flocculate

Flocculation is, in the field of chemistry, a process where colloids come out of suspension in the form of floc or flakes. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution.

Term definition

According to the IUPAC definition, flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters." Flocculation is synonymous with agglomeration and coagulation.^[1]

Applications

Surface chemistry

In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or can be readily filtered from the liquid.

Physical chemistry

For emulsions, flocculation describes clustering of individual dispersed droplets together, whereby the individual droplets do not lose their identity.^[2] Flocculation is thus the initial step leading to further aging of the emulsion (droplet coalescence and the ultimate separation of the phases).

Civil engineering/earth sciences

In civil engineering, and in the earth sciences, flocculation is a condition in which clays, polymers or other small charged particles become attached and form a fragile structure, a floc. In dispersed clay slurries, flocculation occurs after mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractions between negative face charges and positive edge charges.

Biology

In biology, the process is used to refer to the asexual aggregation of microorganisms, most commonly brewing yeast at the end of a brew.

Water treatment

Flocculation and sedimentation are widely employed in the purification of drinking water as well as sewage treatment, stormwater treatment and treatment of other industrial wastewater streams.

Flocculants

Particles finer than 0.1 µm (10^-7m) in water remain continuously in motion due to electrostatic charge (often negative) which causes them to repel each other. Once their electrostatic charge is neutralized by the use of coagulant chemical, the finer particles start to collide and agglomerate (combine together) under the influence of Van der Waals's forces. These larger and heavier particles are called flocs.

Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. Flocculants are used in water treatment processes to improve the sedimentation or filterability of small particles. For example, a flocculant may be used in swimming pool or drinking water filtration to aid removal of microscopic particles which would otherwise cause the water to be turbid (cloudy) and which would be difficult or impossible to remove by filtration alone.

Many flocculants are multivalent cations such as aluminium, iron, calcium or magnesium.^[3] These positively charged molecules interact with negatively charged particles and molecules to reduce the barriers to aggregation. In addition, many of these chemicals, under appropriate pH and other conditions such as temperature and salinity, react with water to form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping small particles into the larger floc.

Long-chain polymer flocculants, such as modified polyacrylamides, are manufactured and sold by the flocculant producing business. These can be supplied in dry or liquid form for use in the flocculation process. The most common liquid polyacrylamide is supplied as an emulsion with 10-40% actives and the rest is a carrier fluid, surfactants and latex. Emulsion polymers require activation to invert the emulsion and allow the electrolyte groups to be exposed.

The following chemicals are used as flocculants:^{[citation needed]}

• alum
• aluminium chlorohydrate
• aluminium sulfate
• calcium oxide
• calcium hydroxide
• iron(III) chloride
• iron(II) sulfate
• polyacrylamide
• polyDADMAC
• sodium aluminate
• sodium silicate

The following natural products are used as flocculants:^{[citation needed]}

• Chitosan
• Moringa oleifera seeds
• Papain
• A species of Strychnos (seeds)
• Isinglass

Deflocculation

• A deflocculant is a chemical that is added to prevent a colloid from coming out of suspension. (In this case deflocculation is a desired effect).
  
• Deflocculation is also used to describe the undesired effect in an activated sludge basin if the sludge is subjected to high-speed mixing. Generally, deflocculation can be prevented or reduced by applying gentle mixing(e.g., by using submersible propeller mixers that utilize large/wide propeller blades and operate at low rotational speed).

See also

• Aggregation
• Algaculture
• Clay-water interaction
• Coagulation (disambiguation)
• Deflocculant
• Drilling mud
• Isoelectric point
• Lamella clarifier
• Saltwater flow
• Sedimentation
• Soil structure
• Yeast flocculation

References

Further reading

• John Gregory (2006) Particles in water: properties and processes, Taylor & Francis, ISBN 1-58716-085-4

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