Organic Copper Speciation by Anodic Stripping Voltammetry in Estuarine Waters With High Dissolved Organic Matter

Abstract

The determination of copper (Cu) speciation and its bioavailability in natural waters is an important issue due to its specific role as an essential micronutrient but also a toxic element at elevated concentrations. Here, we report an improved anodic stripping voltammetry (ASV) method for organic Cu speciation, intended to eliminate the important problem of surface-active substances (SAS) interference on the voltammetric signal, hindering measurements in samples with high organic matter concentration. The method relies on the addition of nonionic surfactant Triton-X-100 (T-X-100) at a concentration of 1 mg L−1. T-X-100 competitively inhibits the adsorption of SAS on the Hg electrode, consequently 1) diminishing SAS influence during the deposition step and 2) strongly improving the shape of the stripping Cu peak by eliminating the high background current due to the adsorbed SAS, making the extraction of Cu peak intensities much more convenient. Performed tests revealed that the addition of T-X-100, in the concentration used here, does not have any influence on the determination of Cu complexation parameters and thus is considered "interference-free." The method was tested using fulvic acid as a model of natural organic matter and applied for the determination of Cu speciation in samples collected in the Arno River estuary (Italy) (in spring and summer), characterized by a high dissolved organic carbon (DOC) concentration (up to 5.2 mgC L−1) and anthropogenic Cu input during the tourist season (up to 48 nM of total dissolved Cu). In all the samples, two classes of ligands (denoted as L1 and L2) were determined in concentrations ranging from 3.5 ± 2.9 to 63 ± 4 nM eq Cu for L1 and 17 ± 4 to 104 ± 7 nM eq Cu for L2, with stability constants logKCu,1 = 9.6 ± 0.2–10.8 ± 0.6 and logKCu,2 = 8.2 ± 0.3–9.0 ± 0.3. Different linear relationships between DOC and total ligand concentrations between the two seasons suggest a higher abundance of organic ligands in the DOM pool in spring, which is linked to a higher input of terrestrial humic substances into the estuary. This implies that terrestrial humic substances represent a significant pool of Cu-binding ligands in the Arno River estuary.

How do certain atmospheric aerosols affect Cu-binding organic ligands in the oligotrophic coastal sea surface microlayer?

“…To overcome this, we considered the application of a desorption step applied aer the accumulation 54 and the addition of competing hydrophobic surfactant such as T-X-100. 33 SAS normally desorb from the electrode surface at potentials more negative than −1.3 V. 24 We applied desorption potential, E d −1.4 V for 1 s (Fig. S3A †), which provided higher Cu reoxidation peak and this step was therefore used in all ASV measurements.…”

“…29 In addition, allochthonous ligands such as terrestrial humic substances appear as relatively strong Cu-binding ligands in coastal and estuarine waters. 33 In this study, we investigated the inuence of atmospheric deposition on the organic Cu complexing capacity (CuCC) parameters (concentration of organic ligand classes, L i , and their apparent stability constants, K i ) of the surface layers (SML and underlying water, ULW from ∼1 m depth) at the central Adriatic coast. The objective was to answer the following questions: (i) how do the CuCC parameters change in the upper surface layers of the coastal waters from winter to summer, and (ii) to what extent were Cu-binding ligands affected by specic atmospheric depositions related to registered atmospheric events such as open-res of local vegetation, pollination period and Saharan dust intrusion.…”

Sources, sinks, and cycling of dissolved organic copper binding ligands in the ocean

“…In estuaries, dissolved Cu concentrations typically show conservative behavior, governed by the mixing of high dissolved Cu river water with lower dissolved Cu seawater (Fig. 1) [108][109][110][111][112][113][114][115] . Deviations from the conservative trend have also been observed, as a result of sorption to solids, colloidal flocculation and/or biological uptake 41,108,110,116,117 , or Cu inputs along the estuary 20 .…”

Trace Metals

“…For dissolved metal speciation (organic ligand) samples, filtered samples (0.2 µm) should be stored in acid-clean bottles, kept at natural pH (without acidification) and either stored in the fridge (+4 o C) or frozen (-4 o C or -20 o C) until voltammetric analysis in the home laboratory or measured 'fresh' directly on-board ship (Bruland et al 2000;Buck et al 2012;Pađan et al 2020;Sander et al 2005). In all cases, speciation samples must be stored in the dark in order to prevent photodegradation of the prevalent ligands, and it should be verified that the pre-cleaning procedure does not result in leaching of acid into the sample (i.e.…”