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With the rapid increase of industrial sewage discharge, a large amount of heavy metal pollution is discharged into the environment. Under certain conditions, some heavy metals (such as mercury) can also be converted to more toxic organic substances under the action of certain microorganisms. In addition, toxic heavy metals can stay and accumulate in the environment for long periods of time, accumulate step by step through the food chain, and eventually enter the human body. Even infants are infested through heredity or breast milk. The main manifestation is the accumulation of chronic poisonings in certain organs of the human body. Therefore, heavy metal pollutants treatment technology has become a hot research, the results have great practical significance.
introduction
With the continuous development of society, people are more advocating the harmonious development of industry and natural environment than ever before. This concept has been constantly penetrating into all disciplines and should be sought in the development of pollution control technologies. Give full play to the nature of the natural self-purification function, pollution prevention and environmental remediation in the development of environmental protection technology embodiment, it is the full use of natural self-purification function response. This article describes the hazards of heavy metals, their sources and their forms of existence, and focuses on the treatment of heavy metal pollutants.
Sources of heavy metal contaminants in wastewater
1. The source of lead. Lead is often used as a raw material in batteries, electroplating, pigments, rubber, pesticides, fuels and other manufacturing industries. Acid wastewater (pH3 = highest lead concentration and waste water lead concentration produced by electroplating waste) is also discharged from lead board making process.
2. The source of cadmium. Cadmium is an off-white metal that is mainly in the form of a divalent element in nature. Cadmium plating for steel, iron and other corrosion protection to provide a protective layer, with good adsorption and coating uniform and smooth and so on, so the industrial 90% blessing for electroplating, pigments, plastic stabilizers, alloys and batteries Industry, sources of cadmium wastewater also include metal mining mining, smelting, electrolysis, pesticides, pharmaceuticals, electroplating, textile printing and dyeing industries such as the production process.
3. Nickel source. Sources of Nickel in Wastewater Nickel in wastewater is mainly present as divalent ions, such as nickel sulfate, nickel nitrate, and nickel salts with many inorganic and organic complexes. There are many industrial sources of nickel-containing wastewaters, mainly the electroplating industry. In addition, nickel is also contained in wastewater discharged from industries such as mining, metallurgy, petrochemicals, textiles, and steel mills, printing and other industries.
4. The source of silver. Common silver salts are the only soluble silver nitrate, silver is also the main component of wastewater. Silver nitrate is widely used in radio, chemical, machinery manufacturing, ceramics, photography, electroplating and ink manufacturing industries, the main source of silver-containing wastewater is the electroplating industry and photographic industry.
Heavy metal pollutants exist in the environment
Heavy metal pollutants in the atmosphere, water, sediment, soil, plants and other systems are distributed in different systems exist in different forms. The heavy metals in the soil exist in the form of soil heavy metal pollution is mainly caused by the use of sewage sludge and sewage irrigation, industrial wastewater effluent accounted for 60% to 80%, and the composition is complex, varying degrees contain difficult to biodegradation of heavy metals.
Heavy metals exist in the form of water. In recent years, the Chinese Academy of Sciences conducted a more in-depth study on the background value of heavy metals in the Yangtze River water environment. The results show that most of the elements in the river water mainly exist in the form of suspended particles, while the dissolved heavy metal concentrations are lower and the total amount is more partial High elements, the higher the proportion of suspended particles exist. This feature is closely related to the regional conditions. When the weathering is intense, the suspended matter content directly affects the elemental concentration distribution in the water environment. At the same time, weak chemical weathering makes the element difficult to release, and the low alkalinity of the water makes the dissolved heavy metal concentration low.
Heavy metals exist in sediments. The vast majority of heavy metals that enter the aquatic environment through a variety of routes are rapidly transferred to sediments or suspended sediments by sedimentation with the physical, chemical, biological and physicochemical effects. The occurrence and characteristics of heavy metals in sediments are as follows: Pb mainly tends to combine with Fe / Mn hydrate oxides and carbonate, while Cu mainly forms the residue phase and organic phase, while Zn is easily bound to Fe / Mn hydrate oxide, Acid salts; Pb, Zn non-residue phase as the main component, the residue of Cu as the main component.
Commonly used method of heavy metal wastewater treatment
There are many ways to treat heavy metal wastewater, which can be divided into two categories: one is to convert the soluble heavy metal into insoluble or insoluble metal compounds, so as to remove it from the water. The other is concentrated without changing the chemical form of heavy metals separation, such as reverse osmosis, electrodialysis, ion exchange, evaporation concentration and so on.
Hydroxide precipitation method The method is that the metal ions are reacted with the light base by adding an alkaline precipitating agent (such as milk of lime and sodium carbonate, caustic soda, etc.) to the heavy metal wastewater to generate a precipitate of a hardly soluble metal hydroxide so as to be separated.
Sulfide precipitation method. In the method, a vulcanizing agent is dosed into the waste water to react the metal ions with the sulfide to generate a precipitate of the insoluble metal sulfide so as to be separated. Sulfide can be sodium sulfide, hydrogen sulfide or ferrous sulfide and so on. The advantage of this method is that the solubility of metal sulfides generated is smaller than the solubility of metal hydroxides, the treatment effect is better than that of hydroxide precipitates, and the amount of residue is small and the water content is low, so as to facilitate the recovery of useful metals. The disadvantage is the high price of sulfide.
3 reduction method. The method is that the reducing agent is dosed into the waste water so as to reduce the metal ions into the metal or the low-valent metal ions and then add the lime to precipitate the metal hydroxide so as to be separated. Reduction method can be used for copper, mercury and other metal ion recovery, commonly used in the treatment of lead-containing wastewater.
4. Ion exchange method. Ion exchange method is the use of exchanging groups from the exchange agent exchange with the metal ions in the wastewater reaction, the metal ions are replaced by the exchange agent to be removed. Use of ion exchange treatment of heavy metal wastewater, such as Cu2 +, Zn2 +, Cd2 +, etc., can be used cation exchange resin; and anionic form of metal ion complexes or acid (HgCl2-, Cr2O72, etc.), you need to use anion exchange resin Remove.
Ferrite method. Ferrite is an oxide composed of ferric ions, oxygen ions and other metal ions. It is a kind of ferromagnetic semiconductor. The use of ferrite method for the treatment of heavy metal wastewater is based on the principle of ferrite manufacturing, the use of ferrite reaction, the wastewater in the divalent or trivalent metal ions filled ferrite spinel lattice go Get precipitation separation method.
Electrolysis Electrolysis is the use of electrodes and heavy metal ions electrochemical action to eliminate its toxicity. In accordance with the different types of anodes, the electrolytic method is divided into electrolytic precipitation and recovery of heavy metal electrolysis two types. Electrolysis equipment is simple, small footprint, easy operation and management, and can recover valuable metals. However, power consumption, water quality is poor, wastewater treatment capacity is small.
7. Membrane separation side
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