Combined Application Of Phosphate Rock And Phosphate-solubilizing Bacteria For Immobilization Pb In Contaminated Soil

Application of phosphate rock?PR?have been considered as an alternative technique for the remediation of Pb contaminated soil.However,current researches revealed that the efficiency of phosphate rock on the immobilization of heavy metals was limited mainly due to its low solubility.Phosphate solubilizing bacteria?PSB?,which have exhibited an ability of dissolving phosphorus from insoluble P compounds,enhanced the concentration of insoluble P compounds in soil solution.Therefore,the objectives of this study are:?i?to screen high-efficiency phosphate solubilizing bacteria and evaluate their P solubilizing capacity and solubilization mechanisms by determining the types and content of organic acid in their metabolites;?ii?to examine whether the inoculation of PSB can promote the immobilization of Pb by phosphate rock,and to reveal the mechanism of the dissoluble P on the immobilization of Pb by SEM-EDX.The major results of this paper were summarized as follows:?1?Pantoea ananatis HCR2 and Bacillus thuringiensis GL-1 showed a high efficiency of P solubilization from phosphate rock.The maximum concentration of dissoluble phosphorus in the National Botanical Research Institute's Phosphate?NBRIP?growth agar medium with phosphate rock as solo P source reached up 85.54 and 97.51 mg·L^-1,and P solubilizting rate were 35.21%and 40.24%,after 7 days of cultivationby Pantoea ananatis HCR2 and Bacillus thuringiensis GL-1,respectively.Moreover,the mechanism of P solubilization from phosphate rock may be attributed to the acidification by Pantoea ananatis HCR2and Bacillus thuringiensis GL-1 by releasing glucose acid,?-Ketoglutaric acid and citric acid.?2?When lead nitrate was added and Pantoea ananatis HCR2and Bacillus thuringiensis GL-1 were inoculated into the NBRIP medium with phosphate rock as solo P source,both phosphate dissolution and significant decrease of Pb²⁺concentration in solution were observed.SEM analysis found that the surface of the particles of phosphate rock became more smooth and smaller than before,and some prills were found adsorbed on the surface of phosphate rock powder,and square particles were discovered between the powders.These prills and square particles were composed of amount of Pb as illustrated by EDS analysis,which indicated that the adsorption of Pb in NBRIP medium with phosphate rock and the formation of new precipitate were observed.?3?Sterile soil cultivation experiment revealed that the combined addition of phosphate rock and PSB?Pantoea ananatis HCR2andBacillus thuringiensis GL-1?to the multi-metal contaminated soil significantly increased the pH value and the available phosphorus content,but reduced the available Pb concentration in soils.The soil pH and available phosphorus content proportionally increased,while available Pb accordingly reduced with the increasing of phosphate rock dosage.In addition,the inoculation of Pantoea ananatis HCR2or Bacillus thuringiensis GL-1 resulted more exchangeable Pb become unavailable.?4?Pot experiment shown that the combined application of phosphate rock and PSB?Pantoea ananatis HCR2and Bacillus thuringiensis GL-1?obviously improved plant growth and biomass of lettuce?Lactuca sativa L.?and Chinese flowering cabbage?Brassica campestris L.?.Compared to the control soil,the combined-addition of phosphate rock and PSBs can remarkably decrease the soil available concentrations of Pb by 4.40%-69.05%,and reduced shoot Pb in lettuce and C hinese flowering cabbage by 9.34%-45.06%and12.37%-88.32%,respectively.