course instructor: scott fendorf 301 green; 3-5238; fendorf@pangeafendorf@pangea teaching...
TRANSCRIPT
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Course Instructor: Scott Fendorf
301 Green; 3-5238; Fendorf@pangea
Teaching Assistants: Ben Kocar
325 Green; 3-4152 kocar@pangea
Jim Neiss
325 Green; 3-4152 jneiss@pangea
Meeting Times:
Lecture: 9 – 10:15 pm Tuesday, Thursday
Recitation: 2:15 –3:30 pm Thursdays
Location:
131 Green or A25 Mitchell
GES 166/266, Soil Chemistry
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Course Website
“http//soils.stanford.edu/classes/GES166.htm”.
Course Objectives:
• To define the chemical composition of soil materials
• To comprehend the chemical (and biochemical) factors functioning within soil systems
• To define the chemical factors influencing the fate of elements (contaminant and nutrient) within soils
Recommended Text and Reading Assignments:
Environmental Chemistry of Soils by Murray B. McBride, 1st Edition, Oxford Press.
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Grading and Exams: • Grading
• Participation
• Philosophy
• Recitation
• Graduate (266) Credit
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Mn+
Mn+x
ReductionOxidation
Mineral
Bacteria
Soil ProfileSoil Profile
Organic ligand
Surface complex
adsorption
desorption
complexation
degradation
Aqueous Metal Ion
Metal-Organic Complex
OrganicMatter
release
deposition
biomineralization
Mineralogical transformation
precipitationprecipitationdissolutiondissolution
GES166/266: Soil ChemistryGES166/266: Soil Chemistry
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Salt Affected Soils
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Acid Soils
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Arsenic in Bangladesh
Largest Mass Poisoning in History: A Result of Arsenic in Drinking Water
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Bangladesh: Water-Use History
• Subsurface wells installed in early 1970s
- avoids surface pathogens
• Irrigated agriculture initiated mid-1970s
• Arsenic poisoning detected late-1980s, extensive exposure noted in 1990s
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125,000 people ( 0.1%)
3,000-7,000 people/y
1,860,000 people (1%)Arsenicosis
Skin Cancer
Internal Cancers(projection)
Exposure(> 50 ppb)
36,000,000 people (19%)
Conditions in Bangladesh
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Bangladesh
Average Total Arsenic: < 40 mg/KgExposure to Hazardous Levels: 36 Million
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Mississippi River Valley
Average Total Arsenic: 90 mg/KgExposure to Hazardous Levels: None reported
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Dissolved Arsenic Profiles
Average Well-Depth: 30 m
Harvey et al. (2002)
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Bangladesh
Where does the arsenic come from?
FeAsSFeAsS
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Chemistry of Arsenic
• Arsenic generally persists as As(III) or As(V) within surface and subsurface environments
- lower valent states, such as As(0), occur
• Retention Characteristics
Arsenate (HxAsO4x-3):
- binds to broad class of oxic solids- adsorption increases with decreasing pH
Arsenite (HxAsO3x-3):
- binds to Fe-oxides- adsorption maximum between pH 7 and 9- reacts with sulfides
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Release of Arsenic
• Release of As to the aqueous phase is promoted by:
1. High pH conditions (pH > 8.5)
2. Competing anions (e.g., phosphate)
3. Transition to anaerobic state- arsenic reduction
- mineralogical changes
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Bangladesh: Dry Season
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Bangladesh: Monsoonal Season
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Anaerobic ConditionsAnaerobic Conditions
• Arsenic is strongly retained within most aerated soilsArsenic is strongly retained within most aerated soils– Arsenate forms strong surface complexesArsenate forms strong surface complexes
• Upon a transition from aerobic to anaerobic conditions:Upon a transition from aerobic to anaerobic conditions:
(i) conversion of arsenate to arsenite(i) conversion of arsenate to arsenite
(ii) reductive dissolution of Fe(III)-(hydr)oxides(ii) reductive dissolution of Fe(III)-(hydr)oxides
Is the fate of arsenic tied to Fe?Is the fate of arsenic tied to Fe?
• Generation of sulfide and sulfide minerals will impact As Generation of sulfide and sulfide minerals will impact As sequestrationsequestration
Mobility of arsenic is commonly enhanced under reducing conditions. Why?
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Fe(OH)3
AsO33-
AsO43-
Al(OH)3 AsO43-
Red.
Red.
Fe(OH)3AsO4
3-
Fe2+
AsO33-
AsO43-
+
Adsorbate Reduction Adsorbent Reduction
Red.
Red.
Possible Mobilization Processes
Fe(OH)3 AsO33-
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x y
z
xy
z
xy
z
x
y
z
Fe(OH)3•nH2O goethite
magnetite
siderite
Iron Biomineralization
Fe(II) aq
Low(< 0.3 mM)
MediumMedium(> 0.3 mM)(> 0.3 mM)
IRB+ S(-II)
green rust
iron sulfide
+ HCO3-
conversion
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0
0.5
1
1.5
2
2.5
3
3.5
Mag Lep Goe F Fs
Ad
sorp
tio
n C
apac
ity
(Mo
les/
Kg
)
Arsenate
Arsenite
Arsenic Retention Capacities
Iron Reductive Transformation
pH 7
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Conclusions: Reductive Transformations
As(V)-SolidLimited FeOx
As(III) aq
if
As(V)-Fe(OH)3
As(III) -FeOOH + As(III) aq
Low [Fe2+]
As(III) –Fe3O4
+ As(III) aq
As(III) –Fe3O4
+ As(III) aq
Mod [Fe2+]
As(III) –GR + As(III) aqAs(III) –GR + As(III) aq
High [Fe2+]
[S(-II)]
As2S3As2S3
FeSx
As-FeSx
(AsFeS)
+ As(III) aq
As-FeSx
(AsFeS)
+ As(III) aq
Reduction
(high S:Fe)
(low S:Fe)
Carbon Addition