Characteristics and treatment processes of electroplating wastewater

Due to the prosperity and continuous development of the world economy, as well as the rapid advancement of science and technology, the scale of the electroplating industry has been expanded, resulting in a huge amount of electroplating wastewater discharged annually from industrial production. The harm of electroplating wastewater is significant, especially the serious damage to water bodies and the environment. The longer the time, the stronger the toxicity, which further causes great damage to the ecological environment. Compared to other types of pollution, the harm caused by electroplating wastewater far exceeds that of other types of pollution. Therefore, it is very important to adopt scientific and reasonable treatment methods to purify and treat electroplating wastewater.

Due to different electroplating processes and product functional requirements, there are also various types of plating types, plating solution components, operating methods, process conditions, etc., which correspondingly make the pollutants in electroplating wastewater more complex. Overall, the main pollutants in electroplating wastewater are heavy metal ions such as chromium, nickel, copper, etc; Next are acidic and alkaline substances; Some also use toxic substances such as cyanide. In addition, impurities such as oil, grease, dust, etc. that are washed out during the electroplating pretreatment process are also carried into the electroplating wastewater, making the composition of the electroplating wastewater very complex. There are also various techniques for treating electroplating wastewater, which can be generally divided into four categories: chemical methods, physical methods, physicochemical methods, biochemical methods, etc. This article takes cyanide containing wastewater, chromium containing wastewater, and comprehensive wastewater as research objects, and analyzes and compares the advantages and disadvantages of electroplating wastewater treatment technologies.

1、 The source of electroplating wastewater

The main sources of electroplating wastewater are water used for cleaning plated parts, filtering plating solution, passivation wastewater, pickling wastewater for plated parts, wastewater from scrubbing floors and plates, and wastewater generated by "running, emitting, dripping, and leaking" due to poor operation or management during the electroplating production process. In addition, there is also the discharge of self use water during wastewater treatment and laboratory drainage. The properties of electroplating wastewater are mainly determined by the properties of chemical cleaning solutions and electroplating solutions, and can generally be divided into four categories: cyanide containing wastewater, chromium containing wastewater, acidic wastewater, and alkaline wastewater. The main pollutants in wastewater are various metal ions, followed by acidic and alkaline substances. Some electroplating solutions also use pigments and other substances, most of which are organic compounds.

Characteristics and hazards of electroplating wastewater

A large amount of toxic substances are generated in the electroplating production process and exist in industrial wastewater, causing environmental pollution. Scientific and effective treatment processes are needed to reduce pollution. This article mainly analyzes and studies the treatment process of electroplating wastewater in order to improve the effectiveness of wastewater treatment.

Electroplating can change the surface properties of metal or non-metal products, such as corrosion resistance, decorative appearance, conductivity, wear resistance, weldability, etc. It is widely used in the mechanical manufacturing industry, light industry, electronic and electrical industry, etc. Some special functional coatings can also meet the needs of cutting-edge national defense technology products. Modern new electroplating technology has become an important technical foundation for the development of high-tech fields and an indispensable key link in the development of many high-tech industries. It can be foreseen that in the coming period, electroplating enterprises will not only not shrink, but also have a high-level development with the rapid development of manufacturing technology and high-tech.

The pollutants in electroplating wastewater are relatively complex, and the water quality composition is difficult to control. However, overall, it can be divided into heavy metal ion wastewater, acid-base wastewater, and oily wastewater, which often contain multiple pollutants at the same time. Toxic and harmful substances include cadmium, lead, chromium, nickel, tin, zinc, acid, alkali, suspended solids, petroleum substances, nitrogen-containing compounds, surfactants, and phosphates. In addition, manufacturers currently using cyanide electroplating technology have a large amount of cyanide in their electroplating wastewater. Untreated discharge of electroplating wastewater can pollute drinking water and industrial water, posing a threat to the ecological environment; Acidic and alkaline wastewater can damage the living environment of microorganisms in water and affect the acidity and alkalinity of normal water sources; Cyanide containing wastewater is highly toxic and can cause death in trace amounts; Heavy metal ions are highly toxic substances that are carcinogenic, teratogenic, or mutagenic. If a large amount of electroplating wastewater containing heavy metal ions is discharged without treatment, it will accumulate in the human body through the food chain and cause serious health problems. Chromium, tin, and copper can lead to lung cancer; The toxicity of Cr (IV) is second only to tin, but if ingested in large quantities, it can cause acute poisoning, and long-term ingestion can also cause chronic poisoning; Nickel and lead have an accumulation effect in the human body, and long-term exposure can cause chronic poisoning. Tin, chromium, lead, and aluminum are all classified as Class I hazardous substances in China, while copper and zinc have relatively low toxicity and are classified as Class II hazardous substances.

According to relevant data, due to the rapid expansion of the electroplating industry, many harmful substances have been added to water bodies, causing widespread pollution of water resources. Therefore, in recent years, the electroplating industry has been selected as one of the most severely polluted industries in the world.

The treatment process of electroplating wastewater

1. Preprocessing

The main pre-treatment methods currently used include but are not limited to chemical treatments such as oil removal and rust removal, as well as electrochemical treatments such as electrochemical oil removal and erosion, and activation treatments such as acid pickling and rust removal. During the entire oil removal process, alkaline chemicals such as sodium hydroxide and sodium carbonate are often used, while acid pickling and rust removal mainly use acidic compounds such as sulfuric acid and hydrochloric acid. In summary, the main sources of pollution generated by pre-treatment wastewater for plating are acids, alkalis, oils, rust, and other substances.

2. Chemical precipitation method

Chemical precipitation methods mainly include hydroxide precipitation method and sulfide precipitation method, which involves adding strong alkaline hydroxides or sulfides to electroplating wastewater to form hydroxide or sulfide precipitates with heavy metals. However, for electroplating wastewater, in order to ensure uniform and shiny coating, strong chelating agents such as EDTA, cyanide, citric acid, and some mono -, di -, and multi ligand acids are often added to the plating solution. Heavy metals are not easily precipitated with hydroxides and sulfides after complexing with these substances, and the removal effect of this method is limited. The chemical precipitation method is simple to operate, not affected by temperature, metal type, and concentration, and has good removal effect on free metal ions. However, it requires a large amount of reagents and sludge production, and has poor treatment effect on complex electroplating wastewater.

3. Adsorption method

Adsorbents, due to their porous surface with a large specific surface area and numerous active groups, can be used to adsorb pollutants from wastewater. Common adsorbents include zeolite, granular and powdered activated carbon, alumina, fly ash, etc. According to the principle of adsorbents, adsorbents can be divided into physical adsorbents and chemical adsorbents. The adsorption method is simple to operate, does not require any flocculants for direct removal, has low cost, selectivity, and in some cases, the adsorbent is renewable. However, there are also drawbacks such as a large amount of adsorbent added and a sharp decrease in adsorption capacity after saturation.

4. Ion exchange method

Ion exchange method is to exchange various ions in wastewater with exchange groups in the exchange agent, in order to remove pollutants from wastewater. The ion exchange method commonly uses ion exchange resins, which are mainly divided into four types: anion exchange resins, cation exchange resins, humic acid exchange resins, and chelating exchange resins. The ion exchange method has a high removal rate, easy control of operating equipment, and low frequency of regeneration and cleaning, but the equipment is complex and the investment cost is high.

5. Biological methods

Biological methods utilize the physiological characteristics of organisms, such as coagulation, absorption, accumulation, and enrichment of microorganisms and plants, to inhale heavy metals from wastewater into the organism, thereby achieving the goal of removing metals from water. By utilizing the characteristics of organisms, a suitable environment for microbial growth can be created, which can cultivate composite functional bacteria with high selectivity, high adsorption capacity, and high degradation ability for adsorbing and degrading pollutants in wastewater. Biological methods have the advantages of strong adaptability, wide sources, low operating costs, low sludge volume, and no secondary pollution. However, they also have disadvantages such as slow bacterial screening and reproduction, low treatment efficiency, and toxic effects of heavy metals on microorganisms.

6. Membrane separation method

Membrane separation method refers to the technology of using external energy or chemical potential difference to form a pressure difference on both sides of the membrane, and selectively allowing metal ions in electroplating wastewater to pass through the membrane to achieve separation from the wastewater. At present, membrane separation methods mainly include electrodialysis, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Membrane separation method has high removal efficiency and simple operation, but the membrane renewal rate is fast and the cost is high.

7. Advanced oxidation technology

Due to the difficulty in removing organic compounds that are complexed with heavy metals in electroplating wastewater, advanced oxidation technology can be used to break down this chelation reaction, and then assist traditional chemical precipitation technology to reduce pollutant concentrations to within emission standards. Precious metals can also be recycled and reused, which has certain economic benefits. The most significant feature of advanced oxidation technology is the reaction between hydroxyl radicals as the main oxidant and organic matter. The organic radicals generated in the reaction can continue to participate in the chain reaction of HO, or further undergo oxidative decomposition reaction until they are degraded into the final products CO2 and H2O, thereby achieving the goal of oxidative decomposition of organic matter and dissociation of heavy metals from complexes. Advanced oxidation technology has fast reaction speed, high removal efficiency, low requirements for reaction environment, and thorough reaction. It can be used as a separate treatment or combined with other treatment technologies (such as pre-treatment for biochemical treatment technology), which can reduce treatment costs.

4、 The Road to Sustainable Development of Electroplating Wastewater

With the increasing demand for ecological environment, traditional electroplating wastewater treatment methods alone can no longer meet strict discharge standards. Emerging treatment technologies have good treatment effects but high treatment costs, and various treatment methods have their own advantages and disadvantages. Therefore, a single treatment method is no longer suitable for current environmental protection requirements and economic development. In the actual production process, appropriate treatment technologies need to be selected based on the properties of electroplating wastewater (concentration, acidity, proportion of various components, etc.), output, environmental conditions, and economic conditions. At the same time, adopting multiple ways to work together can not only meet the requirements of emission standards, but also save costs and promote sustainable economic and ecological development.

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