| Dissolved organic matter（DOM）widely exists in ecosystems and is the one of the important factors controlling the biogeochemical behavior of iron oxides and heavy metals.Its molecular structure and properties can directly affect the migration and transformation of heavy metals.Previous studies have carried out many works on the interaction of"DOM-heavy metal"and"DOM-iron oxide".However,There are few reports on the mechanism of migration and transformation of heavy metals in the"DOM-iron oxide-heavy metal"ternary system from these aspects as follows:hydrologic conditions,molecular weight level of DOM,Fe（II）content and the effects of microorganism.Therefore,in this paper,the microbial effect mechanism of DOM in regulating the bioavailability of heavy metals under different hydrologic conditions was investigated via outdoor simulation experiments.The ultrafiltration technology was used for molecular weight fractionation of DOM to study the effect of DOM on iron oxide adsorption properties and electron transfer capacity（ETC）at the molecular level.Besides,the DOM model humic acid（HA）with high aromatic degree and large molecular weight was selected as the research object,and the coprecipitation of ferrihydrite（Fh）-HA was synthesized in the laboratory.The effects of Fe（II）-abiotic catalyted Fh transformation process on the environmental behavior of adsorbed copper（Cu）and the kinetics of carbon（C）release and redistribution in Fh-HA coprecipitation were investigated.Besides,the ETC of HA in the Fh-HA-chromium（Cr）co-precipitation system was analyzed by electrochemical method,and the microscopic mechanism of Cr migration and transformation in the co-precipitation system in the presence of microorganisms was revealed by XPS and TEM characterization.The main results of this paper were as follows.（1）The results of outdoor pot experiment demonstrated that the p H increased from 5.5 to 6.4 while Eh decreased from 95.8 to 31.6 m V to maximum extent after applying HA under flooded condition.Although planting C.cinerascens improved the soil quality under the stress of Cu and Pb,the use of HA restrained this effect except for Pb under wetted condition.However,the activities of catalase（CAT）and urease（URE）in the soil were significantly enhanced under the influence of DOM.Under the wet and flooded conditions,the CAT activity increased 2.60-fold and 2.39-fold,respectively,and the URE activity increased 68.63%and 59.69%,respectively with the addition of HA at the end of the incubation.The increasement was greater in wet conditions compared to flooded conditions.A similar phenomenon was found in the results for chlorophyll.In addition,microbial diversity and community could be improved with the addition of HA.According to the results of RDA analysis,soil available Cu（DTPA-Cu）,p H and flooding conditions may significantly affect the microbial composition and structure in Cu-Pb co-contaminated soil.（2）The experiment of the effect of DOM on the adsorption affinity and electron transfer ability of iron oxides illustrated that the high molecular weight DOM（H-DOM）with high aromaticity and rich in oxygen showed stronger adsorption affinity for Fe minerals,especially ferrihydrite with poor crystallinity,while this affinity would be weakened as the decrease of molecular weight of DOM.The X-ray photoelectron spectra（XPS）and Fourier transformation infrared spectra（FTIR）revealed that Fe minerals preferentially adsorbed onto the C-O bonds,O-C=O bonds and oxygen-rich functional groups（e.g.phenolic hydroxyls and aromatic ketones）in DOM.In reduction experiments,H-DOM markedly enhanced the effect of microorganism-mediated Fe reduction.In addition,the ETC of DOM largely depends on the molecular weight,wherein the ETC of H-DOM in iron reduction is 27.98μmol（g C）-1,which is 3.4 times than that of L-DOM.Moreover,the efficiency of DOM for microbial iron reduction was determined by the ETC of DOM.（3）The transformation experiments showed that under the catalysis of Fe（II）,Fh would be transformed into more stable hematite and goethite,and the addition of HA inhibited the transformation of Fh,which inhibited more with the increase of OC/Fe in Fh-HA composites.Under acidic conditions,the adsorbed Cu on the mineral surface could be re-released into the solution during the Fe（II）-catalyzed transformation of Fh.The specific surface area（SSA）of Fh-HA co-precipitation is lower than that of pure Fh,resulting in more unstable Cu released.In the presence of2.0 mmol/L Fe（II）,the aqueous Cu concentrations in the Fh and Fh-HA systems increased from initially 11.02-14.06 mg/L to 15.09-17.65 mg/L with an increase of25.5%-36.9%.In addition,the organic carbon in the Fh-HA coprecipitation occurred"release-resorption"kinetic process during the transformation.Based on the3D-EEMs-PARAFAC analysis,the released organics were mainly humic acid-like substances with low molecular weight,closely relating to the biological activity in the environment.The adsorbed Cu on the mineral surface mainly exists in the form of Fe-O-Cu after the conversion experiment via XPS.（4）Shewanella putrefaciens-mediated culture experiments of Fh-Cr and Fh-HA-Cr co-precipitation systems revealed that low p H,sterile conditions and pure Fh showed more promoted effect on Cr adsoption in the co-precipitation system,while the neutral conditions and high content of HA achieved detoxification of Cr mainly via microorganism-mediated redox reactions in the Fh-HA co-precipitation system.The mobility of Cr in the coprecipitation system is the lowest with the combination of high concentration of Fe（II）,high OC/Fe ratio and neutral environment.The CA results indicated that exogenous HA served as an electron shuttle to facilitate electron transfer on the surface of Fh-Cr and Fh-HA-Cr minerals under acidic conditions,with the ETCs of 22.12±1.86 and 15.81±2.68μmol（g C）-1,respectively.In addition,the sterile environment and Fe（II）ions facilitated the transformation of Fh to more stable secondary minerals（maghemite and hematite）during the incubation process.Besides,the co-precipitation of Fh-HA-Cr formed nanoparticles with large aggregates after incubation under sterile conditions,however,Cr might not be incoprated into the structure of minerals,thus causing more Cr resolved.In contrast,the co-precipitation formed loose small particles wth Cr incoprated in the presence of S.putrefaciens.Therefore,the Cr in the Fh-HA-Cr co-precipitation system with high OC/Fe was hard to be resolved via S.putrefaciens induced the reduction of Cr（Ⅵ）.In summary,based on the macroscopic basis of outdoor culture experiments,this paper clarifies the regulation mechanism of DOM on heavy metals and microorganisms.The DOM was classified to investigate the adsorption properties and electron transfer mechanism of DOM on iron oxides at the molecular weight level.Meanwhile,a ternary system with DOM,Fh and heavy metals was constructed to study the mechanism of Cu migration and transformation and the fluorescence characteristics and kinetic process of C release during Fe（Ⅱ）catalyzed the abiotic transformation process of ferrihydrite in Fh-HA-Cu co-precipitation system.In addition,the pathway and microscopic mechanism of Cr removal in Fh-HA-Cr co-precipitation system mediated by dissimilatory reducing bacteria（S.putrefaciens）were clarified.The results of this paper are helpful to understand the biogeochemical process of heavy metals in wetland soil and provide a theoretical basis for solidification/stabilization and phytoremediation technology to control heavy metal pollution in wetland soil. |
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