Thermodynamic Model of Calcium Carbonate System of Soil Solution

Abstract

Carbonate system determines the basic processes of soil formation and evolution. The aim of research was to show the importance of chemical Carbonate Calcium Equilibrium (CCE) in soil solutions. CCE causes a degree of soil solution’s saturation with CaCO₃ as a factor of carbonate geochemical cycle. CCE depends on chemical composition, pH, Eh, buffering properties of liquid phase, dissolution, migration, precipitation of carbonates in the soil profile and landscape, ion exchange processes at the interface of soil solid and liquid phases. At high ionic force in soil solution are formed electrically neutral ion pairs СаСО₃⁰; CaSO₄⁰, MgCO₃⁰, MgSO₄⁰ and charged ion pairs CaHCO₃⁺, MgHCO₃⁺, NaCO₃^-, NaSO₄^-, CaOH⁺, MgOH⁺. The object of research-saline soil of dry steppe zone of Southern Russia. This soil is moderately thick, solonized, humus 2.6%, physical clay 47.7%, clay 29.5%, CaCO₃ 0.15% (up to 3-10% at the depth of 0,8-1,5 m), pH = 7.8-9.0, readily soluble salts in soil solution from 0.2% in 0-5 cm to 3.5% in 70-80 cm, exchangeable cations: Ca²⁺ – 182 mmol kg^-1, Mg²⁺ – 65 mmol kg^-1, Na⁺ – 34 mmol kg^-1. The soil solution was extracted from soil layers at the depth of 0-150 cm. Ion’s composition of the soil solution was determined using standard analytical methods. The mathematical model shows the real state of associated ions at high ionic strength of saline soil solution. On the basis of algorithm CCE a computer program was developed The concentration of free and associated ion form is a result of iteration beginning from analytical ion concentration considering ion material balance, linear interpolation of equilibrium constants, method of ionic pairs, laws of: Initial concentration preservation, operating masses of equilibrium system. Concentration constants of ion pair dissociation were calculated following the law of operating masses. Were determined a quantity of ion free form and the coefficient of ion association γ_e as a ratio of ion’s free form to its analytical content γ_e = C_ass/C_an. Because of association of ions, which differs in soil layers: The chemical equilibrium in soil solution determines high mobility of carbonates, transformation of solution, carbon sink from soil.