Study On Mechanism Of Solidification/Stabilization For Remediation Of Cr(?)-Contaminated Soil Using Mechanochemical Method

The country of China is one of the leading producers of chromate with an annual production of greater more than 160,000 tons.With increasing urbanization,many chromium contaminated sites have been created.Soil surrounding chromate plant was considerably contaminated due to most of the chromium residue accumulating in the air and chromium-containing waste water discharging into the environment.Considering the great hazards of hexavalent chromium pollution to both the environment and human health,chromium contamination has attracted widespread attention from the public and government.Therefore,chromium pollution in soil has become an urgent environmental issue and the remediation of chromium contaminated soil is a critical issue.In our study,mechanochemical reduction method was carried out to remediate the hexavalent chromium contaminated soil with aim of decreasing its toxicity,mobility and leachability.The main content of this dissertation are formed by three parts as follows.?1?The effectiveness of mechanical treatment was evaluated by analyzing corresponding leachable fraction obtained through extraction procedure for leaching toxicity?Sulphuric acid&nitric acid method?.The results indicated mechanical treatment can immobilize the hexavalent chromium in soil,and effectively reduce the hexavalent chromium concentration in leachate.Under a milling time of 6 h,milling speed of 500 rpm,ball-to-powder ratio of 12,the hexavalent chromium leaching concentration significantly decreased from 453.16 mg·L^-11 to 0.29 mg·L^-1,lower than the regulatory limit of 5 mg·L^-1.However,the“solidification capacity”of mechanical treatment is limited.In the case of heavy hexavalent chromium contaminated soil,leaching concentration of hexavalent chromium cannot meet the regulatory standard only through ball milling.?2?In order to achieve the stabilization process,we investigated the mechanochemical reduction effectiveness of four reductants,including FeSO₄·7H₂O,Na₂S₂O₃·5H₂O,CaSx,Na₂S.Taking the high efficiency into consideration,sodium sulfide?Na₂S?was considered to be the optimal reducing agent.Additionally,the relationship between parameters and effectiveness was studied.These parameters include:milling time,milling speed,ball-to-powder ratio and Na₂S dosage.Under a milling time of 2 h,milling speed of 500 rpm,ball-to-powder ratio of 14,Na₂S dosage of 5%,the hexavalent chromium leaching concentration significantly decreased from 663.98 mg·L^-11 to 0.84 mg·L^-1,much lower than the regulatory limit of 5 mg·L^-1.?3?The soil sample before and after mechanochemical treatment was characterized by Scanning electron microscopy?SEM?,Laser particle size analyzer,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?.Meanwhile,Modified BCR sequential extraction procedures was carried out to determine the speciation transformation of Cr in soil.On the basic of experimental results,some conclusions and speculation can be made:Firstly,the BCR sequential extraction procedure revealed that speciation changes of Cr are the root cause for the reduction of leaching concentrations.Secondly,some phenomena,such as the particle refinement,forming stable aggregates,and the defects of crystalline,may be ascribed to explain the immobilization induced by mechanical action.Particle refinement process can provide more adsorption sites for chromium,stable aggregates can encapsulate the chromium,and the defects of crystalline can offer opportunities for chromium to diffuse into the crystal lattice.Lastly,the results of XPS suggested that Cr???can be reduced to Cr???under mechanical force.