Stability of co-existing ZnO and TiO2 nanomaterials in natural water: Aggregation and sedimentation mechanisms.

The use of diverse engineered nanomaterials (ENMs) potentially leads to the coexistence of multiple ENMs in the natural environment. The fate such as colloidal stability, transport, and transformation of individual ENMs are dedicated to the coexistence of other types of ENMs in the environment. Here, we for the first time investigated the sedimentation behaviors of two most widely used ENMs (i.e. ZnO and TiO2 nanomaterials, nZnO and nTiO2) copresented in the natural water of China. Sedimentation rates (Vs), homo-aggregation (khom, crit) and hetero-aggregation (khet, crit) rate of nZnO and nTiO2 were calculated based on Von Smoluchowski-Stokes equation and the sedimentation mechanisms were systematically analyzed. The results showed that the coexistence of like negative charge nZnO and nTiO2 effectively enhanced the stability of either ENM by competing hetero-aggregation with natural colloids (NCs) and reducing to form homo-aggregates by the hindrance effect on particle collision. In the natural water, homo-aggregation, hetero-aggregation between ENMs and NCs, as well as the hetero-aggregation between nZnO and nTiO2 were the main aggregation and sedimentation mechanisms. The coexistence of nZnO and nTiO2 made Vs of nZnO decreased by 30.7-49.1% and Vs of nTiO2 decreased by a factor of 42.4%. Value of khet.crit between nZnO and NCs was 0.084-0.132 L mg-1 day-1, was 0.038 L mg-1 day-1 between nTiO2 and NCs, and was 0.011-0.014 L mg-1 day-1 between nZnO and nTiO2.

Inhibited transport of graphene oxide nanoparticles in granular quartz sand coated with Bacillus subtilis and Pseudomonas putida biofilms.

Increasing production and use of graphene oxide nanoparticles (GONPs) boost their wide dissemination in the subsurface environments where biofilms occur ubiquitously, representative of the physical and chemical heterogeneities. This study aimed at investigating the influence of Gram-positive Bacillus subtilis (BS) and Gram-negative Pseudomonas putida (PP) biofilms on the transport of GONPs under different ionic strengths (0.1, 0.5, and 1.0 mM CaCl2) at neutral pH 7.2 in water-saturated porous media. Particularly, the X-ray micro-computed tomography was used to quantitatively characterize the pore structures of sand columns in the presence and absence of biofilms. Our results indicated that the presence of biofilms reduced the porosity and narrowed down the pore sizes of packed columns. Transport experiments in biofilm-coated sand showed that biofilms, irrespective of bacterial species, significantly inhibited the mobility of GONPs compared to that in cleaned sand. This could be due to the Ca2+ complexation, increased surface roughness and charge heterogeneities of collectors, and particularly enhanced physical straining caused by biofilms. The two-site kinetic retention model-fitted value of maximum solid-phase concentration (Smax2) for GONPs was higher for biofilm-coated sand than for cleaned sand, demonstrating that biofilms act as favorable sites for GONPs retention. Our findings presented herein are important to deepen our current understanding on the nature of particle-collector interactions.

Modeling the transport of TiO2 nanoparticle aggregates in saturated and unsaturated granular media: effects of ionic strength and pH.

This study aims to explore the mechanisms governing the transport and retention kinetics of TiO(2) nanoparticle aggregates (NPAs) in flow-through columns of packed sand, particularly under unsaturated conditions. The study was carried out at different pHs (2.6, 7.1, and 9.6) and ionic strengths (ISs) (1.0, 10, and 50 mM). A two-site kinetic attachment model was used to describe transport behaviors of TiO(2) NPAs. At low ISs (i.e., 1.0 and 10 mM) and in neutral/alkaline conditions, high mobility of TiO(2) NPAs was observed in both saturated and unsaturated conditions. However, the retention of TiO(2) NPAs was substantially enhanced at the high IS (50 mM) and in extremely acidity condition (pH = 2.6), because of increased aggregation and straining of TiO(2) NPAs during their transport course. The breakthrough curves (BTCs) of TiO(2) NPAs under unsaturated and saturated conditions almost overlapped, suggesting that decreasing the water saturation did not enhance the retention of TiO(2) NPAs in sand columns. This was probably due to the repulsive interactions existed between negatively charged air-water and TiO(2) NPAs systems that resulted in unfavorable attachment conditions. The two-site kinetic attachment model provided a good description for the BTCs of TiO(2) NPAs both in saturated and unsaturated conditions. The fitted parameters could successfully explain the transport behaviors of TiO(2) NPAs under various solution chemistries.

[Transport of hydroxyapatite nanoparticles in saturated packed column: effects of humic acid, pH and ionic strengths].

Quartz sand was selected as collector and saturated packed column was constructed to explore the effects of environmental factors (humic acid, pH and ionic strengths of the bulk solution) on the transport and fate of hydroxyapatite nanoparticles (Nano-HAP) through measuring zeta potentials and representative c(i)/c(0) of Nano-HAP. It was suggested that zeta potentials of Nano-HAP colloids became more negative with increasing humic acid concentration and the change in solution composition from 0 to 10 mg/L humic acid yielded an increase in the zeta potentials of Nano-HAP colloids from -15 mV to -55 mV and a sharp decrease in a (attachment efficiency) from 1.0 to 0.012, meanwhile, the increase in bulk solution pH yielded a slight decrease in a which enhancing its transportation in saturated packed column. However, zeta potentials of Nano-HAP colloids became less negative as the ionic strength of bulk solution increased due to the compression of diffuse double layer and yielded an increase in a which greatly impeded its mobility during the pore-water solution, meanwhile, divalent cations have significantly stronger influence on the transport of Nano-HAP than monovalent cations of the bulk solution. The increase in the concentration of monovalent cation (Na+) from 1 to 100 mmol/L yielded an increase in a from 0.030 to 0.13, and divalent cations (Ca2+) from 0.2 to 10 mmol/L yielded a greatly increase in alpha from 0.030 to 1.0. It is important to note that the results could considerably contribute to gain insights in the transport and fate of Nano-HAP in natured and engineered porous media.