Study On Remediation Stability Of Chromium Contaminated Soil And Cr(Ⅲ)reoxidation

As an important industrial raw material,chromium salt is widely used in electroplating,printing and dyeing,leather making,medicine and other industries.As the world’s largest producer of chromium chemicals,the large amount of chromium slag accumulated in history is difficult to meet the disposal standards of"no leakage,no dispersion,and no loss".Under the effects of long-term atmospheric dust and rain washing,the chromium slag storage yard has caused heavy metal chromium pollution in the surrounding atmosphere,water and soil.Hexavalent chromium has strong oxidizing properties,acute toxicity and carcinogenic risk.The high concentration of Cr（Ⅵ）in the soil poses a serious threat to the surrounding ecological environment.At present,my country is remediating and treating chromium contaminated sites.In the soil Cr（Ⅵ）restoration project,it was found that the soil after restoration would produce the phenomenon of"turning yellow".The phenomenon of"return to yellow"represents the situation that the leaching concentration of Cr（Ⅵ）increases again after the restoration reaches the standard.The phenomenon of"return to yellow"has increased the difficulty of soil remediation projects.Therefore,clarifying the mechanism of soil chromium migration and transformation and"returning to yellowing"after remediation,improving the remediation effect,and preventing the rebound of Cr（Ⅵ）concentration are the current difficult topics in the treatment of chromium-contaminated soil.This paper taked Cr（Ⅵ）contaminated soil at a chromium-contaminated site in Jiuquan City,Gansu Province as the research object,and conducts research from three perspectives:the influencing factors of chromium migration and transformation,the influencing factors of"return to yellowing"and the combination of reduction and solidification technology.Conclusion as below:（1）Through total chromium,Cr（Ⅵ）leaching experiment and Tessier heavy metal occurrence form extraction experiment,the influence of p H,organic matter and particle size on the migration and transformation of soil chromium were explored.The results showed that as the soil p H was lowered,the concentration of total chromium and Cr（Ⅵ）were decreased,and the exchange of exchange of state and carbonate binding state gradually decreaseed,and iron manganese oxidation state and organism With the gradual rised,there was no significant change in the residual state;the organic matter was mainly influenced by adsorption and reducing effects,the organic matter content was negatively correlated with the soil chrome leaching concentration,the wors of the stability of chromium contaminated soil,the worse the organic matter The survival form was mainly a transformation of organic state and switchable state;For chromium-contaminated original soil,as the particle size of the soil decreases,the leaching concentration of total chromium and Cr（Ⅵ）both show an upward trend.On the contrary,for reduced soil,the particle size mainly affects the exchangeable state and the carbonate combined state.（2）The oxidation effect of natural oxidant H₂O₂and manganese oxide on soil Cr（Ⅲ）were studied.H₂O₂and manganese oxide are currently known soil Cr（Ⅲ）oxidants.Through the addition of H₂O₂and manganese oxides,the effects on the leaching concentration of soil Cr（Ⅵ）and the occurrence of chromium were explored,and related influencing factors were analyzed.The results showed that Cr（Ⅵ）leaching concentration was added to H₂O₂and the amount was positively correlated.As the H₂O₂content increased,the soil chromium organic binding state was reduced,and the exchange was exchanged;the light conditions could inhibit the oxidation of H₂O₂on Cr（Ⅲ）to a certain extent;In addition,under different soil p H conditions,the oxidation capacity of Cr（Ⅲ）was slightly different,and the degree of reaction in a strong alkali environment was higher than neutral and weak acid weak base conditions.Different valence states of manganese oxides reacted differently with Cr（Ⅲ）.With the increase of the dosage of manganese oxides,Mn（Ⅳ）exhibited a significant oxidation effect on Cr（Ⅲ）,and the occurrence form changed from the exchangeable state to organic combined state and iron manganese oxidation state.Compared with Mn（Ⅳ）,Mn（Ⅲ）had weaker oxidizing ability,and there was no obvious change in the occurrence form.Mn（Ⅱ）has obvious reducing effect on Cr（Ⅲ）.In addition,under the Mn（Ⅳ）-Cr（Ⅲ）reaction,with the increase of soil p H,the leaching concentration of Cr（Ⅵ）first increased and then decreased,reaching the maximum at p H 4;The effect of soil Cr（Ⅲ）content on the Mn（Ⅳ）-Cr（Ⅲ）reaction was mainly manifested in that with the increase of Cr（Ⅲ）content,the degree of Mn（Ⅳ）-Cr（Ⅲ）reaction first increased significantly and then slowly decreases.（3）Used FeSO₄as a reducing agent to repair chromium-contaminated soil,and selected seven common curing agents（superphosphate,sodium dihydrogen phosphate,disodium hydrogen phosphate,calcium magnesium phosphate,aluminum oxide,aluminum hydroxide,and kaolin）to select and solidify.The effective curing agent was compounded with FeSO₄to obtain the optimal solution of the type of combined repair curing agent,the dosage of the agent and the repair sequence.The repair results of FeSO₄combined curing agent showed that the two-step method of 3x FeSO₄+10%aluminum hydroxide and the two-step method of3.5x FeSO₄+10%calcium-magnesium phosphate fertilizer had the best repair effect,and the total chromium and Cr（Ⅵ）leaching concentration were lower than"Chromium Slag"The corresponding leaching standard（total chromium≤4.5mg/L,Cr（Ⅵ）≤1.5mg/L）in the Technical Specifications for Pollution Control and Environmental Protection.The proportion of the active state was reduced to the lowest,and the sum of the proportions of the organic state and the residual state was 2.09 times and 2.15 times of that before repairing.The essence of chromium contaminated soil stabilization was the transformation of chromium occurrence form.XPS characterization analysis further showed that after FeSO₄combined with aluminum hydroxide and calcium-magnesium phosphate fertilizer,the soil Cr（Ⅵ）couls be effectively removed,and the soil had a higher degree of stability.