Course Instructor: Scott Fendorf 301 Green; ; Teaching Assistants: Ben Kocar 325 Green; Jim Neiss 325 Green; 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, 1 st Edition, Oxford Press.

Grading and Exams: Grading Participation Philosophy Recitation Graduate (266) Credit

M n+ M n+x ReductionOxidation Mineral Bacteria Soil Profile Organic ligand Surface complex adsorption desorption complexation degradation Aqueous Metal Ion Metal-Organic Complex Organic Matter release deposition biomineralization Mineralogical transformation precipitationdissolution GES166/266: 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/Kg Exposure to Hazardous Levels: 36 Million

Mississippi River Valley Average Total Arsenic: 90 mg/Kg Exposure to Hazardous Levels: None reported

Dissolved Arsenic Profiles Average Well-Depth: 30 m Harvey et al. (2002)

Bangladesh Where does the arsenic come from? FeAsS

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 (H x AsO 4 x-3 ): - binds to broad class of oxic solids - adsorption increases with decreasing pH Arsenite (H x AsO 3 x-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 Conditions Arsenic is strongly retained within most aerated soilsArsenic is strongly retained within most aerated soils –Arsenate 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 (ii) reductive dissolution of Fe(III)-(hydr)oxides Is the fate of arsenic tied to Fe? Generation of sulfide and sulfide minerals will impact As sequestrationGeneration of sulfide and sulfide minerals will impact As sequestration Mobility of arsenic is commonly enhanced under reducing conditions. Why?

Fe(OH) 3 AsO 3 3- AsO 4 3- Al(OH) 3 AsO 4 3- Red. Fe(OH) 3 AsO 4 3- Fe 2+ AsO 3 3- AsO Adsorbate ReductionAdsorbent Reduction Red. Possible Mobilization Processes Fe(OH) 3 AsO 3 3-

Fe(OH) 3 nH 2 O goethite magnetite siderite Iron Biomineralization Fe(II) aq Low (< 0.3 mM) Medium (> 0.3 mM) IRB + S(-II) green rust iron sulfide + HCO 3 - conversion

Arsenic Retention Capacities Iron Reductive Transformation pH 7

Conclusions: Reductive Transformations As(V)-Solid Limited FeOx As(III) aq if As(V)-Fe(OH) 3 As(III) -FeOOH + As(III) aq Low [Fe 2+ ] As(III) –Fe 3 O 4 + As(III) aq As(III) –Fe 3 O 4 + As(III) aq Mod [Fe 2+ ] As(III) –GR + As(III) aq As(III) –GR + As(III) aq High [Fe 2+ ] [S(-II)] As 2 S 3 FeS x As-FeS x (AsFeS) + As(III) aq As-FeS x (AsFeS) + As(III) aq Reduction (high S:Fe) (low S:Fe) Carbon Addition