At present, the conventional treatment methods of heavy metal wastewater include chemical precipitation method, adsorption method, biological method, etc. its main principle is to convert heavy metals into precipitation or other forms, which is easy to cause secondary pollution to the environment. For example, some projects use Fenton reagent to treat electroplating wastewater, which has good treatment effect, but the cost is high.

In recent years, electrochemical method has attracted much attention as one of the heavy metal wastewater treatment technologies. It has the functions of electrolysis, flocculation, air flotation, redox and micro electrolysis. These functions are often carried out simultaneously in wastewater treatment. The principle of electrolytic reaction mainly includes electrocoagulation, electro redox and electro flotation.

Among them, electro oxidation and electro flotation are used to treat organic wastewater and wastewater containing solid small particles and oil pollution respectively; Electroreduction is the reduction of high valence metal ions into low valence metal ions or metal precipitation through electron gain, so this effect is widely used in the field of heavy metal wastewater treatment; The principle of electrocoagulation is that soluble iron, aluminum and other anodes will lose electrons under direct current to form metal cations Fe2 + and A13 +, and form metal hydroxide colloidal flocculant with OH - in the solution, which can not only effectively remove heavy metal ions in electroplating wastewater, but also reduce the salt content in wastewater.

1.Classification of electroplating wastewater and sources containing heavy metals 

Electroplating wastewater containing heavy metals mainly comes from the waste liquid caused by redundant or non-standard operation in the electroplating treatment process and the waste liquid generated by cleaning plating parts or equipment. The water quality composition is complex and difficult to control. In addition to heavy metal ions such as chromium, nickel, copper and zinc, there are also pollutants such as cyanide and organic matter. According to the current development of electroplating industry, electroplating wastewater containing heavy metals is classified as follows according to the main heavy metals contained.

(1) Chromium containing wastewater: chromium containing wastewater mainly contains chromium (ⅵ) and a small amount of other heavy metal ions;

(2) Nickel containing wastewater: the main pollutants are nickel ions, suspended solids and other substances;

(3) Copper containing wastewater: mainly contains copper ions or complex copper ions;

(4) Heavy metal mixed wastewater: mainly contains chromium and other heavy metal ions, cyanide and suspended solids.

2. electrochemical treatment of electroplating wastewater containing heavy metals

2.1 electrochemical treatment of chromium containing electroplating wastewater

There is a large amount of chromium in electroplating wastewater, and it mainly exists in high valence state, such as cr2o72 - and CrO42 -. Many studies [4-6] use iron carbon micro electrolysis or iron chip internal electrolysis to treat chromium containing electroplating wastewater. When the pH value and residence time are the best, the removal rate of Cr6 + is more than 99%, and the effluent can meet the discharge standard. Liu Zheng et al. Used titanium iron double anode electro flocculation technology to remove chromium (ⅵ) from electroplating wastewater, and the removal rate of chromium (ⅵ) can reach 96.57%.

When electrochemical and biological methods are used, the removal rate of Cr6 + is better, and the organic pollutants in wastewater can be removed at the same time. For example, when the combined process of micro electrolysis / electrolysis biological method is used for treatment, the micro electrolysis / electrolysis method is equivalent to the pretreatment of wastewater, and the removal rate of chromium (ⅵ) can reach more than 90%. After subsequent biochemical process treatment, the removal rate of Cr6 + is as high as 99.9%, and the removal effect is obvious.

The single electrochemical method has obvious effect on the treatment of chromium containing electroplating wastewater, while the post-treatment effect combined with biological method is better, and can effectively reduce the operation cost, which has further popularization value.

2.2 electrochemical treatment of nickel containing electroplating wastewater

In the electroplating industry, nickel plating has been widely used because of its corrosion resistance, wear resistance, weldability and other characteristics, so its industrial volume is second only to surface galvanizing.

If nickel containing electroplating wastewater is discharged directly without treatment, it will not only cause serious pollution to the environment and endanger human health, but also cause a waste of resources. At present, the treatment methods of nickel containing electroplating wastewater are the same as most industrial wastewater treatment methods, which can be roughly divided into physicochemical method, chemical method, biological method or the combination of various methods. Yang Jian [11] discussed the effect of micro electrolysis on the treatment of high concentration nickel containing electroplating wastewater through experiments. The removal rate of nickel can reach 64.09%, which is conducive to subsequent treatment.

Liu cunhai and others studied the nickel containing wastewater in the electroplating workshop of Baoji Changling group by using the combined process of flocculation and electrolysis. After treatment, the nickel ion concentration in the effluent is reduced to 0.365mg/l, and the effluent is lower than the national standard of 0.5mg/l. Therefore, the combination of traditional flocculation process and electrochemical method to treat nickel containing electroplating wastewater can not only make the wastewater meet the standard directly, but also reduce the operation cost.

2.3 electrochemical treatment of copper containing electroplating wastewater

In the electroplating industry, copper plating is often used as the bottom layer of the surface layer of other heavy metals such as chromium plating and nickel plating, which makes copper containing electroplating wastewater very common.

When electrochemical method is used to treat copper containing electroplating wastewater, copper can also be recovered directly. Chen Hao and others use fluidized bed electrode to treat low concentration copper sulfate wastewater. Tian and others use electrolytic method to study the reduction characteristics of Cu2 + ions on stainless steel electrode. Zhang and others use cyclic voltammetry to study the electrodeposition kinetic behavior of Cu2 + ions in acidic environment, so that the effluent can meet the discharge standard. Zhu Youchun used magnetic electrolysis technology to treat industrial wastewater containing copper and found that it can not only effectively treat pollutants in industrial wastewater, but also further recover dense and uniform metal copper on the cathode.

However, due to the influence of metal electrodeposition reduction potential and mass transfer process, electrolytic method has the disadvantages of long treatment time, low treatment efficiency and high energy consumption in the treatment of copper containing electroplating wastewater, which limits the popularization of this method in this field. In order to improve the treatment effect, Wang Gang et al. [17] coupled the two electrochemical methods to treat copper containing wastewater, which is obviously better than the single micro electrolysis and electrolysis, and can promote the rapid progress of the reaction.

2.4 electrochemical treatment of heavy metal mixed electroplating wastewater

The water quality of electroplating wastewater is complex, which usually contains more than one heavy metal, but a variety of heavy metal ions coexist. Ma Hongfang uses iron chip internal electrolysis to treat heavy metal mixed electroplating wastewater. The content of Cr6 + in the effluent is less than 0.5mg/l, and other metal ions meet the discharge standard.

Tan Chaoxiong et al. Used single electrocoagulation to treat electroplating wastewater containing copper and chromium. It was found that under the same parameters, the removal effect of Cu was better than that of CR. Zhang tiaolan et al. Treated heavy metal mixed electroplating wastewater by electro flocculation activated carbon fiber adsorption method, and the removal rate of heavy metal ions reached more than 99.97%.

Huangshan environmental monitoring station and Huangshan environmental engineering company adopt the "micro electrolysis neutralization coagulation sedimentation" process to treat 20t Electroplating Wastewater Containing Cr6 +, Ni2 +, Cu2 + with a total investment of 250000 yuan and a treatment cost of 1.5 yuan / T. Guangdong petrochemical design institute adopts micro electrolysis electrochemical method to treat the wastewater of an electroplating plant in Huizhou. The total investment of the project with q = 80m3 / D is 260000 yuan, and the treatment cost is about 1.05 yuan / T.

When electrochemical method is combined with other methods to treat heavy metal mixed electroplating wastewater, the treatment cost is directly reduced from 3 ~ 10 yuan / T to about 2 yuan / T, and the treatment cost is significantly reduced. From the perspective of cost and operation cost, electrochemical method has extensive popularization and application value.

3. existing problems

With the government's attention to environmental protection and the rapid development and promotion of water treatment technology in recent decades, electrochemical treatment technology has been widely used in various industrial wastewater industries, especially electroplating wastewater industry. However, with the deeper research of electrochemical method, it is found that there are still some problems in this technology, and its popularization is limited. It mainly includes the following aspects:

(1) The electrochemical method has different treatment effects on different heavy metals and adapts to different conditions (pH value, electrode plate spacing, electrode plate, etc.), so it has restrictions on the treatment of heavy metal mixed electroplating wastewater;

(2) When using electrochemical method to treat electroplating wastewater containing heavy metals, although the treatment effect is good, the treatment energy consumption is high, resulting in the increase of operation cost;

(3) Limited by the electrode plate, most electrochemical treatment technologies are still in the pilot stage and have not been widely used in engineering projects.

4. conclusions and Prospects

With the promulgation of the national discharge standard of pollutants in electroplating industry (gb21900-2008), the discharge requirements of various heavy metals in electroplating wastewater have been improved. In order to meet the existing emission standards and cope with the increasingly stringent emission trend in the future, the traditional treatment technologies can not meet the emission requirements and need to adopt advanced treatment, but this greatly increases the treatment cost and leads to a serious waste of resources. In the long run, the recovery of heavy metals from heavy metal electroplating wastewater can not only avoid environmental pollution, but also promote the development of circular economy. It is an important link to fundamentally reduce the impact on the environment.

In order to achieve this goal, electrochemical method can be combined with biological method or physicochemical method to reduce energy consumption and improve treatment effect, so as to achieve the dual purposes of wastewater treatment and heavy metal recovery; Therefore, it has become the development direction of key research and development to focus on developing low-cost electrode plates and multi-dimensional electrochemical reactors, and selecting the best process combination to reduce energy consumption.

Source: Changjiang nonferrous metals network