Shewanella Oneidensis MR-1 For Enhanced Cr(Ⅵ) Removal From Wastewater By Modified Nanoscale Zero-Valent Iron

Chromium（Cr）and its compounds are important chemical raw materials,which is widely used in metal processing,electroplating,printing and dyeing,and pharmaceutical industries.Cr is released into lakes,rivers and groundwater due to leakage or unreasonable discharge from the production processes,which can cause serious damange to the ecological environment and human health.Cr predominantly exists as Cr（Ⅵ）and Cr（Ⅲ）in water.Cr（Ⅵ）is highly toxic,carcinogenic and teratogenic,while Cr（Ⅲ）is less toxic,and recognized as an essential trace element for humans and animals.Therefore,one of the most common and effective approaches to remove Cr（Ⅵ）pollution in water is to reduce it to Cr（Ⅲ）.Nanoscale zero-valent iron（nZVI）has been considered as a promising Cr（Ⅵ）pollution remediation material due to its large specific surface area and strong reducibility.However,nZVI is prone to agglomeration and surface passivation in the actual application processes,which significantly reduces its reactivity,thus reducing the treatment effect of Cr（Ⅵ）and limiting the large-scale application of nZVI remediation technology.Therefore,developing effective methods to solve the agglomeration and surface passivation problems of nZVI is the key to maintain the reactivity of nZVI.In this dissertation,pine cone biochar（PBC）was used as the carrier material,and fulvic acid（FA）as a surface modifier to modify nZVI,respectively,and two kinds of modified nZVI composites were prepared to solve the agglomeration of nZVI.At the same time,Shewanella oneidensis MR-1（MR-1）could use the iron（hydr）oxides in the surface passivation layer of nZVI as an electron acceptor to reduce it to soluble Fe（Ⅱ）,thereby solving the surface passivation of nZVI in the reaction process.Finally,this method can achieve highly efficient and rapid removal of Cr（Ⅵ）in wastewater,and the Cr（Ⅵ）removal mechanism was also explored.The main findings are as follows:1.Preparation of modified nZVI compositesnZVI-PBC and nZVI-FA were prepared by the liquid-phase reduction with PBC as the carrier,and FA as the surface modifier,respectively.For the nZVI composites,the nZVI before and after modification were characterized by SEM-EDS,BET,XRD and FT-IR.The results showed that both PBC and FA could prevent the agglomeration of nZVI particles.Compared with the original nZVI,the two modified composites have better dispersion,larger specific surface area and abundant functional groups on the surface.2.Study on the Cr（Ⅵ）removal performance of two synergistic systemsThe effects of different systems,the dosage of modified nZVI composites,the concentration of MR-1,the initial p H of the solution and the coexisting ions on the Cr（Ⅵ）removal efficiency were discussed.The results showed that the two synergistic systems could effectively solve the agglomeration and surface passivation problems of nZVI,thereby exhibiting excellent Cr（Ⅵ）removal ability.For the nZVI-PBC/MR-1synergistic system,Cr（Ⅵ）can be completely removed within 36 h with the initial Cr（Ⅵ）concentration of 100 mg/L,nZVI-PBC dosage of 0.6 g/L,OD600 of 0.3 and the initial p H of the solution of 6.5.100 mg/L of coexisting metal ions(Na+,Mg²⁺,Mn²⁺,Pb²⁺,Cd²⁺,Cu²⁺ and Zn²⁺)and anions（NO₃-,Cl^-,SO₄2-,PO₄2-and CO₃2-）exerted negligible effects on Cr（Ⅵ）removal.However,when their concentrations increased to 200 mg/L,all these metal ions(Na⁺,Mg²⁺,Mn²⁺,Pb²⁺,Cd²⁺,Cu²⁺ and Zn²⁺)and anions(NO_3-,Cl^-,SO₄^2-,PO₄^2- and CO₃^2-)exhibited different degrees of inhibition on Cr（Ⅵ）removal.For the nZVI-FA/MR-1 synergistic system,Cr（Ⅵ）can be completely removed within 24 h with the initial Cr（Ⅵ）concentration of 100 mg/L,nZVI-FA dosage of 0.8g/L,OD600 of 0.3,and the initial p H of the solution of 6.5.100 mg/L of coexisting metal ions(Na+,Mg²⁺,Mn²⁺,Pb²⁺,Cd²⁺,Cu²⁺ and Zn²⁺)and anions(NO_<sup>2-,Cl^-,SO₄^2-,PO₄^2-and CO₃^2-)had negligible effect on Cr（Ⅵ）removal.However,when their concentrations increased to 200 mg/L,all coexisting metal ions(Mg²⁺,Mn²⁺,Pb²⁺,Cd²⁺,Cu²⁺ and Zn²⁺)and anions(NO_3-,Cl^-,SO₄^2-,PO₄^2- and CO₃^2-)exhibited different degrees of inhibition on Cr（Ⅵ）removal,except for Na+ had negligible effect on Cr（Ⅵ）removal.3.Study on Cr（Ⅵ）removal mechanism of the two synergistic systemsThe Cr（Ⅵ）removal mechanism of the two synergistic systems were revealed by SEM-EDS,XRD,XPS,FT-IR,3D-EEM and XANES.The removal of Cr（Ⅵ）by these two synergistic systems mainly includes reduction,adsorption and complexation:（1）The modified nZVI composites can reduce and adsorb a certain amount of Cr（Ⅵ）;（2）The extracellular secretion of MR-1 can adsorb,reduce and complex a small amount of Cr（Ⅵ）;（3）MR-1 can promote the dissolution of the Fe（Ⅲ）（hydr）oxides passivation layer on the surface of nZVI-PBC and extend the reactivity of nZVI-PBC,promoting the reduction of Cr（Ⅵ）by nZVI.Meanwhile,the adsorbed Fe（Ⅱ）can be used as a strong reducing agent to reduce Cr（Ⅵ）,thus enhancing the removal efficiency of Cr（Ⅵ）.In summary,in these two synergistic systems,most of Cr（Ⅵ）was reduced to Cr（Ⅲ）,which was removed from solution in the form of precipitates such as Cr₂O₃,FeCr₂O₄,and Cr_xFe_1-x（OH）₃;a small amount of Cr（Ⅵ）were removal from solution by adsorption or complexation.Finally,the method can achieve highly efficient and rapid removal of Cr（Ⅵ）in wastewater...