Speciation of metals in the oceans. I. Inorganic complexes in seawater, and influence of added chelating agents

Abstract

The speciation of the metal ions in the surface waters of the open ocean has been computed by using critical stability constants describing all of the inorganic complexes that can be formed by the metal ions and inorganic ligands present. The stability constants used were adjusted to change the literature values from 0.10 M ionic strength to the 0.70 M ionic strength of seawater by using a new empirical technique involving experimentally observed variations of stability constants with ionic strength for complexes of similar charge types. For stability constants not listed in Critical Stability Constant compilations, values were estimated by using known trends of stability constants of similar complexes. The speciation determinations made include equilibria with suspended solid phases such as hydroxides, carbonates, and basic carbonates of the metal ions present. Perturbation of metal speciation resulting from the addition of some common chelating agents is determined and the results obtained are discussed in terms of the effects on seawater by the introduction of these chelating agents into the environment. In the 10 −9 to 10 −7 range of added chelant, EDTA (ethylenediaminetetraacetate), NTA (nitrilotriacetate), TPP (tripolyphosphate) and CIT (citrate) exist in the ocean primarily as their Ca(II) and Mg(II) chelates. EDTA and NTA are the dominant ligands for Cu(II) and Ni(II), while TPP is extensively complexed to Fe(III). CIT strongly influences Cu(II) speciation, while SSA (sulfosalicylate, a well-known Fe(III) reagent) has no influence on any of these metal ions or their complexes.