Redox & Bicarbonate Buffering

What does redox stand for?

reduction + oxidation

What is oxidation?

loosing electrons

What is reduction?

gaining electrons

What does a reducing agent do?

donates electrons

What does an oxidizing agent do?

accepts electrons

What controls redox?

Concentrations of substances, pH,
and temperature

consumption of
oxygen by microbial respiration!!!!

What do microbes do when O2 is depleted?

microbes use oxidized inorganic or organic
substances as e- acceptors in metabolism

Redox potential (EH):

measurement of direction/amount of
electron transfer occurring in oxidation-reduction reactions

mV

High Eh (+400 to +800 mV) =

oxidizing

Oxygen present, decomposition of organic matter (OM) is aerobic

Low Eh (-200 to -400 mV) =

reducing

Oxygen depleted → often because OM has been decomposed, using up the available oxidizers

What is present in Aerobic Respiration

lots of O2 and OM present

Denitrification

nitrate (NO3-) to N2 gas
• Low/no O2, abundant NO3- and OM
• Can release nitrous oxide (N2O)
• Common in aquatic sediments, wetlands

Manganese reduction

MnO2 to Mn2+
• No O2, abundant Mn and OM
• Common in groundwater

Iron reduction

Fe(OH)3 to Fe2+
• No O2, abundant iron
• Locations: wetlands, groundwater

Sulfate reduction

SO42- to HS-
• No O2, abundant sulfate and OM
• Locations: salt marshes, wetlands

Methanogenesis

OM to CH4
• All other electron acceptors are depleted
• Abundant OM
• Common in freshwater sediments, especially in lakes

How do redox and buffering relate to each other?

Redox determines which chemical forms exist

pH affects solubility, toxicity, reactivity, and mobility

pH stability is essential for consistent water quality
Natural buffering systems maintain the pH = predictable chemical behavior

What is a buffering system?

Resists change in pH

Addition of H+ will neutralize OH-

what is present in low pH

only carbon dioxide and carbonic acid present

what is present in intermediate pH

Bicarbonate = dominant

what is present in high pH

bicarbonate and carbonate = dominant

Sedimentary rocks

naturally high carbonate content
• Make up most of the Earth’s
surface
• This buffering system is what
causes weathering
• Provide sufficient hydroxyl ions to
prevent acidification
• Karst

Igneous rocks

naturally low carbonate content
• Carbonate can become exhausted
in these drainages
• Results in rapid pH drops

What is the pH of rain

naturally acidic due to CO2 and naturally occurring sulfate
• Rainwater is neutralized as it moves
through the soil profile
• Catchments with hard rocks, little
buffering capacity, and high SW
inputs
• Stream water will be acidic even without pollution

Alkalinity

Quantity and kinds of compounds that collectively shift the pH into the alkaline range.
• Bicarbonate buffering system is main driver.
• Measures capacity of a solutes in a solution to neutralize acid from a filtered sample

Acid Neutralizing Capacity (ANC)

similar to alkalinity
• determined for an unfiltered sample
• Measures the capacity of solutes + particulates to neutralize acid
Measures of Buffering Capacity

Hardness

Not the same as alkalinity!
• Determined by cations that form insoluble compounds with soap
• Primarily a measure of calcium and magnesium salts
• Ca and Mg occur mainly in combination with bicarbonate, sulfate, and chloride

concentration of certain metal ions like calcium and magnesium in water