Effect Of Iron Minerals And Humic Acid On The Cr(?) Reduction By Shewanella Oneidensis MR-1

Chromium,which is widely used in industrial production,has caused serious environmental problems due to its direct discharge into the soil or water environment.It is a feasible method to repair Cr(?)pollution by using microorganisms to convert highly toxic Cr(?)into low-toxic Cr(?).Bacteria,iron minerals,and humic acid are the main components of the soil matrix.The interaction of these components has an important influence on the biological reduction of Cr(?).In this study,three iron-bearing minerals(goethite,hematite,and nontronite),Shewanella oneidensis MR-1,and soil-derived humic acid are the research objects.The physicochemical properties and structural characteristics of single components and complexes were studied.The reduction kinetic experiments,surface site analysis,Fouriertransform infrared spectroscopy(FTIR),scanning electron microscopy(SEM)combined with energy disperse spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS),and electron paramagnetic resonance(EPR)studies were used to investigate the characteristics and mechanism of Cr(?)reduction through the individual components,binary,and ternary complexes.The effects of iron mineral and humic acid on the Cr(?)bioreduction by S.oneidensis were investigated.The main results are as follows:(1)The surface site concentrations of S.oneidensis,S.oneidensis-humic acid,S.oneidensis-goethite,and S.oneidensis-goethite-humic acid complexes are 3.41 and 3.34,1.98 and 1.71 mmol/g respectively,while their Zeta potentials are:-6.48,-9.30,-8.52,and-9.80 m V,respectively.Goethite increases the negative charge on the surface of the complex and reduces the surface site concentration of the complex.Humic acid has no significant effect on the surface charge and surface-active site concentration of the composite material.The results of Cr(?)reduction kinetics experiments show that S.oneidensis can reduce 65% of 1 m M Cr(?)in solution within 8 hours.Under the mediation of goethite or humic acid,the 8-hour removal rate of Cr(?)dropped to 56%.However,the reduction rate of Cr(?)in the ternary system of S.oneidensis-goethite-humic acid can be increased to 79%.Humic acid reduces the adhesion of bacteria to the surface of goethite,and acts as an electron shuttle,which enhances the electron transfer efficiency,thereby increasing the reduction rate of Cr(?),and the reduction rate of Cr(?)increased by 1.3 times within the same reaction time.The adsorption of chromium on goethite or humic acid is negligible,indicating that the reduction of Cr(?)can be attributed to the biological reduction of S.oneidensis.The degree of Cr(?)reduction decreases with the increase of initial Cr(?)and increases with the increase of p H.When the ratio of S.oneidensis to goethite is 2:1,the reduction rate of Cr(?)is 100%.The reduction rate of Cr(?)by microorganisms decreases with the increase of goethite.A pseudo-first-order kinetic model was used to fit the Cr(?)reduction kinetics data.When the initial chromium concentration is 0.5m M,the Cr(?)reduction rates over the complexes of S.oneidensis-goethite-HA,S.oneidensis,S.oneidensis-goethite,and S.oneidensis-HA were 3.73,2.57,1.07,and 1.06 Kh-1 respectively.Infrared spectroscopy analysis showed that when combined with bacteria,the characteristic absorption peak of goethite was 888 cm-1,which was attributed to the bending vibration of the Fe-O-H surface,while the addition of humic acid did not cause significant changes in surface functional groups.After the bacteria reduced Cr(?),the characteristic absorption peaks(amide I,amide ? and phosphorous oxygen groups)shifted,indicating that the process of microbial reduction of Cr(?)is related to the functional groups on the surface of the bacteria.Scanning electron microscopy results showed that when Cr(?)reacted with S.oneidensis cells,the cell morphology shrank and the cell surface was shrinking,indicating that chromate has a toxic effect on bacteria.The introduction of goethite and humic acid can slow down the toxic effects on bacteria.The XRD results showed that after the reaction with Cr(?),the mineral crystal form and crystallinity in the binary and ternary complexes did not change,indicating that goethite is relatively stable in the short-term reaction.XPS analysis showed that Cr(?)was reduced to Cr(?),and the final product was Cr(OH)3 and Cr2O3 precipitated on the surface of bacterial cells.S.oneidensis reduces Fe(?)in goethite to Fe(?),and then chemically reduces Cr(?).The EDS analysis results also show that the S.oneidensis-goethite-humic acid complex has a significant promotion effect on the reduction of Cr(?),which is consistent with the results of the kinetic reaction.EPR analysis shows that the reduction of Cr(?)by S.oneidensis-goethite-humic acid complex is a two-step electron transfer process,and there is an intermediate valence state during the reduction of Cr(?).(2)The surface site concentrations of S.oneidensis,S.oneidensis-humic acid,hematite,S.oneidensis-hematite,and S.oneidensis-hematite-humic acid are 3.41,3.34,2.13,1.84,and 1.69mmol/g respectively.The surface charges of S.oneidensis-hematite,S.oneidensis-humic acid,S.oneidensis-hematite-humic acid,and S.oneidensis are-10.54,-9.30,-7.50,and-6.48 m V respectively.The addition of hematite increased the negative charge on the surface of the complex,but reduced its surface site concentration,while humic acid had no significant effect on the surface charge of the complex.The binary complex of S.oneidensis-hematite slightly promotes the biological reduction of Cr(?),while the ternary complex of S.oneidensishematite-humic acid significantly enhances the microbial Cr(?)reduction.According to the pseudo-first-order kinetic model,the reduction rates of S.oneidensis-hematite-humic acid,S.oneidensis-hematite,S.oneidensis,and S.oneidensis-humic acid were 3.36,2.62,2.57,and 1.06Kh-1.respectively.The removal of Cr(?)by hematite or humic acid is negligible,indicating that the removal of Cr(?)is mainly due to the biological reduction process of bacteria.In addition,the reduction rate of Cr(?)decreases with the increase of the initial concentration of Cr(?).With the increase of p H value,the microbial Cr(?)reduction by S.oneidensis was enhanced.The S.oneidensis-hematite complex has a slight promotion effect on the biological reduction of Cr(?),while the S.oneidensis-hematite-humic acid complex has an obvious promotion effect on the reduction of Cr(?).The reduction rate of Cr(?)by microorganisms decreases as the proportion of hematite increases.Infrared spectroscopy analysis shows that hematite minerals have the characteristic absorption peaks of Fe-O bond crystal lattice tensile vibration and Fe-O bending vibration at 553cm-1 and 471 cm-1,respectively.There is no significant change in the functional groups on the surface of the complex.After S.oneidensis reduced Cr(?),the characteristic absorption peaks(amide I,amide ?,and P=O)of the bacteria moved,confirming that the functional groups on the surface of S.oneidensis participated in the Cr(?)reduction process.The SEM image shows the toxic effect of chromate on bacteria,and the introduction of hematite minerals and humic acid can reduce the toxic effect on bacteria.After reacting with 1.0 m M Cr(?)for 8 hours,the crystal form and crystallinity of hematite in the binary and ternary complexes did not change,indicating that the long-range ordered structure of hematite minerals is relatively stable in the reduction reaction.XPS analysis shows that the final solid phase products of the Cr(?)reduction process are Cr(OH)3 and Cr2O3 adsorbed on the surface of bacterial cells.Bacteria can also reduce Fe(?)in hematite minerals to Fe(?),thereby chemically reduces Cr(?).EDS analysis shows that the ternary complex of S.oneidensis-hematite-humic acid has a strong reduction effect on Cr(?),which supports the result of the kinetic reaction.EPR analysis shows that Cr(?)reduction is a two-step electron transfer process,in which there is an intermediate valence state during the reduction of Cr(?)by S.oneidensis-hematite-humic acid.(3)Nontronite NAu-2 clay mineral inhibits the microbial reduction of Cr(?).The addition of nontronite increased the negative surface charge of S.oneidensis by 44% and reduced the surface sites by 46%,while humic acid had no significant effect on the surface sites concentration of S.oneidensis.The Cr(?)reduction kinetics experiment shows that since the surface charge of nontronite is negative under all p H conditions,it does not have any Cr(?)removal ability.The microbial Cr(?)reduction by S.oneidensis decreased as the proportion of nontronite increased.The ternary complex of S.oneidensis-nontronite-humic acid has an obvious promotion effect on the reduction of Cr(?).When the ratio of S.oneidensis to nontronite is 2:1,Cr(?)can be completely reduced.According to the pseudo-first-order kinetic model,the Cr(?)reduction efficiencies over the complexes of S.oneidensis-nontronite-humic acid,S.oneidensis,and S.oneidensis-nontronite were 2.90,2.57,and 0.78 Kh-1 respectively.The increase of the initial Cr(?)concentration inhibited the microbial Cr(?)reduction rate.In addition,the ternary complex of S.oneidensis-nontronite-humic acid has an obvious promotion effect on the reduction of Cr(?).When the ratio of S.oneidensis to nontronite is 2:1,Cr(?)can be completely reduced.As the p H value increases,the microbial Cr(?)reduction by S.oneidensis increases.Infrared spectroscopy analysis shows that after the addition of nontronite clay minerals,the composite has new absorption peaks at 1027 cm-1,493 cm-1,and 432 cm-1.These absorption peaks are assigned to pyrophosphate [P2O7]4-groups and the symmetric stretching vibration of Fe O6.XRD analysis showed that after 8 hours of reaction with 1.0 m M Cr(?),the crystallinity,peak position and relative intensity of the binary and ternary complexes did not change,indicating the stability of nontronite clay minerals in the short-term reaction.XPS analysis showed that after Cr(?)reduction,Cr(OH)3 and Cr2O3 accumulated on the surface of bacterial cells.In addition,S.oneidensis can also reduce Fe(?)in nontronite clay minerals to Fe(?),which in turn chemically reduces Cr(?).EDS analysis shows that the ternary complex of S.oneidensis-nontronite-humic acid has a significant promotion effect on the microbial Cr(?)reduction,which confirms the result of the kinetic reaction.Furthermore,EPR analysis demonstrated that an intermediate valence state of Cr(V)appeared during the Cr(?)reduction by S.oneidensis-nontronite-HA.Cr(?),suggesting that the Cr(?)reduction is a two-step electron transfer process.