| Sewage sludge was generated during the wastewater treatment process.It contained high water content,high organic matter content,high heavy metal content,especially the environmental risk of heavy metals in sludge is the bottleneck of the sludge disposal.The heavy metal removal methods include physics,chemistry,biology.Considering that the physical and chemical methods pose several disadvantages,such as high operating cost,large energy consumption,low removal efficiencies and so on.therefore,the biological method cause the researchers attention.bioleaching method has received comprehensive attention with many excellent features such as low acid consumption,low cost bio-additive,high removal rate and no secondary pollution.At present,most researchers were focused on bioleaching of heavy metals from sewage sludge using a pure culture of At.thiooxidans or At.ferrooxidans,including the research of initial pH,solid concentrations,substrate species and concentrations,bacterial inoculum and so on,but there is much less known about optimizing bioleaching of heavy metals using a mixed culture of At.thiooxidans and At.ferrooxidans.The main objective of this work was to optimal the leaching conditions using the mixed bacteria of At.thiooxidans and At.ferrooxidans to improve the removal efficiencies.Afterward,a kinetic study was performed on bioleaching of heavy metals from sewage sludge under optimal conditions using a shrinking core model.Providing technical references with the disposal of sludge using the bacteria.Firstly,strains that removal heavy metals with high efficiencies were isolated from the sewage sludge collected from recirculation loop of a WWTP in Guangzhou.The six strains are named Af-1,Af-2,Af-3,At-1,At-2,At-3.Based on morphological and 16 S rDNA gene sequences analysis,the results indicated that the Af-1,Af-2,Af-3 was identified as At.ferrooxidans,The homology between Af-1 and Acidithiobacillus ferrooxidans strain LY(DQ529309)was 99%.The At-1,At-2,At-3 was identified as At.thioocidans,the homology between At-1 and Acidithiobacillus thiooxidans strain ZJJN-1(JQ259048)was 100%.Secondly,the effect of At.ferrooxidans,At.thioocidans and the mixed culture of At.ferrooxidans and At.thioocidans on removing heavy metals during bioleaching process were compared.The results showed that the removal efficiencies of Cu,Cr,Ni,Zn for the mixed culture of At.ferrooxidans and At.thiooxidans were better than the pure culture of At.ferrooxidans or At.thiooxidans.Besides,Cu,Zn is more easily dissolved and leached out,Ni second,Cr last.Then,the effect of initial p H,solids concentration,sulfur concentration,ferrous iron concentration using the mixed culture of At.ferrooxidans and At.thioocidans on removing heavy metals during bioleaching process was studied by single factor experiments and box-behnken design of response surface methodology experiments.The results of single factor experiment show that initial p H of 4 is good for startup of bioleaching,solids concentration of 5% can effectively remove heavy metals in sludge.The substrate concentration affect the growth of bacteria.Within a certain range,with the increase of Fe SO4·7H2O and S0 dosages will improve the removal efficiencies of removing heavy metals.The results of box-behnken design of response surface methodology show that the optimal conditions were initial pH of 2,solids concentration of 3%,sulfur concentration of 6.14g/L,ferrous iron concentration of 4.55g/L.Maximum removal efficiencies obtained after 14 days of bioleaching under optimal conditions were98.54% for Cu,95.60% for Zn,57.99% for Cr and 60.06% for Ni,respectively.Finally,in order to better understand the mechanism of the leaching reaction,the removal efficiencies of heavy metals,pH,ORP,bacteria number,Fe3+,SO42 concentration were done under optimal conditions,bioleaching kinetics was simulated using a shrinking core model.The results suggested that diffusion step was the rate determining step. |
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