Chromium Magnetic Transformation And Separation Remediation In Soil Induced By Ferrous Ions

China is the world’s largest producer of chromium salts,the global center of tanning and electroplating,and its industrial development is accompanied by serious soil chromium pollution problems.The State Council issued the"Soil Pollution Prevention and Control Action Plan",which clearly proposes to focus on monitoring chromium and other heavy metals in the soil and to comprehensively remediate chromium slag and other stockpile sites.At present,the main idea of chromium-contaminated soil remediation is to reduce the highly toxic and easily migratory hexavalent chromium Cr（Ⅵ）to the low toxicity and low migration of trivalent chromium Cr（Ⅲ）,but the existing reducing agent is difficult to reduce the Cr（Ⅵ）in the soil to the standard limit.And with high concentrations of Cr（Ⅲ）remaining in the soil,Cr（Ⅲ）may be reoxidized,leading to long-term safety risks.The soil after reduction and remediation still needs to be disposed of by expensive and safe landfills.To address the above key issues,a new idea of chromium-contaminated soil remediation is proposed in this paper.Ferrous ions are used to induce chromium reduction and magnetic transformation in soil,and then separate chromium-rich magnetic products to obtain a clean soil with chromium removed.This paper focuses on the morphological distribution characteristics of Cr（Ⅵ）in chromium-contaminated soil,investigates the magnetic transformation mechanism of Cr（Ⅵ）in aqueous solution system,and then explores the mechanism of Cr（Ⅵ）magnetic transformation reaction and extraction remediation induced by subferric ions in two soil systems,wet and dry,respectively.This provides a certain scientific basis for further insight into the environmental geochemical behavior of chromium in soil and the development of new methods for remediation of Cr-contaminated soils.The main findings are as follows:（1）The results of microstructure and elemental chemical analysis of the test soil show that when Cr（Ⅵ）entered the soil,a part of Cr（Ⅵ）was adsorbed by minerals containing exchangeable Ca²⁺and hydrated metal oxides to form a relatively stable adsorbed state of Cr（Ⅵ）,a small part of Cr（Ⅵ）reacted with free state Ca²⁺to form calcium chromate precipitates,and most of Cr（Ⅵ）tended to be reduced by ferrous iron to ferric chromium hydroxide co-precipitates.When the adsorption,precipitation and reduction reactions of Cr（Ⅵ）reach equilibrium,the remaining Cr（Ⅵ）is retained in the soil in its original water-soluble state.The morphological distribution of Cr in soil is closely related to Fe,which facilitates the magnetic transformation of Cr in soil induced by ferrous ions.（2）In the aqueous system,the ferrous-induced magnetic transformation of Cr（Ⅵ）can be divided into two successive reactions including Cr（Ⅵ）reduction to form coprecipitation(Cr_0.25Fe_0.75（OH）₃)and subsequently magnetic transformation of Cr_0.25Fe_0.75（OH）₃ induced by Fe²⁺.The total Fe/Cr mole ratio of 5:1 is at least required for the chromium ferrite transformation.Increasing temperature and p H can enhance the interaction of Fe²⁺with Cr_0.25Fe_0.75（OH）₃ and further promote the formation of chromium ferrite,while suppressing the generation of nonmagnetic by-product goethite.Almost pure chromium ferrite is formed under proposed optimum conditions（Fe（Ⅱ）/Cr（Ⅵ）=7:1,65°C and p H of 9）with Cr（Ⅵ）removal ratio around 100%.（3）In the soil suspension system,ferrous ions were also able to effectively induce the magnetic transformation of Cr（Ⅵ）and Cr（Ⅲ）in the soil.At Fe/Cr=5:1,solid-liquid ratio of 1:1 and reaction temperature of 65°C,the total chromium and Cr（Ⅵ）content in soil was significantly reduced after magnetic product separation,and the recovered magnetic product chromium content could contain up to 38279 mg/kg,which was 5.4times higher than the chromium content in soil after magnetic separation.However,some adsorbed Cr（Ⅵ）still remains failing to be magnetically converted.（3）And this paper also investigated the reduction and magnetic transformation of Cr（Ⅵ）enhanced by dry low-temperature roasting,where temperature,Fe/Cr molar ratio,time and reductant type all affected the reduction and magnetization of Cr（Ⅵ）in soil.When Fe/Cr=5:1 and the roasting time was 60 minutes,the Cr（Ⅵ）concentration of the roasting products at different temperatures ranged from low to high:soil+ferrous ammonium sulfate<soil+ferrous sulfate<soil.When the temperature was 410°C,Fe/Cr=5:1,roasting time was 60 minutes,and ferrous ammonium sulfate was used as the ferrous source to mix with the soil thoroughly and then roasting treatment.The concentration of Cr（Ⅵ）in the roasting product was lower than 1.8 mg/kg,which satisfied the"Soil Environmental Quality Risk Control Standard For Soil Contamination Of Development Land"（GB36600-2018）in the second category of construction land Cr（Ⅵ）screening value is not higher than 5.7 mg/kg.After magnetic separation,the chromium content in the magnetic product can reach 2.5 times the soil chromium content.