Study On Detoxification Of Chromium-Contaminated Soil By Combined Leaching And Reduction

Remediation of Cr???-contaminated soil is currently a hot and difficult problem in the environmental field in China.When FeSO₄ is used to restore and stabilize chromium-contaminated soil,the massive addition of H₂SO₄ destroys the structure of the chromium-contaminated soil and changes the physical and chemical properties of the soil.The soil is prone to hexavalent chromium reoxidation and the cost of remediation is too high.Therefore,from the perspective of efficient remediation,cost reduction,and environmental protection,the combined technology of leaching and reduction is studied to reduce the concentration of chromium in contaminated soil and reduce the cost of remediation.To improve long-term stability and provide a certain theoretical basis and guiding significance for the remediation of chromium-contaminated soil.This article takes light and severe chromium-contaminated soil in the old slag field of Gansu Minfeng Chemical Co.,Ltd.as the research object,and designs three sets of experiments:single reduction,single cleaning?indoor oscillation cleaning?,cleaning and reduction,by optimizing the reducing agent FeSO₄ and adding amount of acid regulator H₂SO₄;experimental conditions such as solid water ratio of clean water cleaning solution,equipment speed,cleaning time,standing time,cleaning times,etc.,compare the detoxification effect,calculate the cost of the experiment,and get the best detoxification effect and the lowest detoxification cost method.To study the reduction and stabilization effect of combined leaching and CaS₅ on chromium in contaminated soil,and the reduction and stabilization effect of three phosphates such as Na₃PO₄,NaH₂PO₄ and?NaPO₃?₆ on CaS₅,the raw soil and treatment were analyzed by Tessier five-state continuous extraction method The morphological changes of the soil were evaluated by the multi-pH parallel leaching method and the MEP method for the long-term stability of the soil after the reduction and stability of CaS₅ and phosphate.the result shows:?1?Single cleaning experiment:The soil hexavalent chromium oscillating cleaning efficiency increases with the number of cleaning times.After two cleanings,most of the water-soluble hexavalent chromium is dissolved,and the efficiency of medium and light cleaning can reach 86.58%,85.41%,the content of hexavalent chromium in the soil is 629.76mg/kg,2212mg/kg,continue to clean,the efficiency does not improve much.?2?Single reduction experiment:When the dosage of FeSO₄ is 25%?moderate?,30%?severe?,and the dosage of H₂SO₄ is 28%,the reduction efficiency is 99.36%,99.73%,moderate-light,severe soil.The content of medium hexavalent chromium is less than 50mg/kg,reaching the restoration target value.?3?Cleaning and reduction experiment:after cleaning twice with moderate and severe chromium contaminated soil,it is reduced and stabilized with FeSO₄.When the dosage of FeSO₄ is 9%?moderate?,12%?severe?,the amount of H₂SO₄ at 8%,the content of hexavalent chromium in moderate to severe soil is less than 50mg/kg,reaching the remediation target value.?4?Cost:When the detoxified soil Cr???reaches a specific standard limit,the cost of the three sets of experiments is calculated,of which the combined cost of cleaning and reduction is the lowest.?5?The reduction efficiency of CaS₅ to hexavalent chromium in moderately and slightly chromium contaminated soil increases with the addition of CaS₅.When the addition amount is8%,the reduction efficiency of hexavalent chromium in soil reaches 92.24%.The content of medium hexavalent chromium is 48.840mg/kg.When the dosage of Ca S₅ reaches 12%,the reduction efficiency of hexavalent chromium in soil reaches 98.08%.?6?Under the same molar ratio of phosphate radical to hexavalent chromium,the removal rate of total chromium and hexavalent chromium in the leaching solution of moderately and lightly polluted soil by three kinds of phosphates increases with time.Within 0?15d,NaH₂PO₄ had a higher removal rate of total chromium and hexavalent chromium in the medium-to-light chromium-contaminated soil leachate than the other two,and the pH of the soil leachate was low,which had little effect on the pH of soil.?7?When the molar ratio of NaH₂PO₄ to Cr?VI?is 8:1,the removal rate of total chromium and hexavalent chromium in medium and light soil leaching liquid reaches 94.16%and 96.26%;When the molar ratio of Cr?VI?is 16:1,the removal rate of total chromium and hexavalent chromium in heavy soil leachate reaches 94.388%and 95.59%,both of which reach the restoration target value.?8?After treatment,the proportion of soil water-soluble and carbonate-bound chromium decreases,and the proportion of residual chromium increases,effectively reducing the environmental risk of contaminated soil on the original site.In short,cleaning and reduction not only reduce the cost of detoxification and reduce the content of chromium in the soil,but also can recycle the chromium in the cleaning solution;NaH₂PO₄ can improve the pH of CaS₅ reduced soil,increase the ratio of residue state,and reduce the leaching of chromium and increase the long-term stability of chromium.