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