Mechanisms Of Transport And Retention Of Tl(Ⅰ) In Water-Saturated Sand Porous Media

In recent years,thallium pollution incidents in China have attracted widespread attention.The highly toxic properties of thallium have caused serious risks to the soil and groundwater environment near thallium-containing tailings.Therefore,exploring the migration behavior of heavy metal thallium（Tl）in a porous medium environment is crucial to the environmental fate of thallium in soil and groundwater and the removal of thallium in water engineering systems.In this paper,by building a porous media migration column,the migration and retention behavior of thallium（I）in water-saturated quartz sand porous media are studied,and the two-point chemical non-equilibrium convection diffusion equation is used to fit the penetration curve of thallium and successfully predict and evaluate the behavior of thallium（I）in porous media.First,the effects of p H,ionic strength,carbonate and phosphate on the transport of Tl（I）were investigated;the results showed that the outflow ratio of Tl（I）from the column decreased with the increase of p H and the decrease ionic strength（IS）,indicating that acidic condition（p H=3.2）and higher IS（Na NO₃=10 mmol/L）are beneficial to the migration of thallium（I）.Besides,the transport of Tl（I）was impeded in the presence of carbonate and phosphate.The pre-adsorbed carbonate and phosphate on the surface of quartz sand increased the retention of Tl（I）in the column（1.365 and 1.279 times）.The fitting of the two-point chemical non-equilibrium convective diffusion equation showed that the fraction of adsorption sites（β）decreases with the increase of p H,indicating that non-equilibrium adsorption was the main adsorption mechanism of Tl（I）under high p H conditions.In addition,in order to clarify the co-transport of typical bio-macromolecule organics and Tl（I）in soil and groundwater environments,this study explored the co-transport of humic acid（HA）and bovine serum albumin（BSA）with Tl（I）.The results showed that,HA slightly inhibited the transport of Tl（I）at p H 3.2,it may be that the low p H condition was conducive to the deposition of humic acid on the quartz sand interface and increased the adsorption of part of Tl（I）.However,under most test conditions,HA promoted the transport of Tl（I）,and the promotion effect was more obvious under alkaline conditions（p H=9.8）,which is related to the Tl-complexes formation and competitive adsorption between Tl（I）and HA.Under the conditions of higher p H and lower ionic strength（Na NO3=1 mmol/L）,HA colloids can obtain higher mobility;Tl（I）complexed with HA,which would shield part of the charge of HA,which inhibit the migration of HA colloid.The migration behavior of HA colloid conformed to the prediction of classic DLVO theory.Regarding the co-transport of BSA and Tl（I）,BSA colloid deposited on the surface of quartz sand and provided more adsorption sites for Tl（I）,so more Tl（I）was trapped in the column,which hindered Tl（I）transport.Due to the large molecular weight of BSA,p H and ionic strength have little effect on the transport of BSA.A two-site chemical non-equilibrium CDE model was used to fit the breakthrough curve of Tl to evaluate the transport behavior of Tl（I）.In the presence of HA,the retardation factor of Tl（I）was higher than that of individual Tl（I）,while the retardation factor of Tl（I）in the presence of BSA was lower than the retardation factor of individual Tl（I）,which showed that transport behavior of Tl（I）can be described well.The transport of Tl（I）is significantly important for predicting and evaluating the environmental fate of Tl（I）in soil and groundwater.Manganese oxide is a common mineral in the soil environment and has a strong affinity with Tl.In order to propose effective strategies for the prevention and control of Tl pollution in soil and groundwater,this paper studied the removal and retention of Tl（I）in manganese oxide-coated quartz sand column.The breakthrough experiment results showed that manganese oxide greatly delayed the breakthrough of Tl（I）.Under the conditions of p H 7.0 and Na NO₃ 10 mmol/L,when the influent water containing Tl（I）was close to 1250 PV,the breakthrough of Tl（I）began;at about 2400 PV,Tl（I）completely penetrated;and divalent manganese ions reduced Tl（I）retention in the column.In addition,the retention of Tl（I）in the column was related to the loading of manganese on the quartz sand surface.The PV numbers of Tl（I）breakthrough in the quartz sand column with low manganese loading was reduced,while higher p H（8.0）can enhance Tl（I）retention,but Ca²⁺reduced the retention of Tl（I）in the column.SEM-EDS and XPS were used to characterize the surface of quartz sand and manganese oxide to explore the mechanism of Tl（I）retention and removal.SEM-EDS data showed that the morphology of manganese oxide on the surface of quartz sand did not change significantly before and after the dynamic experiment.Moreover,after the dynamic experiment,the EDS distribution map of Tl element highly coincided with the EDS distribution map of manganese element.This showed that the manganese oxide-coated quartz sand column has a higher retention of Tl（I）because manganese oxide adsorbed Tl（I）.XPS data showed that,after the dynamic experiment,part of the tetravalent manganese was reduced,and the average oxidation valence state of manganese was reduced and about 90%of Tl（I）is oxidized into trivalent thallium species（Tl₂O₃）.Therefore,manganese oxide had a higher adsorption and removal capacity to Tl,possibly because Tl（I）was oxidized to a less soluble Tl（III）species.In summary,p H,ionic strength,carbonate,phosphate,and macromolecular organic matter have a significant impact on the transport of Tl（I）.This may cause Tl（I）to diffuse in the polluted area,causing further environmental risks.The manganese oxide-coated quartz sand column has a strong ability to trap and fix Tl（I）,which can prevent and control the risk of Tl pollution.An in-depth understanding of the migration and retention behavior of Tl（I）in porous media can help predict and evaluate the environmental risks of Tl（I）pollution in soil and groundwater.In industrial wastewater Tl（I）removal about operations,such as filtration and regeneration,this paper can provide basic research.