Study On The Transport Of Nano-ZnO And The Co-transport Mechanism Of Nano-ZnO With Typical Contaminants In Landfill

As the industrialized application of nanosized zinc oxide(nano-ZnO)increasing,more and more daily commercial products containing nano-ZnO will eventually be disposed of in landfills year by year.As one of the emerging contaminants,the environmental behavior including transformation,transport and finally fate of nano-ZnO in landfill arouse wide attentions.When entering the landfill,nano-ZnO will release into the landfill leachate and transport with the leachate as carrier.At the same time,it will contact with various kinds of contaminants and occur interface effects,which resulting in more complex environmental behavior and joint toxicity.The interactions between nano-ZnO and the co-existing contaminants will become the key factors determining their transport,transformation and bioavailability,and is critical to properly assess and control the environmental risks of nano-ZnO.In this study,dynamic column experiments were used to investigate the transport and co-transport of nano-ZnO with typical contaminants including Cu(II),Cr(VI),bisphenol A(BPA)and diethylhexyl phthalate(DEHP)in leachate.By exploring the interface effects between nano-ZnO and contaminants,the interaction mechanism was revealed.The main contents and conclusions of this research are as follows:(1)The transport ability of nano-ZnO in mixed solid waste is low,at least 60%of nano-ZnO are trapped in the column under both early and aging leachate conditions.But both acetic acid(AA)and humic acid(HA)increased the absolute zeta potential of nano-ZnO and provide spatial steric resistance,thus increasing the electrostatic repulsion between nano-ZnO particles and reducing the aggregation of particles,which making it difficult for nano-ZnO to be trapped in the pores of solid waste;in the young leachate,nano-ZnO is more likely to transport in saturated medium,while in the aged leachate,nano-ZnO have stronger transport in unsaturated medium.The electrostatic attraction between nano-ZnO and solid waste is decisive factor affecting the transport ability of nano-ZnO,however,the surface properties and porosity of solid waste media also affect the transport of nano-ZnO,the lower the roughness of solid waste media and the greater the porosity are,the easier nano-ZnO will be transported out.(2)The transport behavior of nano-ZnO coexisted with Cu(II)and Cr(VI)varied in different leachate stages.In the young leachate,Cu(II)and Cr(VI)inhibited the transport of nano-ZnO,while in the aged leachate,the C/C₀ of nano-ZnO co-transport with Cu(II)or Cr(VI)could reach 1.DLS results indicates that the transport behavior of nano-ZnO is determined by the combined effects of environmental factors and heavy metals on the properties of nano-ZnO and the interfacial effects between nano-ZnO and the porous medium.In the presence of Cr(VI),the Cr(VI)attached to nano-ZnO could co-transport with nano-ZnO which makes the both have strong transport ability;however,when Cu(II)and nano-ZnO coexist,a large number of Cu(II)remain in the quartz sand and cannot co-transport with nano-ZnO due to the electrostatic attraction between Cu(II)and quartz sand.Under the aged leachate conditions,Cu(II)is easy to form stable complexes with humic acid,so it is more likely to be retained in the quartz sand column,while nano-ZnO can significantly promote the transport of Cu(II).Cr(III)ions could form O-Cr(III)bonds with the hydroxyl group on the surface of nano-ZnO,which induced nano-ZnO to form chain complexes with Cr(III),and intensified nano-ZnO aggregation.(3)Both of BPA and DEHP inhibited the transport of nano-ZnO.This is because the positive zeta potential of nano-ZnO and the electrostatic attraction between nano-ZnO and quartz sands increased when co-existed with BPA and DEHP,and the adsorption capacity of BPA and DEHP on nano-ZnO is twice that on the quartz sands in addition.The FTIR spectrum of nano-ZnO showed a weak stretching vibration at the narrow absorption peaks of 1506 cm^-1 and 2427 cm^-1,indicating the chemical adsorption of BPA and DEHP on nano-ZnO.The transport ability of BPA and DEHP in quartz sand columns is weak,and most of them will retain in quartz sand column,and the presence of nano-ZnO had no significant effect on the transport of BPA and DEHP.(4)The transport ability of individual transport nano-ZnO under the young and aged leachate conditions increased by 14%-22%with leachate recirculation which significantly improved the re-transport ability of nano-ZnO.In the presence of Cu(II),the C/C₀ of nano-ZnO in the young leachate could increase by up to 50%leachate recirculation,and the C/C₀ of Cu(II)also increased significantly.The leachate recirculation has no obvious effect on the transport ability of Cr(VI);however,after leachate recirculation,the C/C₀ of nano-ZnO coexisting with Cr(VI)can increase by up to 50%.Leachate recirculation has significant effects on BPA transport and little effects on DEHP,and promote the transport of nano-ZnO in the presence of DEHP.In conclusion,this study systematically elaborated the transport rule and related interaction mechanism of nano-ZnO in the absence or presence of co-existing contaminants in landfill environment.The potential environmental risks of nano-ZnO to the stabilization process in landfill and the surrounding environment are put forward in order to provide reference for the formulation of the management policy of nano-ZnO products entering in the landfill.