Characteristics And Impact Factors Of Nano-ZnO Migration In Municipal Landfill Leachate

Nanostructured metal oxides?NMOs?are the most important nanomaterials in the world with the highest yield and consumption.Landfills have become the most important"sink"and"source"of these nanomaterials in environment.Thus launching researches on them in landfill is of great significance.Nano-ZnO?nZn O?is one of the most widely used nanostructured metal oxides currently,but its environmental behaviors in landfill have been unknown.In this paper,commercial nZnO was used and batch experiments and sand column experiments were conducted to investigate the effects of pH value,Ionic Strength?IS?,Humic Acid?HA?concentration,coexisting Cu?II?and Cr?VI?ions in landfill leachate on the agglomeration,sedimentation and migration of nZnO.The theory of colloidal stability?Derjaguin-Landau-Verwey–Overbeek,DLVO?and the single collector contact efficiency model were employed to discuss the agglomeration and migration characteristics of nZnO and the related mechanisms.The main conclusions are as follows:?1?Landfill leachate pH value can significantly affect the agglomeration,sedimentation and migration of nZnO.When the pH was 5.5,the Zeta potential of nZn O approached zero potential,so it aggregates to form the nano-aggregates with the largest particle size,making it more prone to sedimentation.Meanwhile,nZnO had a strong dissolution effect resulting in the largest total Zn mobility that presented the largest migration distance.With the increase of p H value,the Zeta potential of nZnO departed from zero point potential,and the particle size decreased and the n ZnO sedimentation capacity declined gradually.However,due to the decrease of the solubility of ZnO,the migration capacity decreased with the increase of pH.?2?The agglomeration,sedimentation and migration of nZnO were greatly influenced by landfill leachate IS.When IS rose from 5 mM to 200 mM,the electrical double layer of nZnO was compressed gradually,and nZnO particles broke the DLVO energy barrier easily sothat promoted the agglomeration and sedimentation migrate capacity was r educed.?3?The HA concentration in landfill leachate can affect the agglomeration,sedimentation and migration of nZnO substantially.When HA concentration in the leachate was 100 TOC mg L^-1,it could reduce the Zeta potential of nZn O to-36.8 mV which far away from zero potential,coupled with a strong spatial steric effect of HA,the final agglomeration of nZnO weakened,so it was stable and sedimentation would't occur,but increased the migration distance of n ZnO.?4?Heavy metals like Cu?II?or Cr?VI?existed in leachate affected the agglomeration,sedimentation and migration of nZnO.Under any pH and IS condition,Cu and Cr could provide cations and anions,respectively,to change the Zeta potential of n ZnO,thus affected the subsequent sedimentation and migration behaviors.In general,Cu can promote the sedimentation of nZn O thus inhibiting the migration,while the effect of Cr was the opposite.However,once HA was contained in the solution,Cu and Cr were adsorbed on the HA surface,so their effect on n ZnO agglomeration and sedimentation behavior can be ignored.Therefore,for early landfills,due to the acidic pH,higher IS,and lower HA concentration in leachate,nZnO is more prone to agglomeration and sedimentation,which in turn inhibits its migration.With the increase of landfill years,the pH of the leachate rises or even becomes alkaline,and electrolytes gradually form precipitates,which causes a drop in IS,and especially a significant increase in HA concentration.The nZnO is more likely to exist in a dispersed and stable form with reduced agglomeration and sedimentation capacity and significantly increase migration capacity then increase the potential environmental risk.In addition,the presence of Cu?II?and Cr?VI?in the leachate will provide cations and anions,respectively,which alter the Zeta potential of the nZnO nanoparticles in different pH and IS leachate and thus affect their agglomeration,sedimentation and migration behavior.However,when HA is present in leachate,the effect of heavy metals on the agglomeration and sedimentation behavior of nZnO can be ignored.