Study On The Remediation Of Cadmium Polluted River Sediment By Dispersed Nanoscale Zero-valent Iron And Its Behavior Mechanism

With the rapid development of urbanization and industrialization,the river is more and more disturbed by human activities,and heavy metal pollution is increasingly serious.Heavy metals can be deposited in the bottom mud through adsorption,complexation and sedimentation.When the external environment of the sediment changes,the heavy metals in the sediment are easy to re release to the overlying water,which is easy to cause secondary pollution and poses a great threat to the ecological environment and human health.Therefore,to find a new technology and method for the remediation of heavy metal contaminated sediment has always been a concern in the field of water environment.With the development of environmental nanotechnology,the use of iron nano materials as stable reagents to solve the problem of heavy metal pollution has become a research hotspot.Among them,the nanoscale zero-valent iron(NZVI)has the characteristics of large specific surface area,high reduction activity,fast reaction speed and strong adsorption.It has shown superior performance in the treatment of a series of environmental problems such as heavy metal and organic pollution.However,NZVI is easy to be oxidized in the environment due to its strong reducibility,and due to its own magnetic attraction and van der Waals force between particles,NZVI particles are easy to agglomerate,thus reducing the reaction activity of NZVI and its mobility in the sediment,which is not conducive to the remediation of environmental pollution.Based on this,three modifiers,sodium alginate(SA),rhamnolipid(RL)and graphene oxide(GO),were used to prepare the dispersed NZVI for improving the reaction activity and mobility of NZVI.And taking the sediment polluted by cadmium(Cd)in Changsha section of Xiangjiang River in Hunan Province as the research object,the stable effect and behavior mechanism of the dispersed NZVI materials on the heavy metal Cd in the sediment were studied in order to realize the effective treatment of heavy metal contaminated sediment and the improvement of river ecological environment.The specific research work and creative achievements are as follows.The first part is the synthesis of the dispersed NZVI and its stability research on the Cd polluted river sediment.Sodium alginate modified nanoscale zero-valent iron(SNZVI),rhamnolipid modified nanoscale zero-valent iron(RNZVI)and graphene oxide modified nanoscale zero-valent iron(GNZVI)were successfully prepared and characterized.The prepared dispersed NZVI and unmodified NZVI were applied to repair the river sediment polluted by Cd.In order to evaluate the stabilization effect,the chemical form of Cd,the activity of microbial enzyme and bacterial community structure were measured.The results showed that both the dispersed NZVI and NZVI could effectively transform the unstable state of Cd into the stable state,and the maximum residual percentage of Cd increased by 64.82%,which effectively reduces the mobility of Cd,and its stability performance was GNZVI>RNZVI>SNZVI>NZVI.Through the analysis of microbial enzyme activity,PCR-DGGE and high-throughput sequencing,it was found that the addition of dispersed NZVI significantly activated the sediment enzyme activity,changed the bacterial community structure,increased the diversity of bacterial community and the relative abundance of iron reducing bacteria,and improved the bioavailability of organic matter.This part of research can be concluded that the mechanism of Cd stabilization includes the adsorption and complexation of Cd by the material itself,as well as the increase of the relative abundance of iron reducing bacteria.Thus,the iron oxide bound Cd can be redistributed to the more stable secondary iron minerals by reducing Fe(III)minerals,further reducing the mobility of Cd.The second part is the research on the remediation effect of dispersed NZVI on the Cd polluted river sediment.On the basis of determining the behavior of heavy metal transformation,the leaching toxicity(TCLP)and bioavailability(PBET)of Cd in remediation sediment were investigated to explore the potential release capacity and environmental toxicity of heavy metals to the environment.The results indicated that the addition of three kinds of dispersed NZVI could reduce the concentration of TCLP leaching and PBET extractable concentration of Cd,the immobilization efficiency of Cd was 96.33%and 70.21%respectively,which effectively reduces the solubility and bioavailability of Cd.In addition,the addition of materials will inevitably lead to the change of physical and chemical properties of the sediment.Through the analysis of the changes of pH and E_hin the sediment during the repair process,the mechanism of the effect of different types of dispersed NZVI on the behavior of Cd in sediment was explored.Compared with the control group,the addition of materials increased the pH value of the sediment and decreased the E_hvalue of the sediment.The change of these two factors was conducive to the stability of Cd.The above analysis showed that the stability of the material to Cd includes not only the material itself and the increase of the relative abundance of iron reducing bacteria,but also the change of the pH and E_hvalue of the sediment.The third part is the stability of the dispersed NZVI under different pH conditions to Cd and the risk of iron dissolution.In the process of remediation of heavy metal contaminated sediment,it is not only necessary to investigate whether the leaching toxic concentration of heavy metals in the stabilized sediment meets the standard,but also to consider the change of environmental conditions after remediation,such as the impact of the change of pH value of sediment caused by external environmental factors such as salt alkalization and acid rain leaching on the long-term stable Cd of materials in the sediment,as well as the possible risk of Fe dissolution during remediation,so as to provide risk assessment for the application of dispersed NZVI in remediation of heavy metal contaminated sediment.It was found that the material was beneficial to the stability of Cd under neutral and alkaline conditions,and the leaching concentration of Cd was relatively low in the range of pH7-11.When the pH was 7,the leaching concentration of Cd in GNZVI treatment group was 0.01 mg/L,which had basically reached the level III standard of groundwater quality in China.In this study,the concentration of Fe in the overlying water was lower than the limit value,and the concentration of Fe in the overlying water of each treatment group increases first and then decreases during the whole treatment cycle.The change of Fe concentration in overlying water indicated that the formation of secondary minerals was related to Cd fixation.The fourth part is the research on the behavior mechanism of the dispersed NZVI in the remediation of Cd contaminated sediment.The adsorption isotherm model and adsorption thermodynamics model were constructed,and the structure and element analysis of the repaired materials were carried out to explore the behavior mechanism of the dispersed NZVI in the remediation of Cd contaminated sediment.The results showed that Freundlich linear model could better reflect the characteristics of Cd adsorption.Freundlich model was suitable for describing heterogeneous adsorption sites.The fitting results were consistent with the fact that there exists some iron oxides on the surface of NZVI and dispersed NZVI,which leads to the inhomogeneity of their surface properties.According to K_Fvalue in Freundlich model,the adsorption capacity was GNZVI>RNZVI>SNZVI>NZVI,which indicated that GNZVI had the best adsorption performance for Cd and more adsorption sites.The thermodynamic results showed that the adsorption process of Cd on the material was spontaneous and thermodynamically feasible,and the adsorption process belonged to chemical adsorption.The characterization results of the repaired materials showed that the X-ray diffraction peaks in the original structure of the repaired materials had disappeared,and some new secondary iron minerals were generated,mainly including hematite(Fe₂O₃),magnetite(?-Fe₂O₃),fibro iron(?-Fe OOH)and goethite(?-Fe OOH).The infrared spectrum indicated that these secondary iron minerals had adsorption or coprecipitation reaction with Cd²⁺.The purpose of this paper is to promote the development of remediation technology and theory of heavy metal pollution in sediment,so as to lay a theoretical foundation for the successful application of dispersed NZVI in the remediation of heavy metal polluted river sediment.