Environmental Soil Chemistry - Chapter 4 pptx

The majority of solutes in the soil solution is ions, which occur either as free hydrated ions e.g., Al3+which is expressed as AlH2O63+ or as various complexes with organic or inorganic

Solution–Solid Phase Equilibria

Introduction

There are a number of complex chemical reactions that can occur in

soils These various reactions interact with each other through the soil solution (Fig 4.1) The soil solution is defined as the aqueous liquid 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 free hydrated ions (e.g., Al3+which is expressed as Al(H2O)63+) or as various complexes with organic or inorganic ligands When metal ions and ligands directly interact

in solution (no water molecules are present between the metal ion and the ligand) they form what are known as inner-sphere complexes An outer-sphere complex is formed when a water molecule is positioned between the metal ion and the ligand Outer-sphere complexes are not as tightly bound as inner-sphere complexes An uncharged outer-inner-sphere complex is often referred to as

an ion pair (e.g., Ca2+plus SO42–ions are known to form ion pairs, CaSO40) Soil solutions, therefore, are composed of a variety of ion species, either complexed or noncomplexed The speciation of the soil solution refers to determining the distribution of ions in their various chemical forms

The soil solution is the medium from which plants take up ions (1 in Fig 4.1) and in which plant exudates reside (2) Ions in the soil solution can

be sorbed on organic and inorganic components of the soil (3) and sorbed ions can be desorbed (released) into the soil solution (4) If the soil solution

is supersaturated with any mineral in the soil, the mineral can precipitate (5) until equilibrium is reached If the soil solution becomes undersaturated with any mineral in the soil, the mineral can dissolve (6) until equilibrium is reached Ions in the soil solution can be transported through the soil (7) into groundwater or removed through surface runoff processes Through evaporation and drying upward movement of ions can also occur (8) Microorganisms can remove ions from the soil solution (9) and when the organisms die and organic matter is decomposed, ions are released to the soil solution (10) Gases may be released to the soil atmosphere (11) or dissolved

in the soil solution (12)

Measurement of the Soil Solution

The 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 depleted from the soil solution (capacity factor) are both important properties of a

Soil Solution Free Complexed

Solute Transport, Evaporation & Runoff

Solid Phases

& Minerals

Sorption Plant Uptake

Soil Atmosphere

Organic Matter

& Microorganisms

6 5

4 3 12

11

10 9

Volatilization

Herbivores

FIGURE 4.1. Dynamic equilibria reactions in soils From W L Lindsay, “Chemical Equilibria in Soils.” Copyright © 1979 John Wiley & Sons Reprinted by permission of John Wiley & Sons, Inc.

Measurement of the Soil Solution 117

given soil The measurement of the intensity factor is critical in predicting and understanding reactions in the soil environment such as weathering and the bioavailability, mobility, and geochemical cycling of nutrients and inorganic and organic chemicals in soils (Wolt, 1994) However, the

measurement of soil solution components in situ is very difficult, and,

technically, not possible for most ion species In addition, the actual concentration of the ion species in the soil solution changes with changes in soil 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 a glass column and a displacing solution is added to the soil The displaced soil solution is collected at the bottom of the column and analyzed for its ion content The column displacement method is probably the most reliable method for obtaining a sample of the soil solution However, the procedure

is tedious, requiring an analyst who is experienced in packing the column with soil Furthermore, the procedure is very time-consuming

The centrifugation method uses an immiscible liquid, such as carbon tetrachloride 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 immiscible liquid 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-μm membrane filter to remove suspended clay particles (Wolt, 1994) Centrifugation methods are the most widely used because they are relatively simple and equipment to carry out the procedure

is usually available

In the saturation extract method a saturated soil paste is prepared by adding distilled water to a soil sample in a beaker, stirring with a spatula, and tapping the beaker occasionally to consolidate the soil–water mixture At saturation, the soil paste should glisten Prior to extraction the saturated soil paste should be allowed to stand for 4–16 hr Then, the soil paste is transferred to a funnel containing filter paper The funnel is connected to a flask, vacuum is applied, and the filtrate is collected

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

TABLE 4.1. Total Ion Composition of Soil Solutions from World Soils a

Total ion composition of soil solution (mmol liter –1 )

of soils

United

aAdapted 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.

cFrom 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.

dFrom 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.

eFrom 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.

Speciation of the Soil Solution

While total ion concentration (sum of free and complexed ions) of the soil solution, as measured by analytical techniques such as spectrometry, chromatography, and colorimetry, provides important information on the quantities of ions available for plant uptake and movement through the soil profile, it is important that the speciation or chemical forms of the free and complexed ions be known Ions in the soil solution can form a number of species due to hydrolysis, complexation, and redox (see Chapter 8) reactions Hydrolysis reactions involve the splitting of a H+ion from a water molecule which 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)x–y](n–y)++γH+, (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 hydrolysis for 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]aLb, (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

where Mn+represents the concentration of free hydrated-metal ion species, and MML is the concentration of metal ion associated with the remaining metal–ligand complexes The ligands in the metal–ligand complexes could be inorganic anions or humic substances such as humic and fulvic acids Similar mass balance equations can be written for the total concentration of each metal and ligand in the soil solution

The formation of each metal–ligand complex in the soil solution can be described using an expression similar to Eq (4.2) and a conditional stability

or conditional equilibrium constant, Kcond As mentioned in the previous chapter, conditional stability constants vary with the composition and total electrolyte concentration of the soil solution Conditional stability constants for inorganic complexes, however, can be related to thermodynamic stability

or thermodynamic equilibrium constants (Kº) which are independent of chemical composition of the soil solution at a particular temperature and pressure Box 4.1 illustrates the relationship between thermodynamic and conditional 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

(A)a(B)b

where the small superscript letters refer to stoichiometric coefficients and

A, B, C, and D are chemical species The parentheses in Eq (4.1b) refer

to the chemical activity of each of the chemical species

Thermodynamic equilibrium constants are independent of changes

in the electrolyte composition of soil solutions because they are expressed

in terms of activities, not concentrations Unfortunately, the activities of ions in solutions generally cannot be measured directly as opposed to their concentration The activity and concentration of an ion in solution can be related, however, through the expression

where [B] is the concentration of species B in mol liter–1andγBis a single ion activity coefficient for species B

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

To illustrate how mass balance equations like Eq (4.3) and conditional equilibrium 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 a typical soil solution from a basic soil Assume that the pH of a soil solution

is 7.9 and the total concentration of Ca (CaT) is 3.715 × 10–3 mol liter–1 For simplicity’s sake, and realizing that other complexes occur, let us assume that Ca exists only as free hydrated Ca2+ions and as complexes with the inorganic ligands CO3, SO4, and Cl (i.e., [CaCO03], [CaHCO3+], [CaSO4], and [CaCl+]) The CaT can be expressed as the sum of free and complexed forms as

CaT= [Ca2+] + [CaCO30] + [CaHCO3+] + [CaSO40] + [CaCl+], (4.4) where brackets indicate species concentrations in mol liter–1 Each of the above complexes can be described by a conditional stability constant,

[Ca2+][CO32–]

K2cond = [CaHCO3+]

(4.6) [Ca2+][HCO3–]

K3cond = [CaSO40]

(4.7) [Ca2+][SO42–]

K4cond = [CaCl

+]

[Ca2+][Cl–] Since Eqs (4.5)–(4.8) have [Ca2+] as a common term, Eq (4.4) can be rewritten in terms of the concentrations of [Ca2+] and each of the inorganic ligands (Sposito, 1989):

CaT = [Ca2+] 1 +[CaCO3]+[CaHCO3]

{ [Ca2+] [Ca2+] +[CaSO40]

+[CaCl

+]

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

K° = [C]

cγc

C[D]dγd

c[D]d γc

Cγd

D = Kcond γc

Cγd

D , (4.1d) [A]aγa

A[B]bγb

B [A]a[B]b (γa

Aγb

Aγb B

where Kcond is the conditional equilibrium constant The ratio of the activity coefficients is the correct term that relates Kcondto K° The value

of the activity coefficients, and therefore the value of the ratio of the activity coefficients, changes with changes in the electrolyte composition

of the soil solution In very dilute solutions, the value of the activity coefficients approaches 1, and the conditional equilibrium constant equals the thermodynamic equilibrium constant

TABLE 4.2. Computer Equilibrium Models Used in Geochemical Research a

Chemical Speciation Models REDEQL 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 (Barnes and Clarke, 1969) SOLMNEQ (Kharaka and Barnes, 1973) WATEQ (Truesdall and Jones, 1974)

WATEQ2 (Ball et al., 1979) WATEQ3 (Ball et al., 1981)

aAdapted from Baham (1984) and S V Mattigod and J M Zachara (1996), with permission.

bThese models use an equilibrium constant approach for describing ion speciation and computing ion activities.

c

= [Ca2+] {1 + K1cond[CO32–] + K2cond[HCO3–] + K3cond[SO42–] + K4cond[Cl–]}

As noted earlier, mass balance equations like Eq (4.3) can be expressed for total metal and ligand concentration, MTn+and LTl–, respectively, for each metal and ligand species The mass balance expressions are transformed into coupled algebraic equations like Eq (4.9) with the free ion concentrations as unknowns by substitution for the complex concentrations The algebraic equations can be solved iteratively using successive approximation to obtain the free-ion concentrations by using a number of possible computer equilibrium models (Table 4.2) A common iterative approach is the Newton-Raphson algorithm The computer equilibrium models contain thermodynamic databases (e.g., Martell and Smith, 1976) and computational algorithms Thorough discussions of these models can be found in Jenne (1979), Melchior and Bassett (1990), and Mattigod and Zachara (1996) The thermodynamic databases contain equilibrium constants for aqueous complex species as well as other equilibrium parameters (e.g., solubility products and redox potential, both of which are discussed later)

The free ion concentrations obtained from the iterative approach can then be used to calculate the concentrations of the metal–ligand complexes For example, the complex CaHCO3+forms based on the reaction

Ca2++ H++ CO32–→ CaHCO3+ (4.10) The K2condfor CaHCO3+is 9.33 × 1010at 298K Substituting this value into

Eq (4.6),

9.33× 1010= [CaHCO3+]

[Ca2+][H+][CO32–] and solving for [CaHCO3+] yields

[CaHCO3+] = 9.33 × 1010[Ca2+][H+][CO32–] (4.12) Once the free concentrations of Ca2+, H+, and CO32– are determined, the concentration of the complex, [CaHCO3+], can be calculated using

Eq (4.12)

The calculated concentrations for the various species can be checked by inserting them into the mass balance equation [Eq (4.3)] and determining if the sum is equal to the known total concentration This can be illustrated for

Ca using the data in Table 4.3 Table 4.3 shows the calculated concentrations

of seven metals, six ligands, and metal–ligand complexes (99 total complexes were assumed to be possible) of a basic soil solution To compare the total measured Ca concentration, CaT, to the sum of the calculated Ca species concentrations, one would use the following mass balance equation,

CaT = [Ca2+] + [Ca–CO3complexes] + [Ca–SO4complexes]

+ [Ca–Cl complexes] + [Ca–PO4complexes]

+ [Ca–NO3complexes] + [Ca–OH complexes] (4.13) Using the data in Table 4.3,

CaT = 3.715 × 10–3= [3.395 × 10–3] + [8.396 × 10–5]

+ [1.871 × 10–4] + [1.607 × 10–8] + [9.050 × 10–6] + [2.539 × 10–5] + [3.600 × 10–7],

where all concentrations are expressed in mol liter–1,

CaT= 3.715 × 10–3⯝ 3.717 × 10–3 (4.14) One sees CaTcompares well to the sum of free and complexed Ca species calculated from the GEOCHEM-PC (Parker et al., 1995) chemical speciation model (Table 4.2)

Table 4.4 shows the primary distribution of free metals and ligands and metal–ligand complexes for the soil solution data in Table 4.3 One sees that

Ca, Mg, K, and Na exist predominantly as free metals while Cu mainly occurs complexed with CO3and Zn is primarily speciated as the free metal (42.38%) and as complexes with CO3(49.25%) The ligands Cl and NO3 (>98%) occur as free ions, while about 20% of SO4and 10% of PO4 are complexed with Ca These findings are consistent with the discussion in Chapter 3 (see Box 3.1) on hard and soft acids and bases Hard acids such as

Ca, Mg, K, and Na, and hard bases such as Cl and NO3, would not be expected to form significant complexes except at high concentrations of the metal or ligand (Cl or NO3) However, soft acids such as Cu and Zn can form complexes with hard bases such as CO3

Speciation of the Soil Solution

for a Soil Solution a,b

aAdapted, with permission, from S V Mattigod and J M Zachara (1996) using the GEOCHEM–PC (Parker et al., 1995) chemical speciation model.

bAll concentrations are expressed as mol liter –1

cThe concentrations of the metals complexed with the ligands are those data within the area marked by lines and designated as total metal complexed by each ligand For example, 8.396 × 10 –5 mol liter –1 Ca is

complexed with CO3.

TABLE 4.4. Primary Distribution of Free Metals and Ligands and Metal–Ligand Complexes for a Soil Solution a

Metal–ligand complexes (%)

Metal–ligand complexes (%)

aDistribution for soil solution data in Table 4.3, from S V Mattigod and J M Zachara (1996), with permission.

bPercentages represent the amount of metal or ligand as a free or complexed species For example, 91.36% of Ca occurs as the free metal ion while 2.26% occurs as a Ca–CO3metal–ligand complex, and 5.03, 0.43, 0.23, and 0.68% occur as Ca–SO4, Ca–Cl, Ca–PO4, and Ca–NO3 metal–ligand complexes.

Ion Activity and Activity Coefficients

As illustrated in Box 4.1, the activity of an ionic species in solution is related

to its concentration in solution through its single-ion activity coefficients [Eq (4.1c)] The single-ion activity coefficients are typically calculated using the extended (to account for the effective size of hydrated ions) Debye-Hückel equation (Stumm and Morgan, 1996)

logγi= –A Zi2 I1/2

(1 + BaiI1/2)

The A term typically has a value of 0.5 at 298K and is related to the dielectric constant of water The B term is also related to the dielectric

constant of water and has a value of ⯝ 0.33 at 298K The A and B parameters

are also dependent on temperature The ai term is an adjustable parameter

corresponding to the size (Å) of the ion The Ziterm refers to the valence

(charge) of the ion in solution, and I represents the ionic strength Ionic

strength is related to the total electrolyte concentration in solution, and is a measure of the degree of interaction between ions in solution, which, in turn, influences the activity of ions in solution The ionic strength is calculated using the expression