| Heavy metal Cr is widely used in the production of chromium salt,leather printing and dyeing while the chromium-containing waste gas,waste water and waste residue generated during this processing will pollute the soil.The treatment process for Cr pollution to the soil is a typical area of environmental restoration.The toxicity of Cr is very different which can be attributed to the different morphology,and in particular Cr（Ⅵ）)has carcinogenic and teratogenic effects on humans,which seriously endangers human health and public safety,while low dose of Cr（Ⅲ）is non-toxic.It is also an essential micronutrient in human diet.How to effectively remove Cr（Ⅵ）)or transform into Cr（Ⅲ）to reduce toxicity,and how to remove Cr（Ⅵ）)and Cr（Ⅲ）from soil are the key issues to be considered in soil restoration.Compared with other chemical methods,reduction method is one of the important way used to repair heavy metal contaminated soil.Given the strong reducibility and good adsorption capacity of nano zero-valent iron（nZⅥ）,it can effectively reduce Cr（Ⅵ）)to Cr（Ⅲ）concentrations by transporting it to the target location of restoration,which is a promising technology for repairing soil.However,there are many factors affecting influencing nZⅥ to repair Cr contaminated soil,while the existing research environment is far from the actual site,and the action form is single.It is necessary to carry out relevant research to solve the above problems and provide scientific help for the technology to enter the stage of engineering as soon as possible.Based on the stabilization of nZⅥ,the influence of removal nZⅥ on Cr（Ⅵ）)and related mechanism analysis are major studied by using the following three systems such as static soil sample system,dynamic small soil column system and simulated field system（dynamic large soil column）.In the experiment,the remediation of Cr（Ⅵ）)pollution was carried out in mountain soil,river bank soil,farmland soil and coal mine soil.Through systematic research,the following conclusions are mainly obtained:（1）The dispersion and stability nZⅥ in the reaction process were enhanced by using a composite stable system.Carboxymethyl cellulose sodium（CMC）,acrylic acid（AA）,and bacteria（T.ferrooxidans（Tf）+Thiobacillus ferrooxidans（Tt））were used to construct a composite stable liquid system.CMC and AA jointly undertake the action of conveying and stabilizing nZⅥ particles.The CMC hydrolysis is significantly restrain by using the AA,and the compound flora can promote the dissolution and washing of Cr（Ⅲ）and Cr（Ⅵ）)in addition to the auxiliary stabilization.（2）The optimal operating conditions for removal of Cr（Ⅵ）)by nZⅥ in static soil experiments were explored.The results suggested that the optimal concentration of CMC/AA/bacteria and nZⅥ was 0.6g/L and0.06g/L,respectively.The suitable temperature and the initial pH of the soil system was 35℃and 4,respectively.At the initial stage of nZⅥ,the kinetic reaction was in accordance with the quasi-first-order reaction,and the apparent rate constant kobs was consistent with the rule of Cr（Ⅵ）)removal.The redox potential（ORP）of the reaction system exhibits a certain correlation with the addition of nZⅥ.（3）The optimal operating environment for removal Cr（Ⅵ）)of nZⅥ in dynamic small soil column system was determined.Different soil loading reaction columns show different penetration properties.Among them,the penetration of floodplain soil and farmland soil is slow,and the penetration of mountain soil is fastest.Through the injection of stabilized nZⅥ into the small soil column,Cr（Ⅵ）)was reduced and eluted.The contribution rate of the reduction effect of the reaction system on Cr（Ⅵ）)removal was also studied by adding KH2PO4 to suppress soil adsorption.The research showed that the contribution rate of reduction effect to the total removal rate was 56.87%89.38%in the four soil experiments,and the proportion of reduction effect between riverbank soil and farmland soil was relatively large.The optimal concentration of stabilizer（CMC/AA/bacteria）and nZⅥ in the dynamic small soil column system was 0.8 g/L and 0.08 g/L,respectively.Compared with the static process,the dynamic process has more factors affecting Cr（Ⅵ）)removal.The effect of soil specific organic matter humic acid（HA）on the removal of Cr（Ⅵ）)from soil by nZⅥ was studied.The results showed that the increase of HA concentration was beneficial to the removal of Cr（Ⅵ）)from soil.As for the actual soil organic matter,although the higher the organic matter content,the higher the removal efficiency of Cr（Ⅵ）)in the corresponding system,but the difference was not obvious,and the removal rate could reach98.90%-99.25%.Both X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）analysis showed that the intensity of various forms of Cr on the surface of soil was significantly reduced by removing the Cr（Ⅵ）),which further confirmed the reliability of the removal effect.（4）The operational parameters for nZⅥ to remove Cr（Ⅵ）)efficiently in large dynamic soil column system were determined,and the effects of Cr（Ⅵ）)removal by two special soil column operating modes were studied.In order to achieve the same effect of Cr（Ⅵ）)removal by dynamic large soil columns,the required concentration of nZⅥ is higher.Hence,the removal rate of nZⅥ with 45 times of Fe was relatively good.However,the bacteria added in the process of stabilizing nZⅥ could promote the removal of the high concentration Cr（Ⅵ）).The study on the effect of Cr（Ⅵ）)removal by the new operation mode of unconventional soil remediation reactor provides more options for improving the operation mode of soil remediation site.Among them,the semi-static operation mode had more practical significance for saving the power cost of the system,while the larger reflux ratio of the backflow operation mode was beneficial in the effect of Cr（Ⅵ）)removal.Increasing the liquid reflux ratio was conducive to reducing the amount of CMC,AA and bacteria required in the process of stabilizing nZⅥ,as well as reducing the amount of nZⅥ.（5）A method was established to define the rule of washing transformation in the process of removing Cr from soil.The soil column filled with Cr（Ⅵ）)was washed out by nZⅥ and deionized water（DIW）,and the accumulated amount of Cr（Ⅵ）),Cr（Ⅲ）and Cr（total）was calculated,and the separation and transformation amount of Cr was defined in detail.The inflection point of soil Cr wash-out accumulation was defined as differential Cr separation and transformation when the soil bed volume was 1.5BV.In the four soils,the amount of Cr（total）washed out by DIW was ranged from 32.08%to 44.38%,and the amount of Cr（Ⅵ）)adsorbed in the blank was ranged from 55.62%to 67.92%,and the amount of nZⅥ washed out of Cr（total）was ranged from 15.24%to19.34%,respectively,while the washing out of Cr（Ⅵ）)and Cr（Ⅲ）was relatively small.（6）The study indicated that the residual state is main occurrence state by using the occurrence state of Cr in soil by sequential extraction.The proportion of Cr chemical morphological in soil was affected by a combination of nZⅥ,CMC,AA and bacteria and other comprehensive factors,which was the result of multiple factors.The proportion of various chemical morphological of Cr in soil and the ratio of increase and decrease was calculated when the reaction time was at 48h and 1h,respectively.The results showed that the exchange rate and the adsorption state growth rate were 8.2%-24.3%,12.7%-34.9%for 48h,respectively.The residual state rate reduced by 10.5%-17.4%.In addition to the decrease of mountain soil,the organically bound state increased slightly in the other three soils.The correlation between soil organic matter content and organic binding state increased strongly;the correlation between clay content and adsorption rate increased;the porosity and organic binding rate showed a strong negative correlation.The proportion of various chemical forms of Cr in soil is greatly affected by soil organic matter,clay content and porosity after 48h.TCLP and WET experiments showed that the comprehensive leaching reduction rate of Cr（Ⅵ）)is greater than Cr（total）.（7）The chemical reaction kinetics of Cr（Ⅵ）)removal in dynamic large-scale vertical soil column system was obtained.The variation of Cr（Ⅵ）)concentration of effluent over time conformed to the quasi-first-order reaction kinetics model in the range of 0-30 min.However,the change of concentration was consistent with the quasi-second-order reaction kinetics model when the reaction time further increased to 180 min. |
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