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Soil contamination and
remediation
Soil pH - Adsorption –
Degradation – Partitioning
Soil acidity / soil pH
• presence of H+ ions
• H+ + H2O = H3O+
• pH is probably the single most important factor
affecting the chemistry of the soil
pH
• acidity is expressed in pH scale
• pH = -log[H+], practically pH = -log[H3O+]
• Distilled water 1 x 10-7 M. (M = mol / litr)
• pH distilled water = 7
• pH scale from 0 to 14
• pH = 7 is neutral, ([H3O+ ] = [OH-]),
pH scale
Soil acidity
Active acidity – pH of extracted soil water, immediate amount of H+ at given time
Reserve acidity – exchangable H+ or Al3+
H H H H H+ H+H Ca++ H+Mg Mg++ H+Ca Ca++ H+ H+
H H H Na
soil
Reserve acidity Active acidity
Sources of soil acidity
1. Loss of base cations by their replacements by (potassium chloride, anhydrous Ammonia)
2. Intensive fertilization
Intensive production of CO2 by microorganisms:
CO2 + H2O ----> H2CO3 = H++ HCO3- ;
dissolving of Ca in H2CO3
3. Acid rains
▪ Burning of fossil fuels
▪ Coal power plants (SO2)
▪ Transport (NOX).
▪ These gases and water droplets forms sulphuric and nitric acids
▪ They precipitate as acid rains
• uptake Ca2+, Mg2+,
K+ roots release H+
• pH decreased
4. Plant uptake of base cations
http://ianrpubs.unl.edu/soil/g1503.htm
5. Leaching of base cations
K+K+
Mg2+
Mg2+
Ca2+Ca2+ Ca2+
Ca2+
Al3+
Al3+
Al3+
H+
NH4+
NH4+K+ K+
H+
H+
H+
H+
Ca2+
K+
K+
• leaching Ca2+, Mg2+, K+
from soil profile
• pH decreased
Nutrients
availability
dependent on
pH
pH influence on plants
pH of soil7.2 6.6 6.2 4.7 4.4
Barley
roots
low pH – Al(OH)3 Al3+ toxic
Soil buffer capacity
• Ability of soil to resist to external changes of pH
• Expressed as the amount of acid/base needed to change pH
• Buffer system = weak acids and salts.
• Buffer systems – humic acids, carbonic acid, phosphoric acid, silicic acid and colloids.
• Humus have significant buffer capacity exchange of basic cation for H+:
Buffer soil capacity
• heavy soils (understand clay soils) have higher buffer capacity.
• Example alkality of OH- is buffered by bicarbonate –carbonate system
(HCO3- + OH- CO32- + H2O).
• Soil buffer capacity can be improved by liming or adding organic matter to the soil.
• Soil with content of at least 0,3% CaCO3 a 2% of humusu have usually good buffer soil capacity.
pH of soil in CR
Sorption
Gases or liquids being incorporated into another material of a different state and adhering to the surface of another molecule.
Importance of sorption
Predictions of contaminant transport
Filtration (decontamination) of water and off-gas in remediation technologies
Absorption Adsorption
Soil sorption
Sorption in filtration
Soil sorption
• Mechanical sorption –trapping of
particles and colloids in dead-end pores
and porenecks
• Adsorption on interphase
• Ion exchange
• Chemisorption (complexation)
• Biological sorption (ingestion of the
chem. compound by organisms)
Distribution of Inorganic
contaminants (metals) in soil
• Dissolved in pore water
• Adsorbed on sorption sites
• Specifically adsorbed on
inorganic soil constituents
• Associated with insoluble soil organic matter
• Precipitated as solids
• Present in the structure of minerals
aqueous phase
adsorbed phase
solid phase
Metals – „aqueous phase“
Metals – „aqueous phase“
• Free metal ions (eg. Cd2+, Ni2+, Zn2+, Cr3+)
or
• Complexes (eg, CdSO40, ZnCl+, CdCl3
-)
Ligands Cl-, HS-, OH-, HCO3-, SO4
2-, CO32-
Form soluble complexes with metals
Example:
Zn2+ + Cl- = ZnCl+
ZnCl+ + Cl- = ZnCl2
These reaction can decrease the ionic strength of a solution
and therefore increase solubility of metals ->
increase of contaminant mobility
Metals – „adsorbed phase“
Sorption of inorganicsLaw of mass action
Adsorbed A + B Adsorbed B + A
Eqillibrium is described by following equation:
Keq = (A)[B]
[A](B)
Keq ... Equillibrium constant
(X) Ion activity in solute
[X] Activity of adsorbed ion
= Keq[A]
[B]
(A)
(B)
Metals – „adsorbed phase“
Interaction between cation and clay mineral
Example: 2Cs+ + Ca-clay -> Ca2+ + 2Cs-clay
Cation exchange capacity
CEC
The sum of exchangeable cations
CEC = equivalent charges / mass unit
Units: (me . kg-1 or meq / 100 g)
Sandy soils CEC > 100 meq.kg-1
Clays CEC < 100 meq.kg-1
Peat CEC up to 1500 me.kg-1
Adsorption of contaminants depends on soil CEC
values. The higher is CEC, higher the adsorption cationic
contaminants to the surface the higher the adsorption
Sorption of toxic metals
General order of preference or cations to adsorb
Pb > Cr > Cu > Cd > Ni > Zn
Higher order of pref. Lower order of pref.
Sorption of cation is influenced by:
CEC, PZC, pH, surface area, Eh, Ionic strenght
In practice transport of metals is solved using geochemical models, for example:
PHREEQC(http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/)
Eh – pH diagram
• when pH>7, CdC03
limits the solubility
• In anoxic conditions
CdS limits the
solubility
• Eh – pH diagrams
Eh – pH diagrams – Lead
• Eh – pH diagram
Source : EPA
Organic contaminants
• Hydrocarbons
• Chlorinated hydrocarbons
• Polycyclic aromatic hydrocarbons
• Polychlorinated biphenyls
• Pesticides
Distribution of organic
contaminants in soil
• in vapors
• dissolved in soil/ground water
• adsorbed
• as NAPL
Cw, mg/L, ppm
concentration in water
Cg, mg/L or ppmv
concentration in gas
Cs, mg/kg
adsorbed concentration
Three-phase system(only dissolved and adsorbed contaminants are present)
liquid
solid
air
Solubility in water
Soluble
Insoluble
Solubility depends on temperature, pH,
consolvents, dissolved organic matter
etc.
OCTANOL – WATER patitioning
waterin conc.
octanol in conc.Kow
C
C
w
oct
Partitioning of the contaminants
in system air-water-solid
)(
)(
waterin
solidin
[mg/L]
[mg/kg]K
C
Cd
w
s solid - water
waterin ionconcentrat
gas in ionconcentratH'
C
C
w
gwater – air
Volatilization
H = Henry’s law constant
Adsorption
Kd = Distribution coefficient
Henry’s law constant
• Units of Henry’s law constant
H (atm.m3/mol)
or
H’ (-) dimensionless
R….. gas constant = 8.20575 x 10-5 atm m3/mol °K
T...... temperature in °K
A
w
ρ
ρ
R.T
HH'
w
g
c
pH
Henry’s law constant values
• (10-7 < H < 10-5 atm.m3/mol) low volatilization
• (10-5 < H < 10-3 atm.m3/mol) volatilization is
slow but significant
• (10-3 < H atm.m3/mol) high volatilization
H = higher volatilization
Calculation of Kd
Kd = KOC. (%OC/100)
Soils with OC>1%
KOC ... Distribution coefficient OC/
KOC = 1 to 107
KOC = KOW. 0.41
Relationship between KOC a KOW
Adsorption isotherm
• For low
concentrations
linear
Cs = Kd . Cw
0
5
10
15
20
25
0 5 10 15 20 25CwC
s
Adsorption isotherm
Linear form
log (Cs) = log KF + 1/n log
Cw
0
10
20
30
40
50
60
0 10 20 30 40Cw
Cs
Freundlich isotherm
Cs = KF . Cw1/n
-2
-1.5
-1
-0.5
0
0.5
1
-4 -3 -2 -1 0 1 2
log Cw
log
Cs
Adsorption isotherm
Langmuir
isotherm
b.Kl.Cw
Cs =
1+Kl.Cw
0
1
2
3
4
5
6
7
8
9
10
0 10 20 30 40Cw
Cs
If Kl.Cw << 1 then it becomes
linear
Adsorption isotherm
measurements
“Batch sorption test”
1) Soil suspension in vial
2) Application of contaminant at different concentrations
3) Shaking (usually for 24 h)
4) Sedimentation of suspension in centrifuge
5) Water extraction
6) Chemical analysis of extracted water
7) Adsorption isotherm calculation
Adsorption isotherm measurements
Breakthrough curve (BTC)
1) Soil column
2) Constant water flux
3) Application of a concetration pulse or step function in the inflow water
4) Analysis of the effluent water
5) Relationship conc. vs. time
or conc. vs. Cummulative outflow is called „Breakthrough curve“
6) Inverse modelling ->
Transport parameters
BTC(example pesticides data)
0.0
20.0
40.0
60.0
80.0
100.0
0 1 2 3 4 5 6 7 8 9 10
filled pore volumes (-)
rela
tive c
on
cen
trati
on
(%
)
SMET-OBS
IMID-OBS
BR-OBS
SMET-MODEL
IMID-MODEL
BR-MODEL
Density
r mass/volume
r < 1
LNAPL
NAPL
Density
r mass/volume
r > 1
DNAPL
NAPL
DNAPL spill
NAPL
Source : EPA
Water–gas–NAPL–solid
partitioning
water
Solid phase
air
NAPL
Kd, H
+
KNW = distrib. koeficient NAPL–water
KNG= distribution coefficient
NAPL-gas
Degradation
Decrease of mass of
contaminant molecules in soil
Metabolite products
Often mathematically described
as first order decay
• Degradation database
http://umbbd.ahc.umn.edu/
Biodegradation
Microorganisms need oxygen and
source of energy
Contaminant is often source of
energy for microorganisms
Mostly aerobic degradation
When modeling, the
availability
of a energy source must be
considered
Biodegradation flatten the peaks
a. No sorption
No degradation
b. No sorption
Degradation present
c. Sorption present
No degradation
d. Sorption and
degradation present
Influence of sorption and biodegradation
on contaminant transportko
ncen
trace
vzdálenost
Radioactive decay
Radioactive decayIsotopes have the same number of protons but different
number of neutrons
Isotopes have different mass but similar properties
Vodík Deuterium Tritium
1 proton 1 proton 1 proton
1 neutron 2 neutrons
Carbon – 12 Carbon - 14
6 protons 6 protons
6 neutrons 8 neutrons
Decay halftime Half-life is the time
taken for half the radionuclide's atoms to decay
Half-times of selected radionuclides
Isotope
Natural40K, 226Ra, 222Rn, 235,238U3H, 7Be, 14C, 22Na
Human activity source3H, 90Sr, 137Cs, 239,240Pu60Co, 93,99Zr, 129I
Concentration units
mg/L or Curie (3.7 × 1010 disintegrations per second )
becquerel (symbol Bq) = number of disintegrations per second
Radio nuclides are subject of transport and
adsorption
Some isotopes with environmental occurrence
Radioactive decay
First order decay
k = decay
C = concentration
t = time
dC
dt= - kC
after variables separation and integration .......
C0
C(t)
dC
C= - k dt
0
t
ln C
C0
= - kt nebo C = C0e–kt
time
co
ncen
trati
on
Decay half-time () = time when C = (1/2)C0
After substitution ......
ln .5C0
C0
= - k ln 1
2= - k - ln 1
2= k ln 2 = k
dC
dt= - kC
dC
dt= - C
ln 2
And finally, after substitution
in the original equation.....
k = ln 2
References
• Databáze rozpadu toxických chemických látek
http://umbbd.ahc.umn.edu/
• Paulo C. Gomesa, Mauricio P.F. Fontes*,b, Aderbal G. da Silvab,
Eduardo de S. Mendonçab and André R. Nettoc, Soil Science
Society of America Journal 65:1115-1121 (2001)
• Císlerová M. a Vogel T., Transportní procesy. Skriptum ČVUT
(1998)
• http://www.natur.cuni.cz/~pcoufal/ Separační metody
• http://staff.bath.ac.uk/chsataj/CH10094%20lectures%201-4.pdf
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