Application Of Reduction And Stabilization Technology By Iron-based Substance In The Remediation Of Chromium Contaminated Soil

A comprehensive investigation was conducted on total chromium and chromium(?)in soil with samples from a tannery-sludge landfill in northeastern Zhejiang.The proposed remedy for contaminated soil in this study is coupling chemical reduction and stabilization.The experiment was designed to explore and optimize the reduction eff-iciency with corss-combination of four reducing agents(namely FeS,FeSO4,Fe3O4 and nZVI)and three stabilizers(namely quartz sand,kaolin and limestone).Different conditions,including initial dosage,pH,initial moisture and reaction time,were also investigated.Reduction performance was evaluated based on total chromium and chromium(?)concentration in remedied soil leachate.The main conclusions are as follows:(1)For chemical reduction,results showed that the best reducing agent was FeSO4,followed by nZVI,Fe3O4,and FeS.Specifically,the best mole ration of agent to chromium(?)was three for Fe3O4 and FeS,five for FeS and nZVI.Maximum reduction was achieved in one day for Fe3O4 and FeS,while three days for FeS and nZVI.Lowering soil pH by adding dilute hydrochloric acid will promote chromium(?)reduction.nZVI was influenced the most,with reduction increased from 59.4%(pH=6.2)to 91.8%(pH=5.4).Fe3O4 ranked next,improving from 6.4%to 16.5%.Little improvement was observed with FeSO4 and FeS,with 85.8%to 93.9%and 5.5%to 12,8%respectively.The reducing ability of nZVI improved with increase of soil moisture,while no observable correlation between soil moisture and FeSO4.(2)Three stabilizers(quartz sand,kaolin and limestone)were coupled with above reducing agents.Based on the removal of soil leachate,kaolin was the best,followed by limestone,quartz sand was the worse.The groups of nZVI-quartz sand and nZVI-kaolin achieved better reduction efficiency of Cr(?)in soil.All groups,except for the one with a mass ratio of 10%kaolin addition,had a Cr(?)contamination level exceeding 30 mg/kg,which is the standard of construction land(GB36600-2018)Meanwhile,leachate of all groups with FeSO4?10%limestone,nZVI+10%limestone,nZVI?20%limestone,FeSO4+kaolin and nZVI+kaolin met the standard of hazardous waste(GB5085.3-2007)where restrictions for TCr is 15mg/L and Cr(?)is 5mg/L.In conclusion,the best remedy recipe is nanometer zero-valent iron dosing with a mole ratio(to chromium(VI))of 5,kaolin with a mass ratio of 0.2,with pH=5.4 and 80%soil moisture.However,in the experiment,the soil pH dropped from 7.6 to 6.3 and organic contents dropped from 2.8%to 2.1%,indicating this recipe would inevitably impact soil physical-chemical properties.(3)The speciation analysis of soil chromium before and after remediation showed that chromium species in the exchangeable form and carbonate-bound form decreased,but the residual form was increased.Recipe with FeSO4 increased Fe-Mn-oxide compounds in soil,while it with nZVI reduced them.Limestone,as a stabilizer,observably impacted the organic-bounded species.