Course Instructor : Scott Fendorf 301 Green; 3-5238; Fendorf@pangea

1. GES 166/266, Soil Chemistry 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

2. Course Website • “http//soils.stanford.edu/classes/GES166.htm”. • Course Objectives: • • To define the chemical composition of soil materials • Tocomprehend 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.

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

4. GES166/266: Soil Chemistry Mineralogical transformation biomineralization dissolution precipitation Mn+ Oxidation Reduction Mn+x release Bacteria deposition Organic Matter Mineral adsorption Organic ligand desorption Soil Profile complexation Aqueous Metal Ion degradation Metal-Organic Complex Surface complex

5. Salt Affected Soils

6. Acid Soils

7. Arsenic in Bangladesh Largest Mass Poisoning in History: A Result of Arsenic in Drinking Water

8. 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

9. Conditions in Bangladesh Exposure (> 50 ppb) 36,000,000 people (19%) Arsenicosis 1,860,000 people (1%) 125,000 people ( 0.1%) Skin Cancer Internal Cancers (projection) 3,000-7,000 people/y

10. Bangladesh Average Total Arsenic: < 40 mg/Kg Exposure to Hazardous Levels: 36 Million

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

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

13. Bangladesh Where does the arsenic come from? FeAsS

14. 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

15. Release of Arsenic • • Release of As to the aqueous phase is promoted by: • High pH conditions (pH > 8.5) • Competing anions (e.g., phosphate) • Transition to anaerobic state • arsenic reduction • mineralogical changes

16. Bangladesh: Dry Season

17. Bangladesh: Monsoonal Season

18. Anaerobic Conditions Mobility of arsenic is commonly enhanced under reducing conditions. Why? • Arsenic is strongly retained within most aerated soils • Arsenate forms strong surface complexes • 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 sequestration

19. Red. Red. Fe2+ Red. Red. Possible Mobilization Processes Adsorbate Reduction Adsorbent Reduction AsO43- AsO43- Fe(OH)3 Fe(OH)3 AsO33- Fe(OH)3 + AsO33- AsO43- AsO33- AsO43- Al(OH)3

20. Iron Biomineralization Fe(II) aq IRB + S(-II) iron sulfide Fe(OH)3•nH2O goethite Low (< 0.3 mM) + HCO3- Medium (> 0.3 mM) magnetite siderite green rust conversion

21. Arsenic Retention Capacities pH 7 Iron Reductive Transformation

22. Conclusions: Reductive Transformations Limited FeOx Carbon Addition As(V)-Solid As(III) aq Reduction if As(V)-Fe(OH)3 Low [Fe2+] As(III) -FeOOH + As(III) aq FeSx (low S:Fe) Mod [Fe2+] As-FeSx (AsFeS) + As(III) aq [S(-II)] As(III) –Fe3O4 + As(III) aq (high S:Fe) High [Fe2+] As2S3 As(III) –GR + As(III) aq