Release,Transport,and Co-Transport Of Rare Earth Elements(REEs)in Porous Media

Due to the increasing demand for rare earth elements(REEs),the mining of REEs has expand.The traditional mining process has caused serious damage to the ecosystem of the mining area.It has also caused REEs transport to the surrounding water and soils,causing further pollution and endangering the health of surrounding residents.In recent years,the biogeochemical behavior of REEs in the environment has attracted widespread attention.But most of these researches mainly focused on the transport behavior of REEs nanoparticles(e.g.,cerium dioxide nanoparticles)in porous media or investigated the concentration distribution and association of REEs in soils to assess potential REEs transport in soil.However,the research on the mechanism of the release and transport of REEs in soil and the interface transport mechanism in porous media is still few and unsystematic.Therefore,investigating the transport mechanism of rare earth elements(REEs)in porous media has critical scientific significance for the ecological restoration of REEs mining areas and the prevention and control of pollution in the surrounding environment.This study investigated the transport,co-transport,and competitive retention of three REEs: light REE(La),middle REE(Gd),and heavy REE(Yb),as well as the co-transport of REEs and kaolinite(a representative clay mineral)in porous media.Based on the results of previous studies,the remobilization mechanism of REEs in contaminated soils was explored by simulating the flow interruption and chemical disturbances during rainfall.Through experiments and numerical modeling,the difference in transport behaviors of different ionic REEs were investigated,and the mechanisms of release and remobilization of REEs were analyzed and discussed.The main contents and conclusions are listed as following:1.This study investigated the transport,co-transport,and competitive retention of three ionic REEs(La(a light REE),Gd(a middle REE),and Yb(a heavy REE))in porous media.Through the breakthrough curves(BTCs)and retention profiles(RPs)of saturated quartz sand(QS)column experiments,it was found that a)All ionic REEs exhibited considerable breakthrough and slight retention with almost uniform shapes in quartz sand(QS)owing to the weak affinity of ionic REEs to QS.b)The breakthrough of REEs in all experiments followed the order of La > Gd > Yb,indicating that REE breakthrough increased with decreasing atomic number.c)The same elements exhibited higher breakthrough during the co-transport than single transport.Mathematical modeling indicated that the retention of REEs in QS was a predominantly kinetic process,whereby competitive blocking was the dominant mechanism for the enhanced breakthrough of REEs during co-transport,as compared to single transport.2.The co-transport of REEs and kaolinite(a representative clay mineral)in porous media was studied.The co-transport of REEs and kaolinite demonstrated that kaolinite has a slight influence on the transport of REEs in QS under adsorption kinetics.However,REEs greatly inhibited the transport of kaolinite in QS.Through the adsorption experiment,it was further found that although the adsorption of REEs onto kaolinite was weak under adsorption kinetics,the presence of REEs ions leads to a decrease in the electrostatic repulsion between kaolinite and QS,resulting in the retention of kaolinite in QS.3.The release and remobilization mechanism of REEs in REEs mining area and surrounding contaminated soil during simulated rainfall was studied.In REEs contaminated soils,under the chemical disturbance during rainfall(such as ion exchange and ionic strength reduction),the release and remobilization of rare earth elements mainly through two ways.(1)Release and remobilization caused by ion exchange process.(2)Soil colloids facilitated the transport of REEs.The remobilization of REEs is mainly caused by the role of ion exchange in the mining areas soil which has low clay and low organic matter content.In contaminated farmland soil,light REE(LREE)and middle REE(MREE)also mainly release under the effect of ion exchange,while heavy REE(HREE)will release through the facilitate of soil colloids when the ion exchange is weak.Furthermore,the flow interruption makes it more difficult for REEs remobilization in contaminated farmland soil.This study is expected to provide valuable insights into the transport behaviors of REEs in porous media provide a theoretical basis for preventing the diffusion of REE contaminates.