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

Stability and fate of hollow mesoporous silica nanoparticles in aqueous environment: Effects of pH and electrolyte

Hollow mesoporous silica nanoparticles (HMSNs) and their carboxyl-functionalized counterparts (C-HMSNs) are promising materials for environmental applications, but their stability in aquatic environments remains poorly understood. This study investigated the aggregation behavior of HMSNs and C-HMSNs under varying pH and electrolyte conditions. Experimental results showed that HMSNs had critical aggragation concentrations (CCC) of 35 mM in NaCl and 2 mM in CaCl2, while C-HMSNs exhibited higher CCCs of 52 mM and 3 mM, respectively, indicating greater stability. High concentrations of NaCl and CaCl2 promoted rapid aggregation of HMSNs and C-HMSNs, with CaCl2 being more efficient in promoting aggregation. Both HMSNs and C-HMSNs aggregated extensively in acidic environments, while remaining stable in neutral and alkaline environments. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and molecular dynamics (MD) simulations revealed weakened electrostatic repulsion and reduced total system energy in high ionic strength solutions supporting irreversible aggregation. Simulated environmental conditions showed that HMSNs and C-HMSNs remained stable in rivers, aggregated slightly in groundwater, and aggregated extensively in seawater. These findings provide critical insights into the environmental fate and potential ecological risks of silica nanoparticles, informing future studies on their transport and bioavailability in natural waters.

Comparison of microplastics heteroaggregation with MoS2 and graphene oxide nanosheets: Dependence on the configuration and impacts on aquatic transport

Understanding the behavior and fate of microplastics (MPs) in aquatic environment is crucial for assessing their potential risks. This study investigated the heteroaggregation behaviors of MPs with representative 2D nanosheets, MoS2 and graphene oxide (GO), under various conditions, focusing on the transport behavior of the resulting aggregates. It was found that the destabilization capabilities of 2D nanosheets are notably stronger than those of well-reported nanoparticles. More importantly, the deposition and transport of MPs are highly dependent on the configuration of the resulting aggregates. MoS2 nanosheets conformally coat MPs, forming compact and colloidally stable complexes that completely alter the MPs' surface to the negatively charged MoS2. The interaction resulted in high mobility and minimal deposition in environmental matrices. In contrast, GO nanosheets bridge MPs into large clusters, reducing transport and increasing deposition. This difference in aggregate configuration is attributed to the distinct interactions between the nanosheets and MPs: rigid MoS2 nanosheets adhere via strong van der Waals forces, while GO, with oxygen functional groups on its edges and surfaces, folds and crosslinks between particles upon adsorption. These findings underscore the critical role of 2D materials in shaping the environmental fate of MPs, advancing our knowledge on the aggregation process.

Colloid mobilization and transport in response to freeze-thaw cycles: Insights into the heavy metal(loid)s migration at a smelting site

Smelting sites often exhibit significant heavy metal(loid)s (HMs) contamination in the soil and groundwater, which are inevitably subjected to environmental disturbances. However, there is limited information available regarding the migration behaviors of HMs in a disturbed scenario. Thus, this work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. Ultrafiltration results of soil effluents revealed that 61.5 %, 47.6 %, 68.0 %, and 59.2 % of Zn, Cd, Pb, and As were present in colloidal phase, and co-transported during treatments. Nanoparticle tracking analysis (NTA) further confirmed that freeze-thaw cycles were conducive to the generation of colloidal particles and showed the heteroagglomeration among different particles. Pore-network model (PNM) was used to quantify the soil macropore characteristics (macropore diameter, macropore number, coordination number, and Euler value) after treatments. It is evident that freeze-thaw cycles induced the formation of larger macropores while simultaneously enhancing macropore connectivity, thereby establishing an optimal pathway for colloid migration. These findings underscored the importance of environmental disturbances as a trigger for the release and migration of HMs in the smelting site, offering valuable insights for controlling HMs pollution. ENVIRONMENTAL IMPLICATION: The contaminated site has been subjected to prolonged environmental disturbances, causing the exacerbation of pollutants leaching and frequent occurrences of unstable pollution situations. This work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. The distinct effects of freeze-thaw treatment on colloidal particle number concentration and macropore characteristics may explain the generation and migration of colloid-associated HMs driven by environmental disturbances. This work revealed the underlying mechanisms driving the redistribution of HMs under freeze-thaw cycles, offering valuable insights for risk assessment of soil and groundwater associated with HMs migration.

The colloidal stability of molybdenum disulfide nanosheets in different natural surface waters: Combined effects of water chemistry and light irradiation

With the increasing production and application, more molybdenum disulfide (MoS2) nanosheets could be released into environment. The aggregation and dispersion of MoS2 nanosheets profoundly impact their transport and transformation in the aquatic environment. However, the colloidal stability of MoS2 remains largely unknown in natural surface waters. This study investigated the colloidal stability of MoS2 nanosheets in six natural surface waters affected by both light irradiation and water chemistry. Compared to that of the pristine MoS2 nanosheets, the colloidal stability of MoS2 photoaged in ultrapure water declined. Light irradiation induced the formation of Mo-O bonds, the release of SO42- species, and the decrease in 1T/2H ratio, which reduced negative charge and enhanced hydrophobicity. However, the colloidal stability of MoS2 photoaged in natural surface waters was increased relative to that in ultrapure water not only for the smaller extent of photochemical transformation but more importantly the surface modification by water chemistry. Furthermore, the colloidal stability of MoS2 photoaged in natural surface waters followed the order of sea water > lake water > river water. The abundant cations (e.g., Ca2+ and Mg2+) in sea water facilitated the covalent grafting (S-C bonds) of more dissolved organic matter (DOM) on MoS2 via charge screening and cation bridging, thus inducing stronger electrostatic repulsion and steric effect to stabilize nanosheets. The crucial role of the covalent grafting of DOM was further confirmed by the positive correlation between the critical coagulation concentration values and S-C ratios (R2 = 0.82, p < 0.05). Our results highlighted the dominant role of water chemistry than light irradiation in dictating the colloidal stability of MoS2 photoaged in natural surface waters, which provided new insight into the environmental behavior of MoS2 in aquatic environment.

Different inhibitory mechanisms of flexible and rigid clay minerals on the transport of microplastics in marine porous media

Colloidal interactions between clay minerals and microplastics (MPs) in high salinity seawater are crucial for determining MP fate in marine environments. Montmorillonite (MMT) forms thin and pliable films that tightly cover MPs, while the thick and rigid lamellae of kaolinite (KLT) have limited contact with MPs, resulting in unstable bonding. However, a small quantity of small-sized KLT can create relatively stable heteroaggregates by embedding into the interstitial spaces of MPs. Both MMT and KLT colloids can decrease the mobility of MPs in seawater-saturated sea sand, but their breakthrough curves (BTCs) show distinct phenomena of "blocking" and "ripening", respectively. The "blocking" phenomenon occurs when flexible MMT adheres to the sand surface, depleting attachment sites quickly and inhibiting the retention of subsequent heteroaggregates of MMT-wrapped MPs. The transport of single MMT also experiences colloid competition for attachment sites, but pre-equilibration experiments reveal no competition between MMT and bare MPs for attachment sites. Instead, the attached MMT provides additional attachment sites for MPs. These results suggest that the wrapping of MPs by MMT plays a dominant role in the "blocking" of cotransport. In contrast, rigid KLT forms a three-dimensional stack on the sand surface, offering more attachment sites for subsequent MPs and heteroaggregates.

Redispersion Behavior of 2D MoS2 Nanosheets: Unique Dependence on the Intervention Timing of Natural Organic Matter

The aggregation-redispersion behavior of nanomaterials determines their transport, transformation, and toxicity, which could be largely influenced by the ubiquitous natural organic matter (NOM). Nonetheless, the interaction mechanisms of two-dimensional (2D) MoS₂ and NOM and the subsequent influences on the redispersion behavior are not well understood. Herein, we investigated the redispersion of single-layer MoS₂ (SL-MoS₂) nanosheets as influenced by Suwannee River NOM (SRNOM). It was found that SRNOM played a decisive role on the redispersion of MoS₂ 2D nanosheets that varied distinctly from the 3D nanoparticles. Compared to the poor redispersion of MoS₂ aggregates in the absence or post-addition of SRNOM to the aggregates, co-occurrence of SRNOM in the dispersion could largely enhance the redispersion and mobility of MoS₂ by intercalating into the nanosheets. Upon adsorption to SL-MoS₂, SRNOM enhanced the hydration force and weakened the van der Waals forces between nanosheets, leading to the redispersion of the aggregates. The SRNOM fractions with higher molecular mass imparted better dispersity due to the preferable sorption of the large molecules onto SL-MoS₂ surfaces. This comprehensive study advances current understanding on the transport and fate of nanomaterials in the water system and provides fresh insights into the interaction mechanisms between NOM and 2D nanomaterials.

Improving the Performance of the Lamellar Reduced Graphene Oxide/Molybdenum Sulfide Nanofiltration Membrane through Accelerated Water-Transport Channels and Capacitively Enhanced Charge Density

Graphene is promising in the construction of next-generation nanofiltration membranes for wastewater treatment and water purification. However, the application of graphene-based membranes has still been prohibited by their deficiencies in permeability and ion rejection. Herein, regulating the 2D channel and enhancing the charge density are co-adopted for simultaneous enhancement of the water flux and salt rejection of reduced graphene oxide (rGO) membranes through the intercalation of molybdenum sulfide (MoS₂) nanosheets and external electrical assistance. The fabricated rGO/MoS₂ membranes possess expanded nanochannels with less friction and a higher water molecule transport velocity gradient (from 8.57 to 14.07 s^-1) than those of rGO membranes. Consequently, their water permeance increases from 0.92 to 34.9 L m^-2 h^-1 bar^-1. Meanwhile, benefiting from the high capacitance and negative potential of -1.1 V versus the saturated calomel electrode given to the membranes, their rejection rates toward NaCl reach 87.2% and those toward Na₂SO₄ reach 93.7%. The Donnan steric pore model analysis indicates that the capacitively and electrically increased surface charge density make great contributions to the higher ion rejection rate. This work gives new insights into membrane design for high water flux and salt rejection efficiency.

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.

Boosting photocatalytic efficiency of MoS2/CdS by modulating morphology

CdS-based composites as the highly efficient photocatalyst have been extensively investigated in recent years due to the suitable band gap and high photocatalytic efficiency. In this study, the effect of various factors (pH, U(VI) concentration, contents, and types of photocatalyst) on photocatalytic reduction of U(VI) by MoS₂/CdS composite was investigated. The optimized experimental conditions (e.g., pH 7.0, 20 mg/g U(VI), and 1.0 g/L photocatalyst) was obtained by batch techniques. Approximately 97.5% of U(VI) was photo-catalytically reduced into U(IV) by 2.5 wt% MoS₂/CdS composite within 15 min. After 5 cycles, 2.5 wt% MoS₂/CdS composite still exhibited the high removal efficiency of U(VI) under 50-min irradiation, indicating the good stability. The photo-reduction mechanism of U(VI) on MoS₂/CdS composite was attributed to the O^-₂ radicals by quenching experiments, ESR, and XPS analysis. The findings indicate that CdS-based catalyst has a great potential for the photocatalytic reduction of uranyl in actual environmental remediation.

Heteroaggregation between graphene oxide and titanium dioxide particles of different shapes in aqueous phase

Graphene oxide (GO) inevitably interacts with engineered and/or natural nanoparticles (NPs) after releasing into aquatic environments. It is observed that GO could form heteroaggregates with differently shaped TiO₂ NP_S at pHs 4.0 and 7.0 rather than pH 10.0. Optimal pHs for the maximal heteroaggregation were around 6.63 and 4.92 for TiO₂ spheres and fibers, respectively, which was dominated by electrostatic attraction. The optimal concentration ratio for TiO₂ spheres was 10 times higher than TiO₂ fibers, due to cross-linking configuration and stronger van der Waals force of TiO₂ fibers with GO. Pre-homoaggregation of TiO₂ NPs could promote subsequent heteroaggregation while the promotion was negligible at high ionic strength (e.g., > critical coagulant concentrations), and a longer pre-homoaggregation time showed a stronger promotion (750 s > 150 s > 0 s). Natural organic matter (NOM) obviously inhibited GO-TiO₂ heteroaggregation at pH 4.0 compared with pH 7.0 due to stronger steric hindrance. Moreover, strong disturbance (e.g., sonication) could break down the heteroaggregates, and the disaggregation and re-aggregation processes were shape-independent. This work provides new insights into the role of NPs shapes and homoaggregation on the interaction of GO with both engineered and natural particles/minerals, and the fate of GO in aquatic environments.

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.