Colloid/Solution Partitioning of Metal-Selective Organic Ligands, and its Relevance to Cu, Pb and Cd Cycling in the Firth of Clyde

Abstract

The electrochemical lability of Cu, Pb and Cd and the complexation parameters (stability constants, ligand concentrations) of their organic ligands were determined by anodic stripping voltammetry (ASV) in 14 samples collected in April–May 1994 from the lower Clyde Estuary (S<30·0), through the surface estuarine plume (30·0<S<32·4), and into the Clyde Sea. The titrations were applied to 0·4μm-filtered and cross-flow filtered (∼1nm cut-off) fractions of each sample. In addition, the surface electrical properties of the colloidal matter (1nm<size<0·4μm) were examined in 0·01M NaCl medium as a function of pH by dynamic light scattering (DLS) electrophoresis.Copper was carried to the Clyde Sea by the plume largely in a dissolved form but subsequently reacted with—or formed—colloids. These colloids were either of marine origin or built upin situfrom smaller sub-units such as organic macromolecules originating from the sediments. Overall, the degree of organic complexation for Cu (typically 76–96%) was less than that reported in other inshore marine systems. Biological production of strong ligands did take place in an adjoining loch, but their control on Cu speciation (up to 99·99% Cu in inert forms) remained localized. By contrast, organic complexation was sufficient to maintain pPb (pPb=−log [Pb2+]) as high as 11·7–12·5. Lead in river runoff was extensively and strongly bound to organics. A unique class of ligands ofin situorigin and belonging to the colloidal phase was detected at 40m depth, i.e. under the plume. This colloidal material had limited affinity for Cd and no detectable affinity for Cu, and probably consisted of the remains of micro-organisms which must have been concentrated at this depth by the plume circulation. Despite some affinity for the above-mentioned colloidal ligands (resulting in 54–88% complexation) and for the dissolved ligands produced by phytoplankton (59–64 % complexation), Cd was generally poorly bound (0–30% complexation) and the ligands responsible were mostly of marine origin. Although a fraction of the particulate Pb (associated with Mn-rich particles) was removed from the surface plume by sedimentation, the major modification in the seaward flux seemed to occur at the leading edge of the plume. There was a change in the speciation of Pb across this frontal zone, from a dominance by colloids and macroparticles in the plume to a situation where most of Pb found in the 0·4μm-filtered fraction was also present in the <1nm permeate. In contrast, Cu became less extensively associated with the dissolved phase away from the plume.When interpreted in combination, the ASV and DLS results show that two bulk properties of organic coatings on colloids (electrophoretic mobility and second acidity constant), important in the context of colloid stability, can explain some of the variability in the colloid–solution partitioning of metal-complexing ligands. What is more, the surface electrical state of the colloids appears to have a different effect on the adsorption of Pb-binding ligands than on that of Cu-binding ligands, supporting the conclusion reached from the spatial distribution of ligands that they are metal-selective.