Washing Heavy Metals From Contaminated Soil By Sodium N-lauroyl Ethylenediamine Triacetate(LED3A)

With the rapid development of industry,agriculture and urbanization,soil pollution is becoming more and more serious.Contaminated soil could cause the accumulation of toxic and harmful substances in plants and animals,threatening human health through the food chain.Thus,it is urgent to remedy the contaminated soil.Chemical leaching is widely concerned in soil remediation,where less time is need and high efficiency can be expected.Moreover,chemical leaching can be used with other remediation technologies.A suitable choice of leaching reagents is the key to the efficiency of remediation of contaminated soils,which should be high efficient and environmental friendly.Many leaching agents,such as acid,alkali,salt,chelating agent,and surfactant,etc.,have been studied for the remediation of contaminated soils.Although acid,alkali and salt can effectively remove contaminants,most of them can result in the destruction of soil structure and even secondary pollution.Chelating agent and surfactant are paid much attention by many scholars in recent years.On the one hand,chelating agent is only efficient for heavy metal contaminated soil,and can not effectively remove organic compound in co-contaminated soil.On the other hand,the removal efficiency of heavy metals by traditional surfactant is not good.Therefore,the search for an agent with the functions,chelating heavy metals and solubilizing organic compounds and being environmental friendly,is urgent to chemical leaching remediation of contaminated soils.Novel chelating surfactants not only have all the properties of ordinary surfactants,but also have the ability to chelate heavy metal ions.These surfactants are easily biodegraded,are not toxic to mammals and aquatic organisms,and will not cause secondary pollution.In this paper,sodium N-lauroyl ethylenediamine triacetate(LED3A)was used as the leaching agent,loess in northwest china as the tested soil,and Pb and Zn as the representative pollutants.The mechanism of LED3 A,soil,Pb and Zn were studied systematically.The result could provide theoretical support for the utilization of LED3 A in remediation of contaminated soil,and provide a new technical approach for chemical leaching of contaminated soil.The main research methods and conclusions are as following.(1)The influences of different factors on the stability of LED3 A micelles were investigated by Zeta potential method.The results showed that when the concentration of LED3 A reached 7000 mg·L-1,the absolute value of Zeta potential of LED3 A micelles was the largest and the stability of micelles was the best.The effect of pH on the stability of LED3 A micelles was weak and the optimum pH value was 9.K+ was more likely to make LED3 A micelles stable than Na+,and the influence pattern of Mg2+ on the stability of LED3 A micelles was opposite to those of K+ and Na+.(2)The effects of LED3 A on single and competitive adsorption of Pb and Zn in soils were studied using batch experiments.The experiments of the effect of LED3 A on the adsorption of single Pb and Zn showed that LED3 A could weaken the adsorption of Pb and Zn on soil.The chelating ability of LED3 A micelles was larger than that of LED3 A monomer.The experiments of the effect of LED3 A on Pb and Zn competitive adsorption showed that,in LED3A-soil-Pb-Zn system,the Pb and Zn competitive adsorption processes were similar to those in soil-Pb-Zn system.The presence of LED3 A led to that the equilibrium time of Pb and Zn competition decreased by 2 h and 4 h,respectively.The presence of LED3 A the maximum adsorption capacity of Pb and Zn decreased by 2.56 and 4.97 g·kg-1,respectively.The effect of LED3 A on the competitive adsorption capcity of Pb,Zn on soil was Pb> Zn.(3)The elution effects of LED3 A on single Pb and Zn contaminated soils and Pb-Zn co-contaminated soils were studied by batch experiments.The results showed that the kinetics of elution of single contaminated soil with different concentrations of LED3 A was in accordance with the pseudo-second-order equation model.When the concentration of LED3 A was greater than 3.0 g·L-1,the optimum elution time should be less than 12 h.The maximum removal rates of Pb and Zn were 55.1 % and 65.1 %.The elution of Pb and Zn was endothermic process,and the optimum elution concentration at room temperature was 7.0 g·L-1.The optimum pH was 9.Both K+ and Na+ reduce the removal rate of Pb and Zn by LED3 A,and Na+ had a greater effect on the removal rate.After the elution of LED3 A,the proportion of acid extractable fraction of Pb and Zn was greatly reduced.The proportion of Pb and Zn in the other three fractions(reducible fraction,oxidizable fraction,residual fraction)showed an increasing trend.Acid extractable fraction heavy metals are not only the most easy to remove fraction,but also the first one to be removed.The results of the elution of Pb-Zn co-contaminated soil showed that the time of Pb elution equilibrium was longer than that of Zn,and the elution process conformed to the pseudo-second-order equation model.The competitive chelating ability of LED3 A monomers to Pb and Zn was related to temperature.The increase of temperature was favorable for LED3 A elution of contaminated soil.After elution,the proportion of acid extractable fraction decreased and residual fraction heavy metals increased.Intra-particle diffusion was the rate controlling step of LED3 A eluting Pb and Zn,but it is not the only control mechanism.(4)The effects of different flow rates on LED3 A leaching single and compound Pb,Zn contaminated soils were studied by soil column leaching experiments under dynamic non-equilibrium conditions.The results of single leaching experiment showed that the maximum concentrations of Pb and Zn decreased in leacheate with the increase of the leaching rate.The maximum cumulative removal rates of Pb and Zn were 50.2 % and 60.3 %,respectively.With the increase of leaching depth and the increase of leaching rate,the removal rates of Pb and Zn decreased.The order of removal rate of Pb and Zn in each fraction was related to the leaching rate and depth.The proportion of Pb and Zn in the acid extractable fraction at different depths after LED3 A leaching was significantly reduced,and affected by the flow rate.The proportions of the other three fractions have increased to varying degrees,and affected by the flow rate was low.Therefore,low flow rate leaching can effectively reduce the leaching risk of Pb and Zn in single contaminated soil.Co-contaminated soil leaching results showed that the symmetry of Pb and Zn leaching curves was better than that of single contaminated soil.The chelating ability of LED3 A to Zn was stronger than that of Pb in the competitive system.The maximum removal rates of Pb and Zn at low flow rate were 48.9 % and 52.9 %,respectively.The removal rate of heavy metals in each form is similar to that of leaching single contaminated soil.But there was a phenomenon in the lower column that is Pb and Zn re-adsorption on the soils.The phenomenon of re-absorption was weakened with the increase of leaching rate.The effect of leaching rate on the organic binding fraction,residual fraction Pb and acid extractable fraction Zn at different depths were obvious.While the effect of acid extractable fraction,oxide bound fraction Pb and other fraction of Zn was not obvious.Therefore,the migration of heavy metals Pb and Zn in co-contaminated soils decreased significantly after LED3 A leaching.In summary,the new chelate surfactant LED3 A showed excellent chelating properties for heavy metals Pb and Zn.The removal rate of Pb and Zn was higher during the process of repairing Pb and Zn single and complex contaminated soils by elution and leaching.The environmental risk of heavy metals in soils after leaching is significantly reduced.Therefore,LED3 A can be used as an environment-friendly leaching reagent for chemical leaching repair technology for heavy metals contaminated Soils.