Flocculation of clays by Combining Vatt and Vrep

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Flocculation of clays by Combining Vatt and Vrep
Yaacov Kapiluto
Dept. of Plants, Soils Biometeorology, Utah
State University, Logan, UT
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Introduction

• Clay is an important soil component, which
  present interesting properties like high surface
  area and charge.
• Flocculation of clay is a very important
  phenomena for systems like agriculture and
  environment.

Objectives

• To introduce the forces and the conditions that
  cause flocculation.
• To present a worked example, that combine these
  forces and conditions.

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Flocculation Example
NaCl
CaCl2
0.0001(N)
0.001(N)
0.01(N)
0.05(N)
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Long chains of polymers in two different
solutions. The right picture before
flocculation, the left picture after
flocculation.
Dispersed
Flocculate
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Flocculation
Flocculation test results showing (L to
R)untreated, optimized
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For parallel flat particle there are two types of
forces

• 1) Repulsive
• a) Electrostatic force
• F is the electrical potential of a charged
  particle
• h is distance from the charged particle
• ? is the charge density
• b) Osmotic force
• is the osmotic pressure
• 2) Attractive
• a) Van der Waals forces
  
• A is Hamaker constant

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DVLO Theory
In 1940 Four scientists, two from Russia
Derjaguim and Landau and two from Holland Verway
and Overbeek developed independently a theory
that is related to colloidal stability to the
balance of long-range attractive and double layer
repulsive forces. The name DVLO came from the
initial letters of their names. .
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DVLO (continued)
The theory suggests that the total interaction
between two particles as a function of the
distance between them is simply the sum between
the attractive and the repulsive components
The potential is defined as
It is easy to write
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DVLO (continued)
As we can see the theory doesnt include the core
repulsive term for electron cloud overlap,
because the forces range is too short. In
general, it is not reliable when the distance
between the particles is much smaller than Debye
screening length.
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The potential equation for two flat particles is
Cio is is the Gouy-Chapman coefficient
Debye screening length unit of L-1
Please see appendix A
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Debye Screening Length
Its dimension is (m-1)
Z is the valence of ion
q is the electron charge
C is the concentration of the ion.
are the permittivity in free space and the
relative dielectric, respectively.
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Total interaction energy VT obtained by the sum
of Vrep and Vatt with two different ions
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With help of DVLO theory we can determine under
which conditions flocculation occurs.
This situation occurs while the total potential
is maximum
For parallel flat plate we get
At the maximum point
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From this derivation we pick two important values
The maximum distance between parallel plates, in
order to flocculate is
The critical flocculation concentration is
(To get the CFC term insert the value of h to the
expand equation and write the concentration as a
function of Debye screening length.)
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Exercise
Muscovite mica is a charged clay particle with
one change per every 50?2. Calculate the surface
charge density.
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Solution
Our knowledge of Muscovite structure enables us
to calculate the number of charged site per unit
area. One charge per 50?2 means that
Assuming that each negatively charged site
corresponds to an adsorption site, the surface
charge density is
(Note Please see in the appendix B another
example)
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Exercise (continued)

• Use the charge density that we calculated. In a
  solution of
• 0.1(M) NaCl, or
• 0.1(M) CaCl2 .
  
  We place two identical particles of
  Muscovite mica at a distance of 5 nm.
  
  According to your
  knowledge describe what will happen in every
  system.

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First we will calculate Hamaker constant for two
identical parallel plates
J
Van der Waals potential is
J/m2
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The second step in the question is to calculate
the repulsive force the term is conclude the
osmotic potential and the electric potential.
Repulsive potential for NaCl is
J/m2
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Repulsive potential for CaCl2
J/m2
With a very simple calculation, if we will sum
the potentials, in the solution of NaCl the
potential will be negative and the particles will
repulse each other, while in CaCl2 the potential
will be positive and the particles will
flocculate.