Studies on metal speciation in the natural environment

Abstract

A scheme for metal speciation has been developed and applied to samples of river water, rain and snow. The scheme combines physical speciation by size using ultrafiltration of the soluble metal species with chemical characterization of the metal species by their dissociation kinetics. The kinetics of metal complex dissociation was studied by anodic stripping voltammetry and by ion exchange at Chelex-100 cation-exchange resin, using both the Chelex column and the Chelex batch technique. These kinetic studies cover a time scale of measurement of ca. 2 ms for anodic stripping voltammetry and up to 6 days for Chelex batch technique, a range of about 8 orders of magnitude. The kinetic results suggest that the copper in the snow sample (pH 3.9) is probably bound to different sites having different bonding energies in polyfunctional complexing agents. Four different copper species having dissociation rate constants of 3.1 X 10 −2, 1.6 X 10 −3, 6.2 X 10 −5, and 8.8 X 10 −6, s −1 have been observed. All of these species fall in the small size fractions of ultrafiltration. The Al in the Rideau River water sample (pH 8) is probably bound to the different sites in humic materials to yield dissociation rate constants 1.6 X 10 −4, 1.0 X 10 −6 and < 10 −6 s −1. In the Rideau River water sample, most of the soluble Al species have a large molecular size and are probably associated with humic materials. On the other hand, most of the soluble Al species in the small size fraction are Chelex-nonlabile and are probably Al(OH) − 4. Use of graphite platform furnace atomic absorption spectrometer allows direct determination of trace metals in freshwaters, to be done routinely, rapidly, and inexpensively.