The use of permeation liquid membranes for free zinc measurements in aqueous solution

Abstract

Environmental context The free Zn ion concentration in environmental aqueous systems is an important factor in determining Zn deficiency or toxicity to organisms as this species is directly bioavailable. The permeation liquid membrane technique, a tool to measure either free or bioavailable metal concentrations in solution depending on its setup, was evaluated for the first time for Zn speciation in simplified plant nutrient solutions. The technique is low-cost and applicable to a broad range of aqueous samples. Abstract The bioavailability of Zn in environmental water phases strongly depends on its speciation. One important species in studies on Zn deficiency or toxicity to organisms is the free ion. The permeation liquid membrane (PLM) technique is a tool to measure free metal concentrations with a short analysis time of 1 h and at low cost. However, so far it has only been validated for Cd, Cu, Ni and Pb. In this study we tested the effect of carrier concentrations and pH on Zn transport across the organic PLM membrane and the ability of the technique to measure free Zn in synthetic plant nutrient solution. We found that Zn membrane transport is dependent on the concentration of the carrier molecule lauric acid (LA), whereas variations in the concentration of the other carrier molecule, the crown ether Kryptofix 22DD, showed no effect, suggesting that Zn is not transported by the ‘classical’ PLM transport mechanism by binding to the crown ether. Zn preconcentration increased with increasing pH and decreased with increasing ligand concentrations. Using 0.05 M LA, Zn membrane transport is expected to be rate limiting (permeability criterion <<1) and the free Zn concentration can be measured. Under these conditions, PLM measurements agreed well with speciation calculations and with Donnan membrane technique (DMT) measurements in the presence of ligands forming negatively charged Zn complexes (ethylenediaminetetraacetate or citrate). In the presence of L-histidine higher free Zn concentrations than calculated were measured by PLM and DMT, suggesting that positively charged complexes contributed to cross-membrane transport in both methods.