Co-transport Of Biochar Nanoparticles And Rare Earth Elements In Porous Media

The extensive use of rare earth elements（REEs）in modern industry and agriculture,leading to the inevitably release of REEs into the environment,which results in the REEs contamination in soil and water and harm to the ecological environment and human health.Therefore,research on the transport of REEs in the soil is significantly important for the pollution prevention and control of REEs.As an efficient and environmentally friendly remediation agent,biochar has been widely used in the remediation of heavy metal contaminated soils in recent years,which exhibited excellent adsorption capcities for heavy metals and could inhibit the transport of heavy metals in soils.However,research on the application of biochar to the remediation of REEs contaminated soils is still limited.Therefore,studying the co-transport behavior of biochar nanoparticles and REEs in porous media provides prospective results for the application of biochar nanoparticles to the remediation of REEs contaminated sites,and has important scientific guidance for the prevention and control of REEs pollution.In this study,rice straw biochar nanoparticles and three representative light,medium,and heavy REEs（La,Gd,and Yb）were selected as the research objects.Research on co-transport and adsorption of biochar nanoparticles with different REEs were carried out.Futhermore,since the aging effect of biochar will occur over time,therefore,the aging effect of biochar nanoparticles on the co-transport with REEs was further conducted.First,the adsorption of REEs onto biochar nanoparticles are explored based on batch experiments,adsorption models,FTIR,and other characterization methodsd.The results showed that the adsorption of REEs by biochar is mainly through surface multi-molecular adsorption due to lectrostatic attraction,ion exchange,and functional group complexation.In addition,the adsorption capacities of biochar nanoparticles for the three REEs followed the order of Yb>Gd>La.Second,the co-transport of biochar nanoparticles and REEs in saturated porous media was explored.The retention of biochar nanoparticles increased during co-transport due to the decrease in surface charge of biochar nanoparticles after adsorption with REEs.The transport of REEs followed the order of La>Gd>Yb in co-transport with biochar nanoparticles mainly due to the heterogeneity of the surface charge after the adsorption of different REEs onto biochar nanoparticles.Almost all the biochar nanoparticles were retained in quartz sand mainly due to the weakening of the electrostatic repulsion between the quartz sand and biochar nanoparticles.The transport of REEs followed the order of La>Gd>Yb when single REE or multiple REEs co-transport with biochar nanoparticles,which is reduced to varying degrees in comparison to in the absence of biochar nanoparticles.The transport of multiple REEs was lower than single REE when co-transport with biochar nanoparticles.Because a large amount of REEs are retained due to the retained biochar nanoparticles.Therefore,the added biochar potentially can control the transport of REEs in natural soils.Finally,the co-transport of biochar nanoparticles with different degrees of aging and REEs in porous media was investigated.5%,15%H₂O₂ is used to age biochar to simulate biochar nanoparticles with different degrees of aging in nature.Aging leaded to the increase of oxygen-containing functional groups（hydroxyl,carboxyl）on the surface of biochar nanoparticles,which increased the absorption capacity of REEs with the deepening of aging.These changes leaded to the transport of REEs was weaker than before aging in co-transprt.More REEs were retained in quartz sand.After aging,the transport of biochar nanoparticles increased with the aging of biochar nanoparticles due to the enhanced electrostatic repulsion between biochar nanoparticles and quartz sand.However,more REEs were retained in quartz sand due to the higher adosorption capacities of REEs onto biochar nanoparticles with the aging of biochar nanoparticles.