Chemical method
Chemical methods mainly include chemical precipitation and electrolysis, which are mainly suitable for the treatment of wastewater containing high concentration of heavy metal ions. Chemical method is the main method to treat heavy metal wastewater at home and abroad.
2. 1. 1 chemical precipitation method
The principle of chemical precipitation method is to transform the dissolved heavy metals in wastewater into water-insoluble heavy metal compounds through chemical reaction, and remove the precipitation from the aqueous solution through filtration and separation, including neutralization precipitation method, sulfide precipitation method and ferrite precipitation method. Due to the influence of precipitant and environmental conditions, the effluent concentration of precipitation method often fails to meet the requirements and needs further treatment. The generated sediment must be well treated and disposed of, otherwise it will cause secondary pollution.
2. 1.2 electrolytic method
Electrolysis makes use of the electrochemical properties of metals, and metal ions can be separated from the solution with relatively high concentration during electrolysis, and then used. Electrolysis is mainly used to treat electroplating wastewater. The disadvantage of this method is that the concentration of heavy metal ions in water cannot be reduced to a very low level. Therefore, electrolysis is not suitable for treating wastewater containing low concentration of heavy metal ions.
Physical therapy method
Physical treatment methods mainly include solvent extraction separation, ion exchange, membrane separation technology and adsorption.
2.2. 1 solvent extraction separation
Solvent extraction is a common method to separate and purify substances. Because of liquid-liquid contact, it can be operated continuously and the separation effect is good. When this method is used, a highly selective extractant must be selected. Heavy metals in wastewater generally exist in the form of cations or anions. For example, under acidic conditions, they are complexed with extractant, extracted from water phase to organic phase, and then back-extracted to water phase under alkaline conditions, so that the solvent can be recycled. This requires that the acidity of the water phase should be carefully selected during the extraction operation. Although the extraction method has great advantages, the loss of solvent in the extraction process and a large amount of energy consumption in the regeneration process make this method have certain limitations, and its application is greatly limited.
ion exchange process
Ion exchange method is a method to remove heavy metal ions from wastewater by exchanging heavy metal ions with ion exchangers. Commonly used ion exchangers include cation exchange resin, anion exchange resin and chelating resin. In recent years, scholars at home and abroad have done a lot of research work on the development of ion exchangers. With the continuous emergence of ion exchangers, ion exchange method has shown its advantages in advanced treatment of electroplating wastewater and recovery of high-valent metal salts. Ion exchange method is an important treatment method of electroplating wastewater, which has large treatment capacity, good effluent quality, recovery of heavy metal resources and no secondary pollution to the environment. However, ion exchangers are prone to oxidation failure, frequent regeneration and high operating cost.
2.2.3 Membrane separation technology
Membrane separation technology is a method of separating or concentrating solvents and solutes by using a special semi-permeable membrane under the action of external pressure without changing the chemical form in the solution, including electrodialysis and diaphragm electrolysis. Electrodialysis is a physical and chemical process of separating heavy metal ions from water by using anion-cation exchange membrane to selectively permeate anions in solution under the action of DC electric field. Diaphragm electrolysis is a method of separating the anode and cathode of electrolysis device with membrane, which is actually a method of combining electrodialysis with electrolysis. The above methods all encounter problems such as electrode polarization, scaling and corrosion.
adsorption method
Adsorption is an effective method to remove heavy metal ions from water by using porous solid substances. The key technology of adsorption method is the selection of adsorbent, and the traditional adsorbent is activated carbon. Activated carbon has strong adsorption capacity and high removal rate, but the regeneration efficiency of activated carbon is low, and the treated water quality is difficult to meet the reuse requirements, which is expensive and limited in application. In recent years, various adsorption materials with adsorption capacity have been gradually developed. Related research shows that chitosan and its derivatives are good adsorbents for heavy metal ions. The crosslinked chitosan resin can be reused 10 times, and the adsorption capacity has not decreased significantly. Treating heavy metal wastewater with modified sepiolite has a good adsorption capacity for Pb2+, Hg2+ and Cd2+, and the content of heavy metals in the treated wastewater is obviously lower than the comprehensive wastewater discharge standard. Another document reported that montmorillonite is also a clay mineral adsorbent with good performance. The removal rate of Cr 6+ by Al-Zr pillared montmorillonite under acidic conditions reaches 99%, and the content of Cr 6+ in effluent is lower than the national discharge standard, which has practical application prospects.
Biological treatment method
Biological treatment is a method to remove heavy metals from wastewater through flocculation, adsorption, accumulation and enrichment of microorganisms or plants, including biological adsorption, biological flocculation, phytoremediation and other methods.
2.3. 1 biosorption
Biosorption refers to the method by which organisms adsorb metal ions through chemical action. Algae and microbial cells have good adsorption effect on heavy metals, and have the advantages of low cost, good selectivity, large adsorption capacity and wide application range of concentration. They are relatively economical adsorbents. The research on the removal of heavy metals from wastewater by biosorption has achieved initial results in the United States and other countries. Some researchers pretreated pseudomonas micelle and fixed it on fine magnetite to adsorb copper in industrial wastewater. The results show that when the concentration is as high as 100 mg/L, the removal rate can reach 96%, and 95% of copper can be recovered by acid desorption. Pretreatment can increase the adsorption capacity. However, the biological adsorption method also has some shortcomings, such as the adsorption capacity is easily affected by environmental factors, and the adsorption of heavy metals by microorganisms is selective. However, heavy metal wastewater often contains a variety of harmful heavy metals, which affects the role of microorganisms and its application is limited, so further research is needed.
2.3.2 Biological flocculation
Biological flocculation is a decontamination method that uses microorganisms or metabolites produced by microorganisms to flocculate and precipitate. Although the development of biological flocculation method is less than 20 years, it has been found that more than 17 kinds of microorganisms have good flocculation effect, such as mold, bacteria, actinomycetes, yeast and so on, and most of them can be used to treat heavy metals. Biological flocculation has the advantages of safety, non-toxicity, high flocculation efficiency and easy separation of flocs, and has broad development prospects.
2.3.3 phytoremediation methods
Phytoremediation refers to the use of higher plants to reduce the content of heavy metals in polluted soil or surface water through absorption, precipitation and enrichment, so as to achieve the purpose of pollution control and environmental remediation. Phytoremediation is an effective way to treat the environment by ecological engineering, and it is an extension of biotechnology to treat enterprise wastewater. The treatment of heavy metals by plants mainly consists of three parts:
(1) Use metal accumulation plants or hyperaccumulation plants to absorb, precipitate or enrich toxic metals in wastewater; (2) Using metal accumulation plants or hyperaccumulation plants to reduce the activity of toxic metals, so as to reduce the leaching of heavy metals underground or the diffusion through air carriers; (3) using metal accumulation plants or hyperaccumulation plants to clear the soil.
Heavy metals in soil or water are extracted, enriched and transported to the harvestable parts of plant roots and aboveground parts. Reduce the concentration of heavy metals in soil or water by harvesting or removing plant branches that accumulate and enrich heavy metals. Plants that can be used for phytoremediation technology include algae, herbs and woody plants.
The ability of algae to purify heavy metal wastewater is mainly manifested in its strong adsorption of heavy metals. The absorption capacity of brown algae is 400mg/g, and the removal rate of heavy metal ions such as Cu, Pb, La, Cd and Hg by green algae is 80%~90% under certain conditions. Hao Yuntao and others isolated and screened a Chlorella elliptica with high heavy metal resistance, and studied the effects of different concentrations of heavy metals such as copper, zinc, nickel and cadmium on the growth of Chlorella elliptica and its absorption and enrichment of heavy metal ions. The results showed that algae had high tolerance to zinc and cadmium. The tolerance to four heavy metals is zinc >; Cadmium >; Nickel >: copper. The algae had a good removal effect on heavy metals. After 72h treatment with 15μmol/L Cu2+, 300μmol/L Zn2+, 100μmol/L Ni2+ and 30μmol/L Cd2+, the removal rates reached 40.93%, 98.33% and 97.63% respectively. Therefore, this algae can be applied to the treatment of wastewater containing heavy metals.
There have been many reports on the application of herbs to purify heavy metal wastewater. Eichhornia crassipes is an internationally recognized aquatic floating plant commonly used for pollution control. It has the characteristics of rapid growth, low temperature resistance and high temperature resistance, and can quickly enrich a large number of heavy metals such as Cd, Pb, Hg, Ni, Ag, Co and Cr in wastewater. Zhang Zhijie et al. showed that Eichhornia crassipes with dry weight 1kg could absorb 3.797g of lead and 3.225g of cadmium in 7 ~10 days. Zhou et al. found that the absorption rate of cobalt and zinc by Eichhornia crassipes was as high as 97% and 80% respectively. Typha Pres 1 is also a good herb for purifying heavy metals. It has special structure and function, such as fleshy leaves and developed palisade tissue. Typha plants grow in high-concentration heavy metal wastewater for a long time, forming a special structure to resist the harsh environment and self-regulate some physiological activities to adapt to pollution and poisoning. Zhao et al. studied the stability of cattail constructed wetland system in treating mineral processing wastewater from Fankou lead-zinc mine in Shaoguan, Guangdong. The monitoring results of 10 years show that the system can effectively purify lead-zinc mine wastewater. Untreated wastewater contains high concentrations of harmful metals, such as lead, zinc and cadmium. After the constructed wetland, the outlet water quality was obviously improved, and the purification rates of lead, zinc and cadmium were 99.0%, 97.9% and 94.9% respectively, which were lower than the national industrial wastewater discharge standards. In addition, there are many herbs with purification function, such as Alternanthera philoxeroides, hoses, Sophora alopecuroides, duckweed, Indian mustard and so on.
Using woody plants to treat polluted water bodies has the advantages of good purification effect, large treatment capacity, little influence by climate, and it is not easy to cause secondary pollution, so people pay more and more attention to it. Hu Huanbin and other experimental results show that reed and Taxodium ascendens have strong enrichment ability for heavy metals lead and cadmium, while Taxodium ascendens, a woody plant, has better purification effect than reed. Zhou Qing and others studied the response of five evergreen trees to cadmium pollution stress. The experimental results showed that the physiological and biochemical characteristics of leaves such as chlorophyll content, plasma membrane permeability, catalase activity and cadmium enrichment changed obviously under high concentration of cadmium stress. Among them, Buxus macrophylla, Pittosporum pittosporum and Cunninghamia lanceolata have stronger cadmium pollution resistance than Cinnamomum camphora and Ilex. Heavy metal wastewater treatment technology with woody plants as the main body can cut off toxic and harmful substances from entering the food chain of human body and livestock, avoid secondary pollution, cultivate directionally, beautify the environment while controlling pollution, and obtain certain economic benefits. It is an ideal method for environmental restoration.