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N2
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Cosmic abundance of the elements
Mass number
-2
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70 80 90 100
log
ab
on
da
nce
(ato
me
s / 1
06 Si)
H
He
Li
Be
B
Fe
Ca
Sc
CO
UTh
PbPt
Ni
Tc Pm
N
NeSiS
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s2 d10 p6
n 1
18
1 1.0 1
H 1
2 13 14 15 16 17 4.00 2
He 0
2 6.94 3
Li 1
9.01 4
Be 2
10.8 5
B 6
12.0 6
C –4 –2 0
4
14.0 7
N –3 0 3 5
16.0 8
O –2 0
19.0 9
F –1
20.2 10
Ne 0
3 23.0 11
Na 1
24.3 12
Mg 2
3 4 5 6 7 8 9 10 11 12 27.0 13
Al 3
28.1 14
Si 4
30.1 15
P –3 5
32.1 16
S –2 0 4 6
35.5 17
Cl –1
40.0 18
Ar 0
4 39.1 19
K 1
40.1 20
Ca 2
45.0 21
Sc 3
47.9 22
Ti 4
50.9 23
V 5
52.0 24
Cr 3
54.9 25
Mn 4 3 2
55.8 26
Fe 2 3
58.9 27
Co 2 3
58.7 28
Ni 2
63.5 29
Cu 1 2
65.4 30
Zn 2
69.7 31
Ga 3
72.6 32
Ge 4
74.9 33
As 3,5
79.0 34
Se –2 0 4 6
79.9 35
Br –1
83.8 36
Kr 0
5 85.5 37
Rb 1
87.6 38
Sr 2
88.9 39
Y 3
91.2 40
Zr 4
92.9 41
Nb 3 5
95.9 42
Mo 4 6
98 43
Tc 7
101 44
Ru 3 4
103 45
Rh 2 3 4
106 46
Pd 2 4
108 47
Ag 1
112 48
Cd 2
115 49
In 3
119 50
Sn 4,2
122 51
Sb 3,5
128 52
Te –2 0 4 6
127 53
I –1
131 54
Xe 0
6 133 55
Cs 1
137 56
Ba 2
139 57
La 3
178 72
Hf 4
181 73
Ta 5
184 74
W 4 6
186 75
Re 7
190 76
Os 3 4
192 77
Ir 2 4 6
195 78
Pt 2 4
197 79
Au 1 3
200 80
Hg 2
204 81
Ti 1 3
207 82
Pb 2
209 83
Bi 3 5
209 84
Po 2 4
210 85
At –1
222 86
Rn 0
7 223 87
Fr 1
226 88
Ra 2
227 89
Ac 3
f14 d 140 58
Ce 3
141 59
Pr 3
141 59
Nd 3
145 61
Pm 3
150 62
Sm 3
152 63
Eu 3
157 64
Gd 3
159 65
Tb 3
163 66
Dy 3
165 67
Ho 3
167 68
Er 3
169 69
Tm 3
173 70
Yb 3
175 71
Lu 3
232 90
Th 4
231 91
Pa 5
238 92
U 4 6
237 93
Np 4 5
244 94
Pu 4
243 95
Am 3
247 96
Cm 3
247 97
Bk
3 4
251 98
Cf 3
252 99
Es
257 100
Fm
258 101
Md
259 102
No
260 103
Lr
14.0
N-3,0,3,5
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Nitrogen species
nitrate NO3– N+V stable oxide of N, highly soluble as an anion
nitrite NO2– N+III intermediate between NO3
– and NH4+
nitrous oxide N2O N+I from lightning and internal combustion engines
nitrogen N2 N0 elemental nitrogen gas
hydroxylamine NH2OH N–I intermediate species during oxidation of NH4+
ammonia NH3 N–III un-ionized ammonia gas
ammonium NH4+ N–III ionized ammonia (dominates below pH 9.23)
urea CO(NH2)2 N–III common fertilizer
amino N R–NH2 N–III organic nitrogen as amine, measured as TKN
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-10
-5
0
5
10
15
5 6 7 8 9pH
pe
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
O2/H2O
H2O/H2
SO42–/HS–
NO3–/NO2
–
NO3–/NH4
Eh
(V)
NO3–/N2
NO2–/N2
Fe(OH)3/Fe2+
CO2/CH4
Nitrogen redox
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C106H263O110N16P(S)
Elemental composition of algae
Protein
Essential nutrient
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Fixing nitrogen for photosynthesisC106H263O110N16P(S)
:NN:
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Natural N2 fixation by Rhizobia and
the nitrogenase enzyme(Fe and Fe-Mo proteins)
N2 + 8H+ + 8e− + 16ATP → 2NH3 + H2 + 16ADP + 16 P
adenosine triphosphate (ATP) adenosine diphosphate (ATP)
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And organic N scavenged frombiodegradation
R–NH2 + 2H + NH3
NH3 + O2 NO3–
It’s all about getting NH3 and NO3–.
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NH3
Human intervention into the nitrogen cycle
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Urea from animal waste, urine
CO(NH2)2 + H2O 2NH3 + CO2
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Guano from Chile and the Pacific islands
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NO3–
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Nitroglycerine
4 C3H5(ONO2)3 12 CO2(g) + 10 H2O(g) + 6 N2(g) + O2(g)
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Saltpetre KNO3 from India
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Haber-Bosch, 1908-10
½N2(g) + 3/2H2(g) ⇌ 2NH3(g)
CH4(g) + 2H2O(g) → CO2 (g) + 4H2(g)
N2 and H2 are reacted over a ferric iron catalyst with Al2O3 & K2O at 250 atm and 450-500°C.
ΔG = –16.5 kJ/mol
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Essential nitrogen reactionsDegradation of organic N: –NH2 + H+ NH3
Organically-bound nitrogen is a component of all proteins and plant biomass. Aerobic and anaerobic degradation of such carbon compounds releases this reduced nitrogen in the form of ammonia. Where this occurs in unsaturated materials such as soils or manure, the ammonia can volatilize, or dissolve into water.
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Essential nitrogen reactionsDegradation of organic N: –NH2 + H+ NH3
Organically-bound nitrogen is a component of all proteins and plant biomass. Aerobic and anaerobic degradation of such carbon compounds releases this reduced nitrogen in the form of ammonia. Where this occurs in unsaturated materials such as soils or manure, the ammonia can volatilize, or dissolve into water.
Decomposition of urea: CO(NH2)2 + H2O 2NH3 + CO2
Urea is a common form of organic nitrogen that is produced naturally in animals and industrially. It is often applied as fertilizer in granulated form, and breaks down by a bacterially mediated reaction (urease enzyme) to release ammonia for plants.
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Ionization of ammonia: NH3 + H+ NH4+ KT = 10–9.23
• high solubility of ammonia in water at neutral pH
• at high pH NH3 represents a large fraction of the total ammonia
• NH3 = NH4+ at pH 9.23.
Ammonia transformations
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Ionization of ammonia: NH3 + H+ NH4+ KT = 10–9.23
• high solubility of ammonia in water at neutral pH
• at high pH NH3 represents a large fraction of the total ammonia
• NH3 = NH4+ at pH 9.23.
Volatilization of ammonia: NH3(aq) NH3(g) KH = 101.76
• high Henry’s Law constant for ammonia
• un-ionized NH3 = 246 mg/L at 25˚C for a NH3 partial pressure of one atmosphere
• volatilization from manure, soils and surface waters
• loss from groundwater below the water table is minimal
Ammonia transformations
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Ionization of ammonia: NH3 + H+ NH4+ KT = 10–9.23
• high solubility of ammonia in water at neutral pH
• at high pH NH3 represents a large fraction of the total ammonia
• NH3 = NH4+ at pH 9.23.
Volatilization of ammonia: NH3(aq) NH3(g) KH = 101.76
• high Henry’s Law constant for ammonia
• un-ionized NH3 = 246 mg/L at 25˚C for a NH3 partial pressure of one atmosphere
• volatilization from manure, soils and surface waters
• loss from groundwater below the water table is minimal
Sorption of ammonium: Na–clay + NH4+ NH4–clay + Na+
• cation exchange of ammonium onto clay minerals in soils and aquifers
• erosion of NH4-bearing soils is a major sources of contamination in surface waters
• selectivity coefficient for ammonium varies with the clays and competing cations
• transport of NH4+ in groundwater is retarded.
Ammonia transformations
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Aerobic nitrification of ammonium: NH4+ + 2O2 NO3
– + H2O + 2H+
G°r = –266.5 kJ/mol
• NH4+ can be oxidized to NO3
–by reaction with elemental oxygen (O2)
• significant energy yield is favorable for bacteria
• two step reaction of oxidation to nitrite by a Nitrosomonas, Nitrobacter and Nitrosospira, and oxidation of nitrite to nitrate by Nitrobacter Pseudomonas.
• reaction is restricted to aerobic environments – manure piles, soils and surface waters.
Ammonia oxidation
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Aerobic nitrification of ammonium: NH4+ + 2O2 NO3
– + H2O + 2H+
G°r = –266.5 kJ/mol
• NH4+ can be oxidized to NO3
–by reaction with elemental oxygen (O2)
• significant energy yield is favorable for bacteria
• two step reaction of oxidation to nitrite by a Nitrosomonas, Nitrobacter and Nitrosospira, and oxidation of nitrite to nitrate by Nitrobacter Pseudomonas.
• reaction is restricted to aerobic environments – manure piles, soils and surface waters.
Anaerobic nitrification of ammonium – anammox: 3 NO3– + 5 NH4+ ® 4 N2 + 9 H2O + 2H+ G°r = –282.30 kJ/mol-NH4
+
= –470.50 kJ mol-NO3–
• Recently discovered (1995) less well-known reaction
• thermodynamically very favorable for bacteria
• anaerobic environments with both ammonium and nitrate species are present, such as in waste-water streams, anoxic marine waters and soils.
• NH4+ as an electron donor, with NO3
–, and NO2–, as an electron acceptors, producing N2.
• only known biologically-mediated reaction for conversion of NH4+ to N2.
Ammonia oxidation
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Denitrification: 5CH2O + 4NO3– + 4H+ 2N2 + 5CO2 + 7H2O
G°r = –252.47 kJ/mol
• anaerobic reaction - O2 – free conditions required
• low-pe electron donor such as carbon or sulphide
• nitrate is an electron acceptor with nearly the same energy yield as O2
• Pseudomonas denitrificans reduces NO3– to N2 using fixed carbon (biomass)
• Denitrification can also be mediated by chemotrophs such as Thiobacillus denitrificans, which uses sulfide (H2S or pyrite) as a substrate.
• N2 from denitrification becomes overpressured in water as dissolved nitrogen gas
• anaerobic waters with low nitrate concentrations (NO3– limited), denitrification to
N2 gas may not be complete, resulting in the production of N2O gas.
Nitrate reduction back to N2
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Nitrate Cycle
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UREA HYDROLISIS AND VOLATILIZATION OF AMMONIA
NH3 (gas) CO (NH2) NH3 NH4
+ NO3-
15NH3 + 14NH4+ (aq) 14NH3 (gas) + 15NH4
+ (aq)
Isotope fractionation factor = 1.034
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DENITRIFICATION
4NO3- + 5CH2O + 4H+ 2N2 + 5CO2 + H2O
14NO3- + 5FeS2 +14H+ 7N2 + 10SO4
- + 5Fe+2 + H2O
15NNO3-N2 ~ 15 to 20 permil
18ONO3-H2O ~ 8 permil
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Isotope Data in Nitrate of Different Origins
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Groundwater flow systemRef: Wassenaar, L. 1995. Applied Geochem. 10:391-405
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Nitrate distribution in mg/L as NO3-
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Septic system plume based on Na concentration
From Aravena, R., Evans, M.L., and Cherry, J.A. 1993. Ground Water, 31: 180-186
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Nitrate Concentration in mg/L as NO3-
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15N data (‰) in Nitrate
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Nitrate distribution in mg/L as N
Ref:Aravena, R and Robertson, W. 1998. Ground Water, 36: 975-982
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Oxygen (o) and DOC () concentration profiles
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15N (o) and nitrate () concentration profiles
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Isotope enrichment trend showing denitrification
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Chemical and Isotope Depth Profiles
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WHY RIPARIAN ZONES ARE IMPORTANT
• Nitrate is a major groundwater pollutant in agricultural landscapes
• Riparian zones act as a buffer zones to attenuate nitrate associated to contaminated groundwater discharging in rivers and lakes
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Conceptual Groundwater Flow Regimes in Riparian Zones
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Geological Cross Section of Study Area
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Ref: Cey E.E., Rudolph, D.L., Aravena, R., and Parkin, G et al., 1999. Journal of Contaminant Hydrology, 37: 45-67.
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Buffer strip
Stream
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Manure Spreading
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Instrumentation transect perpendicular to the stream
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Nitrate vs 15N Data
15N vs 18O data
Nitrate vs DOC