Copper speciation and its relationship with dissolved organic matter in a hypersaline coastal lagoon

The Mar Menor (SE Iberian Peninsula) is the largest hypersaline coastal lagoon in Europe. This ecosystem is severely affected by anthropogenic impacts, causing the entry of metals and other contaminants. It is well known that total metal concentrations are not good predictors of their bioavailability, and its chemical speciation should be considered. Copper (Cu) is a metal of environmental concern whose speciation is largely controlled by natural organic ligands. The present work aims at studying the chemical speciation of dissolved Cu in the lagoon and its relationship with dissolved organic matter (DOM). For this purpose, 36 water samples were taken in different locations of the lagoon during six environmental surveys in 2021 and 2022. The chemical speciation of dissolved Cu was studied using cathodic stripping voltammetry (CLE-AdCSV), using salicylaldoxime as competing ligand. DOM was characterized by means of elemental composition (C and N), and optical properties (fluorescence and absorbance) in the same samples. Dissolved Cu speciation could be explained by a one ligand model, with ligand concentrations (L Cu ) ranging between 15.89 and 90.82 nM; and conditional stability constants (log K’ Cu 2+ ) varying between 11.36 and 12.92. L Cu positively and significantly correlated with dissolved organic carbon (DOC) concentration, absorption at 254 nm, and humic-like fluorescence intensity, suggesting that humic materials were the main ligands controlling Cu speciation in this area. In addition, the significant positive correlation found between DOC-normalized L Cu and the aromaticity index SUVA 254 indicates that DOM aromaticity can be a good proxy of the metal binding ability of humic DOM. Despite the very high DOC concentrations present in Mar Menor, Cu complexation was similar to that found in other coastal areas with lower DOC, which may be explained by the hypersaline nature of the lagoon. • Cu speciation is mainly dominated by humic substances in the Mar Menor lagoon. • More than 99 % of dissolved Cu in the lagoon is bound to organic ligands according to CLE-AdCSV. • Similar L Cu in coastal areas with lower salinities and DOC concentrations. • DOM aromaticity can be a good proxy of the metal binding ability of humic DOM.