Chapter 2 Coagulation section 1 Basic Mechanisms and Application

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Chapter 2 Coagulationsection 1Basic
Mechanisms and Application

• Presented by
• Meng Li

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Review

• Basic processes of water treatment

Coagulation
sedimentation
filtration
sedimentation
disinfection
Treated water
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Outlines of this class

• 1 basic concepts of coagulation
• 2 backgrounds for Alum chemistry
• 3 interaction between colloidal particles and
  coagulants
• 4 main factors of particles removal efficiency

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Basic concepts of coagulation

• 1 destabilization(?????)
• Particles lose their stability through charge
  neutralization
• 2 aggregation(??)
• After destabilization, particles collide with
  each other and form larger particles
• 3 coagulation(??)
• Destabilization Aggregation

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Backgrounds For Aluminum Chemistry

• Several Al coagulants are used widely for
  removing contaminants in the raw water AlCl3,
  Al2(SO4)3.nH2O and PACl.
• The classes of Al species can be divided into
  four categories Monomers and small polymers
  such as Al and Al(OH)4- medium polymers, such as
  Al7(OH)174 large polymers, such as Al13(OH)347
  and Al(OH)3 Solids precipitate. And the
  solubility of Aluminum in equilibrium with solid
  Al(OH)3 will be greatly depend on the surrounding
  PH and the concentration of Aluminum.

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Background for colloidal particles

• Average particle size
• 10-9 m
• High charge density
• Small molecular weight
• Hard to be removed

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Backgrounds for colloidal particles(2)

• Carboxyl(??) and phenolic groups(??) provide most
  of the negative charge that adds to the mobility
  of colloidal particles in the raw water.
• Apparently water quality problems related to
  these colloidal particles are color, taste and
  odor, corrosion and biological activity in water
  distribution systems, mobilization of toxic
  elements and micro-pollutants, and the formation
  of disinfection by-products such as THMs.
• Humic substances have a great tendency to adsorb
  to particles and increase their charges and
  stability.

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Basic machanisms of coagulation

• Two important mechanisms for colloidal particles
  removal are
• (1) Charge neutralization-precipitation(CNP). The
  CNP consists of the reaction between soluble Al
  cationic polymers and soluble anionic particles,
  that is to say, adsorption of positively charged
  coagulant species to sites on the negatively
  charged fulvic acid to obtain charge
  neutralization and formation of insoluble
  complexes. Obviously there exists a chemistry
  stoichimetry(????) between coagulant dosage and
  particles and re-stabilization region resulting
  from charge reversal should be possible upon
  overdosing. This mechanisms is generally more
  dominant during the low PH value.

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• (2) Adsorption and simultaneous precipitate( or
  sweep coagulation), that is to say, adsorption of
  colloidal particles to, or enmeshment in, Alum
  hydroxide precipitates. The bonds that form can
  include van der walls interactions, H-bonding,
  hydrophobic bonding and ligand exchange. And
  simultaneous precipitation can be considered to
  be the simultaneous reaction of FA particles with
  both soluble Al polymers and Al(OH)3. This
  mechanism is more dominant at higher PH value.

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Aluminum speciation diagram for AlCl3
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• From this picture
• The data cover a PH range from 4.75 to 8.0. At a
  low PH, monomeric and small polymeric dominate.
  Then, as the PH increases, the medium polymers
  become dominant at P H 6-6.5. Concurrently,
  Al(OH)3 increases while the concentration of
  predominant Al species is Al(OH)3. As PH 8 is
  approached, an increase in monomers is observed
  along with a decrease in the Al(OH)3 due to the
  presence of Al(OH)3 at higher PH.

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Aluminum speciation diagram for Alum Sulfate
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• From this picture
• In terms of speciation diagram for alum sulfate,
  the Al species distribution is distinctly
  different from that for AlCl3. Monomer and small
  polymers concentration decrease sharply at PH 5.
  As PH increases to 6.9, the formation of Al(OH)3
  increases rapidly. The rapid formation of Al(OH)3
  is attributed to the presence of sulfate. Sulfate
  causes Al(OH)3 to be formed at lower PH values.

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Aluminum speciation diagram for PACl
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From this picture 1 The monomer and small
polymer are dominant at low PH and medium polymer
are predominant at PH5.0-6.6. Beyond PH6.6 an
Al(OH)3 precipitates forms and then the
concentration of Al(OH)3 exceeds that of the
medium polymers. 2 The distribution of Al
species between PH 5.5 and 6.5 is representative
of the initial concentrated Al solution.at lower
PH,some small and medium polymers seem to be
dissolved to form monomers.
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Discussion

• 1 The mechanisms of colloidal particles removal
  are directly related to the formation of Al
  speciation in solution. Charge neutralization-prec
  ipitation occurs in the presence of Al monomers
  and polymers. Adsorption takes place when Al(OH)3
  has precipitated from solution. At higher PH
  value at which both Al(OH)3 and Al polymers
  coexist, concurrent reactions by both mechanisms
  appear to cause simultaneous precipitation.

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• 2 Different Al coagulants have been shown to have
  different Al species distributions over a wide
  range of PH, which cause different mechanisms of
  particles removal under the same conditions. So
  analyzing Al species is useful by providing
  insights into the reactions mechanisms between
  colloidal particles and chemicals in the
  suspensions.

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• 3 The use of PACl has been shown to be beneficial
  at low particle concentrations and low PH,
  producing better removals of FA than the other
  kinds of coagulants.
• 4 The addition of Ca during water treatment may
  be a cost-effective means of improving particle
  removal.
• 5 For particles from different sources, there
  does exist different removal effects. The
  influencing factors may include particle
  molecular weight, charge density and components
  and structures of particles.

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Main content of this class

• 1 basic theory of coagulation
• 2 influencing factors of coagulation process
• 3 interaction between colloidal particles and
  coagulants