Coagulation - Flocculation

Principles.
Ion distribution around a colloidal particle (Gouy-Chapman Theory ) :
Louis Georges Gouy in 1910 and David Leonard Chapman in 1913 both observed that capacitance was not a constant and that it depended on the applied potential and the ionic concentration. The "Gouy-Chapman model" made significant improvements by introducing a diffuse model of the DL. In this model the charge distribution of ions as a function of distance from the metal surface allows Maxwell–Boltzmann statistics to be applied. Thus the electric potential decreases exponentially away from the surface of the fluid bulk.

Gouy-Chapman model fails for highly charged DLs. In 1924 Otto Stern suggested combining Helmholtz with Gouy-Chapman. In Stern's model, some ions adhere to the electrode as suggested by Helmholtz, giving an internal Stern layer, while some form a Gouy-Chapman diffuse layer. The Stern layer accounted for ions' finite size and consequently ions' closest approach to the electrode is on the order of the ionic radius. The Stern model had its own limitations, effectively modeling ions as point charges, assuming all significant interactions in the diffuse layer are Coulombic, assuming dielectric permittivity to be constant throughout the double layer and that fluid viscosity is constant above the slipping plane.

with,
. Center: colloidal ion;
. Outside: free ions and electric charge equivalent to that of the opposite ion and colloidal sign forming two layers:

• Ions walls (dense layer or Helmholtz Stern layer) indissoluble colloidal ion that exhibit still lower than that of the colloidal filler ion (the energy level at that distance is characterized by the zeta potential),
• Counter-ions (Gouy-Chapman layer) forming a diffuse layer at a greater distance from the colloidal ion in equilibrium with the solvent, with a progressive depletion of ions of opposite sign to that of ion colloidal, accompanied a corresponding enrichment in ions of the other sign.
  

Reminders about the nature of the colloidal particles and the stability of colloidal dispersions.
The vast majority of natural waters (surface water especially ), contains impurities that affect their appearance and may have adverse effects for the consumer. These solids can be roughly classified as follows:

• dissolved impurities (dissolved mineral salts, organic molecules ...)
• colloids
• suspended solids.

Actually, this covers the classification of particle size intuitively relevant grasped that is the most difficult to remove the finer particles. To this notion of dimension, there are two other more important are those of the surface / volume of colloidal particles which gives them very pronounced adsorption properties and that of their surface charge report.
This charge may come from:

• dissociation of the ionizable groups within the colloid itself,
• of the ions contained in the adsorption solution,
• an isomorphic substitution (replacement of a macromolecular atom with another atom of valence different network); this is the general case of clays.

For their study, colloids are classified arbitrarily into two main groups;

• hydrophilic colloids they originate soluble substances having affinity for water, and adsorbed water of a protective layer bonded;
  In this group are the proteins, carbohydrates and, in general, organic products.
• hydrophobic colloids generally larger and have no protective layer.
  They originate from sparingly soluble or insoluble substances such as metals, their oxides and / or hydroxides.

More generally, these are mineral products.

The existence of colloidal systems depends on the interaction between two particles. It involves two opposing forces:

• repulsive force which tends to separate the particles from one another. This force depends on the charge of the particles which have the same sign;
• force of attraction (van der Waals) which tends to collect the particles to reach the minimum potential energy. This force is depending on the distance between particles of course.

Destabilization of colloids.
To overcome the repulsive forces, two possibilities can be theoretically considered:

• increasing the kinetic energy of the particles (technically inapplicable in practice);
• modification of the resultant of forces involved

Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.

• Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general, different phases have different charge affinities, so that an electrical double layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids.
  In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too weak to overcome the electrostatic repulsion between charged layers of the dispersing phase.
• Steric stabilization consists in covering the particles in polymers which prevents the particle to get close in the range of attractive forces.

A combination of the two mechanisms is also possible (electrosteric stabilization). All the above mentioned mechanisms for minimizing particle aggregation rely on the enhancement of the repulsive interaction forces.
Under these conditions, the particles are close enough to each other so that the van der Waals forces become predominant.
The process leading to this result is usually called coagulation..

Destabilization (flocculation).
Unstable colloidal dispersions can form flocs as the particles aggregate due to interparticle attractions. In this way photonic glasses can be grown. This can be accomplished by a number of different methods:

• Removal of the electrostatic barrier that prevents aggregation of the particles. This can be accomplished by the addition of salt to a suspension or changing the pH of a suspension to effectively neutralize or "screen" the surface charge of the particles in suspension. This removes the repulsive forces that keep colloidal particles separate and allows for coagulation due to van der Waals forces.
• Addition of a charged polymer flocculant. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively-charged colloidal silica or clay particles can be flocculated by the addition of a positively-charged polymer.
• Addition of non-adsorbed polymers called depletants that cause aggregation due to entropic effects.
• Physical deformation of the particle (e.g., stretching) may increase the van der Waals forces more than stabilization forces (such as electrostatic), resulting coagulation of colloids at certain orientations.

Unstable colloidal suspensions of low-volume fraction form clustered liquid suspensions, wherein individual clusters of particles fall to the bottom of the suspension (or float to the top if the particles are less dense than the suspending medium) once the clusters are of sufficient size for the Brownian forces that work to keep the particles in suspension to be overcome by gravitational forces. However, colloidal suspensions of higher-volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels, such as bentonite and toothpaste, flow like liquids under shear, but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.

The coagulating nature of reagents.
Theoretically, any type of electrolyte can be used for coagulation / flocculation of colloid. However, it is recognized, for many years, that the efficiency of a coagulant increases dramatically with the oxidation number (valence) of the cation of the electrolyte.
This is why the aluminum salts and ferric iron containing cations of oxidation number +3 are used almost exclusively in the coagulation / flocculation of water.
The salts of aluminum and iron present in addition to their higher valency, are of interest to hydrolysis in the usual pH range of natural waters, giving an insoluble hydroxide precipitates.
We know, however, recently, that is not the trivalent ion that plays an essential role, but the intermediate hydrolysis products formed.
Aluminum salts of iron and lead in effect, in dilute solution, the following hydroxo-complexes:

(1) X⁺⁺⁺ + OH^- << >> X (OH)⁺⁺

(2) X (OH)⁺⁺ + OH^- << >> X (OH)⁺₂

(3) X (OH)⁺₂ + OH- << >> X(OH)₃

(4) X(OH)₃ + OH- << >> X(OH)^-_{4
where OH = hydroxyl ions, X = Al or Fe}

Although equilibrium, these reactions are substantially complete in the usual pH of the waters, and irreversible at least for the first three.
These hydrolysis products have a very strong tendency to polymerize to give the first (polyhydroxo complexes) and soluble macromolecular colloidal buildings, and insoluble hydroxides finally.
The relative proportion of each of these complexes depend on the environmental conditions (pH, temperature, concentration).
It has been shown that it may be preferable to use an aluminum salt partially neutralized with sodium hydroxide under conditions to promote the formation of a particular complex before injection into the water.
This technique is among others responsible for the marketing of WAC (Water Aluminium Chloride) and PCBA (Polyvinyl Chloride Basic Aluminum).

Influençants flocculation parameters:
There is a pH optimum flocculation for water type and a given flocculant: this pH result from the raw water, the action of the coagulant which consumes OH^- ions and the possible introduction of a reactive controller.
The usual pH values in natural waters, not existing OH^- free ions, they result from the decomposition of bicarbonate (HCO3^-) by the flocculant (and formation of carbon dioxide CO2):

reaction which is superimposed the balance of calcium bicarbonate with its dissociation products.

The overall type of coagulation equation can be written:

therefore, precipitation of a metal hydroxide X(OH)₃ and CO₂ formation.

Recall CO₂ + H₂O <<< >>> H₂CO₃ (carbonic acid).

The appearance of carbonic acid and the subsequent decrease in bicarbonate alkalinity leads to a lowering of the pH of the medium whose magnitude depends on the buffering capacity (initial alkalinity and ionic strength), and the introduced coagulant dose. For this reason, it is essential to consider the pH, alkalinity and mineralization of raw water for the process definition coagulation / flocculation.

Example with the use of aluminum as coagulant:

(MCL = maximum contaminant level)

It may be necessary to obtain a correct flocculation, to adjust the pH of the water by adding either an acid or a base (sodium hydroxide, lime, sodium carbonate).

Other general factors involved in the coagulation-flocculation, are following:

• Complexion of color and turbidity, and relative proportion in the raw water.
  Recall that there is no continuity between measures of color and turbidity, and only the particle size and to some extent their nature, allow to establish an arbitrary classification.
  The large particle size and inorganic nature (clays) are the turbidity.
  Small particles, and certain soluble compounds of organic nature, are responsible for the color.
  The flocculation of the humic acids requires high doses of coagulant proportional to their concentration in water approximately.

  On the contrary, the inorganic colloids are easily flocculated by low doses of coagulant unrelated intensity turbidity.

• Temperature :
  Like any chemical reaction, the flocculation is accelerated by temperature rise. Flocculation cold water is slower and less complete than hot water.
  Furthermore, the rapid increase of viscosity (of water) when the temperature is lowered (the value of the viscosity changes from 1 to 2 between 0 and 25 degrees), against the cheek of the frequency of contact particles in the phase orthokinetic flocculation and decreases the effective number of shocks.
  Finally, the viscosity hinders the settling phenomenon.
  

Main reagents coagulants.
(click on the name for more information)
Aluminum sulphate (or alum) :
Aluminum hydroxides are formed within a fairly narrow pH range, typically: 5.5 to about 7.7 (Optimum 6)
Empirical formula: Al₂(SO₄)₃, n H₂O
(commercial product n = 14, n = 18 per pure product)

It is available in various solid forms (depending on the manufacturer):

• Crushed pieces of 10 cm wide and about 1 cm thick,
• "nutty", particle size of between 2 and 3 and 12 mm or 35 mm,
• powder, common particle size of 0.1 to 3 mm
• aqueous solution of 8.2% Al2O3 (alumina or aluminum oxide)

Ferric chloride (or iron(III) chloride) :
To form over a larger pH range including pH levels lower than are effective for alum, typically: 5.0 to 8.5 (Optimum 8).

Formula: FeCl₃ (pure product).
For the treatment of water, it is only used as an aqueous solution of about 592 g as FeCl3 /L
(41 wt%).
It is stored in tanks, in containers or tanks.
Chlorosulfate ferric:
Its empirical formula is FeClSO₄.
For the treatment of water, it is only used in the form of aqueous solution of about 200 g as FeClSO₄ /L.
(about 41% by weight).
It is stored in tanks, containers or tanks : PVC materials, polyethylene, polypropylene, acid resistant rubber, glass or ceramic.

WAC (PAC or polyaluminium chloride) :
This is a basic aluminum polychloride with an empirical formula: Aln (OH) m-Cl3n m (wherein m/3n is comprised between 0.45 and 0.60).
It is in the form of a liquid with a content of A1203 is about 10%.
The pH range for use is from 6 to 7.5 (optimum 6.5).
It is stored in tanks.

other:

• Aqualenc(polyaluminium chloride hydroxide sulfate)
  polyDADMAC or polyDDA (shortened) : Polydiallyldimethylammonium chloride.

Also include sodium aluminate, alone or in conjunction with alum or ferric chloride, ferric sulfate or ferrous sulfate.


>>links to French official "bulletin" : Circulaire DG 5/VS 4 n° 2000-166 of 28 March 2000, in relation to the products of treatment processes (water intended for human consumption)

Choice of reagents and determination of treatment rates.

• Choice of coagulant:
A number of parameters must be considered:

• Water temperature,
• Characteristics of raw water (including calcocarbonic balance)
• Physico-chemical parameters to include or eliminate priority (turbidity and / or Organic Materials)
• Operations management (stocks, automation, etc ...)
• Product cost,
• Forced choice or "aesthetic considerations".

• Purposefulness of treatment rates:
Coagulation and flocculation are complex phenomena influenced by many parameters:

• water quality (physico-chemical characteristics)
• nature and structure of colloids
• nature and implementation of used product).

also the safest and most effective method to determine in each case the nature and quantity of reagent to be used, will be based on experimentation.
The method which reproduces the small-scale overall process of flocculation-coagulation is the so-called JAR TEST, used in laboratory.
In this research the best possible result (which must also take into account economic considerations), the experience of man shall be assisted by the flocculation test laboratory (JAR-TEST), and possibly by implementation of pilot test.

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