Vertical distribution and chemical nature of strong ligands for copper(II) in the western North Pacific
2012 Volume 63 Pages 31-42
Details
2012 Volume 63 Pages 31-42
We investigated the vertical distribution and chemical nature of strong ligands for copper(II) in the western North Pacific based on measurements of organically complexed copper(II) with two experimental procedures having different properties. The first involved fractionation of copper(II) complexes on the basis of their stability: exclusion of copper(II) complexed with weak ligands and, consequently, extraction of only highly stable copper(II) complexed with strong ligands (conditional stability constant of log K′CuL > 14) by use of a ligand-exchange reaction with ethylenediaminetetraacetic acid. We also detected two weaker ligand classes having log K′CuL = 9-10 (L1) and log K′CuL ∼ 7 (L2) with the copper(II) titration of an aliquot of the concentrated and desalted seawater. The second procedure involved the separation of hydrophobic copper(II) complexes from the original seawater by adsorption onto a resin. We determined the conditional stability constant of the hydrophobic copper(II) complex to be log K′CuL ≤ 10.26 based on measurements of copper(II) complexes collected after copper(II) titration.
We detected strong ligands (log K′CuL > 14) at all depths from the surface to 1000 m, and we determined their concentrations to be 0.02-0.19 nM. The concentrations of hydrophobic ligands were lower than those of strong ligands at most depths, especially in surface waters. The vertical profile of strong ligands showed small maxima in the euphotic zone that did not coincide with chlorophyll a maxima. We estimated strong ligands to be present quantitatively (> 99.9%) in the form of copper(II) complexes throughout the entire 1000-m water column based on the concentrations of inorganic copper(II) measured by means of hydrophobic chromatography. In contrast, the copper(II)-complexed fraction of the hydrophobic ligands (log K′CuL = 10.26) varied from 15% at 120 m to 78% at 1000 m and was positively correlated with the concentration of inorganic copper(II). Strong ligands appeared to have different ligand sites and different structural units in their environments compared to the hydrophobic ligands. The strong ligands appeared to be relatively hydrophilic because their ligand sites were multidentate and were composed of several polar functional groups.
We characterized at least three types of organic ligands with different affinities for copper(II) and different hydrophobic properties. Each ligand was associated with copper(II) with a different efficiency that varied with depth in the oceanic water column. In surface layers it appeared that strong ligands played an important role that enabled copper(II) to be present stably in seawater as a soluble form, whereas at depth, hydrophobic ligands also contributed to the copper(II) speciation. Knowledge of ligand speciation is of noteworthy importance to the investigation of the abundance, distribution, and dynamics of copper(II) and organic ligands in the ocean.
