Download NO 2

Biogeochemical Cycles
• Objective
to understand the principles and importance of
biological systems in the cycling of key elements
and nutrients
• Reference
Butcher S.S. et al
Global Biogeochemical Cycles
Carbon Cycling
• The Major Carbon Reservoirs
CO2 in Air
CO2 in water
Soil Humics
Coal/Oil
Carbonate mineral
Marine Biota
Terrestrial Biosphere
750 Pg
38,000 Pg
1500 Pg
10,000 Pg
107 Pg
3 Pg
560 Pg
P = Peta = (1015)
• The Major Carbon Fluxes
Biological Assimilation
Biological Dissimilation
Fossil Fuel burning
170 Pg
170 Pg
5 Pg
Sulphur Cycle

Differences from N cycle
More valences
spontaneous reactions
Important Oxidation States
Form
Example
S2-
sulphur
proteins
sulphur
sulphite
sulphate
S0
SO32SO42-
Oxidation
State
-2
0
+4
+6
Assimilative
sulphate reduction
-2
H2S
-2
Waste
-2
Animal
protein
-2
-2
plant
protein
SO4
+6
Assimilative Sulphate reduction

Sulphate v. stable

Incorporated into protein

Many organisms in assimilative reduction

SO4
R-SH

Decay
H2S

Desulphurylation
Dissimilatory
Sulphur
Reduction
-2
H2S
intermediate
SO3
SO42-
2- +4
+6
Dissimilatory Sulphate Reduction
Sulphate stable in high pE environments
 If

–
–
–
no oxygen
energy source (eg, H2 or acetate)
Sulphate reducing bacteria (SRB)
then
SO42-
H2S
SO4 electron acceptor
 Competes with methanogens

Sulphate reduction and sulphide very
important

H2S is:
– toxic
 to bacteria, algae, workers (ppm)
– smelly
– reactive with metals
 gives FeS
– precursor for acid formation (when oxidised by
Thiobacillus)
– cause of steel and concrete corrosion
H2S Equilibria and pH

HS- + H+
H2S(aq)
pKa = 7.1
S2- + 2 H+
pKa = 14

H2S also a gas

Only H2S toxic/smelly, others dissolved

pH >8 little H2S

pH <7 mostly H2S
Sulphur
Oxidation
-2
H2S
intermediate
+4
-2
S
0
SO 3
-2
SO4
+6
Sulphur oxidation
H2S
S , spontaneous in O2
– Not fast (only HS- reacts)
 H S and S electron donors
2
 Variety of organisms
– Sulphur Bacteria (Beggiatoa; Thiobaccillus,
Thiothrix)
 require oxygen
– Photosynthetic (Green and Purple sulphur bacteria)
 anaerobic
 Waste Stabilization Ponds (WSP)

Sewer Corrosion
Bacteria in Water Droplets (H2S + O2
H2S
AIR
H2S
Sewage
SO42-
S2-
S2- + H+
H2S
H2SO4 )
Nitrogen cycle

Less spontaneous reactions than S
Form
Species
NH3
ammonia
proteins
N2
NO
NO2NO3-
Nitric oxide
Nitrite
Nitrate
Oxidation
State
-3
0
+2
+3
+5
Nitrogen fixation
0
N2
Plant
Protein
-3
Nitrogen fixation
Energy intensive
 Reduction of nitrogen
 Major path biological

–
–

bacteria
 free-living (Azospirillum)
 and symbiont (Rhizobium)
Blue green algae
Also lightening and feritilizers
Nitrate Reduction and
Ammonification
-3
NH3
-3
Dead
Organic
Matter
Animal
Proteins
-3
-3
Plant &
Bacterial
Proteins
NO3
-
+5
Organic Nitrogen Transformations

Assimilative Nitrate
reduction
N
incorporated into
organic nitrogen.
 minor pathway
 Bacteria, fungi, algae

Ammonification
 Organic
nitrogen into
ammonium
 ammonium taken up
by plants
Organic nitrogen measured as Kjeldahl-N (TKN)
Organic N is digested by H2SO4 to release N as
ammonia.
Ammonia then measured (distillation)
Nitrification
-3
NH3
+3
NO2-
NO3
-
+5
Nitrification


Two Stage Process
–
1 Ammonia oxidation (Nitrosomonas and Nitrosospira)
2NH3 + 3O2
2NO2- + 2H+ + 2H2O
–
2 Nitrite oxidation (Nitrobacter)
2NO2- + O2
2NO3-
Measurement of ammonia
–
–
Nesslers method
 Use Nesslers reagent to get a brown colour
Borate method
NH3 + H3BO3
NH4+ + H2BO3H2BO3- + H+
H3BO3
Denitrification
+3
0
Low O2
N2
Nitrite Reduction
NO2-
NO3
-
+5
Denitrification

Dissimilatory Reduction of NO3- and NO2- to N2
– Variety of bacteria
– Occurs in absence of oxygen
– Small amounts of NO and N2O also formed
Nitrate and Nitrite measured
– ion chromatography (Dionex) - best
 Nitrite
– Indirect Colorimetric method - tedious
 Nitrate
– UV absorbtion - interfering species
– Additional reduction methods
– ion specific electrodes
