Remediation Of Cr(Ⅵ) Contaminated Soil By Chemical Leaching And Reductive Stabilization Using Natural Organic Acids

A mass of Cr（Ⅵ）contaminant was discharged into the environment by the chromium（Cr）salt production and leather industry in China,and many industrial pollution sites with serious Cr（Ⅵ）pollution were formed.The high concentration of Cr（Ⅵ）in soil poses serious threat to the surrounding ecological environment,the remediation is urgently needed.The Cr forms in Cr（Ⅵ）contaminated soil are diverse,traditional leaching or reductive stabilization technology has deficiencies,such as destroying the physical-chemical properties of soil and their incomplete reduction efficiencies.The technology integrating the advantages of the two technologies above needs to be further developed.Natural organic acids（NOAs）with good biodegradability can remove the soil pollutants through leaching and reduction process simultaneously.However,the leaching efficiency of NOAs with low concentration is not well and the combination with other leaching agents is necessary.Transition metal ions such as Fe²⁺and Fe³⁺can catalyze the Cr（Ⅵ）reduction by NOAs,but few studies focused on the remediation of Cr contaminated soil by compound leaching with NOAs.Here,the kinetics,influential factors and mechanism of slow-release iron source in promoting the aqueous Cr（Ⅵ）reduction by citric acid selected as typical NOAs,were investigated.Three typical NOAs were combined with Fe SO₄ as compound leaching agents to remediate Cr（Ⅵ）contaminated soil,respectively.In order to obtain the optimal leaching agent and stabilizer,their Cr（Ⅵ）removal performance was studied and compared with common leaching agents,and the primary influential factors were identified through batch experiments,respectively.Then the optimal combined leaching method was obtained.The surface properties and variation of Cr forms in soil were analyzed,the leaching and reductive stabilization performance of Cr（Ⅵ）contaminated soil based on the application of NOAs was explored and the Cr（Ⅵ）removal mechanism from soil was revealed.The main research conclusions are as follows:（1）As a slow-release iron source,ceramsite particles（CPs）could promote the Cr（Ⅵ）reduction by citric acid.The Cr（Ⅵ）reduction rate by citric acid is very slow,the addition of CPs increased the apparent reduction rate constants from 16.35 at p H=2.6 to44.85 mg·L^-1·d^-1.Increasing p H markedly decreased the CPs’catalytic effect on Cr（Ⅵ）reduction by citric acid and light positively influenced the reduction rate.Soluble Fe（III）dissolved from CPs was identified as the catalyst for Cr（Ⅵ）reduction by citric acid.Under light,soluble Fe（III）complexed with citric acid,and those initiated complexes yielded Fe（II）-citrate and reductive organic radicals that promote Cr（Ⅵ）reduction.In the dark,Fe（III）-citrate complexed with Cr（Ⅵ）to form ring complexes that were responsible for rapid Cr（Ⅵ）reduction.（2）The remediation method was put forward by comparing the leaching efficiencies of Cr（Ⅵ）contaminated soil.At a liquid to solid ratio of 10∶1,Cr（Ⅵ）removal rates by 0.1 mol/L malic acid,citric acid and tartaric acid at 12 hours for the soil with Cr（Ⅵ）content of 310.2 mg/kg were 55.5%,66.1%,and 70.6%,respectively.After compounding with 0.01 mol/L Fe SO₄,Cr（Ⅵ）removal rates increased by 14.4%,7.8%and 15.6%,respectively.The best Cr（Ⅵ）removal and stabilization performance was observed by combination of tartaric acid with Fe SO₄ and Na H₂PO₄,respectively.And Na H₂PO₄ did not markedly affect the physical and chemical properties of soil.For compound leaching agent,tartaric acid concentration and p H of leaching agents have markedly effects on the Cr（Ⅵ）removal in soil.After leaching for 2 hours,the highest Cr（Ⅵ）removal rate（89.5%）was obtained at p H of 1.65,0.1 mol/L tartaric acid and0.01 mol/L Fe SO₄ and with a liquid to solid ratio of 10∶1.The optimal condition of Na H₂PO₄ was leaching for 12 hours,0.1 mol/L and with a liquid to solid ratio of 10∶1.After Na H₂PO₄ leaching,then added tartaric acid and Fe SO4 directly was the best combined leaching method and the highest Cr（Ⅵ）removal rate（93.18%）was obtained.Cr（Ⅵ）and total Cr leaching toxicity of processed soil were 0 and 0.39 mg/L and far lower than the concentration limit（1.5,4.5 mg/L）of the Pollution Control Standard for Domestic Waste Landfill Sites.（3）The leaching mechanism of Cr（Ⅵ）contaminated soil by Na H₂PO₄ combined with tartaric acid+Fe SO₄ is as follows:part of Cr（Ⅵ）on soil surface was washed out by Na H₂PO₄through the ion exchange,the Cr（Ⅵ）in soil particles was released by acid-promoted dissolution and complexation of tartaric acid,meanwhile part of Cr（III）was complexed.Then the Cr（Ⅵ）in leaching solution was reduced to Cr（III）by Fe SO₄and tartaric acid catalyzed by Fe（III）.With the coaction of Na H₂PO₄,Fe（II）and Fe（III）,the exchangeable state,carbonate bound state and iron manganese oxide bound state Cr in soil was converted to more stable organic matter bound state and residual state,which reduced the bioavailability of Cr.