Transport Of Titanium Dioxide Nanoparticles Research Articles

Distinction of facilitating effects between dissolved organic matter derived from corn straw and fulvic acid on transport of titanium dioxide nanoparticles in saturated sand: Dependent on the solution chemistry

The application of organic materials into agricultural soil introduces dissolved organic matter (DOM) into environment. The DOM derived from organic materials would significantly affect the environmental behaviors of nanoparticles. In this study, DOM extracted from corn straw (CSDOM) was characterized and its effects on the transport of titanium dioxides nanoparticles (TiO2 NPs) in porous media were evaluated and compared with those of fulvic acid (FA). The characterization of DOMs indicated marked differences in molecular properties between CSDOM and FA. Weight-average molecular weight (Mw) of CSDOM was 3139 g/mol, higher than 1185 g/mol of FA, whereas CSDOM had a lower aromaticity than FA. Three distinct scenarios of nanoparticle mobility affected by CSDOM and FA were found. Under unfavorable conditions for nanoparticles mobility (pH 4.0 with/without electrolytes, pH 7.0 with electrolytes, and pH 10.0 with CaCl2), both DOMs facilitated TiO2 NPs mobility, and FA with higher aromaticity exerted stronger enhancing effects than CSDOM. However, the promoting effects of DOM on TiO2 NPs mobility were negligible under favorable conditions (pH 10.0 without electrolytes). Under the remaining conditions (pH 7.0 without electrolytes, and pH 10.0 with NaCl), DOM could facilitate the mobility of TiO2 NPs, while there was no difference in the facilitating effects between CSDOM and FA. It could be concluded that distinction of enhancing effects between CSDOM and FA on TiO2 NPs mobility was environmental chemistry-dependent. Furthermore, aromaticity rather than Mw would be a reasonable property determining the enhancing effects of DOM derived from distinct sources on nanoparticle mobility under unfavorable conditions.

Enhancing effects of dissolved and media surface-bound organic matter on titanium dioxide nanoparticles transport in iron oxide-coated porous media under acidic conditions

Natural organic matter (NOM) and iron oxides have been proved to be crucial factors controlling the behaviors of nanoparticles in heterogenous environment. Here, we conducted experimental and modeling study on the transport of titanium dioxide nanoparticles (TiO2 NPs) in iron oxide-coated quartz in the presence of NOM under acidic conditions. Results showed the antagonistic effects of iron oxides and NOM on TiO2 NPs mobility. The inhibition of iron oxides coated on quartz was crystal form-dependent other than quantity-dependent. Amorphous ferric oxyhydroxide with higher specific surface area brought more positive charge and favorable deposition sites onto quartz, and induced more retention of nanoparticles than two crystalline iron oxides, goethite and hematite. Dissolved organic matter (DOM) facilitated TiO2 NPs transport in iron oxide-coated quartz. In comparation with the limited enhancing effects of DOM, the NOM coatings on media surface partially or largely offset the inhibition of goethite on nanoparticles mobility through direct occupation of attachment sites and sites screening due to the steric repulsion of the macromolecules. Owing to the higher steric hindrance, humic acid, both in dissolved and media surface-bound states, exerted stronger facilitating effects on TiO2 NPs mobility relative to fulvic acid.

Competition between fulvic acid and phosphate-mediated surface properties and transport of titanium dioxide nanoparticles in sand porous media

Fulvic acid (FA) is a series of organic macromolecular compounds with many oxygen-containing functional groups and is easy to complex with nanoparticles (NPs) in the environment, thus affecting the stability and mobility of NPs in the water environment. Therefore, it is necessary to investigate the transport behavior of nano titanium dioxide (nTiO2) particles with FA in the presence of phosphate (P). A series of transport experiments were conducted by using nTiO2 suspended in phosphate background electrolyte with and without the addition of FA passing through water-saturated sand columns. The zeta potentials, hydrodynamic radii, and aggregation kinetics were measured, and the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and two-site kinetic attachment model (TSKAM) were employed to explain the underlying mechanism. The experimental results showed that the presence of FA compressed the transport of nTiO2 in phosphate electrolyte due to formation of larger complex via the adsorption of organic FA molecules onto nTiO2, which can strongly compete with phosphate adsorption, leading to reduced electrostatic repulsive forces between nTiO2 and sand. In the absence of FA at low pH (6.0), transport of individual nTiO2 was increased by low phosphate adsorption in response to increased phosphate levels, and then, transport was decreased with the increased number of cations (Na+) from the phosphate (P ≥ 1.0 mM) electrolyte due to the compressed electric double layer. From the goodness-of-fit of the modeling results, the TSKAM provided a good prediction for the retention and transport of nTiO2 in the copresence of FA and phosphate. The compressed transport of nTiO2 in the copresence of FA and phosphate was distinct from the synergistic transport in the copresence of humic acid.

Stability and transport of titanium dioxide nanoparticles in three variable-charge soils

The exposure pathways and environmental impacts of titanium dioxide nanoparticles (TiO2 NPs) released into soils could be significantly influenced by their stability and transport behaviors. The aim of this study was to investigate the stability and transport of TiO2 NPs in three variable-charge soils and to determine the key factors controlling these behaviors. Three surface (0~15 cm) variable-charge soils derived from quaternary red clay, humid ferralsols and stagnic anthrosols collected from Yingtan, Jiangxi Province (YT-H and YT-S, respectively), and humid ferralsols collected from Fuyang, Zhejiang Province (FY-H), were used in this study. Batch sedimentation experiments of TiO2 NPs in soil suspensions were performed for 10 h to quantify their stability. Transport of TiO2 NPs in soil columns was conducted with and without the presence of fulvic acid (FA). Apart from soil organic matter (SOM) and iron oxides, there was no significant difference between the tested soils. Batch experiments showed that TiO2 NPs were more stable in the YT-S soil suspension with high dissolved organic matter than in the YT-H and FY-H soil suspensions. In the column experiments, all TiO2 NPs were retained in YT-H and YT-S, while 11% of the TiO2 NPs were eluted from FY-H with a low amorphous iron oxide content. The significant retention of TiO2 NPs in the soils could be attributed to the straining and adsorption of TiO2 NPs on the surface of soil particles. FA enhanced the transport of TiO2 NPs in YT-H and FY-H by dispersing the TiO2 NPs and reducing their adsorption onto soil particles, while all the TiO2 NPs dispersed in the FA solution were still deposited in YT-S with a high amorphous iron oxide content. The stability of TiO2 NPs in three variable-charge soil suspensions was dependent on the SOM. However, the mobility of TiO2 NPs in soils was not directly related to their stability in the soil suspensions. The difference in amorphous iron oxide content could induce the disparity in mobility of TiO2 NPs in soils.

The limited facilitating effect of dissolved organic matter extracted from organic wastes on the transport of titanium dioxide nanoparticles in acidic saturated porous media

The complexity of natural dissolved organic matter (DOM) motivates the determination of how DOM from diverse sources affects the environmental behaviors of engineered nanoparticles. Here, three types of DOM, DOM extracted from swine manure (SWDOM), sludge (SLDOM) and sediment (SEDOM), were characterized, and their effects on the transport of titanium dioxide nanoparticles (TiO2 NPs, 30 nm in diameter) were evaluated and compared with those of humic acid (HA). Characterization tests showed differences in the aromaticity and weight-average molecular weight (Mw) properties among the three extracted DOM solutions, and greater distinctions were found between the extracted DOM and HA. All the extracted DOM facilitated TiO2 NPs transport in acidic porous media. Nevertheless, the enhancing effects varied among the different extracted DOM types. SWDOM had a promoting effect on TiO2 NPs mobility that was equivalent to that of SEDOM and much higher than that of SLDOM. However, the facilitating effects of all three extracted DOM types were limited compared to that of HA. Based on the combined analysis of DOM properties and TiO2 NPs transport behaviors, it could be concluded that aromaticity and Mw were the key properties determining the limited promoting effects of DOM on TiO2 NPs mobility, and the specific UV absorbance at 280 nm (normalized by concentration, SUVA280) was a facile and useful indicator of the DOM-promoted transport of TiO2 NPs. These findings revealed that transport potential in the presence of DOM would be overestimated if either HA or fulvic acid were chosen as the DOM model in studies.

Assessing titanium dioxide nanoparticles transport models by Bayesian uncertainty analysis

With the rapid growth of nanotechnology industry, nanomaterials as an emerging pollutant are gradually released into subsurface environments and become great concerns. Simulating the transport of nanomaterials in groundwater is an important approach to investigate and predict the impact of nanomaterials on subsurface environments. Currently, a number of transport models are used to simulate this process, and the outputs of these models could be inconsistent with each other due to conceptual model uncertainty. However, the performances of different models on simulating nanoparticles transport in groundwater are rarely assessed in Bayesian framework in previous researches, and these will be the primary objective of this study. A porous media column experiment is conducted to observe the transport of Titanium Dioxide Nanoparticles (nano-TiO2). Ten typical transport models which consider different chemical reaction processes are used to simulate the transport of nano-TiO2, and the observed nano-TiO2 breakthrough curves data are used to calibrate these models. For each transport model, the parameter uncertainty is evaluated using Markov Chain Monte Carlo, and the DREAM(ZS) algorithm is used to sample parameter probability space. Moreover, the Bayesian model averaging (BMA) method is used to incorporate the conceptual model uncertainty arising from different chemical reaction based transport models. The results indicate that both two-sites and nonequilibrium sorption models can well reproduce the retention of nano-TiO2 transport in porous media. The linear equilibrium sorption isotherm, first-order degradation, and mobile-immobile models fail to describe the nano-TiO2 retention and transport. The BMA method could instead provide more reliable estimations of the predictive uncertainty compared to that using a single model.

Effects of Escherichia coli and phosphate on the transport of titanium dioxide nanoparticles in heterogeneous porous media

Transport behaviors of titanium dioxide nanoparticles (nTiO2) were examined in the individual- and co-presence Escherichia (E.) coli and phosphate in heterogeneous sand (uncoated and iron oxyhydroxide-coated sand) columns. The results showed that for the individual presence of phosphate, the degree of nTiO2 deposition was less in uncoated than in iron oxide-coated sands. In contrast, an opposite trend that greater deposition of nTiO2 in uncoated than in coated sands occurred in the individual presence of E. coli. These observations are due to the phosphate adsorption changing the charge of NPs and iron oxyhydroxide-coated sand, or the preferential adhesion of bacterial to coated sand. In the copresence of E. coli and phosphate, interestingly, the phosphate level plays an important role in influencing nTiO2 transport. At a high phosphate concentration (>1.0 mM), the deposition of nTiO2 with the individual presence of E. coli was stronger than nTiO2 in the copresence of both E. coli and phosphate, regardless of sand type. The potential mechanism was that phosphate adsorption led to the formation of more negatively charged NPs-bacteria complexes that have higher mobility in sand columns. At a low phosphate level (≤0.1 mM), a similar observation occurred in uncoated sand. Nevertheless, the deposition of nTiO2 with copresence of E. coli and phosphate was greater than nTiO2 with E. coli in oxyhydroxide-coated sand. It was attributed to the formation of large NPs-bacteria-phosphate clusters (less mobile) and the preferential adhesion of E. coli cells to iron oxyhydroxide coating simultaneously. Taken together, our findings provide crucial knowledge for better understanding the fate, transport, and potential risks of engineered nanoparticles in complicated environmental settings where bacteria and phosphate are ubiquitous.

Facilitated transport of titanium dioxide nanoparticles via hydrochars in the presence of ammonium in saturated sands: Effects of pH, ionic strength, and ionic composition

The widespread use of nanoparticles (NPs) has led to their inevitable introduction into environmental systems. How the existence of hydrochars in crop soils will affect the mobility of nanoparticle titanium dioxide (nTiO2), especially in the presence of ammonium (NH4+), remains unknown. Research is needed to study the effects of hydrochars on the transport and retention of nTiO2 and to uncover the mechanisms of these effects on nTiO2 transport. Column experiments with nTiO2 and hydrochars were performed in various electrolyte (NaCl, NH4Cl, and CaCl2) solutions under a controlled pH (6.0 and 8.0). Additionally, the size distributions and scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the NPs were observed. The experimental results suggested that the mobility of the hydrochars was much better than that of nTiO2. Thus, the mobility of nTiO2 was improved upon their attachment to the hydrochars. The facilitated transport of nTiO2 in the presence of hydrochars was stronger at pH8.0 than at pH6.0, and facilitated transport was nearly independent of the electrolyte cation at pH8.0. However, at pH6.0, the facilitated transport in various electrolytes had the following order: NaCl>NH4Cl>CaCl2. The conversion from a completely reversible to a partially irreversible deposition of nTiO2 in sand was induced by the partially irreversible retention of hydrochars, and this phenomenon was more pronounced in the presence of NH4+ than in the presence of Na+. In particular, the irreversible deposition of nTiO2-hydrochars was enhanced as the cation concentration increased. The increased irreversible retention of nTiO2 was related to the greater k2 value (irreversible attachment coefficients) on site 2 for hydrochars based on two-site kinetic retention modeling. Thus, there is a potential risk of contaminating crops, soil, and underground water when nTiO2 exists in a hydrochar-amended environment, especially when associated with NH4-N fertilizer.

Effect of different-sized colloids on the transport and deposition of titanium dioxide nanoparticles in quartz sand

Colloids (non-biological and biological) with different sizes are ubiquitous in natural environment. The investigations regarding the influence of different-sized colloids on the transport and deposition behaviors of engineered-nanoparticles in porous media yet are still largely lacking. This study investigated the effects of different-sized non-biological and biological colloids on the transport of titanium dioxide nanoparticles (nTiO2) in quartz sand under both electrostatically favorable and unfavorable conditions. Fluorescent carboxylate-modified polystyrene latex microspheres (CML) with sizes of 0.2–2 μm were utilized as model non-biological colloids, while Gram-negative Escherichia coli (∼1 μm) and Gram-positive Bacillus subtilis (∼2 μm) were employed as model biological colloids. Under the examined solution conditions, both breakthrough curves and retained profiles of nTiO2 with different-sized CML particles/bacteria were similar as those without colloids under favorable conditions, indicating that the copresence of model colloids in suspensions had negligible effects on the transport and deposition of nTiO2 under favorable conditions. In contrast, higher breakthrough curves and lower retained profiles of nTiO2 with CML particles/bacteria relative to those without copresent colloids were observed under unfavorable conditions. Clearly, the copresence of model colloids increased the transport and decreased the deposition of nTiO2 in quartz sand under unfavorable conditions (solution conditions examined in present study). Both competition of deposition sites on quartz sand surfaces and the enhanced stability/dispersion of nTiO2 induced by copresent colloids were found to be responsible for the increased nTiO2 transport with colloids under unfavorable conditions. Moreover, the smallest colloids had the highest coverage on sand surface and most significant dispersion effect on nTiO2, resulting in the greatest nTiO2 transport.

Facilitated transport of titanium dioxide nanoparticles by humic substances in saturated porous media under acidic conditions

The transport behavior of titanium dioxide nanoparticles (TiO2 NPs, 30 nm in diameter) was studied in well-defined porous media composed of clean quartz sand over a range of solution chemistry under acidic conditions. Transport of TiO2 NPs was dramatically enhanced by humic substances (HS) at acidic pH (4.0, 5.0 and 6.0), even at a low HS concentration of 0.5 mg L−1. Facilitated transport of TiO2 NPs was likely attributable to the increased stability of TiO2 NPs and repulsive interaction between TiO2 NPs and quartz sands due to the adsorbed HS. The mobility of TiO2 NPs was also increased with increasing pH from 4.0 to 6.0. Although transport of TiO2 NPs was insensitive to low ionic strength, it was significantly inhibited by high concentrations of NaCl and CaCl2. In addition, calculated Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energy indicated that high energy barriers were responsible for the high mobility of TiO2 NPs, while the secondary energy minimum could play an important role in the retention of TiO2 NPs at 100 mmol L−1 NaCl. Straining and gravitational settlement of larger TiO2 NPs aggregates at 1 mg L−1 HS, pH 5.0, and 2 mmol L−1 CaCl2 could be responsible for the significant retention even in the presence of high energy barriers. Moreover, more favorable interaction between approaching TiO2 NPs and TiO2 NPs that had been already deposited on the collector resulted in a ripening-shape breakthrough curve at 2 mmol L−1 CaCl2. Overall, a combination of mechanisms including DLVO-type force, straining, and physical filtration was involved in the retention of TiO2 NPs over the range of solution chemistry examined in this study.

Influence of clay particles on the transport and retention of titanium dioxide nanoparticles in quartz sand.

This study investigated the influence of two representative suspended clay particles, bentonite and kaolinite, on the transport of titanium dioxide nanoparticles (nTiO2) in saturated quartz sand in both NaCl (1 and 10 mM ionic strength) and CaCl2 solutions (0.1 and 1 mM ionic strength) at pH 7. The breakthrough curves of nTiO2 with bentonite or kaolinite were higher than those without the presence of clay particles in NaCl solutions, indicating that both types of clay particles increased nTiO2 transport in NaCl solutions. Moreover, the enhancement of nTiO2 transport was more significant when bentonite was present in nTiO2 suspensions relative to kaolinite. Similar to NaCl solutions, in CaCl2 solutions, the breakthrough curves of nTiO2 with bentonite were also higher than those without clay particles, while the breakthrough curves of nTiO2 with kaolinite were lower than those without clay particles. Clearly, in CaCl2 solutions, the presence of bentonite in suspensions increased nTiO2 transport, whereas, kaolinite decreased nTiO2 transport in quartz sand. The attachment of nTiO2 onto clay particles (both bentonite and kaolinite) were observed under all experimental conditions. The increased transport of nTiO2 in most experimental conditions (except for kaolinite in CaCl2 solutions) was attributed mainly to the clay-facilitated nTiO2 transport. The straining of larger nTiO2-kaolinite clusters yet contributed to the decreased transport (enhanced retention) of nTiO2 in divalent CaCl2 solutions when kaolinite particles were copresent in suspensions.

Transport of titanium dioxide nanoparticles in saturated porous media under various solution chemistry conditions

Because of its wide applications, nanosized titanium dioxide may become a potential environmental risk to soil and groundwater system. It is therefore important to improve current understanding of the environmental fate and transport of titanium oxides nanoparticles (TONPs). In this work, the effect of solution chemistry (i.e., pH, ionic strength, and natural organic matter (NOM) concentration) on the deposition and transport of TONPs in saturated porous media was examined in detail. Laboratory columns packed with acid-cleaned quartz sand were used in the experiment as porous media. Transport experiments were conducted with various chemistry combinations, including four ionic strengths, three pH levels, and two NOM concentrations. The results showed that TONP mobility increased with increasing solution pH, but decreased with increasing solution ionic strength. It is also found that the presence of NOM in the system enhanced the mobility of TONPs in the saturated porous media. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to justify the mobility trends observed in the experimental data. Predictions from the theory agreed excellently with the experimental data.

Influence of Carboxymethyl Cellulose for the Transport of Titanium Dioxide Nanoparticles in Clean Silica and Mineral-Coated Sands

The transport properties of titanium dioxide (anatase polymorph) nanoparticles encapsulated by carboxymethyl cellulose (CMC) were evaluated as a function of changes in the solute chemical properties in clean quartz, amorphous aluminum, and iron hydroxide-coated sands. While pristine anatase TiO2 nanoparticles (ANTNPs) were completely immobile, the presence of CMC significantly enhanced their mobility. The magnitude of the surface charge exhibited by the CMC-coated anatase TiO2 nanoparticles (CMC-ANTNPs) significantly exceeded that of the uncoated ANTNPs, thereby leading to a negative surface charge over the pH range investigated (2-10). The mobility of CMC-ANTNPs was retarded by the presence of amorphous Fe and Al hydroxide, Na+ (30 mM), and Ca2+ (30 mM). Removal of CMC-ANTNPs was more significant in the presence of either Ca2+ or Fe-hydroxide. More retardation and incomplete breakthrough of the CMC-ANTNPs was observed in the mineral-coated sands. This is possibly due to an order of magnitude increase in the surface area of mineral-coated sands compared with the clean quartz sand grains and the potential for chelation between CMC bound to ANTNPs and Fe and Al hydroxides. Chemical-colloidal interactions such as chemicomplexation and ligand exchange were the most important factor controlling mobility of CMC-ANTNPs in mineral-coated sands.