Clay minerals affect the stability of surfactant-facilitated carbon nanotube suspensions

Formation of cation bridges and its promoting mechanism for sorption of sulfamethoxazole by montmorillonite

The coexistence of metal cations is often accompanied by organic pollution and could affect the environmental fate of organics by mediating the formation of cation bridges. However, the environmental fate and risk of organics in cation co-existing environments are poorly understood due to the lack of accurate identification of cation bridge formation and stability. In this study, the sorption of sulfamethoxazole (SMX) on montmorillonite (MT) with the coexistence of three different valence metal cations (Na+, Ca2+, and Cr3+) was investigated. Ca2+ and Cr3+ can significantly promote the sorption of SMX on MT for about 5∼10 times promotion, respectively, while Na+ bridges displayed little effect on the sorption of SMX. The sorption binding energy of SMX with MT-Ca (-44.01 kcal/mol) and MT-Cr (-64.57 kcal/mol) bridges was significantly lower than that with MT-Na (-38.45 kcal/mol) and MT (-39.39 kcal/mol), indicating that the sorption affinity of SMX on Cr and Ca bridges was much stronger. The higher valence of the cations also resulted in a more stable adsorbed SMX with less desorption fluctuation. In addition, the relatively higher initial concentration of SMX and the valence of cations increased the bonding density of the cation bridges, thus promoting the apparent sorption of SMX on MT to a certain extent. This work reveals the formation and function of cation bridges in the sorption of SMX on MT. It lays a theoretical foundation for further understanding the environmental fate and risk of organics.

Asymmetric response of transition metal cationic orbitals to applied electric field

Understanding the quantum mechanical mechanisms underlying atomic/ionic interfacial processes and phenomena, particularly their dependence on the electronic orbital rearrangement of atoms/ions in an external electric field, remains a significant challenge. This study investigated the asymmetric response of transition metal (TM) cationic orbitals when subjected to an applied electric field. Quantum mechanical calculations were employed to quantify the newly formed hybrid orbitals and evaluate the corresponding orbital energies of the TM cations. Analysis of the quantitative contribution of asymmetric orbital hybridization to TM-surface interactions showed a significant change in orbital energy and increased effective charges of TM cations at the charged surface. This asymmetric response, induced by a negative external electric field generated from the structural charges of clay minerals (e.g., montmorillonite), repels electrons from the outer-shell orbital. This repulsion consequently increases the electron binding energy of the inner-shell orbitals, leading to new surface reactions, polarization-enhanced induction force, and polarization-induced covalent bonding between the TM cations and the charged surface. Our theoretical predictions regarding TM-clay mineral interactions are consistent with the experimental observations of TM cation adsorption. This finding has significant implications for the adsorptive removal of TM cations from wastewaters and for enhancing the catalytic efficiency of TM-surface catalysts. The unique physical and chemical characteristics exhibited by TMs at charged particle surfaces, resulting from their asymmetric response, can play pivotal roles in environmental and chemical engineering.

Colloidal and Thermal Stability of Three-Component Hybrid Materials Containing Clay Mineral, Polysaccharide and Surfactant

The paper presents the colloidal and thermal stability of the three-component hybrid materials containing halloysite, polysaccharides (alginic acid, cationic cellulose and hydroxyethyl cellulose) and Tritons. TX-100, TX-165 and TX-405 were used as non-ionic surfactants. Stability and other properties of the hybrid materials were tested by the following methods: UV-Vis, TGA (thermogravimetric analysis) and DSC (differential scanning calorimetry), CHN (elemental analysis), SEM-EDX (scanning electron microscopy with energy dispersive X-ray spectroscopy) and tensiometry. According to the results with the increasing polymer concentration the colloidal stability of the tested systems also increases. Moreover, the addition of the surfactants causes the increase of polysaccharide adsorption but the colloidal stability of the tested systems decreases due to large weights of formed aggregates. As follows from the thermal analysis, the comparison of the TG/DTG-DSC curves obtained for the investigated polymers confirms that their thermal decomposition courses have some common features. The obtained results have the application potential in the formation of the materials for the pollutants removal from water and sewages.

Biosurfactant-affected mobility of oxytetracycline and its variations with surface chemical heterogeneity in saturated porous media

Herein, the influences of rhamnolipid (a typical biosurfactant) on oxytetracycline (OTC) transport in the porous media and their variations with the surface heterogeneities of the media (uncoated sand, goethite (Goe)-, and humic acid (HA)-coated sands) were explored. Compared to uncoated sand, goethite and HA coatings suppressed OTC mobility by increasing deposition sites. Interestingly, rhamnolipid-affected OTC transport strongly depended on the chemical heterogeneities of aquifers and biosurfactant concentrations. Concretely, adding rhamnolipid (1-3 mg/L) inhibited OTC mobility through sand columns because of the bridging effect of biosurfactant between sand and OTC. Unexpectedly, rhamnolipid of 10 mg/L did not further improve the inhibition of OTC transport owing to the fact that the deposition capacity of rhamnolipid reached its maximum. OTC mobility in Goe-coated sand columns was inhibited by 1 mg/L rhamnolipid. However, the inhibitory effect decreased with the increasing rhamnolipid concentration (3 mg/L) and exhibited a promoted effect at 10 mg/L rhamnolipid. This surprising observation was that the increased rhamnolipid molecules gradually occupied the favorable deposition sites (i.e., the positively charged sites). In comparison, rhamnolipid facilitated OTC transport in the HA-coated sand column. The promotion effects positively correlated with rhamnolipid concentrations because of the high electrostatic repulsion and deposition site competition induced by the deposited rhamnolipid. Another interesting phenomenon was that rhamnolipid's enhanced or inhibitory effects on OTC transport declined with the increasing solution pH because of the decreased rhamnolipid deposition on porous media surfaces. These findings benefit our understanding of the environmental behaviors of antibiotics in complex soil-water systems containing biosurfactants.

Surfactant-mediated mobility of carbon dots in saturated soil: comparison between anionic and cationic surfactants

Understanding the mobility, retention, and fate of carbon dots (CDs) is critical for the risk management of this emerging carbon material. However, the influences of surfactants on CDs' transport through subsurface media are still poorly understood. Herein, column experiments were conducted to explore the different influences of an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB), on the CDs' transport in water-saturated soil. In the Na⁺ background electrolyte, both surfactants facilitated the transport of CDs at pH 7.0. The trend stemmed from steric hindrance, a decline in the straining effect, and competitive deposition between CDs and surfactant molecules. Additionally, SDBS increased the electrostatic repulsion of CDs and soil. Interestingly, in the divalent cation background electrolytes (i.e., Ca²⁺ or Cu²⁺), SDBS suppressed CDs' mobility, whereas CTAB had the opposite effect. The transport-inhibited effect of SDBS was mainly due to anionic surfactant ion (DBS^-) precipitation with metal cations and the formation of adsorbed SDBS-Cu²⁺/Ca²⁺-CDs complexes. The enhanced effect of CTAB resulted from the CTAB coating on soil grains, which suppressed the cation bridging between CDs and soil. Furthermore, the magnitude of the SDBS promotion effect was pH-dependent. Surprisingly, CTAB could inhibit CDs' mobility at pH 9.0, owing to the binding cationic surfactant's strong hydrophobicity effect on the soil surface. Moreover, the experimental breakthrough curves of CDs were well described using a two-site transport model. Overall, the observations obtained from this study shed light on the relative mobility of CDs with different surfactants in typical groundwater conditions.

The aggregation of natural inorganic colloids in aqueous environment: A review

Natural inorganic colloids (NICs) are the most common and dominant existence in the ecosystem, with high concentration and wide variety. In spite of the low toxicity, they can alter activity and mobility of hazardous engineered nanoparticles (ENPs) through different interactions, which warrants the necessity to understand and predict the fate and transport of NICs in aquatic ecosystems. Here, this review summarized NICs properties and behaviors, interaction mechanisms and environmental factors at the first time. Various representative NICs and their physicochemical properties were introduced across the board. Then, the aggregation and sedimentation behaviors were discussed systematically, mainly concerning the heteroaggregation between NICs and ENPs. To speculate their fate and elucidate the corresponding mechanisms, the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO (X-DLVO) theories were focused. Furthermore, a range of intrinsic and extrinsic factors was presented in different perspective. Last but not the least, this paper pointed out theoretical and analytical gaps in current researches, and put forward suggestions for further research, aiming to provide a more comprehensive and original perspective in the fields of natural occurring colloids.

Surfactants-mediated the enhanced mobility of tetracycline in saturated porous media and its variation with aqueous chemistry

Knowledge of the mobility of tetracycline (TC) antibiotics in porous media is critical to understand their potential environmental influences. The transport characteristics of TC in sand columns with three different surfactants, including Tween 80, sodium dodecylbenzene sulfonate (SDBS), and didodecyldimethylammonium bromide (DDAB) under various conditions were investigated in this study. Results demonstrated that all surfactants enhanced TC transport under neutral conditions (10 mM NaCl at pH 7.0). The observation was attributed mostly to deposition site competition, higher electrostatic repulsion between TC molecules and sand grains, steric hindrance, and the increase of TC hydrophilicity. Furthermore, the order of the transport-enhancement effects was generally observed as follows: DDAB > SDBS > Tween 80. The trend was controlled by the variation in the physicochemical properties of surfactants. It was noticed that the presence of Cu²⁺ (a model divalent cation) in the background solution, the cation-bridging contributed to the promotion effects of DDAB or Tween 80 on TC mobility. Interestingly, SDBS considerably suppressed TC transport due to the precipitation of SDBS-Cu²⁺ complexes onto sand surfaces. Moreover, the enhancement order of surfactants at pH 5.0 was similar to that pH 7.0. However, DDAB could inhibit TC transport in sand columns at pH 9.0, which were mainly caused by the decrease of electrostatic repulsion and the hydrophobicity induced by the binding cationic surfactant. Findings from this work provide novel insight into involvement of surfactants in antibiotic transport behaviors in the subsurface environment.

Exposed facets mediated interaction of polystyrene nanoplastics (PSNPs) with iron oxides nanocrystal

Nanoplastics (NPs), which are often detected in the natural environment, are regarded as a group of emerging pollutants. Hematite is a substance that exists widely in the surface environment and has an important impact on the environmental behavior of pollutants. Clarifying the migration of NPs requires an in-depth understanding of intrinsic interaction mechanisms of NPs with iron-containing minerals. The interaction process of polystyrene nanoplastics (PSNPs) on the hematite exposed facets was systematically studied by experiments under different conditions, adsorption isotherm curves, Fourier Transform infrared (FTIR) spectroscopy and two-dimensional correlation spectroscopy (2D-COS) analyses. We found that PSNPs were adsorbed on the three exposed faces of hematite ({001}, {012}, and {100}) by electrostatic interaction, respectively, but the capacities for PSNPs were different. Adsorption models were established to explore the preferred interaction surface dependent on the exposed facets, and it was found that {012} surfaces were more favorable for PSNPs adsorption, while {001} surface has better adsorption capacity for PSNPs than {100} surface, which is due to the different density and proportion of hydroxyl groups on the exposed facets of hematite. These findings elucidated the dependence of PSNPs adsorption on the hematite facets, and illustrated t the effect of hematite on the migration of PSNPs in the environment.

Aging significantly increases the interaction between polystyrene nanoplastic and minerals

With the massive use and discarding of plastic products, plastic particles, including nanoplastics (NPs), which are continuously released under the action of environmental factors, are posing greater risk to the ecosystem and human health. NPs exposed to the environment experience aging, which can significantly change their physical and chemical properties and affect their environmental behavior. Here, we examined the adsorption behavior of polystyrene nanoplastic (PSNP) aging by ultraviolet (UV) exposure on different minerals (goethite, magnetite, kaolinite and montmorillonite). Aging not only changes the surface morphology of PSNP, but also increases the surface negative charge and produces a large number of oxygen-containing functional groups (OFGs). Incubation of aged PSNP with minerals indicated that iron oxides (goethite and magnetite) showed stronger interactions with aged PSNP than pristine PSNP, and there was an interaction between clay minerals and aged PSNP. The adsorption experiments and scanning electron microscopy (SEM) suggested that the higher adsorption capacity of a mineral surface to aged PSNP may be related to electrostatic attraction and ligand exchange. The Fourier transform infrared (FTIR) spectra after adsorption showed that the adsorption affinity between the functional groups was different, and two-dimensional correlation spectroscopy (2D-COS) analysis further indicated that the mineral preferentially adsorbed the aged PSNP in accordance with the order of OFGs. The findings provide a theoretical basis for scientific evaluation of ecological risks of NPs in the environment.

Transport of ZIF-8 in porous media under the influence of surfactant type and nanoparticle concentration

Knowledge of the fate and transport of metal-organic frameworks (MOFs) in porous media is essential to understanding their environmental impacts. However, to date, the transport mechanisms of MOFs are not fully revealed. Meanwhile, surfactants can promote MOFs dispersion by forming a stable suspension. They also allow MOFs to migrate in the aqueous environment, which would increase the risks of MOFs being exposed to human health and the ecological environment. In this study, the effect of surfactants type and nanoparticle (NP) concentrations (50, 100, and 200 mg/L) were investigated using a sand column to study the transportability of ZIF-8 NPs in saturated porous media. Surfactants used were categorized into three groups, including cationic surfactants (CTAB, DTAB), anionic surfactants (SDBS, SDS), and nonionic surfactants (Tween 80, Tween 20). Experimental results showed that the ionic surfactants significantly increased the transportability of ZIF-8 NPs. Furthermore, a low concentration of NPs tended to break through the column under ionic surfactant conditions, and the maximum effluent recovery of ZIF-8 NPs (50 mg/L) was 87.4% in the presence of SDS. Nevertheless, ZIF-8 NPs tended to deposit in the inlet of the sand column in the presence of nonionic surfactants due to hydrodynamic bridging and straining. This research provides a comprehensive understanding of the deposition mechanism of ZIF-8 NPs as affected by surfactant types and NP concentrations. Most importantly, the study highlights those ionic surfactants had a significant impact on the mobility of ZIF-8 NPs, which arouses attention to the ecological and human health risk assessment related to the manufacturing of MOFs with the aid of various dispersing agents.

Nanoplastics interaction with feldspar and weathering originated secondary minerals (kaolinite and gibbsite) in the riverine environment

Despite the massive accumulation of nanoplastics (NPs) in the freshwater system, research so far has highly focused on the marine environment. NPs interaction with mineral surfaces can influence their fate in freshwater, which will further impact their bioavailability and transport to the oceans. Current work focuses on understanding NPs interaction with weathering sequence of minerals in freshwater under varying geochemical conditions. Primary mineral feldspar and weathering originated secondary minerals, i.e., kaolinite and gibbsite, were investigated for interaction with NPs under batch mode under relevant environmental conditions. Minerals-NPs interaction was also investigated in natural water samples. Results showed that the amorphous nature, small particle size, and positive surface charge of gibbsite resulted in multi-fold sorption of NPs (108.1 mg/g) compared to feldspar (7.7 mg/g) and kaolinite (11.9 mg/g). FTIR spectroscopy revealed hydrogen bonding and complexation as major players in gibbsite-NPs interaction suggesting the possibility of their co-precipitation. The continuous adsorption-desorption and limited sorption capacity of feldspar and kaolinite can be attributed to their negative surface charge, larger size, crystalline nature, and physical sorption. Therefore, both minerals may co-transport and enhance the mobility of NPs.

A critical review on the role of abiotic factors on the transformation, environmental identity and toxicity of engineered nanomaterials in aquatic environment

Engineered nanomaterials (ENMs) are at the forefront of many technological breakthroughs in science and engineering. The extensive use of ENMs in several consumer products has resulted in their release to the aquatic environment. ENMs entering the aquatic ecosystem undergo a dynamic transformation as they interact with organic and inorganic constituents present in aquatic environment, specifically abiotic factors such as NOM and clay minerals, and attain an environmental identity. Thus, a greater understanding of ENM-abiotic factors interactions is required for an improved risk assessment and sustainable management of ENMs contamination in the aquatic environment. This review integrates fundamental aspects of ENMs transformation in aquatic environment as impacted by abiotic factors, and delineates the recent advances in bioavailability and ecotoxicity of ENMs in relation to risk assessment for ENMs-contaminated aquatic ecosystem. It specifically discusses the mechanism of transformation of different ENMs (metals, metal oxides and carbon based nanomaterials) following their interaction with the two most common abiotic factors NOM and clay minerals present within the aquatic ecosystem. The review critically discusses the impact of these mechanisms on the altered ecotoxicity of ENMs including the impact of such transformation at the genomic level. Finally, it identifies the gaps in our current understanding of the role of abiotic factors on the transformation of ENMs and paves the way for the future research areas.

The dichotomy of nanotechnology as the cutting edge of agriculture: Nano-farming as an asset versus nanotoxicity

The unprecedented setbacks and environmental complications, faced by global agro-farming industry, have led to the advent of nanotechnology in agriculture, which has been recognized as a novel and innovative approach in development of sustainable farming practices. The agricultural regimen is the "head honcho" of the world, however presently certain approaches have been imposing grave danger to the environment and human civilization. The nano-farming paradigm has successfully elevated the growth and development of plants, parallel to the production, quality, germination/transpiration index, photosynthetic machinery, genetic progression, and so on. This has optimized the traditional farming into precision farming, utilising nano-based sensors and nanobionics, smart delivery tools, nanotech facets in plant disease management, nanofertilizers, enhancement of plant adaptive potential to external stress, role in bioenergy conservation and so on. These applications portray nanorevolution as "the big cheese" of global agriculture, mitigating the bottlenecks of conventional practices. Besides the applications of nanotechnology, the review identifies the limitations, like possible harmful impact on environment, mankind and plants, as the "Achilles heel" in agro-industry, aiming to establish its defined role in agriculture, while simultaneously considering the risks, in order to resolve them, thus abiding by "technology-yes, but safety-must". The authors aim to provide a significant opportunity to the nanotech researchers, Botanists and environmentalists, to promote judicial use of nanoparticles and establish a secure and safe environment.

Evaluation of the dispersion of metakaolin-graphene oxide hybrid in water and cement pore solution: can metakaolin really improve the dispersion of graphene oxide in the calcium-rich environment of hydrating cement matrix?

Graphene oxide (GO) is a promising candidate for reinforcing cement composites due to its prominent mechanical properties and good dispersibility in water. However, the severe agglomeration of GO nanosheets in the Ca2+ ion loaded environment of a freshly mixed cement composite is the main obstacle against the mentioned goal. Recent studies, based on the SEM images, have shown that the incorporation of pozzolans can ameliorate the GO agglomeration in cement matrix. Considering the fact that, for identifying the GO dispersion in cement matrix, SEM characterization is not preferred due to the hydrated cement matrix complexity and presence of small dosages of GO, this research has investigated the potential of Metakaolin (MK) as a highly reactive pozzolan against GO agglomeration in the non-hydrated environment of simulated cement pore solution (SCPS) for different MK/GO weight ratios. Additionally, the interaction between MK and GO in water is evaluated through different characterization methods. Visual investigation and UV-vis spectroscopy revealed that there should be a probable interaction between MK particles and GO nanosheets in water which was interpreted by Lewis acid-base interaction and further examined by FTIR spectroscopy. Moreover, the zeta potential measurements indicated that the increase in MK/GO weight ratio could lead to higher adsorption of GO on the surface of MK particles which was confirmed by the particle size analysis. Almost all of the conducted experiments on the MK-GO hybrid in simulated cement pore solution showed that different dosages of MK particles were incapable of preventing GO agglomeration; thus, despite the proposed mechanisms in previous studies, MK cannot effectively restrict the unfavorable effects of Ca2+ ions on GO dispersion in SCPS and analogously in the hydrating cement matrix.

Stronger impacts of long-term relative to short-term exposure to carbon nanomaterials on soil bacterial communities

Environmental impacts of carbon nanomaterials (CNMs) have been attracting increasing concerns in recent years. Knowledge on how short-term exposure to CNMs influences soil microbial communities is available. However, little is known about the possible difference in effects of long-term versus short-term exposure of CNMs on soil microbial communities. In this study, we systematically compared effects of fullerene (C₆₀), single-walled carbon nanotubes (SW), and graphene (GR) on soil bacterial communities over short (30 d) and long (360 d) term exposure durations. Our findings revealed that short-term exposure to all CNMs significantly increased the alpha diversity of soil bacterial communities. SW and GR exposure for 360 d relative to that for 30 d more significantly decreased their alpha diversity. Compared to short-term exposure, a long term exposure to CNMs more strongly altered the beta diversity of soil bacterial communities. LEfSe analysis showed that, GR relative to C₆₀ and SW exposure more strongly altered soil bacterial community composition especially for long-term duration at various taxonomic levels; more taxa were also identified by LEfSe analysis as biomarkers upon long-term GR exposure. More OTUs were affected by long-term GR exposure. These differences resulted from both distinct physicochemical properties of various CNMs and their exposure durations.

Non-monotonic contribution of nonionic surfactant on the retention of functionalized multi-walled carbon nanotubes in porous media

The concentration of nonionic surfactants like Triton X-100 (TX100) can influence the transport and fate of emerging contaminants (e.g., carbon nanotubes) in porous media, but limited research has previously addressed this issue. This study investigates the co-transport of functionalized multi-walled carbon nanotubes (MWCNTs) and various concentrations of TX100 in saturated quartz sand (QS). Batch experiments and molecular dynamics simulations were conducted to investigate the interactions between TX100 and MWCNTs. Results indicated that the concentration ratio of MWCNTs and TX100 strongly influences the dispersion of MWCNTs and interaction forces between MWCNTs and QS during the transport. Breakthrough curves of MWCNTs and TX100 and retention profiles of MWCNTs were determined and simulated in column studies. MWCNTs strongly enhanced the retention of TX100 in QS due to the high affinity of TX100 for MWCNTs. Conversely, the concentration of TX100 had a non-monotonic impact on MWCNT retention. The maximum transport of MWCNTs in the QS occurred at an input concentration of TX100 that was lower than the critical micelle concentration. This suggests that the relative importance of factors influencing MWCNTs changed with TX100 sorption. Results from interaction energy calculations and modeling of competitive blocking indicate that the predictive ability of interaction energy calculations and colloid filtration theory may be lost because TX100 mainly altered intermolecular forces between the MWCNT and porous media. This study provides new insights into the co-transport of surfactants and MWCNTs in porous media, which can be useful for environmental applications and risk management.

Insights into the mutual promotion effect of graphene oxide nanoparticles and tetracycline on their transport in saturated porous media

In this study, batch and column tests were performed to investigate the co-transport of graphene oxide (GO) nanoparticles and tetracycline in saturated porous media under various solution chemistry conditions. Research indicated that GO and tetracycline had mutual promotion effect on their transport in the porous media under all the tested conditions, which was ascribed to the high adsorption capacity of tetracycline onto GO and the increased electrostatic repulsion as well as their competition for deposition sites on sand surfaces. Interestingly, the mutually promoting function of GO and tetracycline under acidic conditions was greater than that under alkaline conditions, the dominant mechanism was that the increased solution pH decreased the sorption of tetracycline onto GO and weakened the deposition site competition. Furthermore, the mutually promoting effect of GO and tetracycline was Na⁺ or Ca²⁺ concentration-dependent. Specially, increased Ca²⁺ concentration weakened the promoting effect of GO on tetracycline transport but magnified the promoting effect of tetracycline on GO transport. This is because higher Ca²⁺ concentration could cause a decrease in the adsorption of tetracycline on GO and facilitate more tetracycline molecules to occupy the deposition sites on sand surfaces. Additionally, sodium dodecyl sulfate had enhancement effect on co-transport of GO and tetracycline. Findings from this study clearly indicated that antibiotics and carbon based nanomaterials may transport together under various solution chemistry conditions, and consequently affect their fates in aquatic environments.

Charge mediated interaction of polystyrene nanoplastic (PSNP) with minerals in aqueous phase

A large number of plastic products potentially become smaller particles, including nanoplastics, under multiple actions in the environment. The interactions between nanoplastic particles and constituents in the environment, such as minerals, would greatly affect the transport, fate and toxic effects of nanoplastics. In this study, the interactions of polystyrene nanoplastic (PSNP) with four typical minerals, including goethite, magnetite, kaolinite and montmorillonite, in aqueous phase were investigated. The stability of PSNP colloidal suspension decreased in the presence of the positively charged goethite or magnetite, while it was not affected by the negatively charged montmorillonite and kaolinite, suggesting that there was a strong electrostatic attraction between PSNP and the two iron oxides. Incubation of PSNP with other three metal oxides with different surface charges, MnO₂, Al₂O₃ and SiO₂, confirmed the importance of electrostatic interaction in the stability of PSNP suspension. The transmission electron microscopy (TEM) analysis and batch adsorption experiments indicated that PSNP was effectively adsorbed on goethite or magnetite due to the strong electrostatic attraction between them. The Fourier transform infrared spectra (FTIR) and two-dimensional correlation spectroscopy (2D-COS) analyses indicated that there was strong hydrogen bonding between the -OH (γ-FeOOH) of goethite and PSNP, contributing to the higher adsorption of PSNP on goethite than magnetite. These findings shed light on the interactions of PSNP with mineral surfaces, and potential fate of PSNP under natural conditions in the water environment.

Nanotechnology in agriculture: Current status, challenges and future opportunities

Nanotechnology has shown promising potential to promote sustainable agriculture. This article reviews the recent developments on applications of nanotechnology in agriculture including crop production and protection with emphasis on nanofertilizers, nanopesticides, nanobiosensors and nano-enabled remediation strategies for contaminated soils. Nanomaterials play an important role regarding the fate, mobility and toxicity of soil pollutants and are essential part of different biotic and abiotic remediation strategies. Efficiency and fate of nanomaterials is strongly dictated by their properties and interactions with soil constituents which is also critically discussed in this review. Investigations into the remediation applications and fate of nanoparticles in soil remain scarce and are mostly limited to laboratory studies. Once entered in the soil system, nanomaterials may affect the soil quality and plant growth which is discussed in context of their effects on nutrient release in target soils, soil biota, soil organic matter and plant morphological and physiological responses. The mechanisms involved in uptake and translocation of nanomaterials within plants and associated defense mechanisms have also been discussed. Future research directions have been identified to promote the research into sustainable development of nano-enabled agriculture.

Adsorption of Bisphenol A on Peanut Shell Biochars: The Effects of Surfactants

Bisphenol A (BPA) is a typical endocrine-disrupting chemical. The removal of BPA has raised much concerns in recent years. This paper examined the adsorption behavior of BPA to biochars and the different effects of cationic, anionic, and nonionic surfactants. The results indicated that peanut shell biochars prepared at 300°C (BC300), 500°C (BC500), and 700°C (BC700) showed strong adsorption affinity for BPA, and the adsorption affinity of biochars increased with the increase of pyrolysis temperature. The range of log Kd values was 2.83∼3.71, 2.91∼4.57, and 3.24∼5.50 for BC300, BC500, and BC700, respectively. Both the type of surfactants and the properties of biochars could affect the adsorption behavior of BPA. Cetyltrimethyl ammonium bromide (CTAB) showed negligible effect on the adsorption of BPA on BC300, and the inhibition effect of CTAB was stronger with the increase of biochar pyrolysis temperature. Tween 20 and sodium dodecyl benzene sulfonate (SDBS) showed stronger inhibition effect than CTAB, especially on BC300. This is likely because the inhibition effect caused by competition of CTAB may be counterbalanced by the enhancement caused by the partitioning effect by adsorbed CTAB and the bridge effect between the –NH4+ group of CTAB and the phenol group on BPA/O-functional groups of biochars, whereas Tween 20 and SDBS do not have this bridge effect advantage. This study could provide insightful information for the application of biochars in removal of BPA.

Transport and retention of engineered silver nanoparticles in carbonate-rich sediments in the presence and absence of soil organic matter

The transport and retention behavior of polymer- (PVP-AgNP) and surfactant-stabilized (AgPURE) silver nanoparticles in carbonate-dominated saturated and unconsolidated porous media was studied at the laboratory scale. Initial column experiments were conducted to investigate the influence of chemical heterogeneity (CH) and nano-scale surface roughness (NR) arising from mixtures of clean, positively charged calcium carbonate sand (CCS), and negatively charged quartz sands. Additional column experiments were performed to elucidate the impact of CH and NR arising from the presence and absence of soil organic matter (SOM) on a natural carbonate-dominated aquifer material. The role of the nanoparticle capping agent was examined under all conditions tested in the column experiments. Nanoparticle transport was well described using a numerical model that facilitated blocking on one or two retention sites. Results demonstrate that an increase in CCS content in the artificially mixed porous medium leads to delayed breakthrough of the AgNPs, although AgPURE was much less affected by the CCS content than PVP-AgNPs. Interestingly, only a small portion of the solid surface area contributed to AgNP retention, even on positively charged CCS, due to the presence of NR which weakened the adhesive interaction. The presence of SOM enhanced the retention of AgPURE on the natural carbonate-dominated aquifer material, which can be a result of hydrophobic or hydrophilic interactions or due to cation bridging. Surprisingly, SOM had no significant impact on PVP-AgNP retention, which suggests that a reduction in electrostatic repulsion due to the presence of SOM outweighs the relative importance of other binding mechanisms. Our findings are important for future studies related to AgNP transport in shallow unconsolidated calcareous and siliceous sands.

Prediction of Cd toxicity to Daphnia magna in the mixture of multi-walled carbon nanotubes and kaolinite

In this study, we investigated cadmium toxicity created by adsorption kinetics in several mixtures containing two types of multi-walled carbon nanotubes (COOH-MWCNT and NH₂-MWCNT) and natural kaolinite. Characteristics of two types of MWCNTs were measured by zeta potential and ATR FT-IR graphs and TEM images. The solution of CNTs and kaolinite was tested to study Cd adsorption kinetics and mechanisms of differentiation-associated toxicity using Daphnia magna in a binary system (Cd-MWCNTs and Cd-kaolinite) and a ternary system (Cd-MWCNTs-kaolinite). In the binary system, Cd removal efficiency was nearly 100% and 40% for MWCNTs and kaolinite because of surface charge, respectively, with increasing sorbent concentration. In the ternary system, the trend of adsorption rate was similar to that of binary system. In comparison with percent mortality in the binary system, the solution in the ternary system showed higher toxicity due to the interaction of MWCNTs-kaolinite coagulated particles, thereby decreasing Cd adsorption onto CNTs and kaolinites. Overall, kaolinite can affect the adsorption process of Cd on MWCNTs in negative ways, depending on adsorption state. In conclusion, our studies suggest that kaolinite differs with adsorption ability of Cd by MWCNTs, and toxicity is likely to be produced by multivariable regression in the adsorption state.

Aggregation of oxidized multi-walled carbon nanotubes: Interplay of nanomaterial surface O-functional groups and solution chemistry factors

The fast-growing production and application of carbon nanotube (CNT) materials in a variety of industrial products inevitably lead to their release to wastewater and surface water. CNT would experience oxidization in wastewater treatment plant due to the presence of large amount of disinfectants, such as H₂O₂ and O₃, which in turn affects the environmental fates and risks of CNT. In this study, oxidized CNT materials (O-CNTs) were prepared by treating CNT with H₂O₂/UV and O₃ (denoting as H₂O₂-CNT and O₃-CNT, respectively). A variety of characterizations indicated that oxygen containing groups were generated on CNT surface upon the oxidation, and the O/C ratio increased in the order of pristine CNT < H₂O₂-CNT < O₃-CNT. In the presence of Na⁺, K⁺ and Mg²⁺, the O-CNTs displayed better colloidal stability than the pristine CNT, and the stability increased with the oxidation degree (indicated by O/C ratio). This could be explained by the more negative surface charge and stronger hydrophilicity of the O-CNTs. Unexpectedly, in the presence of Ca²⁺, the most oxidized O₃-CNT exhibited the poorest colloidal stability. The abundant carboxyl groups in O₃-CNT provided effective binding sites for cation bridging effect through Ca²⁺ and led to stronger aggregation. Increasing pH was more favorable to disperse CNTs (both O-CNT and pristine CNT) in the presence of Na⁺, but much less effective in inhibiting the aggregation of O₃-CNT in presence of Ca²⁺. This could be explained by the stronger cation bridging effect due to enhanced deprotonation the -COOH groups at higher pH conditions. The calculated Hamaker constants of the CNTs decreased with the oxidation degree, implying that there was lower van der Waals force between the O-CNTs. The Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation confirmed that O-CNTs had to overcome higher energy barrier and thus showed better colloidal stability than the pristine CNT in the presence of Na⁺.

Rapid and versatile pre-treatment for quantification of multi-walled carbon nanotubes in the environment using microwave-induced heating

The concerns regarding potential environmental release and ecological risks of multi-walled carbon nanotubes (MWCNTs) rise with their increased production and use. As a result, there is the need for an analytical method to determine the environmental concentration of MWCNTs. Although several methods have been demonstrated for the quantification of well-characterized MWCNTs, applying these methods to field samples is still a challenge due to interferences from unknown characteristics of MWCNTs and environmental media. To bridge this gap, a recently developed microwave-induced heating method was investigated for the quantification of MWCNTs in field samples. Our results indicated that the microwave response of MWCNTs was independent of the sources, length, and diameter of MWCNTs; however, the aggregated MWCNTs were not able to convert the microwave energy to heat, making the method inapplicable. Thus, a pre-treatment process for dispersing bundled MWCNTs in field samples was crucial for the use of the microwave method. In the present paper, a two-step pre-treatment procedure was proposed: the aggregated MWCNTs loaded environmental samples were first exposed to high temperature (500 °C) and then dispersed by using an acetone-surfactant solution. A validation study was performed to evaluate the effectiveness of the pre-treatment process, showing that an 80-120% recovery range of true MWCNT loading successfully covered the microwave-measured MWCNT mass.

Effects of multi-walled carbon nanotubes on pyrene adsorption and desorption in soils: The role of soil constituents

Once entering soil, carbon nanotubes (CNTs) can influence the fate of hydrophobic organic compounds (HOCs) in soil due to its strong adsorption capacity. This process may be influenced by the interactions between CNTs and soil constituents. The mechanisms therein were investigated in the present study by examining pyrene adsorption/desorption on one CNTs, two soils (black soil and paddy soil), and mixtures thereof. CNTs' amendment enhanced soil site heterogeneity and adsorption capacity of pyrene while it was less than that predicted by the sum of the individual adsorption on soils and CNTs, which was more obvious at low aqueous concentrations. This could be due to the interactions between soil constituents (dissolved organic matter (DOM) and clays) and CNTs. Modification of CNTs by DOM attenuated pyrene adsorption by 14.9%-66.1%, which was ascribed to occupying of surface adsorption sites, pore blockage of CNTs' aggregates, enhancement of surface polarity, and enhancement of pyrene solubility in aqueous phase. The coexistence of clay (kaolinite) also showed inhibition on pyrene adsorption onto CNTs with a reduction of 19.2%-40.2%. This could be ascribed to that the attachment of clay particles on CNTs' aggregates could cover the surface adsorption sites and enhance the surface polarity of CNTs. The effect of CNTs amendment on pyrene desorption hysteresis differed among soils. The hysteresis index of the black soil doubled after CNTs' amendment while that of paddy soil remained unchanged. The results of this study provide insights into the possible effects of CNTs on the fate of HOCs in real soil environment.

Co-transport of multi-walled carbon nanotubes and sodium dodecylbenzenesulfonate in chemically heterogeneous porous media

Multi-walled carbon nanotubes (MWCNTs) are increasing used in commercial applications and may be released into the environment with anionic surfactants, such as sodium dodecylbenzenesulfonate (SDBS), in sewer discharge. Little research has examined the transport, retention, and remobilization of MWCNTs in the presence or absence of SDBS in porous media with controlled chemical heterogeneity, and batch and column scale studies were therefore undertaken to address this gap in knowledge. The adsorption isotherms of SDBS on quartz sand (QS), goethite coated quartz sand (GQS), and MWCNTs were determined. Adsorption of SDBS (MWCNTs » GQS > QS) decreased zeta potentials for these materials, and produced a charge reversal for goethite. Transport of MWCNTs (5 mg L^-1) dramatically decreased with an increase in the fraction of GQS from 0 to 0.1 in the absence of SDBS. Conversely, co-injection of SDBS (10 and 50 mg L^-1) and MWCNTs radically increased the transport of MWCNTs when the GQS fraction was 0, 0.1, and 0.3, especially at a higher SDBS concentration, and altered the shape of retention profile. Mathematical modeling revealed that competitive blocking was not the dominant mechanism for the SDBS enhancement of MWCNT transport. Rather, SDBS sorption increased MWCNT transport by increasing electrostatic and/or steric interactions, or creating reversible interactions on rough surfaces. Sequential injection of pulses of MWCNTs and SDBS in sand (0.1 GQS fraction) indicated that SDBS could mobilize some of retained MWCNTs from the top to deeper sand layers, but only a small amount of released MWCNTs were recovered in the effluent. SDBS therefore had a much smaller influence on MWCNT transport in sequential injection than in co-injection, presumably because of a greater energy barrier to MWCNT release than retention. This research sheds novel insight on the roles of competitive blocking, chemical heterogeneity and nanoscale roughness, and injection sequence on MWCNT retention and release.

Adsorption of phenanthrene and 1-naphthol to graphene oxide and L-ascorbic-acid-reduced graphene oxide: effects of pH and surfactants

In this study, reduced graphene oxide (RGO) was synthesized by _L-ascorbic acid reduction, which was a relatively mild and environmental friendly reduction method, and the adsorption of organic contaminants was compared to graphene oxide (GO) to probe the potential adsorption mechanisms. The morphology properties of GO and RGO were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared transmission (FTIR), Raman spectrometer, transmission electron microscope (TEM), and scanning electron microscopy (SEM). The adsorption affinities of GO and RGO for phenanthrene and 1-naphthol were studied in batch experiments. The effects of pH and surfactants were also assessed. The results demonstrated that RGO reduced by _L-ascorbic acid show significantly greater adsorption affinity for both phenanthrene and 1-naphthol than GO, and even greater than most of RGOs that reduced by the strong reductive reagents. This was mainly attributed to the hydrophobic interaction, π-π interaction, and H-bonding between graphene sheets and organic contaminants. Both GO and RGO showed stronger adsorption to phenanthrene than to 1-naphthol. The adsorption of 1-naphthol increased with decreasing pH and reached a maximum around pH = 7.34. The surfactants, sodium dodecyl benzene sulfaonate (SDBS) and cetyltrimethyl ammonium bromide (CTAB), had negligible influence on adsorption to GO. Note that CTAB significantly inhibited the adsorption of phenanthrene/1-naphthol on RGO, which could be attributed to the pore blockage effect. In addition, RGO could be regenerated and reused with high recyclability over five cycles. The present study suggests that RGO obtained via _L-ascorbic acid reduction can be deemed as a promising material for organic contaminated wastewater treatment.

Melt-Processed Poly(Ether Ether Ketone)/Carbon Nanotubes/Montmorillonite Nanocomposites with Enhanced Mechanical and Thermomechanical Properties

The agglomeration problem of nanofillers, for instance, carbon nanotubes (CNTs) in a poly(ether ether ketone) (PEEK) matrix, is still a challenging assignment due to the intrinsic inert nature of PEEK to organic solvents. In this work, organically modified montmorillonite (MMT) was introduced as a second filler for improving the dispersion of CNTs in the PEEK matrix and enhancing the mechanical properties, as well as reducing the cost of the materials. The nanocomposites were fabricated through melt-mixing PEEK with CNTs/MMT hybrids, which were prepared in advance by mixing CNTs with MMT in water. The introduction of MMT improved the dispersion stability of CNTs, as characterized by sedimentation and zeta potential. The CNTs/MMT hybrids were maintained in PEEK nanocomposites as demonstrated by the transmission electron microscope. The mechanical and thermomechanical measurements revealed that CNTs together with MMT had a strong reinforcement effect on the PEEK matrix, especially at high temperature, although it had a negative effect on the toughness. A maximum increase of 48.1% was achieved in storage modulus of PEEK nanocomposites with 0.5 wt% CNTs and 2 wt% MMT at 240 °C, compared to that of neat PEEK. The differential scanning calorimetry results revealed that CNTs accelerated the crystallization of the PEEK matrix while a further addition of MMT played an opposite role. The nucleation activity of the fillers was also evaluated by the Dobreva method.

Removal of ciprofloxacin from aqueous solutions by ionic surfactant-modified carbon nanotubes

Ionic surfactants may impact removal efficiency of organic contaminants from aqueous solution, but research regarding the adsorption mechanisms on surfactant-modified carbon nanotubes (CNTs) was limited. In this study, three multi-walled and one single-walled CNTs were used as adsorbents to investigate the adsorption behavior and mechanisms of ciprofloxacin (CIP) on CNTs modified by ionic surfactants (cationic CTAB (Cetyltrimethylamnonium bromide) or anionic SDS (Sodium dodecyl sulfate)). More than 80% (82-88%) of the total removed CIP on CTAB-modified CNTs occurred within the first 6 h, much higher than that on SDS-modified CNTs (57-78%). Modeling adsorption kinetics demonstrated that CIP adsorption on surfactant-modified CNTs was controlled by multiple and faster processes, and both external mass transfer and intraparticle diffusion are limiting factors. Relative to SDS, CTAB was significantly (P < 0.001) concentration-dependent in suppressing CIP removal. Besides, the increase in 1/n values of Freundlich model with increasing CTAB concentration suggested that CTAB could be a stronger competitor for CIP adsorption. Hydrophobic interactions predominated zwitterionic CIP adsorption on all CNTs tested, while electrostatic interactions could help control ionizable CIP adsorption on surfactant-modified CNTs depending upon pH. CIP adsorption on modified SWCNTs significantly declined with increasing ionic strength from 1 mM to 100 mM relative to those multi-walled CNTs because the more favorable aggregation of SWCNTs reduced the CIP adsorption, irrespective of which surfactant was added. Significant desorption hysteresis of adsorbed CIP released by SDS and water was observed, but not by CTAB, by which 32.6-54.4% of adsorbed CIP were removed. For SDS-modified CNTs, the mean release ratio (RR) followed an order of MWCNTs (0.075) > MHCNTs (0.058) > SWCNTs (0.057) > MCCNTs (0.049), significantly (P < 0.001) lower than CTAB-CNTs (0.37-0.56). It can be predicted that the tested surfactants co-existing with CNTs depress removal efficiency of diverse contaminants similar to CIP in aqueous systems.

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.

Transport and retention of surfactant- and polymer-stabilized engineered silver nanoparticles in silicate-dominated aquifer material

Packed column experiments were conducted to investigate the transport and blocking behavior of surfactant- and polymer-stabilized engineered silver nanoparticles (Ag-ENPs) in saturated natural aquifer media with varying content of material < 0.063 mm in diameter (silt and clay fraction), background solution chemistry, and flow velocity. Breakthrough curves for Ag-ENPs exhibited blocking behavior that frequently produced a delay in arrival time in comparison to a conservative tracer that was dependent on the physicochemical conditions, and then a rapid increase in the effluent concentration of Ag-ENPs. This breakthrough behavior was accurately described using one or two irreversible retention sites that accounted for Langmuirian blocking on one site. Simulated values for the total retention rate coefficient and the maximum solid phase concentration of Ag-ENPs increased with increasing solution ionic strength, cation valence, clay and silt content, decreasing flow velocity, and for polymer-instead of surfactant-stabilized Ag-ENPs. Increased Ag-ENP retention with ionic strength occurred because of compression of the double layer and lower magnitudes in the zeta potential, whereas lower velocities increased the residence time and decreased the hydrodynamics forces. Enhanced Ag-ENP interactions with cation valence and clay were attributed to the creation of cation bridging in the presence of Ca²⁺. The delay in breakthrough was always more pronounced for polymer-than surfactant-stabilized Ag-ENPs, because of differences in the properties of the stabilizing agents and the magnitude of their zeta-potential was lower. Our results clearly indicate that the long-term transport behavior of Ag-ENPs in natural, silicate dominated aquifer material will be strongly dependent on blocking behavior that changes with the physicochemical conditions and enhanced Ag-ENP transport may occur when retention sites are filled.

Attenuation of bacterial cytotoxicity of carbon nanotubes by riverine suspended solids in water

The impact of solid particles on ecotoxicity of nanomaterials in water environments is poorly understood. This study investigated the effect of natural riverine suspended solids (SPS) on the cytotoxicity of single-walled carbon nanotubes (SWCNTs) towards a bacterium, Ochrobactrum sp. in water. Compared with SWCNT suspension without SPS, the presence of SPS at different concentrations ranging from 20 to 400 mg L^-1 markedly increased the survival rates of bacteria exposed to 50 mg L^-1 SWCNTs and bacterial survival rates increased with SPS concentrations by a power law. Sedimentation experiments and field emission scanning electron microscopy revealed the occurrence of heteroaggregation between SWCNTs and SPS, probably responsible for the reduced SWCNT toxicity. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (ExDLVO) calculation showed the mitigated toxicity might also result from the decreased SWCNT-bacterium interaction energy with the increased SPS concentrations and the stronger SPS-SWCNT interaction than the SWCNT-bacterium interaction. This work provides new insights into our understanding of environmental hazards of engineered nanomaterials in aquatic systems.

Designer carbon nanotubes for contaminant removal in water and wastewater: A critical review

The search for effective materials for environmental cleanup is a scientific and technological issue of paramount importance. Among various materials, carbon nanotubes (CNTs) possess unique physicochemical, electrical, and mechanical properties that make them suitable for potential applications as environmental adsorbents, sensors, membranes, and catalysts. Depending on the intended application and the chemical nature of the target contaminants, CNTs can be designed through specific functionalization or modification processes. Designer CNTs can remarkably enhance contaminant removal efficiency and facilitate nanomaterial recovery and regeneration. An increasing number of CNT-based materials have been used to treat diverse organic, inorganic, and biological contaminants. These success stories demonstrate their strong potential in practical applications, including wastewater purification and desalination. However, CNT-based technologies have not been broadly accepted for commercial use due to their prohibitive cost and the complex interactions of CNTs with other abiotic and biotic environmental components. This paper presents a critical review of the existing literature on the interaction of various contaminants with CNTs in water and soil environments. The preparation methods of various designer CNTs (surface functionalized and/or modified) and the functional relationships between their physicochemical characteristics and environmental uses are discussed. This review will also help to identify the research gaps that must be addressed for enhancing the commercial acceptance of CNTs in the environmental remediation industry.

Transport of multi-walled carbon nanotubes stabilized by carboxymethyl cellulose and starch in saturated porous media: Influences of electrolyte, clay and humic acid

This study investigated the transport behaviors of carboxymethyl cellulose (CMC) and starch stabilized multi-walled carbon nanotubes (MWNTs) through a saturated quartz sand column in the presence of electrolytes, model clays, and natural organic matter (humic acid) through column breakthrough experiments and model simulations. Both stabilizers, CMC and starch, greatly enhanced the breakthrough of MWNTs, with a full breakthrough plateau (C/C₀) ranging from 0.69 to 0.90 at ionic strength from 0.3 to 10mM. Between the two stabilizers, CMC was more effective in resisting particle deposition, and thus CMC-stabilized MWNTs were more transportable through the medium. While non-stabilized MWNTs were much less transportable and were vulnerable to electrolyte effects (especially Ca²⁺), the stabilized counterparts were much more resistant to the coagulation effects of electrolytes. The presence of colloidal clay particles showed contrasting effects on the transport of bare and stabilized MWNTs. The full breakthrough C/C₀ of bare MWNTs was suppressed by kaolinite and montmorillonite particles from 0.33 to <0.15 with 5mg/L clay, indicating that the presence of both clays enhanced the aggregation and deposition of MWNTs. However, kaolinite particles facilitated the transport of stabilized-MWNTs, while montmorillonite weakened the breakthrough of stabilized MWNTs. Humic acid had less effect on the mobility of stabilized-MWNTs than that of bare MWNTs. The advection-dispersion transport model incorporated with the filtration theory was able to simulate the breakthrough curves and quantitatively interpret the particle deposition. The results can facilitate our understanding of fate and transport of stabilized carbon nanotubes in the environment.

Effects of Various Surfactants on the Dispersion of MWCNTs-OH in Aqueous Solution

Dispersion of carbon nanotubes (CNTs) is a challenge for their application in the resulting matrixes. The present study conducted a comparison investigation of the effect of four surfactants: Alkylphenol polyoxyethylene ether (APEO), Silane modified polycarboxylate (Silane-PCE), I-Cationic polycarboxylate (I-C-PCE), and II-Cationic polycarboxylate (II-C-PCE) on the dispersion of hydroxyl functionalized multi-walled carbon nanotubes (MWCNTs–OH). Among the four surfactants, APEO and II-C-PCE provide the best and the worst dispersion effect of CNTs in water, respectively. Dispersion effect of MWCNTs–OH has been characterized by optical microscope (OM), field emission-scanning electron microscope (FE-SEM), and Ultraviolet–visible spectroscopy (UV–Vis).The OM images are well consistent with the UV–Vis results. Based on the chemical molecular structures of the four surfactants, the mechanism of MWCNTs–OH dispersion in water was investigated. For each kind of surfactant, an optimum surfactant/MWCNTs–OH ratio has been determined. This ratio showed a significant influence on the dispersion of MWCNTs–OH. Surfactant concentration higher or lower than this value can weaken the dispersion quality of MWCNTs–OH.

Adsorption, Aggregation, and Deposition Behaviors of Carbon Dots on Minerals

The increased production of carbon dots (CDs) and the release and accumulation of CDs in both surface and groundwater has resulted in the increasing interest in their research. To assess the environmental behavior of CDs, the interaction between CDs and goethite was studied under different environmental conditions. Electrokinetic characterization of CDs suggested that the ζ-potential and size distribution of CDs were affected by pH and electrolyte species, indicating that these factors influenced the stability of CDs in aqueous solutions. Traditional Derjaguin-Landau-Verwey-Overbeek theory did not fit well the aggregation process of CDs. Results of the effects of pH and ionic strength suggested that electronic attraction dominated the aggregation of CDs. Compared with other minerals, hydrogen-bonding interactions and Lewis acid-base interactions contributed to the aggregation of CDs, in addition to van der Waals and electrical double-layer forces. Adsorption isotherms and microscopic Fourier transformed infrared spectroscopy indicated that chemical bonds were formed between CDs and goethite. These findings are useful to understand the interaction of CDs with minerals, as well as the potential fate and toxicity of CDs in the natural environment, especially in soils and sediments.

Heteroaggregation of graphene oxide nanoparticles and kaolinite colloids

Graphene oxide (GO) is a material with rapid production growth, and consequently GO nanoparticles are expected to eventually penetrate subsurface formations, where fine mineral particles are in abundance. This study examines the heteroaggregation of GO nanoparticles with kaolinite (KGa-1b) colloids under various conditions. Dynamic batch experiments were conducted in solutions with different pH values (pH=4, 7, and 10), different ionic strengths (I_S=7, 12, and 27mM), and at three controlled temperatures (8, 14, and 25°C). The experimental results showed that a relatively small amount of GO nanoparticles (5-20% of the initial concentration) attached immediately onto KGa-1b colloids, and reached equilibrium in <20min. It was shown that neither temperature nor pH played a significant role in the attachment of GO nanoparticles onto KGa-1b colloids. In contrast, the attachment of GO nanoparticles onto KGa-1b colloids was shown to increase with increasing I_S. Additionally, time-resolved dynamic light scattering (DLS) was used to identify the influence of I_S on heteroaggregation between GO nanoparticles and KGa-1b colloids. The critical coagulation concentration (CCC) for the interaction between GO nanoparticles and KGa-1b colloids was 152mM (NaCl). The interaction energies were calculated, for all experimental conditions, by using measured zeta potentials and applying the classical DLVO theory. The equilibrium experimental data were fitted with a Freundlich isotherm, and the attachment kinetics were described very well with a pseudo-second-order model. Furthermore, thermodynamic analysis revealed that the attachment process was nonspontaneous and exothermic.

Review of key factors controlling engineered nanoparticle transport in porous media

Nanotechnology, an emerging technology, has witnessed rapid development in production and application. Engineered nanomaterials revolutionize the industry due to their unique structure and superior performance. The release of engineered nanoparticles (ENPs) into the environment, however, may pose risks to the environment and public health. To advance current understanding of environmental behaviors of ENPs, this work provides an introductory overview of ENP fate and transport in porous media. It systematically reviews the key factors controlling their fate and transport in porous media. It first provides a brief overview of common ENPs in the environment and their sources. The key factors that govern ENP transport in porous media are then categorized into three groups: (1) nature of ENPs affecting their transport in porous media, (2) nature of porous media affecting ENP transport, and (3) nature of flow affecting ENP transport in porous media. In each group, findings in recent literature on the specific governing factors of ENP transport in porous media are discussed in details. Finally, this work concludes with remarks on the importance of ENP transport in porous media and directions for future research.

Enzymatic degradation of aromatic hydrocarbon intermediates using a recombinant dioxygenase immobilized onto surfactant-activated carbon nanotube

This study examined the enzymatic decomposition of aromatic hydrocarbon intermediates (catechol, 4-chlorocatechol, and 3-methylcatechol) using a dioxygenase immobilized onto single-walled carbon nanotube (SWCNT). The surfaces of SWCNTs were activated with surfactants. The dioxygenase was obtained by recombinant technique: the corresponding gene was cloned from Arthrobacter chlorophenolicus A6, and the enzyme was overexpressed and purified subsequently. The enzyme immobilization yield was 62%, and the high level of enzyme activity was preserved (60-79%) after enzyme immobilization. Kinetic analyses showed that the substrate utilization rates and the catalytic efficiencies of the immobilized enzyme for all substrates (target aromatic hydrocarbon intermediates) tested were similar to those of the free enzyme, indicating that the loss of enzyme activity was minimal during enzyme immobilization. The immobilized enzyme was more stable than the free enzyme against abrupt changes in pH, temperature, and ionic strength. Moreover, it retained high enzyme activity even after repetitive use.

Time-dependent effect of graphene on the structure, abundance, and function of the soil bacterial community

The increased application of graphene raises concerns about its environmental impact, but little information is available on the effect of graphene on the soil microbial community. This study evaluated the impact of graphene on the structure, abundance and function of the soil bacterial community based on quantitative real-time polymerase chain reaction (qPCR), pyrosequencing and soil enzyme activities. The results show that the enzyme activities of dehydrogenase and fluorescein diacetate (FDA) esterase and the biomass of the bacterial populations were transiently promoted by the presence of graphene after 4 days of exposure, but these parameters recovered completely after 21 days. Pyrosequencing analysis suggested a significant shift in some bacterial populations after 4 days, and the shift became weaker or disappeared as the exposure time increased to 60 days. During the entire exposure process, the majority of bacterial phylotypes remained unaffected. Some bacterial populations involved in nitrogen biogeochemical cycles and the degradation of organic compounds can be affected by the presence of graphene.

Effect of inorganic nanoparticles on 17β-estradiol and 17α-ethynylestradiol adsorption by multi-walled carbon nanotubes

With extensive application of diverse engineered nanoparticles (NPs), multiple NPs would inevitably be released into the environment. However, much emphasis in most previous studies on the interactions of pollutants with NPs has been placed on only one type of NPs at a time. This study investigated the impact of inorganic NPs (I-NPs) on the adsorption of 17β-estradiol (E2) and 17α-ethynylestradiol (EE2) by multi-walled carbon nanotubes (CNTs). The presence of I-NPs inhibited the adsorption and increased the equilibrium time of E2 and EE2 by CNTs. Moreover, the effect of Al2O3 was stronger than that of SiO2, because electrostatic attraction enhanced the interaction between oppositely charged Al2O3 and CNTs. The addition sequence of I-NPs and pollutant also influences adsorption. This is among the first studies investigating the effect of I-NPs on pollutants adsorption by CNTs, which is useful for understanding the transport and fate of CNTs and contaminants in natural aquatic systems.