| With the development of tanning,electroplating and chromium salt production industries in our country,the discharge of chromium pollutants has been increased rapidly.Among them,the problem of chromium pollution of soil is particularly serious,and there are many chromium pollution sites,which has become a general environmental issue.The situation of polluted soil remediation is severe,and the remediation technology has become research hotspots.Comprehensive remediation technology has become the main trend of soil remediation.In this paper,we researched the comprehensive remediation technology to chromium-contaminated sites based on the actual situation of existing industrial chromium-contaminated sites and the different degrees of chromium pollution.Ex-situ leaching repair technology was applied in high-concentration chromium-contaminated surface soil,while the in-situ chemical reduction stabilization repair technology was applied in low-concentration chromium-contaminated deep soil.The appropriate process routes and conditions,as well as the feasibility of industrialization was explored.(1)The ex-situ leaching technology was adopted to repair the high-concentration chromium-contaminated surface soil.The suitable leaching agents was selected,and the different influence factors was discussed such as solid-liquid ratio,soil particle size,leaching time,p H value,and concentration on the leaching effect.A multi-factor orthogonal test was carried out on the appropriate leaching agent to determine the best leaching conditions.The recycling of the eluent was discussed,and the influence of the conditions of the reduction reaction and the flocculation reaction on the recycling effect of the eluent was investigated.The research results showed that salt leaching agents(sodium sulfate,sodium carbonate,potassium chloride)were significantly better than the acid leaching agents(oxalic acid,citric acid,tartaric acid).Both of the p H value and ion exchange effect play an important role to desorption of chromium ions especially the p H value.Sodium sulfate was the best suitable eluent.The sequence of the influence factors on the eluent effect obtained by orthogonal experiment was decreased according to p H value>stirring temperature>stirring speed>stirring time>ultrasonic time.The optimal conditions obtained by the optimization experiment were that the soil particle size was 100 mesh,solid-liquid ratio was 1:5,p H value was 12,stirring temperature was 50 ℃,stirring speed was 300 r/min,and the stirring time was 30 min.The content of hexavalent chromium in the restored soil meet the requirements of national standards.In the recycling of the eluent,sodium bisulfite was selected as the reducing agent while the p H was 2-3,the reduction time was 20 minutes and the amount of reducing agent added was 3-4 times of the theoretical amount.In the flocculation reaction,the precipitating agent was potassium hydroxide.And the best recycling effect of eluent was gained when The p H value of the flocculation reaction was 10.5-11.5,and the reaction time was 30 minutes.(2)The in-situ chemical reduction and stabilization technology was used to repair the deep low-concentration polluted soils.The suitable reducing agents were screened and the influence factors was also discussed such as the amount of reducing agent added,reduction time,and p H value on the reduction effect of chromium-containing solutions.On this basis,through the soil column simulation experiment,the reduction effect of different reducing agents on the chromium-contaminated soil in the soil column was explored,and the appropriate reducing agent and conditions were determined.The stabilizers were added for stabilization,and the stabilization conditions were determined through soil toxicity leaching experiments,and the stabilization effect of reduction stabilization was explored.The research results showed that the selected reducing agents were ascorbic acid,ferrous sulfate,and sodium bisulfite.The amount of reducing agent added to the soil were3:1 in molar ratio,the p H value was 2-4,the reaction time was 20 minutes,and the three reducing agents had the best reduction effect on the chromium-containing solution.The reduction effect of three reducing agents on chromium-containing soil was significantly reduced under this reducing condition.Among them,ferrous sulfate had the best reduction effect,which can reduce the chromium content in the soil to the national standard within30 minutes.The soil column simulation experiment showed that the hexavalent chromium in the soil could be completely reduced to meet the national standard requirements 10 days later when the ferrous sulfate was used as the reducing agent over the molar ratio of 4:1.Two days after the stabilizer added no hexavalent chromium was founded in the acetic acid extract.(3)Based on the experimental data,a comprehensive remediation process for industrial chromium-contaminated sites with treatment of 1,000t/d soil was designed.The ex-situ leaching technology was used to remediate the 0-9m deep soil layer with high-concentration chromium-contaminated.The in-situ chemical reduction stabilization technology was applied to repair the 9-25 m deep low-concentration chromium-contaminated soil layer.Process route selection,process design,material heat balance calculation,and main equipment design selection were all completed.Draw process material flow chart(PFD),pipeline and instrument flow chart(PID)and workshop layout drawing were also drawn.The process technology route was reasonable and feasible,which meet the requirements of national soil environmental protection standards,and had certain industrial promotion and application value. |
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