Flocculation

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IUPAC definition [1]

Flocculation (in polymer science): Reversible formation of aggregates in which the particles are not in physical contact.

Contents

Agglomeration (except in polymer science)
Coagulation (except in polymer science)
Flocculation (except in polymer science)
Process of contact and adhesion whereby dispersed molecules or particles are held together by weak physical interactions ultimately leading to phase separation by the formation of precipitates of larger than colloidal size.

  • In contrast to aggregation, agglomeration is a reversible process.
  • The definition proposed here is recommended for distinguishing agglomeration from aggregation. The particles that comprise agglomerates can be dispersed again.
  • This quotation is from the Purple Book (Compendium of Polymer Terminology and Nomenclature: IUPAC Recommendations, 2008). [2]
4x speed video of floc settling after adding flocculant polymers during a jar test.

In colloidal chemistry, flocculation is a process by which colloidal particles come out of suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended, under the form of a stable dispersion (where the internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation) and are not truly dissolved in solution.

Coagulation and flocculation are important processes in fermentation and water treatment with coagulation aimed to destabilize and aggregate particles through chemical interactions between the coagulant and colloids, and flocculation to sediment the destabilized particles by causing their aggregation into floc.[ clarification needed ]

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/coalescence. [3] [4]

Basically, coagulation is a process of addition of coagulant to destabilize a stabilized charged particle. Meanwhile, flocculation is a technique that promotes agglomeration and assists in the settling of particles. The most common used coagulant is alum, Al2(SO4)3·14H2O.

The chemical reaction involved:

Al2(SO4)3 · 14 H2O → 2 Al(OH)3(s) + 6 H+ + 3 SO2−
4 + 8 H2O

During flocculation, gentle mixing accelerates the rate of particle collision, and the destabilized particles are further aggregated and enmeshed into larger precipitates. Flocculation is affected by several parameters, including mixing shear and intensity, time and pH. The product of the mixing intensity and mixing time is used to describe flocculation processes.

Jar test

The process by which the dosage and choice of flocculant are selected is called a jar test. The equipment used for jar testing consists of one or more beakers, each equipped with a paddle mixer. After the addition of flocculants, rapid mixing takes place, followed by slow mixing and later the sedimentation process. Samples can then be taken from the aqueous phase in each beaker. [5]

Applications

Surface chemistry

In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid (creaming), settle to the bottom of the liquid (sedimentation), or be readily filtered from the liquid. Flocculation behavior of soil colloids is closely related to freshwater quality. High dispersibility of soil colloids not only directly causes turbidity of the surrounding water but it also induces eutrophication due to the adsorption of nutritional substances in rivers and lakes and even boats under the sea.

Physical chemistry

For emulsions, flocculation describes clustering of individual dispersed droplets together, whereby the individual droplets do not lose their identity. [6] Flocculation is thus the initial step leading to further ageing of the emulsion (droplet coalescence and the ultimate separation of the phases). Flocculation is used in mineral dressing, [7] but can be also used in the design of physical properties of food and pharmaceutical products. [8]

Medical diagnostics

In a medical laboratory, flocculation is the core principle used in various diagnostic tests, for example the rapid plasma reagin test. [9]

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

Flocculation is used in biotechnology applications in conjunction with microfiltration to improve the efficiency of biological feeds. The addition of synthetic flocculants to the bioreactor can increase the average particle size making microfiltration more efficient. When flocculants are not added, cakes form and accumulate causing low cell viability. Positively charged flocculants work better than negatively charged ones since the cells are generally negatively charged. [10]

Cheese industry

Flocculation is widely employed to measure the progress of curd formation in the initial stages of cheese making to determine how long the curds must set. [11] The reaction involving the rennet micelles are modeled by Smoluchowski kinetics. [11] During the renneting of milk the micelles can approach one another and flocculate, a process that involves hydrolysis of molecules and macropeptides. [12]

Flocculation is also used during cheese wastewater treatment. Three different coagulants are mainly used: [13]

Brewing

In the brewing industry flocculation has a different meaning. It is a very important process in fermentation during the production of beer where cells form macroscopic flocs. These flocs cause the yeast to sediment or rise to the top of a fermentation at the end of the fermentation. Subsequently, the yeast can be collected (cropped) from the top (ale fermentation) or the bottom (lager fermentation) of the fermenter in order to be reused for the next fermentation.

Yeast flocculation is partially determined by the calcium concentration, often in the 50-100ppm range. [14] Calcium salts can be added to cause flocculation, or the process can be reversed by removing calcium by adding phosphate to form insoluble calcium phosphate, adding excess sulfate to form insoluble calcium sulfate, or adding EDTA to chelate the calcium ions. While it appears similar to sedimentation in colloidal dispersions, the mechanisms are different. [15]

Water treatment process

Coagulation-flocculation process in a water treatment system The coagulation and filtration processes at a drinking water treatment plant. (14868618507).jpg
Coagulation-flocculation process in a water treatment system

Flocculation and sedimentation are widely employed in the purification of drinking water as well as in sewage treatment, storm-water treatment and treatment of industrial wastewater streams. For drinking water, typical treatment processes consist of grates, coagulation, flocculation, sedimentation, granular filtration and disinfection. [16] The coagulation and flocculation steps are similar, causing particles to aggregate and fall out of solution, but may use different chemicals or physical movement of water. [17] A variety of salts may be added to adjust the pH and act as clarifying agents, depending on the water chemistry. These include sodium hydroxide, calcium hydroxide, aluminum sulfate, aluminum oxide, ferric sulfate, ferric chloride, sodium aluminate, with flocculant aids polyaluminum chloride, polyferric chloride. [17] A variety of cationic, anionic, and non-ionic polymers are also used, typically with a molecular weight below 500,000. [17] Polydiallyldimethyl ammonium chloride (polyDADMAC) and epiDMA (a copolymer of epichlorohydrin and dimethylamine) are common choices, though these can produce carcinogenic nitrosamines. [17] Sand, powerdered activated carbon, and clay may also be used as nucleating agents; in some cases, these are re-used after extraction. [17]

Biopolymers, especially, chitosan, are increasingly popular as environmentally friendly flocculants. [18] Chitosan is not only biodegradable but also exhibits a unique ability to bind with a wide range of contaminants, including heavy metals and organic pollutants, effectively removing them from water sources. [19]

Flocculation provides promising results for removing fine particles and treating stormwater runoff from transportation construction projects, but are not used by most state departments of transportation in the U.S. This may be due to regulative restrictions or insufficient guidance for soil sampling requirements in light of changing soil characteristics. States that must achieve a numeric turbidity limit are more inclined to use flocculants to ensure the appropriate level of treatment. [20]

Deflocculation

Deflocculation is the opposite of flocculation, sometimes known as peptization. Sodium silicate (Na2SiO3) is a typical example. Usually, in higher pH ranges, in addition to low ionic strength of solutions and domination of monovalent metal cations, the colloidal particles can be dispersed. [21] The additive that prevents the colloids from forming flocs is called a deflocculant. For deflocculation imparted through electrostatic barriers, the efficacy of a deflocculant can be gauged in terms of zeta potential. According to the Encyclopedic Dictionary of Polymers deflocculation is "a state or condition of a dispersion of a solid in a liquid in which each solid particle remains independent and unassociated with adjacent particles (much like emulsifier). A deflocculated suspension shows zero or very low yield value". [21]

Deflocculation can be a problem in wastewater treatment plants, as it commonly causes problems with sludge settling and deterioration of the effluent quality.

See also

Related Research Articles

<span class="mw-page-title-main">Colloid</span> Mixture of an insoluble substance microscopically dispersed throughout another substance

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture. A colloid has a dispersed phase and a continuous phase. The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

An emulsion is a mixture of two or more liquids that are normally immiscible owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid is dispersed in the other. Examples of emulsions include vinaigrettes, homogenized milk, liquid biomolecular condensates, and some cutting fluids for metal working.

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

<span class="mw-page-title-main">Water treatment</span> Process that improves the quality of water

Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.

In polymer chemistry, emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomers, and surfactants. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

<span class="mw-page-title-main">Suspension (chemistry)</span> Heterogeneous mixture of solid particles dispersed in a medium

In chemistry, a suspension is a heterogeneous mixture of a fluid that contains solid particles sufficiently large for sedimentation. The particles may be visible to the naked eye, usually must be larger than one micrometer, and will eventually settle, although the mixture is only classified as a suspension when and while the particles have not settled out.

In materials science, the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves conversion of monomers in solution into a colloidal solution (sol) that acts as the precursor for an integrated network of either discrete particles or network polymers. Typical precursors are metal alkoxides. Sol–gel process is used to produce ceramic nanoparticles.

Electrocoagulation (EC) is a technique used for wastewater treatment, wash water treatment, industrially processed water, and medical treatment. Electrocoagulation has become a rapidly growing area of wastewater treatment due to its ability to remove contaminants that are generally more difficult to remove by filtration or chemical treatment systems, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. There are many brands of electrocoagulation devices available, and they can range in complexity from a simple anode and cathode to much more complex devices with control over electrode potentials, passivation, anode consumption, cell REDOX potentials as well as the introduction of ultrasonic sound, ultraviolet light and a range of gases and reactants to achieve so-called Advanced Oxidation Processes for refractory or recalcitrant organic substances.

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewaters by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank basin. The released air forms tiny bubbles which adhere to the suspended matter, causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device.

<span class="mw-page-title-main">Particle aggregation</span> Clumping of particles in suspension

Particle agglomeration refers to the formation of assemblages in a suspension and represents a mechanism leading to the functional destabilization of colloidal systems. During this process, particles dispersed in the liquid phase stick to each other, and spontaneously form irregular particle assemblages, flocs, or agglomerates. This phenomenon is also referred to as coagulation or flocculation and such a suspension is also called unstable. Particle agglomeration can be induced by adding salts or other chemicals referred to as coagulant or flocculant.

A dispersion is a system in which distributed particles of one material are dispersed in a continuous phase of another material. The two phases may be in the same or different states of matter.

<span class="mw-page-title-main">Clarifier</span> Settling tanks for continuous removal of solids being deposited by sedimentation

Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thickening. Inside the clarifier, solid contaminants will settle down to the bottom of the tank where it is collected by a scraper mechanism. Concentrated impurities, discharged from the bottom of the tank, are known as sludge, while the particles that float to the surface of the liquid are called scum.

<span class="mw-page-title-main">Sediment control</span>

A sediment control is a practice or device designed to keep eroded soil on a construction site, so that it does not wash off and cause water pollution to a nearby stream, river, lake, or sea. Sediment controls are usually employed together with erosion controls, which are designed to prevent or minimize erosion and thus reduce the need for sediment controls. Sediment controls are generally designed to be temporary measures, however, some can be used for storm water management purposes.

The peptization of a liquid mixture is the process of converting the mixture into a colloid by shaking it with a suitable electrolyte called a peptizing agent. That is, the insoluble solid particles which have settled out of the mixture are reformed into microscopic particles suspended in the mixture. Peptization is the reverse of flocculation, the aggregation of colloidal particles into precipitate; as such, it is also known as deflocculation.

<span class="mw-page-title-main">PolyDADMAC</span> Chemical compound

Polydiallyldimethylammonium chloride, also commonly polyquaternium-6, is a homopolymer of diallyldimethylammonium chloride (DADMAC). The molecular weight of polyDADMAC is typically in the range of hundreds of thousands of grams per mole, and even up to a million for some products. PolyDADMAC is usually delivered as a liquid concentrate having a solids level in the range of 10 to 50%. It is a high charge density cationic polymer. The charge density makes it well suited for flocculation. Actually, pDADMAC or DMDAAC, is used as a coagulant - a charge neutralization process that precedes flocculation.

Clarifying agents are used to remove suspended solids from liquids by inducing flocculation, causing the solids to form larger aggregates that can be easily removed after they either float to the surface or sink to the bottom of the containment vessel.

Polyelectrolytes are charged polymers capable of stabilizing colloidal emulsions through electrostatic interactions. Their effectiveness can be dependent on molecular weight, pH, solvent polarity, ionic strength, and the hydrophilic-lipophilic balance (HLB). Stabilized emulsions are useful in many industrial processes, including deflocculation, drug delivery, petroleum waste treatment, and food technology.

A depletion force is an effective attractive force that arises between large colloidal particles that are suspended in a dilute solution of depletants, which are smaller solutes that are preferentially excluded from the vicinity of the large particles. One of the earliest reports of depletion forces that lead to particle coagulation is that of Bondy, who observed the separation or "creaming" of rubber latex upon addition of polymer depletant molecules to solution. More generally, depletants can include polymers, micelles, osmolytes, ink, mud, or paint dispersed in a continuous phase.

In polymer science, dispersion polymerization is a heterogeneous polymerization process carried out in the presence of a polymeric stabilizer in the reaction medium. Dispersion polymerization is a type of precipitation polymerization, meaning the solvent selected as the reaction medium is a good solvent for the monomer and the initiator, but is a non-solvent for the polymer. As the polymerization reaction proceeds, particles of polymer form, creating a non-homogeneous solution. In dispersion polymerization these particles are the locus of polymerization, with monomer being added to the particle throughout the reaction. In this sense, the mechanism for polymer formation and growth has features similar to that of emulsion polymerization. With typical precipitation polymerization, the continuous phase is the main locus of polymerization, which is the main difference between precipitation and dispersion.

<span class="mw-page-title-main">Coagulation (water treatment)</span> In water treatment, the addition of compounds that promote clumping

In water treatment, coagulation and flocculation involve the addition of compounds that promote the clumping of fine floc into larger floc so that they can be more easily separated from the water. Coagulation is a chemical process that involves neutralization of charge whereas flocculation is a physical process and does not involve neutralization of charge. The coagulation-flocculation process can be used as a preliminary or intermediary step between other water or wastewater treatment processes like filtration and sedimentation. Iron and aluminium salts are the most widely used coagulants but salts of other metals such as titanium and zirconium have been found to be highly effective as well.

References

  1. ↑ Slomkowski, Stanislaw; Alemán, José V.; Gilbert, Robert G.; Hess, Michael; Horie, Kazuyuki; Jones, Richard G.; Kubisa, Przemyslaw; Meisel, Ingrid; Mormann, Werner; Penczek, Stanisław; Stepto, Robert F. T. (2011). "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry . 83 (12): 2229–2259. doi:10.1351/PAC-REC-10-06-03. S2CID   96812603. Archived from the original (PDF) on 2013-10-20. Retrieved 2013-07-27.
  2. ↑ Richard G. Jones; Edward S. Wilks; W. Val Metanomski; Jaroslav Kahovec; Michael Hess; Robert Stepto; Tatsuki Kitayama, eds. (2009). Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008) "The Purple Book" (2nd ed.). RSC Publishing. ISBN   978-0-85404-491-7.
  3. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " flocculation ". doi : 10.1351/goldbook.F02429
  4. ↑ Hubbard, Arthur T. (2004). Encyclopedia of Surface and Colloid Science. CRC Press. p. 4230. ISBN   978-0-8247-0759-0 . Retrieved 2007-11-13.
  5. ↑ Operational Control of Coagulation and Filtration Processes (M37): AWWA Manual of Practice. American Water Works Association. 2011-06-01. ISBN   978-1583218013.
  6. ↑ Adamson A.W. and Gast A.P. (1997) "Physical Chemistry of Surfaces", John Wiley and Sons.
  7. ↑ Investigation of laws of selective flocculation of coals with synthetic latexes / P. V. Sergeev, V. S. Biletskyy // ICCS’97. 7–12 September 1997, Essen, Germany. V. 1. pp. 503–506.
  8. ↑ Fuhrmann, Philipp L.; Sala, Guido; Stieger, Markus; Scholten, Elke (2019-08-01). "Clustering of oil droplets in o/w emulsions: Controlling cluster size and interaction strength". Food Research International. 122: 537–547. doi: 10.1016/j.foodres.2019.04.027 . ISSN   0963-9969. PMID   31229109.
  9. ↑ Arora, Satyam; Doda, Veena; Rani, Sunita; Kotwal, Urvershi (2015). "Rapid plasma reagin test: High false positivity or important marker of high risk behavior". Asian Journal of Transfusion Science. 9 (1): 109. doi: 10.4103/0973-6247.150979 . ISSN   0973-6247. PMC   4339923 . PMID   25722593.
  10. ↑ Han, Binbing; Akeprathumchai, S.; Wickramasinghe, S. R.; Qian, X. (2003-07-01). "Flocculation of biological cells: Experiment vs. theory". AIChE Journal. 49 (7): 1687–1701. Bibcode:2003AIChE..49.1687H. doi:10.1002/aic.690490709. ISSN   1547-5905.
  11. 1 2 Fox, Patrick F. (1999). Cheese Volume 1: Chemistry, Physics, and Microbiology (2nd ed.). Gaithersburg, Maryland: Aspen Publishers. pp. 144–150. ISBN   978-0-8342-1378-4.
  12. ↑ Fox, Patrick F. (2004). Cheese - Chemistry, Physics and Microbiology (3rd ed.). Elsevier. p. 72. ISBN   978-0-12-263653-0.
  13. ↑ Rivas, Javier; Prazeres, Ana R.; Carvalho, Fatima; Beltrán, Fernando (2010-07-14). "Treatment of Cheese Whey Wastewater: Combined Coagulation−Flocculation and Aerobic Biodegradation". Journal of Agricultural and Food Chemistry. 58 (13): 7871–7877. doi:10.1021/jf100602j. hdl: 20.500.12207/540 . ISSN   0021-8561. PMID   20557068.
  14. ↑ Brungard, Martin (20 February 2018). "Water Knowledge". Bru'n Water.
  15. ↑ Jin, Y-L.; Speers, R.A.. (1999). "Flocculation in Saccharomyces cerevisiae". Food Res. Int. 31 (6–7): 421–440. doi:10.1016/S0963-9969(99)00021-6.
  16. ↑ Beverly, Richard P (2014-04-17). "Water Treatment Process Monitoring and Evaluation". Knovel. American Water Works Association (AWWA). Retrieved 2015-10-14.
  17. 1 2 3 4 5 Terry L. Engelhardt (April 2010). "Coagulation, Flocculation and Clarification of Drinking Water" (PDF).
  18. ↑ Lamanna, Leonardo; Giacoia, Gabriele; Friuli, Marco; Leone, Gabriella; Carlucci, Nicola; Russo, Fabrizio; Sannino, Alessandro; Demitri, Christian (2023-06-13). "Oil–Water Emulsion Flocculation through Chitosan Desolubilization Driven by pH Variation". ACS Omega. 8 (23): 20708–20713. doi:10.1021/acsomega.3c01257. ISSN   2470-1343. PMC   10268613 . PMID   37332801.
  19. ↑ Pal, Preeti; Pal, Anjali; Nakashima, Kazunori; Yadav, Brijesh Kumar (2021-03-01). "Applications of chitosan in environmental remediation: A review". Chemosphere. 266: 128934. Bibcode:2021Chmsp.26628934P. doi:10.1016/j.chemosphere.2020.128934. ISSN   0045-6535. PMID   33246700.
  20. ↑ Kazaz, Billur (July 2021). "State-of-the-Practice Review on the Use of Flocculants for Construction Stormwater Management in the United States". Transportation Research Record. 2675 (7) – via Sage.
  21. 1 2 Gooch, Jan W., ed. (2007-01-01). "Deflocculation". Encyclopedic Dictionary of Polymers. Springer New York. p. 265. doi:10.1007/978-0-387-30160-0_3313. ISBN   978-0-387-31021-3.

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