A rapid Chelex column method for the determination of metal speciation in natural waters

A simple, rapid Chelex resin column method has been developed for the determination of metal speciation in natural water samples. A water sample (pH 6–8.2) was pumped through a small plug of the Ca-form of Chelex 100 at a flow rate of 48 ± 4 mL/min. The flow regime was optimised to give the shortest possible contact time (0.25 s) but at the same time ensured quantitative uptake of inorganic metal standards onto the resin. Labile metal was calculated as the difference between influent and effluent metal concentrations, measured by some form of atomic spectrometry. In tests with six metal-spiked natural freshwater samples (10 μg/L added metal, pH 7.0–8.2), the labile fraction measured ranged from 87 to 98%, 32 to 64%, 12 to 78%, 38 to 91% and 63 to 89% for cadmium, copper, lead, nickel and zinc, respectively. Application of the method to copper-contaminated waters showed that the results provided a conservative estimate of the fraction inhibiting growth of a copper-sensitive bacterium, thus proving to be a useful measure of bioavailability. Tests on a range of model metal complexes showed that the measured labile metal fraction was inversely proportional to log K, and controlled by the kinetics of metal complex dissociation. Iron-containing colloids, present in natural freshwaters were not retained by the column. The method therefore discriminates trace metal species on the basis of both size and kinetics.