The Soil Chemical Environment

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The Soil Chemical Environment

• Reading
• General background Sparks,Chapter 1, pp. 1-28
• Additional Essington, Chapter 1
• pp. 1-21.

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Chemical Interactions in Soils
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Description of complex interactions involves

• Mineral Chemistry
• Colloidal Chemistry
• Physical Chemistry
• Analytical Chemistry
• Organic Chemistry
• Biochemistry

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Reactions occur at interfaces

• Solid - liquid
• Liquid - gas

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System is open

• fluxes of water and solutes
• fluxes of gases
• fluxes into biota and from decaying biota
• energy fluxes

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Equilibrium vs. Kinetics

• Most soil chemistry is the study of reactions at
  equilibrium.
• This is OK for prediction of fast reactions
  (e.g. ion exchange and many adsorption
  reactions), but soil is mostly a non-equilibrium
  system.

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• Kinetics
• Very important for many reactions (e.g.
  precipitation and dissolution of most minerals).
  Hard to study in a mixed system like soil.
• Kinetics can be used to describe the rates of all
  reactions but in many cases the rates are so slow
  that over the time of interest there is no
  reaction. Kinetics in mixed systems can be very
  complex.

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Soil Chemistry and the Soil Solution

• The soil solution is central to most soil
  chemistry
• Soil chemistry is mostly about the interaction of
  ions and molecules in solution and their
  interaction with the gas phase and the solid
  phases.
• Most concepts similar to those in aquatic
  chemistry courses.

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Soil Solution Interaction with other components
(Sparks Fig. 4.1)
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Inorganic anions and cations in the soil
solutions (Sparks Table 4.1)
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Chemical systems in soils
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Properties of the Elements

• Web Elements has details on the properties and
  behavior of all elements
• Web Elements http//www.webelements.com/

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Chemical Elements in Soil
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Abundance of the elements in soils

• Major vs. minor and trace elements
• In geochemistry and soil chemistry these the
  definitions are a bit fuzzy.
• See also plant nutrition literature
• Mostly trace and minor are used
  interchangeably
• Trace or minor is sometimes defined as lt 1 (10 g
  kg-1 ). Essington says trace is lt 100 mg kg-1
• Some authors consider minor as including higher
  concentrations than trace

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Major elements

• Major Al, Si, C, N, Fe, Ca, O, K, Ti (P in
  plants)
• Remainder are minor (trace) elements
• see Table 2.4 in Sparks

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Periodicity of the elements

• Metals
• Group 1 - alkali metals (I), 1 ions
• Group 2 - alkaline earths (II), 2 ions
• Transition elements
• Non-metals
• Group 6 chalcogens (- II)
• Group 7 halogens (- I), 1- ions

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Heavy metal??

• It is such a vague word, that is often used
  incorrectly.
• Is boron a heavy metal?

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Oxidation states and charge number

• Oxidation state is indicated by Roman numerals.
• E.g. Fe(III), Fe(II), Mn(IV), and Mn(II)
• Charge number on an ion.
• E.g. Ca2 not Ca2  
• SO42- or with oxidation state S(VI)O42-

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Acids and Bases

• See Sparks p. 65

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ACIDS AND BASES

• Arrhenius
• Acid increases H concentration (activity) in
  solution.
• First and least inclusive definition.
• Base increases OH- activity.
• Brönsted definition - typical definition used in
  environmental and soil chemistry
• Acid is a proton donor.
• Base is a proton acceptor
• Includes Arrhenius acids and bases

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Brönsted acids and Bases

• Examples
• HCl H2O H3O Cl-
• acid base conjugate acid
  conjugate base
• NH3 H2O NH4 OH-
• base acid
• CH3COOH OH- CH3COO- H2O
• acid base

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Lewis Acids and Bases

• Lewis Acid a broader definition.
• (Brönsted definition is subset of the Lewis
  definition)
• Acid is an electron pair acceptor
• Base is an electron pair donor.
• Examples
• (see Brönsted acids and bases)
• e.g. H H2O H3O

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LeH is also a Lewis Acid

• H is a proton --- can accept a pair of electrons
  from one of the two unshared pair of electrons in
  water to form a coordinate covalent bond.
• water hydronium ion

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Metal ion complexes

• Lewis defines formation of complexes as acid base
  reactions.
• e.g. Formation of Cu2 amine complex ion.
• Cu2 2NH3 Cu(NH3)22
• Ammonia has one unshared pairs of electrons that
  can be donated to empty bonding orbitals in Cu2.

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Metal ion complexes

• On the last slide NH3 is a ligand
• The complex is also called and adduct (addition
  product). Not a term used very much in
  environmental chemistry.

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Chelation

• Chelation (multidentate complexes)
• Fe(II) can accept 2 pairs of N electrons
• Bidentate complex
• This red complex is useful for detection of
  reduced Fe.

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EDTA complexes
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• N atoms can donate pairs of electrons
• Negatively charged O atoms can donate pairs of
  electrons.
• Negative charges contribute to ionic bonding
• Can form hexadentate complexes
• e.g. Cu2 and other first row transition metal
  cations.
• Complexes with EDTA can be very strong

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• EDTA, is shown in 3-D at
• EDTA
• Fe DTPA, a similar complex is at
• Fe-EDTA

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Systeme International (SI) Units
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Based on mks system

• Basic units m, kg and s
• Examples
• Concentration in a solid, mol kg-1
• Rate of reaction, mol L-1 s-1
• Writing units
• mol L-1 s-1 not mol/L/ s

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Essington Table 1.5
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Some non SI (derived) units

• We use many units that are not strictly part of
  the S.I. system
• e.g. L for liter and ha. for hectare.
• Land application - kg ha-1

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Some equivalences

• Mg m-3 g cm-3
• mg kg-1 or mg L-1   ppm
• µg kg-1 or µg L-1   ppb
• c molc kg-1 same as meq/100g

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Conversion of units

• Example Reporting 5 mg L-1 Ca extracted from a
  10 g soil in 200 mL solution in units of
  cmolckg-1
• 0.5 mmolc kg-1

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Essington Table 1.6
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In class exercise

• What is the common language unit for g T-1
  (gram per ton)?

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Summary

• Soil solution is central to soil chemistry.
• Soil chemistry concentrates on a small fraction
  of the periodic table.
• Knowledge of periodicity is very useful.
• We will mostly use the Brönsted definition of
  acid and bases.
• Will use mostly SI units and a few derived from
  S.I

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