| The metallurgical industries have brought the serious environmental pollution caused by the heavy metals(Cd,Hg,As,Cu,Pb,etc.)while creating social value.Especially,the serious soil pollution caused by the most toxic and extremely harmful heavy metal pollutants Hg and As,is urgently needed to be treated.In this paper,the chemical leaching,thermal desorption and stabilization methods were used to safely and effectively remediate the Hg and As contaminated soils from the abandoned coking plant and the nonferrous metallurgical tailings,and the research on clean treatment for the leachate of Hg-contaminated soil produced by the leaching process and high-efficiency extraction method of Hg were also proposed.The simple chemical leaching method was used to remediate the serious Hg-contaminated soil containing HgS,which collected from the waste coking plant.The feasibility of aqua regia digestion method for total Hg detection was verified by thermodynamic calculation.The total Hg content was effectively determined as 31.8 mg/kg by the aqua regia digestion and ICP-MS detection,which belongs to serious pollution soil(Index of geoaccumulation is 5.9).The leaching agents,such as KI,Na2S2O3 and Na2S were selected to remediate the Hg-contaminated soil.The study found that the combination of Na2S2O3 and Na2S has the better leaching effect,and the total mercury content of the soil can be about 3.0 mg/kg.The combined leaching effect of Na2S2O3 and Na2S was significantly better than any of them when Na2S concentration was low(<0.05 mol/L).However,the complexation of Na2S3O3 and Hg was completely inhibited with the high alkaline leachate,resulting in the leaching effect of complex agents is only similar to that of single Na2S leaching when Na2S concentration exceeded 0.3 mol/L.Then the systematical investigation of Na2S leaching process was further carried out,and found that the total Hg content has dropped to 0.8 mg/kg,with an optimal experimental condition that the Na2S concentration was 0.5 mol/L,the liquid-solid ratio(mL/g)was 10:1,and the leaching time was 10 h,which reached the requirement of national standard.This is mainly attributed to most of the mercury compounds in the soil,including stable HgS,were transformed into more stable HgS22-(stability constant is 2×1055)ions by complexing with the S2-in alkaline sodium sulfide solution.The chemical leaching and thermal desorption were used to remediate the severe Hg-contaminated soil containing HgS and the mechanism was systematically studied.The content of total Hg in heavy Hg-contaminated soils was 168.0 mg/kg,which belonged to severe pollution soil(Index of geoaccumulation is 8.3).It was found that the Na2S leaching had a significant removal role on the total Hg and the harmful moblile Hg;72.4%of the total Hg and 86.3%of the mobile Hg could be removed with the Na2S/soil(mL/g)ratio of 10:1,Na2S concentration of 0.7 mol/L,leaching time of 4 h and leaching temperature of 25 ?.The total Hg content in the soil after leaching was 46.4 mg/kg,which needed to be treated subsequently.The directly thermal desorption method was applied in view of the incompleteness of Na2S leaching on Hg,and the Hg content was only reduced to 32.4 mg/kg when the thermal desorption temperature rose to 550 ? for 60 min.Due to the Hg compounds such as HgS and HgSO4,etc.,which have the higher decomposition temperature could be effectively removed by Na2S leaching,the Na2S leaching assisted thermal desorption method was utilized,and found that Hg content could be reduced to a safety content of 1.0 mg/kg when the thermal desorption temperature was 350 ?and the time was 60 min.Hence,the severe Hg-contaminated soil has been mildly remediated at lower desorption temperature.The cement solidification method and Hg extraction method were adopted to effectively treat the high concentration(3.120 mg/L)Hg leachate with high alkalinity(pH=13.5)produced from the Na2S leaching process for serious Hg-contaminated soils.(1)It was observed from the solidified experiment that only the portland cement can be used to effectively solidify leachate(Hg concentration was 3.120 mg/L)and its concentrate leachate(Hg concentration was 9.243 mg/L).When the ratio of liquid/cement(mL/g)was between 2:10 and 6:10,the TCLP leaching concentration of Hg in the solidified cement from Hg leachate and its concentrate leachate were both less than 0.00016 mg/L,which met the safe disposal of criterion(<0.2 mg/L).When the liquid-solid ratio was between 4:10 and 8:10,both of them behaved the better block-forming property,and their compressive strengths could exceed 7 MPa,(2)In order to efficiently recycle the high content Hg in the leachates,the effect of temperature and pH value on the extraction of Hg was subsequently carried out.It was shown that a little effect on the extraction of Hg by changing temperature,whereas the pH value had a significant effect on Hg extraction ratio.The Hg concentration in the treated leachate was reduced from 3.120 to 0.005 mg/L when the pH was decreased to 2.98,which was obviously lower than the limit of the national standard.It was attributed to the large amount of soluble HgS22-complexing ions had been transformed to the low toxic HgS.The stabilization method was used to treate the severely As contaminated soil collected from the non-ferrous metallurgical tailings dam.The total As content in the soil was 26084.2 mg/kg,which is significantly higher than the current research level(Index of geoaccumulation is 10.4).The main valence states of As in the soil were As(?)and As(?).It has been found that the combination of non-toxic organosulfur TMT-15(Na3S3C3N3 with a mass of 15%)and FeCl3·6H2O stabilizers presented better stabilization of As in soil than a single stabilizer,and could decrease the toxic leaching concentration of As in soil to 57.6 mg/L.Further,H2O2 was used to oxidize the highly mobile and toxic As(?)in the soil,resulting the complete transformation from As(?)to As(V).When the addition amount of TMT-15 was 2 mL and the amount of FeCl3·6H2O was 0.2 g,the stabilizing efficiency of As in the soil reached up to 99.6%,and the As toxic leaching concentration was 1.6 mg/L,thus it could be deposited safely.Because As(?)in the soil has completely oxidized to As(?),and As(?)can be easily converted into a stable scorodite and stabilized by the synergy of TMT-15.Besides,the solubility of scorodit can have a significantly reduction for a certain amount of CaSO4,hence,the severely As contaminated soil has been successfully stabilized,which can give a reference for effectively treating the high level As contaminated soil.In this paper,simple and effective remediation for seriously Hg-polluted soils with different concentrations and efficient stabilization for As-contaminated soil in metallurgical industries have been realized,which provides theoretical reference for efficient remediation of soils polluted by heavy metals in metallurgical industries. |
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