| Chromium(Cr)is widely used in industries such as leather,textiles,printing and dyeing,and metal electroplating.Chromium has special chemical properties,with valence states ranging from hexavalent to divalent.Among those species,the Cr(Ⅵ)with the highest valence state is carcinogenic and teratogenic,and it is highly soluble in the environment and tends to migrate to other places with water bodies,posing significant risks to environments and human health.Due to the discharge of electroplating wastewater and solid wastes from electroplating factories,a large number of electroplating contaminated sites have emerged.Therefore,suitable methods for remediating the contaminated soils around the electroplating sites are needed.At present,many different remediation technologies have been proposed for heavy metal polluted soils,including:engineering techniques(including soil replacement methods,deep ploughing and tillage methods),physical and chemical methods(including electrothermal methods and leaching methods),solidification/stabilization methods(S/S methods)and bioremediation methods(including phytoremediation methods and microbial remediation methods).Among them,solidification/stabilization technology is widely used in remediation of heavy metal polluted soil due to its low cost,wide applicability,and more stable soil after treatment compared to other remediation methods.Moreover,the selection of solidification/stabilization agents has become the main research topic of many scholars.At present,common solidification/stabilization agents include cement,biochar,clay,etc.However,there are also some problems with the solidification method,such as the presence of pollutants in the soil and the risk of secondary pollution.Therefore,in the practical application of remediating the contaminated soils at electroplating sites,it is necessary to improve the traditional solidification remediation method.Nano-sized zero valent iron(nZVI),as a nanomaterial with a typical core-shell structure,has a great specific surface area and surface energy.However,due to the small size,easy oxidation,and aggregation of nZVI particles,their activity is limited.Therefore,it is necessary to modify nZVI to improve its dispersibility,stability,and retain its reaction activity.The commonly used methods for modifying nZVI include physical methods(including physical stirring and ultrasonic dispersion),chemical methods(including bimetallic modification),and loading nZVI on a proper substrate.Clay minerals are a good support material with a relatively stable structure and low price.The low-cost environment-friendly clay minerals montmorillonite(MT)is used as the substrate to prepare montmorillonite-loaded nano zero valent iron(MT-nZVI),which can achieve the purpose of efficient adsorption and removal of Cr(Ⅵ).Based on this,this study focuses on the chromium contaminated soil at electroplating contaminated sites,and evaluate the solidification/stabilization performance of MT supported nZVI.Batch experiments and different characterization methods are used to reveal the adsorption mechanism of Cr(Ⅵ)on MT supported nZVI and the effects of Cr solidification/stabilization in the soil.A surface complexation model is established to predict the distribution of Cr(Ⅵ)morphology in the soil.Based on the above investigations,the optimal adsorption conditions for Cr(Ⅵ)by MT supported nZVI are explored and applied to contaminated soils collected from a electroplating site.The specific research results are as follows:(1)The adsorption edge experiments were conducted on 10 different soils collected from all over the country,and the adsorption edge curves under different p H and different background ion concentrations were fitted with the CD-MUSIC model.The fitting results are of statistical significance.The results show that the metal oxides(hydroxides,goethite)and organic matter in the soil have greater impacts on the adsorption of Cr,and Cr is mainly adsorbed on the active site of these components,At the same time,the concentration of background ions also has a certain impact on the adsorption of Cr.The model established in this study can theoretically be applied to other soils to evaluate the distribution of Cr on different soil components,and can be used in predicting the migration,transformation,and biological activities of Cr in the environments.(2)The effect of MT-nZVI on the removal of Cr(Ⅵ)in solutions with different solution chemistry conditions was studied.The batch experiment results indicate that MT-nZVI has a strong removal efficiency for Cr(Ⅵ),and the removal mechanism is an adsorption-reduction fixation dual mechanism.The removal ability of MT-nZVI for Cr(Ⅵ)is relatively stable within the range of p H=3-11,and the highest removal rates of 88.75%and72.02%for Cr are reached at p H=5 at concentrations of 10 mg/L and 20 mg/L,respectively.The adsorption capacity of MT-nZVI for Cr gradually decreases with the increase of p H.After adsorbing Cr(Ⅵ),the valence state of Fe and Cr in MT-nZVI changed,with the content of Fe0 decreased from 20.11%to 0 and the content of Fe(III)generated by oxidation increased from 79.90%to 100%.The redox reaction was accompanied by 57.74%Cr(III)production.Within the initial 1 hour of the reaction,the adsorption-reduction fixation of Cr(Ⅵ)on the surface of MT-nZVI occurred quickly,showing a straight upward trend and quickly reaching equilibrium.This indicates that the adsorption sites on the surface of MT-nZVI rapidly reacted with Cr(Ⅵ)and reached saturation in a short period of time.The quasi second-order kinetic model fits well with the adsorption kinetics of Cr(Ⅵ)by MT-nZVI(R~2=0.9663),indicating that the adsorption process is mainly chemical adsorption.The Langmuir adsorption isotherm model(R~2=0.9821)fits well with the adsorption isotherms of Cr(Ⅵ)on MT-nZVI,indicating that the MT-nZVI material has uniform adsorption sites on the surface,and single-molecule layer adsorption occurs on the material surface.(3)Cr contaminated soils from an electroplating site in Jingjiang,Jiangsu Province were collected as the experimental object and Cr(Ⅵ)removal experiments were conducted using MT-nZVI.Experiments have shown that under the synergistic effect of other soil components,the effective solidification and stabilization of Cr can achieve by adding MT-nZVI to the soil.The characterization results show that stable Fe Cr compounds are generated during the reaction.The leaching experiment has shown that electroplating contaminated soil treated with MT-nZVI can maintain no detectable Cr(Ⅵ)or total Cr in the leaching within 7 days. |
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