Determination of Free Cd, Cu and Zn Concentrations in Lake Waters by In Situ Diffusion Followed by Column Equilibration Ion-exchange

Abstract

Combining in situ diffusion and column ion-exchange equilibration, we measured free metal ion concentrations (Cd, Cu and Zn) in water samples collected from the epilimnion of 14 lakes in the Rouyn-Noranda area (600 km north-west of Montreal, QC, Canada). Lakes were selected to represent a wide range of physico-chemical characteristics (hardness, pH, dissolved organic matter-DOM, degree of metal contamination), to determine the influence of these parameters on metal speciation. Total dissolved metal concentrations, as determined within the diffusion cells, varied over one to two orders of magnitude: [Cd] 0.19-2.9 nM; [Cu] 36-190 nM; [Zn] 7-2,800 nM. The proportion of total dissolved metal present as free Cd 2? and Zn 2? was relatively constant for the 14 selected lakes, despite the wide pH (4.5-8) and DOM (3-23 mg C/L) ranges, probably reflecting the inverse relationship observed between pH and DOM; this proportion did, however, vary with DOM and pH for Cu. Our experimental free metal ion concentrations were compared with those calculated with the thermodynamic models WHAM (Windermere Humic Aqueous Model VI) and ECOSAT 4.7 (incorporating the NICA-Donnan model). Measured and calculated values were in reasonable agreement for both Cd and Zn although measured values were generally slightly higher, i.e. less than one order of magnitude. For several lakes, measured free Cu concentrations were, however, much higher than the calculated values, suggesting that these models overestimate Cu complexation. The gap between measured and calculated free metal ion concentration becomes more important as the total metal concentration decreases and as pH increases.

Metal speciation from stream to open ocean: modelling v. measurement

“…Samples are titrated with metal between voltammetric scans to estimate concentrations of labile metal, from which free-ion concentrations are calculated with inorganic side reaction coefficients, and then a linear transformation model is applied to determine conditional stability constants for one or two classes of ligands. [26] In the ion-exchange column technique (IET), [27,28] a column of sulphonic acid-type resin is initially equilibrated with an electrolyte solution containing defined concentrations of Na, Mg, K and Ca. The resin is then calibrated by equilibrating it with solutions containing defined concentrations of the trace metal(s) of interest at a range of pH and a fixed ionic strength.…”

Trace metal speciation predictions in natural aquatic systems: incorporation of dissolved organic matter (DOM) spectroscopic quality

“…pH and concentrations of DOC and major cations) is known to be minimal within many of the sampled lakes. [3] Lake water was collected passively using equilibrium diffusion sampling as described in Fortin et al [3] and Mueller et al [20] Briefly, 250-mL polypropylene jars (Nalgene, Nalge Nunc International Corporation, Rochester, NY) topped with custom-made plastic lids fitted with a 0.2-mm filter membrane (HT Tuffryn Membrane, Pall Canada Ltd, Ville St-Laurent, QC, Canada) were filled with Milli-Q (Millipore, EMD Millipore, Billerica, MA) ultrapure water (.18 MO cm) and fixed to plastic rods that were anchored to the lake bottom. The samplers were installed at a depth of 1 m from the lake surface and left to equilibrate for 13 to 14 days.…”

“…In natural freshwater systems, trace metals may interact with a variety of ligands, including common inorganic anions (e.g. OH À , HCO 3 À , CO 3 2À , Cl À , F À and SO 4 2À ), simple monomeric organic molecules of natural or anthropogenic origin (e.g. metabolites such as citrate or glycine, or polycarboxylates such as nitrilotriacetic acid) and dissolved humic substances (fulvic acid (FA) and humic acid (HA)).…”

“…For example, the equilibrium ion exchange technique (IET) has been employed to determine Cd 2þ , Ni 2þ and Zn 2þ concentrations in environmental samples. [2][3][4] Various electrochemical techniques have also been used, including potentiometry with ion-selective electrodes (ISEs) [5,6] and cathodic stripping voltammetry, [5,7] as has the Donnan membrane technique. [8,9] However, given the complexity of the analytical matrices in which the free metal ion is found, and the often very low ambient metal concentrations, the determination of [M zþ ] in natural waters is not a routine measurement.…”

“…[3,12,13] Lofts and Tipping [14] have discussed possible reasons for these differences, including errors in measuring the free metal ion, errors in measuring the variables that are used as input data for the model and differences between the DOM present in the natural waters and the types of (isolated) FA and HA that have traditionally been used to calibrate or parameterise the chemical equilibrium models. In the present paper we have explored this latter factor.…”