Synthesis Of Redox-Switchable Surfactant And Its Flushing And Separating Effects For Nitrobenzene Contaminated Soil

Soil is one of the important components in the ecosystem,which is related to the survival and growth of various organisms.In recent years,soil has been seriously polluted by Non-aqueous phase liquids（NAPLs）,among which common pollutants include nitrobenzenes,halogenated hydrocarbons,petroleum hydrocarbons and so on.These organic pollutants have low water solubility,exist in the environment for a long time,and are easily adsorbed on the surface of soil particles,making their removal more difficult.Surfactant Enhanced Remediation（SER）technology is a cost-effective remediation method for the removal of NAPLs from contaminated soil.This technology enhances the removal rate of NAPLs through the desorption,solubilization,and emulsification of pollutants by surfactants,thereby enhancing the effectiveness of traditional soil flushing remediation.However,the current technologies for handling the flushing solution containing pollutants still have shortcomings such as cumbersome operation and easy secondary pollution,which cannot effectively solve the bottleneck problem of separation and recovery of surfactants.In response to the above phenomenon,a redox-switchable surfactant,11-ferrocenyl undecanoyl trimethylammonium bromide（Fc COC10N）,was synthesized.Based on its switching reversibility,it was emulsified and demulsified with organic pollutants in a cyclic manner,achieving efficient remediation of contaminated soil flushing,pollutant separation,and surfactant recycling.The specific research contents are as follows:Firstly,the chemical structure and physical properties of Fc COC₁₀N were characterized,and the differences in various properties of Fc COC₁₀N after reversible conversion were compared and analyzed.The effects of various factors on the switching conversion rate of Fc COC₁₀N were investigated to determine its optimal conversion conditions and switching performance.Then,with nitrobenzene as the target pollutant,the conditions for the formation of stable emulsions between Fc COC₁₀N and nitrobenzene and the demulsification mechanism of Fc COC₁₀N solution induced by oxidation reaction were explored,and the emulsification and demulsification circulation ability of both were evaluated.Finally,elution experiments with different influencing factors were carried out on nitrobenzene contaminated media to comprehensively evaluate the flushing and separation effects of Fc COC₁₀N enhanced nitrobenzene heavily contaminated soil.The following main conclusions were reached:（1）The hydrophilic head group of Fc COC₁₀N is a quaternary ammonium group with good hydrophilicity,and the hydrophobic tail chain contains a redox-response group,ferrocene group,which is hydrophobic in the reduced state.At this time,Fc COC₁₀N is hydrophilic at one end and hydrophobic at the other end,with good surface activity,which is called"on".The oxidation of ferrocene groups carries a positive charge,resulting in the surfactant being hydrophilic at both ends and having poor surface activity,which is called"off".Fc COC₁₀N has a low critical micelle concentration and surface tension,and has good surface activity.It can spontaneously form stable micelles in aqueous solution with an average particle size below 10 nm.The contact angle between the Fc COC₁₀N droplet and the hydrophobic glass gradually decreases with increasing concentration,and the wettability gradually increases.Its wetting ability far exceeds that of water,and the contact angle of hydrophobic slides is reduced nearly twice compared with that of water,which can significantly reduce the interfacial tension between solid and liquid phases and change the solid surface properties from oil wetting to water wetting.Fc COC₁₀N is converted to Fc COC₁₀N⁺by oxidation,which is hydrophilic at both ends and has reduced wetting ability for hydrophobic solids,weakened surface activity,increased surface tension and CMC,disintegration of original micelles,increased number of monomer molecules in solution,and reduced apparent particle size.After reduction,all physical properties are restored to their original state.Therefore,Fc COC₁₀N is reversible under oxidation/reduction and has the reversible characteristics of oxidation/reduction.（2）The optimal dosage of oxidizer H₂O₂ and reducer hydroxylamine hydrochloride are 3 times and 11 times of Fc COC₁₀N equivalent,respectively.Excessive H₂O₂ will make Fc COC₁₀N⁺slowly change back to Fc COC₁₀N which will reduce the conversion rate of"off".An increase in the concentration of Fc COC₁₀N will increase the upper limit of the energy barrier,resulting in a lower switching conversion rate.In addition,Cl^-and SO₄^2-ions can inhibit the reduction process and reduce the"on"conversion rate,among which SO₄^2-has a stronger inhibitory effect;However,Na⁺has no significant effect on switching conversion.The switching conversion rate of Fc COC₁₀N decreases with the increase of the number of cycles,but after 5 switching cycles,its conversion rate can still reach more than 50%,so it has a good switching reversibility performance.（3）The emulsion formed by Fc COC₁₀N solution with nitrobenzene is an O/W emulsion.Stable emulsion can be formed when the ratio of water to oil is greater than2:8,and nitrobenzene can be completely emulsified when it is greater than 1:1.Stable emulsions can be formed when the concentration of Fc COC₁₀N solution is greater than1.5 mmol/L,and with the gradual increase of the concentration,the diameter of the emulsion droplets gradually decreases and the emulsion system becomes more and more stable.After Fc COC₁₀N is oxidized to Fc COC₁₀N⁺,its hydrophilicity increases,making it easy to desorb from the oil-water interface into the aqueous phase.The interfacial tension between nitrobenzene and the aqueous phase increases rapidly,and nitrobenzene droplets polymerize,resulting in demulsification.With the increase of water-oil ratio and surfactant concentration,the demulsification rate decreases somewhat,but the overall demulsification rate can reach over 90%.Fc COC₁₀N solution has good emulsification and demulsification circulation ability with nitrobenzene.After5 cycles,it can still form a stable emulsion,and the demulsification rate remains above80%.（4）When flushing contaminated soil with Fc COC₁₀N,all systems can form stable emulsions.The higher the water-soil ratio under the same media conditions,the higher the efficiency of Fc COC₁₀N solution for soil flushing,and the highest can reach 94.70%in medium sand.As the water-oil ratio increases from 3:1 to 7:1,the flushing rate of nitrobenzene by Fc COC₁₀N solution first rises and then decreases,with a maximum of94.76%.When the concentration of Fc COC₁₀N solution is within the range of 5 to 25mmol/L,the nitrobenzene elution first increases and then decreases with the increase of concentration.When the concentration of Fc COC₁₀N solution is 15 mmol/L,the elution rate can reach 97.07%.In the range of experimental investigation,different water-soil ratio and water-oil ratio systems can achieve better separation of nitrobenzene and Fc COC₁₀N solution during demulsification,so that the Fc COC₁₀N solution can be fully recycled.However,at different concentrations of Fc COC₁₀N solution,demulsification becomes increasingly difficult as the concentration increases,resulting in the inability to separate some Fc COC₁₀N and nitrobenzene.