Dissolved organic carbon speciation

In addition, dissolved organic carbon (DOC) is also an important soil solution solute affecting speciation and bioavailability of many trace elements in soil solution. Many trace elements and heavy metals complex with dissolved organic carbon. This is especially important in arid and semi-arid environments since high soil pH increases the solubility of organic molecules and accordingly increases concentrations of dissolved organic carbon in soil solution. 

An approach such as that suggested for the re-examination of the molecular weight distribution of dissolved organic carbon in seawater (Carlson et al., 1985) has been successfully adapted to evaluate the performance of ultrafiltration in the study of radionuclide speciation ... 

Chemical Speciation Models. Using the stability constants derived by us for copper complexes with hydroxo and carbonate ligands (Table I) and for natural organic ligands (Table II), the Newport and Neuse Rivers were modeled for copper speciation as a function of pH, total copper, carbonate alkalinity and total dissolved organic matter. Speciation models were calculated from the equation ... 

Reduction of As(V) can occur in the presence of air if samples contain dissolved organic carbon (DOC), arsenate-reducing bacteria, and no preservatives (Bednar et al., 2002 Hall et al., 1999 Inskeep et al., 2002). Arsenic(V) can then be reduced rapidly, within a few days. Storage at 3-5 °C and in the dark helps to preserve the speciation (Hall et al., 1999 Lindemann et al., 2000). Ideally, speciation studies for arsenic or selenium should involve a minimum of time between sampling and analysis. 

Moreover, the availability of metals for uptake by organisms under field will vary from site to site and is highly dependent on the speciation of the metal and the water chemistry (e.g. pH, Dissolved Organic Carbon [DOC]). Hence it is of utmost importance to take the speciation and/or bioavailability into account. 

The purpose of this chapter is to review our present knowledge of the mineralogy and chemical processes controlling dissolved Mo in soils. This chapter emphasizes speciation, adsorption and desorption, and the precipitation and dissolution processes of dissolved Mo. In addition, the importance of dissolved organic carbon in these processes is discussed. 

Equilibrium complexation constants for Cu reactions with natural organic matter and the details of Cu speciation are bound to remain somewhat uncertain, since the composition of the complexing molecules varies from site to site. What is not in dispute is that the fraction of dissolved copper present as free aquo Cu is probably very small in any natural water. In extremely pristine waters, hydroxide and carbonate complexes may dominate, but organic complexes usually dominate in waters containing more than a few tenths of a mg/L organic carbon. 

Presently, the precise determination of the true dissolved Th fraction in water samples remains a challenge. Results from ultrafiltration experiments on organic-rich water samples from the Mengong river tend to demonstrate that Th concentration is less than 15 ng/L in absence of DOC (Table 2 and Viers et al. 1997), and that Th is still controlled by organic carbon in the final filtrate of the ultrafiltration experiments. The latter conclusion is also supported by the results obtained for the Kalix river (Porcelli et al. 2001). These results therefore not only raised the question of the determination of the amount of dissolved Th in water but also of the nature of Th chemical speciation. 

Finally, models were calculated which predict the variations in chemical speciation of copper resulting from changes in the chemical parameters pH, carbonate alkalinity, concentration of dissolved organic matter, and concentration of total dissolved copper. 

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