Experimental Study On The Impact Of Heavy Metals Produced By In-situ Mining Of Oil Shale On Groundwater Environment

Near-critical water method and local chemical method oil shale in-situ mining technology has successfully extracted shale oil in the test site,but the possible impact of heavy metals produced by oil shale in-situ mining on the groundwater environment is still seldom studied.The literature on the impact of heavy metals on groundwater produced by in-situ mining of shale near critical water and local chemical methods has not yet been reported.Because Huadian oil shale has high oil content and great development potential,this paper takes the oil shale in Huadian City,Jilin Province as an example.Selecting As,Hg,Pb,Mn and Cr as the representative heavy metal elements,using the method of laboratory experiments,the oil shale and direct dry distillation are systematically studied for the local chemical method and the near-critical water method.The impact of heavy metal content,various occurrence states,and longterm release in water on the groundwater environment in the residue after extraction of shale oil by method,local chemical method and near-critical water method.Relying on the National Natural Science Foundation of China "Research on the Impact of Oil Shale In-situ Mining on Groundwater Environment(41572216)",this paper systematically analyzes the content,occurrence state and water-rock interaction of heavy metals in oil shale and residues from different mining methods.The occurrence and long-term release laws of heavy metals have made innovations in the comparative analysis of the content changes of heavy metals,the analysis of the influencing factors of their occurrence state,the analysis of the influencing factors of their long-term release and the analysis of their impact on the groundwater environment.The main conclusions are as follows:(1)The contents of As,Pb,Mn and Cr in the residues of dry distillation and local chemical methods increased,and the contents of Hg decreased.the contents of Hg and Pb in the residues of near-critical water method increased,and the contents of As,Mn and Cr decreased.The content of heavy metals except Hg in the local chemical residue is the largest,and the content of heavy metals in the near-critical water residue is the smallest.(2)After pyrolysis of oil shale,the main occurrence state of heavy metals in dry distillation residues did not change,but the main occurrence states of Hg in local chemical residues changed,and the main occurrence states of As and Hg in near-critical water residues changed.The bioavailable bound state content of heavy metals in the residue has a good linear correlation with the pyrolysis temperature and pyrolysis time,and different in-situ mining methods have obvious effects on the occurrence state content of heavy metals.(3)Use the bioavailability index method to evaluate the degree of pollution to the groundwater environment caused by the bioavailable binding state of heavy metals in the residue.As,Pb and Mn in the residue are in a polluted state,and the heavy metals in the near-critical water residue will have relatively little impact on the groundwater environment.Impact.(4)When the release experiment was carried out for 120 days,the p H value of the oil shale aqueous solution was neutral,the p H value of the dry distillation residue aqueous solution was strongly alkaline,the p H value of the local chemical residue was weakly alkaline,and the p H value of the near-critical water residue was strong.Alkaline.The p H value of the residue has a good linear relationship with the pyrolysis temperature and pyrolysis time.(5)The release concentration of heavy metals in oil shale and residues tends to be stable with the prolongation of soaking time.The concentrations of As,Hg and Cr in the dry distillation residues changed greatly within 20 days,and the concentrations of Pb and Mn increased rapidly within 75 days.The concentration of As,Hg and Mn in the near-critical water residues changed greatly within 75 days,Pb increased rapidly within 60 days,and Cr changed greatly within 90 days.There is a good correlation between the release of heavy metals in the residue and the pyrolysis temperature and pyrolysis time.The effect of the in-situ mining method on the long-term release of heavy metals is shown in the release rate of residual heavy metals.The stable release rate of As and Hg in the near-critical water method is the smallest,and the stable release rate of Pb,Mn and Cr in the local chemical method is the smallest.According to the long-term release law of heavy metals in oil shale and residues,the chemical kinetic equation was fitted,and the fitting effect was good.(6)Use the water quality index method to judge the impact of heavy metals in oil shale and residues on the groundwater environment.Near-critical water residue heavy metals do less harm to the groundwater environment.Attention should be paid to the pollution of groundwater caused by the release of As,Pb and Mn in the residue,and the possible pollution of the groundwater environment caused by dry distillation and Cr in local chemical residues.(7)With reference to the standard concentration of class III groundwater,use the accumulation formula and the steady release rate of heavy metals in the residue to analyze and calculate the impact of the release of heavy metals in the residue on the groundwater environment.The calculation results show that with the circulation of groundwater,Mn in the residue has the longest impact on the groundwater environment,Cr has the shortest impact time,and the local chemical method has the shortest time to pollute the groundwater.