Study On Remediation Of Chromium Contaminated Soil By Bentonite-Supported Nanoscale Zero-Valent Iron

With the rise of industries containing chromium pollution such as metallurgy,tanning and electroplating,the discharge of chromium（Cr）-containing wastewater and the stacking of chromium slag can bring pollution to the surrounding environment and soil.Cr enters the human body and can cause respiratory diseases,digestive tract diseases and even cancer.Chromium in soil mainly exists in the form of Cr（Ⅵ）and Cr（Ⅲ）.Compared with Cr（Ⅵ）,Cr（Ⅲ）has less toxicity and lower mobility in soil,so it is a feasible method to passivate Cr by transforming Cr（Ⅵ）into Cr（Ⅲ）in soil.Nanoscale zero-valent iron（nZVI）has the characteristics of large specific surface area,small particle size and high reducibility,and can be used for in-situ remediation of Cr-contaminated soil.However,due to its large reactivity and surface electrostatic force,nZVI is prone to oxidation and agglomeration.Bentonite has the characteristics of adsorption,suspension and dispersion,and is nontoxic,cheap,abundant in reserves and environmentally friendly.In this paper,bentonite was used to supported nanoscale zero-valent iron（nZVI@Bent）and the best preparation conditions were determined through the analysis of passivation effect,dispersibility and suspension.By controlling different reaction conditions and characterizing the surface changes after nZVI@Bent passivated Cr（Ⅵ）in soil suspension,the passivation characteristics and mechanism of nZVI@Bent on Cr（Ⅵ）in soil suspension were determined.The passivation ability of nZVI@Bent on Cr in soil and the recovery of soil function after remediation were determined by analyzing the effect of in-situ remediation and the impact on the growth of pakchoi,to determine whether it is feasible or not.The main research results are as follows:1.Synthesis of bentonite-supported nanoscale zero-valent ironWhen the ratio of Fe to bentonite in the bentonite-supported nanoscale zero-valent iron was 1:1,the passivation rate of Cr（Ⅵ）in the soil suspension was 95.33%.At t=60min,the absorbance of nZVI@Bent（nZVI:Bent=1:1）at 508nm was 66.93%lower than that at t=0,which was lower than nZVI,nZVI@Bent（nZVI:Bent=1:0.5,nZVI:Bent=1:2 and nZVI:Bent=1:3）.The nZVI concentration of nZVI@Bent（nZVI:Bent=1:1）crossing silica sand was 9.2 times of that of bare nZVI when the added water was 5PV.Therefore,bentonite can load nZVI on its own surface to effectively prevent nZVI from agglomerating.Bentonite-supported nanoscale zero-valent iron can enhance the dispersibility and agglomeration of nZVI.In summary,the best ratio of Fe to bentonite for nZVI@Bent is 1:1.2.Influencing factors and mechanism of removal of Cr（Ⅵ）from soil suspended liquid by bentonite-supported nanoscale zero-valent ironWhen the dosage is 2-6 g·L^-1 and the temperature is 15-35℃,the passivation rate of nZVI@Bent to Cr（Ⅵ）in soil suspension increases with the increase of dosage and temperature.When the environmental p H is in the range of 3-11,the passivation rate of nZVI@Bent to Cr（Ⅵ）in soil suspension decreases with the increase of p H.The reaction temperature has the greatest effect on the ability of nZVI@Bent particles to passivate Cr（Ⅵ）in the soil suspension.When the temperature rises from 15℃to 35℃,the removal rate of Cr（Ⅵ）in soil suspension increases from 68.88%to 99.95%.Scanning by SEM,it was found that the granular structure on the surface of nZVI@Bent changed into a flaky structure after the reaction with Cr（Ⅵ）in the soil suspension.XRD and XPS scanning showed that Cr（Ⅵ）in nZVI@Bent passivated soil suspension was divided into two processes:reduction and adsorption,and the reduction process was greater than adsorption process.The Fe⁰ in nZVI@Bent will react with Cr（Ⅵ）to form(Cr_xFe_1-x)（OH）₃and Cr_xFe_1-xOOH precipitation to reduce the mobility of Cr.3.In-situ remediation of contaminated soil and effect on the growth of pakchoi by bentonite-supported nanoscale zero-valent ironIn the in-situ remediation of Cr contaminated soil by bentonite,nZVI and nZVI@Bent,the passivation rates of nZVI@Bent on Cr_total and Cr（Ⅵ）in soil were significantly higher than those of bentonite and nZVI,which were 84.16%and 95.00%respectively.nZVI@Bent could reduce the content of exchangeable Cr in soil and increase the content of Fe-Mn oxidized Cr.The content of Cr in pakchoi grown in Cr-contaminated soil after nZVI@Bent remediation was lower than that in the above-ground and underground parts of the unrepaired soil,which were 79.16 mg·kg-1 and 228.73 mg·kg-1,respectively.The Fe content of the ground and underground parts of pakchoi grown in uncontaminated soil were 1905.80mg·kg^-1 and 6600.26 mg·kg^-1,respectively.The Fe content of pakchoi grown in Cr-contaminated soil after nZVI@Bent remediation was not significantly different from that of pakchoi grown in uncontaminated soil.The SOD and POD activities of pakchoi grown in Cr-contaminated soil after nZVI@Bent remediation are not significantly different from those of pakchoi grown in uncontaminated soil.The root and stem lengths of pakchoi grown in Cr-contaminated soil remediated by nZVI@Bent were 5.44 times,2.94 times and 2.03 times as long as those grown in un remediated soil,Cr-contaminated soil remediated by bentonite,or in Cr-contaminated soil remediated by nZVI.The dry biomass of pakchoi grown in Cr-contaminated soil remediated by nZVI@Bent was 0.2679g.The root length,stem length and dry weight of pakchoi grown in Cr-contaminated soil remediated by nZVI@Bent were not significantly different from those grown in non-contaminated soil.