Coupled effects of solution chemistry and hydrodynamics on the mobility and transport of quantum dot nanomaterials in the vadose zone

Transport and retention of functionalized graphene oxide nanoparticles in saturated/unsaturated porous media: Effects of flow velocity, ionic strength and initial particle concentration

The widespread use of nanomaterials has raised the threat of nanoparticles (NPs) infection of soils and groundwater resources. This research aims to investigate three parameters including flow velocity, ionic strength (IS), and initial particle concentration effects on transport behavior and retention mechanism of functionalization form of graphene oxide with polyvinylpyrrolidone (GO-PVP). The transport of GO-PVP was investigated in a laboratory-scale study through saturated/unsaturated (Saturation Degree = 0.91) sand columns. Experiments were conducted on flow velocity from 1.20 to 2.04 cm min-1, initial particle concentration from 10 to 50 mg L-1, and IS of 5-20 mM. The retention of GO-PVP was best described using the one-site kinetic attachment model in HYDRUS-1D, which accounted for the time and depth-dependent retention. According to breakthrough curves (BTCs), the lower transport related to the rate of mass recovery of GO-PVP was obtained by decreasing flow velocity and initial particle concentration and increasing IS through the sand columns. Increasing IS could improve the GO-PVP retention (based on katt and Smax) in saturated/unsaturated media; katt increases from 2.81 × 10-3 to 3.54 × 10-3 s-1 and Smax increases from 0.37 to 0.42 mg g-1 in saturated/unsaturated conditions, respectively. Our findings showed that the increasing retention of GO-PVP through the sand column under unsaturated condition could be recommended for the reduction of nanoparticles danger of ecosystem exposure.

Experimental measurements and numerical simulations of the transport and retention of nanocrystal CdSe/ZnS quantum dots in saturated porous media: Effects of electrolytes, organic ligand, and natural organic matter

This study explores the transport and retention of CdSe/ZnS quantum dot (QD) nanoparticles in water-saturated sand columns as a function of electrolytes (Na+ and Ca2+), ionic strength, organic ligand citrate, and Suwannee River natural organic matter (SRNOM). Numerical simulations were carried out to understand the mechanisms that govern the transport and interactions of QDs in porous media and to assess how environmental parameters impact these mechanisms. An increase in the ionic strength of NaCl and CaCl2 increased QDs retention in porous media. The reduction of the electrostatic interactions screened by dissolved electrolyte ions and the increase of divalent bridging effect are the causes for this enhanced retention behavior. Citrate or SRNOM enhanced QDs transport in NaCl and CaCl2 systems by either increasing the repulsion energy barrier or inducing the steric interactions between QDs and the quartz sand collectors. A non-exponential decay characterized the retention profiles of QDs along the distance to the inlet. The modeling results indicated the four models containing the attachment, detachment, and straining terms - Model 1: M1-attachment, Model 2: M2-attachment and detachment, Model 3: M3-straining, and Model 4: M4-attachment, detachment, and straining - closely simulated the observed breakthrough curves (BTCs) but inadequately described the retention profiles.

Colloidal stability and aggregation kinetics of nanocrystal CdSe/ZnS quantum dots in aqueous systems: Effects of ionic strength, electrolyte type, and natural organic matter

Understanding the stability and aggregation of nanoparticles in aqueous milieu is critical for assessing their behavior in the natural and engineered environmental systems and establishing their threat to human and ecosystems health. In this study, the colloidal stability and aggregation kinetics of nanocrystal quantum dots (QDs) —CdSe/ZnS QDs—were thoroughly explored under a wide range of aqueous environmental conditions. The z-average hydrodynamic diameters (z-avg. HDs) and zeta potential (ξ potential) of CdSe/ZnS QDs were measured in monovalent electrolyte (NaCl) and divalent electrolyte (CaCl2) solutions in both the absence and presence of natural organic matter (NOM)—Suwannee River natural organic matter, SRNOM to assess the dynamic growth of these nanoaggregate-QD-complexes, and the evaluation of their colloidal stability. Results show that CaCl2 was more effective to destabilize the QDs compared to NaCl at similar concentrations. An increase in NaCl concentration from 0.01 to 3.5 M increased the z-avg. HD of QD aggregates from 61.4 nm to 107.2 nm. The aggregation rates of QDs increased from 0.007 to 0.042 nm·s−1 with an increase in ionic strength from 0.5 to 3.5 M NaCl solutions, respectively. In the presence of Na+ cations, the aggregation of QDs was limited as steric forces generated by the original surface coating of QDs prevailed. In the presence of CaCl2, the aggregation of QDs was observed at a low concentration of CaCl2 (0.0001 M) with a z-avg. HD of 74.2 nm that significantly increased when the CaCl2 was higher than 0.002 M. Larger sizes of QD aggregates were observed at each level of CaCl2 concentration in suspensions of 0.002–0.1 M, as the z-avg. HDs of QDs increased from 125.1 to 560.4 nm, respectively. In the case of CaCl2, an increase in aggregation rates occurred from 0.035 to 0.865 nm·s−1 with an increase in ionic strength from 0.0001 M to 0.004 M, respectively. With Ca2+ cations, the aggregation of QDs was enhanced due to the bridging effects from the formation of complexes between Ca2+ cations in solution and the carboxyl group located on the surface coating of QDs. In the presence of SRNOM, the aggregation of QDs was enhanced in both monovalent and divalent electrolyte solutions. The degree of aggregation formation between QDs through cation-NOM bridges was superior for Ca2+ cations compared to Na+ cations. The presence of SRNOM resulted in a small increase in the size of the QD aggregates for each of NaCl concentrations tested (i.e., 0.01 to 3.5 M, except 0.1 M), and induced a monodispersed and narrower size distribution of QDs suspended in the monovalent electrolyte NaCl concentrations. In the presence of SRNOM, the aggregation rates of QDs increased from 0.01 to 0.024 nm 1 with the increase of NaCl concentrations from 0.01 to 2 M, respectively. The presence of SRNOM in QDs suspended in divalent electrolyte CaCl2 solutions enhanced the aggregation of QDs, resulting in the increase of z-avg. HDs of QDs by approximately 19.3%, 42.1%, 13.8%, 1.5%, and 24.8%, at CaCl2 concentrations of 0.002, 0.003, 0.005, 0.01, and 0.1 M, respectively. In the case of CaCl2, an increase in aggregation rates occurred from 0.035 to 0.865 nm·s−1 with an increase in ionic strength from 0.0001 to 0.004 M, respectively. Our findings demonstrated the colloidal stability of QDs and cations-NOM-QD nanoparticle complexes under a broad spectrum of conditions encountered in the natural and engineered environment, indicating and the potential risks from these nanoparticles in terms of human and ecosystem health.

Experimental measurements and numerical simulations of the transport and retention of nanocrystal CdSe/ZnS quantum dots in saturated porous media: effects of pH, organic ligand, and natural organic matter

The risks of environmental exposures of quantum dot (QD) nanoparticles are increasing, but these risks are difficult to assess because fundamental questions remain about factors affecting the mobility of QDs. The objective of this study is to help address this shortcoming by evaluating the physico-chemical mechanisms controlling the transport and retention of CdSe/ZnS QDs under various environmental conditions. The approach was to run a series of laboratory-scale column experiments where QDs were transported through saturated porous media with different pH values and concentrations of citrate and Suwannee River natural organic matter (SRNOM). Numerical simulations were then conducted and compared with the laboratory data in order to evaluate parameters controlling transport. QD suspensions were injected into the column in an upward direction and ICP-MS used to analyze Cd²⁺ concentrations (C) in column effluent and sand porous media samples. The increase in the background solution pH values enhanced the QD transport and decreased the QD retention. QD transport recovery percentages obtained from the column effluent samples were 2.6%, 83.2%, 101.7%, 96.5%, and 98.9%, at pH levels of 1.5, 3.5, 5, 7, and 9, respectively. The effects of citrate and SRNOM on the transport and retention of QDs were pH dependent as reflected in the influence of the electrostatic and steric interactions between QDs and sand surfaces. QDs were mobile under unfavorable deposition conditions at environmentally relevant pHs (i.e., 5, 7, and 9). Under favorable pH conditions for deposition (i.e., 1.5), QDs were completely retained within the porous media. The retention profiles of QDs showed a non-exponential decay with distance to the inlet, attributed to multiple deposition rates caused by the QD particles and surface heterogeneities of the quartz silica sand. Results of the diameter ratios of QDs to the median sand grains, in suspensions of DI water at pH 1.5, of citrate at pH 1.5, and of citrate at pH 3.5 indicate straining as the dominating mechanism for QD retention in porous media. The blocking effect and straining were significant under favorable deposition conditions and the detachment effect was non-negligible under unfavorable deposition conditions. Physico-chemical attachment and straining are the governing mechanisms that control the retention of QDs. Overall, experimental results indicate that aggregation, deposition, straining, blocking, and DLVO-type interactions affect the advective transport and retention of QDs in saturated porous media. The simulations were conducted using models that include terms describing attachment, detachment, and straining terms-model 1: M1-attachment, model 2: M2-attachment and detachment, model 3: M3-straining, and model 4: M4-attachment, detachment, and straining. The results from simulations with M2-attachment and detachment and M4-attachment, detachment, and straining matched best the observed breakthrough curves, but all four models inadequately described the retention profiles. Our findings demonstrate that QDs are mobile in porous media under a wide range of physico-chemical conditions representative of the natural environment. The mobility behavior of QDs in porous media indicated the potential risk of soil and groundwater contamination.

Mobility and transformation of CdSe/ZnS quantum dots in soil: Role of the capping ligands and ageing effect

The increasing application of Quantum Dots (QDs) is cause of concern for the potential negative effects for the ecosystem, especially in soils that may act as a sink. In this study, soil leaching experiments were performed in quartz sand packed columns to investigate the behavior of core-shell CdSe/ZnS QDs coated with either small ligands (TGA-QDs) or more complex polymers (POAMA-QDs). Fluorescence emission was compared to mass spectrometric measurements to assess the nanoparticles (NPs) state in both the leachate (transported species) and porous media (deposited amounts). Although both QDs were strongly retained in the column, large differences were observed depending on their capping ligand stability. Specifically, for TGA-QDs elution was negligible and the retained fraction accumulated in the top-columns. Furthermore, 74% of the NPs were degraded and 38% of the Se was found in the leachate in non-NPs state. Conversely, POAMA-QDs were recovered to a larger extent (78.1%), and displayed a higher transport along the soil profile. Further experiments with altered NPs showed that homo-aggregation of the QDs prior injection determined a reduced mobility but no significant changes in their stability. Eventually, ageing of the NPs in the column (15 days) caused the disruption of up to 92% of the original QDs and the immobilization of NPs and metals. These results indicate that QDs will accumulate in top-soils, where transformations phenomena will determine the overall transport, persistency and degradation of these chemicals. Once accumulated, they may act as a source for potentially toxic Cd and Se metal species displaying enhanced mobility.

Ligands and media impact interactions between engineered nanomaterials and clay minerals

The exponential growth in technologies incorporating engineered nanomaterials (ENMs) requires plans to handle waste ENM disposal and accidental environmental release throughout the material life cycle. These scenarios motivate efforts to quantify and model ENM interactions with diverse background particles and solubilized chemical species in a variety of environmental systems. In this study, quantum dot (QD) nanoparticles and clay minerals were mixed in a range of water chemistries in order to develop simple assays to predict aggregation trends. CdSe QDs were used as a model ENM functionalized with either negatively charged or zwitterionic small molecule ligand coatings, while clays were chosen as an environmentally relevant sorbent given their potential as an economical water treatment technology and ubiquitous presence in nature. In our unbuffered experimental systems, clay type impacted pH, which resulted in a change in zwitterionic ligand speciation that favored aggregation with kaolinite more than with montmorillonite. With kaolinite, the zwitterionic ligand-coated QD exhibited greater than ten times the relative attachment efficiency for QD-clay heteroaggregation compared to the negatively charged ligand coated QD. Under some conditions, particle oxidative dissolution and dynamic sorption of ions and QDs to surfaces complicated the interpretation of the removal kinetics. This work demonstrates that QDs stabilized by small molecule ligands and electrostatic surface charges are highly sensitive to changes in water chemistry in complex media. Natural environments enable rapid dynamic physicochemical changes that will influence the fate and mobility of ENMs, as seen by the differential adsorption of water-soluble QDs to our clay media.

Pristine graphene induces cardiovascular defects in zebrafish (Danio rerio) embryogenesis

The multiple effect of pristine graphene (pG) toxicity on cardiovascular developmental defects was assessed using zebrafish as a model. Recently, the nanotoxicity is emerging as a critical issue, and it is more significant in embryogenesis. Especially, graphene and its derivatives have attracted a lot of interest in biomedical applications. However, very little is known about the toxic effects of pG which has been widely used carbon nanomaterial according to concentration and its effects on biological and cardiovascular development. In the present study, we examined the development of zebrafish embryos by exposing to pG (5, 10, 15, 20 and 25 μg/L) under different developmental toxicity end-points such as cardiotoxicity, cardiovascular defect, retardation of cardiac looping, apoptosis and globin expression analysis. For this, the developmental cardiotoxicity of pG at different concentrations and the specific cardiovascular defects thereof were elucidated for the first time. As a result, the exposure to pG was found to be a potential risk factor to cardiovascular system of zebrafish embryos. However, a further study on the variations of physical, molecular properties and mechanisms of nanotoxicity which vary depending on production method and surface functionalization is required. In addition, the potential risks of pG flakes to aquatic organisms and human health should be considered or checked before releasing them to the environment.

Transport and retention of TiO2 and polystyrene nanoparticles during drainage from tall heterogeneous layered columns

Recent developments in nanotechnology have seen an increase in the use of manufactured nanomaterials. Although their unique physicochemical properties are desirable for many products and applications, concern continues to exist about their environmental fate and potential to cause risk to human and ecological health. The purpose of this work was to examine one aspect of nanomaterial environmental fate: transport and retention in the unsaturated zone during drainage. The work made use of tall segmented columns packed with layers of two different porous media, one medium sand and one fine sand. The use of tall columns allowed drainage experiments to be conducted where the water table remained within the height of the column, permitting control of final saturation profiles without the need for capillary barrier membranes which can potentially complicate analyses. Experiments were conducted with titanium dioxide (TiO₂) and polystyrene nanomaterials. For the strongly negatively-charged polystyrene nanomaterials, little retention was observed under the conditions studied. For the TiO₂ nanomaterials, results of the work suggest that while saturated fine sand layers may retain more nanomaterials than saturated coarse sand layers, significantly greater retention is possible in unsaturated media. Furthermore, unsaturated medium sand layers exhibited significantly greater retention than adjacent saturated fine sand layers when present at low saturations high above the water table. Retention by unsaturated media were found to correlate strongly with elevation. Free drainage experiments including both primary and secondary drainages in homogeneous columns showed evidence of redistribution during imbibition and secondary drainage, but still showed substantial unsaturated retention of TiO₂ nanoparticles high in the column, despite re-saturation with- and drainage of nanoparticle-free water.

Quantified Pore-Scale Nanoparticle Transport in Porous Media and the Implications for Colloid Filtration Theory

This study evaluates the pore-scale distribution of silver nanoparticles during transport through a sandy porous medium via quantitative synchrotron X-ray computed microtomography (qSXCMT). The associated distributions of nanoparticle flow velocities and mass flow rates were obtained by coupling these images with computational fluid dynamic (CFD) simulations. This allowed, for the first time, the comparison of nanoparticle mass flow with that assumed by the standard colloid filtration theory (CFT) modeling approach. It was found that (i) 25% of the pore space was further from the grain than assumed by the CFT model; (ii) the average pore velocity agreed well between results of the coupled qSXCMT/CFD approach and the CFT model within the model fluid envelope, although the former were 2 times larger than the latter in the centers of the larger pores and individual velocities were upwards of 20 times those in the CFT model at identical distances from grain surfaces ; and (iii) approximately 30% of all nanoparticle mass and 38% of all nanoparticle mass flow occurred further away from the grain surface than expected by the CFT model. This work suggests that a significantly smaller fraction of nanoparticles than expected will contact a grain surface by diffusion via CFT models, likely contributing to inadequate CFT model nanoparticle transport predictions.

Transport of engineered nanoparticles in partially saturated sand columns

The vadose zone is a critical region controlling fate and transport of contaminants in soils and, ultimately, groundwater. It is therefore important to understand the behavior of engineered nanoparticles (ENPs) in this zone, as a potential group of emerging contaminants. Soil is a significant sink for ENPs; however, only a few studies have considered the fate and transport of ENPs in partially saturated systems, representative of the vadose zone. Here, transport behavior of three commonly used ENPs--gold (Au-NPs), silver (Ag-NPs) and zinc oxide (ZnO-NPs)--is investigated in partially saturated sand columns. High mobilities of Au-NPs and Ag-NPs under different water saturation levels and concentrations were observed. The presence of CaCl2 reduces Ag-NP mobility through chemical interactions, similar to behavior reported in saturated systems. Furthermore, transformation of Ag-NPs in the environment may influence their mobility; aging of Ag-NPs following sulfidation was investigated. The silver sulfide (Ag2S-NPs) remained stable in aqueous suspension, and mobile in the partially saturated sand column. In contrast, the positively-charged ZnO-NPs were completely immobilized in the sand column. Significantly, though, addition of humic acid (HA) to the ZnO-NP suspension reverses particle surface charge and thus increases their mobility. Moreover, remobilization of entrapped ZnO-NPs by HA was demonstrated.

The phytotoxicity of ZnO nanoparticles on wheat varies with soil properties

Zn is an essential element for plants yet some soils are Zn-deficient and/or have low Zn-bioavailability. This paper addresses the feasibility of using ZnO nanoparticles (NPs) as soil amendments to improve Zn levels in the plant. The effects of soil properties on phytotoxicity and Zn bioavailability from the NPs were studied by using an acidic and a calcareous alkaline soil. In the acid soil, the ZnO NPs caused dose-dependent phytotoxicity, observed as inhibition of elongation of roots of wheat, Triticum aestivum. Phytotoxicity was mitigated in the calcareous alkaline soil although uptake of Zn from the ZnO NPs occurred doubling the Zn level compared to control plants. This increase occurred with a low level of Zn in the soil solution as expected from the interactions of Zn with the soil components at the alkaline pH. Soluble Zn in the acid soil was 200-fold higher and shoot levels were tenfold higher than from the alkaline soil correlating with phytotoxicity. Mitigation of toxicity was not observed in plants grown in sand amended with a commercial preparation of humic acid: growth, shoot uptake and solubility of Zn from the NPs was not altered by the humic acid. Thus, variation in humic acid between soils may not be a major factor influencing plant responses to the NPs. These findings illustrate that formulations of ZnO NPs to be used as a soil amendment would need to be tuned to soil properties to avoid phytotoxicity yet provide increased Zn accumulations in the plant.

Deposition kinetics of quantum dots and polystyrene latex nanoparticles onto alumina: role of water chemistry and particle coating

A clear understanding of the factors controlling the deposition behavior of engineered nanoparticles (ENPs), such as quantum dots (QDs), is necessary for predicting their transport and fate in natural subsurface environments and in water filtration processes. A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to study the effect of particle surface coatings and water chemistry on the deposition of commercial QDs onto Al2O3. Two carboxylated QDs (CdSe and CdTe) with different surface coatings were compared with two model nanoparticles: sulfate-functionalized (sPL) and carboxyl-modified (cPL) polystyrene latex. Deposition rates were assessed over a range of ionic strengths (IS) in simple electrolyte (KCl) and in electrolyte supplemented with two organic molecules found in natural waters; namely, humic acid and rhamnolipid. The Al2O3 collector used here is selected to be representative of oxide patches found on the surface of aquifer or filter grains. Deposition studies showed that ENP deposition rates on bare Al2O3 generally decreased with increasing salt concentration, with the exception of the polyacrylic-acid (PAA) coated CdTe QD which exhibited unique deposition behavior due to changes in the conformation of the PAA coating. QD deposition rates on bare Al2O3 were approximately 1 order of magnitude lower than those of the polystyrene latex nanoparticles, likely as a result of steric stabilization imparted by the QD surface coatings. Adsorption of humic acid or rhamnolipid on the Al2O3 surface resulted in charge reversal of the collector and subsequent reduction in the deposition rates of all ENPs. Moreover, the ratio of the two QCM-D output parameters, frequency and dissipation, revealed key structural information of the ENP-collector interface; namely, on bare Al2O3, the latex particles were rigidly attached as compared to the more loosely attached QDs. This study emphasizes the importance of considering the nature of ENP coatings as well as organic molecule adsorption onto particle and collector surfaces to avoid underestimating ENP mobility in natural and engineered aquatic environments.

Aggregation, dissolution, and stability of quantum dots in marine environments: importance of extracellular polymeric substances

There is an increasing concern that a considerable fraction of engineered nanoparticles (ENs), including quantum dots (QDs), will eventually find their way into the marine environment and have negative impacts on plankton. As ENs enter the ocean, they will encounter extracellular polymeric substances (EPS) from microbial sources before directly interacting with plankton cells. In this study, EPS harvested from four phytoplankton species, Amphora sp., Dunaliella tertiolecta, Phaeocystis globosa, and Thalassiosira pseudonana, were examined for potential interactions with CdSe nonfunctionalized and functionalized (carboxyl- and amine-) QDs in artificial seawater. Our results show that EPS do not reduce the solubility of QDs but rather decrease their stability. The degradation rate of QDs was positively correlated to the protein composition of EPS (defined by the ratio of protein/carbohydrate). Two approaches showed significant inhibition to the degradation of carboxyl-functionalized QDs: (1) the presence of an antioxidant, such as N-acetyl cysteine, and (2) absence of light. Owing to the complexity in evaluating integrated effects of QDs intrinsic properties and the external environmental factors that control the stability of QDs, conclusions must be based on a careful consideration of all these factors when attempting to evaluate the bioavailability of QDs and other ENs in the marine environments.

Transport of silver nanoparticles (AgNPs) in soil

The effect of soil properties on the transport of silver nanoparticles (AgNPs) was studied in a set of laboratory column experiments, using different combinations of size fractions of a Mediterranean sandy clay soil. The AgNPs with average size of ~30nm yielded a stable suspension in water with zeta potential of -39mV. Early breakthrough of AgNPs in soil was observed in column transport experiments. AgNPs were found to have high mobility in soil with outlet relative concentrations ranging from 30% to 70%, depending on experimental conditions. AgNP mobility through the column decreased when the fraction of smaller soil aggregates was larger. The early breakthrough pattern was not observed for AgNPs in pure quartz columns nor for bromide tracer in soil columns, suggesting that early breakthrough is related to the nature of AgNP transport in natural soils. Micro-CT and image analysis used to investigate structural features of the soil, suggest that soil aggregate size strongly affects AgNP transport in natural soil. The retention of AgNPs in the soil column was reduced when humic acid was added to the leaching solution, while a lower flow rate (Darcy velocity of 0.17cm/min versus 0.66cm/min) resulted in higher retention of AgNPs in the soil. When soil residual chloride was exchanged by nitrate prior to column experiments, significantly improved mobility of AgNPs was observed in the soil column. These findings point to the importance of AgNP-soil chemical interactions as a retention mechanism, and demonstrate the need to employ natural soils rather than glass beads or quartz in representative experimental investigations.

Mobility of functionalized quantum dots and a model polystyrene nanoparticle in saturated quartz sand and loamy sand

Quantum dots (QDs) are one example of engineered nanoparticles (ENPs) with demonstrated toxic effects. Yet, little is known about the behavior of QDs in the natural environment. This study assessed the transport of two commercial carboxylated QDs (CdTe and CdSe) and carboxylated polystyrene latex (nPL) as a model nanoparticle using saturated laboratory-scale columns. The influence of solution ionic strength (IS) and cation type (K(+) or Ca(2+)) on the transport potential of these ENPs was examined in two granular matrices - quartz sand and loamy sand. The retention of all three particles was generally low in the quartz sand columns within the range of studied IS (0.1-100 mM) for the monovalent salt (KCl). In contrast, the retention of the three ENPs in the quartz sand was significant in the presence of 10 mM Ca(2+). Moreover, ENP attachment efficiencies (α) were enhanced by at least 1 order of magnitude in columns packed with loamy sand (for IS between 0.1-10 mM KCl). Although all three ENPs used here are carboxylated, they differ in the type of surface coating (e.g., choice of polymers or polyelectrolytes). Regardless of the surface coatings, the three ENPs exhibit comparable mobility in the quartz sand. However, the ENPs demonstrate variable transport potential in loamy sand suggesting that differences in the binding affinities of surface-modified ENPs for specific soil constituents can play a key role in the fate of ENPs in soils.