Redox speciation of copper in the estuarine environment: Towards better understanding of copper water chemistry

An adapted solid-phase extraction method was used for Cu redox speciation in the dissolved fraction in the stratified Krka River estuary in March and July 2023 at a “clean” and a “polluted” site, while anodic stripping voltammetry in differential pulse mode was used for Cu determination in the Cu(I), Cu(II) and Cu_T fractions. Overall, [Cu(I)]/[Cu_T] was significant in the Krka River estuary and varied between 20.7 and 88.7 %. At the “clean” site E1, the surface maximum of [Cu(I)]/[Cu_T] was observed in March. This was assumed to be related to the biological activity of the freshwater biota, as the temperature rose in March (compared to the previous winter months) and the salinity was low. On the other hand, at E1 in July, the maximum of [Cu(I)]/[Cu_T] was observed at the halocline and corresponded to the highest biological production thus we hypothesised it to be the result of Cu(I) complexation with ligands of biological origin. The minimum of [Cu(I)]/[Cu_T] was observed at the surface at E1 in July where terrestrial humic-like substances were present with higher affinity for Cu(II) than for Cu(I). At the “polluted site” E2 high [Cu(I)]/[Cu_T] values were observed at both the surface and the halocline in March. However, [Cu(I)]/[Cu_T] maximum was observed at the halocline at E2 in July and was associated with the highest biological production, as also observed at E1 in July. Nevertheless, the stabilisation of Cu(I) by chloride should not be disregarded as it maintained relatively high [Cu(I)]/[Cu_T] in the seawater layer at both stations in March and July. The fluctuations in [Cu(I)]/[Cu_T] occurred during the day at the surface at E2 in July, with the minimum of [Cu(I)]/[Cu_T] observed in the early morning, followed by an increase in [Cu(I)]/[Cu_T] during the day, indicating the importance of photochemical reactions in the Cu(I) formation and stabilisation.