The CaCO3-CO2-H20 system in soils'

Equations used to develop the CO2 -Ca-pH relationships in calcareous soils are reviewed. The equation Pco, (Ca) --(H) 2 Ke , is used to draw a three-dimensional surface and to derive three partial differential equations to illustrate the relationships between CO 2 partial pressure, Ca activity and pH. lc is a combination of Henry's Law constant, the first and second dissociation constants for carbonic acid and the calcite solubility product. The three dimensional CO2-Ca-pH surface illustrates how the three parameters relate to each other under ideal conditions. The partial differential equations are presented to illustrate how changes in one parameter affect the other two. The CO,-Ca-pH surface provides a graphical method for introducing the idea of three component equilibria, while the partial differential equations provide a mathematical representation of these interactions for those with chemical thermodynamics or strong mathematic or modeling backgrounds. Deviations from this Ideal model in natural systems are discussed for those who wish to extend the discussion to natural systems. Additional index words: Calcite, Aragonite, Vaterite, Carbonate minerals, Calcareous soils, Lime. S OH, solutions and natural waters in contact with calcium carbonate minerals are buffered with respect to carbon dioxide (CO2), pH, carbonate (C0? 4), bicarbonate (HCO;), calcium (Cal, and several complex ion pair activities. Equilibrium in natural calcium carbonate systems is also affected by other ions in solution that produce common ion effects, or activity coefficient changes due to total ion concentrations. Precipitation and dissolution rates and equilibria can also be affected by contamination of mineral surface sites by inorganic ions or organic solutes. Calcium carbonate systems have been described by a number of different approaches, but most models imposed arftificial constraints or simplifications on the system. The constraints make the system easier to describe, but limit the models' ability in describing natural systems. These simplifications are discussed in detail. by Nakayama (1970). Computer models (Robbins et al., 1980; and Tanji and Doifeen, 1966) and graphical calculation methods (Suarez, 1982) have been developed for estimating calcium carbonate equilibrium. It is not always easy to look at these calcium carbonate models and gain an intuitive feel for what is actually happening when solution components are varied. The objective here is to describe a calcareous soil system in a way that is easier to visualize CO 2 , water, and calcium carbonate equilibrium without oversimplification. The system is open with respect to Ca 2+ , H 4 , HCOi, CO;-, CO,, and any other ions, ion pairs, or dissolved gases that may be found in a natural system. This approach allows for CO, addition from biological activity or removal by escape into the atmosphere or as a leachate solute. Cations and anions are allowed to enter or leave the system as solutes, dissolution and precipitation products or reactants, or cation exchange products or reactants. In the initial development, chemical equilibrium is assumed and then later, departure from equilibrium will be discussed. As CO, gas dissolves in water, less than 0.3% of the CO200 is hydrated to H 2 CO 3 at 25°C (Ponnamperuma, 1967). Since it is difficult to distinguish between CO20,0 and H 2 CO 3 , the composite acid H 2 CO: is defined as the analytical sum of CO2(aq) and H 2 CO 3 . The pure acid, H,CO, is much stronger (pKH,co, = 3.8) than the composite, H 2 COP (pIC Elico7 = 6.3) (Stumm and Morgan, 1970). The difficulty in identifying the species involved in the above reaction makes it more practical to use the "apparent" solubility expression H2O + CO2(8) 7-= H 2C0 1: with the equilibrium for the combined reactions expressed as (11 2CM K H Pco, [11 where Pco, is the CO, partial pressure and K H is Henry's Law constant. The carbonic acid thus formed quickly dissociates to form HCO; and H4 ions, and at high pH the second dissociation produces CO3 4 and a second H4 ion. ' Contribution from ARS, USDA, Snake River Conservation Research Center, Kimberly, ID 83341. 'Soil scientist.