Clogging And Main Influencing Factors For The In Situ Remediation Of Cr(Ⅵ)-contaminated Groundwater By Sodium Pyrosulfite

Groundwater is an important water sources for agricultural irrigation and domestic water in China.However,the hexavalent chromium[Cr（Ⅵ）]pollution of groundwater has become more and more serious in recent years,so the remediation work is urgent.In-situ chemical reduction is a commonly used remediation technology for Cr（Ⅵ）-contaminated groundwater,and its effectiveness is affected by not only the type of reductants,but also the effectiveness of mass transfer process.During the remediation,the reductants usually generate precipitates,which cause the problems of plugging pores,forming restoration dead zone and hindering mass transfer.Therefore,it is of great significance to study the clogging phenomenon of different reductants in the remediation of Cr（Ⅵ）-contaminated groundwater.Sodium pyrosulfite（Na₂S₂O₅）has the advantages of low cost,acidic aqueous solution and well stability,this research aims to compare the clogging phenomena in soil pores caused by Na₂S₂O₅,Fe SO₄,Na₂S,Na₂SO₃ and their combinations,and focus on exploring the clogging mechanisms of Na₂S₂O₅ during the in-situ remediation of Cr（Ⅵ）-contaminated groundwater.This research mainly includes three parts:compare the clogging phenomena of reductants by infiltration column experiment;explore the factors aggravating clogging by infiltration column experiment of calcium matrix;explore the factors alleviating clogging by the complexation experiments of S₂O₅^2-with Cr（III）/Fe（II）.The main conclusions are as follows:（1）The infiltration column experiment of soil showed that the reaction rates of Na₂S and Na₂SO₃ with Cr（Ⅵ）is very slow.Na₂S doesn’t cause soil clogging,its permeability coefficient k=6～7 mm/h,while Na₂SO₃ forms CaSO₃·0.5H₂O with available calcium in soil,resulting in the decrease of permeability coefficient（k=1～2mm/h）.The initial permeability coefficient of Na₂S₂O₅ was bigger than that of Fe SO₄.But the permeability coefficient of Fe SO₄ increased to 1.5 mm/h with the time due to the CO₂ gas produced by the dissolution of soil calcium carbonate,opening some water channels in soil columns.The permeability curve of Fe SO₄+Na₂S₂O₅ is similar to Na₂S₂O₅,and the clogging was serious.Fe SO₄-Na₂S can generate Fe S（s）aggravating clogging.In contrast,the combined remediation of Fe SO₄-Na₂S₂O₅（k=1.5～3.0 mm/h）can not only improve the efficiency of mass transfer by preventing Fe（III）precipitation from wrapping Cr（Ⅵ）,but also alleviate soil clogging.Its effect of anti-clogging is better than that of Na₂S₂O₅,Fe SO₄ Fe SO₄-Na₂S and Fe SO₄+Na₂S₂O₅.（2）The infiltration column experiments of calcium matrix show that there are two main factors for clogging.One is the precipitates of Fe（OH）₃ and Cr（OH）₃ produced by the reaction of reducts and Cr（Ⅵ）;the other is the composition change of soil calcium caused by reductants.The formation of CaSO₃·0.5H₂O（s）is the main reason for the clogging of Na₂S₂O₅.For the soils with low calcium contents,the clogging degree of Na₂S₂O₅ is lighter than Fe SO₄ because there is no Fe（OH）₃ occurring.While for the soil of high calcium contents,Na₂S₂O₅ can convert calcium carbonate into CaSO₃·0.5H₂O（s）,resulting in more serious clogging than Fe SO₄.（3）The complex experiments of S2O₅^2-with Cr（III）/Fe（II）show that there are complexation between Na₂S₂O₅ and Cr（Ⅲ）/Fe（II）.The complexation of Na₂S₂O₅-Cr（III）is actually the complexation of Cr³⁺with SO₃^2-which is the decomposition product of S₂O₅^2-.The coordination number of SO₃-Cr（III）is 6.The complexation reduces the amount of Cr（OH）₃ precipitate and alleviates clogging during remediation.Although the complexation of Na₂S₂O₅-Fe（II）is weak,it can also reduce the Fe（OH）₃ precipitation,meanwhile inhibit the formation trend of CaSO₃·0.5H₂O（s）,resulting in the alleviation of the clogging during the remediation of Cr（Ⅵ）-contaminated groundwater.In summary,the mechanisms affecting the clogging of Na₂S₂O₅ include the aggravation caused by the formation of CaSO₃·0.5H₂O and the alleviation caused by the complexation of S₂O₅^2-with Cr（III）/Fe（II）.It means for the soil and groundwater with low calcium contents,Na₂S₂O₅ can alleviate clogging and facilitate mass transfer of reductants,and it is a prospective in-situ reductant.But for the soil and groundwater with high calcium contents,the serious clogging caused by CaSO₃·0.5H₂O should be noticed.A combined method of Fe SO₄-Na₂S₂O₅ can be used to alleviate the clogging.