Thermodynamic Model of Calcium Carbonate System of Soil Solution

Corresponding Author: Tatiana M. Minkina, Southern Federal University, Rostov-on-Don, Russia Tel: +7(918)5531632 Email: tminkina@mail.ru 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 СаСО3 ; CaSO4 , MgCO3 , MgSO4 0 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: Ca – 182 mmol kg, Mg – 65 mmol kg, Na – 34 mmol kg. 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 γe as a ratio of ion’s free form to its analytical content γe = Cass/Can. 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.


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 CaCO 3 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 СаСО 3 0 ; CaSO 4 0 , MgCO 3 0 , MgSO 4 0 and charged ion pairs CaHCO 3 + , MgHCO 3 + , NaCO 3 -, NaSO 4 -, 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%, CaCO 3 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: Ca 2+ -182 mmol kg −1 , Mg 2+ -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

Introduction
The process of formation of the chemical composition of soil solution is very complex. It is influenced by various factors. One such factor is a nature of most important chemical equilibrium in soil solutions-Carbonate Calcium Equilibrium (CCE). CCE depends on the state of 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 solid and liquid phases. The study of CCE is a base for CaCO 3 solubility calculation at given conditions. The uncertainties where found of physical and biogeochemical models of aquatic and terrestrial systems, improvement of models are recomended (Romanou et al., 2013).
Concerning the carbon sequestration and climate engineering the soil as a drain for carbon sink is to be assessed. Uncertainty of Biosphere is linked to geochemical cycle of soil (Bahn et al., 2015;Seneviratne et al., 2012;Reichstein et al., 2013). The heterogeneity of carbon stream is a function of landscape (Ågren et al., 2014). In boreal landscape it is a major source of dissolved organic carbon. In the semiarid and arid catchments the soil solution equilibrium causes the carbon mobility into the soil, thus the possibility of uncontrolled carbon flux from soil and biosphere is high.
The sinking of carbon is one of the key components of the carbon cycle and global climate system (Lima et al., 2013). It is important to quantify the carbon source/sink nature, determine its drivers (Turi et al., 2014). Water saturation with CaCO 3 gives an adverse biogeochemical effect .
Soil solution equilibrium helps to represent properly the link of soil and vegetation (Melton and Arora, 2014). The CCE of water and soil solution influences the growth of leading agents of biosphere -water and soil organisms (Teichert and Freiwald, 2014). Soil solution varies on temporal and spatial scales (Mezbahuddin et al., 2014). Anthropogenic influence on soil is dangerous (Leiber-Sauheitl et al., 2014). The problem of carbon cycle is linked to release of entrapped methane form the soil structure, especially when soil is in a state of disturbance (Nauer et al., 2013). The sinks and sources of carbon are the key point for terrestrial and aquatic ecosystem models, drivers of atmospheric CO 2 concentration, biosphere functioning and climate (Peng et al., 2014).
The soil is an important part of terrestrial system. CCE of the soil solution is important for understanding the origin of Pedosphere variability of soil solution, destruction and synthesis of organics, secondary minerals and compounds (Amakor et al., 2013;Hunenberger and Relf, 2011;Visconti and de Paz, 2012).
The mineralization of soil solution varies because of geological and biological composition of the local biogeosystem, regional and local water-mass-transfer, wetting and drying circles of the soil, biological process in the soil. CCE is a base for modeling the heavy metals transfer in soil (Anisimov et al., 2015;Endovitskii et al., 2014;Endovitsky et al., 2014a;2014b;2015b;Chaplygin et al., 2014).
Under natural conditions not content of НСО 3 -, СО 3 2and H 2 CO 3 in soil solution depends on pH, but namely the ratio of forms of CCE determines the pH value, buffering and redox properties of the soil solution, nature of ion exchange in the system "solid phase-solution", solubility of many mineral and organic compounds. The precipitation or dissolution of CaCO 3 is linked to removal or receipt of Ca 2+ , НСО 3 and СО 3 2 according to CCE. An important characteristic of CCE is a degree of saturation of the soil solutions with CaCO 3 . However, it does not give a clear answer to the question about the formation of CaCO 3 . The studies of carbonate equilibrium in surface waters show that the state of saturation or super saturation of natural waters with CaCO 3 demonstrates only the possibility of a precipitate forming, but not always the CaCO 3 precipitates from supersaturated solutions. The link between solution's saturation with CaCO 3 and its precipitation depends on physicochemical and biochemical processes, which stably hold CaCO 3 precipitation.
The existence of solutions supersaturated with CaCO 3 is a property of the CCE. Conditions and sustainability of solution's super-saturation with CaCO 3 varies. One cause is related to the properties of solutions of calcium hydro-carbonate. It is the formation of a solid phase which differs in chemical composition from the solution. The second-at extremely low concentration of СО 3 2ions in solution the spatial orientation is required for onset of crystallization, which is probably hampered by the huge number of surrounding ions НСО 3 -. Another reason which determines the high degree of calculated saturation of soil solution with СаСО 3 is the complexation of Са 2+ with organic matter.
For the most cases of soil solution are suitable the basic laws of ion association solution thermodynamics and thermodynamic models of macro-processes of saturation in soil solution, transfer, metamofization and accumulation of salts in the three-dimensional soil continuum (Minkina et al., 2012a).
The aim of the research is to show the role of CCE in soil solution on the basis of thermodynamics mathematical model of CCE of soil solution.

Study Area
The South-East of the Russian Federation, Rostov region. Lower Don.
Object of research-the dry steppe chestnut saline solonetzic soil of Southern Russia. The climate is arid, annual precipitation of 300-350 mm. The parent rocks are Carbonate and Carbonate-sulfate loess-like loam and clay.

Sampling and Analysis
The soil section has been made in semihydromorphic landscape. Soil species were sampled from a section wall down to the depth of 1 m, samples from the deeper soil layers were made by the soil auger, drill cup diameter 5 cm.
In procedure of soil samples preparation the soil was crushed and sifted, openings 2 mm, than mixed with quartz sand in ratio 1:2 (Carter and Gregorich, 2007;Minkina et al., 2012b), than soil from every soil layer was charged into glass tube, inner diameter of 3.4 cm, length of 100 cm. At the bottom of the tube was mounted a solution draining off outlet. Soil solution was allocated directly with ethyl alcohol poured on the top of soil column. The volume of soil solution emitted from every soil column was 20-60 mL.
Soil solution was analyzed by standard methods (Carter and Gregorich, 2007;Visconti and de Paz, 2012).
Moisture of soil was determined by thermostat 105°C method. Dry residual of soil was determined by thermostat 105°C method. pH was measured in thermostat (20±0,2°C) by pH-meter with a glass electrode. The carbonate and bicarbonate anions were titrated directly by 0.01 M hydrochloric acid detenting titration, endpoint on color change of indicators-phenolphthalein and methyl orange. The chloride ion was detected by argentometric method with potassium chromate. The total content of Ca 2+ and Mg 2+ was measured by complexometric titration. In another aliquot Ca 2+ was measured in complexometric. Mg 2+ was calculated as a difference. The sulfate was analyzed by BaSO 4 sedimentation method. Na + by flame photometric detection.
The experiment was performed in triplicate. All statistical calculations were performed using Microsoft Excel 2010.

Results
Carbonate system is complex and important chemical system of soil solution because it influences the nature of many basic processes of formation and evolution of a certain type of soil. Carbonate system includes a number of dynamic equilibriums (Fig. 1).
The CCE of soil solution is an adsorption-hydration balance between solution, gas phase, bioorganic phase. This balance includes step dissociation of carbonic acid; CCE between solution, soil absorbing complex, sediments of CaCO 3 and solid phase, the ion equilibrium of water. An important characteristic of the CCE is a degree of saturation of solution with CaCO 3 . The deposition or dissolution of CaCO 3 caused by receipt or removal of Ca 2+ , HCO 3 and CO 3 2from solution, as well as carbonate equilibrium shift, change the ionic composition of the soil solution. Thus the type of migration and accumulation of carbonates in soil profile becomes special.
The analytical composition of typical dry steppe chestnut saline solonetzic soil of Southern Russia is given in Table 1.
It was shown here above that analytical composition of solution is suitable characteristic at low of main ions concentration only. The measure of real chemical reactions is ions activity.
Was determined the real state of the main ions in soil solutions on the basis of ionic strength and ion's association in soil solution. The model of soil solution equilibrium was proposed (Endovitskii and Minkin, 1979;Minkin and Endovitsky, 1986;Endovitskii et al., 1985;2009).
The algorithm and computer programs were developed.  To calculate the soil solution main ion's forms were used the analytical data (Table 1), Method of Ionic Pairs (MIP) (Adams, 1971): The law of initial concentration preservation, the law of the operating masses of chemical equilibrium system.
The iteration was used: To solve a system of algebraic equations of material balance of ions; linear interpolation to calculate values of tabulated equilibrium constants according calculated data. The equations of main ions material balance are as follows Equation Na Na NaCO NaSO   ; .
NaCO NaSO Na CO Na SO K K NaCO NaSO The equilibrium concentration of ionic pair was replaced in Equation 1-6 with its value according to relevant dissociation constant from Equation 7-9.
Thermodynamic constants of dissociation from (Lurie, 1986;Handbook of chemist 21;Sposito, 1989): The equilibrium concentrations of ion's free forms were used as 6 unknowns of equation's system. Material balance according analytical concentration of chemical elements.
The equilibrium concentrations of free ions was iterated according dissociation constants (7-9).
The effective ionic force according to equilibrium concentration of all ion forms Equation 18 At the first iteration step the concentration constants of dissociation were calculated (16). The next iteration was made on the new set of equations of material balance (10-15). 87 The coefficient of ion association γ e is proposed as a ratio of calculated ion free form to its analytical content Equation (19) Where: C ass = Calculated ion content in solution taking into account its association with another ions C an = analytical concentration of an ion For calculation were used the own software products (Endovitskii et al., 1985;2009).

Discussion
The procedures specified by Equation 7-19 were used to calculate the soil solution ions association in soil horizons at the given soil moisture and analytical composition ( Table 1).
The data of calculation (Table 2) represents the variation of ions association in soil layers. The real equilibrium concentration of ion forms in soil solution depends on concentration and composition of soil solution. The higher salinity of the soil solution, the more ions are associated.
On the other hand, in the non-saline horizon 0-5 cm of the soil at rather soil humidity the free ions forms are observed.
Because of ion association the mobility of soil solution components of dry steppe chestnut saline solonetzic is high in comparison to former analytical concentration based estimations for carbonates. It causes intensive preference solution fluxes in the soil continuum and losses of soil material to vadose zone, lateral pedosphere redistribution of salts. There is a danger of excessive carbonates loss from soil which influences the mechanical structure of soil, the calciumcarbon source for soil organisms and plants. Moreover, the migration ability of organic matter and mineral nutrients, which closely linked to soil carbonates, can be assumed significantly higher than it was previously thought because the association of ions in soil solution causes the formation of complexes of carbonates with organic and inorganic matter as a universal process for various chemicals.
In experiment, the soil solution removed from upper slightly saline soil horizon has been in diluted state. But sufficiently high soil moisture, observed in experiment, is not always common in situ. On the contrary, for the soils of southern dry regions the low soil moisture is typical. At this moisture the concentration of soil solution in the upper low saline soil horizons can be significantly higher compared to the value given in Table  1. So the soil solution material composition of upper layer of soil may be significantly influenced by association of ions in the way as it has been shown experimentally and by mathematical modeling for deep saline layers of soil. The soil system of upper soil layer is possible to describe in the model only, because now there is no way to remove the solution from dry soil.
Association of ions is fundamental base to promote innovative solutions for soil water regime correction, water saving, nutrients loss reducing, elimination of soil degradation, increase of stability and biological productivity of biosphere. It is consistent in a view of the recent data on soil moisture sensors system, corresponds to the determination of true soil solution composition (El Marazky et al., 2011). The soil moisture in artificial soil system and at the irrigation is too high from the point of view of ions association and soil solution migration (Kalinichenko, 2014a;Seitkaziev et al., 2015).    The model proposed is applicable at I <1.0. At this ionic strength of soil solution this model outlines the nature of the studied object on the qualitative and quantitative level. In saline soil it is possible to have the conditions of significantly greater ionic strength of the soil solution in which the need to involve other instruments to describe the structure of concentrated solution will appear (Nicholson and Quirke, 2003;Reznikov and Shaposhnik, 2005). Exotic models of this kind are very original and in special circumstances, for which they are designed, remain the exceptional way of research because are the only method to describe the process. But for all their originality, for the object of our studies those models are redundant. To describe the leading transport phenomena of matter in the soil are necessary and sufficient the thermodynamic models operating at I <1.0.
The nature of the CCE in soils explains the origin of high calculated saturation degree of soil solutions with CaCO 3 at analytical concentration of solution. Reduction of calculated supersaturation of soil solutions with CaCO 3 taking into account an ionic strength is for 10 ... 50 times. A complexation of Ca 2+ with soluble organic substance reduces the degree of calculated soil solution saturation with CaCO 3 to 1.5 ... 2.0 times.
The ion association can explain the excess of analytical concentration of ions in soil solution over the ion thermodynamic solubility product.

Conclusion
The association and activity of ions, calculation of soil solution saturation degree provides new knowledge of migration and accumulation of salts and CaCO 3 in soils and landscapes.
Thus, the study of most important chemical equilibria and ionic structure of soil solution help to determine the formation and nature of chemical composition of saline soil. The approach proposed permits to consider the processes of migration and accumulation of salts, to predict the genesis, evolution and different methods of reclamation of soil and landscape systems.
The association of ions is a thermodynamic precondition of CaCO 3 oversaturation and sedimentation in saline soils at higher concentration of Ca 2+ and CO 3 2in soil solution than it was considered before. At high ionic force in soil solution are formed electrically neutral ion pairs СаСО 3 0 ; CaSO 4 0 , MgCO 3 0 , MgSO 4 0 , charged ion pairs CaHCO 3 + , MgHCO 3 + , NaCO 3 -, NaSO 4 -, CaOH + , MgOH + . It causes a high mobility of CaCO 3 in soil continuum and landscape.
Our scheme of soil solution system modeling is of high scientific and practical prospect even compared to up-to-date direct methods of soil solution research. The system luck of soil solution probing is the new state of solution after its extraction from soil, an artificial system does not reflect the real soil solution.
The idea of ions association in the soil solution is a method to model quantitatively the equilibria processes in soil solution, soil and landscape. The research fulfilled on example of dry steppe chestnut saline solonetzic soil of Southern Russia shows dangerous for biosphere uncertainty of terrestrial carbon sink from soil. To the great extend it is linked to high mobility of carbon in the soil solution in form of carbonates. Concerning preferable water flows (Kalinichenko et al., 1997;Shein et al., 2014;Shein, 2015;Shein et al., 2015), the soil geochemical barriers for carbonates are less stable as it was previously thought. Thus the association of ions in soil solutions is a driver of carbon loss from soil to other geospheres Kalinichenko, 2014b;Kalinichenko and Starcev, 2015). An excess of carbon sink from biosphere through the soil is of high probability. Correction of phases and stages of carbon in soil for stabilizing of climate is a challenging task (Glazko and Glazko, 2015;Kalinitchenko et al., 2014a;2014b;Kalinichenko, 2015a;2015b;Sobgayda and Solodkova, 2015;Yuan et al., 2014).