Effect Of Phosphate On The Transport And Retention Of TiO₂ NPs In Sand Column

TiO₂ Nanoparticles（TiO₂NPs）are widely used in various fields in recent years.However,upon their release to nature environment,potential threat and harm will be produced to human health and ecosystem.Moreover,the widely use of phosphatic fertilizer make the phosphorous pollution more serious.Therefore,it is crucial to investigate the transport and retention of TiO₂NPs in the presence of phosphate.It benefits to assess the risk of nanoscale pollutants and phosphate to enter into groundwater and soil environment.In this study,the aggregation kinetics,hydraulic radius,zeta potential and breakthrough curves of TiO₂NPs in the presence of phosphate were investigated to explore the likely mechanism for their transportability.At last,the two site kinetic retention model and the Derjaguin-Landau-Verwey-Overbeek（DLVO）theory were employed to explain specific behavior.The main results are concluded as follows:（1）The transport behavior of hydrochar-modified TiO₂NPs in the presence of phosphate:The enhancement of hydrochar-modified TiO₂NPs by phosphate is stronger than TiO₂NPs alone.The likely mechanism is that hydrochar has the good mobility and can carry the attached TiO₂NPs to transport together.The facilitated transport of TiO₂NPs is possibly due to the more negative charges of TiO₂NPs at higher p H（9.0）.The breakthrough curves（BTCs）of TiO₂NPs were simulated well by two site kinetic retention model.The compression of the first-order attachment coefficients on Site 1（k₁）and the detachment coefficients on Site 1(k_1d)occur with the increase of IC,while the irreversible attachment coefficients on Site 2（k₂）increases accordingly.It suggests that ionic concentration（IC）has a signification impact on the transport of TiO₂NPs.（2）The transport mechanism of bacteria-modified TiO₂NPs in the presence of phosphate:The promoted transport of E.coli-modified TiO₂NPs by phosphate is less than TiO₂NPs alone.It is due to the adsorption of phosphate on E.coli resulting in the compressed dispersion of TiO₂ NPs by the presence of phosphate.Thus,the electrostatic repulsion between NPs is reduced,which results in the lower mobility of TiO₂ NPs.E.coli increases the transport of TiO₂NPs.It attributes to good transportability of E.coli and the attachment TiO₂ NPs onto the surface of E.coli occurs.Phosphate facilitates the mobility of B.subtilis-modified TiO₂NPs in electrolytes and enlarges the electrostatic repulsion between NPs,which leads to better transportability of B.subtilis-modified TiO₂NPs.The adsorption of phosphate on B.subtilis is larger than E.coli.Therefore,the promoted transport of E.coli-modified TiO₂NPs by phosphate is more than B.subtilis-modified TiO₂NPs.（3）The classic DLVO theory to explain and predict the transport behavior of TiO₂NPs under various phosphate conditions:The value of the substantial repulsive energy barrier(maximum energy barrier,φ_max)declines with the increase IC even though the weak attractive force(the secondary minimum,φ_min2)is very low,which implies that the electrostatic repulsion governs for the transport of TiO₂ NPs.The height ofφ_max enhances with the increase of phosphate,humic acid and hydrochar.The rising p H induces the enhancement transport of TiO₂ NPs due to the more negative Zeta potential on the surface of TiO₂ NPs leading to the higher repulsion energy between NPs.