Soil chemistry

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Soil chemistry

• Environmental Chemistry II, KJM5700
• Rolf D. Vogt
• og Hans M. Seip

2
Soil profile

• A vertical section of sediments between the
  earth surface and bedrock
• Divided into horizons with notations depending
  on their relative placement and physical and
  chemical characters
• The profiles are classified after the presence
  or absence of specific types of horizons

Soil horizons
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Organic soil

• On the top of the soil profile
• H-layer
• Histosol
• Bog or peat soil
• Water saturated
• gt40cm.
• O-Horizon
• gt 35 Organic
• Raw moder, mull
• Dry
• lt40cm

4
Mineral soil

• A-horizon
• Top of the mineral soil
• Contain lt35 organic material as fine particles
  or as coating
• Black colour
• E-horizon
• Lighter layer between the O- and B-horizons
• Eluvial layer, i.e. silicate clays, iron and
  aluminium have been washed out.
• Humic substances transport
• Occurs in poor soil (minor amounts of Fe and
  poorly weatherable primary material)

5
Mineral soil cont.

• B-horizon
• Primary material is changed
• Contain one of the following
• Illuvial accumulation of
• Silicate clay minerals (Bt),
• Iron, aluminium (Bs) and/or
• Humic material (Bh)
• Residual of
• Sesquioksides (oxides/hydroxides) (Bs)
• Alteration of primary material
• Clay (Bw)
• Oxides (Bs)
• C-horizon
• Deepest in the profile
• Unaltered parent material
• R-horizon
• Bedrock

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Composition
Outline of lecture
(Yong et al., 1992)

• Solid phase
• Inorganic mineral particles
• Organic material
• together in aggregates
• Living material
• Liquid phase
• Gas phase
• In a network of pores

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Solid phaseNatural Organic Material
Solid phase

• Humus End product of chemical and biological
  decay
• Poorly defined
• Many functional groups
• Divided into humic, fulvic, humin

Fenol Carboxyl Amin Alchohol Sulfhydryl
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Possible structire of a humic molecule
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Solid phaseMineral soil
Solid phase

• Primary minerals
• Igneous Rocks
• Granite, Basalt
• Sedimentary Rocks
• Sandstone, limestone
• Metamorphic Rocks
• Gneiss, Marble
• Mechanic erosion of rock
• Frost expansion
• Wind, wave and glacial
• Secondary minerals
• Incongruent precipitation products of chemical
  weathering processes
• K-feldspar Kaolinite

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Silicates
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Silicates
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Silicates
Solid phase

• Put together units of Si-tetrahedral ()

Clay, Mica
Quarts
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Phyllosilicates
Solid phase

• Adobt a layer of Al-octahedrals (O)
• Clay type
• Often lt 2?m
• 11 type (l-O)
• 12 type (l-O-l)
• Others
• Mica group
• Biotite, muscovite

Kaolinite
Illite
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Water phase

• The ionic composition in the soil water is
  determined by
• Distance to sea (Sea-salts)
• Natural emissions
• Anthropogenic loading
• Mineral composition of the soil

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Hydrolysis and complexation

• In solution there are numerous chemical reactions
  that are all in equilibrium with each other
• Concidering only the major ions H, Ca2, Mg2,
  Na, K, Fe3, Al3, F-, Cl-, NO3-, SO42- and
  HCO3- there are more than 60 different species in
  equilibrium

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Concentrations of Al3 and Al(OH) complexes in
equilibrium with different types of gibbsite
(Al(OH)3)
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Equilibrium constants for Al-OH species.Note
values may not corresond exactly to the figure.
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Redox processes
Water phase

• Respiration reduces the oxidants
• In an aerobic environmentO2 is used as energy
  source it is reduced to H2O (O2 4H 4e-
  ?2H2O)
• C106H263O110N16P1138O2?106CO216NO3-HPO42-122H
  2O18H
• In an anaerobic environment other oxidants -
  electron acceptors NO3-, MnO2, FeOOH etc
  are used as energy sources

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Redox potential
Water phase

• The environments redox potential in a solution is
  expressed by EH (in mV relative to SHE) or p?
  where
• EH0.0592 ? -loge- 0.0592p?
• The redox potential in nature cannot be measured,
  nor calculated
• This is because chemically the processes are slow
  so that the redox processes become biochemically
  conditioned
• p? from the ratios between redox pairs in a
  natural solution will therefore vary

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Redox sequences
Water phase
Example at pH 7
A
O
B
C
E
G
H

• Reaction combinations
• AL Aerobic respirationBL Denitrification DL
  Nitrate reductionFL FermentationGL
  Sulphate reductionHL Methane fermentation

• AM Sulphide oxidationAO NitrificationAN
  Iron oxidationAP Manganese oxidation

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? Species in soil solution
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Some examples

• AL Aerobic respiration CH2OO2 ? CO2 H2O
• BL Denitrification4NO3?(aq) 5CH2O (aq)
  4H(aq) ? 2N2(g) 5CO2(g) 7H2O(l)
• AM sulphide oxidation2O2 HS? ? SO42? H

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Chemical interactions between soil and water

• Processes
• Solubility
• Hydrolysis
• Sorption
• Ion exchange
• Adsorption
• Complexation

• Factors
• Type of Solid phase
• Clay
• Oxides
• Organic
• pH (p?)
• Kinetics

2
1
1
B
3
4
A
C
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Negatively charged surfaces
Soil / Water interactions

• pH independent charge
• Isomorphic substitution
• Si4 is replaced by Al3 or Si4 and Al3 is
  replaced by Me2
• Error in the lattice structure
• pH dependent charge
• Protonization/deprotonization
• -X-O-s Haq ? -X-OHs
• (-X-OHs Haq ? -X-OH2s)

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Point of Zero Charge (pzc)
Soil / Water interactions

• At pH gt pzc -X-O-sgt-X-OH2s ? net charge
• At pH lt pzc -X-O-slt-X-OH2s ? net charge
• At pzc -X-O-s -X-OH2s
• In soil the pH is usually gt pzc, except for Fe
  and Aloxides/hydroxides
• The soil has therefore more negative than
  positive charged sites, i.e. Cation exchanger

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Diffuse Double Layer (DDL)
Soil / Water interactions

• Surface charge compensated by counter ion layer
• Minimum free energy is achieved as a compromise
  between lowest energy (a) and highest entropy (b)

Concent-ration of counter- ions in DDL
Lowest energy-Highest entropy-Compromise
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Repetition

• Soil profile
• Divided into H or O, A, (E,) B, C and R horizons
• Solid phase
• Mineral soil divided into
• Primary minerals
• Secondary minerals
• Clay minerals Phyllosilicates
• Composed by Si-tethraeders ()
  Al- octaheders(O)
• Natural Organic Matter
• Weak acids (COOH, ROH)
• Complexbinder

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Repetition

• Liquid phase
• Hydrolysis and complexation
• Redox
• Potential is bio-chemically mitigated
• Reactions between soil and water
• Dissolution/precipitation
• Sorption
• Physical adsorption
• pH dependent and independent surface charge
• DDL
• Cation exchange
• CEC
• Base saturation
• Salt effect
• Chemical sorption
• Complex binding
• Chelates
• Hydrolysis (AO H2O ? AOH OH-)

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Cation exchange
Soil / Water interactions

• Cations in the DDL are exchanged for equivalent
  amounts in solution Soil ADDL B?Soil BDDL A
• Cation exchange capacity is an operationally
  defined parameter
• Potential CECP
• Locked pH (8.2) NH4COOH
• Effective CECE
• Variable pH BaCl2
• Measure either loss of NH4 or Ba or amount of
  desorbed cations
• Ca2,Mg2,Al3,Na,H,K,(Fe3)

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Base saturation
Soil / Water interactions
Cation exchange

• Base cations Cations that exchange with H on
  the ion exchanger
• Associated with strong bases (i.e. NaOH or KOH)
• Na, K, Ca2, Mg2 and NH4
• Al3 is considered an acid cation because it can
  generate protons through hydrolysis
• Base saturation The relative equivalent (or
  molar) amount of base cations on the ion
  exchanger
• Base saturation and CEC are method dependent
• Exchangeable base cations remain the same while
  CEC increases with increasing pH

pH dep.charge
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CEC calculations
Soil / Water interactions
Cation exchange

• The relationship between the equivalent fraction
  of two cations on the ion exchanger (S-X) and
  their activity in solution (X) is given by
  either of the two expressions
• Gapon (sites) 1S-Ca 0.5 Na?S-Na½Ca2
• Gaines-Thomas (cations) S2-Ca 2Na ?2S-Na
  Ca2
• where S-X is the equivalent fraction

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E.g. Ca vs. Al Gaines-Thomas
Soil / Water interactions
Cation exchange

• We have
• Expressed by equilibrium equation
• KGT is empirically determined on the basis of the
  composition of the ion exchanger and soil
  solution
• Then we can simulate how Al3 changes with
  changes in the base saturation (EcaMgNaK)

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Salt effect
Soil / Water interactions
Cation exchange

• Desorption of polyvalent ions increase as ionic
  strength increase
• Since
• 10 increase in Ca2 will lead to an 30
  increase in Al3
• An increase in ionic strength will in
  additionlead to a greater activity reduction
  the higher the valence
• pH will also decrease

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Adsorption

• Physical adsorption
• Non-specific adsorption in Gouy-Chapman diffuse
  double layer or in the outer sphere of the
  Stern Model
• X-O-M(H2O)4n ? X-O-M(H2O)4n
• This is conceived as a ordinary reversible cation
  exchange
• Chemical sorption
• Specific sorption in the inner sphere of the
  Stern model

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Chemical sorption

• Specific species are bound selectively to pH
  dependent active surface sites
• The ligands are lost
• Hydrogen- and covalent bindings are created
• -X-O-H M(H2O)nz ? -X-O-M(z-1) HnH2O
• Selectivity determined by
• Electronegativity
• Ability to polarise
• Hydrolysis ability or Ionization potential
  (valence/radii)
• Hydrated radii
• Concentration etc.
• Favourable for transition elements
• CdgtNigtCogtZnCugtPbgtHg

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Sorption isotherms
Chemosorption

• A simple linear relationship
• can be described by a single distribution
  coefficient (Kd) as e.g. KOW
• Nonlinear relationship
• Empirical description of the distribution between
  a solid and a mobile phase at a given temperature
  
• Freundlich
• Langmuir
• The constants n or sm K are determined by
  empirically fitting the data to the equationsm
  gives the number of sites K gives the binding
  strength
• E.g. logslogKFnlogC

Border limit
Difficult to find vacant sites and the best are
occupied
Ample vacant sites
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Chelates
Chemosorption

• Chelate Complex where the central metal ion
  is coordinated with more than one binding site
  on a large molecule, called a ligand, so that a
  ring is formed
• 2(X-O-H)Mz?(X-O)2-M(z-2)2H
• This provides great stability
• Such bindings are very strong
• NOM has big capacity to form chelates
• PbgtCugtFeAlgtMnCogtZn

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Soil pores

• The soils porosity is to a large extent
  determined by the particle size distribution
• Most pores in
• Soils with a small fraction of finer particles
• Particle size distribution
• Sand 2mm 20um
• Silt 20um 2um
• Clay lt2um
• Soils that have poorly sorted soil material
• The pores in the soil are very important for the
  liquid- and gas transport

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Gas phase

• Gas transport through pores by diffusion
• Macro- and micropores
• Macropores gt 10µmMicropores lt 10µm - 300nm
• Micropores are able to hold capillary water
• Unsaturated- and saturated zone

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Partial pressure of CO2 (PCO2) in soil

• 0.035 of the atmosphere is CO2
• pCO2-logPCO2-log0.000353.5
• Respiration causes decreased pCO2
• 0.1 - 3.5 of the soil gas is CO2
• pCO2-logPCO2-log0.0351.5

pCO2 is correlated to the evapotranspiration (te
mp. humidity)
pCO2 varies in soil from 3.0 to 1.5 The darker,
the higher pCO2
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Carbonate system

• The main production of H in soil originate from
  the hydration of CO2
• CO2 hydrate
• CO2 gH2O ? H2CO3 pKH 1.5
• and produce H2CO3 that protolyze
• H2CO3 ? HCO3-H pK1 6.35
• HCO3- ? CO32-H pK2 10.3
• Which then dissolve minerals
• CO2(g)H2OCaCO3? Ca2 2HCO3-

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Carbonate minerals

• Carbonate minerals in the soil has large
  influence on the soil- and water chemistry
• Render soil with high BS and
• Soil solution with high pH alkalinity

Amount of important chemical species relative to
the total amount of dissolved material
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Physical and biological weathering processes
Erosion

• Glacial
• Fluvial
• Eolian
• Marine
• Exfoliation
• Pressure relief
• Freeze-thaw cycle
• Frost bursting
• Biological
• Roots force cracks open

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Chemical weathering processes
Erosion

• Dissolution
• Soluble salts and minerals
• Chelation
• Reactions that make the minerals more soluble
• Oxidation
• 2FeO O2 gt Fe2O3
• Hydrolysis (H2O is split)
• AB HOH ? AHaq BOHaq
• E.g. Reaction between an oxide and water
• Anhydride (oxide) water ? hydride(-oxide)CaOH2
  O ? Ca(OH)2Fe2O3 H2O ? 2Fe(OH)3
• Hydration (H2O is not split)
• Formation of crystal water (CuSO4H2O)
• Formation of aqvo-ligands (Al(H2O)63)

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Hydrolysis due to weak acids
Erosion

• Hydroxides and carbonates are brought easier into
  solution by acid base reaction
• Ca(OH)2 2H ? Ca2 2H2O
• CaCO3 2H ? Ca2 2H2CO3
• Weak acids are therefore very important for the
  chemical erosion
• Carbonic acid pKH2CO3 6.35 Humic acids pK 2.5
  10

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Congruent and incongruent dissolution

• Congruent dissolution
• Mg2SiO44H2CO3?2Mg24HCO3-H4SiO4aq
• Incongruent dissolution
• When one mineral dissolves simultaneously with
  the precipitation of another
• Na0.58Ca0.42Al1.42Si2.58O8 4.45H2O 1.42CO2 ?
  0.42Ca2 0.58Na 1.16H4SiO4o
  0.71Al2Si2O5(OH)4 1.42HCO3-
• Hydrolysis of primary silicate minerals produce
  clay
• In non-acid regions this clay is then slowly
  depleted of SiO2, which is more soluble than
  Al(OH)3,
• KAlSi3O8H2CO37H2O ? Al(OH)3
  s3H4SiO4aqKHCO3-
• Clay is formed as an intermediate product

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Silicate erosion
Erosion

• Total (congruent) dissolution in the northern
  regions by hydrolysis and complexation
• Gradual and incongruent dissolution and formation
  of
• clay in acid (pHlt5) regions and
• (hydr)oxides in non-acid regions
• Total dissolution and neo-formation of clay and
  oxides in equatorial regions

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Soil development

• Determined by
• Parent material
• Erosion speed
• Climate
• Temperature, Humidity

• Time (History)
• Last ice age
• Topography
• Hydrology
• Vegetation
• Deciduous or coniferous
• Soil fauna

Climatic boundaries of morphogenetic regions
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Physical soil developing processes

• Translocation
• Movement of material
• Dissolved or as clay
• Aggregation
• Cementing of particles
• Freezing and melting
• Solifluction
• Expansion and shrinking
• Clay

Solifluction lobes and terraces, Newfoundland
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Chemical soil developing processes

• Gleying
• Reduction
• Peat development
• Podzolization
• Redistribution
• Elution
• Washing out
• Calcification
• Sclerosis

• Salinization
• Increase in salts
• Solodinization
• Increase in Na
• Ferratilization
• Hydrolysis
• Laterization
• Oxidation

Latitudal sonation of soil types

-

8
9
1
3
4
5
6
7
2
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Soil soilwater interactions Eks. Aluminium

• Important when pHlt5 or BSlt20
• Many mechanisms control pAl
• Assume solubility of gibbsite (Al(OH)3) control
  Al3
• Al(OH)3 3H ? Al3 3H2Ogives pAl pK 3pH

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Soil acidification

• 2 definitions of soil acidification
• Reduction in BS
• Low CEC and BSgt20 most sensitive
• Reduction in soil pH
• Low CEC and BSlt20 most sensitive
• Fluxes and reservoirs
• Causes for soil acidification
• Biological uptake
• Elution with anions
• Natural with HCO3-, A- and Cl-
• Anthropogenically with SO42- and NO3-

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Buffer capacity

• Weathering consumes H
• Weathering speed has consequence for the buffer
  capacity
• Different pH ranges have different buffer
  systems

pH gt 6.2 Calcite or other carbonates present
Large capacity to buffer acid 6.2 gt pH gt 5.0
Si-Al minerals erode 5.0 gt pH gt 4.2 Cation
exchange Al minerals dissolve and
Al-species buffer the solution 4.2 gt pH gt
2.8 Al-minerals dissolve pH lt 3.8 Fe-minerals
dissolve 10 gt pH gt 2.5 Organic acids
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Fluxes of ions through the soil


-
0


0

0
-

• Mobile anions
• Move easily through the soil
• ? Cl-
• ? SO42-
• P NO3-
• taken up by vegetation
• P Organic anions
• Usually less mobile, but may contribute to the
  transport in natural acid soil

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N-effects
NH3 O2 lt-gt NO2- 3H 2e- NO2- H2O lt-gt NO3-
2H 2e-
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Critical loads

• A quantitative estimate of the exposure of one or
  more pollutants that do not have significant
  damaging effect of specified sensitive parts of
  the environment according to today's knowledge