Influence Of Several Crucial Groundwater Components On The Toxicity Of Nanoscale Zero-valent Iron Towards Microorganisms

Nano zero-valent iron(NZâ…¥)is a promising material for efficient removal of common contaminants from groundwater.Despite its excellent effectiveness in groundwater remediation,the application of NZâ…¥ aroused great attention due to its potential environmental risk(e.g.,cytotoxicity).So far,the specific effects of groundwater composition on the cytotoxicity of zero-valent iron nanoparticles and the potential mechanisms of effects have not been well explored,and relevant literature reports are limited.In this paper,we investigated the effects of nano zero-valent iron on E.coli(Escherichia coli)cytotoxicity in the presence of dissolved oxygen(DO),heavy metal ions(hexavalent chromium and divalent cadmium),inorganic anions(phosphate)and organic matter(humic acid)in groundwater,and systematically discussed the specific mechanisms of action through comparing the results of different effects under aerobic and anaerobic conditions.These studies are of great relevance and reference value for the practical application of environmental risk assessment in the process of NZâ…¥ groundwater remediation.Based on the results of the study,the main conclusions obtained are as follows:(1)The cytotoxicity of nano-zero-valent iron to E.coli was largely influenced by dissolved oxygen.The toxicity of nano zero-valent iron to E.coli under anaerobic conditions was higher than that under aerobic conditions,and the inactivation mechanisms were different.The presence of dissolved oxygen in aerobic conditions allowed the rapid oxidation of part of the zero-valent iron nanoparticles to generate iron oxides(hydroxides),which were wrapped around the unreacted zero-valent iron nanoparticles surface.Therefore,it could continuously and stably generate Fe(â…¡)and trigger the generation of reactive oxygen species(ROS),which further led to oxidative stress damage in E.coli.At the same time,it not only inhibited the rapid corrosion of nano-zero-valent iron to generate large amount of Fe(â…¡)in a short period of time.It also weakened the adhesion of nano-zero-valent iron to E.coli cells.Therefore,the cell damage was weaker,so that E.coli could still maintain cell integrity in a short time,and resist oxidative stress damage induced by reactive oxygen species generated by nano-zero-valent iron to some extent.In addition,it is obvious from the transmission electron microscopy images and Fe(â…¡)concentration monitoring that the integrity of the cell membrane is rapidly disrupted under anaerobic conditions,and the main lethal mechanism of nano-zero-valent iron to E.coli at this time may be the rupture of cell membrane damage caused by direct physical action.(2)Cr(â…¥)and Cd(â…¡)interacted with the nano-zero-valent iron in different ways,such as reduction or adsorption,thus causing different effects on its cytotoxicity.Cr(â…¥)was first adsorbed onto the nano zero-valent iron particles,and was subsequently reduced to Cr(â…¡I)by the redox reaction,which consumed Fe(0)to form a complex precipitate attached to the unreacted nano-zero-valent iron surface.This reduces the cytotoxicity of zero-valent iron nanoparticles to E.coli.The reaction was particularly pronounced under anaerobic conditions,suggesting a toxic antagonism between Cr(â…¥)and zero-valent iron nanoparticles.However,Cd(â…¡),which itself had a certain cytotoxicity to E.coli,not only adsorbed on the surface of zero-valent iron nanoparticles and E.coli,but also be transferred to E.coli cells together with zerovalent iron nanoparticles with excellent adsorption properties as carriers,thus producing a toxic stacking effect on E.coli and reducing the survival rate of E.coli cells.(3)The complexation reaction of phosphate or humic acid with Fe(â…¡)greatly reduces the toxicity of zero-valent iron nanoparticles,especially under aerobic conditions.This is due to: firstly,phosphate and humic acid would be adsorbed onto the surface of nano zero-valent iron,increasing the electrostatic repulsion and spatial site resistance and inhibiting the direct contact between nano-zero-valent iron and E.coli;secondly,the free Fe(â…¡)complexed by phosphate could not enter the interior of E.coli through the damaged cell membrane,and also inhibited the further production of ROS species,thus suppressing the oxidative stress damage;finally,the generation of Fe-phosphate/humic acid complexes would also occupy the reactive sites on the surface of zero-valent iron nanoparticles,further hindering the contact of zero-valent iron nanoparticles with E.coli.