Study On The Reduction And Stabilization Effect Of Microorganism In Soil On Hexavalent Chromium

Heavy metals are the main pollutants and widespread in the soil.It causes the soil environment to be destroyed and has a serious threat to human health and safety.Heavy metals are common,lagging behind,hidden and irreversible mostly,so it is difficult to control.Although the traditional repair techniques have a certain effect,there are still some disadvantages such as secondary pollution,destruction of soil structure and so on.Hence,bioremediation technology,especially microorganism,has become a new trend in environmental remediation because of its flexibility and environmental friendliness.Cr?VI?has a strong toxic effect and a great harm to the environment.Therefore,Cr?VI?pollution is taken as an example.Firstly,screening high efficient reducing bacteria.The factors affecting the reduction of Cr?VI?by the strains were studied and the optimum reduction conditions were explored.Then,the mechanism of reduction was investigated.Finally,the environmental factors affecting the reduction and stabilization in soil were studied.The results can provide some theoretical basis and technical support for remediation of Cr contaminated soil in situ.The main conclusions are as follows:?1?Four strains with ability of enduring Cr?VI?were isolated from the heavy metal contaminated area of a typical contaminated site in Suzhou.According to the required nutrient sources,they are numbered PDA-Cr₂,PDA-Cr₄₂,N-Cr₁ and G-Cr₂ respectively.The removal of Cr?VI?by any one of the four strains was dominated by reduction reaction.The reduction ability from strong to weak is N-Cr₁>PDA-Cr₄₂>PDA-Cr₂>G-Cr₂,so the strain N-Cr₁ was selected as the experimental bacteria.The results of physiological and biochemical indexes and molecular biological identification showed high homology with bacillus.The metabolism was the most active at 20 hours.Moreover,the maximum tolerance concentration of Cr?VI?was 200 mg/L.When the pH=7,30?and concentration of salt was 5 g/L,the strain grew well.?2?The results of factors affecting the reduction of Cr?VI?by N-Cr₁ in aqueous solution showed that N-Cr₁ had the best reduction effect when pH=7,the concentration of N-Cr₁ was 1 g/L and the reaction time was 48 h.The reduction rate was about 87.7%while the concentration of Cr?VI?was 50 mg/L.It was reducted by N-Cr₁ during metabolic process.High concentration of Cr?VI?might cause certain toxicity to N-Cr₁.Unsuitable pH would cause the disorder of the growth of strains,and ultimately affected the reduction ability.In addition,it was a great influence on the reduction of Cr?VI?by N-Cr₁ if there were coexistent ions.The effect of cations on the reduction could be sorted as Cd²⁺>Zn²⁺>Cu²⁺,and anions effect was SO₄^2->NO₃^->HPO₄^2-.In general,the inhibition effect of the coexisting metal ions was more obvious than that of anions.?3?Through studing the remaining Cr?VI?and total Cr of intracellular and extracellular of N-Cr₁ cells,it was found that most of the Cr?VI?was restored by N-Cr₁outside the cell mainly.Moreover,it was also indicated that the surface of cells was smooth,plump and without precipitation when there was no Cr?VI?.In contrast to the blank group,the cells surface appeared irregular folds and attachment between some cell ends when there was Cr?VI?in the system.But the growth state of N-Cr₁ was still well and the cell morphology was intact.According to the reducing ability,it can sort in order as follows:the whole cell>the intracellular crude enzyme solution>the dormant cell>the extracellular metabolite.It can be preliminarily concluded that reduction reaction is the result of the interaction of cell components and the reduction reaction mainly depends on the functional proteins secreted to the outside by the cell.?4?The effects of soil factors and the stability results showed that the removal rate of water-soluble Cr?VI?was 75%within 2 d when the moisture content was 25%.It also showed that the higher the content of Cr?VI?,the lower the reduction rate of bacteria at the same time,and the longer the time required to reach equilibrium.N-Cr₁ was the most active when the content of Cr?VI?was 100 mg/kg.The results indicated that the reduction effect of bacteria in neutral and acidic soil was better than other conditions which was similar to the results in aqueous solution.The blank group without bacteria still removed 27.6%of Cr?VI?,which may be related to the adsorption and complexation ability of other microorganisms or soil particles.The more bacteria dosage,the more shorter of the time for restoring all of Cr?VI?.Considering the time and cost,5%was selected as the optimum dosage of bacteria.The higher the content of organic matter,the shorter the time-consuming.By the oxidation of potassium permanganate,water-soluble Cr?VI?was partial recovery under the three variables which were moisture content,inoculation amount and organic matter,but it still maintained a good stability overall.It showed that microorganisms have better effect on stabilizing Cr?III?in soil.The results of the experimental groups treated with moist heat and dry heat sterilization also indicated that the normal decay of microorganisms would not affect the final effect of the treatment.In conclusion,the reduction efficiency of Cr?VI?is not the same in different environmental media.The reduction of Cr?VI?in soil is more complex than that in aqueous solution,which may be related to the complex environment of the soil.