Chemical Contaminants in the Soil Environment
By Peder Maribo, 2007.
Introduction
Chemical substances can be grouped and characterized according to how they behave in the soil
environment, more precisely their characteristics in relation to:
1) Transport and spreading in the soil environment
2) Decomposition or transformation
3) Possible adverse effect on e.g. human health or the environment.
The above constitute the key elements in the environmental fate of a chemical substance and can be
described as a function of:
a) the characteristics of the chemical in question, and
b) the characteristics of the soil environment in question.
The following note is an introduction to some key characteristics of chemicals and the soil
environment that are of importance for the phase distribution of the chemicals and thus their
environmental fate. A short introduction to heavy metals and organic chemicals is given in this
perspective.
Figure 1:
Environmental fate of chemicals in the soil environment – a paradigme.
1. Phase distribution of chemicals in the soil environment
The phase distribution of a chemical is essential to the potential transport of the substance in the soil
environment. Soil is made up of three phases:
 an air (or gas) phase
 a solid phase (the soil particles), and
 a water phase
In addition the chemical itself may constitute a phase of it’s own, e.g. if it is present as a non
aqueous phase liquid (NAPL) or as a solid in it’s pure form, see figure 2.
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(Seperate phase)
Air
Water
Soil particles
Soil particles
Water
Air
Free phase chemical
Figure 2.
Phase distribution of a chemical in the soil environment. From Kjeldsen (1996).
In the soil environment a chemical present as vapour in the gas phase or dissolved in the soil water
may move convectively with the soil air or with the water phase respectively. Chemicals present in
these two phases can furthermore be spread due to molecular diffusion or dispersion1. Chemicals in
the soil environment may also move as a NAPL. Finally chemicals can be present in the solid phase
e.g. sorbed to soil particles. Particles in the soil upper crust can be moved by wind as dust.
The phase distribution of chemicals are widely a consequence of chemical or physical properties
associated with the chemical, but the physical and chemical properties of the soil particles and the
soil water also plays an important role. In the following some important physical/chemical
characteristics will be described.
Polarity and water solubility
A molecule is composed by elements sharing electrons in covalent bonds. The electronegativity
(EN) of an element is it’s ability to attract the electrons in the covalent bond (see enclosure 1 for a
periodic table of the elements with electronegativity). For example Chlor is more electronegative
than hydrogen, and in hydrochloric acid (HCl) the common pair of electrons in the covalent bond
between the two elements is more attracted to the chlor atom than to hydrogen atom. The
consequence of this is an uneven charge distribution, and the chlor end of the molecule becomes
slightly negative while the hydrogen end becomes slightly positive. The bond between chlor and
hydrogen is said to be polar.
A molecule can contain one or more polar or non-polar covalent bonds. If the molecule has an
uneven or asymmetrical distribution of electrons it will have a positive end and a negative end. The
1
Diffusion is the interchange of molecules with neighbouring molecules in a liquid or a gas due to the temperature
induced vibration of the molecules. Diffusion always occurs toward lower concentration of the chemical along
concentration gradients.
Dispersion is spreading of a chemical in a moving liquid or gas due to differences in velocity in the moving medium.
Laminar movements results in the highest dispersion and is the dominant flow type in soil gas and water.
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molecule is thus said to be electrically polar due to the internal electrical poles. HCl is a polar
molecule where as H2 is not.
A polar molecule thus has to contain polar bonds, but the spatial composition of the molecule is
also of significance. If we look at the chemical structure of the CO2 molecule (figure 3) there is a
difference in electronegativity between C (having a EN = 2.5) and O (EN = 3.5) resulting in a
different “pull” in the electrons. However the CO2 molecule is linear and the “pull” of the two
oxygen atoms are annihilating each other. Consequently the CO2 molecule is non-polar.
Figure 3.
Model of the CO2 molecule. The geometrical structure is linear and symmetrical
resulting in a non-polar molecule despite differences in electronegativity between the
C and O atoms.
If we look at the H2O molecule we see also here a difference in elektronegativity between hydrogen
(EN = 2.1) and oxygen (EN = 3.5). The presence of 8 electrons in the outer shell of the oxygen
atom results in an angle between the two covalent bonds to the hydrogen atoms of approx 105
degrees, so the molecule is not linear, and the difference in electronegativity results in a polar
molecule (figure 4).
Figure 4.
Model of the water molecule. The oxygen atom (shaded) has two lone pair of electrons
and two shared pairs that form covalent bonds with hydrogen atoms (white). These
pair of electrons result in the non-linear structure of the molecule. The nonlinear
structure and the difference between the electronegativity of the O and H atoms jointly
results in a polar molecule with H-atoms slightly positively charged and the O atom
slightly negatively charged. The total net molecule charge is zero.
The chemical substance water (H2O) exerts relatively strong intermolecular forces between the
individual molecules due to the polarity of the molecule. The negative sites of one molecule is
attracted to the positive site of the neighbouring molecule. These bonds between the water
molecules is the reason for the relative high boiling point of the small molecule. It takes a lot of
energy to break the intermolecular bonds and hence the water temperature (the intermolecular
kinetic energy) must be high before the individual molecular bonds are broken and the molecules
break apart as a gas.
Looking at the CH4 molecule one will see complete symmetry and this molecule is consequently
not polar.
Hydrocarbons (CxHy) are general non polar. As an example we can look at the butane molecule
C4H10 (figure 5).
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Figure 5.
A model of the butane molecule C4H10 (Carbon atoms shown as black and hydrogen as
white).
This molecule is not entirely symmetrical, but the C-H bond is only slightly polar with a difference
in electronegativity of only 0.4. Furthermore, the H atoms are somewhat evenly distributed over the
carbon “backbone” and the weak pull in the electrons is even and exerted from opposite sides. The
result is a non polar molecule.
It is of great significance for a chemical component weather it is has a polar or non-polar character.
A liquid of a polar substance will easily mix with other polar molecules, and can thus dissolve other
polar substances.
Water is an excellent solute for other polar chemical compounds. An ion is an example of a highly
polar substance and water is a fine solute of salts.
In the following we are going to look at the solubility of organic matter in water. What increases the
water solubility is the presence of polar, so called hydrophilic (“water loving”) functional groups in
the organic molecule, whereas non-polar or hydrophobic (“water rejecting”) groups reduces the
solubility (cf. table 2 above).
Hydrophilic (polar) groups are: -O-H ; -O-O-H; =N-H and =C=O groups.
Hydrophobic (non-polar) groups are: =C-H ; =C-Cl =C-Br and =C-I groups
It is the presence of hydrophilic and hydrophobic groups that determine the water solubility of the
chemical as can be seen from the following four examples:
Ethanol:
Butanol:
Octanol:
Buthanechloride:
CH3CH2OH
CH3CH2CH2CH2OH
CH3(CH2)7OH
CH3CH2CH2CH2Cl
Soluble in water under all conditions
Moderately soluble in water
Very low water solubility
Extreme low water solubility
The question of water solubility is not and either or, it is a continuum and depending on how many
hydrophobic groups and how many hydrophilic groups the molecule contains.
A “Rule of thumb” is: It takes approximately four (4) carbon atoms with hydrophobic
groups to even out the presence of one hydrophilic group.
Non-polar substances will not easily dissolve in water and non polar liquids as oil will not mix with
water and are only slightly soluble in water. An example of a non polar solute is tetrachlorethylene
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(TCE), which has been used in the dry cleaning industry for removal of non polar stains of grease
and oil and in the metal industry for removal of oil from metal surfaces.
NAPL’s
Chemicals that have a low water solubility and are present as liquids in pure form are referred to as
Non Aqueous Phase Liquids or NAPL’s. Mineral oil is one such NAPL. If the specific gravity of
the NAPL is higher that that of water (> 1.0 kg/L) the liquid is referred to as a Dense NAPL or a
DNAPL and if the density if the liquid is lower than that of water we refer to it as a Light NAPL or
LNAPL.
The heavy metal Mercury (Hg), heavy components of mineral oil such as tar, and chlorinated
solvents such as tetrachlormethane (TCM) are examples of DNAPL’s.
Most components in petrol, such as benzene, Toluene, ethylene and Xylene are examples of
components belonging to the LNAPL group of substances, although some of these are in fact
relatively well soluble in water.
Soap and detergents
Soaps are typical a sodium salt of a long organic molecule with a carboxylic acid group in the one
end, e.g. CH3(CH2)14COONa. In water the soap dissolve and split in to the negatively charged
organic molecule and the positive sodium ion. The organic molecule is hydrophobic (non-polar) in
the one end and polar (hydrophilic) in the other end. The hydrophilic end will mix with water if
possible and the hydrophobic end will mix with any non-polar material available, e.g. grease and in
this way the soap molecules make a grease particle water soluble.
Detergents – e.g. sulphur soap – consist of a sodium ion and a long organic molecules with a
benzene ring and a –SO3- group in one end. Such detergents can “dissolve” the non-polar chemicals
into a water phase.
Some soap and detergent molecules place themselves at the water surface with the non polar end of
the molecule sticking out of the water. The surface tension of the water is in this way reduced.
Solid phase materials (e.g. soil particles)
Mineral soil particles are solids with a varying surface charge on the atomic scale. Soil particles has
(like glass) an ability to sorb ions to their surface and exchange these ions with a surrounding liquid.
Mineral soil particles are thus moderately polar in their nature. A plastic material as Teflon – the
coating on non-stick frying pans – is an example of a solid (organic) material with a non-polar
character. A drop of water will “pearl” on a Teflon surface with a small contact angle between
Teflon and water. A water drop on a glass surface will have a much bigger contact angle between
glass and water surface due to the more polar nature of the glass surface.
Sorption and distribution coefficients Kd and KOW
Many organic chemicals are hydrophobic, which indicates that these substances have a low affinity
for solution in water, and prefer solution in non-polar liquids. These pollutants are readily taken up
in the soil organic matter, as this organic matter has many non polar components. It has been shown
that the tendency to become adsorbed on the surface of the soil particles is related to the wateroctanol distribution coefficient KOW.
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The distribution coefficient between water and octanol is obtained in a separatory funnel as shown
in the Fig. 6. The organic chemical is introduced into a funnel containing water and octanol. The
funnel is shaken and the two phases are separately collected. Analysis of the concentrations in the
water and octanol gives CW and CO respectively, from which the distribution coefficient
KOW=CO/CW is obtained.
Figure 6:
set-up for measurement of octanol-water distribution coefficient. From:
http://www.water.tkk.fi/wr/kurssit/Yhd-12.126/oppimateriaali/kd_e.htm
Example:
Naphthalene (an aromatic hydrocarbon C10H8) has a KOW coefficient of 2340 (T. Madsen and F.
Pedersen, 2000). If 2341 mg of naphthalene is put into a bottle with one litre of octanol and one
litre of water it will partition with 2340 mg in the octanol phase and 1 mg in the water phase. The
substance is thus very much non-polar. The water solubility of naphthalene is 30 mg/L
(http://www.chemicalland21.com/petrochemical/NAPHTHALENE.htm).
Water solubility of a chemical is correlating with it’s KOW , see figure 7.
Figure 7:
Correlation between Octanol/water distribution coefficient KOW and water solubility S.
(Curtis et al 1986).
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Sorption of organic matter to soil particles – or the distribution between water and particle bound
organic matter - is characterised by the so called soil-water distribution coefficient Kd.
Kd is defined as the ratio between chemical sorbed to the soil particles and chemical dissolved in the
soil water. For chemical X this can be expressed as:
Kd = Cx,soil / Cx,water
[L/kg]
Example:
10 mg of xylene (mxylene) is put in a flask containing 1.0 kg of dry loamy soil ( msoil) and one litre of
water (Vwater) and shaked. After some time the concentration of xylene is measured in the water
phase (Cxylene,water). The concentration in the water phase is found to be 1.7 mg. The remaining
matter must be sorbed to the particles (provided that the water solubility is not exceeded and xylene
is not present in free phase). In order to calculate the concentration of xylene in the particle phase
(Cxylene,soil) and the Kd one can set up the mass balance for the system:
mxylene = Vwater ·Cxylene,water + msoil ·Cxylene,soil
Here the only unknown is Cxylene,soil. This concentration of xylene in the particle phasecan thus be
found to be (10 [mg]– 1.7[mg/L]·1 [L] )/1,0 kg = 8.3 mg/kg. The Kd is thus 8.3/1.7 = 4.9 L/Kg.
Besides depending on the chemical substance and the polarity of it, Kd depends on the type of soil,
and primarily the content of organic matter in the soil. The surface of some clay minerals may have
ability to bind (sorp) some organic material, but by far it is the soil organic matter that is
accountable for the sorption of organic chemicals in the soil environment.
The distribution coefficient KOC (organic carbon distribution coefficient) is defined as the Kd
divided by the fraction of organic carbon fOC in the soil for which Kd is measured.
KOC = Kd / fOC
or
Kd = KOC fOC
Example
If the loamy soil mentioned above the content of organic carbon in the soil is 2 % (equivalent to fOC
= 0.02) and for xylene with Kd of 4.9 L/kg the KOC is: 4.9 L/Kg / 0.02 = 244 L/Kg.
The KOC is a much more universal constant than the Kd as the content of organic matter typically
varies with the depth in a given soil. If KOC for a organic chemical is known and the variation of
organic carbon with the depth of the soil is known, then the fractioning of the chemical in various
depth of the soil Kd can be calculated. Soil organic matter is typically high (1 – 3 %) in the top soil
and decreases a factor 10 or more below the till layer (40 – 50 cm from the surface).
The fraction of carbon in soil organic matter is typically 50 %, so if the organic matter content of a
soil is known it can be converted to an equivalent quantity of organic carbon by a division with 2.
The KOW distribution coefficient is highly correlated with the KOC, see figure 8.
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Figure 8
Correlation between organic carbon/water distribution coefficient for soil KOC and the
octanol/water distribution coefficient KOW . (Curtis et al, 1986)
[For further description of sorption and retardation factor R refer to note from Dan Ferrante (1996)
on the sorption process
http://ewr.cee.vt.edu/environmental/teach/gwprimer/sorp/sorp.html ]
Example:
Consider e.g. the distribution coefficient for benzene in an aquifer which has 1 % organic carbon,
i.e. foc=0.01. Log(Koc) of benzene is 1.58, which implies that Kd= 0.01*101.58= 0.38. The retardation
coefficient R=1 + (d /n)* Kd = 1+(1.5/0.3)*0.38 = 2.90. Here dry bulk density d =1.5 g/cm3 and
porosity n=0.3.
The table 1 below presents calculated distribution coefficient Kd and retardation coefficient R for
several fuel- and chlorinated solvent-related chemicals as a function of organic carbon content fOC.
From table 1 it can be seen that R can vary over two orders of magnitude depending on the
chemical in question.
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Table 1.
Calculated distribution coefficients Kd and retardation factors R for several fuel- and
chlorinated solvent-related chemicals for two different values of organic carbon
content foc (0.1 and 1 %). Dry bulk density d =1.5 g/cm3 and porosity n=0.3. From
http://www.water.tkk.fi/wr/kurssit/Yhd-12.126/oppimateriaali/kd_e.htm
Vapour pressure
The vapour pressure characterises the vapour – liquid equilibrium of a pure substance. This
equilibrium is highly dependant on the temperature. The boiling point is the temperature at which
the vapour pressure a substance equals the surrounding (atmospheric) pressure.
The vapour pressure is one way to define the concentration of a substance in the gas phase if the
substance is present in pure form and in connection with a gas phase. A film (free phase) of mineral
oil in the soil in equilibrium with the soil air is present in the soil air in a concentration equivalent to
the vapour pressure. The vapour pressure (in e.g. [Pa]) can be converted to a concentration in
[mol/L] via the ideal gas equation.
Px·V = nx ·R ·T
In which:
Px is the vapour pressure of substance x,
V is the volume,
nx is the quantity of the substance x in [mol],
R is the gas constant (R = 0.082057 atm·L/(mol·K) = 8.3144 Pa·m3/(mol·K)), and
T is the absolute temperature in [K].
The concentration of substance x in the gas phase with vapour pressure Px can thus be expressed at
temperature T as
Cx = nx/V = Px/( R·T).
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Example:
The vapour pressure of phenol at t = 20 oC (equalling T = 293 K) is registered as 25 Pa. Phenol in
free phase at 20 oC will thus have an equilibrium gas phase concentration of
Cphenol = 25 Pa /[8.3144 Pa·m3/(mol·K) · 293 K] = 0.0103 mol/m3
In atmospheric air [Ptot = 1013 HPa], phenol in equilibrium with the atmosphere would result in a
volumetric concentration of phenol in the air of Pphenol/Ptot = 25 Pa/101300 Pa · 100% = 0.025 %.
Volatilization from dry soil is determined by the vapour pressure and the sorption coefficient Kd.
Henrys law: the equilibrium between the gas phase and the water soluble phase.
If one fill a closed container with equal amounts of water and air and add 100 molecules of
benzene, the molecules will distribute themselves between the water and air phases with 25
molecules in the air phase and 75 in the water phase. This distribution is characteristic for the
chemical benzene, and all chemicals capable of existing as dissolved in water or present as vapour
has a characteristic distribution between the two phases.
This equilibrium is characterised by the so called Henrys Law and the relation between the
concentration of the chemical in the two phases is called Henrys constant KH'. Henrys law is
expressed in one form in equation 1:
KH' = Cx,air /Cx,water
(1)
In the above stated form of Henrys law the constant KH' is dimensionless as the concentration in the
two phases water and air is measured in the same unit, e.g. mole/L or mg/L.
I the soil environment water and air is in contact in the vadose zone and chemicals soluble in water
and capable of being present as vapours (having a vapour pressure, i.e. not ions) will be in a
dynamic equilibrium between the two phases.
Example:
Benzene is a non-polar hydrocarbon only slightly soluble in water and with a relatively high vapour
pressure (10 kPa). Henrys constant KH' for the four common aromatics the BTEX (sesection 3 in
this note) is approximately 0.25. Water solubility of benzene is approximately 1.8 mg/L.
If the soil is contaminated with benzene to an extend at which benzene is present as a free phase
(NAPL) the soil atmosphere will contain benzene in a concentration corresponding to the benzene
vapour pressure (if the water and atmosphere is in equilibrium2).
If the benzene contamination is not present as a NAPL but only dissolved in the water phase, the
atmosphere concentration of benzene is given by Henrys law, and with the KH‘ of benzene
being 0.25 the soil atmosphere concentration will be one quarter of the present in the water phase.
2
Equilibrium is a fair assumption for most soil atmospheres if the transport of chemicals to/from the soil water and the
atmosphere is slow compared to the transport across the water/air interphase. This is often the case in soil since air
and water phases normally move only very slowly.
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In a soil atmosphere the benzene concentration is measured to 1,0 mg/L. If this soil atmosphere is
equilibrium with the soil water, we can be calculate the concentration here as Cbenzene,g =
KH‘·Cbenzene,aq = 0.25 ·1 mg/L = 0.25 mg/L.
For methane (CH4) the dimensionless Henrys constant is approximately 30. Thus for methane 30
molecules exist in the gas phase for each one dissolved in the water phase given equilibrium
between the two phases. Methane thus by far "prefer" the gas phase to the water phase.
A gas concentration can be given as partial pressure in [Pa]: The concentration of a gas given this
way is independent of the total pressure3 . At a given temperature the partial pressure is equivalent
to a concentration in the gas phase4. The vapour pressure is equal to the partial pressure for a
substance present in free phase, so for a chemical dissolved in the water phase, the partial pressure
is lower than the vapour pressure.
The ideal gas equation gives us the relationship between total (or absolute) pressure P, absolute
temperature T, Volume V and gas quantity n (in [mole]) as follows:
P·V = n·R·T
(2)
With R being the universal gas constant (8.314 J/(mol·K) or 0.08206 atm·L/(mol·K))
For a gas component x this equation can be written
Px·V = nx·R·T
(3)
with Px being the partial pressure of x and nx the number of moles of gas per volume V.
The gas concentration Cx,g can be expressed Cx,g = nx/V. The ideal gas equation can be written as
n/V = P/(R·T) and the Henrys law can be reformulated as:
KH = Px / Cx,water
(4)
In which KH = KH‘·R·T.
KH is not dimensionless and KH is depending on the temperature. In the literature many recordings
of Henrys constant are specified with a dimension (a unit, e.g. [Pa·m3/mole]) and it is valid for the
above second statement of Henrys law (equation 4).
2. Heavy metals
3
The atmospheric pressure is the sum of all the partial pressures of the components in an atmosphere. The normal
atmosphere surrounding us is composed of approx 21 % O 2, some 78 % N2 and small contributions of water, CO2 , Ar
and other gases. At a normal atmospheric pressure at sea level of some 1013 HPa, the partial pressure of O 2 is 21 %
hereof or 213 HPa.).
4
The pressure of gas is constituted by the collision of the gas molecules with the surroundings and thus depend on the
number of gas molecules per unit volume and the temperature (speed of the molecules). The gas composition type of
molecules) is not significant.
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Several definitions exist for the group of elements referred to as the heavy metals. An often used
definition is metals with an atomic weight above that of Sodium (Na) and with a specific gravity
above 5 g/cm3 (John Jensen, 2000). This group comprises some 70 metals. A few of these and some
of their characteristics are listed in table 2.
Heavy metals can exist in different oxidation stages in various inorganic forms or incorporated in to
organic molecules. Some of the metals are naturally present in the soil environment, but most of
them are typically primarily found in geological formations outside the typical soil environment and
the biosphere in general. Many of the metals are essential for plants and animals as micronutrients,
but most of them are toxic if the concentration becomes moderate or high. The metals have been or are still - used in various products and processes of the industrialised society, and have been
transferred to the soil environment to a greater or smaller extent in the industrialised part of the
world.
Metal
Arsenic
Symbol Density [g/cm3]
As
5.73
Lead
Pb
11.35
Cadmium
Cd
8.64
Mercury
Hg
13.55
Iron
Fe
7.86
Chromium
Cr
7.18
Mangan
Mn
7.21
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Function / toxicity
Essential for mammals and some algae. 0.02 – 7.5
mg/L is toxic to plants. Has been used in pressure
creosoted wood and is naturally found in connection
with steel and coal and herefore often found associated
with gas and tar production facilities. Normal
oxidation stages are -3, 0, +3 and +5.
Non essential. 3 – 20 mg/L is toxic to plants. Widely
used in batteries, as additive in PVC plastics, as
additive in petrol, in paints, in fireworks, and used in
the construction business and may other industrial
processes. Sale and usage of lead based products was
banned in Denmark since 2000. Normal oxidation
stages are +2 and +4.
Non essential. 0.2 – 9 mg/L is toxic to plants. Critical
concentration in plant tissue 15 mg/kg. Used in metal
alloys, paints, plastics and batteries (Ni-Cd type).
Normal oxidation stages are +2.
Highly toxic to all organisms. Used in thermometers
and barometers, batteries and as seed grain fungicide.
Normal oxidation stages are 0, +1 and +2.
Essential in the synthesis of chlorophyl (plants).
Component in many enzymes (also human). Normal
oxidation stages are 0, +2 and +3.
Essential for mammals. 0.5 – 10 mg/L is toxic to
plants. Used widely in the metal industry (for plating
and in alloys e.g. stainless steel), in pressure creosoting
of wood, and for leather tanning. Normal oxidation
stages are 0, +3 and +6.
Essential in to plants. Normal oxidation stages are +2,
+4, +6 and +7.
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Table 2.
Some heavy metals and some of their properties. From John Jensen,(2000) and Ib
Johnsen (1988).
One significant characteristic of most heavy metals is their ability to form complexes with sulphur
and phosphate groups. These complexes are important for many functional groups in living
organisms e.g. enzymes and proteins in cell membranes. This ability to form complexes also make
the metals toxic in higher concentrations.
The metals are elements and can thus not be degraded to something else, only the oxidation stage
and the chemical species they take part in can vary. Once introduced to a soil environment the
heavy metals either remain present or are moved to a different environment.
Most heavy metals often form salts with low water solubility at neutral pH. Oxidation stage and pH
are the key factors determining solubility. Oxidation stage and complexation or bonding with
organic chemicals are the key parameters in the toxicity of the metal.
Plants and other living organisms take up minerals present in the root zone as part of their life and
may also take up heavy metals present in the soil environment. Heavy metals present in organic
material can be accumulated through the food chain as it may be difficult for the organism to
excrete the metal once taken up in an organism.
3. Organic chemicals
Organic substances are defined as chemicals composed of a carbon structure with ability to “burn”
– i.e. become oxidised – resulting in the formation of CO2, water and various other mineral
components.
Naturally occurring organic matter.
All living organisms are composed of organic material (fat, carbohydrates, proteins etc.), and such
organic material is naturally formed and decomposed in the soil environment.
The most significant environmental effect of naturally occurring organic matter is oxygen depletion
in receiving waters, if organic matter is discharged to e.g. a brook or a stream. Any organic matter
may be decomposed (oxidised) with help from micro organisms, and the preferred oxidizing agent
is molecular oxygen O2. Since oxygen is not very soluble in water it does not take much organic
pollution in a body of water to result in a depletion of the oxygen and thus the removal of a vital
condition for any higher life (insects or animals) in the water body.
Organic matter is therefore often quantified (measured) by the amount of oxygen consumed in a
decomposition process. BOD (biological oxygen demand) is a measure stating the amount of
oxygen used for decomposition of organic matter in a water sample by micro organisms over at
period of 5 days (at 20 oC).
If leachate from a landfill has a BOD contend of 500 mg/L and infiltrate and mix with oxygen rich
groundwater or surfacewater, one litre of leachate will use in the order of 500 mg O2 in a five day
decomposition process, equivalent to the oxygen in approximately 50 L of oxygen saturated water5.
5
At 10 oC water in equilibrium with the atmosphere can contain up to 11 mg O2/L.
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Hydrocarbons – CxHy
Hydrocarbons are in their basic form composed of carbon and hydrogen represented by the general
chemical formula CxHy. They are the basic component for most industrially produced chemicals and
occur naturally as components in mineral oil.
Hydrocarbons can be divided into a number of subgroups depending on the structure of the carbon
“skeleton”. In the group aliphatic hydrocarbons the carbon skeleton is chain formed with or without
sidechains. Octan (C8H18) and propen (CH2=CH-CH3) are examples of two aliphatic
hydrocarbnons.
In the group of alicyclic hydrocarbons the carbon “skeleton” is cyclic, and comprises one or several
ring structures, possibly with carbon side-chains. The aromatic hydrocarbons contain one or several
benzene rings. A benzene ring in it’s pure form is a ring of six carbon atoms with 6 hydrogen
atoms, see figure 9.
Figure 9
The aromatic hydrocarbon Benzene. The covalent bond between the six carbon atoms
consist of single and double bonds or more precisely 18 shared or dislocated
electrons, shown as the ring in the molecule structure.
Another factor that greatly diversifies the characteristics of hydrocarbons is substitution
(replacement) of one or several H atoms by other elements or groups of elements, so called
functional groups. Hydrogen can thus be substituted by elements from the 7th main group of the
Periodic Table of the Elements the so called halogens (comprising F, Cl, Br, I and At) increasing
the weight of the chemical, reduceing the ability to burn and often increasing the toxicity. In table 3
some important H substitutes and their main effect to the chemical characteristics are listed.
H substitute
(functional group)
Cl,
I,
Br
-OH
-O(Or: =C=O)
-OOH
-NH2
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Typical resulting physical/chemical characteristics of the organic
molecule as function of the functional group
The halogen (Cl. I, Br, At) increases the specific gravity of the molecule.
The vapour pressure and boiling point is reduced
The ability to burn is reduces (ignition temperature increased)
The biodegradability is reduced.
The toxicity is typically increased (especially for chlorine)
Water solubility is reduced
The hydroxide group increases the polarity of the molecule and thus the
water solubility
The oxygen atom increases the water solubility
The combination of the above two groups form the carboxyl acid group.
This dramatically increases the water solubility of the chemical and
makes it an (weak) acid.
The amino group increases the water solubility.
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-NO2
Table 3
Nitro group. Increased watersolubility
Hydrocarbon H-substitutes and some of their typical resulting characteristics
BTEX
BTEX is the acronym for the group of monoaromatic hydrocarbons constituted by the Benzene,
Toluene, Xylene and Ethylbenzene. The five most common BETX are shown in figure 10. The
BTEXes are characterised as light NAPL, and although thy have a low solubility in water (an Kow
above 100) it is not without significance. Benzene is the most water soluble of the BTEX. It has a
solubility in water of 1780 mg/L and a vapour pressure of 10 kPa. Gasoline has a BTEX content of
approximately 16 % (by weight). Diesel oil has a BTEX content of approximately 30 %.
Figure 10
The five most common BTEX compounds.
Phenols
Phenols consist of a benzene ring with at least one hydroxyl (-OH) group. The mother substance
phenol is shown in figure 11. The mother substance phenol and derivates hereof are widely used in
the chemical industry. They are toxic and several phenols are used as pesticides. The hydroxyl
group makes the phenols more polar and thus increases the solubility in water and decreases their
vapour pressure. The mother substance Phenol is a weak acid, has a water solubility of 84,000
mg/L, and a vapour pressure of 25 Pa (at 20 oC).
Figure 11
a) Phenol and b) 3-methyl-phenol
PAH’s
Polycyclic aromatic hydrocarbons (PAH) are hydrocarbons containing many benzene rings, see
figure 12. The PAH’s are found in tar products, the heavy part of mineral oil and a large number of
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natural or industrial produced chemicals belong to the group. The term aromatic refer to the
characteristic taste and odour by the substances. The black part of grilled meat contain lots of
PAH’s. PAH’s are relatively large and non polar molecules with high Kow values. Many PAH’s are
known as carcinogenic.
Figure 12
The structure of some of the most common PAH’s
4. References
Jensen, John: Tungmetaller i miljøet. In Kemiske stoffer i miljøet. A.Helweg (red.). Teknisk forlag,
2000.
Johnsen, Ib: Tungmetaller i jord. I Kemiske stoffer i landjordmiljøer. A.Helweg (red.). Teknisk
forlag, 1988.
Madsen, Torben and Finn Pedersen: Vandmiljøet og kemiske stoffer i vand. In Kemiske stoffer i
miljøet. A.Helweg (red.). Teknisk forlag, 2000.
Curtis (1986)
Kjeldsen, Peter: Organic Pollutants and their Phase distribution in soil and ground water. Lecture
note, DTU 1996.
Mygind, Helge. Kemi 2000, B-niveau. Haase, 2000.
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Enclosure 1
Electronegativity Table
H
2.1
Li Be
1.0 1.5
B C N O F
2.0 2.5 3.0 3.5 4.0
Na Mg
0.9 1.2
Al Si P S Cl
1.5 1.8 2.1 2.5 3.0
K Ca Sc
0.8 1.0 1.3
Ti V Cr Mn
1.5 1.6 1.6 1.5
Fe Co Ni Cu Zn Ga Ge As Se Br
1.8 1.9 1.9 1.9 1.6 1.6 1.8 2.0 2.4 2.8
Rb Sr Y
0.8 1.0 1.2
Zr Nb Mo Tc
1.4 1.6 1.8 1.9
Ru Rh Pd Ag Cd In Sn Sb Te I
2.2 2.2 2.2 1.9 1.7 1.7 1.8 1.9 2.1 2.5
Cs Ba La-Lu Hf Ta W Re
0.7 0.9 1.0-1.2 1.3 1.5 1.7 1.9
Os Ir Pt Au Hg Tl Pb Bi Po At
2.2 2.2 2.2 2.4 1.9 1.8 1.9 1.9 2.0 2.2
Fr Ra Ac
0.7 0.9 1.1
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Th Pa U Np-No
1.3 1.4 1.4 1.4-1.3
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