Model Predictions of Copper Speciation in Coastal Water Compared to Measurements by Analytical Voltammetry

Trace metal toxicity to aquatic biota is highly dependent on the metaĺs chemical speciation. Accordingly, metal speciation is being incorporated in to water quality criteria and toxicity regulations using the Biotic Ligand Model (BLM) but there are currently no BLM for biota in marine and estuarine waters. In this study, I compare copper speciation measurements in a typical coastal water made using Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) to model calculations using Visual MINTEQ. Both Visual MINTEQ and BLM use similar programs to model copper interactions with dissolved organic matter-DOM (i.e., the Stockholm Humic Model and WHAM-Windermere Humic Aqueous Model, respectively). The total dissolved (<0.4 μm filter) copper concentration, [Cu_T] in the study sites ranged from <10 nM close to the open Baltic Sea to ca. 50 nM in the vicinity of a marina in the Stockholm Archipelago. The corresponding free copper concentration [Cu²⁺], measured by CLE-ACSV ranged from 10^–13.2 M to 10^–12.0 M for the reference and marina sites, respectively, whereas the corresponding [Cu²⁺] modeled calculations ranged from 10^–12.5 M to 10^–11.6 M. The low copper to DOM ratios (similar to 0.0004 mg Cu per mg DOC) in these coastal waters ensured that ambient dissolved copper was overwhelmingly chelated to strong Cu–binding ligands (12 < log <i>K</i>_{CuL₁,Cu²⁺}^Cond >14). The modeled [Cu²⁺] could be fitted to the experimental values better after the conditional stability constant for copper binding to fulvic acid (FA) complexes in DOM in the SHM was adjusted to account for higher concentration of strong Cu-binding sites in FA