Study On The Interaction Characteristics Of “DOM-iron Reducing Bacteria-Fe(Ⅲ)” And Its Reduction Of Cr(Ⅵ)

As an important part of geochemical cycle,microbial dissimilatory iron reduction can not only affect the valence of iron,but also can effectively degrade heavy metals.Dissolved organic matter is an active organic component in nature,which contains hydroxyl,quinone and other active groups.Its redox properties can potentially promote the migration and transformation of heavy metals through electron transfer.The treatment of Cr（VI）by iron-reducing bacteria is a new research direction in the field of chromium remediation.In addition to its own reduction ability of Cr（VI）,the Fe（II）produced by iron-reducing bacteria is also a good reducing agent of Cr（VI）.In this study,a strain of iron-reducing bacterium PY16isolated from Poyang Lake wetland were identified.Moreover,growth conditions and carbon source utilization capacity by the PY16 were analyzed.In addition,the interaction characteristics of"DOM-iron reducing bacteria-Fe（III）"system and its reduction and transformation of Cr（VI）were investigated,through batch anaerobic culture experiment,combined with UV-Vis,EEMs,XRD,SEM-EDS and XPS characterization technology.We obtain the following results:（1）A gram-negative iron-reducing strain PY16 was isolated from Poyang Lake sediments through enrichment culture.It was identified as Enterobacter sp.PY16 by PCR-16S r DNA sequence analysis.Strain PY16 was easier to grow at 35℃and p H7.0.Glucose and sucrose were the dominant carbon sources of strain PY16.（2）DOM could promote dissimilatory iron reduction by the PY16.DOM from different sources（carex,sediment,rhizosphere soil）had no significant difference in promoting Fe（III）reduction.However,different concentrations of DOM had slightly different promoting effects on Fe（III）reduction,showing that the amount of Fe（II）generation increases with the increase of DOM concentration.The content of Fe（II）increased obviously with the increase of iron content when the addition level of ferrihydrite was 0～1000 mg·L^-1.According to the EEMs,DOM from the three sources were mainly composed of humic acid,fulvic acid and protein-like substances（tryptophan and tyrosine）.Small molecules in DOM could provide nutrients for strain PY16,while large molecules（humic acid and fulic acid）could act as electron shuttle substances to promote dissimilatory iron reduction process.XRD analysis of the ferrihydrite before and after the reaction showed that no new crystalline minerals were formed.（3）Strain PY16 could directly reduce Cr（VI）.However,when Cr（VI）concentration exceeded 10 mg·L^-1,the reduction reaction was inhibited.In the presence of ferrihydrite,namely the occurrence of microbial dissimilatory iron reduction reaction,alleviated the toxicity of Cr（VI）to a certain extent and promoted the reduction of Cr（VI）by strain PY16.After 7 days of culture,the reduction rate of Cr（VI）increased by 2.78%to 42.99%.In addition,15 mg·L^-1Cr（VI）in the system was almost completely degraded within 72 hours with the addition of DOM.DOM and Fe（II）generated in the reduction process of dissimilatory iron reduction could be used as electron shuttles to reduce Cr（VI）to Cr（III）through chemical reaction,and their synergistic effect had stronger redox and electron transfer capacity.SEM-EDS and XPS characterization confirmed the presence of Cr（VI）and Cr（III）on the surface of thallus and ferrihydrite,and the reduction reaction of Cr（VI）and Fe（III）took place.It also indicated that Cr₂O₃and Cr（OH）₃precipitates were formed in the reduction process of Cr（Ⅵ）.