In general, the following two basic approaches are adopted to remediate heavy metal pollution in water. One is to reduce the migration ability and bioavailability of heavy metals in water bodies; the other is to completely remove heavy metals from polluted water bodies. Clear.
2.1 Physical and chemical methods
2.1.1 Dilution method
The dilution method is to mix water contaminated by heavy metals into unpolluted water, thereby reducing heavy metal pollution concentration and reduce the degree of heavy metal pollution. This method is suitable for the treatment of water bodies that are lightly polluted by heavy metals. This method cannot reduce the total amount of heavy metal pollutants discharged into the environment, and because heavy metals have a cumulative effect, when the concentration of heavy metal pollutants in these water bodies reaches To a certain extent, the organisms living in it will be affected by heavy metals, causing disease and death, so this treatment method is gradually being denied.
2.1.2 Coagulation and precipitation method
Many heavy metals mainly exist as cations in water solutions. Adding alkaline substances will increase the pH value of the water, which can cause the formation of most heavy metals. Hydroxide precipitates. In addition, many other anions can also precipitate the corresponding heavy metal ions. Therefore, adding lime, NaOH, Na2S and other substances to heavy metal-polluted water bodies can precipitate and remove many heavy metals, reducing the harm of heavy metals to water bodies. This is a commonly used method in the country to deal with heavy metal pollution. For example, Huang Ming et al. [5] used chemical classification methods to treat electroplating wastewater containing chromium, copper, and nickel, and achieved good results.
2.1.3 Ion reduction method and exchange method
The ion reduction method uses some easily available reducing agents to reduce heavy metals in the water body to form pollution-free or lightly polluted metals. compounds, thereby reducing the mobility and bioavailability of heavy metals in water bodies to reduce the pollution of water bodies by heavy metals. For example, electroplating wastewater often contains hexavalent chromium ions (Cr6+), which exist in the form of chromate ions (Cr2O72-). They are difficult to precipitate under alkaline conditions and are highly toxic, while trivalent chromium is much less toxic than hexavalent chromium. , but hexavalent chromium is easily reduced to trivalent chromium under acidic conditions. Therefore, ferrous sulfate and sulfur trioxide are often used to reduce hexavalent chromium to trivalent chromium.
The ion exchange method uses heavy metal ion exchangers to exchange heavy metal substances with polluted water bodies to exchange heavy metals out of the water bodies to achieve the purpose of treatment. After ion exchange treatment, the heavy metal ions in the wastewater are transferred to the ion exchange resin, and after regeneration, they are transferred from the ion exchange resin to the regeneration waste liquid. This type of method is less expensive and the operators are not directly exposed to heavy metal pollutants, but its scope of application is limited and it is easy to cause secondary pollution.
2.1.4 Electrodynamic remediation technology
The electrodynamic remediation method is a remediation technology developed in the late 1990s for heavy metal pollution in water. Its basic principle is to treat both water bodies polluted by heavy metals. A DC electric field is applied to the end, and the electric field migration force is used to migrate heavy metals out of the water body. Ridha et al. [6] proposed a technology for removing copper, chromium and nickel from industrial wastewater by electrodeposition on a carbon felt electrode. In addition, electroflotation can be used to purify industrial wastewater containing heavy metals such as copper, nickel, chromium and zinc. In addition, in recent years, some people have applied electrodialysis membrane separation technology to the practice of heavy metal treatment in sewage [7].
2.2 Bioremediation method
There are currently many studies at home and abroad using bioremediation to remediate heavy metal pollution in water. According to the different biological objects used, they can be divided into the following three types.
2.2.1 Phytoremediation method
Phytoremediation refers to the general term for the technology that uses specific plants to implement environmental pollution control, through the special enrichment of heavy metal elements or organic substances by plants. and degradation ability to remove pollutants in the environment, or eliminate the toxicity of pollutants, to achieve the purpose of pollution control and ecological restoration.
Since the American scientist Chaney [8] first proposed the idea of ??using plants to remove heavy metal pollution in 1983, many countries have carried out research and application of phytoremediation technology and made great progress. A key issue restricting the development of phytoremediation technology is to select plant species that can both tolerate heavy metal pollution and accumulate heavy metals in large quantities.
So far, many scholars at home and abroad have carried out research on the use of plants to remediate heavy metal polluted water bodies, and have obtained many valuable results. The more common plants used include sunflower, oats, barley, peas, tobacco, and Indian mustard. , lettuce, etc. Salt et al. [9] pointed out that Indian sunflower can accumulate different heavy metals from sewage. Chen Jun et al. [10] pointed out that Lishi's grass is suitable for growing in a wet environment and can have a strong enrichment effect on a variety of heavy metals. It has shown broad application in the remediation of water polluted by heavy metals such as Cr, Cu, and Ni. application prospects. Eichhornia crassipes and cress can effectively remove heavy metals such as Cd, Cr and Cu in sewage [11].
2.2.2 Animal remediation method
Apply some preferred fish and other aquatic animal species to absorb and enrich heavy metals in the water body, and then expel them from the water body to Achieve the purpose of remediation of heavy metal pollution in water. Shellfish, crustaceans, annelids, etc. among the benthic animals in the water body also have a certain enrichment effect on heavy metals. For example, spinnaker clams and river clams have obvious natural purification capabilities for heavy metals (Pb2+, Cu2+, Cr2+, etc.). However, this method has a long treatment cycle and high cost. Therefore, at present, aquatic animals are mainly used as indicator organisms for environmental heavy metal pollution, and not many are used for pollution control. Niu Mingfen [12] found that earthworms significantly enriched Cd in river sediments. Earthworms can also affect the types, quantities, and activities of soil microorganisms [13]. There are also complex interactions between microorganisms and heavy metals, which affect the types and effectiveness of heavy metals. Therefore, they can change the absorption of heavy metals by plants. and transfer. Lasat believes that studying the interactions between soil animals, microorganisms and plants is of great significance to the further development of phytoremediation technology [14].
2.2.3 Microbial remediation method
The bioremediation mechanism of heavy metal contaminated water mainly includes the fixation and form transformation of heavy metals by microorganisms. The former involves microorganisms adsorbing heavy metal ions on the surface of charged cells, or actively absorbing heavy metal ions by taking in necessary nutrients, and enriching heavy metals on the surface or inside the cells; the latter involves changing the form of heavy metals or reducing heavy metals through the life activities of microorganisms. Bioavailability, thereby reducing heavy metal pollution, such as Cr6+ being converted into Cr3+ and reducing toxicity, As, Hg, Se, etc. being reduced to elemental states and volatilizing, microbial secretions producing passivation effects on heavy metals, etc. [7]. Studies have shown that cyanide bacteria and algae lint can effectively remove heavy metals from sewage. Sulfate reducing bacteria produce H2S and reduce heavy metal ions into extremely low water-soluble sulfides such as ZnS, CdS and CuS, which are precipitated to achieve the purpose of controlling heavy metal pollution