Environmental Geochemistry

1
Environmental Geochemistry

• January 26, 2007

2
What is geochemistry?

• The study of
• -chemical composition of the Earth and other
  planets
• -chemical processes and reactions that govern the
  composition of rocks and soils
• -the cycles of matter and energy that transport
  the Earth's chemical components in time and space
• -and their interaction with the hydrosphere and
  the atmosphere.

3
Outline of Topics

• Formation of the elements
• Composition of Earth
• Aqueous Solutions
• Chemical Equilibrium
• Acid-Base Equilibria
• Redox
• Biogeochemistry
• Stable Isotopes
• (with comments on weathering, sorption,
  pollution)

4
Formation of the Elements
5
Composition of Earth
6
(No Transcript)
7
Aqueous Solutions
Water is special
8
(No Transcript)
9
Ionic Strength
I 1/2 ?mz2
10
Chemical Equilibrium
Exists when a system is in a state of minimum
energy (G)

• - Often not completely attained in nature (e.g.,
  photosynthesis leaves products out of chemical
  equilibrium)
• - A good approximation of real world
• Gives direction in which changes can take place
  (in the absence of energy input.)
• Systems, including biological systems, can only
  move toward equilibrium.
• -Gives a rough approximation for calculating
  rates of processes because, in
• general, the farther a system is from
  equilibrium, the more rapidly it will move
• toward equilibrium however, it is generally not
  possible to calculate reaction rates from
  thermodynamic data.

11
Reaction Rates/Equilibrium
12
Acid-Base Equilibria
Bronsted-Lowry definition acid donates H base
accepts H
In aqueous systems, all acids stronger that
H2O generate excess H ions (or H3O) all bases
stronger than H2O generate excess OH-
13
Acid-Base
Many reactions influence pH Photosynthesis and
respiration are acid base reactions. aCO2(g)
bNO3- cHPO42- dSO42- f Na gCa2 hMg2
iK mH2O (b 2c 2d -f -2g - 2h -
i)Hlt-----gt CaNbPcSdNafCagMghKiH2Ombiomass (a
2b)O2 Oxidation reactions often produce
acidity. Reduction reactions consume acidity
pH influences many processes -weathering (Fe and
Al more soluble at lower pH) -cation exchange
(leaching of base cations from soil due to acid
rain) -sorption (influences surface charge on
minerals and therefore what sticks to them)
14
Acid-Base
Alkalinity ANC
Alkalinity ?(base cations) - ?(strong acid
anions) Any process that affects the balance
between base cations and acid anions must affect
alkalinity.
15
Redox

• The oxidation state of an atom is defined with
  the following
• convention
• The oxidation state of an atom in an elemental
  form is 0.
• In O2, O is in the 0 oxidation state.
• When bonded to something else, oxygen is in
  oxidation
• state -2 and hydrogen is in oxidation state of 1
  (except for
• peroxide and superoxide).
• In CO32-, O is in -2 state, C is in 4 state.
• The oxidation state of a single-atom ion is the
  charge on
• the ion.
• For Fe2, Fe is in 2 oxidation state.

16
Redox
Redox reactions tend to be slow and are often out
of thermodynamic equilibrium - but life exploits
redox disequilibrium.
Oxidation - lose electrons Reduction - gain
electrons
Fe was oxidized, Mn was reduced
17
Why do we care about redox rxns?

• Oxidation state can impact
• Sorption/desorption
• Solubility
• Toxicity
• Biological uptake
• etc.

Measure of oxidation-reduction potential gives us
info about chemical species present and microbes
we may find.
18
Accumulation of O2 in the Atmosphere
Fe2 Fe(II) slightly soluble in sea water
with no O2 present Add O2 - oxidizes
Fe(II)--gtFe(III) Very small O2 required Fe3
Fe(III) extremely insoluble in water
Essentially all of the oxygen in the atmosphere
came from photosynthesis
19
Biogeochemistry
20
(No Transcript)
21
Nitrification ammonia? nitrite ?
nitrate Denitrification nitrate ? nitrite ?
nitric oxide ? nitrous oxide ? N2 N Fixation N2
?ammonia
22

23
What is an isotope?

• Isotope- line of equal Z. It has the same
  protons (ie. they are the same element) but a
  diff. of neutrons.

14N
15N
12C
13C
14C
10B
11B
24
How did all this stuff get here?

• 4 types of isotopes, based on how they formed
• Primordial (formed w/ the universe)
• Cosmogenic (made in the atmosphere)
• Anthropogenic (made in bombs, etc)
• Radiogenic (formed as a decay product)

25
Stable Isotopes
Light isotopes are fractionated during chemical
reactions, phase changes, and biological
reactions, leading to geographical variations in
their isotopic compositions FRACTIONATION
separation between isotopes on the basis of mass
(usually), fractionation factor depends on
temperature Bonds between heavier isotopes are
harder to break
26
Stable Isotope Examples

• Rayleigh fractionation light isotopes evaporate
  more easily, and heavy isotopes rain out more
  quickly

d (Rsample Rstandard) / Rstandard x 103
27
Stable Isotope Examples

• d18Ocarbonate in forams depends on d18Oseawater
  as well as T, S
• d18Oseawater depends on how much glacial ice
  there is
• Glacial ice is isotopically light b/c of Rayleigh
  fract.
• More ice means higher
• d18Oseawater

28
Stable Isotopes

• C in organic matter, fossil fuels, and
  hydrocarbon gases is depleted in 13C gt
  photosynthesis
• used as an indicator of their biogenic origin and
  as a sign for the existence of life in Early
  Archean time ( 3.8 billion years ago)
• N isotopic composition of groundwater strongly
  affected by isotope fractionation in soils plus
  agricultural activities (use of N-fertilizer and
  discharge of animal waste)
• Particulate matter in ocean enriched in 15N by
  oxidative degradation as particles sink through
  water column
• Used for mixing and sedimentation studies
• S isotopes fractionated during reduction of SO42-
  to S2- by bacteria
• didnt become important until after 2.35 Ga when
  photosynthetic S-oxidizing bacteria had increased
  sulfate concentration in the oceans sufficiently
  for anaerobic S-reducing bacteria to evolve
  (photosynthesis preceded S-reduction which was
  followed by O respiration)

29
Stable Isotope Examples

• Stable isotopes can also tell you about biology
• Organisms take up light isotopes preferentially
• So, when an organism has higher
• ?30Si, it means that it was feeding from a
  depleted nutrient pool

30
Stable Isotopes
Boron isotopes measured in forams used for
paleo-pH d11B depends on pH (Gary
Hemming) Nitrogen isotopes used for rapid temp.
changes in ice cores d15N depends on temp.
gradient in firn (Jeff Severinghaus) Stable
isotopes are also used to study magmatic
processes, water-rock interactions, biological
processes and anthropology and various aspects of
paleoclimate
31
References

• http//mineral.gly.bris.ac.uk/Geochemistry/
• http//mineral.gly.bris.ac.uk/envgeochem/
• http//www.soest.hawaii.edu/krubin/gg425-sched.htm
  l
• http//geoweb.tamu.edu/courses/geol641/notes.html
• http//www.imwa.info/Geochemie/Chapters.HTML
• (WM White Geochemistry Ch9 - Stable Isotopes)
• Isotopes Principles and Applications - Faure
  Mensing
• How to Build a Habitable Planet - Wally Broecker