Estuarine waters, colloidal stability

Kramer, C.3.M. and Duinker, 3.C., 1984a. Complexation capacity and conditional stability constants for copper of sea- and estuarine waters, sediment extracts and colloids. In C.3.M. Kramer and 3.C. Duinker (eds), Complexation of Trace Metals in Natural Waters. Nijhoff/3unk Publ., the Hague, pp. 217-228. 

As this figure illustrates, increasing ionic strength (addition of salts) compresses the electric double layer and decreases the colloid stability. Thus colloids tend to be less stable in estuarine and ocean waters. 

The speculation continues here into estuarine waters, bounded in this analysis at both inlet and outlet by waters that can, in the presence of suitable NOM, yield stable colloids. Diffuse layer thicknesses in these waters are small, on the order of 1 nm at 7 = 0.1. There can be sufficient salt in these waters to prevent classical DLVO electrostatic stabilization this is the conventional view. There may also be insufficient salt to form a thick layer of adsorbed NOM by screening of intra- and intermolecular repulsive interactions of the molecules of NOM. The result would then be a region of ionic strength or salinity in an estuary within which colloidal particles have a minimum stability and a maximum sticking probability. This possibility is shown by the proposed relationship between a and ionic strength shown in Figure 12. 

For trivalent metals the influence of humic substances present in diluted seawater is shown by the formation of negatively charged species, which might be attributed to metal-humic substance complexes (especially pronounced for fulvic acid). The presence of humic substances, even at lower concentrations, solubilizes iron. This is partly evident by the formation of the cationic species, which are not found in estuarine or seawater unless humic substances are added. These effects might also be attributed to the stabilization of iron colloids in water rich in humic substances. 

---