Key Concepts in Environmental Chemistry || Soil Chemistry

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  • Chapter 8Soil ChemistryKe

    CoWhat is a soil scientist? What do they do? How do they contribute to the field of

    Environmental Chemistry? Soil scientists (or pedologists) study the upper portion of

    the Earths crust in terms of its physical and chemical properties, distribution,

    genesis and morphology, and plant and animal material. Like any applied science

    discipline, a strong background in the physical and biological sciences and

    mathematics is advantageous. Advanced knowledge of soil chemistry is vital in

    studying the accumulation and transport of contaminants, managing soils for crop

    production, watershed rehabilitation, hazard remediation, and erosion control

    management. Graduates may go on to be employed by universities, research

    institutions, mining industries, state or federal conservation services, consulting

    firms, and regulatory and land management agencies.

    FIGURE 8.1y Concepts in Environmental Chemistry. DOI: 10.1016/B978-0-12-374993-2.10008-1

    pyright 2012 Elsevier Inc. All rights reserved. 245

    http://dx.doi.org/10.1016/B978-0-12-374993-2.10008-1

  • 246 Chapter | 8 Soil Chemistry8.1. INTRODUCTION TO SOIL CHEMISTRY

    Soil chemistry is a young but growing branch of soil science with varying defi-

    nitions anddegrees of interpretation. In general terms it is the study of the chemicalcharacteristics of soils. However, physical concepts such as the study of particledensity and size, soil depth and pore spaces, bulk density, and soil moisture andtemperature complement pure chemical concepts. In this chapter, attentionwill begiven to such processes as weathering, adsorption, precipitation, complexation,and ion exchange, both in the soil solution and the solid-liquid interface. Inaddition, we will explore soil biochemistry, the causes of soil acidity, soil buff-ering, and reactions between soil andwastes, pesticides, andmetals. It is importantto remember that this chapter is only an overview to an ever-growing field ofinformationwith awidevariety of applications in agriculture and the environment.8.2. SOIL FORMATION, COMPOSITION, AND STRUCTURE

    What is soil? Lets begin with an accurate definition of soil developed by Joffein 1936, but one that is still relevant today (Joffe, 1936):

    The soil is a natural body, differentiated into horizons of mineral and organic constit-

    uents, usually unconsolidated, of variable depth, which differs from the parent material

    below in morphology, physical properties and constitution, chemical properties and

    composition, and biological characteristics.

    Any journey into the study of soil chemistry must begin with the kinetics of soilformation, i.e., knowledge of the variety of reactions that are likely to occurduring the transformation of soil parent material to a soil profile (Barshad,1965). Soil parent material is the material that soil develops from, and may besolid rock that has decomposed, or alluvial material that has been deposited bynatural forces (e.g., wind, water, or ice). As expected, the chemical compositionof the parent material plays an essential role in influencing soil properties,especially during the early stages of soil formation. The soil profile is the verticaldisplay of soil horizons arising from continued weathering. Soils horizons arecomposed of a wide range of materials from organic rich surface layers tounderlying layers high in rock matter. Complete descriptions of the master soilhorizons are found in Table 8.1. In-depth coverage of soil organic matter isprovided in subsequent sections. As can be surmised from the descriptions aboveand information contained in Table 8.1, climate and topography have significantimpacts on soil formation. For example, the parent material necessitates rela-tively undisturbed soil horizon processes for proper vertical development. Heavyprecipitation shuffling across the surface strips parent material away, hinderingsoil formation. Topography helps determine water runoff; its orientation affectsthe microclimate, which in turn affects vegetation growth and cover.

    Structured soil in solid form consists of aggregates as depicted inFigure 8.2. Soil aggregates are groups of soil particles that bind to each other by

  • TABLE 8.1 Master Soil Horizons and Associated Properties

    Soil Horizon* Characteristics

    O The top portion of the soil profile dominated by high organicmatter content (or humus) consisting of partially decomposedlitter (e.g., leaves, twigs, moss, and lichens) that hasaccumulated on the surface.

    A A surface horizon that is characterized by a mix of mineral(inorganic products of weathering) and organic matter.Eluviation (the removal of inorganic and organic matter byleaching) occurs in this horizon.

    E A horizon dominated by the removal of silicate clay, iron, andaluminum, ultimately leaving a layer of sand and silt particles.No major rock structure present.

    B A horizon dominated by illuviation, a process where fineparticle material is accumulating in a downward fashionthrough, for example, percolation of soil drainage water.Often associated with increased concentrations of carbonateprecipitates and residual concentrations of iron and aluminumoxides.

    C A horizon composed of unconsolidated parent material, eithercreated at source or transported into its present location.C horizons are characteristically composed of sediments,saprolite, and bedrock.

    R A horizon characteristically composed of hard bedrock (e.g.,granite, basalt, limestone, and sandstone).

    * Note: Base symbols to which other characters are added to complete the designation.

    247Chapter | 8 Soil Chemistryclay particles and organic matter. The space between the aggregates providespore space for retention and exchange of air and water. This allows soil to drainappropriately, while retaining enough moisture to promote and uphold healthyplant growth. As to be expected, sandy soils (sand particles range from 0.05mm for fine sand to 2.0 mm for course sand) have limited structure but aretypically free-draining. Those soils with higher clay content (clay particles aretypically 0.001e0.002 mm in size) demonstrate increased soil structuralcharacteristics but do suffer from decreased drainage ability (Williams, et al,1983). Ultimately, the amount of soil pores and the pore size relate to thedrainage capacity of the soil. Also, as expected, when soil structure varies indifferent soil types and the horizons of these soil types, water flow andcontaminant transport in soils will be impacted. For example, the extent ofgroundwater contamination caused by pesticides largely controlled by waterflow is strongly dependent upon the composition of soil structure componentsresulting from soil formation and management.

  • TABLE 8.2 Chemical Compositions and Physical Properties of Selected

    Clays in Soil

    Type

    Chemical

    Formula

    Particle

    Size

    (mm)

    Surface

    Area

    (m2/g)

    Cation

    Exchange

    Capacity

    (meq/100 g)

    Kaolinite Al2(OH)4Si2O5 0.10e5.0 10e25 5e15

    Illite K0-2Al4(Si8-6Al0-2)O20(OH)4

    0.10e2.5 100e200 15e40

    Montmorillonite Al2(OH)2Si4O10 0.10e1.0 650e800 75e100

    248 Chapter | 8 Soil ChemistrySoils that are predominately composed of clay are called fine textured soils,while those dominated by larger particles are termed coarse textured soils. Claysare typically characterized as layered silicates, metal oxides and hydroxides,amorphous and allophones, and crystalline chain silicates. The most importantproperty of clay fractions is that of cation adsorption and exchange, given thatthis property largely determines the storage capacity of a soil for plant nutrients(Reynolds, et al., 2002). Table 8.2 provides relevant information on selected claytypes, including the cation-exchange capacity (CEC): a measure of the quantityof sites on soil surfaces that can retain cations by electrostatic forces. Thesecations are in turn easily exchangeable with other cations in the soil solution.TEXTBOX 8.1

    ME or meq represents 1 milligram of exchangeable H. For example, in soil witha CEC of 1, every 100 grams of soil contain an amount of negative sites equal to the

    amount of positive ions in 1/1000th of a gram of H (or its equivalent). CEC esti-mates are made by determining the extractable cations (K, Ca2, Mg2, and Na)and estimating H from soil and buffer pH measurements.

    Negative surface charge on clay surfaces makes them extremely reactive.They are developed in two ways: 1) isomorphic substitution (permanentcharge) and 2) pH dependent charge through deprotonation of surface func-tional groups. For example, substitution of a cation of lower valence (e.g.,Al3) for one of higher valence (e.g., Si4) in the tetrahedral layer results inpermanent charge. In regards to pH dependent charge, as the pH of the soilenvironment increases, for example, weak acid functional groups (e.g.,carboxylic acids) donate a proton and generate negative charge as follows:

    COOH OH#COO H2O (8.1)

  • 249Chapter | 8 Soil ChemistryExchange capacity is routinely measured in milligram equivalents, abbreviatedME or meq. One meq of negative charge on a clay particle is neutralized by onemeq of associated cation. Note that cmolc/kg (centimoles of charge per kilo-gram of dry soil) may also be reported. Universally, the more clay and organicmatter in the soil, the higher the CEC. Organic matter has an estimated CEC ofabout 150 meq/100 g.

    Example Problem 8.1

    Using concepts discussed in Textbox 8.1, a clay material with a CEC of 1 meq/

    100 g is capable of adsorbing 1mg of H (or its equivalent) for every 100 g of clay.What would the equivalent amount of Mg be in mg?

    Answer: Mg has a 2 charge (2 charges) and an atomic weight of 24.31.Therefore, Mgwith an atomic weight of 24.31 has an equivalent weight of 24.31/212.1. Hence, 12.1 mg is the weight of 1 meq of Mg.FIGURE 8.2 Schematic representation of a typical soil aggregate held together by organic matterand clay. The space between the aggregates provides pore space for retention and exchange ofair and water.8.3. SOIL ORGANIC MATTER AND BIOCHEMICAL ASPECTS

    Lets reflect on the importance of soil organic matter (SOM) in more detail.Any discussion on SOM must include biochemical concepts, as they play vitalroles in decomposition of complex organic compounds. Recall from Chapter 1that the carbon cycle illustrates how carbon is distributed through the atmosphere,biosphere, hydrosphere, and pedosphere. As also mentioned, dead organic

  • 250 Chapter | 8 Soil Chemistrymatter (OM) of the soil is colonized by microorganisms, which obtain energy forgrowth from the oxidative decomposition of organic molecules (Federle, et al.,1986). It is during the decomposition process whereby inorganic elements areconverted from organic compounds, a process called mineralization (Nierop,et al., 2006). For example, organic nitrogen and phosphorus are mineralized toammonia and orthophosphate, respectively, thereby providing nutrients for bioticuptake and growth. In addition, carbon is converted to carbon dioxide. Theresidue carbon substrate used by the microorganisms is incorporated into theircell substance (biomass),which is called immobilization (Alexander, 1977). Theincorporated minerals are immobilized and freed after the organisms die off.

    Research Application XeHydrogen Peroxide Decomposition in Soils:A Role for Organic Matter?

    Recall from Chapter 7 that the photolysis of HCHO in the aqueous phase is

    a dominant source of hydrogen peroxide (H2O2) in wet deposition processes. A

    substantial amount of H2O2 can be introduced to soils through such processes. In

    fact, rainwater measurements have been reported to show H2O2 concentrations as

    high as 40 mM. What impact will this deposited oxidant have on soil chemistry?

    What role does organic matter play in its decomposition? To answer this, a study by

    Petigara et al. critically examined the mechanisms of H2O2 decomposition in

    soils (Petigara, et al., 2002). As they described, its impact could be significant

    if its decomposition within the soil leads to the formation of OH radical through

    Fenton-type chemistry:

    H2O2 Mn/OHOH Mn1

    And based on our discussion in Chapters 3 and 7, the OH radical is an

    exceptionally powerful oxidant. As a result, oxidative processes initiated by OH

    could significantly modify the nature and speciation of the organic and inorganic

    constituents within the soil. The investigators used a sensitive method to examine

    the formation of OH radical as a possible decomposition intermediate. Interest-

    ingly, they found that in surface soils with higher organic matter, H2O2 decayed

    rapidly, with the formation of OH radical representing less than 10% of the total

    H2O2 decomposed. Although H2O2 decayed at a slower rate in soils with lower

    organic matter content, OH radical was a major product. Ultimately, the yield of

    OH radical varied widely and depended on numerous soil parameters. More detail

    on the mechanisms behind such processes can be found in the body of the original

    published study referenced above.

    8.3.1. An Introduction to Humic Substances

    The process of humification results in the formation (primarily through micro-bial degradation) of humus from decaying organic matter of plant, animal, andmicrobial origin. The biological composition, the main fraction of natural humicmatter, is the humic substances, which largely contain humic acids and fulvicacids (Martin, et al., 1998). Humic acids (see proposed model structure in

  • 251Chapter | 8 Soil ChemistryFigure 8.3) are heterogeneous, high molecular weight organic materials thatdemonstrate a variety of components including quinone, phenol, catechol, andsugar moieties (Stevenson, 1994). The functional groups that contribute most tosurface charge and reactivity of humic acids are believed to be phenolic andcarboxylic groups. In fact, by looking at the model structure proposed bymultiple research groups, the matrix appears to be substituted by alkyl chains oraromatic moieties (both acting as monosubstituents) that correspond to esterifedaliphatic monocarboxylic acids, alkanol, or aromatic acids. In all cases, theconsideration of fatty acid and alkanol moieties indicates a possible bacterialinput (Grasset & Amblea, 1998). As expected, the characteristics of humic acidschange with soil depth, which in turn affect their sportive behavior in soil.

    Example Problem 8.2

    What is the significance of the presence of carboxylate and phenolate groups in

    humic acids?

    Answer: For example, when a carboxyl group is deprotonated, its conjugate

    base, a carboxylate anion, is produced. In addition, the phenol molecule has

    a tendency (in a slightly acidic environment) to lose the H ion from the hydroxylgroup, resulting in the highly water-soluble phenolate anion. Thus, complexation

    with metal ions (e.g., Ca2, Fe2, Fe3, and Mg2) is possible. (See section 8.4 forgreater detail on the interaction of metals with humic acids.)

    Fulvic acids (see proposed model structure in Figure 8.4) can be character-ized as those components of SOM that can form water-soluble and water-insoluble complexes with metal ions and hydrous oxides, interact with claymaterials, and combine with alaka...

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