Evaluation of the effects of natural dissolved and colloidal organic ligands on the electrochemical lability of Cu, Pb and Cd in the Arran Deep, Scotland

Abstract

The physicochemical speciation of Cu, Pb and Cd was determined by voltammetric titration procedures on filtered (F; 0.4-μm) and cross-flow filtered (CFF; 3000 Da) fractions of 15 samples collected at three sites in the Firth of Clyde, on Scotland's west coast. The colloidal material — defined on the basis of size (size <0.4 μm) and molecular weight (NMW>3000 Da) — was characterized by dynamic light scattering in terms of its electrophoretic behaviour with changing pH in a 0.01 M NaCl medium. The particulate material (size >0.4 μm) was analysed for its total metal content (Cu, Pb, Cd, Mn, Fe) by atomic absorption spectrometry. The top 20 m of the water column in the Arran Deep was influenced by freshwater inputs (32.9<S<33.2/Sbottom=33.7) and showed elevated organic complexation for Cu, Pb and Cd. Dissolved organic ligands were responsible for producing pCu (−log [Cu2+]) values of 10.2–11.9 and pCd values of 11.2–11.6. Colloidal ligands were largely responsible for the elevated pPb values (11.9–12.5) observed in the top 20 m of the water column. Unexpected features in (a) the speciation and size fractionation of Cu and Pb, and (b) the electrophoretic mobility (uE) of colloids, occurred at depths situated 35 m above the bottom at the three sites. At these depths, uE measured at any given pH showed minima, coinciding with maxima in the concentrations of colloidal ligands with strong affinity for Cu and Pb, and with maxima in the colloid/solution partitioning of Cu. No such anomalies were observed in the vertical profiles representing the acid–base properties of the colloids or in any of the key physicochemical variables such as S, T, pH or suspended particulate matter (SPM). These observations suggest that (1) the deep water (80–150 m) cycle of Cu and Pb may be partly coupled to that of their organic ligands via the in situ formation of colloids at a depth situated around (Zbottom−35) m, and (2) these colloids contain weakly bound (cation-bridged) assemblages made up from the same dominant organic components as found in solution at other depths in the water column.