| Chromium occupies an important position in the booming industry year by year,involving production activities in several important fields.A large amount of chromium-containing waste is discharged into water bodies and soil,seriously threatening human safety and ecological health.Bacterial mineralization refers to the conversion of free metal ions into stable mineral precipitates by the metabolism of bacteria.Using the mineralization behavior of bacteria to deal with heavy metal pollution is an environmentally friendly and low-cost strategy.However,in the lastly 20 years of exploration,scientists have not discovered the bacterial mineralization of chromium,and the products of different bacteria processing hexavalent chromium have been revealed as organo-trivalent chromium.Whether the chromium mineralization behavior of bacteria exists has become a huge controversy.There are four main reasons why bacteria produce organic trivalent chromium.(a)The main mechanism for bacteria to reduce hexavalent chromium is an enzyme-mediated reaction.Trivalent chromium,as a reduction product in the enzyme mechanism,is usually combined with an electron donor and it is organo-trivalent chromium;(b)Organo-trivalent chromium helps bacteria to survive the toxicity of hexavalent chromium,and can enhance the bacterial detoxification of hexavalent chromium;(c)Bacteria usually need to deal with the hexavalent chromium under acidic or neutral conditions.And bacteria also requires external organic matter as an electron donor to maintain activity during treatment of hexavalent chromium.But this medium environment is conducive to the production of organic trivalent chromium;(d)Coordination competition experiments show that trivalent chromium ions are more inclined than bacterial fragments in combination with small organic molecules.However,inspired by a phenomenon that the amorphous chromium nanoparticles produced by bacteria in the system of chromium and tellurium,and a natural mineral(like chromite),we suspected that the chromium mineralization of bacteria may need to occur in a more complex environment,such as co-exist with one or more heavy metals.In this research,based on the bottleneck problem of microbial treatment of chromium pollution,this paper selects Bacillus cereus with excellent metal ion tolerance and mineralization potential as the research object.This study is divided into the following two parts:(1)In the preliminary exploration of the treatment of hexavalent chromium by Bacillus cereus 12-2,it was found that hexavalent chromium severely inhibited the growth of its growing cells,while resting cells showed strong hexavalent chromium processing ability.The results of batch experiments showed that the bacteria had the highest reduction rate of hexavalent chromium in the presence of manganese and calcium ions,and 88.6% of hexavalent chromium was removed within 120 h.When magnesium ions coexist,the total chromium fixation rate is the highest,reached at 64.4%.The aluminum group produced the least amount of soluble trivalent chromium,only 4.1 mg/L.In general,bacteria perform poorly when cadmium ions coexist,and both the reduction rate of hexavalent chromium and the immobilization rate of total chromium are the lowest.SEM and XRD analysis results showed that Bacillus cereus 12-2 did not detect minerals in the environment where these 9metal cations coexisted.(2)Bacillus cereus 12-2 significantly enhances the ability of hexavalent chromium reduction and total chromium removal under the stimulation of copper ions.The best is achieved at a copper ion concentration of 64 mg/L,that is,almost 100% of hexavalent chromium is removed at 48 h,and almost 100% of total chromium in the supernatant is removed at 72 h.The results of XPS,SEM,TEM,EDX and other technologies show that after copper ions enter Bacillus cereus 12-2,they stimulate the activity of enzymes,thereby promoting the reduction of hexavalent chromium.Under the synergistic effect of chromium and copper,the bacteria adjust the intracellular and extracellular p H to alkaline,creating media conditions conducive to mineralization.Furthermore,after the chromium and copper elements are co-precipitated in the cell,the chromium copper oxide is discharged into the extracellular space by the secretion behavior of the bacteria,completing the chromium mineralization of the bacteria.This chromium copper oxide will not be dissolved and released into the environment as the bacteria break down and die.The actual electroplating wastewater test showed the extremely strong application potential of Bacillus cereus 12-2.In summary,this work explored the changes in the ability of Bacillus cereus 12-2 to treat hexavalent chromium under the coexistence of other metal cations.In this process,the phenomenon of bacteria-mediated chromium copper mineralization was discovered and its mechanism was analyzed.This work breaks through the unique understanding of chromium products processed by bacteria,and will provide new reference ideas for the biogeochemical cycle of chromium elements,and also provide new strategies for the treatment of chromium-containing wastes. |
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