Chapter 10 Distribution of species in aquatic systems

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Chapter 10Distribution of species in aquatic
systems

• CH350/EV350
• Spring 2008

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Chemical Distribution

• Knowing the distribution of chemical is important
  in environmental chemistry
• At pH lt 5 aluminum exists as free aluminum Al3
  (hydrated by water), can be taken up by plants,
  and is toxic to plants
• In slightly alkaline water aluminum exists as
  partially deprotonated hydroxy species or in
  complexes with organic ligands, these are not
  considered very bioavailable, and are less toxic

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Phosphate

• Can solve these equations to determine the amount
  of each species at different pH (H
  concentrations)
• Calculations assume no interactions with other
  particles (low ionic strength water) and would
  not be applicable in sea water

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Phosphate distribution at various pH

• Eastern part of Lake Victoria (Kenya) 12 mg/L P
  as phosphate pH 8.2 (a)
• 92 HPO42-, 8 H2PO4-
• Pore water of forest soil in Canada has 62 mg/L P
  as phosphate pH 4.3 (b)
• 100 H2PO4-

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Cadmium complexes with chloride

• Can solve these equations to determine the amount
  of each species at different Cl- concentrations

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Cadmium distribution at varying chloride
concentrations
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Cadmium distribution at varying chloride
concentrations in the Chesapeake Bay

• Outlet of Susquehanna River, salinity 0.1,
  Cl-0.02 mol/L (a)
• 29 Cd2, 64 CdCl, 7 CdCl2
• Halfway down bay, surface salinity is 10, Cl-
  0.16 mol/L (b)
• 4 Cd2, 52 CdCl, 33 CdCl2, 10 CdCl3-, 1
  CdCl4-
• Mouth of bay, surface salinity is 23, Cl-
  0.37 mol/L (c)
• 26 CdCl, 39 CdCl2, 29 CdCl3-, 6 CdCl4-
• Cd2 more toxic than forms bound with Cl-

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Electron activity

• pH -log aH -logH
• pE -log ae
• Large negative value of pE indicates a large
  value of electron activity in solution
• Reducing conditions
• Anoxic water
• Swamps and sediment pore water
• Large positive value of pE indicates a low value
  of electron activity in solution
• Oxidizing conditions
• Well-aerated surface water
• pE varies from -12 25 in water

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Calculating pEo for a chemical reaction

• Can use Eo, Keq, or DG to calculate pEo for a
  chemical reaction

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Calculating pE (nonstandard conditions) for a
chemical reaction

• Can use pEo and reaction quotient to calculate pE
  for a chemical reaction at non standard conditions

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Chromium in tannery wastes

• Leather tanning involves teating hides with
  aqueous chromium (III)
• Cr3 becomes toxic when oxidized to Cr2O72-
• Water system with waste water from tannery
• 26 mg/L Cr3
• Equilibrium with atmospheric oxygen
• pH 6.5

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Chromium in tannery wastes

• When Cr3 is released into well aerated waters,
  it is converted to the toxic form of Cr2O72-

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pE/pH diagram

• Water limits the boundaries on a pE/PH diagram
• Reducing conditions
• 2H2O 2e- ? H2(g) 2OH-(aq)
• For this reaction pE -pH
• Oxidizing conditions
• 6H2O ? 4H3O(aq) O2(g) 4e-
• For this reaction pE 20.80 - pH

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pE/PH values of common solution and soil
environments

• Well oxygenated environments
• Acid mine water, rain, lakes, rivers, and oceans
• Anoxic environements
• Submerged soils, swamps, marine sediments.

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Sulfur

• pH controlled reactions (vertical lines)
• SO42- ? HSO4-
• HS- ? H2S
• Mostly pE controlled reactions (horizontal lines)
• HSO4- ? So
• So ? H2S

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Application of sulfur pE/pH diagram

• Acid mine waste pH 2.5 exposed to atmospheric
  oxygen (X)
• Sulfide released from ore is oxidized to sulfate
  (SO42-)
• Rice paddy with high organic matter content pH
  6, pE -3 (Y)
• H2S common form of sulfur

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Measurement of pE

• Measure potential on inert Pt electrode immersed
  in soil or water relative to the reference
  electrode