Study On The Transport Of Nano Phosphorus Iron Material In Lead And Cadmium Contaminated Simulated Soil

The lead and cadmium pollution of soil in China is large and serious,which poses a serious threat to the ecological environment and human health.nano phosphorus iron material has strong adsorption and solidification ability for lead and cadmium.At the same time,due to the magnetic effect,after repairing the soil polluted by lead and cadmium,the material can also be separated from the soil and reused after treatment.More importantly,the adsorbed and solidified heavy metals can be completely separated from the soil to achieve the goal of no environmental risk.However,in the process of remediation,the particle size of nano phosphorus iron material is small,which is similar to the colloidal properties in the soil.It will carry the pollutants in the soil to form co-transport and cause secondary pollution.In this paper,nano phosphorus iron material（nHAP/Fe）is taken as the research object to study the transport characteristics and retention mechanism of nano phosphorus iron material in simulated soil and the interaction between the material and the transport behavior of lead and cadmium.It has important engineering application value for the application of nano phosphorus iron material in the in-situ remediation of lead and cadmium contaminated soil,and can provide a theoretical basis for the transport behavior and application safety evaluation of nano phosphorus iron material in the soil environment.In this paper,nano phosphorus iron material were prepared by ultrasonic-sol-gel method with independent intellectual property rights.The materials were characterized by BET,TEM,XRD,FTIR and hysteresis loop methods.The results show that nHAP/Fe has high specific surface area and pore volume.The particles are ellipsoidal,with a long half axis of 5.8nm and a sub half axis of 1.6nm.nHAP/Fe with different iron phosphorus ratio has obvious Fe-O group characteristic peaks.Fe₃O₄is successfully loaded,nHAP/Fe has good magnetic properties and the saturation magnetization of nHAP/Fe1,nHAP/Fe2 and nHAP/Fe3 are 2.02,3.34 and 5.88 emu/g respectively.The adsorption process of lead and cadmium by nano phosphorus iron material is more in line with the quasi second-order kinetic model.The adsorption of Pb²⁺and Cd²⁺involves three processes:membrane diffusion,particle diffusion and chemical adsorption.The isothermal adsorption of Pb²⁺by phosphorus iron material is closer to the Langmuir model,and the reaction is dominated by dissolution precipitation,coprecipitation and ion exchange.The fitting results of Langmuir and Freundlich models for the adsorption of Cd²⁺have high correlation coefficients.The adsorption mechanism of Cd²⁺by materials has the synergistic effect of ion exchange,surface complexation and other adsorption processes.In this paper,the transport test-bed of saturated column and unsaturated column with external magnetic field was built,and the iron phosphorus ratio of nano phosphorus iron material and the effect of external magnetic field on the transport of material in simulated soil were studied.The results show that the iron phosphorus ratio of nano phosphorus iron material inhibits its transport ability in the simulated soil,and the external magnetic field promotes the transport ability of the material in the simulated soil;Compared with nHAP/Fe1,the maximum outflow ratio and mass recovery of nHAP/Fe3 are reduced by 70%and 16.59%respectively.The hydrodynamic diameter of the material increases from 447 nm to 837 nm,and the increase of hydrodynamic diameter is easy to be intercepted;When the external magnetic field is 2000 Gs,the maximum outflow of nHAP/Fe1,nHAP/Fe2 and nHAP/Fe3increases by 10.5%,13%and 18%respectively,and the mass recovery increases by 6.58%,7.75%and 3.96%respectively.The study on the effect of suspension chemical conditions on the transport behavior of nano phosphorus iron material in saturated and unsaturated simulated soil shows that:Humic acid promotes the transport ability of nano phosphorus iron material.When HA concentration increases from 1mg/L to 20mg/L,the maximum outflow ratio of nHAP/Fe1 increases from0.05 to 0.29,the maximum transport distance L_maxincreases from 0.46m to 1.15m.HA increases the negative charge on the surface of nano phosphorus iron material and river sand,the electrostatic repulsion between nano phosphorus iron material particles（C-C）,material and solid-liquid interface（C-SWI）and material and gas-liquid interface（C-AWI）,and enhances the stability of suspension;Acidic soil has little effect on the transport behavior of nHAP/Fe1,but under alkaline conditions,the negative charge of the solution increases,and the particles are not easy to agglomerate,so they are easy to migrate;Ionic strength inhibits the transport ability of nano phosphorus iron material,When the IS concentration increased from 1 m M/L to 100 m M/L,the maximum outflow ratio of nHAP/Fe1 decreased from 0.15 to0.06,the maximum transport distance L_maxdecreased from 0.75 m to 0.46 m.With the increase of ionic strength,the zeta potential of nano phosphorus iron material and river sand decreases,the particle electric double layer is compressed,the electrostatic repulsion between nano phosphorus iron material and the interface decreases,and the Van der Waals gravity increases,resulting in the increase of material retention;The decrease of simulated soil water saturation inhibits the transport ability of nano phosphorus iron material.The existence of gas phase enhances the pore strain and reduces the pore throat,which makes it difficult for particles to pass through.At the same time,it produces potential retention sites related to gas-liquid interface（AWI）and gas-solid-liquid（AWS）surface.In addition,in unsaturated simulated soil,nano phosphorus iron material can also be retained in a thin water film wrapped with solid particles,As a result,the retention of nano phosphorus iron material on simulated soil increases and the transport ability decreases.The paper also carried out the research on the interaction between nano phosphorus iron material and the transport behavior of Pb and Cd in simulated soil.The results show that the coexistence of lead,cadmium and coexistence of lead,cadmium inhibits the transport of nano phosphorus iron material,and the inhibitory effect of lead on the transport of nano phosphorus iron material is stronger than that of cadmium.Most of lead and cadmium react with nHAP/Fe1 to increase the particle size,reduce the amount of surface negative charge,reduce the electrostatic repulsion,increase the Van der Waals gravity,and the particles are easy to be intercepted.Nano phosphorus iron material significantly inhibits the transport behavior of Pb and has little effect on the transport behavior of Cd.Compared with no nHAP/Fe1,the maximum effluent concentration and mass recovery of Pb in the effluent decreased by 99%and 97%respectively in the presence of nHAP/Fe1.Most of the lead was adsorbed on the surface of nano phosphorus iron material and intercepted by simulated soil;In the presence of nHAP/Fe1,the maximum effluent concentration and mass recovery of cadmium were only7.1%and 3.6%lower than that of cadmium alone;When lead and cadmium coexist,the mass recovery of Cd and Pb increases by 57.71%and 29.87%respectively compared with that of lead and cadmium alone,When lead and cadmium coexist,nHAP/Fe1 significantly promotes the transport ability of Cd and Pb,indicating that the more kinds of heavy metals in the soil,the stronger the ability of nano phosphorus iron material to promote the transport of heavy metal ions,and the greater the risk of pollution to soil and groundwater;The desorption rates of lead and cadmium on nHAP/Fe1 are 38.88%and 46%respectively.In practical application,it should be considered that the desorption of lead and cadmium from the material surface causes secondary pollution to the soil environment.