Toxicity Study Of Modified Nanoscale Zero-valent Iron Toward Escherichia Coli

In general,nZVI can enhance its own reactivity and specific surface area by surface modification to obtain better contmainant removal.However,the potential risks of modified nZVI to humans and the environment remain unclear.Therefore,in order to evaluate the toxicity effects of modified nZVI,Escherichia coli was used as the test organism in this study to investigate the toxicity of Fe/Ni?Fe/Ni?bimetallic/starch-modified Fe/Ni bimetallic?S-Fe/Ni?nanoparticles and sulfide-modified nZVI?S/nZVI?toward Escherichia coli?E.coli?.The experimental results of this study can be divided into the following two parts:Part ?:This section evaluates the toxicity of Fe/Ni and S-Fe/Ni nanoparticles toward E.coli.The experimental results showed that the inactivation of E.coli by Fe/Ni nanoparticles was concentration-and time-dependent.The Fe/Ni nanoparticles exhibited stronger toxicity than the unmodified nZVI?i.e.,Ni=0 wt%?,indicating that the loading of Ni increased the toxicity of nZVI.However,when the content of Ni in Fe/Ni increased from 1 wt%to 5 wt%,the toxicity of the Fe/Ni nanoparticles did not increase with increasing Ni content,revealing that there is no positive correlation between Ni content and the toxicity of Fe/Ni nanoparticles toward E.coli.Starch coating can alleviate the toxicity of Fe/Ni to E.coli,depending on starch concentration.The results of transmission electron microscopy?TEM?showed that membrane disruption and cellular internalization of nanoparticles occurred for E.coli after exposure to the Fe/Ni nanoparticles.S-Fe/Ni nanoparticles did not cause significant cell membrane disruption because starch was coated on the surface of Fe/Ni and could inhibit the close contact of the nanoparticles with E.coli by providing electrostatic repulsion and steric hindrance.E.coli was analyzed using Fourier transform infrared spectroscopy,these starch modified/unmodified Fe/Ni nanoparticles might react with carboxyl groups,ester groups and amine groups on the surface of E.coli.In addition,N-acetyl-L cysteine?NAC,an reactive oxygen species scavenger?was added to coexisting suspension of E.coli and nanoparticles,the results showed that the presence of NAC could significantly reduce toxicity of Fe/Ni/S-Fe/Ni nanoparticles to E.coli,which indicated that the ROS-induced oxidative stress should be the primary mechanism for the inactivation of E.coli.The aging experiments results showed that the aged nanoparticles exhibited lower toxicity to E.coli.Part ?:This part investigates the toxicity of S/nZVI to E.coli.The experimental results showed that sulfidation could alleviate the toxicity of nZVI to E.coli,and S/nZVI exhibited weaker toxicity at lower Fe/S molar ratio,probably due to its lower Fe⁰ content,higher sulfate and iron oxides.In the presence of N-acetyl-L cysteine in the nanoparticles-E.coli system,the toxicity of S/nZVI toward E.coli was significantly reduced,indicating that ROS-induced oxidative stress should be the main mechanism by which S/nZVI inactivate E.coli.Furthermore,transmission electron microscopy images showed that the cell membrane of E.coli exposed to S/nZVI was disputed and S/nZVI nanoparticles were present on the surface and cytoplasm of E.coli.The analysis results of fourier transform infrared spectroscopy indicated that S/nZVI might interact with amine,carboxyl and ester groups on the surface of E.coli.However,the presence of vidual groundwater components(such as Ca²⁺,SO₄²-,HCO₃^-and humic acid?HA?)in the reaction system could more or less alleviate the toxicity of S/nZVI to E.coli.In addition,the S/nZVI showed a slight toxic effect?the result was<0.15-log after 60 min?to E.coli in the presence of mixed groundwater components and simulated groundwater.For the aged S/nZVI nanoparticles,it exhibited weaker toxicity to E.coli in aqueous solution?the experimental result was<0.15-log after 60 min?,indicating that S/nZVI could lose its toxicity over time.