The role of humic acids on the iron speciation and the competitive effect with copper in seawater

Abstract

The role of humic acids (HA) on the iron speciation and the competitive effect with copper were studied in artificial seawater as a function of the ionic strength (0.1 – 0.7 mol L-1), pH (6.0 – 8.5) and temperature (5 – 30ºC) by using the geochemical numerical code Visual Minteq. The simulations were in a good agreement with the data collected from the literature in terms of Fe speciation, where the presence of humic acids produced the formation of Fe organic species (Fe-L) that showed a higher contribution to the total iron species when HA where higher than 1000 nmol L-1 in artificial seawater as a function of ionic strength, pH and temperature. The presence of copper produced a competitive effect with the Fe speciation due to the formation of Cu-organic complexes (Cu-L) that were more abundant than the Fe-L species at pH higher than 7.5. These ligands are considered weak ligands in the ocean and their Fe and Cu complexing capacities are defined by the conditional stability constants and their hydrolysis constants with the major Fe and Cu species in seawater. The current numerical code also allows to study the Fe mineral formation under the physico-chemical conditions previously considered as well as their solubility index. In this sense, the oversaturated Fe minerals in artificial seawater were Fe(OH)2.7Cl3, ferrihydrite, goethite, hematite, lepidocrocite, maghemite and magnesioferrite. After the copper addition, the Fe-Cu mineral cupric ferrite was formed with a saturation index above 14. These minerals could be found in deep seafloor and marine sediments. The numerical codes are useful geochemical toolboxes in order to predict the trace metal speciation in seawater and to better understand the competitive mechanisms between metals and organic ligands, as well as the mineral formation. In this sense, most of the numerical codes have been developed at low ionic strength and future experiments should improve the existing databases.