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