Bioleaching Of Cu And Zn In Soil By Hyper-accumulative Plant Endophytes

Heavy metals are one of the important factors leading to soil pollution.Several methods already used to remove these kinds of contaminants from soil,but most of them are costly and difficult to get optimum results.As an effective and reasonable restoration technology,biological leaching has the characteristics of environmental protection and low cost.Plant rhizosphere bacteria transform metals into bioavailable and soluble forms through the action of siderophores,organic acids,biosurfactants,biomethylation,and redox processes.Due to the strong tolerance of endophytes to heavy metals in super-accumulated plants and it has the same mechanism of remediation of heavy metals in soil as rhizosphere bacteria,we apply endophytes directly to the soil and use direct or indirect effects of endophytes on the soil.We use endophytes as eluents to explore their effects on the bioavailability of heavy metals in the soil and to try the possibility of bioleaching.In this study,endophytes extracted from hyperaccumulator(Solanum Nigrum L.)plants were used as biological eluents to investigate the effects of different endophytic bacteria on different forms of Cu and Zn in soil,and the effects of soil-liquid ratio and leaching time on the leaching effect.The main research results are as follows:1.Through the grinding of Solanum nigrum L.,a variety of endophytic bacteria were isolated,and four strains with distinct color forms,numbered E1,E2,E3,and E4,were selected and analyzed for biological analysis.E1 was identified as Bacillus cereus,E2 was identified as Staphylococcus cohnii subsp.Urealyticus,E3 was identified as Staphylococcus epidermidis,E4 was identified as Sphingomonas yabuuchiae;2.Four strains of bacteria were used as bioleaching agents to occur in soil,and their leaching performance and optimal leaching times were explored.The results showed that compared with the leaching effect of aseptic water,four endophytic bacteria E1,E2,E3 and E4 as bioleaching agents significantly enhanced the leaching of mobile Cu and Zn in soil through six leaching,and E2 leached the most mobile Cu and Zn,with the total content basically unchanged.E2 has the highest total activation rate of 36.47% for Cu and 30.81% for Zn under six elutions.E2 mainly acts on the ex-three times extractions,and the elution effect in the second extractions is significantly improved.The main reason is that endophytic bacteria convert other forms into mobilisable during the first rinsing and convert the sealed mobilisable into mobile by the activation of endophytic bacteria,resulting in significant improvement of the elution effect.Therefore,four strains can convert part of the mobilisable components and part of the residue components of Cu and Zn into the mobile in the soil sample.3.By high performance liquid chromatography(HPLC)analysis of E2,three kinds of organic acids were identified,namely propanedioic acid,acetic acid and citric acid.4.Three kinds of E2 organic acid products were quantified to prepare a standard mixture of the same concentration,and the soil was subjected to one-component and multi-component leaching.The results show that under the three leaching,the total activation rate of E2 to Cu and Zn was 31.51% and 38.45% respectively.Citric acid promoted the leaching of mobile Cu,while acetic acid and propanedioic acid played an inhibitory role in the leaching of mobile Cu.Therefore,it can be considered that citric acid as a product of E2 plays a very important role in the activation of Cu in soil.The total activation rate of Zn by mixed elution of propanedioic acid,acetic acid and citric acid is 54%.The effect of single component elution of propanedioic acid,acetic acid and citric acid on mobile zinc is not as good as that of aseptic water.Mixing of propanedioic acid,acetic acid and citric acid can promote mobile Cu and Zn,indicating that there may be synergistic effects among the three.E2 plays an antagonistic role in the elution of mobile zinc due to the existence of other substances.Therefore,the activation effect of E2 on mobile Zn is not as good as the standard mixture of three small organic acids.This indicates that small molecular organic acids can also increase the fluidity of heavy metals such as Cu and Zn.5.The leaching effect of E2 with different leaching time and soil-liquid ratio was preliminarily investigated.The results showed that the maximum activation rate of Cu and Zn in a short time was achieved based on the factors of repair time and cost,and the adaptation time of bacterial solution.It was considered that the elution time of 12 hours was the most reasonable.Considering the economic benefits,it is considered that the optimum soil-liquid ratio of E2 to Cu is 7:12,and the optimum soil-liquid ratio of Zn to Zn is 1:1.