| Among the many in-situ remediation technologies of Cr(?)-contaminated groundwater,in situ chemical reduction has attracted more and more attention because of its fast reaction and good effect.However,at present,there are some problems such as low utilization rate,poor mobility and easy clogging of the reducing agents,which makes the activity of reducing agent become low,and the remediation effect becomes bad.Therefore,it is of great significance to improve the remediation efficiency of groundwater pollution.As a new type of high efficient reducing agent,carbon dioxide anion radical can be used for the reduction of organic and inorganic compounds,but the research is only limited to the traditional field of water treatment,and there are few reports on the Cr(?)reduction and the remediation of groundwater.Based on the above background,the thesis established an advanced reduction remediation technology based on carbon dioxide anion radical setting Cr(?)in groundwater as the target pollutant,medium sand as the aquifer media,persulfate and formate as the reaction reagents,heat as the activation means.The experiment studied on Cr(?)reducing capacity of thermally activated persulfate/formate system,migration and distribution of Cr(?)in the aquifer,and remediation efficiency of Cr(?)-polluted groundwater by thermally activated sulfate/formate system,to expect to obtain the feasibility and effectiveness of carbon dioxide anion radical for remediation of Cr(?)-polluted groundwater.Specific research results are as follows:(1)An advanced reduction reaction system was established to reduce Cr(?)by thermally activated persulfate/formate system,whose results showed that exceeding 99%of Cr(?)was reduced at 70 ?,30 mM PS,and 30 mM FA,for an initial Cr(?)concentration of 50 mg·L-1;The mechanism of Cr(?)reduction by control experiment and radical identification experiment,namely,thermally activated persulfate to produce sulfate radical,and react with formate to produce carbon dioxide anion radical,then reduce Cr(?)to Cr(III);dynamics model establishment and verifying experiment showed that Cr(?)reduction by carbon dioxide anion radical accorded with the pseudo first order reaction kinetics,and Cr(?)reduction was almost complete when the dosage ratio of sulfate and formate is less than or equal to 1,and low pH was beneficial to Cr(?)reduction,and high temperature could promote the reduction reaction.(2)Migration and distribution of Cr(?)in the aquifer were studied under the condition of homogeneity,stratified heterogeneity and heterogeneity with lens.The results showed that the migration of Cr(?)was mainly affected by gravity,hydrodynamic effect and diffusion effect in the homogeneous aquifer,and the migration rate in each direction was gradually decreasing,and Cr(?)pollution plume has obvious concentration zone;in layered heterogeneous aquifer,the layer with low permeability seriously blocked the migration of Cr(?),which made the migration of Cr(?)in the vertical direction obviously accelerate compared with homogeneous situation;in the heterogeneous aquifers containing lens,the lens with low permeability hindered the migration of Cr(?),which made the migration of Cr(?)in vertical direction slow down compared with homogeneous case;in these three cases,migration distance of Cr(?)with time showed the same rule that the relationship could be fitted to a double BoxLucas model,which could be used to describe the migration difference between high concentration and low concentration in pollution plume.(3)A 2-D sand tank was used to simulate the remediation of Cr(?)-polluted groundwater by thermally activated persulfate/formate system.The results showed that the remediation system could form a 24-cm wide in-situ advanced reduction reaction zone in the area around the heating rod,which had a good effect on the removal of hexavalent chromium(94.43%)and total chromium(75.36%)in the simulated aquifer.In the 16-hour remediation process,Cr(?)pollution plume continuously shrunk,and the change of total chromium pollution plume was relatively slow down because of the influence of adsorption;high temperature area in thermal field by the heating rod directly determined the distribution of effective reaction zone;changes of the persulfate content and formate content in the aquifer were basically the same,and both had a gradually decreasing trend,but affected by low temperature in the bottom of aquifer there were residues in the reaction zone;changes of pH and ORP in the reaction zone were basically the same,and significantly reduced after the reaction,but because of the impact of persulfate residue in the bottom,in the late reaction pH of this region was low,while ORP was high;generally speaking,changes of temperature,persulfate,formate,pH and ORP in the aquifer were coupled with the removal of hexavalent chromium and total chromium in the whole,which showed that the thermally activated persulfate/formate system had ideal effect for Cr(?)-contaminated groundwater. |
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