Carbon Species in Water

The distribution of 5 C-values with water depth is mainly controlled by biological processes Conversion of CO2 into organic matter removes C resulting in a C enrichment of the residual DIG. In turn, the oxidation of organic matter releases C-enriched carbon back into the inorganic reservoir, which results into a depth-dependent isotope profile. A typical example is shown in Fig. 3.21. 

North Atlantic Deep Water (NADW), which is formed with an initial 5 C-value between 1.0 and 1.5%c, becomes gradually depleted in C as it travels southward and mixes with Antarctic bottom water, which has an average 8 C-value of 0.3%c (Kroopnick 1985). As this deep water travels to the Pacific Ocean, its C/ C ratio is further reduced by 0.5%o by the continuous flux and oxidation of organic matter in the water column. This is the basis for using 8 C-values as a tracer of paleo-oceanographic changes in deep water circulation (e.g., Curry et al. 1988). 

The uptake of anthropogenic CO2 by the ocean is a crucial process for the carbon cycle, resulting in changes of the 5 C-value of dissolved oceanic bicarbonate (Quay et al. 1992 Bacastow et al. 1996 Gruber 1998 Gruber et al. 1999 Sonnerup et al. 1999). Quay et al. (1992) first demonstrated that the 5 C-value of dissolved bicarbonate in the surface waters of the Pacific has decreased by about 0.4%c between 1970 and 1990. If this number is valid for the ocean as a whole, it would allow a quantitative estimate for the net sink of anthropogenically produced CO2. Recent accounts estimate that the Earth s ocean has absorbed around 50% of the CO2 emitted over the industrial period (Mikaloff-Fletcher et al. 2006). 

Particulate organic matter (POM) in the ocean originates largely from plankton in the euphotic zone and reflects living plankton populations. Between 40°N and 40°S 

Jeffrey et al. (1983) interpreted this trend as the loss of labile, C-enriched amino acids and sugars through biological reworking which leaves behind the more refractory, isotopically light lipid components. 

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Figure 3.10 illustrates the important relationships among dissolved CO2 in water, dissolved carbonate species (HCOj, CO3"), and solid carbonate minerals, particnlarly limestone (CaCOj) and dolomite (CaCOj MgCOj). These species are very important in the chemistry of the hydrosphere. As examples, many minerals are deposited as salts of COj" ion and algae in water utilize dissolved CO2 and HCO3 in the synthesis of biomass. The equilibrium relationships among CO2 gas in the atmosphere, dissolved CO2 and carbonate species in water, and solid carbonate minerals largely determine the pH, alkalinity, and hardness of water. 

Rightmire, C. T., Hanshaw, B. B., Relationship between the carbon isotope composition of soil C02 and dissolved carbonate species in qroundwater. Water Resour. Research, 9(4), 958-567 (1973). 

Figure 3.2 Diurnal changes in pH and concentrations of carbonate species in the flood-water in a ricefield (Mikkelsen et al., 1978). Reproduced by permission of Soil Sci. Soc. Am.

The hydration rate constant of C02, the dehydration rate constant of carbonic acid (H2C03), and p pK2 values (pTf, =6.03, pTf2 = 9.8 at 25 °C, 7=0.5 M) (63) are such that nearly 99% of dissolved carbon dioxide in water at pH < 4 exists as C02. However, these four different species may be considered as the reactive species under different pH conditions which can react with aqua metal ions or their hydroxide analogues to generate the metal carbonato complexes. The metal bound aqua ligand is a substantially stronger acid than bulk H20 ( )K= 15.7). Typical value of the p of H20 bound to a metal ion may be taken to be 7. Hence the substantial fraction of such an aqua metal ion will exist as M-OH(aq)(ra 1) + species at nearly neutral pH in aqueous medium. A major reaction for the formation of carbonato complex, therefore, will involve pH controlled C02 uptake by the M-OH(" 1)+ as given in Eq. (17). 

The original parfait method rested on the use of vacuum distillation—lyophilization to concentrate the poorly volatile species in water. It might be expected that the removal of water under vacuum should be simple and straightforward. Vacuum distillation and lyophilization do indeed recover the poorly volatile contaminants from unfractionated surface waters. However, the compounds are often obtained in an intractable, insoluble form. These intractable precipitates are believed to form when bicarbonate dissociates under vacuum to form metal carbonate precipitates that trap organic polymers and lipids (4, 5). The parfait method prevents the formation of these precipitates by removing metal ions on an acidic cation-exchange bed. 

Depending upon the composition of the water, other products may be formed. Typically the dissolved species in water such as chloride, sulfate or carbonate, present in small concentrations, give rise to the chloride, sulfate or carbonate compounds of copper. 

Neutral molecules have activity coefficients essentially equal to unity in solutions of less than 10 mM ionic strength. At higher salt concentrations, most neutral molecules are increasingly salted out of water that is, the activity coefficient > 1, so that a, /c, < 1 for molecules in higher ionic strength solutions. In our discussion of dilute aqueous acids and bases, we will assume ideal behavior of the neutral species. The importance of salting out of dissolved CO2 will be reflected in considering dissolved carbonic species in seawater (Chapter 4). 

The equilibrium of sulfide in water, the percentages of H2S, HS, and species, is dependent on the pH. Figure 1 shows the distribution of each species at various pH. At a pH of approx 5.7, the sulfide species in water would be near 100% H2S and at approx pH 7, 50% of the sulfide species in water would be H2S and the other 50% would be HS species. The H2S species are volatile as a result, the aeration process effectively removes it from the water. Therefore, the removal efficiency of sulfide depends on pH. As the pH increases, aeration becomes less effective because there are fewer sulfides in the form of H2S, which is readily removed by aeration. This process is utilized by both municipalities and chemical industries. In water treatment, the process is called degasification, and is effectively used to remove both H2S and carbon dioxide from well water and product water from the reverse osmosis process. 

The plot in Fig. 4.2 demonstrates the relative importance of the three carbonate species in seawater as a function of pH. At pH = pKj the concentrations of GO2 and HGO3 are equal and at pH = pK the concentrations of HGO3 and GOg" are equal. Since we know that the pH of surface waters is about 8.2, it is clear that the dominant carbonate species is HGO3. What has been done so far, however, does... 

We will now take a more rigorous approach to this question and compute the distribution diagram for carbonic acid species in water as a function of pH, but ignore ion activity coefficients. First let us define the total carbonate, Cj, where... 

In practice the measured content of P species in fresh and marine waters is higher than dissolved carbon species. This apparent inconsistency arises from sources of nutrient supply. We have seen (Chapter 3, Sections I and 2), that carbonate can be re-supplied by atmospheric CO2. A similar explanation applies to nitrogen since this element can be fixed by blue-green algae with corresponding increasing of dissolved nitrogen species in waters. 

Manganese solubility is controlled by the redox potential and pH of the soil. The Mn ion is a very soluble species in water, forming hydroxide and carbonate precipitates only at high pH (>7). However, as the pH is raised above 6 in soils. 

There have been several studies of the thermal decomposition of titanyl oxalates this year. The general pattern is initial loss of water followed by decomposition of the oxalate groups with the liberation of CO and COj and the formation of carbonate species. In some cases, solids containing entrapped CO2 are formed. If the decomposition is carried out under vacuum or inert atmosphere conditions, partial reduction of the metal may occur e.g., Pb"->Pb . The final stage in all the decompositions is the formation of the metal metatitanate an oxidizing environment must be maintained throughout for pure metatitanate to be obtained. 

The worldwide economic impact of undesirable cyanobacterial blooms in municipal water systems is unknown. Increased costs can occur due to increased amounts of activated carbon used in water treatment facilities to counteract higher levels of off-flavor compounds in the water. In addition, filters and filtration equipment can become clogged with certain types of filamentous cyanobacteria and result in significant down time to correct these problems. Certain species of cyanobacteria (e.g., Lyngbya wollei)... 

The predominant carbonate species in normal seawater is bicarbonate (HCO3 ), close to 70 % of the dissolved inorganic carbon (DIC). About 10 % of the total calcium in normal sea-water exists as CaS04 complex. This complex is rather insignifi-... 

FIG. 22.6. Fraction of uranium species in water with natural carbon dioxide content and at different pH, showing hydrolysis and C03 complexation. 

The concentrations of inorganic carbon species in sea water are controlled not only by the chemical reactions outlined above (i.e., eqn [I]) but also by various physical and biological processes, including the exchange of CO2 between ocean and atmosphere the solubility of CO2 photosynthesis and respiration and the formation and dissolution of calcium carbonate (CaC03). 

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