D.C. Grégoire

Kinetic studies of metal speciation using chelex cation exchange resin : application to cadmium, copper, and lead speciation in river water and snow

Kinetic studies of the rate of uptake of the metal ions Cd, Cu, and Pb by the Chelex batch technique have been made on model solutions containing the metal ions and the complexants EDTA, NTA, and fulvic acid. The metal ions were quantitated by inductively coupled plasma mass spectrometry, and the rates of uptake of the metal ions were analyzed by the iterative deconvolution method. In the presence of an excess of EDTA or NTA, all the metal ion was complexed with the chelating agent, forming a slowly dissociating complex. In the presence of fulvic acid, these metals formed strongly bound complexes with a range of slow dissociation rates. The results reveal the importance of [fulvic acid]/[metal ion] ratio and, hence, the extent of occupation of binding sites in fulvic acid (a polyfunctional complexes

Kinetic studies of aluminum and zinc speciation in river water and snow

Abstract Kinetic studies of chemical speciation by the Chelex-100 batch technique revealed aluminum is present both as an aquo ion and as simple complexes of inorganic ligands in snow, but probably bound to macromolecule complexants and/or colloidal substances in the Rideau River surface water. Two kinetically distinguishable components of aluminum having dissociation rate constants of 1.1 × 10−2, ≤ 5.4 × 10−4 s−1 and three kinetically distinguishable components of aluminum having dissociation rate constants of 5.5 × 10−3, 8.4 × 10−4, ≤6.3 × 10−5 s−1 were observed in snow and river surface water, respectively. One kinetically distinguishable component of zinc having a dissociation rate constant of 9.1 × 10−3 s−1, and three kinetically distinguishable components of zinc having dissociation rate constants of 8.5 × 10−3, 2.5 × 10−3, ≤ 6.0 × 10−5 s−1, were observed in snow and river surface water, respectively. Use of inductively-coupled plasma mass spectrometry (ICP-MS) with the solution nebulizer to follow the kinetics of the uptake of aluminum and zinc by the Chelex resin in the Chelex batch technique allowed collection of many more data points in the same time range than were obtainable by graphite platform furnace atomic absorption spectrometry (GFAAS) with an automatic sampler, with the result that ICP-MS had a greater ability to resolve labile complexes than GFAAS. ICP-MS is therefore preferable to GFAAS for kinetic studies of metal speciation in aqueous samples.

Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method

Abstract Publications on the binding characteristics of metals with humic acid (HA) are sparse. Here we investigated the release of nickel from Ni(II)–HA complexes using model solutions of three different [Ni(II)]/[HA] mole ratios at three different pH values; we also compared the results with those of [Ni(II)]/[FA] complexes from previous work in this laboratory. Ligand exchange kinetics using the competing ligand exchange method (CLEM) were studied using two different techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime as the competing ligand to measure the rate of dissociation of Ni(II)–HA complexes. The results of the kinetic studies showed that as the [Ni(II)]/[HA] mole ratio was decreased, the rate of dissociation of Ni(II)–HA complexes decreased, and the proportion of free Ni2+ ions plus very labile nickel complexes decreased while the proportion of the less labile kinetically distinguishable components increased. Generally, the rate of dissociation of Ni(II)–HA complexes was slower than that of Ni(II)–FA complexes. Studies on the validity of the kinetic model showed that the concentrations of chemical species varied in a reasonable way with pH and the [Ni(II)]/[HA] mole ratios, indicating that the kinetically distinguishable components have chemical significance and the kinetic model is valid.

Effect of the competition of copper and cobalt on the lability of Ni(II)–organic ligand complexes. Part I. In model solutions containing Ni(II) and a well-characterized fulvic acid

Abstract The competitive binding of Cu(II), Co(II) and Ni(II) ions by a well-characterized fulvic acid (FA) in model aqueous solutions has been investigated by employing the competing ligand exchange method (CLEM) with Chelex-100 as the competing ligand. The reaction rate between a metal ion and a ligand is dependent on the rate of exchange of the coordinated water, which varies greatly for the above metals. Fulvic acid is a polyfunctional, polyelectrolytical, physically-heterogeneous, organic complexant having binding sites that can be roughly classified into two categories: minor (∼1–10%), strong sites, and major (∼99–90%), weak sites. The strong binding sites are first occupied, and after all the strong binding sites are occupied, the weak binding sites are occupied. Experiments have been done using model aqueous solutions containing various concentrations of Cu, Co and Ni, and the above well-characterized FA. Graphite furnace atomic absorption spectrometry and inductively-coupled plasma mass spectrometry were employed to monitor the rate of uptake of the metals by Chelex-100. The above metals, in the metals/FA mole ratios used in this study, have been found to compete with one another for the relatively small number of strong binding sites of the FA, and Cu(II) and Co(II) which have much greater rate constant for water exchange than that of Ni(II) win the competition over Ni(II). In the absence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Ni(II), forming strong complexes, which are inert, whereas, in the presence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Cu(II); Co(II), Ni(II), and the remaining Cu(II) occupy the weak binding sites of the FA, forming weak complexes, which are labile. The enhanced lability of the Ni–FA complexes in the presence of Cu(II) and Co(II) indicates that Cu(II) and Co(II) successfully compete with Ni(II) for the strong binding sites of the FA.

Methods for kinetic analysis of simultaneous, first-order reactions in waters: The kinetic model and methods for data analysis

Abstract The graphical method and the iterative method for analysing kinetic data for metal speciation in waters are described. The graphical method involves successive subtractions of one component from the total concentration of the metal remaining, beginning with the slowest component. The iterative method uses nonlinear regression of the experimental data assuming different numbers of components to obtain the weighted residuals. The number of components which gives a minimum to the sum of squares of the weighted residuals represents the number of kinetically distinguishable components. The weighted residuals should also have a normal distribution throughout the course of the reaction. These methods were applied to simulated data for systems containing three components whose rate constants differed by a factor of two and for some cases by a factor of three. When the concentrations of each component were equal and the ratio of the rate constants equal to two, the values of the rate constants obtained by these analyses ranged from 2 to 19% of the assigned values, but the values for the initial concentrations were as much as 40% different from the assigned values. When the concentration of one component was altered the reliability of its recovered rate constant and initial concentration decreased. The advantage of the iterative method is that no prior knowledge of the number of components, rate constants and initial concentrations is required. It is demonstrated that both of these methods provide simple, reliable ways of analysing kinetic data for characterization of metal species in waters.

Measurements and analysis of dissociation rate constants of metal–fulvic acid complexes in aqueous solutions: Part II: measurement of decay rates by inductively-coupled plasma mass spectrometry and determination of rate constants for dissociation

Abstract Rates of removal of metal ions from metal–fulvic acid complexes in aqueous solutions by Chelex-100 cation exchange resin have been measured using inductively-coupled plasma mass spectrometry (ICP–MS). When metal ions are complexed by the complexing agents, fulvic acid and ethylenediaminetetraacetic acid (each complexing agent is taken separately), the rate of uptake of metal ions by Chelex-100 has been related to the rate of dissociation of the metal complexes. The data were analyzed using the statistical methods described in Part 1. Complexes of lead with fulvic acid have a broad distribution of rate constants for dissociation, whereas complexes of aluminum with fulvic acid have a narrow distribution of dissociation rate constants. The rate constants for dissociation of both of these complexes decrease as the concentration of fulvic acid is increased at constant concentration of the metal. The results show that neither Zn nor Cd form strong complexes with fulvic acid.

Measurements and analysis of dissociation rate constants of metal-fulvic acid complexes in aqueous solutions. Part I : Simulation of decay curves obtained by inductively coupled plasma-mass spectrometry to evaluate a method to measure the distribution of first-order rate constants

Abstract A method for the analysis of exponential decay curves involving several kinetic components has been evaluated using simulated data. The method was originally developed for determination of fluorescence lifetimes and is applied here to data similar to those obtained in the measurement of rates of dissociation of metal–fulvic acid complexes in aqueous solutions. The method allows for a distribution of rate constants for dissociation of complexes instead of the single value for each complex obtained from the analysis used in previous studies from this laboratory. The signal-to-noise ratio of the data was a crucial factor in the distribution of the rate constants recovered from the analysis. Complexes with rate constants for dissociation differing by a factor of five were easily recovered, but when the difference was only a factor of two the resolution was not achieved even with a low level of noise. These tests were made with equal concentrations of each kinetic component. When the concentration of one kinetic component was reduced by a factor of 10, this species was not recovered in the analysis. These results clearly show the limits of resolution for individual kinetic components and the uncertainty expected in the values of the rate constants obtained by statistical analysis.

Evaluation and characterization of a commercial zeeman-modulated tungsten-strip electrothermal atomic absorption spectrometer

Abstract The operating parameters and the quantitative of an AAZ-2 Zeeman-modulated tungsten-strip atomic absorption spectrometer are reported. The figures of merit for Ag, Au, Cd, Co, Cu, Fe, Mn, Ni, Pb, Tl, and Zn are reported. Detection limits obtained with the AAZ-2 were comparable to those obtained with other metal atomizers reported in the literature. A maximum heating rate of 12 K ms−1 was measured for the tungsten-strip atomizer.