| Hexavalent chromium(Cr(Ⅵ))has been attracting much attention due to its high toxicity,high mobility and strong oxidizing properties.Converting it into trivalent chromium(Cr(III))with low toxicity and low migration ability is currently a common technical method.The combination of adsorption method and chemical reduction method as an effective technology has been widely used in the treatment of Cr(Ⅵ)polluted water and soil.Nano-zero-valent iron(n ZVI)has large specific surface area,strong reduction performance,high reactivity,and can efficiently reduce Cr(Ⅵ);but on the other hand,n ZVI has shortcomings such as easy agglomeration,less reactive specific surface area,and low mechanical strength,which limit its large-scale application.Loading modified n ZVI can inhibit the agglomeration of n ZVI particles,and there is a reduction and adsorption synergy between the carrier material with functional groups and n ZVI,which can improve the specific surface area of the modified material and the stability of the nanomaterial.Walnut shell is an ideal carrier material,which has the characteristics of low-cost and easy-to-obtain,hard and porous and a large number of oxygen-containing functional groups.Therefore,in this study,walnut shell powder(WSP)was used as a carrier to successfully prepare three kinds of shell iron than walnut shell loaded nano-zero-valent iron composite materials(2WSP@n ZVI、WSP@n ZVI and WSP@2n ZVI),which were used for the removal of Cr(Ⅵ)from aqueous solution and remediation of Cr(Ⅵ)in soil.(1)The best shell-to-iron ratio was screened out;(2)The influence of factors such as p H,dosage,initial concentration and reaction time on the removal of Cr(Ⅵ)from water was discussed;(3)fitting the adsorption kinetic model,analyzing the removal mechanism by combining the characterization before and after the reaction between the composite material and the Cr(Ⅵ)solution;(4)discussing the influence of dosage,soil-water ratio and remediation time on the passivation remediation effect of Cr(Ⅵ)contaminated soil;(5)the possible remediation mechanism was explored by analyzing the changes of Cr(Ⅵ)concentration in soil leachate,soil Cr(Ⅵ)content,total soil Cr content and soil Cr speciation before and after treatment,as well as the internal relationship between them.The main conclusions of this study were as follows:(1)The best shell-to-iron ratio of the walnut shell loaded nano-zero-valent iron composite was 2:1.For the removal of Cr(Ⅵ)from aqueous solution,the removal rate of Cr(Ⅵ)from aqueous solution by 2WSP@n ZVI was higher than that of pure n ZVI and pure WSP.For the removal of Cr(Ⅵ)from soil,2WSP@n ZVI and WSP@n ZVI had the best effect on the removal of Cr(Ⅵ)from soil leachate.2WSP@n ZVI had a significant cost advantage while ensuring the removal rate.Both SEM and XRD characterizations proved the successful preparation of 2WSP@n ZVI composites.(2)2WSP@nZVI had a good removal effect on Cr(Ⅵ)from aqueous solution.The removal rate of Cr(Ⅵ)from aqueous solution increased with the decrease of p H value,increase of dosage,decrease of initial concentration,extension of reaction time and increase of temperature.When the p H value was 3.0,the dosage was 0.5 g/L,the initial concentration was 20 mg/L,the reaction time was 2 h and the temperature was 25℃,the removal effect was the best,and the removal rate reached nearly 100%.The fitting curve of the reaction kinetics conformed to the pseudo-second-order kinetics,which was dominated by chemisorption.The adsorption thermodynamics parameters showed that the adsorption process of Cr(Vl)by2WSP@n ZVI was a spontaneous endothermic process.The analysis results of SEM,FT-IR and XPS before and after the reaction of the composite material with the Cr(Ⅵ)solution confirmed that the removal mechanism was the synergistic effect of adsorption-reduction-coprecipitation:(i)The n ZVI on the surface of the composite material had electrostatic attraction to Cr(Ⅵ);(ii)n ZVI reduced Cr(Ⅵ)to Cr(III);(iii)Co-precipitation of Cr(III)and Fe(III)to form Cr(III)-Fe(III)hydroxide,attached to the surface of 2WSP@n ZVI.PPMS confirmed that 2WSP@n ZVI had high-intensity magnetism and obvious superparamagnetism before and after the reaction with Cr(Ⅵ)solution.(3)2WSP@nZVI had a good removal effect on Cr(Ⅵ)from soil,which will be better if the dosage increases,the soil-water ratio decreases and the contact time increases.When the dosage was 7.5 mg/g,soil water ratio was 1/10,contact time was 2 days,the removal rates of Cr(Ⅵ)in soil leachate and in soil were 96.9% and 86.7% respectively,and the removal capacity were 15.6 mg/g and 222 mg/kg respectively;the total Cr content in soil was similar to that in original Cr(Ⅵ)-contaminated soil;the proportion of acid-extractable Cr and reducible Cr decreased by 27% and 12%,while oxidizable Cr and residual Cr increased by24% and 15%.2WSP@n ZVI could adsorb,reduce and immobilize Cr(Ⅵ)in soil,which was similar to the removal mechanism of Cr(Ⅵ)from aqueous solution.After treatment with2WSP@n ZVI,Cr speciation in soil mainly existed in stable and low-toxic oxidizable and residual state. |
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