Charge balance soils

Perez, D. V., de Campos, R. C., and Novaes, H. B. (2002). Soil solution charge balance for defining the speed and time of centrifugation of two Brazilian soils. Commun. Soil Sri. Plant Anal. 33(13—14), 2021-2036. 

An interesting case in mineral equilibria is the presence in a soil-water system of two minerals with a common ion. An example of such a case is barium sulfate (BaS04) plus calcium sulfate (CaSO. Which mineral would be controlling SOj- in the system Two conditions would need to be met in such a system one is mass-balance while the second is charge balance. The mass-balance is given by... 

FIGURE 4.8 Charge balance test for a kaolinitic soil in LiCl solutions. Electrolyte concentration circles, 0.001 M triangles, 0.005 M and squares, 0.001 M. Error bars are only given for 0.001-M electrolyte. Bars are similar in the other two cases. Data provided by J. Chorover. 

Examples of this behavior were given by Chorover and Sposito29 for kaolinitic soils from Brazil and by Schroth and Sposito34 for two Georgia kaolinites, shown in Figure 4.8. Equation 4.10 is a test for charge balance and consistency. Deviations from the behavior described by the equation reveal data inconsistency and indicate inaccuracy or inappropriateness of any of the methods used to measure the charge components. The equation is also very useful to correct relative aH versus pH curves.29 If astr and Aq are known, at least one pH value, the absolute aH, can be calculated at that pH, and the whole relative aH versus pH curve can be corrected. 

The SMART model (De Vries et al., 1989) estimates long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. The model structure is based on the mobile anion concept, incorporating the charge balance principle. SMART 2 adds forest growth and biocycling processes, which allow soil... 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

Figure 4.1. Cross section of the planar (001) surface of gibbsite depicting bridging (charge-balanced) OH groups. (From M. B. McBride. 1989. Reactions controlling heavy metal solubility in soils. In B. A. Stewart (ed.), Advances in Soil Science 10 1-56.)...

Assuming that is the predominant base cation in soil solution and NOf is the only significant anion besides HCOJ (soluble A1 species, fulvic acid, CO3, and OH" are ignored for the sake of simplicity), then charge balance in soil solution requires the equality ... 

Consider now the case where strong acids have been added to the water—for example, HNO3 from acid rain. Since equation 5.57 reveals that an increase in [H ] must cause [HCOf] to decrease for any fixed CO2 pressure in the soil, it is necessary to conclude that in this situation, [H ] > [HCOj ]. In fact, if [H" ] [HCOr], charge balance (equation 5.58) requires that [H ] [NO ] that is, [H ]... 

The soil solution differs from other aqueous solutions in that it is not electrically neutral and usually contains more cations than anions. The net negative charge of soil clay particles in most soils extends electrically out into the sod solution, and the charge is balanced by an excess of cations in the soil solution. These cations belong to the solid but are present in the sod solution. Sods in old and heavily weathered soils, as in parts of Australia, Africa, and South America, or in soils of volcanic origin, as in Japan and New Zealand, may have a net positive charge. There the soil solution has an excess of anions. 

Numerous processes take place in soil solution, including plant uptake, ion complexation, adsorption and desorption, and precipitation and dissolution (Figure 2.1). As shown in Figure 2.1, Mo solid phases dissolve upon contact with water and provide dissolved Mo in soil solution. The free molybdate ion (Mo04 ) reacts with metals to form complexes and ion pairs in soil solution. Plants absorb dissolved Mo, mainly as Mo04 from soil solution. Removal of Mo04 by plants disrupts the electroneutrality of a soil solution. This causes desorption and adsorption of Mo by oxides, as well as dissolution and precipitation of Mo solid phases in soil solution, until charge balance is reached. The speciation of dissolved Mo in soil solutions must be understood in order to quantitatively describe the availability, toxicity, adsorption, and pre-... 

The positive adsorption of metal cations by the solid phases in soil can involve the formation of either inner-sphere or outer-sphere surface complexes, or the simple accumulation of an ion swarm near the solid surface without complex formation. These adsorption mechanisms are implied in the development of the concept of surface charge balance (Eq. 3.3) and were illustrated, for the case of surface complex formation, in Figs. 1.8 and 1.10. The quantitative relationship between these mechanisms and measured surface excesses of metals on soil minerals is taken up in Chap. 5. In the present section, emphasis is placed on the qualitative... 

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