Co-transport of gold nanospheres with single-walled carbon nanotubes in saturated porous media

Insights on the Dynamics and Toxicity of Nanoparticles in Environmental Matrices

The manufacturing rate of nanoparticles (10–100 nm) is steadily increasing due to their extensive applications in the fabrication of nanoproducts related to pharmaceuticals, cosmetics, medical devices, paints and pigments, energy storage etc. An increase in research related to nanotechnology is also a cause for the production and disposal of nanomaterials at the lab scale. As a result, contamination of environmental matrices with nanoparticles becomes inevitable, and the understanding of the risk of nanoecotoxicology is getting larger attention. In this context, focusing on the environmental hazards is essential. Hence, this manuscript aims to review the toxic effects of nanoparticles on soil, water, aquatic, and terrestrial organisms. The effects of toxicity on vertebrates, invertebrates, and plants and the source of exposure, environmental and biological dynamics, and the adverse effects of some nanoparticles are discussed.

Co-transport of negatively charged nanoparticles in saturated porous media: Impacts of hydrophobicity and surface O-functional groups

Graphene oxide (GO) and polystyrene nanoplastic (PSNP) are typical carbonaceous nanomaterials which likely co-exist in soil and sediment. Here, we describe the transport of GO, irradiation reduced GO (RGO) and PSNP in saturated quartz sand both in single and binary systems. In the single transport system, the materials exhibited mobility in the order of GO > RGO > PSNP, due to increased hydrophobicity and decreased negative surface charges. Nevertheless, the co-transport of (R)GO and PSNP in the binary transport system was much more intricate. In Na⁺ saturated porous media, PSNP preferred to interact with (R)GO relative to the highly negatively charged quartz sand, thus (R)GO carried PSNP to break through the sand column. However, in Ca²⁺ saturated porous media, the transport of both (R)GO and PSNP was depressed, attributed to the particle-collector and particle-particle bridging effects between Ca²⁺ and the metal-complexing moieties of the nanoparticles and sand grains. Moreover, GO influenced the co-transport of PSNP to a larger extent than RGO, especially at relatively high ionic strength, because of the more abundant surface O-functional groups on GO providing more complexion sites with Ca²⁺. These results demonstrated that the transport of negatively charged nanomaterials was greatly related to the hydrophobicity and surface O-functional groups.

Graphene oxide nanoparticles and hematite colloids behave oppositely in their co-transport in saturated porous media

Since iron oxide minerals are ubiquitous in natural environments, the release of graphene oxide (GO) into environmental ecosystems can potentially interact with iron oxide particles and thus alter their surface properties, resulting in the change of their transport behaviors in subsurface systems. Column experiments were performed in this study to investigate the co-transport of GO nanoparticles and hematite colloids (a model representative of iron oxides) in saturated sand. The results demonstrated that the presence of hematite inhibited GO transport in quartz sand columns due to the formation of less negatively charged GO-hematite heteroaggregates and additional deposition sites provided by the adsorbed hematite on sand surfaces. Contrarily, GO co-present in suspensions significantly enhanced the transport of hematite colloids through different mechanisms such as the increase of electrostatic repulsion, decreased physical straining, GO-facilitated transport of hematite (i.e., highly mobile GO nanoparticles served as a mobile carrier for hematite). We also found that the co-transport behaviors of GO and hematite depended on solution chemistry (e.g., pH, ionic strength, and divalent cation (i.e., Ca²⁺)), which affected the electrostatic interaction as well as heteroaggregation behaviors between GO nanoparticles and hematite colloids. The findings provide an insight into the potential fate of carbon nanomaterials affected by mineral colloids existing in natural waters and soils.

Role of Electrostatics in the Heterogeneous Interaction of Two-Dimensional Engineered MoS2 Nanosheets and Natural Clay Colloids: Influence of pH and Natural Organic Matter

Few-layered molybdenum disulfide (MoS₂) nanosheets are poised to be at the core of low-voltage electronic device development. Upon environmental release, these two-dimensional (2D) structures can interact with abundant natural geocolloids. This study probes the role of dimensionality in modulating the aggregation behavior of 2D MoS₂ nanosheets with plate-like geocolloids (i.e., homoionized kaolinite and montmorillonite clays). MoS₂ nanosheets were exfoliated using an ethanol/water mixture, and aggregation kinetics were investigated with time-resolved dynamic light scattering at low monovalent salt concentrations and at three pH levels, in the presence and absence of Suwannee River humic acid (SRHA). Results indicate that pH and particle ratios are key to modulating the stability of MoS₂/clay systems. At pH 4, aggregation of MoS₂ increased with increasing MoS₂/clay ratios and approached maximum values of 0.09 and 0.06 nm/s in the binary systems with montmorillonite and kaolinite, respectively. Electrostatic attraction facilitates heteroaggregation at pH values of 4 and 6; differences in the clay structures (i.e., face-face or face-edge aggregates) might explain the resulting MoS₂/clay aggregate configurations, which were probed via the evolution of particle size distribution. The presence of only 0.1 mg/L SRHA drastically suppresses the heteroaggregation propensity of MoS₂ nanosheets with geocolloids (to less than 0.01 nm/s at all pH values tested). The high stability of these heterogeneous systems under environmentally relevant conditions can increase the likelihood for cellular uptake and long-distance transport of MoS₂.

Nanostructured Raman substrates for the sensitive detection of submicrometer-sized plastic pollutants in water

We prepared novel Raman substrates for the sensitive detection of submicron-sized plastic spheres in water. Anisotropic nanostar dimer-embedded nanopore substrates were prepared for the efficient identification of submicron-sized plastic spheres by providing internal hot spots of electromagnetic field enhancements at the tips of nanoparticles. Silver-coated gold nanostars (AuNSs@Ag) were inserted into anodized aluminum oxide (AAO) nanopores for enhanced microplastic (MP) detection. We found that surface-enhanced Raman scattering (SERS) substrates of AuNSs@Ag@AAO yielded stronger signals at the same weight percentages for polystyrene MP particles with diameters as small as 0.4 μm, whereas such behaviors could not be observed for larger MPs (diameters of 0.8 μm, 2.3 μm, and 4.8 μm). The detection limit of the submicrometer-sized 0.4 μm in our Raman measurements were estimated to be 0.005% (∼0.05 mg/g =50 ppm) along with a fast detection time of only a few min without any sample pretreatments. Our nano-sized dimensional matching substrates may provide a useful tool for the application of SERS substrates for submicrometer MP pollutants in water.

Long-term fate of ZnO-FexOy mix-nanoparticles through the saturated porous media under constant head condition

The objective of this study is to evaluate the long-term fate of the nZnO-nFe_xO_y mix nanoparticles through the natural sediment in the presence of humic acid (HA) under varying pH and natural groundwater conditions. To achieve the objectives, a series of experiments were carried out where 50 mg/l of nZnO-nFe_xO_y mix suspensions were injected for 5 pore volumes (PVs) in that porous media in the presence of 10 mg/l HA under varying pH (6 and 8) followed by flushing with deionized and natural groundwater for another 95 PVs under constant head condition. The outcome of the study suggests that during the injection of the nZnO-nFe_xO_y mix suspension, more nZnO particles retain when the suspension is prepared at pH 6 (>90%). With an increase in pH of nZnO-nFe_xO_y mix suspension and background water, the long-term release of retained nZnO particles has increased significantly (from 29.97% at pH 6 to 95.89% at pH 8). The surface charge and the electrostatic repulsion are likely to govern the detachment and release of nZnO particles. Certain fraction (3.58-7.97%) of the Zn was also found to be dissolved and eluted at the outlet when the pH of background water is maintained at 6. In the case of nFe_xO_y, extensive retention of particles is observed during injection at both pHs (6 and 8). The release of the retained particles is limited (6.34%) specifically at lower pH (pH 6). There is an increase in the release of nFe_xO_y particles (24.76%) with an increase in the pH (pH 8) of both the suspension and background solution. When groundwater is used as the background water, a slight reduction in the release of Zn (22.04%) and Fe (2.06%) is observed at a pH of 6. However, at higher pH (pH 8), significantly less amount of retained particles (2.24% of Zn and 4.96% of Fe) are released. This is mainly due to the presence of co-ions in the groundwater which resulted in less negative charge of ENPs thus having less detachment and release of Zn and Fe particles. Overall, it could be concluded that there is a risk of release of Zn and Fe (especially at high pH) in the long run in the presence of organic matter when exposed in the porous media. The extent of release of Zn and Fe would be more at higher pH and might be less in the presence of other ions and under groundwater conditions.

Transformation pathways and fate of engineered nanoparticles (ENPs) in distinct interactive environmental compartments: A review

The ever increasing production and use of nano-enabled commercial products release the massive amount of engineered nanoparticles (ENPs) in the environment. An increasing number of recent studies have shown the toxic effects of ENPs on different organisms, raising concerns over the nano-pollutants behavior and fate in the various environmental compartments. After the release of ENPs in the environment, ENPs interact with various components of the environment and undergoes dynamic transformation processes. This review focus on ENPs transformations in the various environmental compartments. The transformation processes of ENPs are interrelated to multiple environmental aspects. Physical, chemical and biological processes such as the homo- or hetero-agglomeration, dissolution/sedimentation, adsorption, oxidation, reduction, sulfidation, photochemically and biologically mediated reactions mainly occur in the environment consequently changes the mobility and bioavailability of ENPs. Physico-chemical characteristics of ENPs (particle size, surface area, zeta potential/surface charge, colloidal stability, and core-shell composition) and environmental conditions (pH, ionic strength, organic and inorganic colloids, temperature, etc.) are the most important parameters which regulated the ENPs environmental transformations. Meanwhile, in the environment, organisms encountered multiple transformed ENPs rather than the pristine nanomaterials due to their interactions with various environmental materials and other pollutants. Thus it is the utmost importance to study the behavior of transformed ENPs to understand their environmental fate, bioavailability, and mode of toxicity.

Co-transport of graphene oxide and titanium dioxide nanoparticles in saturated quartz sand: Influences of solution pH and metal ions

Increasing production and application of nanomaterials lead to their environmental release possible. The nanomaterials with different properties may transport together in porous media, and consequently affect their environmental fates. In this study, column experiments were conducted to investigate the co-transport of two typical nanomaterials, graphene oxide (GO) and nano-titanium dioxide (nTiO₂), in saturated quartz sand in NaCl and CaCl₂ electrolyte solutions under both favorable and unfavorable conditions. The breakthrough curves as well as the retained profiles of single and binary nanoparticles were examined. The results indicated that nTiO₂ significantly enhanced the GO retention under all examined conditions, especially at lower pH, higher ionic strength and the presence of divalent cation Ca²⁺. This might be attributed to the formation of less negatively charged and larger-sized GO-nTiO₂ agglomerates as well as the increased retention sites on sand surface by preferentially deposited nTiO₂. However, GO merely slightly enhanced the transport of nTiO₂ in NaCl solutions, whereas had negligible effect on nTiO₂ transport and retention in CaCl₂ solutions. The highly hydrophilic and mobile GO served as a carrier and facilitated the transport of nTiO₂ in NaCl solutions. In CaCl₂ solutions, the strong attachment affinity between positively charged nTiO₂ and negatively charged quartz sand (at pH 4.5), and dramatical accumulation of large nTiO₂ agglomerates near the column inlets (at pH 6.5) led to significant deposition of nTiO₂ on quartz sand. The co-presence of GO failed to counteract the retention of nTiO₂ particles on sand.

Effect of fullerol nanoparticles on the transport and release of copper ions in saturated porous media

While the application and discharge of carbon nanomaterials (CNMs) increased rapidly, the research on the environmental safety of CNMs is also increasing. The high dispersity and mobility of modified CNMs in environmental media may have impacts on the environmental behavior of heavy metals. This work mainly studied the effect of fullerol nanoparticles (C₆₀(OH)_n) on Cu²⁺ transport, sorption, and release in water-saturated porous media. The results showed that due to the strong adsorption capacity of C₆₀(OH)_n for Cu²⁺, the transport of Cu²⁺ could be facilitated. However, with the pre-existence of C₆₀(OH)_n in porous media, the transport of Cu²⁺ was also slightly enhanced. In addition, when loaded into the pre-contaminated porous medium, the C₆₀(OH)_n also enhanced the release of retained Cu²⁺, which implies a high environmental risk of C₆₀(OH)_n.

Cotransport of biochar and Shewanella oneidensis MR-1 in saturated porous media: Impacts of electrostatic interaction, extracellular electron transfer and microbial taxis

Biochar widely applied to soil can influence microbial community composition and participate in extracellular electron transfer (EET). However, little is known about the cotransport behaviors of bacteria and biochar in aquifer and soil-water environments, which can affect the fate and application performance of biochar. In this study, we found that in comparison to their individual transport behaviors, the mobilities of cotransporting Shewanella oneidensis MR-1 and biochar colloid (BC) were significantly inhibited. The decreasing colloidal mobilities at higher ionic strengths signified the importance of electrostatic interaction between cell and BC in cotransport. Moreover, the less suppressed cotransport of BC and mutants defective of EET and the elevated inhibition effects on cotransport by adding exogenous electron donor suggested the importance of EET. Difference in cotransport behavior was also observed with BC having different redox states. Compared with oxidized BC, reduced BC with higher hydrophobicity led to easier aggregation with cell and higher retention in column. More importantly, MR-1 exhibited EET-dependent taxis towards biochar, which also contributed to the enhanced heteroaggregation and decreased mobilities of cell and biochar. Our results highlight that metabolic activities of microbes towards abiotic colloids cannot be neglected when assessing their transport behaviors, especially in subsurface environments abounded with redox-active inorganic particles and microbes performing extracellular respiration.

Transport and retention behavior of carbonaceous colloids in natural aqueous medium: Impact of water chemistry

Carbon based materials are emerging as a sustainable alternative to their metal-oxide counterparts. However, their transport behavior under natural aqueous environment is poorly understood. This study investigated the transport and retention profiles of carbon nanoparticles (CNPs) and graphene oxide quantum dots (GOQDs) through column experiments in saturated porous media. CNPs and GOQDs (30 mg/L) were dispersed in natural river water (RW) and passed through the column at a flow rate of 1 mL/min, which mimicking the natural water flow rate. After every 10 min, the column effluents were collected and the mass recovery and retention profiles were monitored. Results indicated that the transport of both carbonaceous colloids was predominantly controlled by surface potential and ionic composition of natural water. The CNPs with its high surface potential (-40 mV) exhibited more column transport and was less susceptible to solution pH (5.6-6.8) variation as compared to GOQDs (-24 mV). The results showed that, monovalent salt (NaCl) was one of the dominating factors for the retention and transport of carbonaceous colloids compared to divalent salt (CaCl₂). Furthermore, the presence of natural organic matter (NOM) increased the transport of both carbonaceous colloids and thereby decreases the tendency for column retention.

Cotransport of nanoplastics (NPs) with fullerene (C60) in saturated sand: Effect of NPs/C60 ratio and seawater salinity

Nanoplastics (NPs) have been identified as newly emerging particulate contaminants. In marine environments, the interaction between NPs and other engineered nanoparticles remains unknown. This study investigated the cotransport of NPs with fullerene (C₆₀) in seawater-saturated columns packed with natural sand as affected by the mass concentration ratio of NPs/C₆₀ and the hydrochemical characteristics. In seawater with 35 practical salinity units (PSU), NPs could remarkably enhance C₆₀ dispersion with a NPs/C₆₀ ratio of 1. NPs behaved as a vehicle to facilitate C₆₀ transport by decreasing colloidal ζ-potential and forming stable primary heteroaggregates. As the NPs/C₆₀ ratio decreased to 1/3, NPs mobility was progressively restrained because of the formation of large secondary aggregates. When the ratio continuously decreased to 1/10, the stability and transport of colloids were governed by C₆₀ rather than NPs. Under this condition, the transport trend of binary suspensions was similar to that of single C₆₀ suspension, which was characterized by a ripening phenomenon. Seawater salinity is another key factor affecting the stability and associated transport of NPs and C₆₀. In seawater with 3.5 PSU, NPs and C₆₀ (1:1) in binary suspension exhibited colloidal dispersion, which was driven by a high-energy barrier. Thus, the profiles of the cotransport and retention of NPs/C₆₀ resembled those of single NPs suspension. This work demonstrated that the cotransport of NPs/C₆₀ strongly depended on their mass concentration ratios and seawater salinity.

Deposition of engineered nanoparticles (ENPs) on surfaces in aquatic systems: a review of interaction forces, experimental approaches, and influencing factors

The growing development of nanotechnology has promoted the wide application of engineered nanomaterials, raising immense concern over the toxicological impacts of nanoparticles on the ecological environment during their transport processes. Nanoparticles in aquatic systems may undergo deposition onto environmental surfaces, which affects the corresponding interactions of engineered nanoparticles (ENPs) with other contaminants and their environmental fate to a certain extent. In this review, the most common ENPs, i.e., carbonaceous, metallic, and nonmetallic nanoparticles, and their potential ecotoxicological impacts on the environment are summarized. Colloidal interactions, including Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO forces, involved in governing the depositional behavior of these nanoparticles in aquatic systems are outlined in this work. Moreover, laboratory approaches for examining the deposition of ENPs on collector surfaces, such as the packed-bed column and quartz crystal microbalance (QCM) method, and the limitations of their applications are outlined. In addition, the deposition kinetics of nanoparticles on different types of surfaces are critically discussed as well, with emphasis on other influencing factors, including particle-specific properties, particle aggregation, ionic strength, pH, and natural organic matter. Finally, the future outlook and challenges of estimating the environmental transport of ENPs are presented. This review will be helpful for better understanding the effects and transport fate of ENPs in aquatic systems. Graphical abstract ᅟ.

Differences in transport behavior of natural soil colloids of contrasting sizes from nanometer to micron and the environmental implications

Transport behaviors of nanoparticles (<100nm) and clay fractions (clay particles, <2μm; coarse clay particles, 1-2μm and fine clay particles, 0.1-1μm) extracted from two natural soils (Inceptisol from Jilin and Oxisol from Hainan, China) were investigated in saturated sand columns at 1-30mM NaCl and pH5-9. Increasing NaCl concentrations decreased the mobility, while increasing pH increased the mobility of soil particles of various sizes. At pH5 and 30mM NaCl, nanoparticles and clay fractions exhibited the different transport behaviors, and ripening was observed for Inceptisol nanoparticles while blocking for Oxisol nanoparticles in breakthrough curves (BTCs). The effluent mass recoveries (MRs) of nanoparticles were much more than that of clay particles for both two soils (>1.9-fold) at all tested conditions, except for Inceptisol at pH5 and 30mM NaCl (with comparable MR). According to Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations and particle-collector size ratios, both secondary energy minimum and physical straining led to the retention of clay fractions at pH5 and 30mM NaCl, whereas primary energy minimum and straining induced by simultaneous aggregation caused the retention of nanoparticles. The experimental attachment efficiency between soil particles of various sizes and sand collector for both two soils was in the order nanoparticles<fine clay particles<clay particles<coarse clay particles, indicating that soil nanoparticles had greater mobility than clay fractions. Consequently, nanoparticles are suggested to have greater risks to transport contaminants to surface and groundwater, once they are released from soils.

Aggregation Behavior of Multiwalled Carbon Nanotube-Titanium Dioxide Nanohybrids: Probing the Part-Whole Question

Multiwalled carbon nanotube-titanium dioxide (MWNT-TiO2) nanohybrids (NHs), a promising support for electrocatalysts, have a high likelihood of environmental release. Aggregation of these NHs may or may not be captured by the sum of their component behavior, thus necessitating a systematic evaluation. This study probes the "part-whole question" by systematically evaluating the role of TiO2 loading (C:Ti molar ratios of 1:0.1, 1:0.05 and 1:0.033) on the aggregation behavior of these NHs. Aggregation kinetics of these in-house synthesized (using a sol-gel method) NHs and the components is investigated with time-resolved dynamic light scattering in the presence of mono- and divalent cations and with and without Suwannee River humic acid. A deviation in the aggregation behavior from classical electrokinetic theory has been observed which indicates that the material complexity has a strong influence in the observed behavior; hence other material attributes (e.g., fractal dimension, surface roughness, charge heterogeneity, etc.) should be carefully considered when studying such materials. The sum of the aggregation behavior of the parts may not capture that of the whole (i.e., of the NHs); aggregation depends on the TiO2 loading and also on the hybridization process and the background aquatic chemistry.

Oxytetracycline increases the mobility of carbon nanotubes in porous media

The effect of engineered nanoparticles on the mobility of co-existing contaminants has been increasingly studied, while the reverse effect receives little attention. This study provides results from investigating the effect of oxytetracycline (OTC) on the mobility of oxidized multi-walled carbon nanotubes (O-MWCNTs) in quartz sand (QS) columns at various solution ionic strengths (ISs) and pHs. The mobility of O-MWCNTs in QS columns was significantly enhanced by the presence of OTC under all of the tested solution conditions (IS: 0.1, 1.0, and 10mM; pH: 3.0, 5.5, and 8.5), with an increase of 8.6-50.9%. Such enhancement was nonlinear over OTC concentration, which firstly increased (0 to 2.5mgL^-1 OTC) and then decreased (2.5 to 20mgL^-1OTC) at pH5.5. The major contributor to the OTC-enhanced O-MWCNTs mobility was competition of the two analytes for adsorption sites on the QS surface. Batch attachment results show that the adsorption of O-MWCNTs in the presence of OTC onto QS was also nonlinear with OTC concentration (firstly decreased and then increased with increasing OTC) at pH5.5, which gave the plausible explanation for the nonlinear enhancement of O-MWCNTs transport in QS columns by the presence of OTC. In turn, both the carrier and competition actions of O-MWCNTs determined the mobility of OTC in QS columns and the carrier action was stronger when more OTC was associated with O-MWCNTs in the influent. These results imply that the mobility of O-MWCNTs in OTC polluted water and soil can be significantly stronger than that in non-polluted area.

CAPSULE

OTC can increase the migration of O-MWCNTs mainly through the competition for adsorption sites on collectors.

Insights into Metal Oxide and Zero-Valent Metal Nanocrystal Formation on Multiwalled Carbon Nanotube Surfaces during Sol-Gel Process

Carbon nanotubes are hybridized with metal crystals to impart multifunctionality into the nanohybrids (NHs). Simple but effective synthesis techniques are desired to form both zero-valent and oxides of different metal species on carbon nanotube surfaces. Sol-gel technique brings in significant advantages and is a viable technique for such synthesis. This study probes the efficacy of sol-gel process and aims to identify underlying mechanisms of crystal formation. Standard electron potential (SEP) is used as a guiding parameter to choose the metal species; i.e., highly negative SEP (e.g., Zn) with oxide crystal tendency, highly positive SEP (e.g., Ag) with zero-valent crystal-tendency, and intermediate range SEP (e.g., Cu) to probe the oxidation tendency in crystal formation are chosen. Transmission electron microscopy and X-ray diffraction are used to evaluate the synthesized NHs. Results indicate that SEP can be a reliable guide for the resulting crystalline phase of a certain metal species, particularly when the magnitude of this parameter is relatively high. However, for intermediate range SEP-metals, mix phase crystals can be expected. For example, Cu will form Cu₂O and zero-valent Cu crystals, unless the synthesis is performed in a reducing environment.

Fullerol-facilitated transport of copper ions in water-saturated porous media: Influencing factors and mechanism

The environmental safety of carbon nanomaterials (CNMs) has become a hot spot of worldwide research work. The high stability, mobility and adsorption capability of CNMs may give rise to the enhancement of heavy metal transport. However, the understanding of facilitated transport of heavy metal ions by CNMs is still limited. In this research, fullerol nanoparticles (C₆₀(OH)_n) were used as a typical CNM to investigate its effect on Cu²⁺ transport in the water-saturated porous media. Column experiments showed that C₆₀(OH)_n could facilitate the transport of Cu²⁺. Meanwhile, flow velocity showed little impact on the facilitated transport while pH and fullerol concentration played an important role. The mechanism of fullerol-facilitated transport of Cu²⁺ was mainly due to the much higher adsorption capacity of C₆₀(OH)_n than that of the porous media. Besides, the existence of C₆₀(OH)_n decreased the adsorption kinetics of Cu²⁺ on the porous media, which led to a decreasing chance for Cu²⁺ to be retained and thus enhanced the transport of Cu²⁺.

Continuum-based models and concepts for the transport of nanoparticles in saturated porous media: A state-of-the-science review

Environmental applications of nanoparticles (NP) increasingly result in widespread NP distribution within porous media where they are subject to various concurrent transport mechanisms including irreversible deposition, attachment/detachment (equilibrium or kinetic), agglomeration, physical straining, site-blocking, ripening, and size exclusion. Fundamental research in NP transport is typically conducted at small scale, and theoretical mechanistic modeling of particle transport in porous media faces challenges when considering the simultaneous effects of transport mechanisms. Continuum modeling approaches, in contrast, are scalable across various scales ranging from column experiments to aquifer. They have also been able to successfully describe the simultaneous occurrence of various transport mechanisms of NP in porous media such as blocking/straining or agglomeration/deposition/detachment. However, the diversity of model equations developed by different authors and the lack of effective approaches for their validation present obstacles to the successful robust application of these models for describing or predicting NP transport phenomena. This review aims to describe consistently all the important NP transport mechanisms along with their representative mathematical continuum models as found in the current scientific literature. Detailed characterizations of each transport phenomenon in regards to their manifestation in the column experiment outcomes, i.e., breakthrough curve (BTC) and residual concentration profile (RCP), are presented to facilitate future interpretations of BTCs and RCPs. The review highlights two NP transport mechanisms, agglomeration and size exclusion, which are potentially of great importance in controlling the fate and transport of NP in the subsurface media yet have been widely neglected in many existing modeling studies. A critical limitation of the continuum modeling approach is the number of parameters used upon application to larger scales and when a series of transport mechanisms are involved. We investigate the use of simplifying assumptions, such as the equilibrium assumption, in modeling the attachment/detachment mechanisms within a continuum modelling framework. While acknowledging criticisms about the use of this assumption for NP deposition on a mechanistic (process) basis, we found that its use as a description of dynamic deposition behavior in a continuum model yields broadly similar results to those arising from a kinetic model. Furthermore, we show that in two dimensional (2-D) continuum models the modeling efficiency based on the Akaike information criterion (AIC) is enhanced for equilibrium vs kinetic with no significant reduction in model performance. This is because fewer parameters are needed for the equilibrium model compared to the kinetic model. Two major transport regimes are identified in the transport of NP within porous media. The first regime is characterized by higher particle-surface attachment affinity than particle-particle attachment affinity, and operative transport mechanisms of physicochemical filtration, blocking, and physical retention. The second regime is characterized by the domination of particle-particle attachment tendency over particle-surface affinity. In this regime although physicochemical filtration as well as straining may still be operative, ripening is predominant together with agglomeration and further subsequent retention. In both regimes careful assessment of NP fate and transport is necessary since certain combinations of concurrent transport phenomena leading to large migration distances are possible in either case.

Effect of reduced humic acid on the transport of ferrihydrite nanoparticles under anoxic conditions

Humic acid (HA) occurs ubiquitously in the subsurface environments and is well-known to play a critical role in the fate and transport of ferrihydrite nanoparticles (NPs) and NPs-associated contaminants. Under anoxic conditions, HA can readily be reduced by microorganisms or geochemical reducing species, and the mechanisms and kinetics of ferrihydrite reduction by reduced HA (HA_red) are well-documented; however, the role of these redox reactions on the transport of ferrihydrite NPs is largely underestimated. This study provides new knowledge regarding the role of HA (both reduced HA (HA_red) and oxidized HA (HA_ox)) of environmentally relevant concentrations (0-50 mg C/L) on the transport of ferrihydrite NPs in anoxic sand columns. Our findings show that, regardless of the redox state, the presence of a low concentration of HA (3 mg C/L) inhibited ferrihydrite NP's transport due to enhanced aggregation (and deposition) between positively charged ferrihydrite NPs and negatively charged HA molecules. In contrast, higher HA (both HA_red and HA_ox) concentration (≥10 mg C/L) significantly enhanced the mobility of ferrihydrite NPs, primarily due to the enhanced electrostatic and steric stabilization originating from excessively adsorbed HA molecules. Interestingly, the transport of ferrihydrite NPs is substantially lower in the presence of HA_red than in the presence of HA_ox. This distinct effect (HA_red vs. HA_ox) on the particle transport is attributed to the fact that reductive dissolution of ferrihydrite NPs occurs in the presence of HA_red (ferrihydrite dissolves and thus total breakthrough decreases), but not in the presence of HA_ox. Furthermore, the abatement extent of ferrihydrite NPs transport triggered by the presence of HA_red is dependent on dissolved HA_red concentration. Taken together, our findings provide direct, and much needed insights into the distinct roles of redox state of HA on the transport of redox-sensitive metal-bearing NPs in porous media.

Impact of Redox Reactions on Colloid Transport in Saturated Porous Media: An Example of Ferrihydrite Colloids Transport in the Presence of Sulfide

Transport of colloids in the subsurface is an important environmental process with most research interests centered on the transport in chemically stable conditions. While colloids can be formed under dynamic redox conditions, the impact of redox reactions on their transport is largely overlooked. Taking the redox reactions between ferrihydrite colloids and sulfide as an example, we investigated how and to what extent the redox reactions modulated the transport of ferrihydrite colloids in anoxic sand columns over a range of environmentally relevant conditions. Our results reveal that the presence of sulfide (7.8-46.9 μM) significantly decreased the breakthrough of ferrihydrite colloids in the sand column. The estimated travel distance of ferrihydrite colloids in the absence of sulfide was nearly 7-fold larger than that in the presence of 46.9 μM sulfide. The reduced breakthrough was primarily attributed to the reductive dissolution of ferrihydrite colloids by sulfide in parallel with formation of elemental sulfur (S(0)) particles from sulfide oxidation. Reductive dissolution decreased the total mass of ferrihydrite colloids, while the negatively charged S(0) decreased the overall zeta potential of ferrihydrite colloids by attaching onto their surfaces and thus enhanced their retention in the sand. Our findings provide novel insights into the critical role of redox reactions on the transport of redox-sensitive colloids in saturated porous media.