Thread Content
Preface: Water resource issues are becoming increasingly prominent and have become one of the most focused problems in the world. At present, the water resources situation in our country is extremely severe. Due to the rapid economic development and swift population growth, water scarcity and water pollution have become increasingly severe issues, and they have now become one of the main bottlenecks hindering our country’s progress on the path to sustainable development. The water use efficiency in most industrial enterprises in our country is unsound, resulting in an increasing amount of wastewater being discharged, which further exacerbates the problem of water scarcity. In recent years, membrane technology has been increasingly widely used in the treatment of industrial wastewater. The widespread use of the combined technology of physico-chemical and biochemical methods with dual membranes (microfiltration/ultrafiltration–reverse osmosis) provides new approaches for the resource utilization of wastewater and its sustainable reuse. Among these methods, reverse osmosis can convert approximately 75% of the water from the secondary sedimentation tank into reclaimed water; however, about 25% of the reverse osmosis concentrate still needs to be treated promptly, as this water contains a large amount of recalcitrant dissolved small-molecule organic substances, such as intermediates containing dyes and surfactants, biological metabolites, and concentrated inorganic salts. Compared to traditional water treatment processes, reverse osmosis is one of the most effective methods for obtaining clean water. At the same time, it features easy operation, small footprint, and high efficiency, and has been widely applied in various fields such as seawater and brackish water desalination, urban and industrial water treatment, production of pure and ultra-pure water, industrial wastewater treatment, food processing, and aerospace. In recent years, reverse osmosis water treatment technology has become the most successful, rapidly developing, and widely adopted membrane technology in China, bringing substantial economic and social benefits. At the same time, this also leads to an increase in the production of reverse osmosis concentrate. The water quality of reverse osmosis concentrate from various types is complex, posing new challenges to researchers in the field of environmental science while also posing potential risks to the ecological environment. Concentrated solutions contain not only organic pollutants but also high levels of chloride ions, sulfate ions, and fluoride ions. Fluorine is one of the most widely distributed elements in the world; it has the highest electronegativity and the greatest reactivity, and can react with almost all elements. As a result, fluorine exists mostly in compound form, with no elemental fluorine present. Fluorine is an essential element for the human body; it is ingested in trace amounts primarily through water and food. The absorption rate remains at 80%–97%. Trace amounts of fluorine are crucial for the growth of teeth and bones in humans. However, if the fluoride content is too high, it can cause a series of problems. According to the guidelines set by the World Health Organization (WHO), the fluoride content in drinking water should not exceed 1.5 mg/L. In China, the fluoride level in drinking water must be below 1.0 mg/L. Long-term consumption of drinking water with a fluoride content lower than 0.3 mg/L can lead to tooth decay in children, while it can cause osteoporosis and increased fragility in the bones of the elderly. Excessive intake of fluoride can affect the body’s normal metabolism of proteins, vitamins, minerals, and carbohydrates. When the fluoride content in drinking water is between 1.5 and 2.0 mg/L, infants and young children can develop fluorosis, which may even lead to tooth defects and loss of teeth ； When the fluoride content in drinking water is between 3 and 6 mg/L, adults may develop fluorosis, leading to functional disorders or even paralysis. High doses of fluoride can affect the endocrine functions of the thyroid gland, gonads, and pituitary gland, and in severe cases, it can be life-threatening. If fluorine-containing wastewater is discharged directly into the natural environment without proper treatment, it can cause severe ecological pollution, lead to localized fluoride levels that are too high, and pose a direct threat to human health. The fluorine chemical industry is a highly polluting and dangerous sector; moreover, fluoride-containing wastewater can corrode equipment, accelerating its depreciation rate and increasing the financial burden on companies. The \"Comprehensive Wastewater Discharge Standards\" (GB 8978—1996) stipulate that the secondary discharge standard for fluoride in industrial wastewater must be less than 10 mg/L. Therefore, addressing the issue of fluoride-containing wastewater, achieving clean production, and pursuing a green and environmentally friendly approach represent challenges for the fluorine chemical industry. 1 Sources of fluorine-containing industrial wastewater: The fluoride in China’s surface water mainly comes from groundwater and human industrial activities. Fluoride-containing minerals are continuously washed away and weathered, spreading to local water bodies and causing an increase in fluoride levels ； The fluorine chemical industry is an important branch of new chemical materials in China. Fluorine-based materials are essential for industries such as military manufacturing, metallurgy, aerospace, optics, and the automotive sector, and they hold a strategic position that cannot be replaced in future industrial production. In modern industry, applications such as the use of hydrofluoric acid, steel production, the electrolytic refining of aluminum, the production of sulfur-based fertilizers and sulfuric acid, the smelting of rare earth metals and non-ferrous metals, and the reprocessing of fluorides all inevitably generate a certain amount of fluoride-containing wastewater. Currently, the discharge of fluoride-containing wastewater is increasing in some areas, and it is not being properly treated, resulting in many places becoming areas with high fluoride levels and frequent cases of fluorosis. Therefore, in light of issues such as the long treatment process, high treatment costs, difficulties in filtration, and the challenge of recovering fluorine resources from fluoride-containing wastewater, it is urgent to find fast and effective treatment processes for such wastewater. 2 Fluoride removal processes for fluorine-containing industrial wastewater. The composition of fluorine-containing industrial wastewater is very complex; the fluoride present in water exists mainly in the form of HF and fluorosilicates, along with a wide range of other pollutants such as organic substances and inorganic salts. This makes the treatment process quite challenging. It is necessary to reduce the COD level to 75 mg/L through primary treatment, and then to bring the fluoride content in the wastewater below the allowable levels (less than 10 mg/L) through secondary treatment. Numerous studies have been conducted today on processes for removing fluoride from fluoride-containing wastewater, including ion exchange, membrane separation (electrodialysis and reverse osmosis), adsorption, and precipitation methods (chemical precipitation and coagulation precipitation). For industrial-level treatment processes, adsorption and precipitation methods are primarily used to meet emission standards; other methods are less employed. Nevertheless, many researchers are still working to overcome issues such as low fluoride removal efficiency and high treatment costs. 2.1 Fluoride removal process by ion exchange The principle of fluoride removal using ion exchange lies in the use of certain anions present in the resin to exchange with fluoride ions, thereby adsorbing and capturing these fluoride ions from wastewater and achieving fluoride removal from it. The advantage of the ion exchange resin method is its simplicity, ease of operation, and recyclability. However, due to the high production costs of resins and the high costs associated with regeneration, the problem of secondary pollution is difficult to resolve, and other harmful ions may appear in the treated wastewater, which limits its industrialization. 2.2 Fluoride removal process by membrane separation The membrane separation method involves using different pressure differences to drive fluoride ions through a semipermeable membrane, thereby enabling the liquid phase separation and removing fluoride from wastewater. Both electrodialysis and reverse osmosis belong to membrane separation methods, and they are also widely used in the industrial sector today. 2.2.1 Fluoride removal by electrodialysis: Electrodialysis was first used for desalinating seawater; it makes use of an applied direct current to enable the cations and anions in water to migrate in a directed manner through ion exchange membranes made of ion exchange resins. Using electrodialysis for fluoride removal eliminates the need to add external chemicals; it not only achieves effective fluoride removal but also reduces the salt content in wastewater, offering advantages that other methods cannot match. However, this method removes other beneficial minerals along with fluoride from drinking water; people also need to be vigilant about the polarization phenomenon that occurs during fluoride removal, and must continuously switch the anode and cathode to maintain the stability of the fluoride removal process. 2.2.2 Fluoride removal by reverse osmosis The reason why fluoride removal using the reverse osmosis method is called reverse osmosis is that it operates in the opposite direction to natural osmosis; it also relies on the principle of osmotic pressure differences in solutions, utilizing the selective permeability of semipermeable membranes to retain fluoride ions while allowing water molecules to pass through, and it falls under the category of physical filtration. This method is easy to operate, provides thorough fluoride removal, requires no space, and is also one of the most advanced technologies available today. However, reverse osmosis membranes are expensive, prone to clogging and contamination, which results in high maintenance costs and imposes additional financial burdens on enterprises; as a result, it is difficult to carry out systematic fluoride removal on an industrial scale. In summary, membrane separation methods have advantages such as a simple operating environment (it can be carried out at normal temperature and pressure), low space requirement, and low energy consumption. However, membrane modules are expensive themselves, have a weak tolerance to changes in temperature and pH levels, and are prone to contamination during the membrane separation process; therefore, pretreatment steps are necessary, which results in high operating and maintenance costs and increases the operational expenses for enterprises. 2.3 Fluoride removal process by adsorption Method The fluoride removal by adsorption method involves utilizing the functional groups of the adsorbent to facilitate ion exchange of fluoride ions, or relying on physical adsorption or chemical reactions to make the fluoride ions adhere to the adsorbent, thereby achieving the purpose of fluoride removal. The adsorbent itself can be regenerated to restore its adsorption capacity, enabling recycling. Currently, most are used for treating drinking water, and less frequently for treating wastewater with high fluoride concentrations. The main adsorbents currently in use include aluminum-containing adsorbents (activated alumina, molecular sieves, bauxite, polyaluminum salts, etc.) ； Natural polymer adsorbents (chitosan adsorbents, functional fiber adsorbents, tea-based iron adsorbents, etc.) ； Other adsorbents (activated carbon, activated magnesium oxide, zeolites, bone char, etc.). Among them, activated alumina is the most widely used adsorbent at present. The advantages of the adsorption method are easy availability of raw materials and low cost, as well as the ability to obtain adsorption materials with excellent properties, resulting in stable water quality after fluoride removal. The disadvantages include the need to pre-treat fluoride-containing wastewater in order to avoid increased costs, the limited adsorption capacity of the adsorbents, a certain degree of reduction in capacity after regeneration, complicated regeneration processes, and issues such as secondary pollution. 2.4 Fluoride removal by precipitation method 2.4.1 Fluoride removal by chemical precipitation Chemical precipitation for fluoride removal involves adding a precipitant to fluoride-containing wastewater, causing the fluoride ions to react chemically with the precipitant to form insoluble fluoride precipitates or fluoride complexes; thereafter, liquid-solid separation is carried out to achieve fluoride removal. Chemical precipitation is widely used and suitable for treating high-concentration fluoride-containing wastewater. The solubility product constant Ksp for CaF2, which is the product of fluoride and calcium concentrations, is only 2.7×10-11. This indicates that calcium salts are very effective at removing fluoride. Additionally, calcium salts are relatively inexpensive, which is why the precipitation method using calcium salts is the most commonly used approach for fluoride removal. When wastewater contains a certain amount of sodium chloride, potassium chloride, and sodium sulfate, the solubility of calcium fluoride in water increases, thereby reducing the fluoride removal efficiency. The chemical precipitation method is effective for removing fluoride, is simple to operate, and has low costs; however, attention should be paid to the solubility of calcium fluoride and the issue of secondary pollution. 2.4.2 Fluoride removal by coagulation-precipitation The fluoride removal method via coagulation-precipitation involves adding a coagulant to fluoride-containing wastewater; the coagulant binds with fluorides to form precipitates, or it undergoes hydrolysis, resulting in an adsorption effect that causes the fluorides to aggregate and precipitate. Subsequently, these precipitates are removed from the wastewater through sedimentation and filtration. This method is suitable for the large-scale treatment of fluorine-containing wastewater. Common inorganic coagulants include aluminum chloride, ferrous sulfate, polyaluminum chloride, and polyferric sulfate ； The commonly used organic coagulant is acrylamide (PAM). The coagulation sedimentation method is simple in process, easy to operate, space-saving, and requires low investment. However, operating temperature, acidity, settling time, stirring intensity, and ions such as Cl- and SO42- can all have a significant impact on the defluorination effect. When treating wastewater with high fluoride content, this leads to an increased amount of coagulant required, thereby raising the treatment costs. Therefore, coagulation is generally used for low-concentration fluoride-containing wastewater that fails to meet the standards after treatment with the calcium salt method. 3 Introduction to the Haip Process: Jiangsu Haip Functional Materials Co., Ltd. is located in the picturesque Suzhou Industrial Park. It is a high-tech enterprise dedicated to the research and development of high-performance adsorbents, catalysts, and related processing technologies. Jiangsu Haipu is committed to providing internationally leading products, technologies, and comprehensive solutions for industries such as environmental protection, resource recycling, new energy, chemicals and pharmaceuticals, food, and printing and dyeing. Jiangsu Haipu Functional Materials Co., Ltd. possesses advanced technical expertise in the treatment of adsorption materials; its accompanying adsorption treatment processes are efficient and stable, having helped numerous leading companies in various industries across China to overcome various environmental challenges. The principle of the Heip adsorption process is to use the special adsorbent materials developed by our company to selectively adsorb the components or substances that need to be removed. When the adsorbent becomes saturated, a specific desorbent is used to remove the adsorbed substances, allowing the adsorbent to be regenerated. This process repeats continuously. When using Heip’s adsorption process to treat fluorine-containing wastewater, the wastewater is first filtered to remove any suspended and particulate matter, after which it enters the adsorption tower. The special adsorption material filled in this tower selectively adsorbs the fluoride present in the wastewater, concentrating it within the adsorption material, thereby reducing the fluoride concentration in the treated water. After reaching adsorption saturation, the adsorbent material is desorbed to allow it to be regenerated and able to adsorb again, and this cycle repeats continuously. The process flow is as follows: http://www.jshpfm.com/uploads/allimg/210307/1-21030H15405356.png Figure 1 Process flow diagram. Case 4: A company in Shandong processed its 600 t/d of fluoride-containing wash water using the HEP adsorption process; as a result, the fluoride content in the water was reduced, without any introduction of other metal ions or impurity ions that could affect reuse, with a removal rate of over 90%. The fluoride concentration, in ppm, was 28.8 in the inlet water and 1.33 in the outlet water, resulting in a removal rate of 95.3%. http://www.jshpfm.com/uploads/allimg/210307/1-21030H1545UM.png Figure 2 shows the appearance of the outlet water (left) and the raw water (right). As can be seen from the figure and table above, after treatment with special adsorbents, the outlet water is colorless, and almost all of the fluoride in the wastewater is removed. The experiments demonstrate that the use of special adsorbents can effectively reduce the fluoride concentration in wastewater.