[SGC54-P01] Distribution and speciation of copper in seawater of East China Sea and its surrounding areas
Keywords:Copper, Speciation, East China Sea
Copper is an essential micronutrient for all living organisms as it plays an important role in electron transfer in many life-supporting systems, and is present in many enzymes and proteins. However, at high concentrations in seawater, Cu2+ is known to be toxic to marine microorganism. Therefore, many phytoplankton and bacterial species have the ability to release Cu-complexing ligands to decrease the concentration of Cu2+ and reduce its toxicity. As a result, at the eutrophic surface waters, more than 99% of Cu are complexed with organic ligands. Hence, in this study, we determined the total dissolved Cu concentrations, Cu speciation parameters and Cu2+ concentrations at 15 stations in the East China Sea and its surrounding areas.
II. Sampling and Methods
Seawater samples were collected using acid-cleaned, Teflon-coated X-type Niskin samplers mounted on conductivity-temperature-depth carousel multi-sampling system (CTD-CMS) onboard R/V Shinsei Maru during KS-15-6 cruise (June-July 2015) and R/V Hakuho Maru during KH-15-3 cruise (October 2015). The samples were collected in low-density polyethylene bottles through a 0.2 µm-pore size filter. Samples for total Cu analysis were acidified to a pH of less than 1.8 using ultrapure HCl, and stored. Another set of samples, for CLE/CSV analysis, was frozen at -18oC immediately after sampling. The samples were brought back to the laboratory and analyzed using CLE-CSV with salicylaldoxime (SA) as the competing ligand (Campos and van den Berg, 1994).
Samples used for total dissolved Cu were placed under UV radiation for 60 minutes to destroy all organic ligands prior to analysis. Frozen samples for Cu speciation analysis were allowed to thaw for 24 hours at 4oC, and then placed at room temperature for 4-8 hours. 10mL of sample, borate buffer, and a known concentration of Cu were added into two sets of 10 Teflon vials, left for at least 2 hours to allow the natural ligands to equilibrate with the added Cu, and the competing ligand, SA, was then added into the solution. The vials were then left to equilibrate overnight before analysis. 5 µM SA and 1 µM SA were used as the competing ligand for each of the two titrations, respectively.
III. Results and Discussion
The highest concentration of total dissolved Cu was found at the surface (10m) of Stn.05, where [Cu] = 4.65nM and the lowest concentration was at Stn.13 ([Cu] = 0.47 nM at 100m). Surface waters at Stn.03, Stn.04 and Stn.05 have relatively low salinities when compared to those of other stations, and this can be attributed to the freshwater discharge from Yangtze River, which is likely a major source of copper into the East China Sea (Koshikawa et al., 2007; Abe et al., 2003). Two classes of ligands were found in the surface waters in this study. The concentration of the stronger ligand, L1, ranged from 0.63 nM to 6.11 nM, with an average log K value of around 14, whereas for the weaker ligand, L2, the concentrations were in the range of 4 nM to 27 nM, with an average log K value of around 12.3. Cu2+ concentrations remain constant at all stations (~10-14 M) despite the large fluctuations in total Cu concentrations. The large ligand pool had probably buffered against changes in Cu concentration in seawater.
Abe, K., Ishihi, Y., Watanabe, Y., 2003. Dissolved copper in the Yellow Sea and the East China Sea - Cu as a tracer of the Changjiang discharge. Deep-Sea Research II 50, 327-337.
Campos, M.L.A.M., and van den Berg, C.M.G., 1994. Determination of copper complexation in sea water by cathodic stripping voltammetry and ligand competition with saliclyaldoxime. Analytica Chimica Acta, 284, 481-496.
Koshikawa, M.K., Takamatsu, T., Takada, J., Zhu, M., Xu, B., Chen, Z., Murakami, S., Xu, K., Watanabe, M., 2007. Distributions of dissolved and particulate elements in the Yangtze estuary in 1997-2002: Background data before the closure of the Three Gorges Dam. Estuarine, Coastal and Shelf Science 71, 26-36