Soil contamination and remediation
Introduction to soil chemistry
Chemistry background – History –
chemical reactions – Colloids - soil
pH – soil’s buffer capacity
Chemistry - background
•
•
•
•
atom, molecule, chem. substance
periodic table (eg. http://en.wikipedia.org/wiki/Periodic_table)
atomic number, atomic mass ~ molar mass of a substance (g)
molecules (ionic and covalent chemical bonds)
oxidation no. of element in molecule = sum of positive and
negative charges
–
–
–
–
–
–
Basic rules:
sum of oxidation number is zero for neutral molecule (eg. NaCl),
otherwise is equal to the molecule charge (např. CO32-)
oxidation number of an element in free state is equal to O (O in O2)
oxidation number of alkali metals (Group 1: H, Li, Na, K, Rb, Cs and
Fr) = +1
oxidation number of alkaline earth metals (Group 2: Be, Mg, Ca, Sr,
Ba and Ra) = +2
oxidation number of oxygen is almost always equal to -2 (except v
H2O2 where is -1)
oxidation number of hydrogen is almost always equal to +1
Chemistry - background
http://en.wikipedia.org/wiki/Periodic_table
Units of concentration
Molar concentration cA (M)
Moles per unit volume 1M = 1 (mol / L)
1 mol is amount of substance of a system which has 6.0225 x 1023
elementary entities
Mole fraction xi
number of moles of solute (ni) /
total number of moles in a solution (-)
Milliequivalents per Liter (meq/L)
equivalent weight = atomic mass / oxidation number
Example: Ca2+ atomic mass 40 / ox. number 2 = 20 g
Units of concentration
"Parts-per" notation
amount of one substance in another
ppm, ppb, ppt (parts per million, billion...)
nebo ppmv (parts per million volume)
Mass per unit volume
Mass of solute per unit volume of solution
mg / L – common unit
1 L distilled water represents 1 000 000 mg
so 1 ppm ≈ 1 mg / L
Mole = gram formula weight
6 x 10 23 atoms
Formula weight – add up atomic weights
2 Hydrogen = 2 x 1 gram = 2 grams
1 Oxygen = 1 x 16 grams = 16 grams
1 mole H20 = 18 grams
Molarity = moles / L of solution
Molality = moles / kg of solution
But 1000 g of water = 1 liter of water …
For dilute solutions (up to 0.01 molal) molality = molarity
Molarity = (mg/L x 10-3 ) / formula weight in grams
Concentration units
Example 1:
What will be the concentration in ppm, when
1g is diluted in 999,999 liters of water?
1 ppm
Concentration units
Example 2:
What concentration expressed in ‰
is equal to 2000 ppmv
2‰
Concentration units
Example 3:
How many kilograms of Atrazin would have to escape to Orlik
water dam (300 mil. m3), to reach the concentration 3 ppb
of Atrazine in water
900 kg
Concentration units
Example 4:
What concentration in ppm equals to
molar concentration 0,001M Ca2+ in water
40 mg/L = 40 ppm
Brief history of soil chemistry
• 1819 Italian chemist Gazerri
early leaching experiments
• J. Thomas Way reported that soils
retain cations NH4+, K+ a Na+
dissolved in leaching water and
release cation Ca2+
„Father of Soil Chemistry”
• F. Stohmann a W. Henneberg
introduced adsorption
isotherm
c on solid
Brief history of soil chemistry
c in solution
• 1859 S. Johnson found that organic matter is
capable to absorb NH4+ even more than soil’s
inorganic part
Elementary soil composition
Prvek
%
O
49,0
Si
33,0
Al
6,7
Fe
3,2
Ca
2,0
Na
1,1
Mg
0,8
Prvek
%
K
1,8
Ti
0,5
P
0,08
Mn
0,08
S
0,04
C
1,4
N
0,2
(URE a BERROW, 1982)_
• Hydroxides, clays
• Oxides, hydroxides, organic
matter soil air
• Quartz, silicates, clay
minerals
Soil water
• Water is an exceptionally
good solvent
• Charges strong enough
to cause dissociation of
molecule
• Groundwater naturally
contains dissolved
cations and anions
Major, Minor, and Trace Solutes Dissolved in Groundwater
Major (> 5mg/L)
Ca 2+
Mg 2+
Na +
HCO3 SO4 2Cl Si
Minor (.01 – 5 mg/L)
Calcium
Magnesium
Sodium
Bicarbonate
Sulfate
Chloride
Silicon
B 2+ / 3+
Fe 2+ / 3+
NO3 NH4 +
K+
Sr 2+
Mn 2+
Trace (<.01 mg/L)
Everything Else!
Boron
Iron
Nitrate
Ammonia
Potassium
Strontium
Manganese
Chemical reaction in soil
(Biogeochemical weathering)
(6) Types:
All involve water!!!
1. Hydration
2. Hydrolysis
3. Dissolution
4. Carbonation
5. Complexation
6. Oxidation-Reduction REDOX
Chemical reaction in soil
•
Hydration: addition of “whole” water to a
mineral (Adsorption) e.g. clay & mica
•
Hydrolysis: reaction between H+ a OH-, the
products of water molecule dissociation
mineral + water = acid + base
CaCO3 + H20 = Ca 2+ + HCO3- + OH -
hydrolysis products form clays
Chemical reaction in soil
•
Carbonation: formation of carbonic acid from
dissolved CO2 gas (from organism
respiration)
CO2 + H20 = H2CO3 = HCO3- + H+
•
Complexation: organic acids from
(decomposed OM) interact with metal
ions to form organo-metal complexes
(chelates)
Chemical reaction in soil
Oxidation
Loss of electrons, increased (+)
valence
• Example: Fe2+ Æ Fe3+ + e• Oxidation releases energy
• Lost e- must go somewhere, so always
paired with a reduction
Chemical reaction in soil
Reduction
• Gain of electrons. Occurs where
oxygen supply is low and biological
demand is high
• Example: Fe3+ + e- Æ Fe2+
• Reduction often consumes H+,
decreasing soil acidity (raising pH)
Key things to remember
• O2 > NO3- > Mn4+ > Fe3+ > SO42- > CO2
• The longer the soil is saturated, the further to
the right the system moves. So VERY little
energy to be gained in permanently saturated
systems.
• There has to be an energy source (O.M. or
sugars) for any redox to take place, because
reduction (think wet) is MICROBIALLY
mediated
Chemical reaction in soil
Oxidation and reduction
in soil, most influenced element is Fe, e.g.:
4Fe2+ + O2 + 4H+ = 2 H2O + 4Fe3+
Reaction consists of two half reactions:
4Fe2+ = 4Fe3+ + 4e-
oxidation
O2 + 4H+ + 4e- = 2 H2O
reduction
Standard electrical potential of a half reaction is the
voltage represented by the flow of electrons
when reaction is at equilibrium
http://www.fr.ch/mhn/images/mineraux/goethit.jpg
REDOX
Together the oxidation and reduction is often
called (REDOX)
Oxygen is not a single electron acceptor
Electron Donors
Sulfur (as sulfide, S 2- )
Iron (as ferrous, Fe 2+ )
Nitrogen (as ammonia, NH4+)
Carbon (as CH2O)
Electron Receptors
Oxygen (as gas)
Sulfur (as sulfate, SO4 2- )
Iron (as ferric, Fe 3+ )
Nitrogen (as nitrate, NO3 -)
Carbon (as CO2)
REDOX potential
Result of REDOX reactions is a flux of electrons
electrical potential
Oxidation Potential Eh (mV)
positive if oxidizing,
negative if reducing
In soils: from -200mV to 750mV
REDOX potential
Eh measurements
http://www.soil.ncsu.edu/wetlands/wetlandsoils/RedoxWriteup.pdf
Chemické procesy
Dissolution
• Higher temperature = higher solubility
examples :
Dissociation of Salt
NaCl = Na+ + ClGypsum dissolution:
CaSO4(s) • 2H2O
(gypsum)
Ca2+(aq) + SO42-(aq) + 2H2O
(solute) (solute)
dissolving of minerals by
All chemical weathering processes occur
water,and
allare
previous
reactions
simultaneously
interdependent
are dissolution
Redox
Hydratation
Dissolution
Carbonation
Hydrolysis
Complexation
Layer silicate clays
ƒ Product of biogeochemical weathering
Surface charge; ability to hold and exchange ions;
physical properties (stickiness and plasticity)
Clay minerals structures
• Silica tetrahedron
SiO4+
one silicon surrounded
by four O2-
• Tetrahedral sheets
tetrahedra are joined
by shared oxygen
Clay minerals structures
• Octahedral sheet
one Al3+ or Mg2+
surrounded by four O2or OH• Octahedral sheets
• Octaherda are joined
by shared O2- or OH-
(OH-)
Al3+ (Mg2+)
Clay minerals structures
• Clay particles are formed
by octahedral and
tettrahedral sheets
stocked one on the other.
• Isomorhpous
substituition:
tetrahedral sheets Al3+ for
Mg2+
octahedral sheets
Si4+ for Al3+
Unbalanced negative
charges
Clay minerals
1:1
ƒ
ƒ
ƒ
ƒ
ƒ
Kaolinite group
No effective layer charge
No internal surface
Several sheets form crystal
Small specific surface ~15 m2/g
Kugler, R.L. and Pashin, J.C., 1994, Reservoir
heterogeneity in Carter sandstone, North Blowhorn
Creek oil unit and vicinity, Black Warrior basin,
Alabama: Geological Survey of Alabama Circular
159, 91 p.
Clay minerals
1:1
Kaolinite group
Clay minerals
2:1
vermiculite
vermiculite
limited shrink-swell
vermiculite
Typy jílových minerálů
2:1:1
ƒ Chlorite: octaherdal-like sheet of hydroxides
forms the interlayer, no swelling
ƒ Nonexpanding
Clay minerals
2:1
ƒ Chlorite
Clay minerals
2:1
ƒ
ƒ
ƒ
ƒ
Smectite
Substitution of Al for Mg
Expanding
Water and ions adsorption
ƒ Small crystals
ƒ Large speficic
surface area
800m2/g
Clay minerals
2:1
ƒ Smectite
Clay minerals summary
Soil Colloids
ƒ Chemical properties of colloids =
chemical properties of soil (adsorb
water and ions)
ƒ Size < 2 µm
ƒ Large surface area > 10 m2/g (outer)
až 800 m2 (inner + outer)
mineral (clay minerals)
Colloids
organic (humus, humic acid)
organic-mineral
Colloids
Colloids
Net negative (adsorbs
cations)
Net positive (adsorbs
anions)
Variable charge (depends on pH)
pH
... positive
pH
...negative
core – negative charge
Solution
Stern layer
Diffuse layer
Acidoidní
koloid
Electric double layer
Ca 2+
Ca 2+
Ca 2+
SO42Ca 2+
Ca 2+
Ca 2+
SO42-
Ca 2+
Ca 2+
SO42-
Ca 2+
SO42-
Ca 2+
Ca 2+
Ca 2+
Stern
layer
Diffuse
layer
solution
... colloid transport
Have a look at movies on colloid
transport.
http://www.bee.cornell.edu/swlab/colloid
s/videos/
References
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http://old.mendelu.cz/~agro/af/agrochem/multitexty/html/agrochemie
_pudy/ (in Czech)
Kutílek a kol. Hydropedologie, - skriptum (in Czech)
Fitzpartick, E.A. Soils
Sharma, H.D., Reddy, K.R. Geoenvironmental engineering, Wiley
and Sons, 2004
Kugler, R.L. and Pashin, J.C., 1994, Reservoir heterogeneity in
Carter sandstone, North Blowhorn Creek oil unit and vicinity, Black
Warrior basin, Alabama: Geological Survey of Alabama Circular
159, 91 p.