Copper binding by terrestrial versus marine organic ligands in the coastal plume of River Thurso, North Scotland

It is generally assumed that terrestrial humic matter contains a high density of metal complexing ligands spanning a wide range of stability constants. From equilibrium modelling it would be anticipated that carboxyl groups, which are known to be abundant in humic matter, should be capable of binding Cu quantitatively in humic-rich estuarine systems such as the river plume outflow of River Thurso, on Scotlandʹs north coast, where concentrations along the full mixing gradient typically range for Cu: 1−25 nM and for DOC (Dissolved Organic Carbon): 150−1500 μM. Using a combination of reverse and forward titrations monitored by adsorptive cathodic stripping voltammetry (ACSV), we show that terrestrial humic substances were the likely source of high-affinity (class 1), low abundance ligands (1−4 nM) found throughout the coastal plume, but that weaker (class 2), more abundant ligands (90−170 nM) appeared to be produced largely in situ, probably as a biological response to Cu inputs into the estuary. Conditional stability constants for Cu-binding were respectively log K1′ = 14.9–15.9 and log K2′ = 11.8–12.6. Class 2 ligands were evenly distributed between the low and high molecular weight fractions of dissolved organic matter (DOM), both in March and May. Class 1 ligands were more abundant in the low molecular weight fraction in March, but not in May. The latter could be related to seasonal differences in the nature of peatland derived organic matter carried by River Thurso. Since class 1 ligands (terrestrial, humic) were saturated with Cu throughout the plume, it effectively fell upon class 2 ligands (marine, biological) to control Cu speciation in Thurso Bay, in particular keeping the free Cu concentration [Cu2+] within a relatively narrow range (10−13.6 M to 10−12.4 M). On the other hand, class 1 ligands were responsible for solubilising small amounts of Cu derived from this peatland catchment and effectively transporting Cu, in dissolved and colloidal forms, to the ocean.