| In recent years,the problem of heavy metal contaminated soil has become more and more serious,and witch will threatens the soil environmental quality,animal and plant health,and human health.Electrokinetic remediation,as a new soil remediation technology,has the characteristics of simple operation,less time-consuming,and in-situ operation,making it a popular technology for soil heavy metal remediation.However,electrokinetic remediation will be applied on a large scale by more research owing to excessive power loss and electric field distribution choices.At the same time,the influence of electric field on soil organic matter(especially the properties of organic matter for sustainable use of soil)during the electrokinetic remediation also needs further study.In this study,heavy metals Cr,Cd,Pb and Ni are used as the research objects to conduct electrokinetic remediation experiments.Two power supply systems of DC-Electrokinetic remediation(DEKR)and Solar Energy-Electrokinetic remediation(SEKR)with enhancement methods including exchanging electrode and replacing electrolyte regularly are adopted to study the removal and residual distribution of heavy metals.The effects of external electric field on the properties of soil organic matter were studied by measuring the content of soil organic matter and its spectral(UV,3D fluorescence and infrared)characterization during DEKR and SEKR treatment,and investigating the power loss and the impact of different heavy metals on energy consumption competition through the relevant mathematical model.Construct a SEKR project that simulates a remediation unit in an actual contaminated site,and perform engineering verification of the effectiveness of the laboratory test.The main results of this research are as follows:(1)In DEKR,the enhancement methods such as exchanging electrodes and renewing electrolyte bring new current peaks in traditional electric fields and two-dimensional electric fields.Electrolysis water reaction and electromigration lead to the decrease of soil p H and the increase of soil p H from anode to cathode.The removal rates of Cr,Cd,Pb and Ni in traditional electric fields were 13.38%,72.36%,34.76%and56.42%respectively;the removal rates of Cr,Pb and Ni in two-dimensional electric fields were similar to those by left and right electric fields,only the removal rate of Cd decreased by 15%;while the removal rates of Cd,Ni and Pb in vertical electric fields were about 12%higher than those by traditional electric fields.The low removal rate of Cr in DEKR is mainly due to the reduction of Cr(VI)to Cr(III)and the insoluble Cr(III).The highest removal rate of heavy metals in traditional electric field and two-dimensional electric field is Cd,but Pb in vertical electric.(2)In SEKR,the voltage first increases due to the influence of light intensity,and reaches the peak value of about 45V at 11am,then decreases continuously,but maintains below 5V in rainy days.The distribution trend of soil p H was similar to that of DEKR,but decreased by 5.5 units on average with a greater decline.In SEKR,the removal rates of Cr,Cd,Pb and Ni in traditional electric fields were 43.20%,92.97%,54.11%and 90.39%,respectively.The removal rate of Cr in vertical electric field decreased by nearly 30%compared with that by traditional electric fields,but the removal rate of Pb increased by 25%.The overall removal rate of heavy metals in two-dimensional electric field is 15%lower than that in one-dimensional electric field.The dominant heavy metal removal by SEKR did not change.(3)After remediation,the enrichment trend of organic matter was opposite to that of Cd,Ni and Pb,but the same as that of Cr.The difference of organic matter content in SEKR is smaller than that in DEKR,but the difference of organic matter content in SEKR is larger than that in DEKR.The results show that the electric field has little effect on the molecular structure and humification degree of fulvic acid,but the molecular weight of humic acid increases from cathode to anode in one-dimensional electric field of DEKR and SEKR,and decreases at two poles in two-dimensional electric field.The three-dimensional fluorescence of fulvic acid and humic acid were mainly humic peak(A)and fulvic acid peak(C).Meanwhile,the three-dimensional fluorescence characteristic parameters of fulvic acid and humic acid showed that the endogenous components of fulvic acid were higher in SEKR,while humic acid was mainly imported from outside,and the humification degree was higher in SEKR.The infrared spectra of fulvic acid and humic acid are similar.However,under the action of electric field,the O-H stretching vibration absorption peaks of alcohol or carboxylic acid molecules of fulvic acid become stronger and wider,and the stretching vibration of aromatic ring and benzene ring skeleton of humic acid become stronger.(4)In the two power supply modes,the energy consumption generated and used by SEKR is higher than that of DEKR,but the cost of SEKR is lower.The remediation energy costs of traditional electric fields,vertical electric fields and two-dimensional electric fields of DEKR are536.21,110.13 and 369.84$/m~3 respectively.Through the improved mathematical model,it can be concluded that the competitive energy consumption of Ni has been the main advantage in the migration process of the four heavy metals,while the energy consumption of Cr is small due to its low initial concentration and the main form conversion in electrokinetic remediation.(5)The pilot project of SEKR shows that Cd,Ni and Pb are enriched in the cathode region,while Cr is enriched in the anode region.At the same time,compared with the pilot project,total Cr has a higher migration amount in the anode and Cr(VI)has a higher reduction amount.The migration amount of Cd,Ni and Pb decreased slightly compared with the small scale test due to the less electrolyte supply.At the same time,according to the three-dimensional fluorescence characteristic parameters,fulvic acid and humic acid had higher endogenous growth contribution in the pilot test.The results showed that the humification degree of fulvic acid and humic acid was less affected by pilot electric field,and the molecular weight of fulvic acid decreased,while that of humic acid increased in heavy metal enrichment area. |
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