engleski

Copper Speciation by Anodic Stripping Voltammetry in the Surface Layers of the Oligotrophic Sea Influenced by Atmospheric Deposition

Chemical speciation of trace metal ions in the marine environment is important for understanding their toxicity, reactivity, transformation and biogeochemical behavior. Copper ions (Cu2+) speciation is considered especially significant due to its very narrow concentration range between having a positive nutritional and a negative toxic effect to phytoplankton. As more than 99.9 % of Cu2+ ions in seawater exist in the form of organic complexes, interaction with organic ligands mostly controls their impact to marine life. We studied the effect of atmospheric organic matter deposition on the Cu2+ speciation within the surface layers, differentiating between the sea-surface microlayer (SML, top 1 mm) and the underlying water (ULW, 1 m depth) of the middle Adriatic Sea (Šibenik archipelago), especially sensitive to such external inputs during the period of retrieval of the sea surface oligotrophic conditions (February-July 2019). We applied anodic stripping voltammetry (ASV) in conjunction with static mercury drop working electrode in titration measurement step. The obtained experimental data from the titration curves were fitted into the Langmuir/Gerringa nonlinear mathematical model built in the ProMCC software to determine Cu complexing capacity parameters (organic ligand class, its concentration and apparent stability constant) and estimate free (bioavailable) Cu2+ concentrations. We examined the optimal experimental conditions for titration of samples with Cu2+ standard solution in ASV and finally selected: Ea -0.6 V, ta 120 s, Edes -1.4 V for 1 s, [Triton-X- 100] = 1 mg/L. The presence of L2 ligand class ([L2] = 28 - 392 nM) was determined in all SML samples, while only in four samples L1 ligand class with concentrations [L1] = 41 - 76 nM were found. L1 ligand class were not found in the ULW, and the determined concentrations of L2 ligand class were lower compared to SML and ranged from 20 – 40 nM. Values of conditional stability constants for the SML samples were in the range logKCuL1 = 10.34 - 11.11 and logKCuL2 = 8.32 - 10.02, and for the ULW samples logKCuL2 = 9.51 - 10.18. The SML showed enrichment by organic ligands with a factor of 0.9 - 9.1, with the highest concentrations of organic ligands present in SML (392 and 276 nM) not originating from the water column, but from specific atmospheric sources in April 2019. The range of free Cu concentrations was calculated to be 2.3 - 46.1 pM, which is in the range of the estimated toxic Cu concentration limits of about 10 pM. Alongside biogeochemical results such as the influence of the specific atmospheric depositions on the Cu speciation and the free Cu concentrations in oligotrophic marine areas, this work also provided valuable information regarding experimental parameters. Specifically, preparation of the samples containing very high total organic carbon concentrations for ASV measurement of Cu complexing capacity parameters and the importance of accurate titration around the inflection part of the titration curve will be additionally discussed.

Atmospheric deposition ; Sea-surface microlayer ; Electrochemistry ; Cu speciation

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