13.3 Mechanism of Coagulation/Flocculation Process
Although the coagulation/flocculation process follows a complicated method to be
accomplished, four mechanisms have been found to more appropriately describe the
whole process (Nharingo and Moyo 2016). These four mechanisms are as follows:
1. Electrical double layer (EDL) compression. The EDL plays an important role in
the settling of the colloidal particles, primarily when the ionic strength of the
solution is raised. The EDL constitutes a layer of cations bound around the
negative charge particle surface and a diffuse layer encompassing a set of cations
and anions that extend out into the solution. The ionic strength in surrounding
water affects the decay function of the electrostatic potential (Sawyer et al. 2003).
In other words, it can be inferred that high ionic concentration in surrounding
water compresses the diffuse layer towards the surface of the colloid. As a result,
the diffused layer is sufficiently compressed, and the net attractive force, van der
Waals force, will predominantly prevail across the entire area of influence, and no
energy barrier will exist. This ionic layer compression occurs in nature when
water containing high turbidity increases the ionic content and thereby resulting
in the settling of the particles (Ghernaout et al. 2015). Formation of delta around
the oceans represents an excellent example of ionic compression occurring in
nature (Miller et al. 2008).
2. Neutralization of charge and adsorption. Since most colloidal particles of surface
water are negatively charged, the process of charge neutralization occurs by
adsorption of positively charged cations or polymers. The metal salts or cationic
organic polymers get ionized in water and produce cationic ions that interact with
the negatively charged surface of the particles and neutralize them, leading to
destabilization of the particles (Ghernaout et al. 2015). Here, the dose of the
metallic coagulants and cationic polymers plays an essential role in the destabilization of colloidal particles. With the properly optimized dose, the particles
come together, but if the dose exceeds the optimum level, the particles, instead of
being neutralized, will attain the re-stabilization state (Sawyer et al. 2003).
3. Interparticle bridging. When non-ionic polymers or long-chain low-surface
charge polymers are added alone or in addition with metallic salts, they dissociate
and form larger molecules. These polymers might have a linear or branched
structure having high surface reactivity. Thus, a particle gets adsorbed on the
chain of one polymer, and other available active surface sites of other particulates
may adsorb on the remainder of the polymers. As a result, the polymer-colloid
groups may form enmeshment, leading into the formation of the bridge between
the particles (Duan and Gregory 2003) and, thus, make the particles heavier that
settles down the flocs. Again, the dose of the polymers plays a key role in the
formation of the enmeshed polymer matrix. Overdosing of polymers may further
lead to re-stabilization of the colloidal particles. Aggressive mixing or extended
agitation can also break the interparticle bridging of polymer-colloid matrix and
leads to re-stabilization of the colloids.
13 Role of Coagulation/Flocculation Technology for the Treatment of Dye. . .
307
Although the coagulation/flocculation process follows a complicated method to be
accomplished, four mechanisms have been found to more appropriately describe the
whole process (Nharingo and Moyo 2016). These four mechanisms are as follows:
1. Electrical double layer (EDL) compression. The EDL plays an important role in
the settling of the colloidal particles, primarily when the ionic strength of the
solution is raised. The EDL constitutes a layer of cations bound around the
negative charge particle surface and a diffuse layer encompassing a set of cations
and anions that extend out into the solution. The ionic strength in surrounding
water affects the decay function of the electrostatic potential (Sawyer et al. 2003).
In other words, it can be inferred that high ionic concentration in surrounding
water compresses the diffuse layer towards the surface of the colloid. As a result,
the diffused layer is sufficiently compressed, and the net attractive force, van der
Waals force, will predominantly prevail across the entire area of influence, and no
energy barrier will exist. This ionic layer compression occurs in nature when
water containing high turbidity increases the ionic content and thereby resulting
in the settling of the particles (Ghernaout et al. 2015). Formation of delta around
the oceans represents an excellent example of ionic compression occurring in
nature (Miller et al. 2008).
2. Neutralization of charge and adsorption. Since most colloidal particles of surface
water are negatively charged, the process of charge neutralization occurs by
adsorption of positively charged cations or polymers. The metal salts or cationic
organic polymers get ionized in water and produce cationic ions that interact with
the negatively charged surface of the particles and neutralize them, leading to
destabilization of the particles (Ghernaout et al. 2015). Here, the dose of the
metallic coagulants and cationic polymers plays an essential role in the destabilization of colloidal particles. With the properly optimized dose, the particles
come together, but if the dose exceeds the optimum level, the particles, instead of
being neutralized, will attain the re-stabilization state (Sawyer et al. 2003).
3. Interparticle bridging. When non-ionic polymers or long-chain low-surface
charge polymers are added alone or in addition with metallic salts, they dissociate
and form larger molecules. These polymers might have a linear or branched
structure having high surface reactivity. Thus, a particle gets adsorbed on the
chain of one polymer, and other available active surface sites of other particulates
may adsorb on the remainder of the polymers. As a result, the polymer-colloid
groups may form enmeshment, leading into the formation of the bridge between
the particles (Duan and Gregory 2003) and, thus, make the particles heavier that
settles down the flocs. Again, the dose of the polymers plays a key role in the
formation of the enmeshed polymer matrix. Overdosing of polymers may further
lead to re-stabilization of the colloidal particles. Aggressive mixing or extended
agitation can also break the interparticle bridging of polymer-colloid matrix and
leads to re-stabilization of the colloids.
13 Role of Coagulation/Flocculation Technology for the Treatment of Dye. . .
307