as diffusion coefficient in the membrane, Dm, partition coefficient, Kp, between the solution and the membrane, and permeability coefficient, P, have been evaluated by studying the effect of pH and membrane thickness on HQ transport. The results showed that the neutral HQ species is transported efficiently across the membrane and that the charged protonated species are not transported. Values of Dm = 3.0 × 10−7 cm2 s−1 and P = 2.47 × 10−3 cm s−1 were found. The partition coefficient, Kp, of HQ in phenylhexane/toluene mixture and toluene were also determined using the classical liquid/liquid extraction method for comparison. The results revealed that partition coefficient in toluene was higher than that in phenylhexane/toluene mixtures. However, the two values obtained for Kp were comparable as expected.