soil chemistry and biogeochemical processes in soilfolk.uio.no/rvogt/cv/presentations/soil chemistry...
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10/27/2006 Depatment of Chemistry 1
Soil chemistry and biogeochemical processes in soil
10/27/2006 Depatment of Chemistry 2
Soil horizons
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
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
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
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
10/27/2006 Depatment of Chemistry 7
Solid phase
SilicatesStructure Anionic formula Examples
10/27/2006 Depatment of Chemistry 8
Solid phase
SilicatesStructure Anionic formula Examples
10/27/2006 Depatment of Chemistry 9
Solid phase
Silicates
Put together units of Si-tetrahedral (|)
Clay, Mica
Quarts
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
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
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
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
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
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
10/27/2006 Depatment of Chemistry 16
Water phase
Water phase
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
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
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
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
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
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
10/27/2006 Depatment of Chemistry 23
In solution
Between liquid and solid phase
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
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
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+
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
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)
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
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
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 →
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
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
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
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.
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
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.
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 +
+
+
≠
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
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.
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
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
pε
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
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)
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+
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.
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
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
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
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
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.
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.
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
*
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
-
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
10/27/2006 Depatment of Chemistry 55
Calculate TIC at different pH when PCO2 is constant
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,
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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
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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