Researches On The Reactivity,Colloidal Stability And Microbial Toxicity Of Nanoscale Zero-valent Iron

Nanoscale zero-valent iron(NZVI)technology is quickly becoming the most widely used nanotechnology for environmental remediation.Nevertheless,fundamental questions still remain on its reactivity,colloidal stability and health risks in the environment.This thesis systematically studied the reactivity of NZVI on the removal of contaminants(e.g.selenium),the effects of carboxymethyl cellulose(CMC)on the stability and cytotoxicity of NZVI.Firstly,batch experiments were performed to compare the sequestration of Se(?)and Se(?)by nanoscale zero-valent iron(NZ?)particles under different solution conditions.The comparison was conducted at three pH values(4.0,6.0 and 8.0)in ultrapure water.Generally,the removal of Se(?)/Se(?)by NZVI was more rapid under acidic conditions and the removal efficiency of Se(?)was much higher than that of Se(?).Moreover,the variation of solution pH exhibited much larger influence on the sequestration of Se(?)than that of Se(?)by NZVI.X-ray photoelectron spectroscopy(XPS)and X-ray diffraction(XRD)were used to characterize the reaction products of selenium removal by NZVI.It was found that both the Se(?)and Se(?)were reduced to SeO and Se2-,while NZVI was transformed into iron(hydr)oxides.When the selenium-NZVI reactions occurred in the synthetic groundwater,all the reaction systems were inhibited in varying degrees.Then batch tests were conducted to evaluate the individual effects of humic acid(HA)and typical inorganic ions in groundwater on the kinetics of Se(?)/Se(?)removal by NZ?.It seems that HA could substantially hinder the sequestration of Se(?)compared with that in ultrapure water,while sulfate(SO42-)and bicarbonate(HCO3-)inhibited the Se(?)removal significantly.Notably,the presence of cations(i.e.,Na+ or Ca2+)ions did not cause obvious interference to the Se(?)/Se(?)removal by NZ?,while the presence of Ca2+ could alleviate the adverse effect of HA on Se(?)removal to some degree.With the development of NZVI techniques,modifiers are usually used as surface coating(e.g.,polyelectrolyte)to mitigate the particle aggregation and sedimentation in liquid phase during the environmental application.However,the surface coating may not only influence the particle stabilization but also the particle cytotoxicity.In this study,we investigated the dual effects of CMC on the colloidal stability and cytotoxicity of NZVI towards gram-negative Escherichia coli(E.coli).The effect of CMC concentration,ionic strength(Ca2+)and aging treatment on the particle cytotoxicity were also examined.Specifically,the aqueous stability of NZVI suspensions with CMC ratio dose-dependently strengthened within 1h.The inactivation of E.coli by bare NZVI was significant and concentration-and time-dependent.On the contrary,an increasing reduction in cytotoxicity of NZVI with CMC ratio increasing was observed.Transmission Electron Microscopy(TEM)analysis demonstrates the membrane disruption and the cellular internalization of nanoparticles after exposure of E.coli to NZVI.However,in the case of CMC-modified NZVI(CNZVI),the bacterial cell wall displays an outer shell of a layer of nanoparticles attached around the outer membrane,but the cell membrane was kept intact.The presence of Ca2+ can either increase or decrease the cytotoxicity of NZVI and CNZVI,depending on the concentration.The NZVI and CNZVI particles aged for 15 days and 30 days did not seem to present obvious bactericidal effect due to the transformation of Fe0 to the less toxic or non-toxic iron oxides(i.e.,magnetite or/and maghemite and lepidocrocite),as indicated by the XRD analysis.