Physicochemical Description of Aquatic Colloids
A unified description and parameterization for COM should include properties of proton and electron reactive and hydrophobic sites, as well as the composition of major and minor components of COM. Models used in particle and pollutant (including radionuclide) in relation to chemical, physical, and biological processes that are acting on colloids would greatly benefit from such parameterization. As a starting point, thermodynamic constants and other physicochemical parameters of COM that control the behavior and its components on the self-cleansing capacity of natural waters need to be defined.
Parameters should, for example include the pKa values of proton-reactive sites, electrophoretic mobilities, isoelectric points, or relative hydrophobicities. Santschi et al. [9] reported a total concentration of 1.4meqg-1 of proton-reactive sites in marine colloids from the Gulf of Mexico, which is intermediate between that of humic and fulvic acids (10-16meq g-1) and polysaccharides (0.3-0.8meq g-1). These proton-reactive sites are often assumed to have pKa (negative log of equilibrium constant for acid dissociation) values of 2, 4, 6, 8, and 10. Actual values of pKa 1-3 have been determined for aquatic colloids, which could suggest sulfate or polyphosphate as possible ligands.
Values for pKa2 are more variable and increase from about 4 to 9 across the salinity gradient in the Firth of Clyde estuary, suggesting that the nature of colloids changes dramatically across an estuarine salinity gradient. Interestingly, log K values, as well as fractions of metal-specific ligands (CL) in the colloidal fraction (colloidal Cd-binding ligand, CL) of selected metals in coastal waters, can be significantly related to the pKa2 of the colloids; Cu showed a positive relationship for both log K and % colloidal CL with pKa2, whereas for Pb, a negative relationship was found.
This is possibly due to the different affinities of these trace metals to the two predominant forms of freshwater colloids, that is the smaller organic carbon-rich (enriched in Cu) and larger Fe-rich (enriched in Pb) colloids, both of which are also found in coastal waters. In agreement with the pKa values, measured electrophoretic mobilities of aquatic colloids are in the negative range. Electrophoretic mobilities, determined by dynamic light scattering at pH 8, were measured, ranging from about (-2 to -3.4) x 10-8 m2 v-1 s-1. Their values were weakly related to the colloidal fraction of metal-specific ligands (colloidal Cdtotal, CL); Cu and Pb were related in opposite ways.
In accordance with the relative high concentration of moderately strong acidic sites, for example values of acid dissociation constants, pKa1 of ~3 or below for marine colloids, isoelectric points of about 2-4 for marine colloids have been determined by Quigley et al., Alvarado Quiroz et al., and Chuang et al., suggesting that the surface charge is dominated by relatively strong, negatively charged, acidic groups. Conversely, positively charged ligand groups such as those of amino groups in proteins can vary.
Date added: 2025-02-13; views: 18;