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Ch 4 - Oxidation-Reduction Reactions

Hanford, WA

Na2S2O4 reduces Fe3+ in soil to Fe0

Fe

CrO42-(aq) + Fe(s) + 4H2O(l)  Cr(OH)3(s) + Fe(OH)3(s) + 2OH-(aq) Fe catalyst ClHC=CCl2(aq) H2C=CH2 + HCCH + Cl- (unbalanced)

Definitions of Oxidation and Reduction Historical Definitions: Oxidation means adding oxygens to the molecule while the number of hydrogens decreases: e.g. CH4 + O2  CO2 + H2O Reduction means adding hydrogens to the molecule while the number of oxygens decreases: e.g.

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Modern Definitions: Oxidation means losing electrons: e.g. Reduction means gaining electrons:

e.g.

Both definitions are the same, and can be used together when oxygen and hydrogen are present

Oxidation-Reduction Diagrams The Stability of H2O H2O can be oxidized according to the following reaction: O2 (in H2O)

2O2

H2O can be reduced according to this reaction: 2H+ (in H2O)

+

2e′ 

H2

These reactions determine the stability limits of water in the environment as a function pH and Eh.

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pH = -log [H+] e.g. [H+] = 0.10 M

Eh is the potential in volts produced by a redox reaction occurring in nature. e.g. 4Fe2+ (aq) + 3O2(g)  2Fe2O3(s) 1. A positive Eh means oxidation, negative means reduction 2. Oxygen is the primary substance in natural waters that oxidizes other species 3. Carbonaceous material produces reducing conditions

Water quality meter: pH, Eh, conductivity,….

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Oxidation and Reduction in Nature

Oxidizing (aerobic)

Reducing (anaerobic)

http://www.atlas.keystone.edu/edu/Community_Resources/basics/03habitats.htm

Limits of pH and Eh in nature

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Fe, Pb, Zn, Au containing ore

Sulfur Eh-pH Diagram

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Iron Eh-pH Diagram

Lead Eh-pH Diagram

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Gold Eh-pH Diagram

Open Pit Copper Mines e.g. Globe, AZ “Supergene Enrichment”

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Copper Eh-pH Diagram

Tenorite CuO

Cuprite Cu2O

Native Copper Cu

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Including CO2 makes new minerals

malachite

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Case Study 4-1 Sources of High Dissolved Manganese Concentrations in Mildly Acidic Runoff from a Coal Mine in Eastern Tennessee

MnO2

FeCO3

Rhodochrosite

1. Manganese in coal mine acidic mine waters, coal low in sulfur 2. 1 year after opening at a pH = 4, mine runoff was 60 mg L-1, exceeding regulations 3. Sources of Mn were from (a) adsorption onto clay particles, and (b) siderite (FeCO3) concretions and cement

4. H2SO4 from oxidation of pyrite neutralized from hydrolysis of the siderite FeCO3 + H2O = Fe2+ + HCO3- + OH- or the net ionic equation is CO32- + H2O = HCO3- + OHResults – based on water sample pH and Eh – Fe forms Fe(OH)3 flocculant at mine water , but the manganese is soluble Solutions: •Limit oxidation of pyrite in mine water “spoil” •Siderite spoils up hydraulic gradient to prevent hydrolysis •Increase the pH and Eh of mine spoil to induce Mn-hydroxide precipitate

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Microorganisms and Energy Pathways Examples of Oxidation-Reduction Reactions Mediated by Microorganisms Aerobic Degradation – breakdown of dead plant and microbial material in the presence of oxygen and microorganisms CH2O is the general formula of a carbohydrate CH2O(aq) + O2(g)  CO2(g) + H2O Reaction very fast when microbiologically mediated, very slowly without e.g. another example – sulfide oxidation and acid mine drainage, Catalyzed by Thiobaccillus thiooxidans 2FeS2(pyrite) + 7O2(g) + 2H2O  2Fe2+(aq) + 4SO42- + 4H+(aq)

Then the iron is catalyzed by Thiobacillus ferrooxidans 4 Fe2+(aq) + O2(g)  4Fe3+(aq) + 2H2O

Anaerobic Degradation – oxygen deficient environments, e.g. mangrove swamps, wetlands, etc. 2CH2O(aq)  CH4(g) + CO2(g) One of the major sources of greenhouse gases

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Case Study 4-2 Bacterial Dissimilarity Reduction of Arsenate and Sulfate in Mono Lake, CA •Alkaline and hypersaline with high concentrations of dissolved As •Reduction of As5+ (arsenate) to As3+ (arsenite) in transition zone between oxidizing surface waters and deeper anaerobic bottom waters, e.g. depth of 11 to 17 meters

Source of As – anthopogenic such as mine drainage, pesticides, leaching from wall rocks Biochemical reduction - microorganisms that reduce As5+ to As which is then expelled from the cell

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Case Study 4-6 The Effect of a Hydrocarbon spill on the Sediments of the Contaminated Aquifer •USGS Toxic Substance Hydrology study site •1979 oil pipeline rupture = 1 meter thick oil slick floating on the groundwater •Organics + microorganisms + Fe3+  Fe2+ •Source of the Iron is iron-rich minerals found in the aquifer •A series of sediment cores collected along the plume axis

Contaminant plume and redox conditions from aerobic to anaerobic

•Organics in the form of low molecular weight aliphatic and organic acids •Samples collected from pore waters closest to the plume were highest in Fe2+ and showed the greatest loss from the sediments •Supports conclusion that the Fe3+ + microorganisms + organics  Fe2+ according to –

C7H8 + 36 Fe(OH)3 + 65 H+  7 HCO3- + 36 Fe2+ + 87 H2O

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U.S. Uranium Deposits

Uranium-Vanadium Eh-pH Diagram (w/carbonate)

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Colorado Plateau Uranium Deposit

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