FLOCCULATION

FLOCCULATION

Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no formation of a cake, since all the flocs are in the suspension.
Flocculation by definition means a process in which individual particles of a suspension form aggregates. In the water treatment industry, the terms coagulation and flocculation imply different mechanisms. Flocculants consist of various molecular weight anionic, nonionic and cationic polymers. They are used to increase the efficiency of settling, clarification, filtration and centrifugation operations.
There are many types of flocculants and flocculents including flocculant agents and flocculant chemicals. You can also use various flocculant aids such as pH adjustment.
(Flocculants are the chemicals or substances that facilitate flocculation or floccing up of suspended solids in liquid, the resulting floc or wooly mass itself is called flocculent with an "e" however sometimes the terms are used interchangeably.)

FLOCCULATION
FLOCCULATION is used to describe the action of polymeric materials which form bridges between individual particles. Bridging occurs when segments of a polymer chain adsorb on different particles and help particles aggregate. Flocculants carry active groups with a charge which will counterbalance the charge of the particles. Flocculants adsorb on particles and cause destabilization either by bridging or charge neutralization.
An anionic flocculant will usually react against a positively charged suspension (positive zeta potential). That is the case of salts and metallic hydroxides.
A cationic flocculant will react against a negatively charged suspension (negative zeta potential) like silica or organic substances.
However the rule is not general. For example, anionic flocculants agglomerate clays which are electronegative.
Three groups of flocculants are currently used
1.1 - MINERAL FLOCCULANTS
They are colloidal substances. Adsorption and charge neutralization play some part in the flocculation mechanism. They are:
• activated silica.
• certain colloidal clays (such as bentonite),
• certain metallic hydroxides with a polymeric structure (alum, ferric hydroxide)
1.2- NATURAL FLOCCULANTS
They are water soluble anionic, cationic or nonionic polymers. Nonionic polymers adsorb on the suspended particles. The most common natural flocculants are:
• the starch derivatives: mostly pregelatinized hence water-soluble. They are corn or potato-starches. They can be natural starches, anionic oxidized starches or amine treated cationic starches. The use of this class of products has decreased in water treatment but remains important in the paper industry.
• the polysaccharides: usually guar gums and mostly used in acid medium.
• the alginates: anionic and used in potable water treatment.
1.3- SYNTHETIC FLOCCULANTS
Polyacrylamides
The most common polymers are those based on polyacrylamide, which is a nonionic polymer. Their effect is due to bridging between particles by polymer chains.
Polymers can be given anionic character by copolymerizing acrylamide with acrylic acid. Cationic polymers are prepared by copolymerizing acrylamide with a cationic monomer. All available acrylamide based polymers have a specific amount of ionic monomer giving a certain degree of ionic character.
They have a specific average molecular weight (i.e. chain length) and a given molecular distribution.
For each suspension, a certain degree of anionic, cationic or nonionic character is beneficial. Usually, the intrinsic flocculating power increases with the molecular weight.
Polyacrylamides have the highest molecular weight among the synthesized industrial chemicals in the range of 10-20 millions. Other polymers display specific properties and are used under specific conditions.
They are mostly:
• Polyethylene-imines
• Polyamides-amines
• Polyamines
• Polyethylene-oxide
• Sulfonated compounds

POLYELECTROLYTES

Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions (water),
making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (high molecular weight compounds), and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches[1] to describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while their unique properties are being exploited in a wide range of technological and industrial fields. One of their major roles, however, seems to be the one played in biology and biochemistry. Many biological molecules are polyelectrolytes. For instance, polypeptides , glycosaminoglycans, and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
A major focus in our research group over the years has concerned the basic
physical properties of polyelectrolytes in solution; scattering, interactions,
conformations, and hydrodynamics. In the past it was often thought that polyelectrolyte
solutions were almost impossible to measure reproducibly by light scattering, especially
at low ionic strength. While it is true that the scattering from polyelectrolyte solutions
decreases dramatically as ionic strength decreases, which can lead to artifacts in solutions
which are either not in equilibrium or poorly prepared, advances in modern light
scattering practice and sample preparation technology have made possible high quality,
quantitative scattering measurements.

 

 


 

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