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10/27/2006 Depatment of Chemistry 1 Soil chemistry and biogeochemical processes in soil

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Page 1: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 1

Soil chemistry and biogeochemical processes in soil

Page 2: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 2

Soil horizons

Page 3: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 3

Lecture outline:Soil composition

Solid phaseInorganic mineral particlesOrganic material

together in aggregates

Living material

Liquid phaseGas phase

In a network of pores

Rocks + soil = the lithosphereLithosphere + plants and animals which live on it = the terrestrial environment

Outline of lecture

Soil

Liquid phase Solid phase Gas phase

Water withdissolved componants

Organic materialOrganic soil

Inorganic materialMineral soil

Organisms Humus Sand size particles Silt size particles

Air and gases

Clay size particles

Clay Amorphous material(Hydr)oxides

Fulvic acids

Humic acids

Page 4: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 4

Solid phase

Solid phase composition

Soil typically consists of ~ 95 w/w % inorganic and ~ 5% organic material

The inorganic material is in turn composed of 3 primary particles

Sand 2–0.05 mm,

Silt 0.05–0.002 mm

Clay <0.002 mm.

In loam soils, no single component dominates.

Organic matter and clays are dominant in determining the soil properties

Due to: large surface area

associated charge (usually negative).

Average Elemental Percentage Composition of Soils by Weight:

O 49 Na 0.7Si 33 Mg 0.6Al 7 Ti 0.5

Fe 4 N 0.1C 1 P 0.08

Ca 1 Mn 0.08K 1

Page 5: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 5

Solid phase

Solid phaseMineral soil

Primary mineralsIgneous Rocks

Granite, Basalt

Sedimentary RocksSandstone, shale, limestone

Metamorphic RocksGneiss, Marble

Secondary mineralsIncongruent precipitation products of chemical weathering processes

Page 6: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 6

Solid phase

Mineral structure

Silicates are the main rock-forming minerals

Most rocks (with the exception of carbonate rocks) are composed wholly or in part of silicates

The basic SiO42- tetrahedron forms

a rich variety of structureschains (1-D), rings and sheets (2-D) or frameworks (3-D)

O

SiO O

O

4-

Si4+ + 4O2- SiO44-

O

SiO O

O

SiO

SiO

O O

O O

Page 7: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 7

Solid phase

SilicatesStructure Anionic formula Examples

Page 8: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 8

Solid phase

SilicatesStructure Anionic formula Examples

Page 9: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 9

Solid phase

Silicates

Put together units of Si-tetrahedral (|)

Clay, Mica

Quarts

Page 10: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 10

Solid phase

Aluminium

Aluminium is the other element abundant in the earth's crust that can form a complex anion:

Aluminium can exist in fourfold (tetrahedral) and sixfold (octahedral) (O) coordination sites.

Kaolinite:

HYDROXIDE

HYDROXIDEALUMINIUM

ALUMINIUMOXIDE / HYDROXIDE

OXIDE / HYDROXIDE

SILICON

SILICON

OXIDE

OXIDE

Hydrogen bonding

Page 11: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 11

Solid phase

Phyllosilicates

Adobt a layer ofAl-octahedrals (O)

Clay type Often < 2µm

1:1 type (l-O)1:2 type (l-O-l)

Others: Mica group

Biotite, muscovite

Kao

linite Ill

ite

Page 12: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 12

Solid phase

Solid phaseNatural Organic Material

Humus: End product of chemical and biological decayPoorly defined:

Divided into: humic, fulvic, humin

Page 13: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 13

Functional groups on NOM

Poorly defined: Many functional groups

OH

C OH

O

N C

H

CC OH

H

S CC

Phenol Carboxyl Amin Alchohol Sulfhydryl

Page 14: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 14

Fractionation of organic matter

Organic matter is commonly extracted from soils and sediments with 0,5M NaOH.

Humin: What is not extractedHumic matter: Precipitates when the resulting dark colored solution is brought to pH=1 with HClFulvic acids: The remaining fraction

Page 15: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 15

Congruent and incongruent dissolution

Congruent dissolutionMg2SiO4+4H2CO3

*→2Mg2++4HCO3-+H4SiO4aq

Incongruent dissolutionWhen dissolution of one mineral leads to a simultaneously precipitation of anotherNa0.58Ca0.42Al1.42Si2.58O8 + 4.45H2O + 1.42CO2 →0.42Ca2+ + 0.58Na+ + 1.16H4SiO4

o + 0.71Al2Si2O5(OH)4 s + 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,

KAlSi3O8+H2CO3*+7H2O →

Al(OH)3 s+3H4SiO4aq+K++HCO3-

– Clay is formed as an intermediate product

Page 16: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 16

Water phase

Water phase

Page 17: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 17

Water phase

Soil water composition

The ionic composition in water is determined by:

Distance to sea (sea-salts)Natural emissionsAnthropogenic loadingMineral composition of the soil

Sea salts

Natural emission

Anthropogenic emission

Catchment

HCO3- Bicarbonate

H+ Hydronium Ca2+ Calsium Na+ Sodium SO4

2- Sulphate Cl- Chloride Al3+ Aluminium Mg2+ Magnesium K+ Potassium NO3

- Nitrate Fe2+ Iron A- Organic acids

Page 18: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 18

Water phase

Carbonate minerals

Carbonate minerals (Limestone and Dolomites) in the soil has large influence on the soil- and water chemistry

React easily with groundwater and give water 'hard' character. high pH & alkalinity

Render soil with high %BS

Amount of important chemical species relative to the total amount of dissolved material

Page 19: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 19

Water phase

Concentration vs. Activity{X}=γX · [X]

{X} is the activity to X[X] is the concentration to XγX is the activity coefficient to X

γX is dimensionless It is determined by:

• The diameter (å) of the hydrated X

• Its valence (nX)• The ionic strength (I)

n=1

n=2

n=3

n=4

• when I →0 γ→1when I<10-5M γ ≈ 1

Anions + cations

Not possible to calculate further than

I=0.1

Page 20: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 20

Water phaseDebye Huckel(DH) equation

For ionic strengths (I) < 0.1M the γX can be calculated by means of the e.g. Debye Huckel equation:

I < 0.1

I < 0.005

0.5 & 0.33 are temperature dependent table values

Presented values are for 25°C

åX is a table value for the specie in question

))33.01/((5.0log 2 IåIn xxx +=− γ

Spesier åH 3O + 9Na(H 2O)6

+ 4K(H 2O)6

+ 3Cl(H 2O)6

- 3

M g(H 2O)62+ 8

Ca(H 2O)62+ 6

Ni(H 2O)62+ 6

Cu(H 2O)4+ 6

Zn(H 2O)42+ 6

Pb(H 2O)62+ 5

Al(H 2O)62+ 9

Fe(H 2O)62+ 9

Inxx25.0log =− γ

1)005.033.01( <<+ xå

Page 21: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 21

Complexation reactions;

Outer sphere complexes

Physical, non-specific adsorptionMetal ion retains coordinated waterHeld together by electrostatic attractive forces (Coloumb forces)

Ion Pairs• formed solely by electrostatic

attraction• ions often separated by coordinated

waters• short-lived association• no definite geometry

Page 22: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 22

Complexation reactions;

Inner sphere complexes

Chemical, specific adsorptionCovalent binding by a metal ion to a ligand possessing a free ion pair

• Displacement of coordinating water molecules

• E.g.: Hydrolysis– Chemical process in which a

molecule is cleaved into two parts by the addition of a molecule of water

– NaAc + H2O = Na+ + CH3COOH +OH-

Coordination complexes• Large covalent component to the

bonding• Ligand and metal joined directly• Longer-lived species• Definite geometry

Page 23: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 23

In solution

Between liquid and solid phase

Page 24: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 24

H2O 1.00E+00H(+) 2.51E-06Al(3+) 1.18E-10Ca(2+) 9.42E-06Cl(-) 1.00E-05CO3(2-) 1.10E-07Fe(3+) 1.57E-19F(-) 9.93E-07K(+) 1.00E-06Mg(2+) 9.34E-06Na(+) 9.97E-06NH4(+) 1.00E-06NO3(-) 1.00E-05SO4(2-) 9.96E-06OH- 4.00E-09Al(OH)2(+) 1.49E-09Al(OH)3 7.46E-10Al(OH)4(-) 2.97E-11AlOH(+2) 4.82E-10CaOH 9.47E-13FeOH 4.04E-16FeOH2 5.32E-14Fe2(OH)2+4 4.39E-30FeOH3 2.49E-16FeOH4 9.91E-19Fe3(OH)4+5 4.88E-41MgOH 6.03E-12CaHCO3 5.55E-07H2CO3 3.32E-02HCO3 5.89E-03MgHCO3 6.46E-07NaHCO3 3.30E-08H2F2 3.65E-17HF2 1.39E-14HF 3.68E-09NH3 2.23E-10HSO4 2.43E-09AlF2 6.56E-10AlF3 1.21E-11AlF 1.20E-09AlF4 6.04E-15AlF5 7.21E-20AlF6 3.59E-26AlSO4 1.23E-12Al(SO4)2 9.76E-16CaCO3 1.46E-09CaF 8.15E-11CaSO4 1.91E-08FeCl2 2.12E-27FeCl3 2.12E-33FeCl 4.74E-23MgCO3 9.78E-10NaCO3 2.03E-11FeF 2.47E-19FeF3 1.54E-23FeF2 9.77E-21FeSO4 1.30E-20Fe(SO4)2 4.10E-24MgF 6.13E-10NaF 1.61E-12KSO4 7.05E-11MgSO4 1.65E-08NaSO4 4.98E-10NH4SO4 1.28E-10

Water phase

Hydrolysis and complexation

In solution there are numerous chemical reactions that are all in equilibrium with each otherConsidering only the major ions: H+, Ca2+, Mg2+, Na+, K+, Fe3+, Al3+, F-, Cl-, NO3

-, SO4

2- and HCO3- there are

more than 60 different species in equilibrium

Page 25: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 25

Water phase

Inorganic complexes

Common ligands in natural systems:

OH-, HCO3-, CO3

2-, Cl-, SO42- & F-

In anoxic environment: HS- & S2-

Dominating species in aerobic freshwater at pH 8 are:

Metal ion Dominating species % Mn+aq of

total amount of M

Mg(II) Mg(H2O)62+ 94

Ca(II) Ca(H2O)62+ 94

Al(III) Al(OH)2(H2O)4+, Al(OH)3(H2O)3

0, Al(OH)4(H2O)2- 1•10-7

Mn(IV) MnO2(H2O)20 -

Fe(III) Fe(OH)2(H2O)4+, Fe(OH)3(H2O)3

0, Fe(OH)4(H2O)2- 2•10-9

Ni(II) Ni(H2O)62+, NiCO3(H2O)5

0 40 Cu(II) CuCO3(H2O)2

0, Cu(OH)2(H2O)20 1

Zn(II) Zn(H2O)42+, ZnCO3(H2O)2

0 40 Pb(II) PbCO3(H2O)4

0 5

Page 26: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 26

Water phase

Hydrolysis

85.22 H4Al(OH)OH4Al

25.17 H3Al(OH)OH3Al

55.10 H2Al(OH)OH2Al

954 H Al(OH) OH Al

6.5 H(OH)O)Al(H(OH)O)Al(H

75.6 H(OH)O)Al(H(OH)O)Al(H

6.5 H(OH)O)Al(H(OH)O)Al(H

954 H(OH)O)Al(HO)Al(H

43214aq

42aq3

3213aq 0

32aq3

212aq22aq3

11aq2

2aq3

4aq422aq0

332

3aq0

332aq242

2aq242aq2

52

1aq2

52aq3

62

=+++=+↔+

=++=+↔+

=+=+↔+

==+↔+

=+↔

=+↔

=+↔

=+↔

+−+

++

+++

+++

+−

++

+++

+++

pKpKpKpKp

pKpKpKp

pKpKp

.pKp

pK

pK

pK

.pK

β

β

β

β

Inner sphere complexationHydrolysis reactions are controlled by {H+}

E.g. Aluminium

– Al3+aq denotes Al(H2O)6

3+

Page 27: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 27

Water phase

Equilibrium constants E.g. Al-OH speciesNote: values may not correspond exactly to the figure

gibbsite)for common is (-8gibbsite) (amorphous 10 - gibbsite), (synthetic 6- pK

3pH pK }{H 3log-K -log}{Al log-}{HK}{Al

}{H}{AlK

O(l)3H(aq)Al(aq)3H (s)Al(OH)

3

33

3

3

23

3

=

+==

⋅=

=

+↔+

++

++

+

+

++

85.22 H4Al(OH)OH4Al

25.17 H3Al(OH)OH3Al

55.10 H2Al(OH)OH2Al

954 H Al(OH) OH Al

6.5 HAl(OH)OHAl(OH)

75.6 HAl(OH)OHAl(OH)

6.5 HAl(OH)OHAl(OH)

954 HAl(OH)OHAl

43214aq

42aq3

3213aq 0

32aq3

212aq22aq3

11aq2

2aq3

4aq42aq0

3

3aq0

32aq2

2aq22aq2

1aq2

2aq3

=+++=+↔+

=++=+↔+

=+=+↔+

==+↔+

=+↔+

=+↔+

=+↔+

=+↔+

+−+

++

+++

+++

+−

++

+++

+++

pKpKpKpKp

pKpKpKp

pKpKp

.pKp

pK

pK

pK

.pK

β

β

β

β

Page 28: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 28

Water phase

Concentrations of Al3+ and Al-OH complexes in equilibrium with different types of gibbsite (Al(OH)3)

Page 29: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 29

Calculating concentration of hydrolysis species

Water phase

{Fe3+} is determined by replacing each of the other parts of the mass equation with their equilibrium expression:

Then the other species can be determined from the {Fe3+} and β

E.g.;

}{Fe(OH)}{Fe(OH)}Fe(OH){}{Fe(OH)}{Fe

7.22 H4Fe(OH)OH4Fe

8.13 H3Fe(OH)OH3Fe

31.6 H2Fe(OH)OH2Fe

05.3 H Fe(OH) OH Fe

4

032

23

43214aq

42aq3

3213aq 0

32aq3

212aq22aq3

11aq2

2aq3

−+++

+−+

++

+++

+++

++++=

=+++=+↔+

=++=+↔+

=+=+↔+

==+↔+

C

pKpKpKpKp

pKpKpKp

pKpKp

pKp

β

β

β

β

++++= ++++

+4

43

32

213

}{H}{H}{H}{H1}{Fe ββββC

2

32

2 }{}{)( +

++ =

HFeOHFe β

2

32

2

3

22

2

aq22aq3

}H{}{Fe}{Fe(OH)

}{Fe}H{}{Fe(OH)

H2Fe(OH)OH2Fe

+

++

+

++

+++

=

•=

+↔+

β

β

Page 30: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 30

Concentrations of dissolved Fe3+ species

Water phase

FeT = 10-4 M

%Fe

0

20

40

60

80

100

FeT = 10-2 M

pH1 2 3 4

%Fe

0

20

40

60

80

100

Fe3+

FeOH2+

Fe(OH)2+

Fe2(OH)24+

Fe3+

FeOH2+

Fe(OH)2+

Two total Fe concentrations, FeT = 10-4M and FeT = 10-2M

3,05

Page 31: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 31

Water phase

Dissolved Organic Matter (DOM)

Concentrations range (mg/L)

Soilwater: 1- 50Ground water: 0,1-10Stream: < 60

Low molecular weight (LMW)

< 1000Da (e.g. C32H80O33N5P0.3)

High molecular weight1000 - > 100 000DaHumic substance

• Very complex and coloured substances

Increasing DOM →

Page 32: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 32

Water phase

Role of DOM in the environment

Mineral weatheringNatural soil acidificationC-sequestration to the seaFlux of macro-nutrients to surface waterEnergy and nutrients for heterotrophic micro-organismsFlux of heavy metals and organic pollutantsPhotic zone Aquatic flora and fauna since DOM are weak natural xenobiotics

Page 33: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 33

Water phase

pH dependent charge

Charge on humic and fulvicacids is strongly pH dependent

The charge is also dependent on the ionic streangth

Increase in I will decrease pH

Page 34: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 34

Water phase

Buffering capacity

OH

C OH

O

N C

H

CC OH

H

S CC

Phenolicgroups have pKa~10

Carboxylic groups have pKa ~ 4

Page 35: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 35

Water phase

Redox processes

Reduction and oxidation processes exert an important control on the distribution of species in groundwater

O2, Fe 2+, H2S, CH4

Redox processes play a major role in aquifer pollution problems such as:

nitrate from fertilizers, leaching from landfills, acid mine drainage and the mobility of heavy metals.

Page 36: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 36

Water phase

Redox processes

Respiration reduces the oxidantsIn an aerobic environmentO2 is used as energy source; it is reduced to H2O (O2 + 4H+ + 4e- 2H2O)

C106H263O110N16P1+138O2106CO2+16NO3

-+HPO42-+122H2O+18H+

In an anaerobic environment other oxidants are electron acceptors; NO3

-, MnO2, FeOOH etc

The oxidants are used as energy sources

Page 37: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 37

Standard electron potentialsExample calculation2Fe2+ + MnO2 + 4H+ = 2Fe3+ + Mn2+ + 2H2O

In order to obtain the E° for this reaction, we simply subtract the E° for the two half-cell reactions

E° = 0.77 - 1.23 = - 0.46 VoltThe negative voltage indicates that the reaction should proceed spontaneously to the right when all activities are equal to one.

Page 38: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 38

Water phase

Redox potential

The environments redox potential in a solution is expressed by EH(in mV relative to SHE) or rather pε where:

EH=0.0592 ⋅ -log{e-} = 0.0592pε

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

• pε from the ratios between redox pairs in a natural solution will therefore vary

2

2

3

2

MnOMn

FeFe +

+

+

Page 39: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 39

Water phase

Redox reactions- kinetic constraints

Redox reactions implicate the electron transfer from one atom to another.

Electron transfer reactions are often very slow, which indicates that kinetics also play a significant role Many reactions proceed only at significant rates when mediated by bacterial catalysis

Page 40: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 40

Water phase

Redox measurements

Eh determinations should in theory express the distribution of all redox equilibria, similar to how the pH expresses the distribution of all acid-base equilibria. However, in contrast to pH, pεis very difficult to measure unambiguously in most natural waters.

Page 41: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 41

Limitations in applicability of thermodynamic calculations on redoxreactions

Red-ox reactions are generally slowEquilibrium not establishedSpeciation mediated by bio-chemical processes

Thermodynamic calculations merely express limit values

Page 42: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 42

Water phase

Redox sequences

Reaction combinationsA+L: Aerobic respirationB+L: DenitrificationD+L: Nitrate reductionF+L: FermentationG+L: Sulphate reductionH+L: Methane fermentation

A+M: Sulphide oxidationA+O: NitrificationA+N: Iron oxidationA+P: Manganese oxidation

Electrone acceptors

O2 13.75 NO3

- (to N2) 12.65 MnO2 8.9 NO3

- (to NH4+) 6.15

FeOOH - 0.8 Organic material - 3.01 SO4

2- - 3.75 CO2 - 4.13

Example at pH 7A

BOC

E

G

H

↑Species in soil solution

Page 43: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 43

Water phase

The Redox ladder

H2O

H2

O2

H2ONO3

-

N2 MnO2

Mn2+Fe(OH)3

Fe2+SO4

2-

H2S CO2

CH4

Oxic

Post - oxic

Sulfidic

Methanic

The redox-couples are shown on each stair-step, where the most energy is gained at the top step and the least at the bottom step. (Gibb’s free energy becomes more positive going down the steps)

Page 44: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 44

Water phase

Some examples

A+L Aerobic respiration: CH2O+O2 → CO2 + H2O

B+L: Denitrification:4NO3

−(aq) + 5CH2O (aq) + 4H+(aq) →2N2(g) + 5CO2(g) + 7H2O(l)

A+M sulphide oxidation2O2 + HS−→ SO4

2− + H+

Page 45: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 45

Water phase

Redox zones downstream of a landfill

Baedecker and Back, 1979a

Decomposition of organic rich material, domestic waste, sewage, discarded chemicals, produces an organic-rich leachate which may enter an ”oxic” aquifer.

Page 46: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 46

Water phase

Water buffers

In soil water and streams/lakes with pH > 5,5 the bicarbonate system is important for buffering.

For a simple system with carbonate, bicarbonate and salts, we have (ANC: Acid Neutralizing Capacity (alkalinity))

(Concentrations in mol/L)When pH is below about 5.5, the alkalinity is low.

Dissolved organic acids (mostly humic substances) constitute another important buffering system.Al-hydroxides buffer in acid waters

ANC= [HCO3-] + 2[CO3

2-] + [OH-] – [H+] = [Na+] + [K+] + 2[Ca2+] + 2[Mg2+] – [Cl-] – 2[SO4

2-] – [[NO3-]

= Σ Strong base cations – Σ Strong acid anions

Page 47: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 47

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 <2um

Soils that have poorly sorted soil material

The pores in the soil are very important for the liquid- and gas transport

Page 48: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 48

Gas phase

Gas transport through pores by diffusion

Macro- and microporesMacropores > 10µmMicropores < 10µm - 300nm

• Micropores are able to hold capillary water

Unsaturated- and saturated zone

Page 49: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 49

Partial pressure of CO2 (PCO2

) in soil

0.035% of the atmosphere is CO2pCO2=-logPCO2

=-log0.00035=3.5

Respiration causes decreased pCO2

0.1 - 3.5% of the soil gas is CO2pCO2=-logPCO2=-log0.035=1.5

pCO2 is empirically correlated to the evapotranspiration(temp. & humidity)

pCO2 varies in soil from 3.0 to 1.5

The darker, the higher pCO2

Page 50: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 50

Calcite solubility in pure water

CaCO3 = Ca2+ + CO32-

Kcalcite (25°C)={Ca2+} + {CO32-}= 10 -8.48 = 3.3 10-9

All Ca2+ and CO32- - must come from

dissolving calcite, thusm Ca++ = m CO3--

Assumes that a = m

mCa2+ = √ 10-8.48 = 10-4.24

mCa2+ = 0.00006 mol/l = 0.06 mmol/l

• Field data from carbonate aquifers show, however, that Ca2+

concentrations can be as high as 1-5 mmol/L, i.e. almost a hundred times higher than predicted by the above dissolution reaction.

Page 51: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 51

Reaction with carbonic acid, H2CO3

The higher Ca2+ concentrations observed in the field are a result of reaction with carbonic acid H2CO3derived from respiration of organic matter. The acid provides protons (H+) which associate with the carbonate-ion (CO3

--) from calcite to form bicarbonate (HCO3

-) This is similar to complexation of Ca2+

and SO4-- ions leading to an increase

in the solubility of gypsum.

Page 52: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 52

CO2 - H2O system

CO2 gas will always dissolve in water, as it is rather soluble in water.

Rainwater will be in contact with atmospheric CO2, Soil water will be in contact with soil gas CO2, etc.

CO2(g) -> CO2(aq)

and subsequently:CO2(aq) + H2O -> H2CO3

For convenience of calculations, CO2(aq) is included in H2CO3

H2CO3* = CO2(aq) + H2CO3

and the overall reaction becomes:CO2(g) + H2O -> H2CO3

*

Page 53: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 53

Carbonatesystem

The main production of H+ in soil originate from the hydration of CO2

CO2 hydrateCO2 g+H2O H2CO3

* pKH= 1.5

and produce H2CO3that protolyze:

H2CO3* HCO3

-+H+ pK1= 6.35HCO3

- CO32-+H+ pK2= 10.3

Which then dissolve mineralsCO2(g)+H2O+CaCO3Ca2+ + 2HCO3

-

Page 54: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 54

Carbonic acid species vs pH when PCO2 is constant

K1 = {H+} {HCO3-}/{H2CO3*}

log(HCO3-) = log K1 + log(H2CO3) + pH

β2 = {H+}2 {CO3--}/{H2CO3*}

The TIC increases above pH 6.3log(CO3

--) = log β2 + log(H2CO3*) + 2pH

Page 55: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 55

Calculate TIC at different pH when PCO2 is constant

Page 56: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 56

Alkalinity

The alkalinity of a water sample is equal to the number of equivalents of dissociated weak acids.

Guiyang - Summer 2004

It is often determined by titration with a HCl or H2SO4 solution of known normality towards an endpoint pH of ca. 4.5

The most accurate way to determine alkalinity is the so-called Gran titration

In practice only the dissolved carbonic acid is of quantitative importance for the measured alkalinity.Alk ≈ Ac = [HCO3

-]+ 2[CO3--]

Although organic- and phosphoric acid as well as hydrolysis of aqueous metal ions may contribute to some extent,

Page 57: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 57

Availability of elementControlling Processes

1. Desorption or dissolution

2. Diffusion and convection

3. Sorption or precipitation at new sites

4. Adsorption by roots - rhizosphere effect

5. Translocation in plants

Page 58: Soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/CV/Presentations/Soil chemistry - Introduction.pdf · 10/27/2006 Depatment of Chemistry 3 Lecture outline: Soil

10/27/2006 Depatment of Chemistry 58

Mobility of Element The Controlling Factor

Mobility depend on Desorption, dissolutionDiffusion, convection

Relative mobility depend on:1) Chemical form and nature of the element 2) Chemical and mineralogical nature of the soil

- Clay, oxides or humus- pH and pε

3) Physical and biological environment of the soil