Fluorine-containing wastewater treatment technology
Fluorine is one of the indispensable trace elements for the human body to maintain normal physiological activities. An appropriate amount of fluoride can promote the calcification of teeth and bones, and contribute to the conduction of nerve excitation and the metabolism of enzymes in the body, but excessive intake of fluoride by the human body can lead to fluorosis, osteoporosis, and arthritis.
The World Health Organization stipulates that the upper limit of fluoride content in drinking water is 1.5mg/L. my country’s “Sanitation Standards for Drinking Water Quality” stipulates that the limit of fluoride content in drinking water is 1mg/L, and the content of fluoride ions in industrial wastewater should be less than 10mg. /L.
Electroplating, aluminum electrolysis, semiconductors, iron and steel industry, glass manufacturing, phosphate fertilizer production, thermal power plants, fluorite beneficiation, fluoride salts and hydrofluoric acid, and many other production processes will discharge a large amount of fluorine-containing wastewater, the content of which is above 100mg/L. In some industries, the fluoride ion content is even as high as several thousand mg/L. Therefore, fluorine-containing wastewater must be treated and discharged to the outside after reaching the standard.
At present, the reported methods for removing fluorine from fluorine-containing wastewater include chemical precipitation, coagulation precipitation, adsorption, ion exchange, membrane filtration, electrochemistry, and induced crystallization. Among the many methods, chemical precipitation, coagulation precipitation, and adsorption have attracted much attention due to their strong practicability. This paper mainly introduces the research progress of these three methods in the field of fluoride removal in China in recent years and points out the direction of future efforts.
Fluorine-containing wastewater treatment method
1.1 Chemical precipitation method
The chemical precipitation method for fluoride removal is to add calcium chloride, calcium hydroxide and calcium oxide and other chemical substances to the fluorine-containing wastewater to form calcium fluoride precipitation with fluoride ions to achieve the purpose of removing fluoride. At present, this method is generally suitable for the treatment of large-scale high-concentration fluorine-containing wastewater due to its simple operation, low investment, and obvious fluoride removal effect.
However, calcium fluoride itself has a certain solubility and is co-dissolved with calcium hydroxide, which often leads to the fluorine content in the treated wastewater still being 20-30 mg/L, which is difficult to meet the discharge standard, and there is a large amount of sludge. , serious secondary pollution and other problems. Therefore, it is often necessary to perform the secondary treatment or even multiple treatments on wastewater to meet the discharge requirements.
1.2 Coagulation sedimentation method
The coagulation and sedimentation method for fluorine removal is a method that is widely used and is suitable for the large-scale treatment of wastewater.
The principle is to add a coagulant with a coagulation effect to the fluorine-containing wastewater and then adjust the pH to an appropriate value, and the fluoride in the wastewater is adsorbed by the formed colloid or precipitate, thereby achieving the purpose of removing fluoride ions.
Flocculants can be divided into three categories: organic flocculants, inorganic flocculants, and microbial flocculants. Among them, iron salts and aluminum salts are common flocculants.
Calcium chloride, polyaluminum chloride and polyacrylamide are added to the secondary coagulation reaction pool and the tertiary coagulation reaction pool respectively, and the fluoride ion content can directly reach the emission standard by 1000mg/L before treatment.
When calcium chloride and calcium sulfate are present in the water, the defluorination capacity increases due to the same ion effect.
Two organic flocculants, chitosan, and acrylamide-modified chitosan were synthesized, and their fluorine removal properties were compared. The results show that in laboratory simulated fluorine-containing wastewater, acrylamide-modified chitosan has better fluorine removal performance, and the optimal fluorine removal temperature of both flocculants is 25℃.
The high fluorine content wastewater produced by a solar cell manufacturer was studied, and it was found that under the optimized process conditions, the F-content could be reduced from 7456mg/L before being untreated to below 10mg/L, reaching the first level in GB8978-2002. Emission Standards.
The optimized process conditions are as follows: the dosage of Ca2+ is twice the amount of F-, the pH of the coagulation and precipitation process is 8 to 9, and the dosage of coagulant poly aluminium chloride and coagulation aid polyacrylamide is 400mg/ L and 4 mg/L.
Research shows that: FCZJ-15 is not suitable for defluorination treatment alone, but it is suitable for compounding with aluminum salts for defluorination.
In areas with high fluorine content, the modified microbial flocculant FCZJ-15 can reduce the usage of aluminum salts to about 1/2 of the original usage.
The advantage of the coagulation sedimentation method is that the dosage of flocculant in the coagulation sedimentation is small, and a large amount of wastewater can be treated at one time. While removing F-, harmful substances in the flocculant are introduced, and there is the possibility of secondary treatment.
1.3 Adsorption method
The adsorption method is the most widely used fluorine removal method, which can be directly used for low fluorine content wastewater treatment and can also be used as an advanced treatment after chemical precipitation and coagulation precipitation.
According to the different raw materials used, the adsorbents are divided into conventional adsorbents and new high-efficiency adsorbents. Table 1 lists adsorbent types and some examples.
1.3.1 Conventional adsorbents
Most of the polymer-based adsorbent materials are derived from natural biomass and its derivatives, such as chitosan and diatomaceous earth.
Although natural chitosan can remove fluoride ions in water through surface adsorption, complexation and ion exchange, it usually needs to be loaded and modified to improve the fluoride removal performance.
The main chemical composition of diatomite is SiO2, and contains a small amount of impurities such as Al2O3, CaO, MgO, etc. It has a huge specific surface area and silicon and fluoride ions in water form stable fluorosilicic acid, which will enhance the defluorination effect.
Natural minerals such as zeolite, bentonite, etc. have low cost and have good development prospects as fluoride removal adsorbents.
However, both original zeolite and original bentonite need to be modified properly to further improve the fluorine removal performance. Take the fluorine-containing wastewater produced by a chemical enterprise as an example.
The hydroxy complex Al(OH)2+ rich in positively charged aluminum is the key to improving the fluorine removal performance. The modified zeolite not only has high fluorine removal performance but also has better regeneration ability and recycling efficiency.
The original bentonite was not effective in removing fluoride ions in the simulated wastewater, but the performance was greatly improved after modification with poly dimethyl diallyl ammonium chloride and sulfuric acid.
When the dosage of bentonite is 30g/L, the pH is 4, the reaction temperature is 25℃, and the adsorption time is 25min, the highest removal rate of F- by modified bentonite can reach 97%, and the residual content of fluorine after treatment reaches the national first-class pollution. emission standards.
Metal-based adsorbents are mainly oxides or hydroxides of metals such as aluminum, iron, and magnesium. Activated alumina is the earliest metal oxide used for fluorine removal. It has a large specific surface area, high mechanical strength, good high-temperature resistance, and corrosion resistance. It has an ideal fluorine removal effect in acidic solutions, and its adsorption capacity is generally 0.8 ~ 2.0mg/g.
Iron-based adsorbents have similar properties to aluminum-based adsorbents but are more stable, while MgO needs to be pre-treated due to its certain defluorination ability.
Activated alumina is the most commonly used fluoride removal agent. Although it has the advantages of low raw material price, stable fluorine removal capacity, and stable effluent quality, activated alumina also has low adsorption capacity, difficult separation, and rapid decline in adsorption capacity after repeated regeneration. main disadvantage.
Using Fe3O4-TiO2 as a composite carrier to support activated alumina effectively solves the problem of separation and adsorption capacity. Under the same operating conditions, Fe3O4-TiO2·nH2O·Al has the same adsorption effect on simulated wastewater and actual wastewater. The fluoride ion content can reach the emission standard of <10mg/L.
1.3.2 New high-efficiency adsorbents
In order to improve the shortcomings of traditional adsorbents such as low adsorption capacity and low mechanical strength when absorbing fluorine in wastewater, rare earth elements with high affinity for F-, large adsorption capacity and high adsorption rate can be added to fluorine removal materials.
The results show that the hydroxyl groups on the surface of the adsorbent participate in the reaction, and the temperature rises to facilitate the progress of the adsorption. The material has a large adsorption capacity and a wide adsorption pH range.
The researchers found that the adsorption rate of the Mg-Al-La composite oxide prepared by the double-drop coprecipitation method was mainly controlled by the intra-particle diffusion of the adsorbent and the adsorption reaction.
Its maximum saturated adsorption capacity can be as high as 54.2 mg/g, which is slightly higher than that of Mg-Al-Zr composite oxides and Mg-Al-Ce composite oxides, and significantly higher than other adsorbents without rare earth elements. Divalent and trivalent anions will affect the defluorination performance to a certain extent.
Resin contains dense pore structure, huge specific surface area, and various active groups. It has the characteristics of large adsorption capacity, less pollution, easy modification, and convenient separation. It has been widely used in separation, environmental protection, catalysis, and medical treatment.
Hu Jiapeng et al. applied the prepared hydroxylanthanum modified resin to simulated fluorine-containing wastewater in the laboratory, and found that the hydroxylanthanum modified resin had better selective adsorption performance for F-, which not only enabled the treated wastewater to meet the national discharge standard, and can improve the pH of discharged wastewater.
In general, whether using traditional adsorbents or new adsorbents, the advantage of the adsorption method is that the content of fluoride ions can be reduced to drinking water levels, but the main disadvantage is that in practical applications, fluoride ions compete with other coexisting anions in wastewater. , the adsorbent must have preferential selectivity for fluoride ions.
In addition, how to prepare adsorbents with large adsorption capacity, high adsorption rate, good economic benefits, and multiple regenerations and reuse are worthy of further research.
Conclusion
Fluoride ion is one of the more difficult substances to remove in wastewater.
Considering that other ions may be present in the wastewater, fluoride removal becomes more difficult. From the existing successful practice, for large-scale fluoride-containing wastewater with high concentration.
Domestically, the multi-stage precipitation method combining chemical precipitation method and coagulation precipitation method is mostly used, and whether to use the adsorption method for secondary treatment is determined according to the actual treatment effect. Exchange method to make it meet the emission standards.
For the coagulation type precipitation method, the removal mechanism and effect will be significantly different due to the different types of flocculants added.
Therefore, it is very important to develop green and environmentally friendly high-efficiency flocculants in the future to realize the comprehensive utilization of fluorine resources.
For the adsorption method, the preferential selectivity of the adsorbent to fluoride ions, the adsorption mechanism and the regeneration and utilization of the adsorbent need to attract more attention from researchers.