Environmental Soil Chemistry || Soil Solution-Solid Phase Equilibria

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  • 115

    4 Soil SolutionSolidPhase Equilibria

    Introduction

    There are a number of complex chemical reactions that can occur insoils. These various reactions interact with each other through thesoil solution (Fig. 4.1). The soil solution is defined as the aqueousliquid phase of the soil and its solutes (Glossary of Soil Science Terms, 1997).The majority of solutes in the soil solution is ions, which occur either as freehydrated ions (e.g., Al3+ which is expressed as Al(H2O)6

    3+) or as various complexeswith organic or inorganic ligands. When metal ions and ligands directly interactin solution (no water molecules are present between the metal ion and theligand) they form what are known as inner-sphere complexes. An outer-spherecomplex is formed when a water molecule is positioned between the metal ionand the ligand. Outer-sphere complexes are not as tightly bound as inner-sphere complexes. An uncharged outer-sphere complex is often referred to asan ion pair (e.g., Ca2+ plus SO4

    2 ions are known to form ion pairs, CaSO40).

    Soil solutions, therefore, are composed of a variety of ion species, eithercomplexed or noncomplexed. The speciation of the soil solution refers todetermining the distribution of ions in their various chemical forms.

  • The soil solution is the medium from which plants take up ions (1 inFig. 4.1) and in which plant exudates reside (2). Ions in the soil solution canbe sorbed on organic and inorganic components of the soil (3) and sorbedions can be desorbed (released) into the soil solution (4). If the soil solutionis supersaturated with any mineral in the soil, the mineral can precipitate (5)until equilibrium is reached. If the soil solution becomes undersaturated withany mineral in the soil, the mineral can dissolve (6) until equilibrium isreached. Ions in the soil solution can be transported through the soil (7) intogroundwater or removed through surface runoff processes. Throughevaporation and drying upward movement of ions can also occur (8).Microorganisms can remove ions from the soil solution (9) and when theorganisms die and organic matter is decomposed, ions are released to the soilsolution (10). Gases may be released to the soil atmosphere (11) or dissolvedin the soil solution (12).

    Measurement of the Soil SolutionThe concentration of a particular ion in the soil solution (intensity factor)and the ability of solid components in soils to resupply an ion that is depletedfrom the soil solution (capacity factor) are both important properties of a

    116 4 Soil SolutionSolid Phase Equilibria

    SoilSolution

    Free Complexed

    Solute Transport,Evaporation & Runoff

    Solid Phases& Minerals

    Sorption

    Plant Uptake

    Soil Atmosphere

    Organic Matter& Microorganisms

    1 2

    8 7

    6

    5

    4312

    11

    10

    9

    Volatilization

    Herbivores

    FIGURE 4.1. Dynamic equilibria reactions in soils. From W. L. Lindsay, ChemicalEquilibria in Soils. Copyright 1979 John Wiley & Sons. Reprinted by permission of JohnWiley & Sons, Inc.

  • Measurement of the Soil Solution 117

    given soil. The measurement of the intensity factor is critical in predictingand understanding reactions in the soil environment such as weathering andthe bioavailability, mobility, and geochemical cycling of nutrients andinorganic and organic chemicals in soils (Wolt, 1994). However, themeasurement of soil solution components in situ is very difficult, and,technically, not possible for most ion species. In addition, the actualconcentration of the ion species in the soil solution changes with changes insoil moisture content (Wolt, 1994).

    A number of laboratory methods are used to obtain samples of the soil solution. These are described in detail by Adams (1971) and Wolt (1994) and will be discussed only briefly here. The methods can be classified as displacement techniques including column displacement(pressure or tension displacement, with or without a displacing solution),centrifugation (perhaps with an immiscible liquid), and saturation extracts(e.g., saturation pastes).

    In the column displacement method, a field moist soil is packed into aglass column and a displacing solution is added to the soil. The displaced soilsolution is collected at the bottom of the column and analyzed for its ioncontent. The column displacement method is probably the most reliablemethod for obtaining a sample of the soil solution. However, the procedureis tedious, requiring an analyst who is experienced in packing the columnwith soil. Furthermore, the procedure is very time-consuming.

    The centrifugation method uses an immiscible liquid, such as carbontetrachloride or ethyl benzoylacetate, to physically remove the soil solution(which is essentially water) from the solid matrix of the moist soil. During centrifugation (>1 MPa for from 30 min to 2 hr), the immiscibleliquid passes through the soil and soil solution is displaced upward. The displaced soil solution is then decanted and filtered through a phase-separating filter paper (to remove traces of the immiscible liquid) and then through a 0.2-m membrane filter to remove suspended clayparticles (Wolt, 1994). Centrifugation methods are the most widely usedbecause they are relatively simple and equipment to carry out the procedureis usually available.

    In the saturation extract method a saturated soil paste is prepared byadding distilled water to a soil sample in a beaker, stirring with a spatula, andtapping the beaker occasionally to consolidate the soilwater mixture. Atsaturation, the soil paste should glisten. Prior to extraction the saturated soilpaste should be allowed to stand for 416 hr. Then, the soil paste istransferred to a funnel containing filter paper. The funnel is connected to aflask, vacuum is applied, and the filtrate is collected.

    Table 4.1 shows the average ion composition of soil solutions for soilsfrom around the world. One will note that Ca is the most prevalent metalcation in the soil solution, which is typical for most soils. Nitrate, chloride,and sulfate are the common anions (ligands) in more acidic soils whilecarbonate can be important in some basic soils.

  • TABLE 4.1. Total Ion Composition of Soil Solutions from World Soilsa

    Total ion composition of soil solution (mmol liter1)

    Metals Ligands

    Location pH Ca Mg K Na NH4 Al Si HCO3 SO4 Cl NO3of soils

    Californiab 7.43 12.86 2.48 2.29 3.84 0.86 3.26 5.01 3.07 21.36

    Georgiac 6.15 0.89 0.29 0.07 0.16 0.0004 0.13 1.18 0.21 0.10

    UnitedKingdomd 7.09 1.46 0.12 0.49 0.31 0.01 0.34 0.32 0.75 0.69

    Australiae 5.75 0.27 0.40 0.38 0.41 2.60 0.85 0.87 1.67 0.32

    a Adapted from J. Wolt, Soil Solution Chemistry. Copyright 1994 John Wiley & Sons. Adapted by permission of John Wiley & Sons, Inc.b From Eaton et al. (1960); soil solution obtained by pressure plate extraction at 10 kPa moisture. The pH and total composition for each ion

    represent the mean for seven soils.c From Gillman and Sumner (1987); soil solution obtained by centrifugal displacement at 7.5 kPa moisture. The pH and total composition for

    each ion represent the mean for four Ap (surface) horizons.d From Kinniburgh and Miles (1983); soil solution obtained by centrifugal displacement at field moisture contents. The pH and total

    composition for each ion represent the mean for 10 surface soils.e From Gillman and Bell (1978); soil solution obtained by centrifugal displacement at 10 kPa moisture. The pH and total composition for each

    ion represent the mean value for six soils.

    118 4 Soil SolutionSolid Phase Equilibria

    Speciation of the Soil SolutionWhile total ion concentration (sum of free and complexed ions) of the soilsolution, as measured by analytical techniques such as spectrometry,chromatography, and colorimetry, provides important information on thequantities of ions available for plant uptake and movement through the soilprofile, it is important that the speciation or chemical forms of the free andcomplexed ions be known. Ions in the soil solution can form a number ofspecies due to hydrolysis, complexation, and redox (see Chapter 8) reactions.Hydrolysis reactions involve the splitting of a H+ ion from a water moleculewhich forms an inner-sphere complex with a metal ion (Baes and Mesmer,1976). The general hydrolysis reaction for a hydrated metal ([M(H2O)x]

    n+)in solution is

    [M(H2O)x]n+ [M(OH)y (H2O)xy]

    (ny)+ + H+, (4.1)

    where n+ refers to the charge on the metal ion, x the coordination number,and y the number of H+ ions released to solution. The degree of hydrolysisfor a solvated metal ion [extent of the reaction in Eq. (4.1)] is a function of pH. A general complexation reaction between a hydrated metal[(M(H2O)x]

    n+ and a ligand, L1, to form an outer-sphere complex is

    a[M(H2O)x]n+ + bL1 [M(H2O)x]a Lb, (4.2)

    where a and b are stoichiometric coefficients.Analytically, it is not possible to determine all the individual ion

    species that can occur in soil solutions. To speciate the soil solution, one must apply ion association or speciation models using the total concentration data for each metal and ligand in the soil solution. A mass

  • Speciation of the Soil Solution 119

    balance equation for the total concentration of a metal, MTn+ in the solution

    phase can be written as

    MTn+ = Mn+ + MML, (4.3)

    where Mn+ represents the concentration of free hydrated-metal ion species,and MML is the concentration of metal ion associated with the remainingmetalligand complexes. The ligands in the metalligand complexes could beinorganic anions or humic substances such as humic and fulvic acids. Similarmass balance equations can be written for the total concentration of eachmetal and ligand in the soil solution.

    The formation of each metalligand complex in the soil solution can bedescribed using an expression similar to Eq. (4.2) and a conditional stabilityor conditional equilibrium constant, Kcond. As mentioned in the previouschapter, conditional stability constants vary with the composition and totalelectrolyte concentration of the soil solution. Conditional stability constantsfor inorganic complexes, however, can be related to thermodynamic stabilityor thermodynamic equilibrium constants (K) which are independent ofchemical composition of the soil solution at a particular temperature andpressure. Box 4.1 illustrates the relationship between thermodynamic andconditional equilibrium constants.

    BOX 4.1 Thermodynamic and Conditional Equilibrium Constants

    For any reaction (Lindsay, 1979)

    aA + bB cC + dD, (4.1a)a thermodynamic equilibrium constant, K, can be written as

    K =(C)c (D)d

    , (4.1b)(A)a (B)b

    where the small superscript letters refer to stoichiometric coefficients andA, B, C, and D are chemical species. The parentheses in Eq. (4.1b) referto the chemical activity of each of the chemical species.

    Thermodynamic equilibrium constants are independent of changesin the electrolyte composition of soil solutions because they are expressedin terms of activities, not concentrations. Unfortunately, the activities ofions in solutions generally cannot be measured directly as opposed to theirconcentration. The activity and concentration of an ion in solution can berelated, however, through the expression

    (B) = [B] B, (4.1c)

    where [B] is the concentration of species B in mol liter1 and B is a singleion activity coefficient for species B.

    Using the relationship in Eq. (4.1c), one can rewrite Eq. (4.1b) interms of concentrations rather than activities,

  • 120 4 Soil SolutionSolid Phase Equilibria

    To illustrate how mass balance equations like Eq. (4.3) and conditionalequilibrium constants can be used to speciate a metal ion in a soil solution,let us calculate the concentration of the different Ca species present in atypical soil solution from a basic soil. Assume that the pH of a soil solutionis 7.9 and the total concentration of Ca (CaT) is 3.715 10

    3 mol liter1. For simplicitys sake, and realizing that other complexes occur, let us assume that Ca exists only as free hydrated Ca2+ ions and as complexes withthe inorganic ligands CO3, SO4, and Cl (i.e., [CaCO3

    0], [CaHCO3+],

    [CaSO40], and [CaCl+]). The CaT can be expressed as the sum of free and

    complexed forms as

    CaT = [Ca2+] + [CaCO3

    0] + [CaHCO3+] + [CaSO4

    0] + [CaCl+], (4.4)

    where brackets indicate species concentrations in mol liter1. Each of theabove complexes can be described by a conditional stability constant,

    K1cond =

    [CaCO30]

    (4.5)[Ca2+][CO3

    2]

    K2cond =

    [CaHCO3+]

    (4.6)[Ca2+][HCO3

    ]

    K3cond =

    [CaSO40]

    (4.7)[Ca2+][SO4

    2]

    K4cond =

    [CaCl+] . (4.8)

    [Ca2+][Cl]

    Since Eqs. (4.5)(4.8) have [Ca2+] as a common term, Eq. (4.4) can berewritten in terms of the concentrations of [Ca2+] and each of the inorganicligands (Sposito, 1989):

    CaT = [Ca2+] 1 +

    [CaCO30]

    +[CaHCO3

    +]{ [Ca2+] [Ca2+]+

    [CaSO40]

    +[CaCl+]

    (4.9)[Ca2+] [Ca2+] }

    K =[C]c cC [D]d dD = [C]

    c [D]d cC dD = Kcond cC dD , (4.1d)

    [A]a aA [B]b bB [A]a [B]b ( aA bB ) aA bBwhere Kcond is the conditional equilibrium constant. The ratio of theactivity coefficients is the correct term that relates Kcond to K. The valueof the activity coefficients, and therefore the value of the ratio of theactivity coefficients, changes with changes in the electrolyte compositionof the soil solution. In very dilute solutions, the value of the activitycoefficients approaches 1, and the conditional equilibrium constantequals the thermodynamic equilibrium constant.

  • TABLE 4.2. Computer Equilibrium Models Used in Geochemical Researcha

    Chemical Speciation ModelsREDEQL series modelsb

    REDEQL (Morel and Morgan, 1972)REDEQL2 (McDuff et al., 1973)MINEQL (Westall et al., 1976)GEOCHEM (Sposito and Mattigod, 1980)GEOCHEM-PC (Parker et al., 1995)REDEQL.EPA (Ingle et al., 1978)MICROQL (Westall, 1979)REDEQL.UMD. (Harris et al., 1981)MINTEQ (Felmy et al., 1984)SOILCHEM (Sposito and Coves, 1988)

    WATCHEM series modelsc

    WATCHEM (Barnes and Clarke, 1969)SOLMNEQ (Kharaka and Barnes, 1973)WATEQ (Truesdall and Jones, 1974)WATEQ2 (Ball et al., 1979)WATEQ3 (Ball et al., 1981)

    a Adapted from Baham (1984) and S. V. Mattigod and J. M. Zachara (1996), with permission.b These models use an equilibrium constant approach for describing ion speciation and computing ion activities.c These models use the Gibbs free energy minimization approach.

    Speciation of the Soil Solution 121

    = [Ca2+] {1 + K1cond [CO3

    2] + K2cond [HCO3

    ]

    + K3cond [SO4

    2] + K4cond [Cl]}.

    As noted earlier, mass balance equations like Eq. (4.3) can be expressedfor total metal and ligand concentration, MT

    n+ and LTl, respectively, for each

    metal and ligand species. The mass balance expressions are transformed intocoupled algebraic equations like Eq. (4.9) with the free ion concentrations asunknowns by substitution for the complex concentrations. The algebraicequations can be solved iteratively using successive approximation to obtainthe free-ion concentrations by using a number of possible computer equilibriummodels (Table 4.2). A common iterative approach is the Newton-Raphsonalgorithm. The computer equilibrium models contain thermodynamicdatabases (e.g., Martell and Smith, 1976) and computational algorithms.Thorough discussions of these m...

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