GES166/266: Soil Chemistry

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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+xReductionOxidationMineralBacteriaSoil ProfileOrganic ligandSurface complexadsorptiondesorptioncomplexationdegradationAqueous Metal IonMetal-Organic ComplexOrganic MatterreleasedepositionbiomineralizationMineralogical transformationprecipitationdissolutionGES166/266: Soil Chemistry

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Salt Affected Soils

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Acid Soils

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Arsenic in BangladeshLargest Mass Poisoning in History: A Result of Arsenic in Drinking Water

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Bangladesh: Water-Use HistorySubsurface 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/y1,860,000 people (1%)ArsenicosisSkin CancerInternal Cancers (projection)Exposure (> 50 ppb)36,000,000 people (19%)Conditions in Bangladesh

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BangladeshAverage Total Arsenic: < 40 mg/Kg Exposure to Hazardous Levels: 36 Million

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

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Dissolved Arsenic ProfilesAverage Well-Depth: 30 mHarvey et al. (2002)

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BangladeshWhere does the arsenic come from? FeAsS

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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: High pH conditions (pH > 8.5) Competing anions (e.g., phosphate) 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 ConditionsArsenic 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 sequestrationMobility of arsenic is commonly enhanced under reducing conditions. Why?

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Fe(OH)3AsO33- AsO43- Al(OH)3AsO43- Fe(OH)3AsO43- AsO33- AsO43- +Adsorbate ReductionAdsorbent ReductionPossible Mobilization ProcessesFe(OH)3AsO33-

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Fe(OH)3•nH2OgoethitemagnetitesideriteIron BiomineralizationFe(II) aqLow (< 0.3 mM)Medium (> 0.3 mM)IRB+ S(-II)green rustiron sulfide+ HCO3-conversion

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Arsenic Retention CapacitiesIron Reductive TransformationpH 7

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Conclusions: Reductive TransformationsAs(V)-SolidLimited FeOxAs(III) aqifAs(V)-Fe(OH)3As(III) -FeOOH + As(III) aqLow [Fe2+]As(III) –Fe3O4 + As(III) aqMod [Fe2+]As(III) –GR + As(III) aqHigh [Fe2+][S(-II)]As2S3FeSxAs-FeSx (AsFeS) + As(III) aqReduction(high S:Fe)(low S:Fe)Carbon Addition

Last Updated: 8th March 2018