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THE GEOCHEMISTRY OF NATURAL WATERS

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Title: THE GEOCHEMISTRY OF NATURAL WATERS


1
THE GEOCHEMISTRY OF NATURAL WATERS
  • BIOTRANSFORMATIONS OF ORGANIC COMPOUNDS - II
  • CHAPTER 7 - Kehew (2001)

2
LEARNING OBJECTIVES
  • Classify microorganisms according to their
    characteristics and metabolic processes.
  • Understand the implications of various metabolic
    processes for natural and accelerated
    bioremediation of contaminated aquifers.

3
AEROBIC RESPIRATION
  • The dominant form of metabolism in the presence
    of dissolved oxygen because it yields the
    greatest amount of energy.
  • Saturation limit of O2 in water is 13 mg L-1 so
    aerobic respiration is limited by availability of
    O2.
  • Assimilative capacity - The amount of a
    xenobiotic compound that can be metabolized by
    aerobic respiration.

4
ENERGY YIELDS FROM METABOLIC PROCESSES
5
CALCULATION OF ASSIMILATIVE CAPACITY
  • Assume benzene is metabolized to CO2 without
    producing additional microbial cells.
  • C6H6 7.5O2 ? 6CO2 3H2O
  • According to this reaction 7.5 moles ? 32 g mol-1
    240 g O2 are required to metabolize 1 mole ? 78
    g mol-1 78 g of benzene.
  • This yields a mass ratio of 240/78 3.081.
  • The assimilative capacity of an aquifer with 10
    mg L-1 O2 is therefore (10 mg L-1)/(3.08) 3.25
    mg L-1.

6
ANAEROBIC RESPIRATION WITH NITRATE REDUCTION
  • Nitrate reduction can be performed by obligate
    anaerobes or facultative anaerobes.
  • Denitrification (reduction of nitrate to
    molecular oxygen) occurs via a series of steps
    involving NO2- and N2O. Each step requires a
    specific enzyme catalyst.
  • Nitrate reduction is an important pathway for
    xenobiotic compounds
  • C6H6 6NO3- 6H ? 6CO2 3N2 6H2O
  • To consume 1 mg L-1 benzene requires 1.1 mg L-1
    nitrate-N (20-30 mg L-1 or more in many aquifers).

7
ANAEROBIC RESPIRATION WITH FERRIC IRON REDUCTION
  • Ferric hydroxide coatings on mineral grains yield
    a large assimilative capacity for anaerobic
    metabolism.
  • Metabolism of toluene by iron-reducing bacteria
  • C7H8 36Fe3 21H2O ? 36Fe2 7HCO3- 43H
  • requires 36 moles of ferric iron per mole of
    toluene!
  • Iron concentrations can be greatly increased.
    Ferrous iron may be oxidized by chemolithotropic
    bacteria under aerobic conditions this can cause
    clogging problems in water systems.

8
Metabolic processes controlling fate of iron in
the subsurface.
9
ANAEROBIC RESPIRATION WITH SULFATE REDUCTION
  • The presence of sulfate can significantly augment
    the assimilative capacity of an aquifer for
    metabolism of xenobiotics.
  • C6H6 3.75SO42- 7.5H
  • ? 6CO2 3.75H2S 3H2O
  • A 3.751 sulfate/benzene molar ratio translates
    into a mass ratio of 4.61.
  • It takes 4.6 mg L-1 of sulfate to degrade 1 mg
    L-1 of benzene.

10
METHANOGENESIS
  • Archaebacteria employ methanogenesis as their
    metabolic process.
  • Two main metabolic pathways
  • CO2 reduction pathway
  • 4H2 CO2 ? CH4 2H2O
  • Essential H2 is produced by fermentative bacteria
    (symbiotic relationship).
  • Acetate fermentation (acetate dissimilation)
    pathway
  • CH3COOH ? CH4 CO2
  • A minor but significant pathway
  • C6H6 4.5H2O ? 3.75CH4 2.25CO2

11
CHEMOLITHOTROPIC PROCESSES
12
ACCLIMATION
  • Once a microbial community becomes acclimated to
    a specific xenobiotic, degradation proceeds
    immediately upon addition of more of the
    compound. Before that, there is a period where
    little degradation occurs.
  • Reasons for the acclimation period
  • Proliferation of small populations
  • Presence of toxins
  • Predation by protozoa
  • Appearance of new genotypes
  • Diauxie
  • Enzyme induction

13
  • Proliferation of small populations - When a
    xenobiotic is first introduced, the proportion of
    microbes that can degrade it may be small. It may
    take several doublings of the population before a
    change in xenobiotic concentration is noticeable.
  • Diauxie - Sequential utilization of two or more
    carbon substrates. Bacterial community feeds on
    substrate with the highest energy yield first.
    The acclimation period could represent when the
    bacteria are using the primary substrate.
  • Enzyme induction - The time required for this is
    usually less than the acclimation period.

14
GROWTH-LINKED METABOLISM - I
  • When substrate concentration is low, all of the
    available substrate is used for cell maintenance
    cell growth does not occur.
  • Only when the substrate concentration exceeds a
    threshold does growth occur. Above the threshold,
    exponential growth results in more rapid
    substrate metabolism.
  • Monod equation
  • where µ is the population growth rate, µmax is
    the maximum growth rate and Ks is the substrate
    concentration at 1/2 the maximum growth rate.

15
Kinetic models for declines in concen-tration of
compounds metabolized by micro-organisms.
S lt Km
Non-growth-linked conditions
S gt Km
S ? Km
S gt Ks
Growth-linked conditions
S ? Ks
S lt Ks
16
Growth rate of a population vs. substrate
concentration. The hyperbolic relationship is
described by the Monod equation.
17
GROWTH-LINKED METABOLISM - II
  • Logarithmic decline - Occurs when the substrate
    concentration is very high. Once the population
    reaches a critical value, the substrate
    concentration will decline rapidly.
  • Logistic decline - Occurs when the substrate
    concentration is very low. Growth rate is
    linearly proportional to substrate concentration,
    and doubling of population takes longer as
    substrate concentration decreases.
  • Monod, with growth - Intermediate case.
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