Estuarine colloids

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

Sigleo, A.C. and Helz, G.R., 1981. Composition of estuarine colloidal material major and trace elements. Geochim. Cosmochim. Acta, 45 2501-2509. 

Means, J. C., and R. D. Wijayaratne. 1984. Chemical characterization of estuarine colloidal matter implications for adsorptive processes. Bulletin of Marine Science 35 449. 

Wijayaratne, R.D. and J.C. Means (1984). Affinity of hydrophobic pollutants for natural estuarine colloids in aquatic environments. Environ. Sci. Technol., 18 121-123. 

Wijayaratne R. D. and Means J. C. (1984) Adsorption of polycyclic aromatic hydrocarbons by natural estuarine colloids. Mar. Environ. Res. 11, 77-89. 

As discussed elsewhere in this volume, another field where the use of U-series disequilibria has proven to be very useful is the study of chemical transport in waters, either marine (see Cochran and Masque 2003 Henderson and Anderson 2003), estuarine (Swarzenski et al. 2003) or continental waters. In the continental domain, in addition to characterization of transfer processes related to groundwater flows (Porcelli and Swarzenski 2003), radioactive disequilibria have also helped in constraining chemical exchanges between particulate, dissolved and colloidal loads of waters, as well as the origin of chemical fluxes carried by waters. 

River inputs. The riverine endmember is most often highly variable. Fluctuations of the chemical signature of river water discharging into an estuary are clearly critical to determine the effects of estuarine mixing. The characteristics of U- and Th-series nuclides in rivers are reviewed most recently by Chabaux et al. (2003). Important factors include the major element composition, the characteristics and concentrations of particular constituents that can complex or adsorb U- and Th-series nuclides, such as organic ligands, particles or colloids. River flow rates clearly will also have an effect on the rates and patterns of mixing in the estuary (Ponter et al. 1990 Shiller and Boyle 1991). 

Removal to sediments. Removal of surface-reactive trace elements from the oceans readily occurs by adsorption onto settling particles, and this process is most pronounced in the typically high-energy, particle-rich estuarine environment. Particles are supplied by rivers, augmented by additions of organic material generated within the estuary. Also, floes are created in estuaries from such components as humic acids and Fe. The interaction between dissolved and colloidal species is enhanced by the continuous resuspension of sediments in... 

Importance of particles and colloids for controlling estuarine uranium... 

Evidence for the association of U with humic acids has been documented elsewhere. Dearlove et al. (1991) showed that U concentrated by ultrafiltration techniques from organic-rich groundwater samples were associated with humic colloids. Humic and fulvic acids have been shown to strongly complex U. Lienert et al. (1994) modeled the distribution of U species in the Glatt River and concluded that U-humate complexes become important at pH <6.8. These results reinforce the conclusions in the estuarine studies that U humate and fulvate complexes may account for the association of U with colloids. 

A series of experiments was also conducted by Bowman et al. [34] to ascertain the effects of differing environmental factors on the sediment-water interactions of natural estrogens (estradiol and estrone) under estuarine conditions. Sorption onto sediment particles was in this case relatively slow, with sorption equilibrium being reached in about 10 and 170 h for estrone and estradiol, respectively. On the other hand, true partition coefficients calculated on colloids were found to be around two orders of magnitude greater that those on sediment particles. Hence, it was concluded that under estuarine conditions, and in comparison to other more hydrophobic compounds, both estrone and estradiol... 

Wen, L.-S., Santschi, P., Gill, G. and Paternostro, C. (1999). Estuarine trace metal distributions in Galveston Bay importance of colloidal forms in the speciation of the dissolved phase, Mar. Chem., 63, 185-212. 

The important forces involved in the adsorption of metals on to particles are attractive electrostatic or van der Waals forces. These concepts explain many of the properties of colloids with respect to the adsorption of contaminants or ion-exchange factors and the aggregation of the colloids into larger particles. These larger particulates may then descend the water column to the sediment. This occurs most notably in estuarine environments, as increases in salinity lead to estuarine silting. Binding of electrolytes to hydrophobic colloids is often used to facilitate their coagulation and precipitation. 

Mayer, L.M. (1982) Aggregation of colloidal iron during estuarine mixing Kinetics, mechanism, and seasonality. Geochim. Cosmochim. Acta 46 2527-2535... 

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