Electroplating wastewater component is very complex, in addition to cyanide (CN -), and an outer waste acid waste, heavy metals electroplating industry wastewater is potentially hazardous waste category great. The heavy metal elements of heavy metals contained in waste water are classified generally be divided into chromium (Cr) waste water, containing nickel (Ni) wastewater containing cadmium (Cd) wastewater containing copper (Cu) wastewater containing zinc (Zn) waste water, Gold-containing (Au) wastewater, silver- containing (Ag) wastewater, and the like. The treatment of electroplating wastewater has been paid more and more attention at home and abroad, and various treatment technologies have been developed to eliminate and reduce heavy metal emissions by transforming toxic treatment into non-toxic and harmful, harmless, recycling precious metals, and recycling water. With the rapid development of electroplating industry and the increasing requirements of environmental protection, at present, electroplating wastewater treatment has begun to enter the stage of clean production process, total quantity control and circular economy integration. Resource recycling and closed circuit are the mainstream of development.
I. Current status of electroplating heavy metal wastewater treatment technology
(1) Chemical Precipitation The chemical precipitation method is a method for converting a heavy metal in a dissolved state into a heavy metal compound insoluble in water, including a neutralization method and a sulfide precipitation method.
1. Neutralization precipitation method Neutralization reaction is carried out by adding a base to a heavy metal-containing wastewater to separate a heavy metal into a water-insoluble hydroxide precipitate form. The neutralization precipitation method is simple in operation and is a commonly used method for treating wastewater. Practice has proved that the following points should be noted in the operation: (1) After neutralization and precipitation, if the pH value in the wastewater is high, it needs to be neutralized before it can be discharged; (2) There are often many kinds of heavy metals coexisting in the wastewater. When containing an amphoteric metal such as Zn, Pb, Sn or Al, the pH value is too high, and there may be a tendency to re-dissolve. Therefore, the pH value should be strictly controlled to carry out staged precipitation; (3) Some anions in the wastewater such as halogen, cyanide, Humic substances and the like may form complexes with heavy metals, so they need to be pretreated before neutralization; (4) Some particles are small and difficult to precipitate, and flocculant is added to assist precipitation.
2. Sulfide precipitation method A method in which a sulfide precipitant is added to remove a heavy metal ion from a waste water to form a sulfide precipitate. Compared with the neutralization precipitation method, the sulfide precipitation method has the advantages that the solubility of the heavy metal sulfide is lower than that of the hydroxide, and the pH of the reaction is between 7 and 9, and the treated wastewater is generally not neutralized. Disadvantages of sulfide precipitation is: sulfide precipitation of small particles easily form a colloid; sulfide precipitation agent itself residual water, hydrogen sulfide gas with acid, secondary pollution. In order to prevent the secondary pollution problem, British scholars have developed an improved sulfide precipitation method, that is, selective addition of sulfide ions and another heavy metal ion in the wastewater to be treated (the sulfide ion balance concentration ratio of the heavy metal is required) The equilibrium concentration of the sulfide of the removed heavy metal pollutant is high). Since the added heavy metal sulfide is more soluble than the heavy metal sulfide in the wastewater, the original heavy metal ions in the wastewater are separated from the added heavy metal ions, and the harmful gas hydrogen sulfide generation and sulfide ion residue problem are prevented. .
(2) Redox treatment
1. Chemical reduction method Cr in electroplating wastewater mainly exists in the form of Cr 6+ ions. Therefore, after adding reducing agent to waste water to reduce Cr 6+ to slightly poisonous Cr 3+ , lime or NaOH is added to produce Cr (OH). 3 precipitation separation and removal. The chemical reduction method for treating electroplating wastewater is one of the earliest applied treatment technologies. It has a wide range of applications in China. Its treatment principle is simple, the operation is easy to master, and it can withstand the impact of large water volume and high concentration wastewater. Depending on the dosage of the reducing agent method can be divided into FeSO 4, NaHSO 3 method, scrap iron method, SO 2 method and the like.
The chemical reduction method is used to treat the wastewater containing Cr. Generally, lime is used in alkalization, but the waste residue is more. When NaOH is used, the sludge is less, but the cost of the medicament is high, and the treatment cost is large, which is a disadvantage of the chemical reduction method. 2. Ferrite method Ferrite technology was developed based on the principle of ferrite production. Excess FeSO 4 is added to the Cr-containing wastewater to reduce Cr 6+ to Cr 3+ , Fe 2+ to Fe 3+ , and the pH value is adjusted to about 8, so that Fe ions and Cr ions are precipitated by hydroxide. The mixture is stirred by air and added with hydroxide to continuously react to form chrome ferrite. Typical processes are intermittent and continuous. The sludge formed by the ferrite method has high chemical stability and is easy for solid-liquid separation and dehydration. In addition to the treatment of Cr-containing wastewater, the ferrite method is particularly suitable for electroplating mixed wastewater containing heavy metal ions. China has applied ferrite method for decades, and the treated wastewater can meet emission standards, and it is widely used in the domestic electroplating industry.
The ferrite method has the advantages of simple equipment, low investment, simple operation, and no secondary pollution. However, heating (about 70 degrees Celsius) is required in the process of forming ferrite, the energy consumption is high, the salinity after treatment is high, and there is a disadvantage that the wastewater containing Hg and complex is not treated.
3. Electrolysis method The treatment of Cr-containing wastewater by electrolysis has been in China for more than 20 years. It has the advantages of high removal rate, no secondary pollution, and the recyclability of precipitated heavy metals. Metal ions in about 30 different wastewater solutions can be electrodeposited. Electrolysis is a relatively mature treatment technology, which can reduce the amount of sludge generated, and can recover metals such as Cu, Ag, Cd, etc., which have been applied to the treatment of wastewater. However, the cost of electrolysis is relatively high, and the economic benefits are usually better after concentration.
In recent years, the electrolysis method has developed rapidly, and the internal electrolysis of iron filings has been deeply studied. The dynamic wastewater treatment device developed by the principle of internal electrolysis of iron filings has a good removal effect on heavy metal ions.
In addition, the High Voltage Electrocagulation System is a new generation of electrochemical water treatment equipment in the world, such as Cr, Zn, Ni, Cu, Cd, CN-, etc. in surface treatment, coating wastewater and electroplating mixed wastewater. Contaminants have significant governance effects. The high-voltage pulse electrocoagulation method increases the current efficiency by 20%~30% compared with the traditional electrolysis method; the electrolysis time is shortened by 30%~40%; the energy saving reaches 30%~40%; the sludge production is small; the heavy metal removal rate can reach 96%. ~99%. [next]
(III) Solvent extraction separation Solvent extraction method is a commonly used method for separating and purifying substances. Because of the liquid-liquid contact, it can be operated continuously, and the separation effect is better. When using this method, the extractant with higher selectivity should be selected. The heavy metals in the wastewater are generally in the form of cations or anions, for example, under acidic conditions, complexation with the extractant, extraction from the aqueous phase to the organic phase. Then, it is back extracted to the aqueous phase under alkaline conditions, and the solvent is regenerated for recycling. This requires careful selection of the acidity of the aqueous phase during the extraction operation. Although the extraction method has great advantages, the loss of solvent in the extraction process and the energy consumption in the regeneration process make the method have certain limitations, and the application is greatly limited.
(4) Adsorption method
Adsorption is an effective method for removing heavy metal ions using the unique structure of the adsorbent. The adsorbents for treating heavy metal wastewater by adsorption are activated carbon, humic acid, sepiolite, and polysaccharide resin. The activated carbon equipment is simple and widely used in wastewater treatment, but the activated carbon regeneration efficiency is low, and the treated water quality is difficult to meet the reuse requirement, and is generally used for the pretreatment of electroplating wastewater. Humic acid is a relatively inexpensive adsorbent. It has been successfully used to treat humic acid as a humic acid resin for treating wastewater containing Cr and Ni. Related studies have shown that chitosan and its derivatives are good adsorbents for heavy metal ions. After cross-linking of chitosan resin, it can be reused 10 times, and the adsorption capacity is not significantly reduced. The modified sepiolite treatment of heavy metal wastewater has good adsorption capacity for Pb 2+ , Hg 2+ and Cd 2+ . The content of heavy metals in the wastewater after treatment is significantly lower than the comprehensive discharge standard of wastewater. It is also reported in the literature that montmorillonite is also a good clay mineral adsorbent. The removal rate of Cr 6+ in aluminum zirconia pillared montmorillonite is 99% under acidic conditions, and the content of Cr 6+ in effluent is lower than national emission. Standard, with practical application of heat.
(5) Membrane separation technology
The membrane separation method is a technique for separating substances by utilizing the selectivity of a polymer, including electrodialysis, reverse osmosis, membrane extraction, ultrafiltration, and the like. The electroplating industrial wastewater is treated by electrodialysis, and the composition of the wastewater after treatment is unchanged, which is beneficial to the use of the return tank. Metal ion wastewaters containing Cu 2+ , Ni 2+ , Zn 2+ , Cr 6+ , etc. are all suitable for electrodialysis treatment, and complete sets of equipment are available. The reverse osmosis method has been widely used for the treatment of Zn, Ni, Cr rinse water and mixed heavy metal wastewater. The reverse osmosis method is used to treat the electroplating wastewater, and the treated water can be reused to achieve a closed circuit cycle. There are many reports on the treatment of electroplating wastewater by liquid membrane method. In some fields, the liquid membrane method has entered the preliminary industrial application stage from basic theoretical research. For example, both China and Austria use emulsion liquid membrane technology to treat Zn-containing wastewater, and also apply to Au-plated waste. In liquid treatment. Membrane extraction technology is a highly efficient, non-secondary separation technology that has made great progress in metal extraction.
(6) Ion exchange treatment
The ion exchange treatment method is a method for separating harmful substances in waste water by using an ion exchanger. The ion exchangers used are ion exchange resins, zeolites, etc., and the ion exchange resins are gel type and macroporous type. The former is selective, and the latter is complicated to manufacture, high in cost, and large in regenerant consumption, and thus is greatly limited in application. Ion exchange is achieved by ion exchange of freely mobile ions carried by the exchanger itself with ions in the treated solution. The driving force for ion exchange is the difference between the concentration of ions and the affinity of the functional groups on the exchanger for ions. In most cases, the ions are first adsorbed and then exchanged, and the ion exchanger has the dual functions of adsorption and exchange. This material is used more and more, such as bentonite , which is a clay composed mainly of montmorillonite. It has good water swelling, large specific surface area, strong adsorption capacity and ion exchange capacity. The ability to adsorb and ion exchange is stronger. However, it is more difficult to regenerate. Natural zeolite has more advantages than bentonite in the treatment of heavy metal wastewater: zeolite is an aluminosilicate mineral with a grid structure, which is porous inside, has a large specific surface area, and has unique adsorption and ions. Exchange ability. Studies have shown that the mechanism of zeolite removal of heavy metal ions from wastewater, in most cases, is the dual role of adsorption and ion exchange. With the increase of flow rate, ion exchange will dominate the adsorption. Pretreatment of natural zeolite with NaCl improves adsorption and ion exchange capacity. Through the adsorption and ion exchange regeneration process, the concentration of heavy metal ions in the wastewater can be concentrated by 30 times. The zeolite removes copper. During the regeneration of NaCl, the removal rate is over 97%, and the adsorption and exchange cycles can be repeated, and the removal rate of copper is not reduced. [next]
(7) Biological treatment technology
Because traditional treatment methods have the disadvantages of high cost, complicated operation, and difficult to deal with harmful pollution caused by large flow and low concentration, after years of exploration and research, biological control technology has been paid more and more attention. With the research progress of heavy metal-tolerant microorganisms, the use of biotechnology to treat electroplating heavy metal wastewater is booming. According to the mechanism of biological removal of heavy metal ions, it can be divided into bioflocculation, biosorption, biochemical and phytoremediation.
1. Bioflocculation method The bioflocculation method is a decontamination method for flocculation and sedimentation using metabolites produced by microorganisms or microorganisms. Microbial flocculants are a class of metabolites that are produced by microorganisms and secreted outside the cell and have flocculation activity. Generally, it is composed of a polymer material such as polysaccharide, protein, DNA, cellulose, glycoprotein, or polyamino acid, and contains a plurality of functional groups in the molecule, so that the colloidal suspension in the water can be coagulated and precipitated. Up to now, there are about a dozen varieties of flocculation of heavy metals. The amino and hydroxyl groups in bioflocculants can form stable complexes with heavy metal ions such as Cu 2+ , Hg 2+ , Ag + and Au 2+ . And precipitated. The application of microbial flocculation method is safe, convenient, non-toxic, non-secondary, with good flocculation effect, fast growth and easy industrialization. In addition, microorganisms can genetically engineer, acclimate or construct strains with special functions. Therefore, the microbial flocculation method has broad application prospects.
2. Biosorption method The biosorption method is a method for adsorbing metal ions dissolved in water by utilizing the chemical structure and composition characteristics of the living body, and then separating the metal ions in the aqueous solution by solid-liquid two-phase separation. The extracellular polymer is used to separate metal ions, and some proteins released during the growth process can be removed by converting soluble heavy metal ions in the solution into precipitates. Biosorbent has been widely used because of its wide source, low price, strong adsorption capacity and easy separation and recovery of heavy metals.
3. Biochemical method Biochemical method refers to the removal of soluble metal ions by inoculating the soluble metal ions into insoluble compounds. Sulfate bioreduction is a typical biochemical method. The method is to reduce the sulfate to H 2 S by the sulfate reduction of the sulfate-reducing bacteria under anaerobic conditions, and the heavy metal ions in the wastewater can react with the generated H 2 S to form a metal with low solubility. The precipitate is removed and the reduction of H 2 SO 4 can increase the pH of the wastewater by taking SO 4 2 as S2. It precipitates due to the small ion product of many heavy metal ion hydroxides. Relevant research shows that the biochemical treatment of wastewater containing Cr 6+ of 30~40mg/L can reach 99.67%~99.97%. Some people also used livestock manure anaerobic digestion sludge to treat heavy metal ions in mine acid wastewater. The results show that the method can effectively remove heavy metals from wastewater. Zhao Xiaohong et al. used the desulfovation of Enterobacter aerogenes (SRV) to remove copper ions from electroplating wastewater. The solution with a copper concentration of 246.8 mg/L had a removal rate of 99.12% at a pH of 4.0.
4. Phytoremediation method The phytoremediation method refers to the use of higher plants to reduce the heavy metal content of existing contaminated soil or surface water by absorption, sedimentation, enrichment, etc., in order to achieve pollution control and environmental restoration. The phytoremediation method is an effective method to use the ecological engineering to control the environment. It is an extension of biotechnology to treat enterprise wastewater. The use of plants for the treatment of heavy metals consists of three main components: (1) the use of metal-accumulated plants or hyperaccumulators to extract, precipitate or enrich toxic metals from wastewater; (2) the use of metal-accumulated plants or hyperaccumulators to reduce the activity of toxic metals, thereby It can reduce the leaching of heavy metals into the ground or through the air carrier: (3) extracting heavy metals in the soil or water by metal accumulation plants or hyperaccumulation plants, enriching and transporting them to the harvestable parts of the plant roots and the branches of the plants. . Reduces the concentration of heavy metals in soil or water by harvesting or removing shoots that have accumulated and enriched heavy metal plants. Plants that can be utilized in phytoremediation techniques include algae, herbaceous plants, woody plants, and the like.
The ability of algae to purify heavy metal wastewater is mainly manifested in the strong adsorption of heavy metals. The use of algae to remove heavy metal ions has been reported extensively. The absorption of Au by brown algae reached 400mg/g. Under certain conditions, the removal rate of heavy metal ions such as Cu, Pb, La, Cd and Hg by green alga was 80%~90%. The adsorption of heavy metals by Sargassum and S. Although it is not as good as green seaweed, it still has good removal ability.
There have been many reports on the use of herbal plants to purify heavy metal wastewater. Eichhornia crassipes is an internationally recognized and commonly used aquatic floating plant for pollution control. It has the characteristics of rapid growth, low temperature resistance and high temperature resistance. It can rapidly and massively enrich Cd, Pb, Hg and Ni in wastewater. , Ag, Co, Cr and other heavy metals. Studies have found that the absorption rates of cobalt and zinc by Eichhornia crassipes are as high as 97% and 80%, respectively. In addition, there are many herbaceous plants that have purifying effects, such as lotus seeds, water dragons, thorns, duckweed, Indian mustard.
Woody plants have the advantages of large processing capacity, good purification effect, little impact by climate, and are not easy to cause secondary pollution. At the same time, it has strong adsorption and accumulation effects on Cd and Hg in soil. The results of Hu Huanbin and other tests show that both reed and cedar have strong enrichment ability for heavy metals Pb and Cd.
Second, the prospect of electroplating heavy metal wastewater treatment technology
With the implementation of the global sustainable development strategy, circular economy and clean production technology have attracted more and more attention. The treatment of electroplated heavy metal wastewater has been developed from the end treatment to the comprehensive prevention and control stage of clean production process, material recycling and wastewater reuse. In the future, the treatment of electroplating heavy metal wastewater will highlight the following aspects:
(1) Implementing circular economy, attaching importance to the development and application of clean production technologies; improving the conversion rate and recycling rate of electroplating materials and resources; reducing the production of heavy metal pollutants from the source, and adopting whole process control and integrated wastewater treatment Finally, zero discharge of wastewater is achieved.
(2) There are many treatment technologies for electroplating heavy metal wastewater. Among them, biotechnology is a technology with great development potential, which has the advantages of low cost, high efficiency and no secondary pollution. With the development and application of technologies such as genetic engineering and molecular biology, high-efficiency and toxic-resistant strains have been successfully cultivated, which provides favorable conditions for the wide application of biotechnology. For the already polluted and wide-ranging external environment, phytoremediation technology can be used to control the pollution, while not only beautifying the environment, but also obtaining certain economic benefits.
(3) Comprehensive integration technology is a hot spot in the future of electroplating wastewater treatment technology. There are many kinds of electroplating wastewater, and various electroplating processes are very different. The use of only one kind of wastewater treatment method often has its limitations and does not achieve the desired effect. Therefore, an integrated technology that combines the characteristics of various governance technologies has emerged.
Third, the conclusion
In summary, although chemical, physical, and biochemical methods can treat and recover heavy metals in wastewater, biochemical treatment of heavy metal wastewater is low in cost, high in efficiency, easy to manage, and does not cause secondary pollution to the environment. It is conducive to the improvement of the ecological environment. However, biochemical methods also have certain limitations. Both plants and microorganisms are generally selective. They only absorb or adsorb one or several metals, and some may cause poisoning when the concentration of heavy metals is high, thus limiting their application. Despite the broad prospects for the research and development of biochemical methods, many scholars have made the organisms have stronger adsorption, flocculation and remediation capabilities through the application of genetic engineering and molecular biology. We should make full use of the synergistic purification of microorganisms and plants in nature, supplemented by physical or chemical methods, to find effective ways to purify heavy metals.
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