Sulfur Biogeochemistry

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Sulfur Biogeochemistry
Sulfur transformations are complex due to number
of redox states for sulfur and the relatively
rapid chemical transformations that can occur.

• Outline
• Redox chemistry of sulfur
• Global balance of sulfur
• Key processes in the sulfur cycle
• ExamplesSulfate reduction and arsenic remediation

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• Principal redox states common forms
• S2- sulfide (H2S, HS-, S2- sulfhydryl
  (R-SH)S0 (elemental sulfur)
• S6 sulfate (SO42-)mineral forms gypsum
  (CaSO42H2O), pyrite (FeS2), native sulfur (S)

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Sulfur in the Atmosphere

• Gaseous component
• no sulfur gas is a long-lived or major
  constituent of the atmosphere, oxidation of SO42-
  ? short residence time

• Aerosols
• particles lt 1um are held a loft by Brownian
  motion
• long transport
• sources volcanic eruptions, ocean (water
  evaporates from bubbles, the salt crystallizes
  to form sea-salt aerosols)

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Sources of Sulfur in the Atmosphere

• Volcanic eruptions
• average over many years 12-30x1012 g S
• e.g. Tambora (Indonesia) in 1815, 1816 - year
  without summer in England, USA, Canada, 50x1012
  g S

• Soil dust

• Biogenic gases
• H2S, dimethyl-sulfide (CH3)2S (DMS), carbonyl
  sulfide COS

• Anthropogenic emissions
• without human effects, net transport through the
  atmosphere carries S from sea to land

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Sulfur in Rivers

• Natural river load
• from weathering of pyrite4FeS215O28H2O?
  2Fe2O38H2SO4
• and gypsum, rainfall

• Human activities affect the transport of S in
  rivers
• 28 of the current content of S in rivers is
  derived from air pollution, mining, erosion, and
  the other human activities
• the current transport is supposed to be about
  double that of pre-industrial conditions

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Marine sulfur cycle

• Ocean is large source of aerosols (sea salts)
  that contains SO42-.
• Most of the flux is re-deposited in the ocean in
  precipitation and dry-fall
• Dimethyl-sulfide (CH3)2S or DMS is the major
  biogenic gases emitted from sea
• mean residence time about 1-2 days - most of S
  from DMS is also re-deposited in the ocean
• The net transport of S from sea to land is about
  20x1012 g S/yr. Ocean receives a net input of S.
• Residence time gt 3,000,000 years.
• Major marine sinks metallic sulfides
  precipitated at hydro-thermal vents, biogenic
  pyrites, the formation of organic sulfur .

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Global Sulfur Cycle
all values in 10 12 g S/yr
Rivers
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Natural weathering and erosion
From Schlesinger W.H. 1997
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Sulfur transformations
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Sulfur transformations
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S0 disproportionation is favorable if H2S
produced is coupled to metal oxidation, such as
MnO2?Mn2
Sulfur transformations and trace elements
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Sulfur transformations and trace elements
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Desulfotomaculumreduces arsenate (AsO43-) to
arsenite (AsO32-), also reducing sulfate to
bisulfide, precipitating arsenic sulfide (As2S3)
in the process. This biomineralization process
can detoxify mobile arsenic in the environment.
Sulfur transformations and trace
elementsExample Arsenic mobility
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Mahomet Glacial Aquifer System
From M. Kirk et al., 2004
0
30 km
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60
SO4 (mg/L)
40
20
600
CH4 (µM)
400
200
50
100
150
200
As (µg/L)
From M. Kirk et al., 2004
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Example Acid Mine Drainage and Approaches
Waters draining areas that have been mined for
coal or sulfide ores become acidic due to the
oxidation of pyrite (or other sulfides). A simple
two-step formulation could be written as follows
However the overall mechanism by which pyrite
oxidation occurs is better represented by a
two-step process whereby pyrite is oxidized by
ferric iron
Compost or Anaerobic Wetland Compost wetlands, or
anaerobic wetlands as they are sometimes called,
consist of a large pond with a lower layer of
organic substrate. The flow is horizontal within
the substrate layer of the basin. Piling the
compost a little higher than the free water
surface can encourage the flow within the
substrate. Typically, the compost layer is made
from spent mushroom compost that contains about
10 percent calcium carbonate. Other compost
materials include peat moss, wood chips, sawdust
or hay. A typical compost wetland will have 12 to
24 inches of organic substrate and be planted
with cattails or other emergent vegetation. The
vegetation helps stabilize the substrate and
provides additional organic materials to
perpetuate the sulfate reduction reactions.
                                                  
                                                  
                                                  
                                                  
     
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SR is important in anoxic habitats, limited by
availability of sulfate in FW systems.
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