Environmental Water-Treatment Knowledge: Meaning and Function of Anionic Surfactants
Surfactants whose surface-active moiety carries a negative charge after ionization in water are called anionic surfactants.
Anionic polyacrylamide (APAM) is a water-soluble polymeric compound, mainly used for the flocculation and settling of various industrial wastewaters and for sedimentation and clarification treatment, such as steel-plant wastewater, electroplating-plant wastewater, metallurgical wastewater, and coal-washing wastewater, as well as sludge dewatering. It can also be used for the clarification and purification of drinking water. Because its molecular chain contains a certain number of polar groups, it can adsorb suspended solid particles in water, causing bridging between particles or coagulation through charge neutralization to form large flocs; thus it accelerates the settling of particles in suspension and has a very obvious effect of speeding up solution clarification and promoting filtration.
On this basis, industry experts have also carried out extensive research and improvement.
In wastewater treatment, sludge thickening and dewatering, ore dressing, coal washing, papermaking, and other aspects, it can fully meet the requirements of various fields.
Coal-washing wastewater treatment scheme: Coal-preparation plants generally adopt a 'cyclone-thickener-filter press (coal-slime settling pond)' treatment process for slime water. Generally, polymeric flocculants (polyacrylamide) are purchased. The polymeric flocculant contacts and acts on the coal-slime particles or coal-slime colloid, neutralizing the surface electric charge of the coal slime, lowering the surface energy, and causing the coal-slime particles to coagulate and settle. The molecular weight of polyacrylamide is generally in the millions; coal-slime waters of different particle-size compositions require flocculants of different molecular weights. Polyacrylamide can be divided into three types: anionic polyacrylamide, cationic polyacrylamide, and non-ionic polyacrylamide. When using polyacrylamide for water treatment, the type must match the pH of the coal-slime water: anionic polyacrylamide is suitable for alkaline coal-slime water, cationic polyacrylamide is suitable for acidic coal-slime water, and mixing anionic and cationic polyacrylamide gives better flocculation and sedimentation of the coal-slime water.
2. Adding a small amount of this anionic polyacrylamide product yields a great flocculation effect. Generally, only 0.01-10 ppm (0.01-10 g/m3) needs to be added to bring its effect into full play.
3. Using this anionic polyacrylamide product together with inorganic flocculants (polyferric sulfate, polyaluminum chloride, iron salts, etc.) shows an even greater effect.
1) For sludge dewatering: according to the nature of the sludge, the corresponding model of this product can be selected to effectively dewater the sludge before it enters the filter press. During dewatering, it produces large flocs that do not stick to the filter cloth, do not disperse during pressure filtration, and yield a thicker sludge cake with high dewatering efficiency and a cake moisture content below 80%.
2) For the treatment of domestic sewage and organic wastewater: this product exhibits cationic charge in both acidic and alkaline media, so it is very effective for the flocculation and clarification of sewage in which the suspended particles carry a negative charge. For example, for grain-alcohol production wastewater, papermaking wastewater, municipal wastewater-treatment-plant effluent, beer wastewater, monosodium-glutamate-plant wastewater, sugar-refining wastewater, high-organic-content wastewater, feed wastewater, and textile printing-and-dyeing wastewater, using cationic polyacrylamide achieves effects several to dozens of times higher than using anionic, non-ionic polyacrylamide, or inorganic salts, because such wastewaters generally carry a negative charge.
3) Used as a flocculant for tap water sourced from river water: low dosage, good effect, and low cost; especially when compounded with inorganic flocculants, the effect is even better. It will become a highly efficient flocculant for waterworks along the Yangtze River, the Yellow River, and other river basins.
It is worth noting that the technologies and standards in this field are also constantly being developed and improved.
It is packaged in 25 kg plastic-lined woven bags or paper-plastic composite bags, and can also be packaged according to user requirements. During storage and transport, guard against heat and moisture, prevent package damage, and note that long-exposed powder products will absorb moisture and cake. The stacking layers must not exceed 20. The effective storage period is 2 years. The particle size of this product is 20-80 mesh, and can also be produced according to user requirements.
An anionic surfactant whose hydrophilic group is a carboxyl group, including potassium, sodium, and ammonium salts of higher fatty acids and triethanolamine salts. The part that exerts the surface-active action after ionization in water is the fatty-acid anion. For example:
In addition, from the perspective of industrial development, market demand is also driving technological progress.
(1) Soap is the most common fatty-acid-salt anionic surfactant. The main performance characteristic of soap is that its aqueous solution has a pH of 9.0-9.8, slightly alkaline, and it has good wetting, foaming, and detergency, so it is widely used as a detergent.
The disadvantage of soap is its poor resistance to hard water; when used in hard water, not only is its washing power poor, but the calcium-soap dirt generated floats in the acidic water and adheres to clothing, making it difficult to remove. Soap reacts with calcium, magnesium, and other ions in hard water to form soap scum, which not only increases soap consumption but also, when bonded to clothing, produces spots that make the fabric stiff. Fabric containing soap scum will cause uneven dyeing during printing and dyeing processing.
Soap turns into water-insoluble free fatty acids in acidic media with pH below 7, making the soap solution turbid and difficult to remove from clothing. Therefore, soap can only be used in neutral and alkaline media. Usually, when using soap, an appropriate amount of soda ash is added to keep the soap solution at a pH around 10, the purpose being to prevent soap hydrolysis and improve washing effect. Note that soap cannot be used to remove acidic dirt or in acidic media.
It is worth noting that the technologies and standards in this field are also constantly being developed and improved.
Lamepon A is often used as a detergent, emulsifier, and dispersant in the textile sectors of wool spinning, silk, synthetic fibers, and printing and dyeing, and can also be used as a metal cleaner and skin cleanser. Because the polypeptide part of its structure is chemically similar to proteins, it has low irritation to the skin and can form a good protective colloid, so it is also suitable for hair products and shampoos or for use in skin-care creams. Washing silk, wool, and other protein-fiber fabrics with it leaves them soft, lustrous, and elastic after washing. It has strong emulsifying power; for example, 22 parts of Lamepon A can emulsify 1000 parts of vegetable oil. And it has strong dispersing power for calcium soap. It is stable in neutral and alkaline media, with even better detergency in alkaline media, but precipitates out at pH below 5. Because of its strong moisture absorption, it is generally not made into a powder product; the commercial product is a yellow-brown viscous liquid with an active-matter content of 32%-40%.
It is worth noting that the technologies and standards in this field are also constantly being developed and improved.
Its early product was sodium tetrapropylene benzene sulfonate (ABS); because the alkyl portion had a branched chain, its biodegradability was poor, so from the 1960s various countries successively switched to producing linear alkylbenzene sulfonate (LAS) made from normal alkanes as raw material. Alkylbenzene sulfonate is not a pure compound; the alkyl composition is not completely identical, so its properties are greatly affected by the number of carbon atoms in the alkyl portion, the degree of branching of the alkyl chain, the position of the benzene ring on the alkyl chain, the position and number of sulfonic groups on the benzene ring, and the type of counter-ion of the sulfonate.
(1) Branched-chain alkylbenzene sulfonate (ABS): When a higher olefin (such as dodecene) reacts with benzene, a branched-chain alkylbenzene is produced, which then undergoes a sulfonation reaction with concentrated sulfuric acid to obtain branched-chain alkylbenzenesulfonic acid; neutralized with an alkali (NaOH), it yields the branched-chain sodium alkylbenzenesulfonate salt, of which sodium dodecylbenzenesulfonate is the most common product.
Sodium dodecylbenzenesulfonate is a synthetic anionic surfactant with excellent performance; it is more readily soluble in water than soap and is a yellow oily liquid. It foams easily, but because its foam has low viscosity, the foam disappears easily. It has good degreasing ability and excellent properties of lowering the surface tension of water and wetting, penetrating, and emulsifying. Its chemical properties are stable; it does not decompose in acidic or alkaline media or under heating. It also does not decompose when mixed with oxidants such as sodium hypochlorite and peroxides. It can be prepared by sulfonating alkylbenzene; the raw materials are abundant, the cost is low, the manufacturing process is mature, and the product purity is high. Therefore, since the National Aniline Company of the United States began producing sodium alkylbenzenesulfonate in 1936, it has remained popular with users and valued by producers for over 60 years, becoming the most-consumed household detergent and also widely used in industrial cleaning.
Its shortcomings are that the fiber washed with it feels poor, and prolonged skin contact causes irritation. It easily forms an adsorption film on the surface of washed objects that remains on them, and this adsorption film is not easily rinsed away with water at low temperature. It foams well, so it is also unwelcome in situations where foam is undesirable.
Sodium dodecylbenzenesulfonate is especially prone to producing a synergistic effect with other substances (mixing two substances to produce an effect better than each alone is called a synergistic effect), so it is often compounded with non-ionic surfactants and inorganic builders to improve detergency.
In hard water it does not form calcium-soap precipitates like soap does, but the calcium alkylbenzenesulfonate generated is not readily water-soluble and can only be dispersed in water, reducing its washing power. When used compounded with chelating agents such as sodium tripolyphosphate to complex the calcium and magnesium ions, it can be used in hard water without affecting its washing effect.
Because the branched-chain-structure sodium alkylbenzenesulfonate is difficult for microorganisms to degrade and causes serious environmental pollution, from the mid-1960s it was gradually replaced by linear alkylbenzenesulfonate.
(2) Linear alkylbenzene sulfonate (LAS): Linear alkylbenzene sulfonate is obtained by synthesizing linear alkylbenzene from linear alkanes and benzene under the action of a special catalyst, followed by sulfonation and neutralization. The typical representative structure is para-linear sodium dodecylbenzenesulfonate; its performance is the same as that of branched-chain sodium alkylbenzenesulfonate, and its advantage is that it is readily biodegradable, making it a superior product from an environmental-protection perspective. All alkylbenzenesulfonates now in use have a linear alkyl structure.
(2) alpha-Olefin sulfonate (AOS): A mixture of surface-active anions obtained by reacting alpha-olefin with SO3 under appropriate conditions, then neutralizing and hydrolyzing; the composition is relatively complex and varies with process conditions and feed amounts. Its main components are alkenyl sulfonates (R-CH=CH-(CH2)p-SO3Na), hydroxyalkyl sulfonates (RCH-(CH2O)p-SO3Na), and a small amount of disulfonates (R'-CH=CH-CH-(CH2)-SO3Na) or R'-CH-(CH2)x-CH-(CH2)y-SO3Na. Its trade name is alpha-olefin sulfonate, abbreviated AOS.
(3) Alkyl sulfonates (AS and SAS): The general formula of alkyl sulfonates is RSO3M (M is an alkali metal or alkaline-earth metal), where R is an alkyl group in the C12-C20 range, of which cetyl sulfonate has the best performance. The sulfonates obtained by reacting normal alkyl groups with SO2 and O2 under the action of an initiator are divided into two types: primary alkyl sulfonates (AS) and secondary alkyl sulfonates (SAS). The structural formula of the secondary alkyl sulfonate is R-CH-R', abbreviated SAS, with the domestic trade name detergent 601; it is a detergent with very good water solubility, wetting power, and degreasing power. The alkyl carbon atoms are generally C14-C18, with the strongest detergency at C15-C16. Its detergency is similar to that of linear alkylbenzene sulfonic acid (LAS), with slightly lower foaming power, and it is a main raw material for heavy-duty liquid detergents. Its toxicity and skin irritation are lower than those of LAS, and it is readily biodegradable. It is often compounded with alcohol ether sulfate (AES) and alpha-olefin sulfonate (AOS) to compensate for SAS's poor foam performance in hard water. It can be used in personal-care toiletry products, various laundry products, and hard-surface cleaners.
MES is an anionic surfactant developed and produced in recent years from natural oils and fats as raw material. It has good biodegradability, is beneficial to environmental protection, is safe to use, and has strong detergency. Its detergency declines little as water hardness increases, so it has good detergency in hard water; for example, replacing LAS with MES in a laundry-powder formula gives markedly better detergency in low-concentration, high-hardness water than a formula using LAS alone. It is also an excellent calcium-soap dispersant; used together with soap, it compensates for soap's disadvantage of poor hard-water resistance and scum formation, so it is a main component of liquid soap. MES has good foaming ability. It has little effect on the activity of alkaline protease and alkaline lipase, making it suitable for enzyme-added laundry powders. It has strong solubilizing power for oil stains and low toxicity and good safety, so it is a new variety with good application prospects. However, its hydrolysis failure in alkaline media should be prevented.
It is worth noting that the technologies and standards in this field are also constantly being developed and improved.
Fatty-acid sulfoalkyl esters (Igepon A) and fatty-acid sulfoalkyl amides (Igepon T) were originally used as textile auxiliaries; in particular, the Igepon T series has the advantages of insensitivity to hard water, good detergency, wetting power, and softening action on fibers, and can be used in acidic media, so it has wide applications in the textile industry. Among them, sodium N-oleoyl-N-methyltaurate is the most important, used for cleaning coarse wool, synthetic fibers, and dyed fabrics, and it has a good softening effect on fibers. These two types of products—sulfoalkyl esters and sulfoalkyl amides—are heavy-duty fine-textile detergents, hand- and machine-dishwashing detergents, and important formulation components of various shampoos, foam baths, and soaps. Usually, sulfoalkyl esters or sulfoalkyl amides of coconut-oil fatty acids and tallow fatty acids are used. Their physical properties and surface activity are shown in Tables 7-7 and 7-8.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
(6) Petroleum sulfonate: Obtained by sulfonating and neutralizing natural petroleum fractions or high-carbon-hydrocarbon by-products from chemical reactions; it is a mixture of various hydrocarbon sulfonation products. Petroleum sulfonate is mainly used as a cleaning dispersant for engine lubricating oil and to disperse sludge, keep metal parts clean, and reduce acidity and inhibit rust. Petroleum sulfonate used for this purpose accounts for about 60% of total production. Metal cleaners formulated with petroleum sulfonate can effectively remove oil stains from metal parts.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
Aerosol OT (penetrant OT) is the earliest succinic-acid diester sulfonate to appear; it is an excellent industrial wetting agent and penetrant. It is a product obtained by sulfonating the monoester formed from fatty-alcohol polyoxyethylene ether and fatty-acid monoethanolamide with maleic anhydride. It is mild, low in skin and eye irritation, and has excellent foaming properties, and its application in personal-care products is growing. Because of abundant raw materials, low production cost, and no three-waste discharge, it has developed greatly in recent years.
Meanwhile, the related supporting processes and equipment are also being continuously optimized and upgraded.
(4) Alkyl glyceryl ether sulfonate (AGS): Its general formula is ROCH2-CH-CH2SO3-M+; it has good water solubility, is stable to acid and alkali, and is an effective wetting agent, foaming agent, and dispersant, but its application and development are limited by its high price.
Meanwhile, the related supporting processes and equipment are also being continuously optimized and upgraded.
FAS is the earliest anionic surfactant to appear after soap; it is obtained by hydrogenolyzing coconut oil to produce C12-C14 fatty alcohol, which is then esterified with sulfuric acid and neutralized. It has suitable solubility, foaming, and detergency. It is widely used in dentifrices, shampoos, foam baths, and cosmetics, and is also an important component in light- and heavy-duty detergents, carpet cleaners, and hard-surface cleaner formulations. For example, sodium lauryl sulfate (C12H25OSO3Na), with the trade name detergent K12, has wetting, foaming, and washing effects in dentifrices; the heavy-metal salts of lauryl sulfate have fungicidal and bactericidal effects; sodium and potassium soaps made from tallow and coconut oil compounded with alkyl sulfate sodium and potassium salts produce rich, fine foam and also prevent calcium-soap formation; high-carbon fatty-alcohol sulfates compounded with amphoteric surfactants produce bar detergents with good abrasiveness and physical properties and a conditioning action.
Meanwhile, the related supporting processes and equipment are also being continuously optimized and upgraded.
(2) Secondary alkyl sulfate (Teepol): It is a product obtained by reacting alpha-olefin with sulfuric acid to produce a secondary alkyl sulfate, then neutralizing; its general formula is R-CH-O-SO3Na, with the trade name Teepol.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
In addition to the above four types of anionic surfactants, there are others, such as amino-acid salts (R-CHNH2COO-), phenates, enolate salts, ketosulfonamide salts ([R-CO-N-SO2-R']-), and coordinated anionic salts (such as [ROCe(NO3)5]-). Their solubility differs at different pH values, and anionic surfactants play many roles in production and daily life. They are an indispensable class of substances in life.
