The electroplating industry discharges a huge amount of pollutants, and the pollutants are highly toxic and difficult to handle, which can easily cause greater environmental damage. Small and medium-sized electroplating enterprises in my country account for a large proportion, their production technology is backward, and the supporting wastewater treatment equipment is not perfect, which causes greater pollution. Electroplating wastewater contains a large amount of heavy metals. These heavy metals tend to accumulate in living organisms and are not biodegradable. Many heavy metal elements are toxic or carcinogenic. When these heavy metals enter the human body, they will cause great harm to the human body. In addition, electroplating wastewater also includes a large number of acids, alkalis and various organic substances, among which some "three causes" substances are serious threats to human health. Therefore, electroplating wastewater needs to be effectively treated and can be discharged after the treatment reaches the standard. The country and society are paying more and more attention to the research of electroplating wastewater treatment technology, in order to realize the harmlessness and resource utilization of waste, and obtain better environmental and economic benefits.
At present, the conventional treatment technologies of electroplating wastewater mainly include physical methods (evaporation concentration method and reverse osmosis), chemical methods (chemical precipitation method, redox method and ferrite method), physical and chemical methods (adsorption method, membrane separation method, ion exchange method) Method and electrolysis method), biological method, combined treatment method and other methods (photocatalytic technology, heavy metal trapping agent), combined with polyacrylamide and other chemical flocculation methods to achieve. This article will introduce the principle, advantages and disadvantages and research status of each treatment method, in order to make prospects for the treatment of electroplating wastewater, and provide basis and direction for research.
Composition and properties of electroplating wastewater
Electroplating wastewater is mainly composed of plated parts cleaning water, waste electroplating solution, equipment cooling water and other wastewater (including washing workshop floor, plate washing water, ventilation equipment condensate water and bath liquid and drainage caused by leakage of plating bath). The quality of wastewater is complex and its composition is difficult to control. It contains different concentrations of iron, copper, zinc, chromium, tin, lead, cadmium, iron and nickel ions, and high concentrations of acid, sulfate, chloride ions, etc. These ions seriously threaten the human body health. In addition, electroplating wastewater also contains many valuable industrial raw materials, which can be recycled.
Electroplating wastewater treatment method
The physical method is a method to separate the suspended pollutants in the electroplating wastewater without changing the chemical properties of the substance, and the representative ones include the evaporation concentration method and the reverse osmosis method. The former, as the name suggests, is to concentrate heavy metals through evaporation. The latter is based on the principle of reverse osmosis, applying a higher pressure to the waste-containing part, allowing water molecules as a solvent to pass through the semi-permeable membrane to separate water from heavy metals and other solutes. Both are physical operations and the process is mature and simple; there is no need to add chemical reagents, no secondary pollution, and can recycle heavy metals and water, generally suitable for chromium, copper and nickel wastewater. However, these two methods are not suitable for the treatment of wastewater with low heavy metal content due to high energy consumption and high cost. Therefore, the physical method is generally used as an auxiliary treatment method and other methods to treat electroplating wastewater. Microfiltration-reverse osmosis process is used to deeply treat electroplating wastewater. The results show that the desalination rate, Cu2+ removal rate, and Ni2+ removal rate in electroplating wastewater reach 95.6%, 98.8%, 98.6%, respectively, the turbidity is almost completely removed, and the effluent quality meets GB 21900-2008 "Emission Standard of Electroplating Pollutants" in the special discharge limit requirements of water pollution.
It is a method of sedimentation, filtration, separation, and removal by adding chemical reagents to combine with pollutants in the wastewater to form a precipitate. It mainly includes sulfide precipitation, hydroxide precipitation, chromate precipitation and ferrite precipitation. As a traditional process, chemical precipitation method has mature application and relatively low cost, so it occupies a large proportion in electroplating wastewater treatment. However, it has problems such as excessive chemical consumption, large amount of waste residue, heavy metals cannot be directly reused, and secondary pollution. Na2S2O5 is used to reduce the chromium ions in the electroplating wastewater to generate trivalent chromium ions with less harmful effects, which can be precipitated by adjusting the pH to achieve the removal of chromium.
Redox method is a method that uses oxidizing or reducing agents to undergo redox reactions with soluble pollutants, thereby converting pollutants into harmless substances. Which mainly includes chemical oxidation and chemical reduction. The redox method has the advantages of wide sources, high efficiency, simple operation, low investment, and wide application. The oxidation-reduction method was used to treat the mixed electroplating wastewater containing cyanide and chromium. The results showed that the indexes of the two kinds of wastewater after mixed treatment were better than the national standard, and the process flow and equipment were simpler than when treated separately.
The principle of the ferrite method is: under suitable temperature conditions and pH conditions, the added ferric sulfate and the metal ions in the electroplating wastewater form ferrite composite oxides, and heavy metal ions are removed through solid-liquid separation. The ferrite method has the advantages of simple process, easy solid-liquid separation, and no secondary pollution. However, this method has high processing cost, difficult to control processing conditions, and produces a large amount of sludge. Ferrite method is used to treat mixed electroplating wastewater. This method can effectively treat electroplating waste liquid containing multiple heavy metal ions, and the treatment price is low.
Physicochemical method/ion exchange method
The exchange group in the exchanger is used to selectively exchange and separate different ions in the wastewater to achieve a method of removing pollutants. At present, this method is mainly suitable for the treatment of electroplating wastewater containing chromium, nickel, and gold. The ion exchange method has advantages in terms of processing efficiency and resource recovery that other methods are difficult to match, but it has the problems of large one-time investment, complicated operation and management, large area, and easy to cause "second pollution". In addition, since the resin column is easily saturated, the ion exchange method is limited in wastewater with a high concentration of heavy metals. Dong Xin et al. used ion exchange method to treat electroplating wastewater containing chromium. The results showed that the concentration of C(r VI) in the effluent after treatment was less than 0.2mg/L, which reached the national discharge standard. In addition, the chromic acid solution obtained can be reconcentrated after being concentrated. It is applied to the plating tank to eliminate the environmental pollution caused by C(r VI).
Electrolysis is a method that uses electrode oxidation and reduction products to chemically react with harmful substances in wastewater to generate precipitation. This method is highly efficient, easy to recycle, and the recovered products generally have reutilization value and have certain economic benefits. At the same time, this method is not suitable for treating low-concentration electroplating wastewater because of its high energy consumption and high cost. Many researchers use electrodialysis to selectively recover zinc and nickel from electroplating wastewater. Guan W et al.  used RuO2/Ti anode and stainless steel cathode combined with electro-oxidation-electrodeposition (EO-ED) system to treat nickel-ammonia complex wastewater, and at the same time realized the complexation of nickel-ammonia complex and the recovery of nickel metal. , The nickel recovery rate is 85-95%, and the ammonia nitrogen removal rate is 65-70%.
Due to the permeation of the membrane, driven by external energy, the selective permeation of certain components in the waste liquid can be achieved, thereby achieving separation, purification and enrichment. These include reverse osmosis (RO), microfiltration, ultrafiltration, and nanofiltration. These methods can not only solve the problem of heavy metal pollution, but also reuse useful metals in the electroplating industry. Membrane separation method is a promising technology with a small area and no secondary pollution, but the membrane is expensive and easily polluted. Dong Jia et al. used membrane separation method to treat electroplating wastewater, and found that under certain conditions (pressure, pH and reflux ratio), the removal rate of chromium ions, copper ions and nickel ions in the wastewater reached more than 98%, and at the same time economic benefits And environmental benefits are extremely good.
The adsorbent has a special structure, and the method of using these unique structures to adsorb and remove heavy metals is called the adsorption method. Activated carbon, chitosan resin, and humic acid are all common adsorbents. Different adsorbents have different adsorption mechanisms, the main ones being physical, chemical and biological adsorption. The adsorption method has the advantages of high removal efficiency, good stability, no or little secondary pollution, and the adsorbent can be reused. Yubochang used Mg(OH)2 to adsorb Ni2+ ions in wastewater. Studies have shown that when the pH is between 4.8 and 8.6, the stirring time is 4 minutes, and the dosage is 1.5g/L, more than 90% of Ni2+ ions can be This method is used for adsorption and removal, and the used Mg(OH)2 can be reused. Taheri R et al. studied the adsorption of electroplating wastewater by MCM-48 mesoporous silica, and the results showed that the prepared adsorbent can remove 99% of Ag. Wang SY et al. used eucalyptus slag to prepare magnetic biochar to treat Cr-containing electroplating wastewater coexisting with metals. Among them, Cr(VI), total Cr, Cu(II) and N(i II) are effectively adsorbed on the magnetic biochar. , The removal rate is 97.11%, 97.63%, 100% and 100% respectively. Moreover, the used magnetic biochar still has the original magnetic separation performance.
Biological method is a method of enriching to remove pollutants through the adsorption function and metabolism of microorganisms or plants. Compared with other physical and chemical methods, biological methods have the advantages of low consumption, economy, and environmental protection, and can further recover heavy metals. However, biological methods are mostly in the experimental simulation stage, and their practical and industrialization still need in-depth research. Liu C et al. used the biosorption of coffee ground waste to effectively reduce and adsorb chromium in electroplating wastewater. During the process, Cr(VI) is completely eliminated, and only a small amount of Cr(III) remains in the solution. Hackbarth FV et al. found that macroalgae (P.algalicula-ta) can be used as a natural electron donor for reducing Cr(VI) to Cr(III) under acidic pH conditions, and as a chelating agent for zinc, iron and trivalent chromium. Natural cation exchanger.
Combination processing technology
There are many types of electroplating wastewater, and various production processes also make the various characteristics of wastewater different, making it difficult to use a single wastewater treatment technology widely. At the same time, it is difficult for a single treatment method to achieve the required indicators, and it is impossible to achieve the unity of treatment effects and economic benefits. Multi-combination technology is used to solve this problem. Multiple technologies learn from each other's strengths and promote each other to achieve better treatment effects and economic benefits. The combined process of physicochemical-biological-membrane method is the mainstream of electroplating wastewater treatment. The physicochemical method has a good effect on removing heavy metal ions in electroplating wastewater, the biological method can effectively remove organic matter, and the membrane method further intercepts pollutants. Combining the advantages of the three in the removal of different pollutants, thereby effectively reducing the treatment cost of electroplating wastewater and increasing the regeneration rate. In addition, other combined methods are also widely used. The micro-electrolysis-A/O process is used to treat electroplating wastewater. The mass concentrations of ammonia, total nitrogen and COD in the effluent meet the emission standards, and the removal effect is significant and stable. Cui J et al. used the ozone oxidation-biological aerated filter (BAF) process to treat cyanide-containing electroplating wastewater. The results showed that the removal rates of CN-, COD, Cu2+ and Ni2+ were 99.7%, 81.7%, 97.8% and 95.3%, respectively. And the effluent concentration reached the discharge standard of electroplating wastewater. In addition, the addition of glucose can improve the pollutant removal efficiency of the biofilter. Ghosh P et al. proposed a combined method of electrochemical method and lime precipitation as an effective method for treating rayon industrial wastewater containing high COD and zinc.
The principle of photocatalytic treatment technology is that photocatalysts undergo a transition under light to generate electron-hole pairs. Among them, electrons can directly reduce heavy metals in electroplating wastewater, while holes can oxidize water into hydroxyl radicals, thereby reducing difficult-to-degrade organics Oxidized to H2O, CO2. The photocatalyst mainly includes TiO2, ZnO, WO3, SrTiO3, SnO2 and Fe2O3. Photocatalytic technology has the characteristics of wide application range, effective treatment, good product degradation, and no secondary pollution. Under ultraviolet light conditions, TiO2 was selected as the catalyst to carry out the photocatalytic reaction of the complex copper wastewater. The results showed that the removal rates of Cu(II) and COD in the complex copper wastewater were 96.56% and 57.67%, respectively, under suitable conditions. .
Heavy metal trap
Under normal temperature environment, most of the heavy metal ions in the wastewater and the heavy metal trapping agent can produce strong chelation, and the resultant product is the polymer chelate salt precipitation, which can be removed by solid-liquid separation. the goal of. This method has the advantages of wide sources, no secondary pollution, high reaction efficiency and good selectivity, and is especially suitable for wastewater with low heavy metal content. The effect of three kinds of supplementing agents on the market on the treatment of Cu2+, Zn2+ and Ni2+ in the actual electroplating wastewater. It was found that trisodium thiocyanate (TMT) is suitable for the treatment of a single copper-containing wastewater; dimethyl dithio Sodium carbamate (Me2DTC) has better applicability, and the removal effect of three kinds of heavy metal ions is better at pH=9.7, and all kinds of ions can meet the standard discharge; sodium diethyldithiocarbamate (Et2DTC) is effective for The Ni2+ treatment effect is not ideal.
Conclusion and Outlook
The country’s requirements for the environmental protection industry are becoming stricter. Electroplating companies need to pay attention to environmentally friendly production and launch green energy-saving technologies. This requires more in-depth research on electroplating wastewater treatment technology. On the other hand, this is also for electroplating wastewater treatment. The research of technology provides opportunities, and the research of various treatment methods becomes more necessary. The optimization of treatment methods and the invention of new treatment methods will make this field have more rapid development. According to the research and governance of electroplating wastewater treatment at home and abroad, the following prospects are put forward:
(1) For the treatment and reuse of waste slag, sludge and electroplating waste liquid, it is possible to consider establishing a centralized recycling center covering a city or a region, plan overall, and collect and reuse in a unified manner to realize the reuse of resources;
(2) The chemical method is used for more than 90% of electroplating wastewater treatment in foreign countries. At the same time, a single treatment method is difficult to achieve the desired treatment effect. According to this, the development of a combined treatment technology with chemical methods as the main and other methods is a kind of The development direction, this combined processing technology has a wide range of applications, while saving resources.
(3) Microbes have received widespread attention due to their low cost and no secondary pollution. In-depth research at the genetic level through molecular biology technology, and the combined treatment of electroplating wastewater with plants or new functional materials will be the future research hotspot.
(4) On the basis of achieving zero discharge of key pollutants from electroplating wastewater, clean production technologies such as heavy metal wastewater recycling technology are adopted to realize the recycling and utilization of useful heavy metals.
(5) The research on the automatic control system of wastewater treatment has certain significance. It will adjust the wastewater treatment equipment in real time according to the different components of the wastewater and the different content of the components to achieve efficient treatment results.