The Carbonate System

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

The carbonate system plays a pivotal role in most global cycles. For example, gas exchange of CO2 is the exchange mechanism between the ocean and atmosphere. In the deep sea, the concentration of COi ion determines the depth at which CaCOs is preserved in marine sediments. 

Dissolved inorganic carbon is present as three main species which are H2CO3, HCOs and CO. Analytically we have to approach the carbonate system through measurements of pH, total CO2 or DIC, alkalinity (Aik), and PcOj- In an open carbonate system there are six unknown species H", OH , PcOj/ H2CO3, HCOs, and CO . The four equilibrium constants connecting these species are K, Ki, Kh, and fCw. The values of these equilibrium constants vary with T, P, and S (Millero, 1995). To solve for the six rmknowns we need to measure two of the four analytical parameters (Stumm and Morgan, 1996). Direct measurement of Pco is the best approach, but if that is not possible then the most accurate and precise pair (Dickson, 1993) is Total CO2 by the coulometric method Johnson et al., 1993) and pH by the colorimetric method (Clayton et ah, 1995). 

By analogy to the carbon systems that contain three atoms, describe the structure of interhalogen species (see Chapter 15) such as I3 . Assuming that only p orbitals are used, describe the bonding in this species. 

The strong increase in atmospheric concentrations of carbon dioxide [ 127] has generated considerable interest in the global carbon cycle [ 128-130]. Techniques for determining the components of the carbonate system have been refined, new techniques have been developed, or both. Among the four measurable parameters (total inorganic carbon), pH, pC02, and total alkalinity... 

Chapter 5 introduces the equations that describe equilibrium between dissolved species of carbon in the ocean. The carbon system is central to so many aspects of ocean, atmosphere, and sedimentary rocks that I wish to make available a general method of solution. This chapter also introduces several housekeeping routines that I have found useful. Examples are routines to read files of specifications, including initial values, to write... 

As an application of these computational helpers I shall also introduce the carbon system and the equilibrium relationships among the species of carbon dissolved in natural waters. 

Program DGC09 solves this version of the carbon system and is discussed in the next section. 

Program OGC09 solves the carbon system in atmosphere, shallow,... 

Program IS0TQ1 solves the carbon system in atmosphere, shallow, and deep sea. Includes a gradual injection of fossil fuel carbon dioxide, read from a file. 

Program ISOT02 solves the carbon system in an evaporating lagoon nrow =5 the number of equations and unknowns ncol = nrow + 1... 

The temperature experiment can be expected to show only the effects of the temperature dependence of the equilibrium constants in the carbonate system. Other possible consequences of changing temperature are not included in the simulation. Figure 6-7 shows little response by the calcium... 

Figure 6-10 shows how the various elements of the carbon system respond to the seasonal change in evaporation rate. Although the fluctuations in carbonate ion concentration cannot be seen on the scale of this figure, examination of the numbers shows that the amplitude of the carbonate fluctuation is about 3 percent, comparable to the amplitude of the... 

In this chapter I explained how isotope ratios may be calculated from equations that are closely related, but not identical, to the equations for the bulk species. Extra terms arise in the isotope equations because isotopic composition is most conveniently expressed in terms of ratios of concentrations. I illustrated the use of these equations in a calculation of the carbon isotopic composition of atmosphere, surface ocean, and deep ocean and in the response of isotope ratios to the combustion of fossil fuels. As an alternative application, I simulated the response of the carbon system in an evaporating lagoon to seasonal changes in biological productivity, temperature, and evaporation rate. With a simulation like the one presented here it is quite easy to explore the effects of various perturbations. Although not done here, it would be easy also to examine the sensitivity of the results to such parameters as water depth and salinity. 

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Chemical addition for the removal of inorganic compounds is a well-established technology. There are three common types of chemical addition systems that depend upon the low solubility of inorganics at a specific pH. These include the carbonate system, the hydroxide system, and the sulfide system. 

In reviewing the basic solubility products for these systems, the sulfide system removes the most inorganics, with the exception of arsenic, because of the low solubility of sulfide compounds. This increased removal capability is offset by the difficulty in handling the chemicals and the fact that sulfide sludges are susceptible to oxidation to sulfate when exposed to air, resulting in resolubilization of the metals. The carbonate system is a method that relies on the use of soda ash (sodium carbonate) and pH adjustment between 8.2 and 8.5. The carbonate system, although... 

For some reactions, has been determined by direct measurement over a broad range of temperature, pressure, and salinities. Enough data exist to formulate empirical equations that enable extrapolation to the exact temperature, salinity, and pressure of interest. This has been done for the chemical reactions in the carbonate system, for the dissociation of water and for the dissolution of gases. These equations have been used to formulate look-up tables, such as those presented in the online appendix on the companion website. 

As a result of these reactions, the carbonate system can buffer against changes in pH caused by addition of acid via two reactions ... 

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