At present, the methods of heavy metal wastewater treatment can be roughly divided into three major categories: (1) chemical method; (2) physical treatment method; (3) biological treatment method. Chemical method mainly includes chemical precipitation method and electrolysis method, which is mainly applicable to the treatment of wastewater containing high concentration of heavy metal ions, and chemical method is the main method of treating heavy metal-containing wastewater at home and abroad at present.
2.1.1 Chemical precipitation method
The principle of chemical precipitation method is to make the heavy metals in the wastewater in a dissolved state through chemical reaction to the insoluble heavy metal compounds, and the precipitates are removed 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, precipitation method is often the concentration of water does not meet the requirements, need to be further treatment, the resulting precipitates must be well treated and disposed of, otherwise it will cause secondary pollution.
2.1.2 Electrolysis
Electrolysis is the use of metal electrochemical properties, metal ions in the electrolysis can be separated from the relatively high concentration of the solution, and then used. Electrolysis is mainly used for electroplating wastewater treatment, the disadvantage of this method is that the concentration of heavy metal ions in the water can not be reduced very low. Therefore, the electrolysis method is not suitable for the treatment of lower concentrations of heavy metal ion-containing wastewater.
2.1.3 chelating method
Chelating method, also known as polymer ion trapping agent method, refers to the wastewater treatment process through the addition of an appropriate amount of heavy metal trapping agent, the use of trapping agent and metal ions lead, cadmium, combined with the formation of the corresponding chelate principle to achieve the removal of lead, cadmium separation. The reaction can occur at room temperature and a wide range of pH (3?11), and at the same time, the trapping agent is not affected by the heavy metal ions in the ****. Therefore, the method is characterized by high removal rate, good flocculation effect, low sludge volume and easy dewatering of the integrated material.
2.1.4 Nano heavy metal water treatment technology
Nanomaterials because of its surface area far more than ordinary materials, so the same material will show different physical and chemical characteristics, many new nanomaterials are constantly in the water treatment industry experiment, practice. Jointly approved by the Ministry of Environmental Protection, the Ministry of Science and Technology, the Ministry of Industry and Information Technology, the Ministry of Finance, four ministries and commissions for the "2011 National Major Scientific and Technological Achievements Transformation Project" --- nano water treatment process and series of products, in Jiangxi Copper Co. Ltd. has made a historic breakthrough in the application of nano water treatment process and series of products, filling the gaps in the domestic market.
The heavy metal wastewater treatment methods usually used in China include lime neutralization and sulfidation. These traditional treatment processes, although the heavy metals in the wastewater can be removed, but the treatment effect is not stable, the quality of water recovered after treatment is still difficult to ensure that the stability of the discharge standards, but also produce secondary pollution. Nano heavy metal water treatment technology can not only make the treated water quality is better than the national emission standards and stable and reliable, low investment costs and operating costs, and heavy metal ions in the water, fast reaction, adsorption, processing capacity of 10 times to 1000 times the ordinary materials, and make the amount of sludge precipitation than the traditional process to reduce the amount of more than 50% of the sludge in the sludge impurities are also fewer, and is conducive to the follow-up treatment and resource recovery. Some data show that the same daily treatment of 300 cubic meters of heavy metal sewage volume, the traditional process to produce 25 tons of lime sludge per day, while the use of nanotechnology produces only 25 tons of nano-metal sludge per month. Of particular concern is that the heavy metal unit content in this sludge is 30 times higher. If we take the wastewater treatment of copper smelter as an example, the grade of its recovered nano-copper sludge has reached 20%, which can be completely reused as copper mining resources. Physical treatment method mainly contains solvent extraction separation, ion exchange method, membrane separation technology and adsorption method.
2.2.1 Solvent Extraction Separation
Solvent extraction is a commonly used method to separate and purify substances. Due to the liquid-liquid contact, it can be operated continuously and the separation effect is better. When using this method, it is necessary to select the extractant with high selectivity, heavy metals in wastewater generally exist in the form of cations or anions, for example, under acidic conditions, complexation reaction with the extractant, from the aqueous phase is extracted to the organic phase, and then under alkaline conditions is counter-extracted to the aqueous phase, so that the solvent is regenerated in order to recycle. This requires that care be taken to select the aqueous phase acidity during the extraction operation. Despite the superiority of the extraction method, however, the loss of solvent in the extraction process and the regeneration process of energy consumption, so that this method has certain limitations, the application of the application is greatly restricted.
2.2.2 Ion exchange method
Ion exchange method is the exchange of heavy metal ions with ion exchangers to remove heavy metal ions from wastewater. Commonly used ion exchange agents are cation exchange resin, anion exchange resin, chelating resin and so on. In the past few years, scholars at home and abroad have carried out a lot of research work on the development of ion exchangers. With the continuous emergence of ion exchangers, the ion exchange method is showing more and more advantages in the deep treatment of electroplating wastewater and the recovery of high-value metal salts. Ion exchange method is an important electroplating wastewater treatment method, large treatment capacity, good effluent water quality, heavy metal recovery, no secondary pollution of the environment, but the ion exchange agent is easy to oxidize failure, regeneration is frequent, high operating costs.
2.2.3 Membrane Separation Technology
Membrane separation technology is the use of a special semi-permeable membrane, under the action of the external pressure, does not change the chemical form of the solution on the basis of the solvent and solute separation or concentration of the method, including electrodialysis and diaphragm electrolysis. Electrodialysis is under the action of direct current electric field, the use of anion and cation exchange membrane on the solution of anion and cation selective permeability of heavy metal ions in aqueous solution and water separation of a physicochemical process. Diaphragm electrolysis is a method of electrolysis by separating the anode and cathode of the electrolysis device with a membrane, which is actually a combination of electrodialysis and electrolysis. All of the above methods encounter problems of electrode polarization, scaling and corrosion in operation.
2.2.4 Adsorption method
Adsorption method is an effective method of removing heavy metal ions from water by adsorption using porous solid substances. The key technology of adsorption method is the selection of adsorbent, the traditional adsorbent is activated carbon. There are clay adsorbent powder, coal ash adsorbent, biomass-based materials and resin-based adsorbent materials. 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 requirements for reuse, and the price is expensive, so the application is limited. In recent years, a variety of adsorption materials with adsorption capacity have been gradually developed. Some relevant studies have shown that chitosan and its derivatives are good adsorbents for heavy metal ions, and after crosslinking of chitosan resin, it can be reused for 10 times without significant reduction in adsorption capacity. The use of modified seafoam to treat heavy metal wastewater on Pb2+, Hg2+, Cd2+ has a very good adsorption capacity, the content of heavy metals in the treated wastewater is significantly lower than the comprehensive sewage discharge standards. It is also reported in the literature that montmorillonite is also a kind of clay mineral adsorbent with good performance, and the removal rate of Cr 6+ by aluminum-zirconium column-supported montmorillonite under acidic condition reaches 99%, and the content of Cr 6+ in effluent is lower than the national emission standard, which has the prospect of practical application. Biological treatment is a method of removing heavy metals from wastewater with the help of flocculation, absorption, accumulation and enrichment of microorganisms or plants, including biosorption, bioflocculation, phytoremediation and other methods.
2.3.1 Biosorption
Biosorption refers to the method of adsorption of metal ions by organisms with the help of chemical action. Algae and microbial organisms have good adsorption effect on heavy metals, and have the advantages of low cost, good selectivity, large adsorption capacity, and wide range of concentration application, which is a more economical adsorbent. The study of removing heavy metals from wastewater by biosorption has shown initial success in the United States and other countries. Some researchers pre-treatment of Pseudomonas aeruginosa bacterial colloid, fixed in fine-grained magnetite on the adsorption of Cu in industrial wastewater, found that when the concentration is as high as 100 mg / L, the removal rate of up to 96%, with the acid desorption, can be recovered 95% of the copper, the pre-treatment can increase the adsorption capacity. However, there are some shortcomings in the biosorption method, such as the adsorption capacity is easily affected by environmental factors, the adsorption of heavy metals by microorganisms is selective, and heavy metal wastewater often contains a variety of harmful heavy metals, which affects the role of microorganisms, the application of the limitations and so on, so it is necessary to carry out further research.
2.3.2 Bioflocculation
Bioflocculation is a decontamination method that utilizes microorganisms or metabolites produced by microorganisms for flocculation and precipitation. Although the development of bioflocculation method is less than 20 years, more than 17 species of microorganisms have been found to have better flocculation function, such as molds, bacteria, actinomycetes and yeasts, etc., and most of the microorganisms can be used to treat heavy metals. Bioflocculation has the advantages of safety and non-toxicity, high flocculation efficiency, easy separation of flocculants, etc., and has a broad development prospect.
2.3.3 Phytoremediation
Phytoremediation refers to the use of higher plants to reduce the content of heavy metals in polluted soil or surface water through absorption, precipitation, enrichment, etc., so as to achieve the purpose of pollution control and environmental restoration. Phytoremediation is an effective method of utilizing ecological engineering to treat the environment, and it is an extension of biotechnology to treat corporate wastewater. The use of plants to deal with heavy metals has three main components:
(1) the use of metal-accumulating plants or hyper-accumulating plants from the wastewater to absorb, precipitate
or enrichment of toxic metals: (2) the use of metal-accumulating plants or hyper-accumulating plants to lower
low activity of toxic metals, which can reduce the heavy metals are leached into the ground or diffused through the
air carriers : (3) Utilizing metal-accumulating plants or hyperaccumulating plants to extract heavy metals from soil
soil or water, enriching and transporting them to the harvestable portions of plant roots and above ground branching portions of plants. Heavy metal concentrations in the soil or water are reduced by harvesting or removing the branches of plants that have accumulated and enriched heavy metals. Plants that can be utilized in phytoremediation techniques include algal plants, herbaceous plants, and woody plants.
The ability of algae to purify heavy metal wastewater is mainly manifested in their strong adsorption of heavy metals. The absorption of Au by brown algae reaches 400mg/g, and the removal rate of heavy metal ions such as Cu, Pb, La, Cd, Hg, etc. by green algae under certain conditions reaches 80%~90%. Hao Yuntao et al. separated and screened a strain of Chlorella ellipsoidea with high heavy metal resistance, and studied the effects of different concentrations of heavy metals Cu, Zn, Ni and Cd on the growth of the alga and its absorption of heavy metal ions enrichment. The results showed that the alga was highly tolerant to Zn and Cd. The tolerance to the four heavy metals was Zn> Cd> Ni> Cu in order. This algae has good removal effect on heavy metals, the removal rate reached 40.93%, 98.33%, 97.62%, and 86.88% with the treatment of 15 μmol/L Cu2+, 300 μmol/L Zn2+, 100 μmol/L Ni2+, and 30 μmol/L Cd2+ concentration for 72h, respectively. It can be seen that this algae can be applied to the treatment of heavy metal-containing wastewater.
There have been many reports on the application of herbaceous plants to purify heavy metal wastewater. Wind-eye
Lotus (Eichhoria crassipes Somis) is an internationally recognized and commonly used aquatic floating plant for pollution control, which has the characteristics of rapid growth, both low and high temperature resistance, and can rapidly and abundantly enrich a variety of heavy metals such as Cd, Pb, Hg, Ni, Ag, Co, Cr, and so on, in wastewater. Zhang Zhijie and other research results show that the dry weight of lkg of the wind-eye lotus in 7 ~ l0d can absorb lead 3.797g, cadmium 3.225g. Zhou Fan and other research found that the wind-eye lotus on cobalt and zinc absorption rate of up to 97% and 80% respectively. Typhao rientaliS Pres1 is also an excellent herb for purifying heavy metals, which has special structure and function, such as fleshy leaves and developed fenestrated tissue. The plant grows for a long time in the high concentration of heavy metal wastewater to form a special structure to resist the harsh environment and can self-regulate certain physiological activities to adapt to the pollution and toxicity. Recruitment of Wenrui et al. studied the stability of the artificial wetland system of broad-leaved cattail in the treatment of lead-zinc ore dressing wastewater in Fankou, Shaoguan, Guangdong, China. The results of 10 years' monitoring showed that the system could effectively purify the lead-zinc mine wastewater. The untreated wastewater contains high concentration of harmful metals lead, zinc, cadmium through the artificial wetland, the outlet water quality improved significantly, of which the lead, zinc, cadmium purification rate of 99.0%, 97.0% and 94.9%, and are in the national industrial wastewater discharge standards. In addition, there are many herbaceous plants with purification effect, such as hi lotus seed grass, water dragon, prickly bitter grass, duckweed, Indian mustard and so on.
The use of woody plants to deal with polluted water, with good purification effect, large treatment capacity, less affected by the climate, is not easy to cause secondary pollution and other advantages, more and more attention. Hu Huanbin and other experimental results show that the two kinds of plants, reed and pond fir, have a strong enrichment capacity for heavy metals lead and cadmium, and the woody plant pond fir has a better purification effect than the herbaceous plant reed. Zhou Qing et al. studied the response of five kinds of evergreen trees to cadmium pollution stress, and the experimental results showed that, under the high concentration of cadmium stress, five kinds of trees leaf chlorophyll content, cytoplasmic membrane permeability, peroxidase activity and cadmium enrichment and other physiological and biochemical characteristics have produced obvious changes, of which, boxwood, Helianthus annuus, fir resistance to cadmium pollution is better than the balsam fir and holly. Woody plants as the main body of heavy metal wastewater treatment technology, can cut off toxic and harmful substances into the human body and livestock food chain, to avoid secondary pollution, can be directed to cultivate, in the treatment of pollution at the same time, but also can beautify the environment, to obtain a certain degree of economic benefits, is an ideal method of environmental restoration.