Competition between copper and iron for humic ligands in estuarine waters

We determined the concentration of iron- and copper-binding humic substances (Fe-HS and Cu-HS) in estuarine waters along with the concentrations of iron- and copper-complexing ligands (LFe and LCu). Suwannee River humic acid (SRHA) was used as a humic standard. The complex stability of Fe with salicylaldoxime (SA) was calibrated for salinities between 4 and 35 and fitted to linear equations to enable Fe speciation in estuarine waters: K′Fe′SA = − 2.98 × 104 × Sal + 4.60 × 106 and log B′Fe′SA2 = − 1.41 × log Sal + 12.85. The concentration of Cu-HS in waters from the Mersey estuary and Liverpool bay was less than the overall ligand concentration ([Cu-HS]/LCu = 0.69 ± 0.05) suggesting that a second ligand was of importance to Cu complexation. The concentration of Fe-HS was virtually equal to the total ligand concentration for Fe ([Fe-HS]/LFe = 0.95 ± 0.16) confirming that humics are responsible for Fe complexation in these waters. The concentration of HS determined from Fe-HS was within 4% of that found from Cu-HS, confirming that the same substance is detected. The average complex stability (log K′Fe′L) was 11.2 ± 0.1, the same as for log K′Fe′-SRHA. Copper additions demonstrated competition between Cu and Fe for the HS-type ligands. This competition was used to determine the complex stability for the Cu-HS species, giving a value of 10.6 ± 0.4 for logK′Cu′HS, which is nearly a unit less than the complex stability, logK′Cu′L = 11.4 ± 0.2, found for all Cu ligands (the HS and the unknown ligand combined). The competition affects the complexation of both metals with HS-type ligands. Extrapolation of the concentration of Fe-HS to an ocean salinity of 35 gives a residual level of 0.05 mg HS L− 1, equivalent to an Fe-binding ligand concentration of 1.5 nM. If HS-type ligands are confirmed to be ubiquitous in coastal or ocean waters, competition reactions could be of importance to the bioavailability of both metals to marine microorganisms.