Evidence for copper-binding humic substances in seawater

Copper-binding humic substances (Cu-HS) in seawater were determined using a novel method based on cathodic stripping voltammetry (CSV). Cu-HS adsorb on the mercury (Hg) surface and are quantified on the basis of the reduction current for complexed Cu. Humic acid reference material (Suwannee River humic acid, HA) was used to calibrate the voltammetric response for the HS. The method was used to determine the concentration of Cu-HS in estuarine and coastal seawater, giving very good agreement with HS determined by UV spectrophotometry. Optimised conditions include a Cu concentration of 20-50 nM, a deposition potential of 0.05 V, differential pulse mode and pHNBS 8.2. The presence of Cu-HS indicates that this may be an important species of Cu in estuarine waters. The complex stability of Cu-HA was determined by two methods giving a value of 12.08 for log K′Cu-HA and a binding capacity of 18 nmol Cu for 1 mg of the HA. This complex stability is less than that (log K′CuL = 14) of the strongest Cu complexes thought to occur in ocean waters suggesting that the Cu-HS could represent the weaker class of Cu-binding ligands. Iron was found to compete with copper for complexation with the HA.