Bioremediation of heavy metal contaminated soil microorganisms is achieved primarily through microbial dissolution of heavy metals, transformation and fixed. Microbial dissolution of heavy metals It is mainly carried out directly or indirectly by producing various low molecular weight organic acids through various metabolic activities. The earliest reports were that fungi can dissolve heavy metals and heavy metal-bearing minerals by secreting amino acids, organic acids, and other metabolites. Comparing the dissolution of heavy metals by microorganisms under different carbon sources, it was found that soil microorganisms can utilize effective nutrients and energy, and secrete organic acids in the soil filtration process to dissolve heavy metals in the soil. In the sterilization test, heavy metals were removed from the unsterilized eluent. The concentration of heavy metal ions in the sterilized eluent was significantly higher than that of the sterilization treatment. Loser et al. used the leaching of indigenous microorganisms to repair the heavy metal pollution of river sediments in the Saxony region of Germany, pointing out that the optimal dosage of the substrate during the treatment was 2%, and the optimum temperature was between 30-40 °C. The rate is up to 98%. Redox conversion of heavy metals by microorganisms In the external environment, the variable metals As, Cr, Co and Au can exist in different valence states, and the bacterial metabolic activities can change their valence state through their redox. Chang et al found in the sewage treatment plant that a sulphate bacterium can reduce Cr6+ to a less toxic, less soluble Cr3+, thereby reducing the toxicity of heavy metals in the water. Soil can also distributed more chromate and dichromate reducing microorganisms, such as Alcaligenes, Bacillus, Corynebacterium, Enterobacter, Micrococcus and Pseudomonas spp, These bacteria can reduce highly toxic Cr6+ to low toxicity Cr3+. Other bacteria, such as sulfur - Bacillus species capable of oxidizing iron As, Cu, Mo and Fe, etc., can Pseudomonas As, Fe, Mn, etc. oxidized, thereby reducing the activity of these heavy metal elements. Biological fixation of heavy metals by microorganisms There are three main modes of action: extracellular complexation, extracellular precipitation, and intracellular accumulation. The molecular structure in the cell wall is active and can sequester the metal on the cell surface. After Papassiopi and other two groups of Cr (VI) contaminated soil samples were biologically fixed, the content of soluble Cr in the soil decreased from 13 mg/kg to less than 0.5 mg/kg, and 120 mg/kg decreased to less than 5.6 mg/kg. The cell wall separated from the Bacillus can sequester a large amount of metal elements from the solution, and when the cell wall is placed in an aqueous solution containing gold chloride, minute crystals can be formed by polymerization on the cell wall. It can regulate the redox reaction of Zn2+, Pb2+ and other metals, such as iron oxide and sulfur. Microbial-plant joint repair Although phytoremediation technology has the advantages of no secondary pollution and low cost, the growth of plants as living organisms is bound by various natural and human factors, and the repair process is usually slow; one plant often only absorbs one. Species or several heavy metals show some poisoning symptoms to other heavy metals in the soil, which limits the application of phytoremediation technology to various heavy metal contaminated soils. At the same time, different soils should be selected for different pollution conditions. Eco-plants: Super-accumulated plants should be used for soils with heavy metal pollution, and heavy-tolerant plants should be planted for less polluted soils. It is also difficult to screen for super-accumulated plants. For microbial remediation, most of the technology is still limited to scientific research and laboratory level. The microbial remediation research in the laboratory is ideal because of the repair conditions, and the interference factors are very few. The repair may be very good, such as once indoors Microbial remediation technology is put into the field conditions, the interference factors are complex, and a series of new problems may appear. At present, more research is on mycorrhiza repair, because mycorrhiza is a combination of fungal hyphae in soil and higher plant roots. Mycorrhiza repair is only a kind of micro-plant combination repair. On the one hand, the epidermal cells of the roots of plants, the release of enzymes and nutrients, provide a better environment for the growth of microorganisms. On the other hand, the activity of mycorrhizal fungi can improve the rhizosphere micro-ecological environment and enhance the disease resistance of plants. Improve the viability of plants under adverse conditions such as drought and toxic substances. Outlook Bioremediation technology is an emerging, efficient, green and inexpensive repair path that minimizes disturbances to the environment during repairs. However, the technology is still not mature enough and needs further improvement. At present, the development of phytoremediation also relies on the development of plant species that efficiently absorb pollutants, soil amendments, and agricultural practices such as optimization of plant cultivation. Plate And Frame Heat Exchanger,Brazed Plate Heat Exchanger,Spiral Heat Exchanger,Air Cooled Heat Exchanger Wuxi Dingfeng pressure vessel Co., Ltd , https://www.wuxidingfeng.com