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 (L-Fe and L-cu). 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 x 10(4) x Sal + 4.60 x 10(6) and log B'(Fe'SA2) = -1.41 x 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]/L-Cu = 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]/L-Fe = 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 HSL-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. (C) 2015 The Authors. Published by Elsevier B.V.