The production wastewater discharged from the electroplating industry is one of the major sources of environmental pollution. By the year 2000, more than 10,000 electroplating factories were established in China, and the annual discharge of electroplating wastewater had reached over 4 billion cubic meters. If not treated or poorly treated, it will result in resource wastage and severely harm the environment. Electroplating wastewater mainly includes acidic and alkaline wastewater, some organic wastewater, and heavy metal wastewater, with heavy metal wastewater being the focus of treatment. 

Currently, conventional methods for treating heavy metal wastewater include chemical precipitation, adsorption, and biological methods. These methods primarily work by converting heavy metals into precipitates or other forms, which can easily lead to secondary pollution of the environment. For example, some projects use Fenton’s reagent to treat electroplating wastewater, which produces good results but at a high cost.

In recent years, electrochemical methods have gained significant attention as one of the key technologies for heavy metal wastewater treatment. These methods include electrolysis, coagulation, flotation, redox reactions, and micro-electrolysis, all of which are often carried out simultaneously in wastewater treatment. The principle of electrochemical reactions mainly includes electrocoagulation, electrooxidation-reduction, and electroflotation.

Among these, electrooxidation and electroflotation are used for treating organic wastewater and wastewater containing solid particles or oil contamination; electroreduction is widely applied in the treatment of heavy metal wastewater, as it reduces high-valent metal ions to low-valent metal ions or metal precipitates through electron gain. Electrocoagulation, on the other hand, involves the use of soluble metals like iron or aluminum as anodes, which lose electrons under direct current to form metal cations (Fe²⁺ and Al³⁺), reacting with OH^- in the solution to generate metal hydroxide colloidal flocculants. This not only effectively removes heavy metal ions from electroplating wastewater but also reduces the salinity of the wastewater. 

1. Sources and Classification of Heavy Metal Electroplating Wastewater

Heavy metal electroplating wastewater primarily arises from excess or improper operation during the electroplating process, as well as wastewater generated from cleaning plated parts or equipment. The composition of the water is complex and difficult to control. In addition to heavy metal ions such as chromium, nickel, copper, and zinc, the wastewater may also contain pollutants such as cyanides and organic substances. Based on the current development of the electroplating industry, heavy metal electroplating wastewater can be classified as follows, according to the main heavy metal content:

a. Chromium-containing wastewater: This wastewater mainly contains chromium (VI) along with trace amounts of other heavy metal ions. 

b. Nickel-containing wastewater: The primary pollutants in this type of wastewater are nickel ions and suspended solids.

c. Copper-containing wastewater: This wastewater mainly contains copper ions or complexed copper ions.

d. Mixed heavy metal wastewater: This wastewater primarily contains chromium and other heavy metal ions, cyanides, and suspended solids.

2. Electrochemical Treatment of Heavy Metal Electroplating Wastewater

2.1 Electrochemical Treatment of Chromium-containing Electroplating Wastewater

Chromium is abundant in electroplating wastewater, and it mainly exists in its high valence state, such as Cr₂O₇²^- and CrO₄²^-. Many studies have used iron-carbon micro-electrolysis or iron scrap internal electrolysis to treat chromium-containing electroplating wastewater. When the pH value and residence time are optimized, the removal rate of Cr⁶⁺ can reach over 99%, and the treated effluent meets discharge standards. Zheng Liu and others used a titanium-iron dual anode electrocoagulation technique to remove Cr⁶⁺ from electroplating wastewater, achieving a removal rate of 96.57%.

When electrochemical treatment is combined with biological methods, the removal rate of Cr⁶⁺ is even higher, and it can also remove organic pollutants in the wastewater. For example, when a micro-electrolysis/electrolysis-biological treatment combined process is used, the micro-electrolysis/electrolysis process acts as a pre-treatment, achieving a Cr⁶⁺ removal rate of over 90%. After the subsequent biological treatment, the Cr⁶⁺ removal rate can reach 99.9%, with a significantly improved removal effect.

While the electrochemical method alone is effective for treating chromium-containing electroplating wastewater, combining it with biological methods yields even better results, effectively reducing operational costs and showing further potential for wide application.

2.2 Electrochemical Treatment of Nickel-containing Electroplating Wastewater

In the electroplating industry, nickel electroplating is widely used due to its corrosion resistance, wear resistance, and weldability, making it the second most commonly used after zinc plating in terms of industrial volume.

If nickel-containing electroplating wastewater is not treated before being discharged, it not only causes severe environmental pollution and harms human health, but also leads to resource wastage. Currently, the treatment methods for nickel-containing electroplating wastewater, like most industrial wastewater treatment methods, are generally divided into physical-chemical methods, chemical methods, biological methods, or a combination of these techniques. Jian Yang explored the effectiveness of micro-electrolysis in treating high-concentration nickel electroplating wastewater, achieving a nickel removal rate of 64.09%, which is beneficial for subsequent treatment. 

Cunhai Liu and others used a combination of coagulation and electrolysis processes to treat nickel-containing wastewater from the electroplating workshop of Baoji Changling Group. After treatment, the concentration of nickel ions in the effluent was reduced to 0.365 mg/L, below the 0.5 mg/L limit set by China’s national discharge standards. Therefore, combining traditional coagulation techniques with electrochemical methods for treating nickel electroplating wastewater can help the effluent meet discharge standards directly while also reducing operational costs.

2.3 Electrochemical Treatment of Copper-containing Electroplating Wastewater

In the electroplating industry, copper plating is often used as the base layer for other heavy metals like chrome and nickel, making copper-containing electroplating wastewater very common. 

When using electrochemical methods to treat copper-containing electroplating wastewater, copper can also be directly recovered. Hao Chen and others used a fluidized bed electrode to treat low-concentration copper sulfate wastewater. Tian and colleagues studied the reduction characteristics of Cu²⁺ ions on stainless steel electrodes using electrolysis. Zhang and others used cyclic voltammetry to investigate the electro-deposition kinetics of Cu²⁺ ions in acidic environments, enabling the effluent to meet discharge standards. Youchun Zhu employed magnetic electrolysis technology to treat copper-containing industrial wastewater and found that it not only effectively treated the pollutants in industrial wastewater but also allowed for the recovery of dense, uniform copper metal on the cathode. 

However, electrolysis suffers from limitations such as long treatment times, low efficiency, and high energy consumption due to the influence of the metal electro-deposition reduction potential and mass transfer processes. These drawbacks limit the application of this method in this field. To improve treatment effectiveness, Gang Wang and others coupled two electrochemical methods for treating copper-containing wastewater, and the results showed significantly better performance than micro-electrolysis or electrolysis alone, also promoting faster reactions.

2.4 Electrochemical Treatment of Heavy Metal Mixed Electroplating Wastewater

Electroplating wastewater has complex water quality, typically containing multiple heavy metal ions rather than just one. Hongfang Ma used the iron scrap internal electrolysis method to treat mixed heavy metal electroplating wastewater, achieving Cr⁶⁺ levels lower than 0.5 mg/L in the effluent, with other metal ions also meeting discharge standards.

Tan Chaoxiong and others used single electroflocculation to treat electroplating wastewater containing both copper and chromium. They found that, under the same parameters, the removal of Cu was more effective than that of Cr. Tiaolan Zhang and others applied a combination of electroflocculation and activated carbon fiber adsorption to treat mixed heavy metal electroplating wastewater, achieving a removal rate of heavy metal ions greater than 99.97%.

The Environmental Monitoring Station of Huangshan, China, in cooperation with the Huangshan Environmental Engineering Company, used a "micro-electrolysis—neutralization—coagulation precipitation" process to treat electroplating wastewater containing Cr⁶⁺, Ni²⁺, and Cu²⁺ with a daily discharge of 20 tons. The total investment was 250,000 Yuan, and the treatment cost was 1.5 Yuan/ton. The Petrochemical Design Institute of Guangdong Province, China, used micro-electrolysis electrochemical methods to treat wastewater from a plating factory in Huizhou, with an engineering project total investment of 260,000 Yuan and a treatment cost of approximately 1.05 Yuan/ton.

When electrochemical methods are combined with other processes to treat mixed heavy metal electroplating wastewater, treatment costs can be reduced from 3-10 Yuan/ton to around 2 Yuan/ton, significantly lowering treatment costs. From both a cost and operational perspective, electrochemical methods have broad potential for promotion and application.

3. Existing Problems

With the Chinese government's increased focus on environmental protection in recent decades, along with the rapid development and promotion of water treatment technologies, electrochemical treatment technologies have been widely applied in various industrial wastewater sectors, particularly in the electroplating wastewater industry. However, deeper research into electrochemical methods has revealed certain limitations that restrict their broader application. The main issues are as follows:

a. Different Treatment Effectiveness for Various Heavy Metals: The electrochemical method has varying effectiveness for different heavy metals, and the optimal conditions (such as pH, electrode plate spacing, and electrode materials) also vary. Therefore, there are limitations in treating mixed heavy metal electroplating wastewater. 

b. High Energy Consumption: When electrochemical methods are used alone to treat heavy metal electroplating wastewater, the treatment effectiveness is good, but the energy consumption is high, leading to increased operational costs.

c. Electrode Plate Limitations: Due to the constraints of electrode plates, most electrochemical treatment technologies are still at the pilot scale and have not yet been widely applied in engineering projects.

4. Conclusion and Outlook

With the implementation of the Emission Standard of Pollutants for Electroplating (GB21900-2008), the discharge requirements for various heavy metals in electroplating wastewater have been raised. To meet the current discharge standards and address the increasingly stringent discharge trends in the future, traditional treatment technologies can no longer meet the required standards and will need to employ advanced treatment methods. However, this results in significantly higher treatment costs and causes severe resource waste. From a long-term perspective, the recovery of heavy metals from electroplating wastewater not only helps prevent environmental pollution but also promotes the development of a circular economy. It is a crucial step in fundamentally reducing the environmental impact.

To achieve this goal, a combination of electrochemical methods with biological or physico-chemical methods can be adopted to reduce energy consumption, improve treatment efficiency, and achieve both wastewater treatment and heavy metal recovery. Therefore, the focus of current research and development is on creating low-cost electrode materials, developing multidimensional electrochemical reactors, and selecting the best process combinations to reduce energy consumption. These areas have become key research directions.