In conventional water treatment systems, the most widely used flocculants are aluminum salts and iron salts, the aluminum salts remaining in the treated water will endanger human health, and the residual iron salts will affect the color of water, etc.; in most In wastewater treatment, it is difficult to overcome the secondary pollution problems such as large amount of sludge and difficult disposal of sludge. Therefore, seeking a natural product that does not cause secondary pollution to the environment to replace aluminum salt and iron salt flocculants is the need for implementing sustainable development strategies today. Natural polymer flocculants have attracted much attention among many flocculants due to their abundant raw material sources, low price, good selectivity, small dosage, safety and non-toxicity, and complete biodegradation. After decades of development, a large number of natural polymer flocculants with different properties and uses have emerged, among which starch, lignin, chitosan and vegetable glue are currently widely used.
Chitosan Properties
Chitosan is a white amorphous, translucent flaky solid, insoluble in water but soluble in acid, which is the deacetylation product of chitin. Generally speaking, chitosan can be called chitosan when the N-acetyl group in chitin is removed by more than 55%. Chitin is the main component of the exoskeletons of animals and insects, and is the second largest natural organic compound on earth after cellulose. As a flocculant, chitosan is natural, non-toxic and degradable. There are many hydroxyl groups, amino groups and some N-acetylamino groups distributed on the macromolecular chain of chitosan, which can form cationic polyelectrolytes with high charge density in acidic solutions, and can also form network-like structures by means of hydrogen bonds or ionic bonds. Cage molecules, thereby complexing and removing many toxic and harmful heavy metal ions. Chitosan and its derivatives have a wide range of uses, not only in textile, printing and dyeing, papermaking, medicine, food, chemical industry, biology and agriculture and many other fields have many application values, but also in water treatment, can be used as adsorbent, flocculation agents, fungicides, ion exchangers, membrane preparations, etc. Chitosan has been approved by the US Environmental Protection Agency as a purifying agent for drinking water due to its unique advantages in water supply applications and water treatment.
Application of Chitosan in Water Treatment
(1) Remove suspended solids in the water body. In natural water, it becomes a negatively charged colloid system due to the existence of clay bacteria, etc. As a long-chain cationic polymer, chitosan can play the dual functions of electric neutralization and coagulation and adsorption and bridging, and has a strong coagulation effect on suspended substances. Compared with traditional alum and polyacrylamide as flocculants, chitosan has better clarifying effect. RAVID et al. studied the effect of flocculation treatment of single kaolin water distribution when chitosan pH value was 5-9, and found that flocculation was greatly affected by pH value, and the effective pH value of turbidity removal was 7.0-7.5. 1mg/L flocculant, the turbidity removal rate exceeds 90%, and the flocs produced are coarse and fast, and the total flocculation sedimentation time does not exceed 1h; but when the pH value decreases or increases, the flocculation efficiency decreases, indicating that only In a very narrow pH range, chitosan can form good polymerization with kaolin particles. Some studies have found that when flocculated bentonite suspension is treated with chitosan, the suitable pH value range is wide. Therefore, when the turbid water contains particles similar to kaolin, it is necessary to add an appropriate amount of bentonite as a coagulant to improve the polymerization of chitosan on the particles. Later, RAVID et al. found that
If there is humus in the kaolin or titanium dioxide suspension, it is easy to flocculate and precipitate it with chitosan, because the negatively charged humus is attached to the surface of the particles, and the humus makes it easy to adjust the pH value. Chitosan still showed superior flocculation properties for natural water bodies with different turbidity and alkalinity.
(2) Remove algae and bacteria from the water body. In recent years, some people abroad have begun to study the adsorption and flocculation of chitosan on biological colloid systems such as algae and bacteria. Chitosan has a removal effect on freshwater algae, namely Spirulina, Oscillator algae, Chlorella and blue-green algae. Studies have shown that for freshwater algae, removal is best at a pH of 7; for marine algae, the pH is lower. The appropriate dosage of chitosan depends on the concentration of algae in the water body. The higher the concentration of algae, the more dosage of chitosan needs to be added, and the increase of the dosage of chitosan tends to cause flocculation and precipitation. faster. Turbidity can measure the removal of algae. When the pH value is 7, 5mg/L chitosan can remove 90% of the turbidity in water, and the higher the algae concentration, the coarser the floc particles and the better the sedimentation performance.
The microscopic examination showed that the algae that were removed by flocculation and sedimentation were only aggregated and adhered together, and were still in an intact and active state. Since chitosan does not cause any negative effects on species in the water, the treated water can still be used for freshwater aquaculture, unlike other synthetic flocculants for water treatment. The removal mechanism of chitosan on bacteria is relatively complicated. By studying the flocculation of Escherichia coli with chitosan, it is found that the unbalanced bridging mechanism is the main mechanism of the flocculation system, and chitosan produces hydrogen bonds on cell debris. Another study showed that the efficiency of chitosan flocculation of E. coli depends not only on the chargeability of the dielectric but also on its hydraulic dimension.
(3) Remove residual aluminum and purify drinking water. Aluminum salts and polyaluminum flocculants are widely used in tap water treatment processes, but the use of aluminum salt flocculants can lead to increased aluminum content in drinking water. Residual aluminum in drinking water is a serious hazard to human health. Although chitosan also has the problem of water residue, because it is a natural non-toxic alkaline aminopolysaccharide, the residue will not cause harm to the human body, and it can be removed in the subsequent treatment process. In addition, the combined use of chitosan and inorganic flocculants such as polyaluminum chloride can reduce the content of residual aluminum. Therefore, in drinking water treatment, chitosan has the advantages that other synthetic organic polymer flocculants cannot replace.
Application of Chitosan in Wastewater Treatment
(1) Remove metal ions. The molecular chain of chitosan and its derivatives contains a large number of amino groups and hydroxyl groups, so it has a chelating effect on many metal ions, and can effectively adsorb or capture heavy metal ions in the solution. Catherine A. Eiden and other studies have shown that the adsorption capacity of chitosan to Pb2+ and Cr3+ (in unit of chitosan) reaches 0.2 mmol/g and 0.25 mmol/g, respectively, and has a strong adsorption capacity. Zhang Ting’an et al. used deacetylated chitosan to remove copper by flocculation. The results showed that when the pH value was 8.0 and the mass concentration of copper ions in the water sample was lower than 100 mg/L, the copper removal rate was over 99%; The mass concentration is 400mg/L, and the mass concentration of copper ions in the residual liquid still meets the national wastewater discharge standard. Another experiment proved that when pH=5.0 and adsorption time was 2h, the removal rate of chitosan to Ni2+ in adsorption chemical nickel plating waste liquid could reach 72.25%.
(2) Treat wastewater with high protein content such as food wastewater. During food processing, wastewater containing a large amount of suspended solids is discharged. The chitosan molecule contains amide group, amino group and hydroxyl group. With the protonation of amino group, it shows the role of cationic polyelectrolyte, which not only has a chelating effect on heavy metals, but also can effectively flocculate and adsorb negatively charged fine particles in water. Chitin and chitosan can form complexes by hydrogen bonding with proteins, amino acids, fatty acids, etc. Fang Zhimin et al. used chitosan, aluminum sulfate, ferric sulfate and polypropylene phthalamide as flocculants to recover protein from seafood processing wastewater. High protein recovery rate and effluent light transmittance can be obtained. Because chitosan itself is non-toxic and has no secondary pollution, it can be used to recycle useful substances such as protein and starch in wastewater from food processing plants for processing and reuse, such as adding to feed as animal feed.
(3) Treatment of printing and dyeing wastewater. Printing and dyeing wastewater refers to the wastewater discharged from cotton, wool, chemical fiber and other textile products in the process of pretreatment, dyeing, printing and finishing. It usually contains salts, organic surfactants and dyes, etc., with complex components, large chroma and high COD. , and develop in the direction of anti-oxidation and anti-biodegradation, which is extremely harmful to human health and the environment. Chitosan contains amino groups and hydroxyl groups, and has a strong adsorption effect on dyes, including: physical adsorption, chemical adsorption and ion exchange adsorption, mainly through hydrogen bonding, electrostatic attraction, ion exchange, van der Waals force, hydrophobic interaction, etc. effect. At the same time, the molecular structure of chitosan contains a large number of primary amino groups, which form an excellent polymer chelating agent through coordination bonds, which can agglutinate dyes in wastewater, and is non-toxic and does not produce secondary pollution.
(4) Application in sludge dewatering. At present, the vast majority of urban sewage treatment plants use cationic polyacrylamide to treat sludge. Practice has shown that this agent has good flocculation effect and is easy to dewater sludge, but its residue, especially acrylamide monomer, is a strong carcinogen. Therefore, it is a very meaningful work to seek its replacement. Chitosan is a good sludge conditioner, which helps to form activated sludge bacteria micelles, which can agglomerate negatively charged suspended matter and organic matter in the solution, and improve the treatment efficiency of activated sludge process. Studies have shown that the polyaluminum chloride/chitosan composite flocculant not only has obvious effect in sludge conditioning, but also compared with the use of a single PAC or chitosan, the sludge specific resistance first reaches a low point, and the filtration rate is higher. It is fast and is a better conditioner; in addition, three kinds of carboxymethyl chitosan (N-carboxymethyl chitosan, N, O-carboxymethyl chitosan and O-carboxymethyl chitosan) are used as The flocculant was tested on the dewatering performance of the sludge, and it was found that the flocs formed were strong and not easy to break, indicating that the effect of the flocculant on sludge dewatering was significantly better than that of ordinary flocculants.
Chitosan and its derivatives are rich in resources, natural, non-toxic, degradable, and have various properties at the same time. They are green water treatment agents. Its raw material, chitin, is the second largest natural organic compound on earth. Therefore, in recent years, the development of chitosan in water treatment has an obvious growth momentum. As a natural polymer that turns waste into treasure, chitosan has been initially applied in many fields, but the performance and application of domestic products still have a certain gap compared with other advanced countries. With the deepening of the research on chitosan and its derivatives, especially the modified chitosan with excellent synthesis properties, it has more and more application value. Exploring the application technology of chitosan in water treatment and developing environmentally friendly products of chitosan derivatives with wider application range will have very broad market value and application prospects.
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Post time: Aug-09-2022