Examination of the range of copper complexing ligands in natural waters using a combination of cathodic stripping voltammetry and computer simulation

Abstract

Knowledge of the detection capabilities of speciation techniques, gained by calculation and computer simulation, can be combined with experimental measurements to arrive at an understanding of trace metal speciation which is less dependent on operational factors than other approaches. Examples of the application of this means of measuring copper speciation to samples from the Humber Estuary are given. Although concentrations of total dissolved copper can approach the estuarine Environmental Quality Standard value of 5 μg 1 −1, there is evidence for a substantial excess of complexing ligands at all locations except the outer estuary, where copper levels are much reduced by dilution. Dissolved copper is therefore present almost totally in the form of organic complexes. The range of different types of ligand is also assessed. In sea water, there appears to be a range of ligands of differing affinities for copper; the complexing capacity ranges from 20 nM [conditional stability constant of the copper complex ( K′) > 10 14] to 70 nM ( K′) > 10 8). For estuarine samples, ligands with a high affinity for copper seem to be predominant and the overall complexing capacity rises to over 200 nM. In freshwater samples, it is likely that the potential for varying combinations of weak and strong complexes will depend on the water quality, but a capacity to complex over 200 nM copper is not unusual.