Transport of contaminants

choppedspleenMechanics

Feb 21, 2014 (3 years and 10 months ago)

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Ecotoxicology

Transport of contaminants

The transport of contaminants in the atmosphere takes place:

Globally

Large
-
scale:

> 1000 km

Meso
-
scale:

10
-

1000 km

Micro
-
scale:

< 10 km

The dispersal of chemicals is a function

of their atmospheric lifetime

Boundary layer →

tropopause:

> 5 days

Whole tropospheric hemisphere
:


> 1 month

Global troposphere
:


> 2 years

Troposphere




Stratosphere
:


> 10 years

Examples

NO

2


(


~ 1 day)

-

only small exchange to the free tropopause

Aerosol from Chernobyl (


~ 1 month)

-

spreading to the whole Northern hemisphere

Metan (

~ 4 years)

-

mixing between both hemispheres

Chlorfluorocarbon (CFC) (

~ 100 years)

-

transport to the stratosphere

The global transport

Equator

South

North

Hadley

circulation

Typical forms and sizes of trace metals in aquatic systems

Metal

species

Free

metal

ions

Inorganic ion

pairs


Inorg. complexes


Low mol. Wt. or
-

ganic complexes

High molecular

org. complexes

Metals
adsorbed

onto inorganic
colloids

Metals adsorbed

Into living cells


Metals adsorbed

Onto or incorpora
-

ted into mineral

soils and precipita
-

tes

Ex.

Mn
2+


Cd
2+

NiCl
+


HgCl
4
2
-

Pb
-
humates

Co
-
MnO
2


Pb
-
Fe(OH)
3

Cu
-
clays

PbCO
3(5)

Soluble Colloidal Particulate

Size
----------------

1 nm
------------------
10 nm
----------------------
100 nm
----------------
1000 nm
----------

15

12

9

6

3

50

100

150

200

250

Concentration of Cr

µ
g / l

Average flow (m
3

s
-
1
)

Dissolved

Suspended

Nitrate

Phosphor

Concentration of chromium in the River Thames as a function of flow rate

Thermic boundary layer

Wind + Coriolis force

Atmosphere
-
land surface exchange

Dry deposition:

particles and gases

Wet deposition:

Rainfall and fog

Flux to surface = Vg
• atmospheric concentration at 1 m



Vg = deposition velocity

Vg

=

=

c

b

a

r

r

r

+

+

=

=

Vg

1

R



-

r
a

: aerodynamic resistance to within 1
-
2 mm of the surface

-

r
b

: the resistance of the individual roughness elements in


the boundary layer (ca 1 mm)

-

r
c

: the resistance of the surface itself to take
-
up of the gas

Flux to surface

Atmospheric concentration at 1 m

Resistance to deposition

Deposition velocity of particles depends on their size

0.001
-
0.1
µm

: Vg high (high diffusivity


like gasses)

0.01
-
1.0 µm

: Vg small (atmospheric lifetime: 7
-
30 days)

1.0
-
100 µm

: Vg high (heavy)

Washout from the atmosphere

=

W(drops) = 200
-
1000

Air:

microgram/m
3

(1 m
3

air weighs 1,2 kg)

Rain:

mg/l

Washout factor
W

Concentration in rain (mg/kg)

Concentration in in air (mg/kg)

The flux between air and sea

Henry’s law:
H=K
·α·P

H
= solubility of gas

K
= constant

α
= the gas’ coefficient of absorption

P
= partial pressure of the gas

C
air

∙ H
-
1

= C
water


: equilibrium (H
-
1

Henry’s law constant)

C
air

∙ H
-
1

> C
water


: net transport air

=
w慴敲
=
C
air

∙ H
-
1

< C
water


: net transport water

=
慩a
=
䥦I
=
=
=
ΔC = C
air

∙ H
-
1



Cwater


The transport of material is:
F = K
(T)W

· ΔC
(K
(T)W

: ”transfer velocity”)

a

W

W

T

Hk

k

K

1

1

1

)

(

+

=

a

where
k
w

and
k
a

are the individual transfer velocities for gasses in water and air

Air


sea transfer velocity for CO
2

Points:

Experimental data


Line:

Calculated

Water


sediment exchange

Dissolved

Suspended

Sediment

Water
-

suspended

K

d

=

Water


sediment

-
less efficient due to limited extent of contact


Suspended


sediment (sedimentation)

most important for the removal of contaminants from the water

Sedimentation rates are expressed as the rate of bottom increase

-
oceans


: mm / 1000 years

-
lakes, estuaries

: cm / year

Distribution coefficient

Concentration in solid phase material (mg/kg)

Concentration in dissolved phase (mg/kg)

Sedimentation

Exchange of

pore water

Physical mixing

of sediment

Upward flow of pore
water

Hydrostatic pressure

Vand


sediment fluxe

)

(

0

S

S

z

C

U

UC

dz

dC

D

P

F

flux

Net

+

+

-

=

=

P

: Porosity (volume fraction occupied by water)

D

: Diffusion coefficient

C

: Concentration in solution

C
S

: concentration in solids

U

: rate of pore water advection

U
S

: Sedimentation rate

z

: Vertical distance

Redox
-
active compounds

Example: Ion and Manganese

Fe
2
O
3

MnO
2

Fe
+++


=

++

Mn
++++


=

++

Oxic

Anoxic

Bound to

sediment

Dissolved in

pore water

Oxic

zone

Anoxic

zone

Atmospheric CO
2
-
concentration

measured in Hawaii (1950
-
1988)

Lead concentration in

ice core from Greenland