Chemical speciation of Cu, Zn, Pb and Cd in rain water

Abstract

A scheme for metal speciation which combines physical characterization by size fractionation and chemical characterization by dissociation kinetics has been used to characterize trace metal species in a sample of rain water. The lability of the metal species in each size fraction of ultrafiltration was determined by differential pulse anodic stripping voltammetry and by ion-exchange with Chelex-100 resin, using both the column and the batch technique. Most of the Pb, Cd, Cu and Zn soluble species were ASV-labile and Chelex-labile, and fell in the small size fractions (< 1000 molecular weight cut off). Kinetic analysis of the rain water sample (multi-component system) revealed the presence of the following kinetically distinguishable components (other than the very labile metal aqua ions) undergoing first-order (or pseudo-first-order) dissociation: three for copper, two for zinc, two for lead and one for cadmium. The lability of metal species found in the rain water sample ranged from labile, moderately labile, slowly labile to inert species, the half-lives range from less than one minute to more than seven days. The results of the kinetic analysis of rain water samples were very similar to those of a snow sample collected at the same site about 8 months earlier. The observed difference between the kinetically distinguishable components in the rain water sample and in the snow sample may be due to differences in the pH of the samples.