2. air water soil interaction 2013

7/30/2019 2. Air Water Soil Interaction 2013

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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).


3/39

Mercury Transport


4/39

Up the Food Chain


5/39

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.


6/39

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


8/39

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


9/39

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


10/39

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


12/39

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


14/39

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


15/39

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


16/39

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


17/39

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


18/39


19/39


20/39

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


21/39

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.


22/39

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.


23/39

pH60 years.


24/39


25/39

CASE STUDY: LAKE NYOS DISASTER


26/39

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.


27/39

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.


28/39

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


29/39

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


30/39

http://www.okeanosgroup.com/blog/aquatic-architecture-2/how-to-transform-a-poisonous-explosive-lake-into-electricity/


31/39

Other Applications/ Phenomena Linked with GasWater Interaction


32/39

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:


33/39

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


34/39

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,


35/39

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


37/39

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


38/39

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


39/39

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

2

33

2. air water soil interaction 2013

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