Soil contamination and remediation

History - Chemistry background

– Chemical reactions – Colloids

- Soil pH – Soil buffer capacity

Introduction to soil chemistry

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”

Brief history of soil chemistry

• F. Stohmann a W. Henneberg

introduced adsorption

isotherm

c in solution

c o

n s

olid

• 1859 S. Johnson found that organic matter is

capable to absorb NH4+ even more than soil’s

inorganic part

Elementary soil composition

Element

%

O

49,0

Si

33,0

Al

6,7

Fe

3,2

Ca

2,0

Na

1,1

Mg

0,8

Element

%

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)_

• Oxides, hydroxides, organic

matter soil air

• Quartz, silicates, clay

minerals

• Hydroxides, clays

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 (e.g. CO3

2-)

– oxidation number of an element in free state is equal to 0 (O in O2)

– oxidation number of alkaline 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

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 (-)

Molar concentration cA (M)

Moles per unit volume 1M = 1 (mol / L)

Units of concentration

Milliequivalents per Liter (meq/L)

equivalent weight = atomic mass / oxidation number

Example: Ca2+ atomic mass 40 / ox. number 2 = 20 g

Units of concentration

ppm, ppb, ppt (parts per million, billion...)

nebo ppmv (parts per million volume)

mg / L – common unit

1 L distilled water represents 1 000 000 mg

so 1 ppm 1 mg / L

"Parts-per" notation

amount of one substance in another

Mass per unit volume

Mass of solute per unit volume of solution

Formula weight – add up atomic weights

Mole = gram formula weight

6 x 10 23 atoms

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

Molarity = (mg/L x 10-3 ) / formula weight in grams

Molality = moles / kg of solution

But 1000 g of water = 1 liter of water …

For dilute solutions (up to 0.01 molal) molality = molarity

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 (parts per million volume)

2 ‰

Concentration units

Example 3:

How many kilograms of Atrazine would have to escape to Orlik

water dam lake (300 mil. m3), to reach the concentration 3 ppb

of Atrazine in water

900 kg

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

Ca 2+ Calcium

Mg 2+ Magnesium

Na + Sodium

HCO3- Bicarbonate

SO42- Sulfate

Cl - Chloride

Si Silicon

B 2+ / 3+ Boron

Fe 2+ / 3+ Iron

NO3- Nitrate

NH4+ Ammonia

K + Potassium

Sr 2+ Strontium

Mn 2+ Manganese

Everything Else!

Major (> 5mg/L) Minor (.01 – 5 mg/L)

Trace (<.01 mg/L)

Chemical reaction in soil(Biogeochemical weathering)

(6) Types:

1. Hydration

2. Hydrolysis

3. Dissolution

4. Carbonation

5. Complexation

6. Oxidation-Reduction REDOX

All involve water!!!

Chemical reaction in soil

• Hydration: addition of “whole” water to a

mineral (Adsorption) e.g. mica

• Hydrolysis: reaction between H+ a OH-, the

products of water molecule dissociation

mineral + water = acid + base

hydrolysis products form clays

CaCO3 + H20 = Ca 2+ + HCO3- + OH -

Chemical reaction in soil

• Carbonation: formation of carbonic acid from dissolved CO2 gas (from organism respiration)

• Complexation: organic acids from

(decomposed OM) interact with metal

ions to form organo-metal complexes

(chelates)

CO2 + H2O = H2CO3 = HCO3- + H+

Oxidation

Loss of electrons, increased (+)

valence

• Example: Fe2+ 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- Fe2+

• Reduction often consumes H+,

decreasing soil acidity (raising pH)

Chemical reaction in soil

Oxidation and reduction

in soil, most influenced element is Fe, e.g.:

http://www.fr.ch/mhn/images/mineraux/goethit.jpg

Chemical reaction in soil

4Fe2+ + O2 + 4H+ = 2 H2O + 4Fe3+

Reaction consists of two half reactions:

4Fe2+ = 4Fe3+ + 4e-

O2 + 4H+ + 4e- = 2 H2O

oxidation

reduction

Standard electrical potential of a half reaction is the

voltage represented by the flow of electrons

when reaction is at equilibrium

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, SO42- )

Iron (as ferric, Fe 3+ )

Nitrogen (as nitrate, NO3-)

Carbon (as CO2)

Key things to remember

• O2 > NO3- > Mn4+ > Fe3+ > SO4

2- > 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

Oxidation Potential Eh (mV)

REDOX potential

In soils: from -200mV to 750mV

Result of REDOX reactions is a flux of electrons

electrical potential

positive if oxidizing,

negative if reducing

REDOX potential

Eh measurements

http://www.soil.ncsu.edu/wetlands/wetlandsoils/RedoxWriteup.pdf

Dissociation of Salt

NaCl = Na+ + Cl-

Chemical reactionsDissolution

Gypsum dissolution:

CaSO4(s) • 2H2O Ca2+

(aq) + SO42-

(aq) + 2H2O

(gypsum) (solute) (solute)

• Higher temperature = higher solubility

examples :

dissolving of minerals by

water, all previous reactions

are dissolution

All chemical weathering processes occur

simultaneously and are interdependent

Hydratation

Hydrolysis

Carbonation Complexation

Redox

Dissolution

▪ Product of biogeochemical weathering

Surface charge; ability to hold and exchange ions;

physical properties (stickiness and plasticity)

Layer silicate clays

Clay minerals structures

• Silica tetrahedronSiO4+

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 O2-

or OH-

• Octahedral sheets

• Octaherda are joined

by shared O2- or OH-

Al3+ (Mg2+)

(OH-)

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▪ 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

1:1Kaolinite group

vermiculite

limited shrink-swell

Clay minerals

vermiculite

2:1 vermiculite

2:1:1▪ Chlorite: octaherdal-like sheet of hydroxides

forms the interlayer, no swelling

Typy jílových minerálů

▪ Nonexpanding

2:1▪ Chlorite

Clay minerals

Clay minerals2:1▪ Smectite

▪ Substitution of Al for Mg

▪ Expanding

▪ Water and ions adsorption

▪ Small crystals

▪ Large speficic

surface area

800m2/g

Clay minerals

2:1▪ Smectite

Clay minerals summary

Soil Colloids▪ Chemical properties of colloids =

chemical properties of soil (adsorb

water and ions)▪ Size < 2 mm

▪ Large surface area > 10 m2/g (outer)

až 800 m2 (inner + outer)

Colloids

mineral (clay minerals)

organic (humus, humic acid)

organic-mineral

Colloids

core – negative charge

Diffuse layer

SolutionStern layer

Colloids Net negative (adsorbs

cations)Net positive (adsorbs

anions)

Variable charge (depends on pH)

pH ... positive pH ...negative

Colloid with net

negative charge

charge neutral = ZPC (zero

point charge)

ZPC is an important property

of clay minerals

Kaolinite ZPC = 4-5

Montmorillonite = <2.5

Electric double layer

Ca 2+

Ca 2+

Ca 2+

Ca 2+

Ca 2+

Ca 2+

Ca 2+

Ca 2+

Ca 2+

Ca 2+

SO42-

SO42-

Ca 2+

Ca 2+

SO42-

SO42-

Stern

layer

Diffuse

layer

solution

Diffuse double layer thickness

depends also on:

Temperature

T (↑T ….. ↑ th.)

`

Electrolyte concentration

n0 (↑ n0 ….. ↓th.)

Cation valence

z (↑ z ….. ↓th.)

... colloid transport

Have a look at movies on colloid

transport.

http://www.bee.cornell.edu/swlab/colloid

s/videos/

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-]),

Soil acidity

Active acidity – pH of extracted soil water, immediate amount of H+ at given time

Reserve acidity – exchangeable H+ orAl3

+

H H H H H+ H+H Ca++ H+Mg Mg++ H+Ca Ca++ H+ H+

H H H Na

půda

Reserve acidity Active acidity

pH scale

Sources of soil acidity1. 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+

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, silica acid and colloids.

• Humus have significant buffer capacity exchange of basic cation for H+:

pH in Czech Republic

References

• 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.