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
(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 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
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
For each suspension, a certain degree of anionic, cationic or nonionic character
is beneficial. Usually, the intrinsic flocculating power increases with the
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:
• Sulfonated compounds
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 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,
at low ionic strength. While it is true that the scattering from polyelectrolyte
decreases dramatically as ionic strength decreases, which can lead to artifacts
which are either not in equilibrium or poorly prepared, advances in modern light
scattering practice and sample preparation technology have made possible high
quantitative scattering measurements.