Theoretical basis of flocculants

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Theoretical basis of flocculants: The theoretical basis is the \"agglomeration\" theory. Flocculants mainly consist of positively (negatively) charged groups that neutralize certain particles or granules in water that carry negative (positive) charges and are difficult to separate; this reduces their electrical potential, putting them in an unstable state. Utilizing their aggregating properties, these particles are brought together and then separated through physical or chemical methods. 　　The agents generally used to achieve this purpose are called flocculants. 　　They are mainly divided into two categories: iron-based formulations and aluminum-based formulations, and of course also the various polymer-based formulations. There are many types of flocculants, all of which share the common characteristic of being able to aggregate and bind the suspended particles in a solution to form large flocculent clumps or masses. 　　An overview of flocculants is as follows:
Inorganic Flocculants
1.1 Classification and properties of inorganic flocculants
Inorganic flocculants can be classified into two main categories based on metal salts: aluminum salts and iron salts ； Aluminum salts are mainly aluminum sulfate and aluminum chloride, while iron salts are mainly ferric sulfate and ferric chloride. Later, on the basis of traditional aluminum and iron salts, new water treatment agents such as polyaluminum sulfate and polyferric sulfate were developed. Their introduction not only reduced treatment costs but also improved efficiency. Such flocculants contain polyhydroxy complex ions, which form multinuclear complex ions through bridging by OH-, thereby becoming large inorganic polymers with a relative molecular mass as high as 1×105. The reason why inorganic polymer flocculants have higher efficiency and better flocculation effects compared to other inorganic flocculants is that they can provide a large number of the complexing ions mentioned above, which enable them to strongly adsorb colloidal particles; through adhesion, bridging, and cross-linking, they facilitate the coagulation of these particles. At the same time, physicochemical changes occur, neutralizing the charges on the surfaces of colloidal particles and suspended solids, thereby reducing the Zeta potential. This causes the colloidal particles to shift from repelling each other to attracting one another, disrupting the stability of the colloid clusters and leading to collisions between these particles. As a result, flocculent coagulation precipitation is formed, and the surface area of the precipitate can reach (200–1000) m2/g, giving it a high adsorption capacity. In other words, polymers have both adsorption and destabilization effects, as well as adhesive, bridging, and flocculating functions. 　　1.2 Modified monocationic inorganic flocculants In addition to the commonly used polyalumins and polyferrics, there are also polyactive silicas and their modified versions, such as polysilicoalumins (ferics) and polyphosphoalumins (ferics). The purpose of modification is to introduce certain highly charged ions in order to enhance the charge-neutralizing capacity, as well as to add hydroxyl groups, phosphate groups, etc. to improve the coordination and complexation ability, thereby altering the flocculation effect. The possible reasons for this are that certain anions or cations can change the morphological structure and distribution of the polymers, or there may be a synergistic effect between two or more types of polymers. 　　In recent years, composite inorganic flocculants and composite inorganic polymer flocculants have been developed in China one after another. Polysilicic acid flocculant (PSAA) is a new type of inorganic polymer flocculant with a simple preparation method, readily available raw materials, and low cost. It possesses a stronger oil-removal capacity for treating the produced water from heavy oil in oil fields, which gives it great potential for development and wide application prospects. The polyferric silicate sulfate (PFSS) flocculant shows that highly polymerized silicic acid, in combination with metal ions, can produce an excellent coagulation effect. By introducing metal ions into polysilicic acid, a coagulant with an average molecular weight of up to 2×105 is obtained, which has the potential to partially replace organically synthesized polymer flocculants in water treatment. In polyphosphoric ferric chloride (PPFC), the high-valent PO43- anion has a strong affinity for Fe3+, exerting a significant influence on hydrolyzed solutions of Fe3+. It can participate in complexation reactions with Fe3+ and act as a bridge between iron atoms, thereby forming multinuclear complexes ； The electroneutralization, adsorption, and bridging effects on the negatively charged diatomite colloids in water are enhanced; meanwhile, the presence of PO43- increases the volume and density of the flocs, thereby improving the flocculation effect. Polyaluminum chlorophosphate (PPAC) is also based on the strong polymerization-enhancing effect of phosphate groups on polyaluminum (PAC); an appropriate amount of phosphate is introduced into polyaluminum, and through this polymerization-enhancing action of the phosphate groups, PPAC gives rise to a new class of highly charged, phosphate-containing polymeric intermediate complexes. Ferrous polysilicate (PSF) not only effectively treats water with low temperatures and low turbidity, but also exhibits significantly superior flocculation properties compared to ferric sulfate. It requires less dosage, can be used within a wider range of concentrations, results in the formation of flocs that are larger in size and easier to settle, and reduces the retention time of water samples within the treatment system. As a result, its efficiency in treating water is improved, with little effect on the pH value of the treated water. 　　1.3 Modified polycationic inorganic flocculants Polyaluminum ferric sulfate chloride (PAFCS) exhibits better performance in drinking water and wastewater treatment compared to alum ； In oily wastewater and printing and dyeing wastewater, PAFCS performs better than PAC, and it also has a superior decolorization capacity ； The flocs have a high specific gravity, settle rapidly, are easy to filter, and yield a high water recovery rate ； Its raw materials are all derived from industrial waste residues, resulting in low costs, making it suitable for industrial water treatment. Aluminum-iron copolymer composite flocculants also fall into this category; their raw materials, aluminum chloride and iron chloride, are both inexpensive traditional inorganic flocculants that are readily available and require simple production processes, which facilitates their development and application. The copolymer of aluminum salts and iron salts is different from a mixture of the two salts; it is a flocculant that more effectively combines the advantages of PAC and FeCl3, thereby enhancing its turbidity-removing effect. 　　As awareness of water treatment continues to improve, the toxic effects of residual aluminum on living organisms have attracted considerable attention, and efforts to reduce secondary pollution are receiving increasing focus. The aluminum content in drinking water produced by existing domestic manufacturing methods is generally 1-2 times higher than that in the raw water. The high levels of residual aluminum and other substances in drinking water may be due to an imperfect flocculation process, which results in some of the aluminum remaining in the water as fine particles of aluminum hydroxide. Adopting enhanced flocculation purification methods, improving the conditions of the flocculation reaction, and extending the slow flocculation time can effectively reduce the levels of aluminum and other substances. Given that inorganic flocculants are corrosive and toxic, and can have adverse effects on human health and the ecological environment, organic polymer flocculants have been developed. 　　Organic polymer flocculants Organic polymer flocculants appeared in the 1950s; they have broad application prospects and have developed very rapidly. It has been used in water purification, demulsification of water/oil systems, treatment of oily wastewater, wastewater reuse, and sludge dewatering, among other applications ； It can also be used as a mud treatment agent in oil field development, a selective water shutoff agent, a thickening agent for water injection, a softener in textile printing and dyeing processes, an antistatic agent, and as a general-purpose bactericide and disinfectant. 　　2.1 Types and properties of organic polymer flocculants Organic polymer flocculants can be divided into two categories: natural polymers and synthetic polymers. Based on their chemical structure, they can be divided into the following 3 types: (1) Polyamine type – low-molecular-weight cationic electrolytes ； (2) Quaternary ammonium type – exhibits a wide range of molecular weights and has a high degree of cationicity ； (3) Copolymers of acrylamide – with relatively high molecular weights, ranging from hundreds of thousands to several million or even tens of millions – are available in emulsive or powdered forms; they are less convenient to use, but possess good flocculation properties. Based on the charge of the particles after dissociation due to different functional groups, they can be classified into three main categories: cationic, anionic, and non-ionic. Organic polymer flocculants can contain hydrophilic groups such as -COO-, -NH-, -SO3-, and -OH- in their macromolecules, and they come in various structures including linear and cyclic forms. Due to their numerous active groups and high molecular weight, cationic flocculants have the advantages of requiring low dosages, producing less sludge, possessing strong flocculation capabilities, enabling easy separation of flocs, and offering effective oil and suspended solids removal. As such, they are widely used in the treatment of refinery wastewater, other industrial wastewater, wastewater with high levels of suspended solids, as well as in solid-liquid separation processes. In particular, acrylamide-based organic polymer flocculants exhibit superior performance in water treatment due to their high molecular weight and strong flocculation and bridging capabilities. 　　2.2 Non-ionic organic polymer flocculants Non-ionic organic polymer flocculants are mainly polyacrylamide. It is obtained by the polymerization of acrylamide. 　　2.3 Anionic organic polymer flocculants (1) Anionic organic polymer flocculants mainly include polymers such as polyacrylic acid, sodium polyacrylate, calcium polyacrylate, and the alkali-hydrolyzed products of polyacrylamide. 　　(2) Copolymers of acrylamide with styrene sulfonate, lignosulfonate, acrylic acid, methacrylic acid, etc. 　　2.4 Cationic Organic Polymer Flocculants 　　2.4.1 Quaternized Polyacrylamide 　　Cationic quaternized polyacrylamides are obtained by hydroxymethylation and quaternization of the -NH2 group; they can be classified into cationization of polyacrylamide and polymerization of quaternized acrylamide. 　　(1) Quaternization of polyacrylamide: Polyacrylamide (PAM) first reacts with an aqueous formalin solution, resulting in the hydroxymethylation of the amide groups; it then reacts with secondary amines to undergo alkanamidation, and finally reacts with hydrochloric acid or quaternizing agents to be quaternized into tertiary amines. 　　(2) Polymerization of quaternized acrylamide Under alkaline conditions, acrylamide first reacts with an aqueous solution of formaldehyde, then with dimethylamine; after cooling, quaternization is carried out using hydrochloric acid. The product is evaporated and concentrated, followed by filtration, to yield quaternized acrylamide monomer. 　　2.4.2 Cationic derivatives of polyacrylamide These products are usually obtained by copolymerizing acrylamide with cationic monomers. 　　2.5 Amphoteric polyacrylamide polymers: Amphoteric polyacrylamide flocculants containing carboxyl and amine groups are synthesized by adding an appropriate amount of formaldehyde and dimethylamine to partially hydrolyzed polyacrylamide, via the Mannich reaction. 　　2.6 Acrylamide Grafted Copolymers Since starch is inexpensive and readily available, and it is itself a polymer compound with hydrophilic rigid chains, flexible polyacrylamide side chains are grafted onto these rigid chains as a backbone. This hybrid network of rigid and flexible components not only retains the properties of conventional polyacrylamide but also possesses some superior characteristics. 　　Since most organic polymer flocculants, either in their original form or as a result of their hydrolysis or degradation, are toxic, and the acrylamide monomer used for their synthesis is also toxic, they can damage the human central nervous system; this limits their areas of application, forcing the development of flocculants that are inexpensive, practical, non-toxic, and highly effective. 　　Microbial flocculants 3.1 Overview of microbial flocculants The commercial production of microbial flocculants abroad began in the 1990s, owing to their lack of secondary pollution, ease of use, and promising application prospects. Microorganisms such as Rhodopseudomonas palustris and the NOC-1 produced from it are currently the best microbial flocculants identified, possessing strong flocculation activity and being widely used in the treatment of livestock wastewater, expanded sludge, and colored wastewater. There are no reports on products of microbial flocculants in our country. 　　Microbial flocculants mainly include those using microbial cell wall extracts, those using metabolic products of microbial cell walls, those using microbial cells directly, and those obtained through cloning technology. The flocculants produced by microorganisms are high-molecular compounds such as glycoproteins, mucopolysaccharides, proteins, cellulose, and DNA, with a relative molecular mass of over 105. 　　Microbial flocculants are new types of water treatment agents that are safe, efficient, and capable of natural degradation; they are obtained through biotechnology by extracting and purifying them from microorganisms or their secretions. Since microbial flocculants can overcome the inherent defects of inorganic polymer and synthetic organic polymer flocculants, thereby enabling pollution-free discharge, research on microbial flocculants has become an important topic in the field of flocculants worldwide today. 　　3.2 Types and properties of microbial flocculants Researchers studying microbial flocculants have long observed that certain microorganisms such as yeasts and bacteria exhibit cell flocculation, but this phenomenon has not received much attention; it has been used merely as a method for cell enrichment. In the past decade or so, cell flocculation technology has come to be widely used in continuous fermentation and product separation as a simple and cost-effective method for separating biological products. Microbial flocculants are a class of polymer organic substances produced by microorganisms that possess flocculation properties. The main components include glycoproteins, mucopolysaccharides, cellulose, and nucleic acids. In terms of its origin, it also belongs to natural organic polymer flocculants; therefore, it possesses all the advantages of natural organic polymer flocculants. At the same time, research on microbial flocculants has progressed from purification and modification to the use of biotechnology to cultivate and select superior strains in order to obtain highly effective flocculants at lower costs; as a result, its scope of research has extended beyond that of traditional natural organic polymer flocculants. Microorganisms with the ability to secrete flocculants are called flocculant-producing bacteria. The earliest flocculant-producing bacterium was isolated by Butterfield from activated sludge. In 1976, Nakamura et al. screened 19 microorganisms with flocculation capabilities from species such as molds, bacteria, actinomycetes, and yeasts, among which the flocculant produced by Aspergillus souae AJ7002 had the best effect. In 1985, Takagi H and others studied the flocculant PF101 produced by the microorganism Paecilomyces sp. l-1. PF101 exhibits excellent flocculation effects on Bacillus subtilis, Escherichia coli, brewer’s yeast, red blood cells, activated sludge, cellulose powder, activated carbon, diatomite, alumina, and others. In 1986, Kurane and colleagues developed the microbial flocculant NOC-1 using Rhodococcus erythropolis. It exhibits excellent flocculation and decolorization effects on Escherichia coli, yeast, mud water, river water, fly ash water, activated carbon powder water, expanded sludge, and pulp wastewater, making it the best microbial flocculant discovered to date. 　　The molecular weight, molecular structure and shape of the flocculant, as well as the functional groups it possesses, all affect its flocculating activity. Generally speaking, the higher the molecular weight, the greater the flocculation activity ； Linear molecules have high flocculation activity; the more branches or crosslinks a molecule has, the worse its flocculation properties ； In the later stages of cultivation, the bacteria that produce flocculants have an increased hydrophobicity on their cell surfaces, which results in higher activity of the flocculants they produce. The surface structure and charge of colloidal ions in water also affect the flocculation effect. Some reports indicate that cations in water, particularly Ca2+ and Mg2+, can effectively reduce the negative charge on the surface of colloids, thereby facilitating the formation of \"bridges\". Furthermore, the presence of high concentrations of Ca2+ can also protect the flocculant from the action of degradation enzymes. Microbial flocculants have a wide flocculation range and high flocculation activity, and they require relatively simple operating conditions; they are largely unaffected by ion strength, pH value, and temperature. Therefore, they can be widely used in the treatment of sewage and industrial wastewater. Thanks to their efficiency, safety, and environmental friendliness, microbial flocculants hold great potential for application in fields such as pharmaceuticals, food processing, and the separation of biological products. The new generation of flocculants – organic polyactive polymer flocculants. The “Tengda” multi-functional flocculant produced by Weifang Ruifeng Industry Company in China is an ideal high-tech, environmentally friendly product for the 21st century; it is a high-quality alternative to polyacrylamide, and represents an improved version of inorganic flocculants, organic flocculants, and microbial flocculants. 　　4.1 Product Overview This flocculant is a transparent gel-like liquid obtained by modifying polyacrylamide with natural polymer materials; it is a new type of organic liquid flocculant with anionic, cationic, and non-ionic properties. Its features include (1) it can effectively reduce COD, BOD, and SS after use ； (2) It requires a small amount in sludge reduction, can rapidly increase the firmness of the sludge cake, kill harmful microorganisms, and foster beneficial microorganisms ； (3) Compared with solid flocculants, it has a fast dissolution rate, uniform dissolution, high utilization efficiency, and no secondary pollution ； (4) Simple process, easy to use, and low cost (saving over 25% compared to solid dry powder). 　　4.2 Mechanism of Action This product primarily aggregates pollutant particles into large, dense particles through various mechanisms such as electrostatic adsorption, electrical neutralization and destabilization, double-layer compression, bridging, and trapping. Under the influence of gravity, these aggregated particles settle, allowing the pollutants to be separated from the water, thereby achieving the purpose of wastewater treatment. 　　4.3 Usage This product is primarily used for the treatment of urban domestic wastewater, as well as wastewater from papermaking, oil fields, coal washing, and textile printing, along with the treatment of the resulting sludge. When in use, dilute it with water at a ratio of 1:20 to obtain a 5% working solution ； 1. The quantitative concentrate can treat wastewater at a volume ratio of 8,000–10,000 times. The optimal dosage should be determined based on the actual process of the wastewater to be treated. 　　4.4 Product Specifications Under optimal experimental conditions, this product has a settling time of 26 minutes and a dewatering rate of 72.6%. The optimal dosage is 1.2 m/L·L-1, and the cost per ton of sludge processed is 25.8% lower compared to PAM (of Japanese origin). 　　Appearance: viscous liquid ； pH value: 6–12 ； Solid content (%): ≥40 ； Viscosity (mpas, 2s.Cbzooktie14# rotor 60zPm): 4000–10000 Conclusion Looking at the current state of flocculants, it can be seen that there is a wide variety of them, ranging from low-molecular-weight to high-molecular-weight ones, and from single-type to composite types. The general trend is toward flocculants that are inexpensive, practical, non-toxic, and highly effective. Inorganic flocculants are inexpensive, but they can have adverse effects on human health and the ecological environment ； Although organic polymer flocculants require less dosage, result in less sludge production, possess strong flocculation capacity, allow for easy separation of flocs, and offer excellent performance in oil and suspended solids removal, the residual monomers of such polymers have carcinogenic, mutagenic, and teratogenic effects, which limits their scope of application ； Microbial flocculants have attractive application prospects due to their lack of secondary pollution and ease of use. Microbial flocculants are likely to replace or partially replace traditional inorganic polymeric and synthetic organic polymeric flocculants in the future. The development and application of microbial flocculants are still in their infancy, and their properties and advantages offer broad prospects for the advancement of water treatment technologies.