The Stabilization Of Chromium-contaminated Soil In Electroplating Plant Site By Using Sulfides And Its Long-term Stability

The soil is generally heavily contaminated with Chromium?Cr?in and surrounding the electroplating plant sites in our country.It is aicidic,and usually contained with a large amount of hexavalent Chromium?Cr????.Great demands for remediation of the soil are presented due to its high risk to hunman health and environemt.Stabilization is the main technique used to remediate the Cr-contamined soil.This technology changes the speciation of Cr,but can not reduce the total amount of Cr mass in soil.Therefore,the long-term stability of stabilizing effect after treatment becomes the technical focus of controversy,and it is the long-term stability that hinders the field application and the adminatrative recognition of the technique.As a sort of reductive chemical agent,sulfides are commonly used to stabilize the Cr in soil.It is,however,rarely reported that they are used to treat the Cr-contaminated soils in electroplating plant sites.In this paper,the acidic Cr-contaminated soils in electroplate plant sites are used and treated by the use of three sulfides,as stabilizers,including calcium polysulfide?CPS?,sodium sulfide?Na₂S?and sodium pyrophosphate?Na₂S₂O₅?.The stabilizing effects are studied and the mechanisms for stabilization of Cr in the acidic soil are analyzed.The long-term performance of the stability of Cr in the treated soil are further tested by:?1?leaching of simulated acid rain,?2?wetting and drying alternately,and?3?freezing and thawing alternately.The change of soil properties,the leaching toxicity,and the fraction distribution of Cr in the soil are analyzed accordingly for the tests.Scientific data are expectedly provided by this study for the sulfides-based stabilization of acidic Cr-contaminated soil in electroplating plant sites.The effects of sulfides on the valence,leaching toxicity and fraction distribution of Cr in acidic soils of electroplating plants were first investigated.The results showed that the reduction of Cr???in the soil was made by the tested sulfides with an ability of CPS>Na₂S₂O₅>Na₂S.when the ratio of sulfides to Cr???in molar was 3,the Cr???in the tested soil was reducted by CPS,Na₂S₂O₅ and Na₂S in an efficiency of 99.2%,74.5%and 55.6%,respectively.Among the three sufides,CPS showed the highest stabilizing effect on the total chromium?TCr?and Cr???.When the molar ratio of CPS to TCr and Cr???was 3,their concentration in the leachate of treated soil was reduced by 99.1%and 99.8%,respectively.With increase of the dosage of CPS and Na₂S?in molar?,the leached concentrations of TCr and Cr???from treated soil were significantly droped.With increase of Na₂S₂O₅,however,the leached Cr???was decreased while the TCr increased.The exchangbale fraction of Cr in the treated soil decreased with increase of CPS and Na₂S,but increased with the dosage increase of Na₂S₂O₅;carbonate fraction increased with the CPS dosage,but decreased with the dosage increase of Na₂S₂O₅ and Na₂S;other fractions changed insignificantly with the sulfides and their dosage.The difference significance analysis demonstrated that the change of exchangeable Cr was the main reason for the change of TCr leached from the treated soil with sulfides.The leaching characterics of Cr in the treated soil were then tested by use of leaching fluids with different pH values,and by leaching action of simulated acid rain.The results showed that the acid-buffering capacity of the CPS-treated soil became larger than the untreated soil.The leached concentration of Cr???increased with the increase of pH value of leaching fluids,while the trivalent chromium?Cr????decreased.Under leaching action of simulated acid rain?SAR?,the TCr and Cr???in CPS-treated soil released roughly in two time periods:rapid release and stable release.The more dosage of CPS used in the treatment,the shorter time was taken to reach the stage of stable release.The cumulative release of both TCr and Cr???in CPS-treated soil can be described,at the rapid stage and stable stage,by the modified Elovich equation and the first order dynamic equation,respectively.When the leaching of SAR lasted for 70 days,the exchangeable fraction of Cr in the CPS-treated soil was reduced significantly,and the iron-manganese oxide fraction was partly changed into carbonate fraction.The effects of wetting-drying and freezing-thawing on soil properties,leaching toxicity and fraction distribution of Cr in the CPS-treated soil were finally studied.The results showed that the alternate freezing-thawing increased the pH value and decreased the Eh value of soil.With the increase of freezing-thawing cycle,the leached Cr from CPS-treated soil increased,but lower than that of the un-frozen soil.Under the action of alternate freezing-thawing,the exchangeable fraction and carbonate fraction of Cr in the soil changed to more stable Fe-Mn oxides and organic fraction,but the residue fraction changed insignificantly.The alternate drying-wetting increased the Eh value of the soil but the pH value changed insignificantly.Under the action of alternate wetting-drying,the leaching toxicity of Cr was lower than the soil with no wetting-drying,and the change of the Cr fraction distribution is consistent with that in the freezeing-thawing tests.