Voltammetric Studies on the Stabilisation of Dissolved Copper in Hydrothermal Vent Fluids

Abstract

Globally significant amounts of hydrothermally-derived, bio-active copper (Cu) are stabilised against precipitation, whether as dissolved or nanoparticulate species, and are laterally transported over thousands of kilometres across the deep-ocean (Sander and Koschinsky 2011). This stabilisation is facilitated by the formation of strong Cu-complexes with hydrothermally-derived ligands, which together influences the biogeochemical cycling of Cu in the ocean (Sander, Koschinsky et al. 2007). However, despite the recognised importance of these stabilisation processes, their extent and mechanisms, and the chemical nature of the hydrothermally-derived ligands are poorly understood. A better understanding of these processes will strengthen oceanic biogeochemical models, as broad assumptions will be lessened due to data on the distribution of hydrothermally-derived Cu-stabilising ligands. This project aims to provide data to rectify the paucity of information about this important phenomenon, which is the stabilisation of hydrothermally-derived Cu in the deep sea. As such, hydrothermally-influenced seawater from different vent sites along the geologically active, southern Mid-Atlantic Ridge were analysed for Cu-complexing ligands using the competing ligand equilibrium - adsorptive cathodic stripping voltammetry (CLE-AdCSV) technique.

In this study, the CLE-AdCSV technique was assessed to determine its ability to quantify Cu complexation in UV-irradiated seawater to which Cu-binding sulphur-ligands glutathione, L-cysteine and sodium sulphide had been added. It was determined that the competing ligand, salicylaldoxime, has an unexpected and deleterious impact on the Cu-complexing abilities of the sulphur ligands, which invalidates the assumption in CLE-AdCSV that the added ligand has negligible effects on the species in solution; except the trace metal of interest (Cu). The CLE-AdCSV method failed to detect more than one Cu-binding ligand class in mixed-ligand solutions. Also, it was unable to correctly quantify Cu-complexation in these solutions, which is suspected to be due to a quantitative artefact of the method. It was conclusively shown that the CLE-AdCSV technique has profound limitations that may lead to severe misrepresentation of Cu-complexation in seawater samples. As such, it is recommended that metal-complexation parameters derived from CLE-AdCSV for seawater samples should be reconsidered, and validated against independent experimental approaches and other speciation analysis methods, for example diffusive gradients in thin films (DGT).

Having assessed the quantitative reliability of the CLE-AdCSV technique, the method was then used in this study to determine Cu-complexing ligands in hydrothermally-influenced seawater, after an acidification pre-treatment. These measurements resulted in characteristically, oddly-shaped Cu-ligand titration plots for the hydrothermal samples, which complicated the quantification of the Cu-complexing ligands by the Gerringa data treatment method. Nonetheless, complexation parameters – ligand concentrations and stability constants, were extracted from the data within the limitations of the data not being ideal.

Total dissolved Cu concentrations in the hydrothermally-derived seawater samples were enriched relative to ambient seawater, with values from 1 to 1563 nM. The concentration of Cu-complexing ligands were measured at 0.014 ±0.001 to 10.6 ±2.16 µM, and with log KCu'L' values ranging from 11.13 ±0.23 to 12.80 ±0.38. The more stable Cu-complexes were associated with samples from low-temperature, diffused vent sites, while samples from the high-temperature vents had much higher concentrations of Cu-stabilising ligands. A significant fraction of the Cu-ligand pool was suspected to be inorganic sulphur species in both the high and low temperature vent samples. It was successfully demonstrated that if elemental sulphur was a major part of the Cu-complexing species, then it could explain the analytical oddities of the CLE-AdCSV technique evident during the analysis of the hydrothermally-influenced seawater samples. Finally, the results reported here are taken to be more representative of the eventual products from the reaction of hydrothermal fluids with seawater, rather than at the exact time and location of sample collection.

This current study reported concentrations of hydrothermally-derived ligands up to 10 µM which is similar to that used by Sander and Koschinky (2011), who used geochemical modelling to simulate the mixing of Cu and Cu-binding ligands from hot black-smoker vents. Sander and Koschinsky reported that up to 4.2% of hydrothermally derived copper can be stabilised by ligand concentrations up to 10 µM. Since the amount to Cu that is stabilised is primarily dependent on the ligand concentrations, the author of this present study proposes that the hydrothermally-derived Cu measured in this study would be stabilised to the same extent reported by Sander and Koschinsky (2011). That is, it is expected that up to 4.2% of the hydrothermal end-member Cu reported in this study can be stabilised, and contribute to the oceanic Cu flux.