Remediation Cu And Cd Contaminated Soil Using Switchable Surfactant Foams

Surfactant leaching and repairing heavy metal contaminated soil has proved to be one of the most effective soil remediation technologies.Because it is difficult to separate surfactants from pollutants,high cost,and easy to cause secondary pollution.Therefore,it is difficult to apply to large-scale use.The researchers proposed to use the switch surfactant solution instead of the conventional surfactant solution as the eluent.The migration and movement of the eluent in the soil is easily affected by the pore effect.It is difficult for the eluent to cover the entire contaminated area.Resulting in waste of eluent and reduced eluent efficiency.This paper uses switch surfactant foam instead of switch surfactant solution as the eluent to wash and repair the contaminated soil,so as to improve the remediation efficiency of the contaminated soil by the eluent.In this paper,Cu and Cd are selected as representatives of heavy metals in contaminated soil.At the same time,CO₂/N₂switch-type surfactant N’-dodecyl-N,N-dimethylaceta-midine bicarbonate（DDAB）,surfactant saponin and organic acid citric acid are used as leaching agents to perform shaking elution test and soil column leaching test,to explore the removal effect of leaching agent on heavy metals Cu and Cd in contaminated soil.The best elution conditions and the best mixture ratio of the eluent system were studied.At the same time,the effects of single eluent and complex eluent on the remediation of Cu and Cd contaminated soil and the CO₂/N₂switch response of DDAB were studied.Through conductivity experiment and surface tension change experiment.The results show that DDAB has good CO₂/N₂reversible conversion characteristics,which can realize the reversible conversion of the switching surfactant substance between the active state and the inactive state,thus improving its effective utilization rate.In addition,the paper further changed the leaching solution system from the solution form to the foam form for soil leaching and repairing.Compared with the DDAB solution leaching and repairing efficiency,the selected DDAB concentration was 0.001 mol·L^-1.The results showed that:The cumulative eluent removal rate of DDAB solution and DDAB foam for Cd was 10.5%and 23.6%,respectively.The eluent efficiency of foam form was significantly higher than that of solution form.Through optimizing the experimental conditions of concussion elution,the optimal citric acid concentration was 0.04 mol·L^-1,the mass fraction of saponin was 5%,the optimal volume ratio of citric acid to saponin was 1:5,and the solution p H was 8.Under this condition,the elution rates of citric acid to Cu and Cd were 60%and 46%;the elution rates of saponin to Cu and Cd were 35%and 34%;When the concentration of citric acid was 0.04mol·L^-1,the mass fraction of saponin is 5%and the volume ratio is 1:5,the elution rates of Cu and Cd are 61%and 48%,respectively;When the concentration of citric acid is 0.04 mol·L and the mass fraction of saponin is5%,the optimal volume ratio of citric acid to saponin is 1:5 and the solution p H was 8,the elution removal rates of Cu and Cd are 74%and 63%,respectively.In addition,further discussion is applied to the leaching and repairing of contaminated soil after the leaching solution system is changed from the solution form to the foam form.The results show that the cumulative leaching removal rates of saponin solution and saponin foam to Cu are 23.6%and 34.2%;the leaching removal rates of saponin solution and saponin foam to Cd were 24.4%and 32.6%,indicating that the eluent efficiency of foam form was significantly higher than that of solution form.The optimal citric acid concentration is 0.04 mol·L^-1,the mass fraction of saponin is 5%,the optimal volume ratio of citric acid to saponin is 1:5,the p H is 8,the concentration of DDAB is 0.001 mol·L^-1.Next,a series of soil column leaching experiments were carried out.The results showed that when DDAB was mixed with citric acid,the cumulative leaching rate of Cu for DDAB and citric acid compound solution and DDAB and citric acid compound foam were 41.2%,56.7%;the cumulative leaching rates of DDAB and citric acid compound solution,DDAB and citric acid compound foam on Cd were 35.9%and 40.7%.In contrast,the overall leaching and repairing effect of the leaching solution of the mixed system of citric acid and DDAB is the best.The amount of Cu and Cd in the leaching solution of the leaching solution are gradually increased with the increase of the cumulative volume.When the compound solution was used to wash the soil column,the mode of action was synergistic.When the system eluent was changed from solution to foam,the efficiency of the foam eluent for Cu and Cd was significantly increased.The results showed that the foam washing efficiency of the mixed surfactant system was higher than that of the leaching solution.From the analysis of the experimental results of the conductivity change of the leaching waste liquid in the compounding system,after the leaching liquid of the DDAB compounding system passes through the polluted soil column,the leaching waste liquid still has good CO₂/N₂reversible conversion characteristics,which can realize the reversible conversion of the switching surfactant substance between the active state and the inactive state.