Kinetics of Trace Metal Competition in the Freshwater Environment:  Some Fundamental Characteristics

Abstract

Freshwaters are recognized as dynamic systems that may be far-removed from equilibrium. A kinetic approach using the competing ligand exchange method with Chelex 100 as the competing ligand and inductively coupled plasmamass spectrometry to measure the dissociation kinetics was used to investigate the chemical speciation of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II) in model solutions of a well-characterized fulvic acid (Laurentian fulvic acid) and a freshwater sample collected from the Grand River (Ontario, Canada). The kinetic distribution of the metal species were quantitatively characterized by their first-order dissociation rate constants. This kinetic speciation approach has the advantage of providing an objective method for estimating the dissociation rate constants without any a priori assumptions about the number of kinetically distinguishable components or the shape of the distribution. Three factors were found to influence the kinetics of trace metal competition in the freshwater environment: (i) metal-to-ligand ratio, (ii) ionic potential (z2/r), and (iii) ligand field stabilization energy. The results illustrate the importance of considering the valence-shell electron configuration in predicting the kinetics of trace metal competition in the freshwater environment. The markedly slow dissociation kinetics of Ni(II) and Cu(II) species suggest that the usual equilibrium assumption for freshwaters may not be valid. This study has demonstrated the ability of the kinetic model to correctly predict the relative rates of trace metal reactions, indicating that the kinetic model provides a chemically significant description of the kinetic processes in natural waters.

An Electrochemical Study of the Interactions Between Trace Metals and Humic Substances in Freshwaters by Anodic Stripping Voltammetry with a Thin Mercury Film Rotating Disk Electrode

“…The general trend observed in this study, in which the results were obtained using an equilibrium approach (i.e. measurements taken under steady-state conditions) is in agreement with the results obtained by Fasfous et al [31], who, using a kinetic approach, based on a competing ligand exchange method to determine the dissociation rate constants of Mn(II), Zn(II), Cd(II) and Pb(II) complexes in model solutions of Laurentian fulvic acid and in freshwater samples from the Grand River, observed a reversal in the trend predicted by electrostatic interactions.…”

“…Consequently, a greater number of strong sites on the FA molecules were available for binding Cu(II), forming nonlabile Cu(II)-FA complexes. Slow dissociation kinetics of Cu(II) complexes with Laurentian FA has been observed by Fasfous et al [31] who used a competive ligand exchange method to measure the dissociation rate constants of metal complexes. Following the lability criterion, the metal species are defined as labile if L ss d ) 1.…”

“…As a consequence, it was not possible to fit the data for the Cu(II)-FA system to equation 10. This loss in lability may be due to the fact that the metal-to-ligand ratio, which has been identified as playing an important role in controlling trace metal complexation by humic substances [30,31], was relatively low compared to that of the freshwater samples. Consequently, a greater number of strong sites on the FA molecules were available for binding Cu(II), forming nonlabile Cu(II)-FA complexes.…”

“…The correlation of metal complex stability constants with structural parameters of the metal ion is expected to produce two trends -one based on ionic potential (z 2 /r), and the other based on Ligand Field Stabilization Energy (LFSE) (for first row transition metals) [31,34]. The stability constants for Zn(II) (3d 10 ) and Cd(II) (4d 10 ), which have closed-shell electron configurations, and the posttransition metal, Pb(II), are commonly expected to show correlation with the ionic potential.…”

Determination of Free Nickel Ion Concentrations Using the Ion Exchange Technique: Application to Aqueous Mining and Municipal Effluents

“…Over the past two decades, many papers have been published on metal speciation in aquatic systems. These publications have mainly focused on the measurement of labile metals (free plus rapidly dissociating complexes) using methods such as Diffusive Gradients in Thin Films Technique (DGT) (Zhang and Davison 2000;Unsworth et al 2005;Unsworth et al 2006;Sigg et al 2006) and competing ligand exchange methods using ion exchange or electrochemical techniques (Mandal et al 2002;Fasfous et al 2004;Apte et al 2005;Chakraborty et al 2006); the exception to this statement is the Donnan membrane technique which is also gaining promise in measuring free metal ion concentration (Temminghoff et al 2000). Although labile metal species may be indicators of metal bioavailability (Batley et al 2004;Apte et al 2005), the free metal ion concentration is believed to be a better predictor of bioavailability (Morel 1983).…”