| Soil is the natural environment and material foundation for the survival and development of mankind.Currently with the development of industrial and agricultural meanwhile soil pollution has the characteristics of latent and cumulative,current soil pollution situation is more serious and cause widespread concern.The soil pollution caused by Cr(VI)which has strong carcinogenic teratogenic can not be ignored.Therefore,the effective remediation method of Cr(VI)contaminated soil become the hotspot.At present,the remediation methods of Cr(VI)contaminated soil mainly include microbial remediation method,leaching method,electric repair method,curing stability method and reduction method.Chemical reduction method has many characteristics,such as low cost,can be large-scale application andcan be carried out in situ repair,so this test chose the chemical reduction method as soil remediation method.The reduction and repair material selected in this study is nano-materials nanometer zero-valent iron(nZVI)with excellent reducing properties.Although nZVI has the advantage of reduction,its shortcomings of easy agglomeration and oxidation will restrict the exertion of its characteristics.This article selected non-toxic harmless environment-friendly materials sodium carboxymethyl cellulose(CMC)as the stabilizer.CMC-nZVI was made by liquid phase reduction method as a reduction repair agent for Cr(VI)contaminated soil.CMC is rich in sources,non-toxic,easy to degrade and at present many scholars has improved their sources of raw materials to further reduce its cost,the costs of soil repair agent can reduce and more conducive to its promotion and application.In order to prove the stability effect of CMC to n ZVI,the dispersion stability test and the absorption spectra of CMC-nZVI and bare-nZVI were carried out respectively,and TEM and SEM were used for characterization.This test was divided into static test and dynamic soil column test.Static test process was as follows the contaminated soil and CMC-nZVI repair agent was mixed as the ratio of 1g: 5ml in the centrifuge tube,shocked for a certain time and then centrifugated,filtered and determinated absorbance.The dynamic experiment is carried out in three modes,such as "Mode one"(simple continuous flow),"Mode two"(intermittent flow)and "Mode three"(circulating flow).In this paper,homemade simulated Cr(VI)contaminated soils were used as test soil samples at a concentration of 102 mg Cr(VI)/ kg soil.The XRD characterization of the tested soil samples showed that the chromium supported on the sand and clay was mainly in the form of Cr2O3.The static test and dynamic soil column test were carried out with self-made CMC-nZVI as the repairing agent for Cr(VI)contaminated soil.The effects of different experimental conditions on soil remediation were studied and the reaction mechanism was analyzed by reaction kinetics.The conclusions of the study was obtained as follows:(1)CMC-nZVI colloid suspension prepared by chemical reduction method with CMC as stabilizer was used as repair agent for Cr(Ⅵ)contaminated soil.Through the dispersion stability test and the TEM and SEM of CMC-nZVI,it is proved that CMC can effectively stabilize the effect of nano-iron particles on its agglomeration.And only the addition of CMC the size of nZVI particles can below 100 nm,play the role of nano-materials.By observing the absorption spectra of CMC-n ZVI and bare-nZVI,it can be seen that the absorbance value of CMC-nZVI is about 13 times higher than that of bare-n ZVI,so it can be proved that CMC can stabilize nZVI.(2)The results of static test show that: The higher the proportion of clay in the soil component,the more detrimental to the removal of Cr(Ⅵ)in the soil.Meanwhile CMC dosage,CMC-n ZVI dosage and pH value will have an effect on Cr(Ⅵ)removal effect.The removal rate of Cr(Ⅵ)increased with the increase of CMC and CMC-nZVI dosage,but excessive CMC-nZVI dosing will destroy the soil structure;acidic environment is more conducive to the removal of Cr(Ⅵ).The optimum reaction conditions were as follows: the molar ratio of CMC to Fe0 was 0.0186,the dosage of CMC-nZVI was 1 g / L,and the pH value was 6.0.Under these conditions,the Cr(Ⅵ)concentration of contaminated soil was 102 mg Cr(Ⅵ)/ kg soil,the removal rate of Cr(Ⅵ)of sand and clay contaminated soil was 91.34% and 85.91%,respectively.(3)The dynamic soil column test results of the "Model one" composite flow system show that: CMC-nZVI dosage,glucose addition,attapulgite filler use,CMC-nZVI pump flow rate will have an impact on the removal of Cr(Ⅵ).Increasing the concentration of CMC-n ZVI in a certain range can increase the Cr(Ⅵ)removal rate,the addition of glucose to soil is not conducive to the repair of Cr(Ⅵ)in soil by zero-valent iron,Cr(Ⅵ)removal rate increases first and then decreases with the increase of flow rate and the addition of attapulgite clay can increase the removal rate of Cr(Ⅵ).The optimum conditions were as follows: CMC-nZVI dosage is 1g / L,glucose addition amount is 0g / kg soil,CMC-n ZVI pump flow rate is 5ml / min and applied attapulgite filler.Under these conditions,the soil concentration of contaminated sand soil was 102 mg Cr(Ⅵ)/ kg,and the removal rate of Cr(Ⅵ)was 98.75%.(4)Under the best experimental conditions of dynamic test,the "Model two" dynamic soil column test was carried out.The results showed that the removal efficiency of Cr(Ⅵ)fluctuates with the increase of intermittent residence time.(5)Under the best experimental conditions of dynamic test,the "Model three" dynamic soil column test was carried out.The results showed that the initial removal rate of Cr(Ⅵ)was the fastest and the initial removal rate was 42.68% and the final removal rate was 98.89%.The removal rate varied from large to small at 2h,1h,6h and 8h,and the final removal rate was better under different cycle time conditions.The results of the "Model three" and "Mode one" dynamic test results are compared.From 0 min to 12 min the Cr(Ⅵ)removal rate of "Mode three" was larger,the initial removal rate was 42.68%,and the removal rate increased more rapidly.The removal rate of Cr(Ⅵ)was almost the same during 12 min ~ 60 min,and both had a large Cr(Ⅵ)removal rate of about 98%.(6)The repair of soil Cr(Ⅵ)pollution by CMC-nZVI can be fitted to the pseudo-first order reaction kinetics model.The trend of kobs with the reaction conditions is as followed,(a)There is a good linear relationship between the kobs value and the addition of CMC-n ZVI,and the kobs value is proportional to the dosage of CMC-nZVI.(b)There is a good linear relationship between kobs and glucose addition,and the kobs value is inversely proportional to the amount of glucose added(R2=0.9211).(c)There is a good linear relationship between kobs and velocity,and kobs is proportional to the velocity(R2=0.9447).(d)The kobs value of the attapulgite filler is greater than the value in the ordinary gravel test. |
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