Carbonate, chemical speciation

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. 

Ferrari, S., Belevi, H. Baccini, P. 2002. Chemical speciation of carbon in municipal solid waste incinerator residues. Waste Management, 22, 303-314. 

Physico-chemical speciation refers to the various physical and chemical forms in which an element may exist in the system. In oceanic waters, it is difficult to determine chemical species directly. Whereas some individual species can be analysed, others can only be inferred from thermodynamic equilibrium models as exemplified by the speciation of carbonic acid in Figure 9. Often an element is fractionated into various forms that behave similarly under a given physical (e.g., filtration) or chemical (e.g., ion exchange) operation. The resulting partition of the element is highly dependent upon the procedure utilised, and so known as operationally defined. In the following discussion, speciation will be exemplified with respect to size distribution, complexation characteristics, redox behaviour and methylation reactions. 

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. 

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 ... 

Figure 15. Chemical speciation model for dissolved copper in the Neuse River at 25°C as a function of carbonate alkalinity at p[Cutot] 7...

The bioavailability of a given metal is influenced by its chemical speciation in the ambient environment. Although some metals occur predominantly in their free or aquo form (that is, the inner coordination sphere of the metal ion is occupied solely by water molecules), most bioactive metals occur as complexes in the natural environment. Hydroxide, carbonate, and chloride anions can all bind transition metals to a significant extent. For example, Fe + forms hydroxide... 

Finally, we must somehow resolve the vexing problem of the chemical speciation of trace elements in seawater what is the chemical nature of the various metal chelators whose existence has been demonstrated by electrochemistry Is the chemistry of several metals in surface seawater really controlled by metallophores released by prokaryotes Or are dissolved metals chiefly present as parts of metalloproteins in the process of remineralization How do metal chelators affect the residence times of metals (in particular, scavenged elements such as iron and cobalt) and in turn how do those chelators influence the global carbon cycle via changes in marine primary productivity ... 

Thus, the empirical characterization of the OV transient produced from OH in seawater leads to a chemical mystery the UV transient definitely interacts with seawater in a pH and CO2-dependent process that occurs in waters of different origin, including artificial seawater. Hence, the effect would appear to be due to the carbonate system itself, not some trace impurity. The following paper tahes up the challenges of finding such an interaction in simpler media than seawater and of explaining the pseudo first-order decay of seawater in terms of these interactions and carbonate system speciation (16). 

Bc2 produced by flash photolysis of seawater decays by parallel first- and second-order reactions. The environmentally important exponential decay is a pseudo first-order reaction of Br2 with the carbonate/ bicarbonate system in seawater. A chemical speciation model for the free ions and ion-pairs in seawater and in solutions at seawater ionic strength allowed us to measure the dependence of the pseudo first-order rate term, a, on individual C02-containing species. A predictive equation based on reaction of Br2 with free C03 and the hgC03°, NaC03 and CaC03 ion pairs accounts for the mean seawater a at pH 8.1 within experimental uncertainty. The reaction productfs) are unknown. 

In order to investigate the dependence of a on dissolved CO2 and pH, we prepared solutions of simpler composition in which the concentrations of several important free ions and ion-pairs were maximized. In order to do this, we calculated the chemical speciation of carbon-ate/bicarbonate in seawater and in solutions of KCl, NaCl, NgCl2 end mixtures of these salts as a function of pH in the range 6.0 to 8.9. Ion strengths were kept equivalent to those of natural seawater (y 0.7 at S = 35 / <). For the seawater calculations, we assumed the concentrations of major cations given by Culkin (5) and a total CO2 concentration of 2.333 x 10 N. We used an iterative con uter program to solve a series of simultaneous equations based on thermodynamic association constants and estimated activity coefficients at ionic strengths of solutions for free carbonate and bicarbonate, and for the carbonate/bicarbonate ion-pairs with Mg, Ca and Na (6). Our calculations did not consider the reduction in available cations... 

TABLE I. Chemical Speciation of Carbonate and Bicarbonate as a Function of pH and... 

The Six Cities and the ACS studies were designed before concerns about health effects of different specific components of PM2.5, including black carbon and PM2.5 species with little ambient mass (such as metals), had become a major research focus. Perhaps for this reason, the only chemically-speciated type of... 

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