Effects Of Escherichia Coli On The Transport And Retention Of TiO₂ Nanoparticles In Porous Media

Nanoparticle of titanium dioxide?nTiO₂?is one of the widely used nano-materials,which raised more and more concerns in recent years.During the production and employment,the intentional or unintentional leakage and release of nTiO₂ into the soil and underlying groundwater may pose a significant risk to human health and the environment.The mobility of nTiO₂ in complex soil matrixes is not well understood,mainly due to combined effects of microorganisms,clay minerals,and phosphate on their subsurface transport.Thus,it is necessary to investigate the relative behavior and underlying mechanisms of nTiO₂ in complex environmental systems.In this study,the influences of Escherichia coli?E.coli?,kaolin?clay material?,phosphate?P?and the surface heterogeneity of quartz sand on the transport and retention of nTiO₂ in the sand columns were investigated.Various auxiliary experiments?such as transport experiments,zeta potential and hydraulics radius,adsorption equilibrium experiments?were also conducted to study the transport mechanism of nTiO₂ particles.The two-site kinetic attachment model?TSKAM?was applied to simulate the process and fit the breakthrough curves?BTCs?of nTiO₂ under various conditions,explaining the transport behavior of nTiO₂.Moreover,the related parameters obtained via the classical Derjaguin-Landau-Verwey-Overbeek?DLVO?theory were used to predict the transport behavior of nTiO₂ in saturated sand columns.The results of the experiment are as follows:?1?The transport mechanism of nTiO₂ in the presence of kaolin and E.coli:At pH9.0,the addition of kaolin facilitated the transport of nTiO₂ in the sand columns.The site competition between the kaolin and nTiO₂ may contribute to this result.The kaolin with larger size preferentially adhered onto the surface of quartz sand,thus reducing the deposition sites for nTiO₂ particles.The mobility of nTiO₂ in sand columns was better in the binary system of kaolin and E.coli.At pH 6.0,the zeta potentials?ZPs?of clusters became more negative due to particles would adhere onto the negatively charged E.coli,and subsequently decreasing the aggregation between particles.The dispersivities of bacteria and particles were greater at alkaline conditions?pH 9.0?.Moreover,the microscale bacteria would occupy the deposition sites of quartz sand surface,inbibiting the retention of kaolin and nTiO₂.The two-site kinetic attachment model?TSKAM?provided a good approximation of the transport behavior of nTiO₂ under various conditions.The prediction of DLVO theory for the transport behavior of the nTiO₂ in the presence of kaolin was consistent with the transport experiment results.However,due to the complex polymer layers of E.coli,there was a deviation between the DLVO theory and experiments.?2?The transport mechanism of nTiO₂ in the presence of kaolin,P and E.coli:The transport of nTiO₂ in the coexistence of kaolin and P was better than that with kaolin alone.It was mainly due to the adsorption of P onto the kaolin and nTiO₂enhanced the electrostatic repulsion between particles,and consequently reducing the aggregation and deposition of particles.In ternary system of kaolin,P and E.coli,the mass recovery of nTiO₂ was slight higher than that in the presence of kaolin and P in high ionic concentration?IC?NaNO₃ solutions?5 and 10 mM?,whereas the results were reversed in low IC?1 mM?.In 1 mM NaNO₃ solutions,the decreasing adsorption capacity of P onto the particles less negatively charged the surface of aggregate,promoting the deposition of nTiO₂.In high IC solutions,however,P would release the previously immobilized E.coli cells.More nTiO₂ could break through the sand columns via adhering onto the surface of suspended E.coli.?3?The transport mechanism of nTiO₂ with P and E.coli in iron oxyhydroxide-coated sand:The addition of E.coli favoured the transport of nTiO₂ in iron oxyhydroxide-coated sand.The surface charge of quartz sands became less negative after coating with positively charged iron hydroxide.As a result,more E.coli cells tended to adhere onto the surface of iron oxyhydroxide-coated sand,reducing the deposition sites for nTiO₂ particles.In binary system of P and E.coli,the ZPs of nTiO₂-E.coli clusters became less negative in low IC NaNO₃ solutions?0.1 and 1 mM?,decreasing the mobility of nTiO₂ in modified quartz sand.However,in 10 mM NaNO₃solutions,the presence of 0.1 mM P enhanced the stability of nTiO₂-E.coli clusters.Furthermore,the percentage of nTiO₂ in effluent was proportional to the P concentration??1 mM?.The increased P in the mobile aqueous phase stabilized the nTiO₂ and E.coli,facilitating the transport of nTiO₂ particles in iron oxyhydroxide-coated sand columns.