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.

Diffusion limitation of zinc fluxes into wheat roots, PLM and DGT devices in the presence of organic ligands

“…[24] Uptake of intact complexes by plants may also explain why histidine caused a larger deviation than citrate between measured Zn fluxes into plants and predictions based on the relationship between root and PLM Zn fluxes in the EDTA and no ligand treatments (Fig. 5b(i)), whereas it cannot be excluded on the other hand that faster complex dissociation was at least partially responsible also for this difference between histidine and citrate treatments.…”

“…The DGT available concentration (C DGT ) was calculated according to the DGT research manual. [33] PLM experiment Hollow fibre PLM (HF-PLM) experiments were set up in the same way as described by Gramlich et al [24] using a carrier concentration of 0.05 M lauric acid (LA) and 0.1 M Kryptofix 22DD in the organic membrane (1/1, v/v, toluene/hexylbenzene mixture) and PLM membranes of 31-cm length. After 2-h exposure of the PLM membranes to the treatment solutions (Table 1), the Zn concentrations of the strip solutions were measured using ICP-OES (Vista-MPX, Varian).…”

“…[33] (c) In PLM, free Zn is transported across the membrane into the strip solution containing a strong ligand by a transporter molecule T (i), a contribution of complexed Zn occurs if free Zn diffusion in the solution is limiting and complexes dissociate (ii) or if positively charged complexes ZnL xþ form ternary complexes with the transporter molecules T (iii). [24] A. Gramlich et al uptake in soils, studies with hydroponics enable investigation of the different effects of single ligands and the effects of diffusion limitations on metal uptake in much more detail than it would be possible in a complex soil system.…”

“…Apart from trans-membrane transport of lipophilic complexes, [23] transport of positively charged complexes is also likely. [24] The important feature of PLM measurements is that the limiting processes are in many respects analogous to those governing metal uptake by living cells. [5,12] PLM has been tested as a sensor for bioavailable cadmium, copper, nickel and lead to microalgae.…”

Free Zn2+ determination in natural freshwaters of the Pyrenees: towards on-site measurements with AGNES

“…Many analytical techniques like ion-exchange methods, [1] the Donnan membrane technique (DMT) [2,3] or permeation liquid membrane (PLM) technique [4] have been developed to determine the free metal ion concentrations in natural samples. These methods agree well with speciation models (Windermere Humic Aqueous Model, WHAM, or Visual Minteq), [5][6][7][8] but require complementary equipment that cannot be moved into the field for on-site measurements.…”