The Aggregation And Bacterial Toxicity Of Graphene Oxide In Synthetic Surface Waters

Graphene oxide(GO),as a kind of new functional nanomaterial,has been widely used for a variety of applications and is inevitably released to the environment,posing a threat to the ecosystem.Many studies have investigated the aggregation behavior of GO in background solution with a single salt(monosalt system)and the toxic effect of GO to bacteria in the ultrapure water and culture media;however,these findings may not be suitable to predict the behavior of GO in the real aquatic environment with multiple coexisting salts(multisalt system).In this study,the aggregation and sedimentation behavior of GO in the synthetic soft water was investigated to explore the aggregation mechanism in the presence of multiple salts.Meanwhile,the toxic effect of GO to E.coli was studied before and after its aggregation in the soft water,moderately hard water,and hard water to reveal the effects of GO aggregation and solution hardness.The main results were as follow.(1)The GO showed a concentration-dependent aggregation(low concentration aggregation,high concentration stability)in the soft water,which was opposite to the aggregation behavior in monosalt systems(NaCl or CaC12 solutions).In the soft water,GO sheets interacted with Ca2+,Mg2+,HCO3-and induced the formation of amorphous CaMg(CO3)2 nanoparticles on the GO surfaces.The formed nanoparticles further promoted the aggregation and sedimentation of low concentrations of GO through bridging action.However,the CaMg(CO3)2 nanoparticles that formed on GO sheets with high concentration were small in quantity and size,and had a weak impact on the stability of the GO suspension.(2)In the soft water,solution pH can alter the concentration-dependence of GO aggregation due to the impact on the concentration of HCO3-and thereby on the formation of CaMg(CO3)2 nanoparticles.Acidic condition was unfavorable for the formation of CaMg(CO3)2 nanoparticles,in which GO aggregated intensively at high concentrations.But it was opposite under neutral and alkaline conditions.With increasing hardness of the solution,the high ionic strength significantly compressed the electric double-layer(EDL)and played a dominant role in GO aggregation.Therefore,high concentration of GO was favorable for aggregation in moderately hard water and hard water,which was in accordance with the trend in monosalt systems.(3)The toxic effect of GO to E.coli increased with increasing solution hardness while decreased after GO aggregation.The 3 h 50%inhibitory concentration(IC50)values of dispersed GO in the soft water,moderately hard water,and hard water were 12.2±2.2,8.5±1.5,and 4.0±1.0 mg/L,respectively.After 24 h shaking in the synthetic surface waters,the dispersed GO aggregated and the 3 h IC50 values of GO aggregates in the three synthetic waters were 40.3±6.9,15.9±2.2,and 7.5±1.5 mg/L,respectively.(4)The dispersed GO sheets wrapped E.coli cells and cut the cell membrane,resulting in the disruption of cell membrane and the cell inactivation.With the increase of water hardness,the interaction between GO and cells was enhanced,resulting in the increase of toxic effect.After GO aggregation,the interfacial and toxic effects with bacteria decreased.