Redox reactions

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Redox reactions the acid base chemistry of
natural waters pollution purification of
water chapter 10
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Natural Waters

• 97 salt water
• ¾ of fresh water is trapped in glaciers ice
  caps
• Lakes rivers (major water supplies for humans)
  only 0.1 total water supply
• Humanity consumes 1/5 accessible (mostly
  agriculture) runoff water, ¾ by 2025
• In 2000 10 lived with water stress of scarcity,
  28 by 2025 (Figure 9-1)

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OxidationReduction Chemistryin Natural
WatersDissolved Oxygen (DO)
Half reaction
Acidic solution
Basic solution
Partial pressure of oxygen is 0.21 atm, it
follows that the solubility of O2 is 8.7 mg/L or
8.7 ppm. The amount of O2 that dissolves at 0C
is 14.7 ppm, at 35C7.0 ppm. The median
concentration of oxygen found in natural,
unpolluted surface waters in the United States
is about 10 ppm. Thermal Polution results in
lower DO. Fish need minimum of 5 ppm DO
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Oxygen Demand

• Organic material is oxidized by O2 (catalyzed by
  bacteria)
• Capacity to consume O2 is called biological
  oxygen demand (BOD)
• Chemical oxygen demand (COD) moles of O2 are 1.5
  mole of chromate (higher than BOD)

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Oxygen Demand

• If BOD exceeds O2 then fish will die
• Total organic carbon (TOC) dissolved suspended
  organic material in raw water typical 1mg/l
  (1ppm) in ground water
• Dissolved organic carbon (DOC) typically 5 mg/l
  (5PPm) in surface water bog swamps 50 ppm
  sewage 100s ppm largest component of DOC are
  carbohydrates, with smaller amounts of proteins,
  low molecular weight ketones, aldehydes, and
  carboxylic acids

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Green Chemistry Enzymatic Preparation of Cotton
Textiles

• 20 million kilograms of cotton produced each year
  
• Scouring (removing the outer fatty/waxy layer) is
  traditionally done using millions of kg of 18-25
  NaOH solution at elevated temperature BOD levels
  generally exceed 1100 mg/L
• Biopreparation uses an enzyme (a pectin lyase)

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Green Chemistry Enzymatic Preparation of Cotton
Textiles

• Biopreparation
• Removes much less organic material lowers BOD by
  20 COD by 80 more robust fibers/higher
  yields
• Eliminates the use of sodium hydroxide solutions
  at elevated temperatures, which in turn
• lowers the pH of the wastewater
• eliminates the need for neutralization with
  acetic acid and the concomitant wastes
• lowers energy requirements, and
• lowers rinsing requirements, reducing water
  consumption by 3050
• reduces risk to workers

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Decomposition of Organic Materials in Water

• Anaerobic conditions
• Disproportionation (fermentation)
• Reduction of iron
• Oxidizing reducing conditions can occur in
  different parts of a lake

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The pE Scale

• Measure of reducing ability
• Low pE values signify that electrons are readily
  available from substances dissolved in the water,
  so the medium is very reducing in nature
• High pE values signify that the dominant
  dissolved substances are oxidizing agents, so few
  electrons are available for reduction processes.

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The pE Scale

• Significant amount of O2 dominant reaction
  determining overall electron availability is
• The pE is
• If water is saturated with O2 then PO2.21 atm
  pH7 pE13.9

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The pE Scale

• Low oxygen availability, pE determined by ions
  such as nitrate or sulfate, extreme case
  -anaerobic conditions found at the bottoms of
  lakes in the summer and in swamps and rice
  paddies, electron availability determined by
  ratio of CH4/CO2 if pressures of the two gases
  are equal and the water is neutral, the pE value
  is -4.1

Lower levels of lakes have - pE values upper
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Sulfur Compounds in the Natural Waters
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Acid Mine Drainage
Yellow boy
http//www.mines.edu/fs_home/jhoran/ch126/yellow.h
tm
S22- from FeS2 iron pyrite or fools gold
Combining these two reactions gives
The oxidation of disulfide ion to sulfate ion in
this process is accomplished to some extent by
the action of Fe3
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Nitrogen Compounds in Natural Waters
Reaction is pH dependent
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Acid-Base Chemistry of Natural Waters

• The CO2-Carbonate System

Carbon dioxide from the atomosphere dissolves in
water to make the water acidic
Carbonate from limestone makes the water basic
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Water in Equilibrium with Calcium Carbonate
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Water in Equilibrium with Calcium Carbonate
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Water in Equilibrium with CaCO3 Atmospheric CO2
CO2 dissolved also equal to S and is 34 times the
amount that dissolves without the presence of
calcium carbonate (see results of Problem 3-14).
Furthermore, the calculated calcium concentration
is four times that calculated without the
presence of carbon dioxide.
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Water in Equilibrium with CaCO3 Atmospheric CO2
Typically, the pH of such calcareous waters lies
in the range of 7 to 9, in reasonable agreement
with our calculations. About 80 of natural
surface waters in the United States have pH
values between 6.0 and 8.4.
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Ionic Concentrations in Natural Waters Drinking
Water

• The abundant ions in fresh water

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Fluoride (F-)Ion in Water

• From 0.01-20 ppm (source fluorapatite Ca5(PO4)3F)
• In some European countries and Mexico fluoride is
  added to table salt
• In US, Canada, New Zealand Australia fluoride
  is added to drinking (fluorosilicic acid, H2SiF6)
  to 1 ppm most toothpastes also have a source of
  fluoride protection from tooth decay

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Bottled Drinking Water Seawater

• Maximum concentrations Table 9-4
• Sodium blood pressure
• Sulfate laxative
• Some brands exceeded arsenic fluoride
• Little chloroform
• Polycarbonate jugs leached bisphenol-A
• Seawater much higher concentrations than
  freshwater predominantly Na Cl also high
  levels of Mg2 and SO4-2 other ions

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Alkalinity Indices for Natural Waters

• Measure of the ability of a water sample to act
  as a base by reacting with protons (resist
  acidification by neutralization when acid rain
  falls)
• Methyl orange indicator is used (pH4)
• Typical values 0.05-2 mmol/L
• Phenolphthalein alkalinityCO32- pH8-9

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Alkalinity

• Alkalinity measure of ability to support plant
  life. Algae extract CO2 for photosynthesis from
  bicarbonate.
• Water with low alkalinity Ca, CO2 is from
  HCO3- forms OH water becomes very basic (as
  high as 12.3

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Hardness Index for Natural Water

• Measured by titration with EDTA
• Most calcium from either CaCO3 in the form of
  limestone or from CaSO4. Much of the magnesium is
  from dolmitic limestone, CaMg(CO3)2.
• Waters high in concentration of Ca Mg are
  classified as hard precipitates soaps

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Aluminum in Natural Waters

• Concentration of aluminum is generally small
  (10-6 M). At typical pH of 6-9 the aluminum in
  rocks is quite insoluble
• At pH6
• Al concentration increase by 1000 with every
  decrease of the pH by 1. At pH4 Al
  concentration is 10-3
• High concentrations of Al may lead to
  neurological damage also precipitates of Al(0H)3
  on gills of fish mobilization of Al in soils
  leading to dieback of trees (Poland, Czech
  Republic, and eastern Germany

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Perchlorates

• Perchlorate ion ClO3- is a pollutant in drinking
  water
• Much of the percholorate pollutants come from
  NH4ClO3 an oxidizing agent in fireworks, rocket
  fuel, air bags batteries
• NH4ClO3 is unstable and washed out of rocket
  booster missiles on a regular basis into
  holding ponds Figure 9-6 shows regions of
  perchlorate use and contamination in the US
• Perchlorates are found in lettuce milk samples
• In high concentrations reduces hormone production
  in the thyroid gland

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