Research On Comprehensive Treatment Methods Of Chromium Contaminated Soil

With the rapidly development of industry,chromium salts are used in many industries,such as dyes,leather,metal coatings,etc.It brings a lot of conveniences for people's life.However,the discharge of chromium-containing wastewater and the random accumulation of chromium residue cause serious industrial soil chromium pollution.Due to different levels of industrial soil chromium pollution and different properties of industrial soil,there is no single method that can effectively control all soil chromium pollution problems,so it is necessary to explore a comprehensive control method of integrating many kinds of technology.According to invesitgation and survey about Cr contaminated soil,the chromium contaminated industrial soil is divided into two parts: heavy chromium contaminated soil and mild chromium contaminated soil,which were treated by ectopic leaching and direct reduction respectively.Heavy chromium contaminated soil was treated by ectopic leaching technology.Proper leaching agent was optimized to carry hexavalent chromium ion out of the soil,to investigate the effects of different eluents(deionized water,citric acid,oxalic acid,EDTA)and reaction temperature on the extraction of hexavalent chromium from soil.The concentration of extracted hexavalent chromium was detected by ultraviolet spectrophotometry.Suitable reducing agents(ferrous sulfate,sodium thiosulfate,sodium pyrosulfite)were selected to reduce hexavalent chromium in the eluent,and the hexavalent chromium with high toxicity was reduced to non-toxic trivalent chromium.The type and optimum amount of the reducing agent were investigated.Calcium oxide was selected as curing agent to stabilize chromium trivalent,and to realize the ectopic leaching and remediation of heavily chrome-contaminated soil.The results showed that there were great differences in the extraction of hexavalent chromium by different eluents,the best effect was the oxalic acid,Under the condition of 1:10 solid-liquid ratio and 40 reaction temperature,the concentration was 0.1mol/L,?and the elution rate of chromium in soil reached 78.95%.While as the poor leaching rate of EDTA and citric acid is 77% and 35.21% respectively as the result of temperature 30°C and concentration 0.1mol/L.The leaching temperature and the concentration of eluent had different effects on the leaching results,the temperature was more important.Among the three reductants.For the three agentis,sodium thiosulphide has the best reduction effect and satisfy the demand of National standard and the poor toxicity with the least amout,which can meet the demand of the economy and enviroment protection.Calcium oxide is used as a curing agent to form a stable chromium hydroxide precipitate after the reduction of trivalent chromium,and the eluent can be recycled.The leaching water can be reconvered or discharged directly without polluting water resources and secondary pollution.Mild chromium contaminated soil was controled by direct reduction technology.Different reducing agents(ascorbic acid,sodium metabisulfite,ferrous sulfate,sodium thiosulfate)were added into the soil.Then the type and the optimum amount of reducing agent were investigated in terms of the reducing effect.The pH of the soil was also under investigation.The results show that ascorbic acid and sodium metabisulfite with the 15% additive amount could reduce the content of hexavalent chromium to less than 30mg/kg,which is mandatory requirement of National standard.However,sodium metabisulfite and ferrous sulfate could not reach that demand level.Specially,the reduction rate of ascorbic acbestid with the 7% additive amount can reach 99.91%,which can meet the demand of National standard.When the pH value of the sample soil was adjusted to 1.1,3.5 and 6.0,the reduction effect of ascorbic acid and sodium thiosulfate was significantly affected by the soil pH value.The lower the soil pH value was,the better the reduction effect of ascorbic acid and sodium thiosulfate was,while the reduction effect of ferrous sulfate was less affected by the soil pH value.Based on the experimental results of ectopic leaching technology to treat heavy chromium-contaminated soil,the design that treat 500 tons of chromium-contaminated soil per day was carried out.The work includes the method selection,the flow process design,the material and heat balance,equipments selection and design.This process design could be used to confirm the feasibility of ectopic leaching technology industrial application.