THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

• CHEMICAL KINETICS
• CHAPTER 2f - Kehew (2001)

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LEARNING OBJECTIVES

• Be introduced to some of the basic principles of
  chemical kinetics.
• Learn what is meant by reaction order.
• Become familiar with some basic rate-law
  expressions.
• Learn about the Arrhenius equation.
• Gain an appreciation for the factors controlling
  mineral precipitation and dissolution rates.

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TEMPERATURE-DEPENDENCE OF RATES

• Arrhenius equation
• Higher temperatures lead to faster chemical
  reactions.
• A general rule of thumb an increase of 10C in
  temperature generally leads to a doubling of
  reaction rate.

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ACTIVATION ENERGY BARRIER
Plot illustrating the physical significance of
the activation energy in the Arrhenius equation.
Ea represents the height of an energy barrier
that must be overcome to get from reactants to
products. The k0 in the Arrhenius equation
represents, in part, the frequency with which an
attempt is made to surmount the energy barrier.
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MINERAL DISSOLUTION - I

• Experimental studies have shown that the rate of
  mineral dissolution is a function of the degree
  of undersaturation, i.e., the following reaction
  applies
• The more undersaturated the solution with respect
  to a mineral, the faster it will dissolve. As the
  solution approaches saturation, the rate of
  dissolution will decrease.

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MINERAL DISSOLUTION - II

• Mineral dissolution rates also depend on surface
  area. Greater surface area leads to faster
  dissolution.
• It is also often found that experimentally
  measured rates of dissolution appear to decrease
  with time after the start of the experiment. Why?
• Partially leached layer on surface retards the
  migration of ions through the altered layer.
• Initial rapid rate due to higher dissolution rate
  of smaller particles rate slows when smaller
  particles have been removed.

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MINERAL PRECIPITATION

• A nucleation barrier exists that must be overcome
  before small mineral precipitates will be stable
  and grow.
• This nucleation barrier means that mineral
  precipitation will not occur until a critical
  degree of supersaturation is attained.
• Some foreign substances can affect precipitation
  kinetics. For example, Mg, phosphate and organics
  can inhibit the precipitation of calcite.

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CRYSTAL NUCLEATION
A plot of the Gibbs free energy of an incipient
crystal precipitating from a saturated solution
as a function of the size of the crystal. At
small sizes the Gibbs free energy increases with
growth of the particle because of the effect of
surface area. After a critical size, surface
effects are no longer important and the stable
nucleus can grow.