course instructor: scott fendorf 301 green; 3-5238; fendorf@pangeafendorf@pangea teaching...
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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
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.
Grading and Exams: • Grading
• Participation
• Philosophy
• Recitation
• Graduate (266) Credit
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
Salt Affected Soils
Acid Soils
Arsenic in Bangladesh
Largest Mass Poisoning in History: A Result of Arsenic in Drinking Water
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
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
Bangladesh
Average Total Arsenic: < 40 mg/KgExposure to Hazardous Levels: 36 Million
Mississippi River Valley
Average Total Arsenic: 90 mg/KgExposure to Hazardous Levels: None reported
Dissolved Arsenic Profiles
Average Well-Depth: 30 m
Harvey et al. (2002)
Bangladesh
Where does the arsenic come from?
FeAsSFeAsS
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
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
Bangladesh: Dry Season
Bangladesh: Monsoonal Season
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?
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-
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
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
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
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