Metal speciation in natural water: Evaluating kinetics versus equilibrium.

摘要

Trace metals are essential micronutrients at low concentrations, but they may become toxic at elevated concentrations. Bioavailability of trace metals in freshwaters is known to be often correlated with free ion activity. The free metal ion activity is dependent on the dissociation of metal complexes of dissolved organic matter (DOC) in freshwaters. The objective of this research was to investigate trace metal competition in freshwaters and in model solutions of well-characterized fulvic and humic acids. Dissociation kinetics of metal-DOC complexes were investigated using Competing Ligand Exchange Method/Adsorptive Cathodic Stripping Volatmmetry and Competing Ligand Exchange Method/Inductively Coupled Plasma-Mass Spectrometry. Three factors were found to influence the kinetics of trace metal competition in the freshwater environment: (1) metal-to-ligand mole ratio, (2) ionic potential (z2/r), and (3) Ligand Field Stabilization Energy. The results show the importance of considering the valence-shell electron configuration in studying the kinetics of trace metal competition in the freshwater environment. The markedly slow dissociation kinetics of Ni(II)- and Cu(II)-complexes suggest that the usual equilibrium assumption for freshwater may not be valid. Exposure of the DOC in freshwaters to UV-B radiation of sunlight for 0, 10, 20, and 30 days resulted in photodegredation of the DOC, releasing the DOC-bound metals. The measured labile metal-complexes concentrations were compared with the predictions made by the applying widely-used computer speciation model, the Windermere Humic Aqueous Model (WHAM) and its improved version WHAM VI. For lake water samples studied, both versions of WHAM provided concentrations of free metal ions: Co2+, Ni 2+, Zn2+, and Cd2+, which were in reasonably-close agreement (within one order of magnitude) with the experimentally measured concentrations of labile metal complexes. The effect of metal competition on the stability constants of metal complexes, was investigated using an equilibrium approach. The heterogeneity of humic substances (humic acid and fulvic acid) in their metal complexation in model solutions, was investigated by using pseudopolarography, resulting in the determination of Differential Equilibrium Functions and Differential Kinetic Functions, which accounted for the heterogeneity of humic substances on the thermodynamic stability and kinetic lability of metal complexes.