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ATMOSPHERE - WATER - SOIL
INTERACTIONS
1. Reactions and partitioning at gas-water interface; Henrys Law
2. Environmental implications
3. A few example problems4. Soil Water Interaction & Alkalinity
CONTENTS:
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Hydrologic
Cycle
Hydrogeochemical
cycles couple
1. Atmosphere,
2. Water3. Land
Atmosphere is an important conveyor belt for many pollutants.
Example: China is a major source of pollution in Japan, Canada is affected byemission from the Northeast USA
Water a minor
component of
the
atmosphere.
Also, only
0.001% of the
total water in
the world is in
cloud or in
atmosphere).
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Mercury Transport
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Up the Food Chain
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O2
Atmosphere (Oxygen)-Surface Water Interactions: Mechanism behind
the Sustenance of Aquatic Life
O2 is central to the sustenance of life
of aerobic life-forms.
Similarly, aquatic ecosystem
depends on the oxygen dissolved
in the water
Oxygen enters the surface water andoceans in two different pathways:
a) Diffusion from atmosphere to water
b) Through photosynthesis by algae in
presence of sunlight.
At any time in steady state
condition, there is an equilibrium
partitioning of oxygen between the
water and the atmosphere.
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CO2
NH3 NO2
DustSO2
Aerosols Organic
acids
RAINWATER DROPLET IN THE AIR
As the droplet passesthrough the atmosphere to
fall on the earth surface,
various gases, liquid and
solids that are either part of
the atmosphere or aresuspended in air, tend to
get inside the water droplet.
Many of the species establish a quick equilibrium, thus imbibing
impurities even before the rain water reaches the earth surface.
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Gas-Water Partitioning: Henrys Law
The Statement: The amount of volatile species (gas) dissolved in a
solution is directly proportional to the partial pressure of the gas
above the solution.
Aaq pA ][For any species A,
[A]aq
pA
AHaq pKA ][
.
:unit][
atm
M
p
AK
A
aq
H
Sealed vessel: liquidand gas phase.
KHis called the Henrys law constant
Gas molecules strike the water surface,
some becomes dissolved. Some of the
dissolved molecules also strike the surface,
and escape in the gas phase.
At dynamic equilibrium, the rate of
dissolution into water equals the rate of
escape into gas phase.
The equilibrium situation is described byHenrys law
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Partial Pressure and Molar Concentration
Ideal Gas
Law:
R= 0.0820578 L.atm/(K.mol)
Molar concentration: n PCV RT
pV nRT
Daltons Law
Partial pressure of each gas in a gas mixture, such as the
atmosphere, is the portion of the total pressure that a particular gas
would exert
Ptot = pA + pB + pC+
= (nA + nB + nC+ )RT/V
= (ntot)RT/V
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The atmosphere:
(by volume)
N2 78%
O2 21%Ar 0.9345%
CO2 0.0314%
Other gases Rest
Partial pressure of O2 = pO2 = 0.21*Ptot=0.21*1 atm =0.21
atm
Partial pressure of CO2
= pCO2
= 0.000314*1 atm =10-3.5 atm
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Example 1.
Partial pressure of oxygen in the atmosphere = 0.21 atm.
Aqueous concentration of oxygen = KH*pO2= 1.26 X10-3 M/atm. * 0.21 atm.
=0.0002646 M
= 8.467 mg/L
Find out the aqueous solubility of oxygen in a fully aerated water
at 25 deg C in a treatment plant. KH of O2 at 25 deg C = 1.26X10
-3
M/atm
[1M O2= 32 g/L]
Is KHor Henrys law constant is really a constant?
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The value of Henrys Law constant changes inversely with
temperature.
Implication: In the summer, the rivers and lakes shall have less
dissolved oxygen as compared to the dissolved oxygen
concentration in winter
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H++HCO3-
H++CO32-
CO2
CO2(aq.)
Carbon dioxide -Water Interaction: Natural Acidity
Unlike dissolution of oxygen, gases like carbon dioxide,
sulfur oxides or nitrogen oxides after dissolution form further
hydrated species which add acidity to the water.
Carbon dioxide is a natural constituent of
atmosphere. The acidity imposed by carbon
dioxide is referred as natural acidity.
When carbon dioxide dissolves in water it
forms aqueous carbon dioxide which is
popularly known as carbonic acid (H2CO3).
Depending upon the solution pH, the species
further dissociates into hydrogen ion and
bicarbonate ion.
Bicarbonate ion can get further dissociated into
hydrogen ion and carbonate ions.
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Carbon dioxide : Natural Acidity (contd)
22
5.1
2 10][ COCOHaq ppKCO
.)( 32
HCOHaqCO
2
33 COHHCO
3.6
.)(2
31 10
][
]][[
aq
aCO
HCOHK
3.10
3
23
2 10][
]][[
HCO
COHKa
According to Henrys Law:
.)()( 22 aqCOgCO KH =10
-1.5 M/atm.
Ka1 =10-6.3
][
10
][
*10
][
][][ 22
8.73.6.)(21
3
H
p
H
pK
H
COKHCO
COCOHaqa
2
1.18
2
8.73.10
322
3
][
10
][
10*10
][
][][ 22
H
p
H
p
H
HCOKCO
COCOa
Ka2 =10-10.3
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Carbon dioxide : Natural Acidity (contd)
Imposing Electro neutrality condition
][][2][][ 233
OHCOHCOH
][
10
][
10
*2][
10
][
14
2
1.188.7
22
HH
p
H
p
H
COCO
In atmosphere, there is 370 ppm of carbon dioxide on mole
basis. atm.10*370 62
COp
][
10
][
10*2][
10][14
2
5.212.11
HHHH
Comparing 1st
and 3rd
term on the right hand side, 3rd
term is atleast 600 times smaller than 1st term. So, it can be neglected
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2
5.212.11
][
10*2
][
10][
HHH
For trial, if the pH is 6 or [H+]=10-6, then left hand side and first term in the
right hand side are dimensionally similar, whereas the last term is about 2000
times smaller than the other two terms. Hence, the last term can be
neglected.
][
10][
2.11
H
H2.112 10][ H
6.510][ H
6.5]log[ HpH
In general, for rainwater containing only CO2 approximately,
2*10*10][ 5.13.62 COpH
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For a can of carbonated drink,
pCO2= 5 atm.
5*10*10][ 5.13.6 H
56.3]log[ HpH
pH?
pCO2= 2*370*10-6 atm. =10-3.13
13.35.13.6 10*10*10][ H
47.5]log[ HpH
Find out the pH if the atmospheric CO2 concentration is
doubled.
This means that rise in atmospheric CO2 concentration does
not change the acidity of rain water by a significant amount
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If CO2is the only species, that affects the rainwaters
acidity, its natural pH is 5.6
When additional acidic species arepresent at appreciable levels due to man-
made activities, pH of rainwater becomes
lower than 5.7ACID RAIN Major contributors to acid rain : H2SO3,
H2SO4 and HNO3
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Find out the pH of acid rain when the atmosphere has 5 ppb of
SO2 along with 370 ppm of CO2 as discussed earlier. KH for
SO2 is 100.096 M/atm , Ka1=10
-1.77 ; Ka2=10-7.21
22
096.0
2 10][ SOSOHaq ppKSO According to Henrys Law:
.)()( 22 aqSOgSO KH =100.096
M/atm.
.)( 32
HSOHaqSO
77.1
.)(2
31 10
][
]][[
aq
aSO
HSOHK
Ka1 =10-1.77
][
10
][
*10
][
][
][
22
674.177.1
.)(21
3
H
p
H
pK
H
SOK
HSO
SOSOHaqa
pSO2 = 5*10-9 atm.
][
10
][
10][
97.9674.1
32
HH
pHSO
SO
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2
33 SOHHSO21.7
210 aK
21.7
3
2
3
2 10][
]][[
HSO
SOH
Ka
2
18.17
2
97.921.7
322
3][
10
][
10*10
][
][][
HHH
HSOKSO a
][][2][2][][][ 232
333
OHSOCOHSOHCOH
Imposing electro-neutrality condition,
][10
][10*2
][10*2
][10
][10][
14
2
18.17
2
5.2197.92.11
HHHHHH
With an initial guess of pH =5, third, fourth and fifth terms on the
RHS are found to be negligible compared to the other terms in the
equation.
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Therefore, considering only significant terms, the previous equation
can be approximated as,
][10
][10][
97.92.11
HHH
96.997.92.112 101010][ H
98.410][ H
98.4]log[ HpH
We observe that the presence of trace amount SO2 can
significantly alter the pH of the rainwater, the acid causing
potential being more than carbon dioxide. NOx have the same
effect as SOx for imparting acidity to rainwater.
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pH60 years.
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CASE STUDY: LAKE NYOS DISASTER
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CASE STUDY: LAKE NYOS DISASTER
LAKE NYOS, CAMEROON
On August 21, 1986, more than 1700 people and numerous wild life were
killed by a silent and mysterious killer, CO2 gas
It is a lake in the volcano crater, 1.2 miles X 0.75
miles in area, but 682 feet deep. Under the bed,
the volcano is leaking carbon dioxide into the
water. This changes the water into carbonic acid.
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Volcanic gases containing CO2 from the underlying
magma seeps into the bottom of lake
High partial pressure of CO2, High
conc. of bicarbonate and low pH
About 230
m deep,
Pressure is
about 25
atm
Due to such a high partial pressure of carbon dioxide, the pH was
substantially low, HCO3- concentration was pretty high.
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Carbon dioxide gas is heavier than atmosphere, unless there is a strong
dispersion forces such as high wind, it tend to sit at the bottom of the
atmosphere, causing asphyxiation http://www.geogonline.org.uk/g1_nyos.htm
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REMEDIATION EFFORTS
The method is simple, consisting of a vertical
pipe between the lake bottom and the
surface. A small pump raises the water in the
pipe up to a level where it starts releasing
the gas from the diphasic fluid. At this point
there is a pressure gradient which causes the
water to rise to the surface and erupt like a
fountain. Therefore, once it has primed thegas lift, the pump is not needed, and the
process becomes self-powered. Isothermal
expansion of gas bubbles drives the flow of
the gas-liquid mixture as long as dissolved
gas is available for ex-solution and expansion.
http://mhalb.pagesperso-
orange.fr/nyos/2006/index2006.htm
http://www.geo.arizona.edu/geo5xx/geos577/
projects/kayzar/html/lake_nyos_disaster.html
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http://www.okeanosgroup.com/blog/aquatic-architecture-2/how-to-transform-a-poisonous-explosive-lake-into-electricity/
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Other Applications/ Phenomena Linked with GasWater Interaction
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SOIL-WATER INTERACTION
CO2 + H2O H+
+ HCO3-
Subsoil
Limestone CaCO3(s) + H+ Ca2++HCO3-MgCO3(s) + H
+ Mg
2++HCO3-
Precipitation
Topsoil
Result: Mobilization of different ions in the groundwater from the
minerals constituting the topsoil and subsoil
EXAMPLE:
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EXAMPLE:
Rainwater falls on a soil surface and is under open atmosphere in
contact with soil containing abundant amount of limestone (calcium
carbonate). What will be the pH in this case? Ksp of calcium
carbonate is 10-8.42.There is abundant quantity of calcium carbonate and it is sparingly soluble
in water. So, the aqueous system is always in equilibrium with solid calcium
carbonate. It is also in equilibrium with the air.
The situation is :CO2
CO2 (aq.)
H+
+HCO3-
Ca2++CO32-
H++CO32-
CaCO3
So, the relevant reactions are:
.)( 32
HCOHaqCO
233 COHHCO
.)()( 22 aqCOgCO
Also,
2
3
2
3 COCaCaCO
Ka1 =10-6.3
Ka2 =10-10.3
KH =10-1.5 M/atm.
KSP =10-8.42
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The electro-neutrality condition is now different from before. We have a
divalent calcium ion.
][2][][][][2 2332
COHCOOHHCa
2
1.18
2
8.73.10
322
3][
10
][
10*10
][
][][ 22
H
p
H
p
H
HCOKCO
COCOa
From CO2 equilibrium,
From solubility of calcium carbonate,
222
268.9
1.18
242.8
1.18
2
2
3
2 ][10
10
][10
10
][
][][
COCOCO
spsp
p
H
p
H
p
HK
CO
KCa
2
1.188.714268.9
][
10*2
][
10
][
10][
][10*2 22
2
H
p
H
p
HH
p
H COCO
CO
Hence,
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2
61.1868.714
6
268.9
][
10*370*10*2][
10*370*10
][
10][10*370
][10*2
HHHH
H
pCO2= pressure exerted by 370 ppm (by volume) gas= 370 * 10-6
atm
2
23.2123.1114241.13
][
10
][
10
][
10][][10
HHHHH
For a test case, lets consider that [H+]=10-7
23.723.47759.0 1010101010
This means that only first term and second term in the equation issignificant for obtaining an approximate solution.
][
10][10
23.11241.13
HH
64.243 10][ H
21.810][ H pH= 8.21
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2
268.92 ][10][
COp
HCa
21.810][ H
atmpCO610*360
2
MCa 46
221.868.9
2 10*05.510*360
]10[10][
LmgCa /20][ 2
This eventually means that the rainwater which has a pH of 5.6, when
comes in contact with a limestone deposit on the surface, the whole
chemistry changes because of the dissolution of limestone into the water.
The water turns alkaline with pH of 8.2 and with calcium being dissolved at
a concentration of about 20 mg/L.
Soil contains multitude of different minerals. The rainwater when comes
in contact with soil , dissolves many metal cations , also anions, into the
water and the pH also rises so that the natural waters, surface as well asunderground, normally have a pH in the envelope of 6.5 to 8.5. Also, the
surface- and groundwater contains minerals many of which are
physiologically significant.
ALKALINITY
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ALKALINITY
Alkalinity is a measure of the acid buffering capacity of water. In other
words, it measures the waters capability to consume hydrogen ion
without making any change in the pH of the system.
What happens then to H+ ions added to the system?
The H+ ions would react with the components in water mainly, HCO3-,
CO32- and OH- according to the following reactions and would get
consumed and will be unavailable for lowering the pH.
.)(23 aqCOHCOH
3
2
3 HCOCOH OOHH 2H
This means that presence of these ions provide some buffer capacity for the
system, so that the pH does not change even if acid is added to the system.
The summation of all these H+ buffering ions is called acid buffering ability or
alkalinity .
][][][2][)( 233
HOHCOHCOMAlkalinity
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EXAMPLE
A sample of water at pH 10 has 32 mg/L of CO32-. Find the alkalinity of
the water in the following units: M as well as mg/L as CaCO3.
SOLUTION Estimate all the acid buffering ions.
pH =10 1010][ H pOH = 4 410][ OH
2
33 COHHCO
3.10
3
2
32 10
][
]][[
HCO
COHKa
Ka2 =10-10.3
97.2
3.10
27.310
2
2
33 10
10
10*10]][[][
aK
COHHCO
32 mg/L of CO32- MM
moleg
Lg 27.333
1010*53.0
/60
/10*32
10427.397.2
2
33
101010*210
][][][2][)(
HOHCOHCOMAlkalinity
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M
HOHCOHCOMAlkalinity
3-10427.397.2
2
33
10*2.246M101010*210
][][][2][)(
1 eq/L CaCO3 = 50 g/L of CaCO3
Alkalinity = 1.71*10-3*50 g/L =85.5 mg/L as CaCO3
Leq
HOHCOHCOLmeqAlkalinity
/10*71.1eq/L10101010
][][][][)/(
3-10427.397.2
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