Co-transport of graphene oxide and heavy metal ions in surface-modified porous media

Aggregation, retention and transport of γ-MnO2 nanoparticles in water-saturated porous media: Impact on the immobility of thallium

Nano-scale Mn oxides can act as effective stabilizers for Tl in soil and sediments. Nevertheless, the comprehensive analysis of the capacity of MnO2 to immobilize Tl in such porous media has not been systematically explored. Therefore, this study investigates the impact of γ-MnO2, a model functional nanomaterial for remediation, on the mobility of Tl in a water-saturated quartz sand-packed column. The mechanisms involved are further elucidated based on the adsorption and aggregation kinetics of γ-MnO2. The results indicate that higher ionic strength (IS) and the presence of ion Ca(II) promote the aggregation of γ-MnO2, resulting from the reduced electrostatic repulsion between particles. Conversely, an increase in pH inhibits aggregation due to enhanced interaction energy. γ-MnO2 significantly influences Tl retention and mobility, with a substantial fraction of γ-MnO2-bound Tl transported through the column. This might be attributed to the high affinity of γ-MnO2 for Tl through ion exchange reactions and precipitation at the surface of γ-MnO2. The mobility of Tl in the sand column is influenced by the γ-MnO2 colloids, exhibiting either inhibition or promotion depending on the pH, IS, and cation type of the solution. In solutions with higher IS and Ca(II), the mobility of Tl decreases as γ-MnO2 colloids tend to aggregate, strain, and block, facilitating colloidal Tl retention in porous media. Although higher pH reduces the mobility of individual Tl, it promotes the mobility of γ-MnO2 colloids, facilitating a substantial fraction of colloidal-form Tl. Consequently, the optimal conditions for stabilizing Tl by γ-MnO2 involve either high IS and low pH or the presence of competitive cations (e.g., Ca(II)). These findings provide new insights into Tl immobilization using MnO2- and Mn oxide-based functional materials, offering potential applications in the remediation of Tl contamination in soil and groundwater.

New Insights into the Bio-Chemical Changes in Wheat Induced by Cd and Drought: What Can We Learn on Cd Stress Using Neutron Imaging?

Cadmium (Cd) and drought stresses are becoming dominant in a changing climate. This study explored the impact of Cd and Cd + drought stress on durum wheat grown in soil and sand at two Cd levels. The physiological parameters were studied using classical methods, while the root architecture was explored using non-invasive neutron computed tomography (NCT) for the first time. Under Cd + drought, all the gas exchange parameters were significantly affected, especially at 120 mg/kg Cd + drought. Elevated Cd was found in the sand-grown roots. We innovatively show the Cd stress impact on the wheat root volume and architecture, and the water distribution in the "root-growing media" was successfully visualized using NCT. Diverse and varying root architectures were observed for soil and sand under the Cd stress compared to the non-stress conditions, as revealed using NCT. The intrinsic structure of the growing medium was responsible for a variation in the water distribution pattern. This study demonstrated a pilot approach to use NCT for quantitative and in situ mapping of Cd stress on wheat roots and visualized the water dynamics in the rhizosphere. The physiological and NCT data provide valuable information to relate further to genetic information for the identification of Cd-resilient wheat varieties in the changing climate.

Fate and cotransport of Pb(II) and Cd(II) heavy ions with bentonite colloidal flow in saturated porous media: The role of filter cake, counter ions, colloid concentration, and fluid velocity

This study examines the transport and retention of colloidal particles and heavy ions in porous sand, focusing on the environmental risks associated with waste from oil and gas drilling. Experimental and numerical models assess the influence of flow rate, external filter cake layer, and ionic strength on bentonite clay particles and heavy ions, such as cadmium (Cd) and lead (Pb), in near-wellbore (high-flux) and far-field (low-flux) scenarios. Colloidal filtration theory and the one-dimensional convection-dispersion equation with two-site kinetic model for attachment and detachment were utilized to calibrate and predict the transport of colloidal suspension in porous media. The research investigates the role of internal and external filter cakes on sand column pressure distribution and heavy ion absorption. Results indicate that the mobility of colloids and heavy ions is influenced by the ionic strength and pH of the carrying fluid. Colloidal clay suspensions show a higher affinity for Pb (II) absorption, while Cd (II) exhibits increased mobility in both clean sand and colloidal environments. Notably, the formation of an external filter cake significantly delays the breakthrough of heavy ions, up to four times longer than in clean sand, and reduces Cd (II) and Pb (II) outlet concentrations by 86% and 93%, respectively. This cake also limits clay concentration and particle size passage. High clay concentrations or injections under high ionic conditions induce clay bridging in pore throats, enhancing internal filtration and heavy ion retention. Conversely, low clay fluxes allow freer particle passage, increasing heavy ion loads and outlet concentrations.

Effects of neonicotinoid insecticides on transport of non-degradable agricultural film microplastics

Mulch film microplastics (MPs) could act as a vector for agricultural chemicals due to their long-term presence in farmland environments. As a result, this study focuses on the adsorption mechanism of three neonicotinoids on two typical agricultural film MPs, polyethylene (PE) and polypropylene (PP), as well as the effects of neonicotinoids on the MPs transport in quartz sand saturated porous media. The findings revealed that the adsorption of neonicotinoids on PE and PP was a combination of physical and chemical processes, including hydrophobic, electrostatic and hydrogen bonding. Acidity and appropriate ionic strength (IS) were favorable conditions for neonicotinoid adsorption of on MPs. The results of column experiments showed that the presence of neonicotinoids, particularly at low concentrations (0.5 mmol L^-1), could promote the transport of PE and PP in the column by improving the electrostatic interaction and hydrophilic repulsion of particles. The neonicotinoids would be adsorbed on MPs through hydrophobic action preferentially, whereas excessive neonicotinoids could cover the hydrophilic functional groups on the surface of MPs. Neonicotinoids reduced the response of PE and PP transport behavior to pH changes. 0.005 mol L^-1 NaCl ameliorated the migration of MPs by increasing their stability. Because of its highest hydration ability and the bridging effect of Mg²⁺, Na⁺ had the most prominent transport promoting effect on PE and PP in MPs-neonicotinoid. This study shows that the increased environmental risk caused by the coexistence of microplastic particles and agricultural chemicals is unneglectable.

Performance of graphene and traditional soil improvers in limiting nutrients and heavy metals leaching from a sandy Calcisol

Given the large amount of Graphene produced in the last years, there is the need to introduce this new material into a green and circular economy loop. In this study, for the first time, the fate of nutrients and heavy metals in a sandy Calcisol amended with Graphene was monitored and compared to other traditional improvers such as Compost, Zeolites, and Biochar. This was performed via saturated and unsaturated columns' experiments with two different fertilization regimes: one with NPK fertilizer and one with an innovative fertigation water (FW) produced from a pilot wastewater treatment plant. The breakthrough curves of each nutrient and heavy metal were analysed to understand the main processes occurring in saturated and unsaturated conditions, comparing the columns amended with the improvers versus the unamended Controls. Mass balances for each nutrient and heavy metal were developed to infer whether the different soil improvers were effective in minimizing leaching. Graphene, for most cases, behaved as the Control in nutrients' leaching for all the saturated and unsaturated experiments, both with NPK and FW. Biochar increased EC, K⁺, and pH of the leaching water, which could be an issue for the growth of some plants. Compost increased NO₃^- leaching in all the experiments. Zeolites showed the best N compounds retention, but great PO₄^3- leaching in saturated conditions. Heavy metals leachates were analysed only for unsaturated columns (as more representative of field conditions) and found at concentrations well below the limits suggested by the U.S. Environmental Protection Agency. Overall, Graphene performed well in minimizing nutrients and heavy metals leaching, respect to classical improvers. This study is a starting point for field studies that will be critical to have a clear understanding of how Graphene behaves in the environment.

Interactions of pristine and aged nanoplastics with heavy metals: Enhanced adsorption and transport in saturated porous media

Interactions of nanoplastics (NPs) with other contaminants are attracting attention, and it is essential to investigate the interaction of aged plastics with heavy metals. We obtained aged nanopolystyrene by UV radiation and investigated the effects of aged NPs on the adsorption and cotransport of Pb-(II) and Cd-(II). The results showed that the UV-aged NPs led to the enhanced adsorption capacity of heavy metals due to the increase in oxygen-containing functional groups, and the promotion of transport by the aged NPs to heavy metals was stronger than that of the pristine NPs. Furthermore, the heavy metals retained in the columns could be freed by the NPs, and the aged NPs were more capable of freed of heavy metals as carriers. In conclusion, the radiation of NPs is correlated with their ability to promote heavy metal transport, and the oxygen content on the surface of NPs plays an essential role in this process to promote the transport of heavy metals in porous media. The ADR equation and DLVO theory simulates the transport behaviour of NPs well. This study is expected to provide a new perspective for assessing the potential risk of aged NPs in soil-groundwater systems.

Transport of silver nanoparticles coated with polyvinylpyrrolidone of various molecular sizes in porous media: Interplay of polymeric coatings and chemically heterogeneous surfaces

Silver nanoparticles (AgNPs) are usually capped with stabilizing agents to protect their activities and improve stability. Polyvinylpyrrolidone (PVP) is one of the most used capping agents of AgNPs, and may affect the transport of AgNPs in porous media. The transport and retention of AgNPs capped with PVPs of different molecular weights (PVP10-AgNP, PVP40-AgNP and PVP360-AgNP) in uncoated, and humic acid (HA)-, kaolinite (KL)- and ferrihydrite (FH)-coated sand porous media were investigated. Among the three AgNPs, PVP360-AgNP exhibited the highest mobility and eluted from all types of porous media. This is because PVPs of higher molecular weight provided stronger steric effect and electrostatic repulsive forces among PVP-AgNPs, inducing stronger blocking and shadow effects. The transport of the PVP-AgNPs increased in the HA-Sand columns, while decreased in the KL- and FH-Sand columns, especially for PVP10-AgNP and PVP40-AgNP. The simulation results using one-site kinetic model indicated that HA-Sand reduced the maximum retention capacity (Smax), while KL- and FH-Sand increased the Smax as well as the first-order attachment rate coefficients (katt), particularly at high ionic strength. The results shed light on the interplay of the capping agents of AgNPs and the surface heterogeneity on the transport of AgNPs in porous media.

Co-transport and co-release of Eu(III) with bentonite colloids in saturated porous sand columns: Controlling factors and governing mechanisms

Accurate prediction of the colloid-driven transport of radionuclides in porous media is critical for the long-term safety assessment of radioactive waste disposal repository. However, the co-transport and corelease process of radionuclides with colloids have not been well documented, the intrinsic mechanisms for colloids-driven retention/transport of radionuclides are still pending for further discussion. Thus the controlling factors and governing mechanisms of co-transport and co-release behavior of Eu(III) with bentonite colloids (BC) were discussed and quantified by combining laboratory-scale column experiments, colloid filtration theory and advection dispersion equation model. The results showed that the role of colloids in facilitating or retarding the Eu(III) transport in porous media varied with cations concentration, pH, and humic acid (HA). The transport of Eu(III) was facilitated by the dispersed colloids under the low ionic strength and high pH conditions, while was impeded by the aggregated colloids cluster. The enhancement of Eu(III) transport was not monotonically risen with the increase of colloids concentration, the most optimized colloids concentration in facilitating Eu(III) transport was approximately 150 mg L^-1. HA showed significant promotion on both Eu(III) and colloid transport because of not only its strong Eu(III) complexion ability but also the increased dispersion of HA-coated colloid particles. The HA and BC displayed a synergistic effect on Eu(III) transport, the co-transport occurred by forming the ternary BC-HA-Eu(III) hybrid. The transport patterns could be simulated well with a two-site model that used the advection dispersion equation by reflecting the blocking effect. The retarded Eu(III) on the stationary phase was released and remobilized by the introduction of colloids, or by a transient reduction in cation concentration. The findings are essential for predicting the geological fate and the migration risk of radionuclides in the repository environment.

Systematic Research on the Transport of Ball-Milled Biochar in Saturated Porous Media: Effect of Humic Acid, Ionic Strength, and Cation Types

Ball-milled biochar (BMBC) is a typical engineering material that has promising application prospects in remediating contaminated soil and water. It is fundamental to rate the transport behaviors of BMBC in the underground environment before extensive use. In this study, the effects of the ubiquitous cations (Na⁺, Mg²⁺, and Al³⁺) and model organic matter (humic acid) on the transport of BMBC were investigated using laboratory column experiments. The results demonstrated the facilitated effect of HA on the transport of BMBC due to the negatively charged surface and steric effect under neutral conditions. HA and ionic strength manifested an antagonistic effect on the transport of BMBC, where the presence of one could weaken the effect from the other. We also found the charge reversal of the BMBC surface in the presence of Mg²⁺, thus enhancing the deposition of BMBC onto the medium surface. On the other hand, the charge reversal from Al³⁺-coupled acid conditions led to the restabilization and transport of BMBC in porous media. Therefore, the rational usage of BMBC is indispensable and more attention should be paid to the composition and change in underground water that might facilitate the transport of BMBC and thus lead to negative environmental implications.

Exploring the potential of graphene oxide nanosheets for porous media decontamination from cationic dyes

Graphene oxide (GO) nanosheets, often embedded in nano-composites, have been studied as promising materials for waste water purification, in particular to adsorb heavy metals and cationic organic contaminants. However, a broader range of potential applications of GO is still unexplored. This work investigated the potential applicability of GO for enhanced in-situ soil washing of secondary sources of groundwater contamination (i.e. the controlled recirculation of a washing GO suspension via injection/extraction wells). The laboratory study aimed at quantifying the capability of GO to effectively remove adsorbed methylene blue (MB) from contaminated sand. The tests were conducted in simplified conditions (synthetic groundwater at NaCl concentration of 20 mM, silica sand) to better highlight the key mechanisms under study. The results indicated a maximum sorption capacity of 1.6 mgMB/mgGO in moderately alkaline conditions. Even though the adsorption of MB onto GO slightly reduced the GO mobility in the porous medium, a breakthrough higher than 95% was obtained for MB/GO mass ratios up to 0.5. This suggests that a very high recovery of the injected particles should be also expected in the field.

Transport of TiO2 and CeO2 nanoparticles in saturated porous media in the presence of surfactants with environmentally relevant concentrations

Nanomaterials are threatening the environment and human health, but there has been little discussion about the stability and mobility of nanoparticles (NPs) in saturated porous media at environmentally relevant concentrations of surfactants, which is a knowledge gap in exploring the fate of engineered NPs in groundwater. Therefore, the influences of the anionic surfactant (sodium dodecylbenzene sulfonate, SDBS), the cationic surfactant (cetyltrimethylammonium bromide, CTAB), and the nonionic surfactant (Tween-80) with environmentally relevant concentrations of 0, 5, 10, and 20 mg/L on nano-TiO₂ (nTiO₂, negatively charged) and nano-CeO₂ (nCeO₂, positively charged) transport through saturated porous media were examined by column experiments. On the whole, with increasing SDBS concentration from 0 to 20 mg/L, the concentration peak of nTiO₂ and nCeO₂ in effluents increased by approximately 0.2 and 0.3 (dimensionless concentration, C/C₀), respectively, because of enhanced stability and reduced aggregate size resulting from enhanced electrostatic and steric repulsions. By contrast, the transportability of NPs significantly decreased with increasing CTAB concentration due to the attachment of positive charges, which was opposite to the charge on the medium surface and facilitated the NP deposition. On the other hand, the addition of Tween-80 had no significant influence on the stability and mobility of nTiO₂ and nCeO₂. The results were also demonstrated by the colloid filtration theory (CFT) modeling and the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction calculations; it might promote the assessment and remediation of NP pollution in subsurface environments.

Effects of physicochemical factors on the transport of aged polystyrene nanoparticles in saturated porous media

Plastic debris, particularly nanoplastics, have attracted substantial attention as an emerging pollutant of global concern. The aging process caused by UV could dramatically alter the physicochemical properties of polystyrene plastics and affect their fate and transport in the subsurface environment. We researched the effects of diverse factors, including flow rate, ionic strength (IS), and cation valence on the transport of aged polystyrene nanoparticles (PSNPs) in saturated porous media and found that their mobility was greatly enhanced by the aging process at all other experimental conditions except coexistence with Al³⁺. Interestingly, we found that the aged PSNPs were polymerized due to the coexistence with Al³⁺, which led the aged PSNPs to exhibit weaker mobility than the pristine. Zeta potential and particle size measurements, FTIR spectroscopy, and XPS were employed to characterize the PSNPs, and the results suggest that UV radiation provides O-containing functional groups for the PSNPs. The experimental results correspond well with the ADR model and the values of Smax and k closely reflect the deposition of the PSNPs in sand columns. Moreover, the Derjaguin-Landau-Verwey-Overbeek (DLVO) theoretical calculation accurately reflects the interaction of the pristine and aged PSNPs and quartz sand. Overall, due to the processes that PSNPs possibly undergo in the environment, their mobility may fluctuate dramatically. These findings help remedy knowledge deficiency regarding nanoplastic mobility being affected by aging processes, further underscore the critical influence of the aging process on the potential risks and environmental fates of nanoplastics.

Effect of nZVI/biochar nanocomposites on Cd transport in clay mineral-coated quartz sand: Facilitation and rerelease

The development and application of nano-biochar synthesized by ball milling technology is still challenging in the field of environmental remediation because of its higher activity with pollutants. The purpose of this study was to investigate the transport behavior of two kinds of biochar nanoparticles (nanobiochar (NBC) and nZVI-modified nanobiochar (nZVI-NBC)) and Cd2+ in clay mineral (kaolinite, illite, and montmorillonite)-coated quartz sand columns. The interaction between biochar nanoparticles and Cd2+ in saturated porous media was studied in cotransport experiments. Then, the effect of biochar nanoparticles on the release of Cd2+ in contaminated media was explored by elution experiments. The cotransport experiments showed that the mobility of Cd2+ was enhanced by two kinds of biochar nanoparticles, while the transport of biochar was limited due to the presence of Cd2+. The elution experiments showed that the transport of biochar nanoparticles can be inhibited by Cd2+ previously immobilized in the sand column, and Cd2+ can be rereleased by biochar nanoparticles. The rerelease ability of nZVI-NBC to Cd2+ is weaker than that of NBC because nZVI is more easily retained in the sand column after oxidation, thus strengthening the immobilization of Cd2+. In general, the recoveries of NBC, nZVI-NBC and Cd2+ in saturated porous media were reduced by the presence of clay minerals. The experimental results describing the stability of biochar nanoparticles in sand columns are consistent with those predicted by the Derjaguin-Landau-Verwey-Overbeek theory. The transport behavior of Cd2+ and biochar nanoparticles in sand columns can be well simulated by advection-dispersion-reaction. These findings reveal the interaction between biochar nanoparticles and heavy metals in the soil environment and provide new insights into the transport and fate of environmental remediation materials and pollutants in the underground environment.

Transport of nanoparticles in porous media and its effects on the co-existing pollutants

Nanomaterials are widely used in daily life owing to their superior characteristics. The release and transport of nanoparticles (NPs) in the environment is inevitable during their entire life cycle, posing a risk to the aquatic environment. Thus, considerable attention has been focused on the fate and behavior of NPs in porous media, as well as the co-transport of NPs with other pollutants. In this review, current knowledge about the retention and transport behavior of NPs in porous media is summarized. NP transport in porous media is dominated by various internal and external factors, including the characteristics of NPs, porous media, and water flow. Generally, NPs with high density, small particle size, and surface coating are easily transported in porous media with the characteristics of large size, smooth surface, and low water saturation. Meanwhile, high pH and velocity, low temperature, and natural organic matter-containing fluids are also conducive to NP transport. Aggregation, adsorption, straining, and blocking are the primary mechanisms by which NPs affect the transport of co-existing pollutants in porous media. Current research on NP transport has been performed predominantly using modal porous media (e.g., sand and glass beads); however, there is a large gap between simulated and natural porous media. Further studies should focus on the transport, fate, and interaction of NPs and coexistent pollutants in natural porous media, as well as the coupling mechanisms under actual environmental conditions.

Co-transport of ball-milled biochar and Cd2+ in saturated porous media

The combination of ball milling technology and biochar materials provides new prospects for environmentally friendly and sustainable environmental pollution control technologies, but comes with opportunities and risks. In this study, column experiments were used to evaluate the environmental behavior of ball‑milled biochar (BMBC). The results of the column experiments showed that BMBC transport increased with a high flow velocity, large medium size, high pH, and low ionic strength. Owing to the strong adsorption of Cd²⁺ by BMBC, the presence of BMBC in the medium led to a decrease in effluent Cd²⁺. The presence of Cd²⁺ in the solution slightly inhibited the transport of BMBC. The transport of Cd²⁺ was facilitated by BMBC due to the high affinity. Therefore, attention should be paid to favorable conditions for BMBC transport. This study provides a perspective to assess the behavior of BMBC in the environment and whether its interaction with Cd²⁺ will introduce new environmental hazards.

Effects of phosphate on the transport of graphene oxide nanoparticles in saturated clean and iron oxide-coated sand columns

In this study, transport behaviors of graphene oxide (GO) in saturated uncoated (i.e., clean sand) and goethite-coated sand porous media were examined as a function of the phosphate. We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e., pH 5.0-9.0 and the presence of 10 mmol/L Na⁺ or 0.5 mmol/L Ca²⁺). The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media. Meanwhile, deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport. Interestingly, when the phosphate concentration increased from 0.1 to 1.0 mmol/L, the transport-enhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand. The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns, which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate. When the background solution contained 0.5 mmol/L Ca²⁺, phosphate could be bind to sand/ goethite-coated sand surface by cation bridging; and consequently, promoted competition between phosphate and nanoparticles for deposition sites, which was an important mechanism for the enhanced effect of phosphate. Moreover, the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate. Taken together, these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment.

Role of solution chemistry in the attachment of graphene oxide nanoparticles onto iron oxide minerals with different characteristics

Given the ubiquity and abundance of the iron oxide minerals and their important roles in affecting the environmental fate of graphene oxide (GO) nanoparticles, the attachment of GO onto three iron oxide minerals (i.e., hematite, goethite, and ferrihydrite) under different solution chemistry conditions was investigated in this study. The main mechanism of the attachment of GO was electrostatic interaction. Calculations based on the DLVO theory showed that the attachment was a favorable process. Interestingly, the affinity of GO towards three iron oxide minerals was in the order of ferrihydrite > goethite > hematite. This result indicates that different characteristics of various iron oxides (e.g., specific surface area, crystal structure, and surface charge, and surface hydroxyl densities) can influence their attachment capacities for GO. The attachment of GO depended on the solution pH and ionic strength. Electrostatic attraction and hydrogen bonding were the important retention mechanisms for GO attachment when pH < pH_PZC (the point of zero charge) and pH > pH_PZC, respectively. The attachment capacities of iron oxides decreased with increasing ionic strength at lower pH because of the decrease of the electrostatic attraction. Meanwhile, the presence of divalent cations (i.e., Ca²⁺ and Cu²⁺) could significantly promote GO attachment mainly by the surface-bridging mechanism. Meanwhile, the enhancement effect of Cu²⁺ was greater than Ca²⁺ due to the greater complexation affinity of Cu²⁺. Furthermore, attachment isotherms showed that the presence of phosphate could inhibit the attachment of GO onto minerals obviously. Because phosphate could form inner-sphere surface complex on the iron oxide surface, and consequently decreased the electrostatic attraction between nanoparticles and minerals. Our study has important implications for predicting the fate of GO in natural environment where amounts of iron oxide minerals are present.

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.

Transport of N-CD and Pre-Sorbed Pb in Saturated Porous Media

Carbon dots (CDs) are a new type of nanomaterials of the carbon family with unique characteristics, such as their small size (e.g., <10 nm), high water solubility, low toxicity, and high metal affinity. Modification of CDs by Nitrogen functional groups (N-CDs) enhances their metal adsorption capacity. This study investigated the influences of pH (4, 6, and 9), ionic strength (1, 50, and 100 mM), and cation valency (Na+ and Ca2+) on the competitive adsorption of Pb to quartz and N-CD surfaces, the transport and retention of N-CDs in saturated porous media, and the capacity of N-CDs to mobilize pre-adsorbed Pb in quartz columns. Pb adsorption was higher on N-CDs than on quartz surfaces and decreased with increases in ionic strength (IS) and divalent cations (Ca2+) concentration. N-CD mobility in quartz columns was highest at pH of 9- and 1-mM monovalent cations (Na+) and decreased with decreases in pH and increases in ionic strength and ion valency. N-CDs mobilized pre-adsorbed Pb from quartz due to the higher adsorption affinity of Pb to N-CD than to quartz surfaces. These findings provide valuable insights into the transport, retention, and risk assessment of lead in the presence of carbon-based engineered nanoparticles.

Co-transport and competitive retention of different ionic rare earth elements (REEs) in quartz sand: Effect of kaolinite

The increasing excavation and utilization of rare earth elements (REEs) have resulted in an elevated release of these elements into the environment. Therefore, investigating the transport behavior of REEs is critical for a comprehensive understanding of their geochemical cycles and to propose potential pollution control strategies. This study investigated the transport, co-transport, and competitive retention of three REEs: La (a light REE), Gd (a middle REE), and Yb (a heavy REE), as well as the co-transport of REEs and kaolinite (a representative clay mineral) in porous media. Both observed and simulated breakthrough curves and retention profiles demonstrated that all ionic REEs exhibited considerable breakthrough and slight retention with almost uniform shapes in quartz sand (QS) owing to the weak affinity of ionic REEs to QS. The breakthrough of REEs in all experiments followed the order of La > Gd > Yb, indicating that REE breakthrough increased with decreasing atomic number. The same elements exhibited their highest breakthrough during the co-transport of the three REEs, followed by co-transport of two REEs, and finally single transport. Furthermore, mathematical modeling indicated that the retention of REEs in QS was a predominantly kinetic process, whereby competitive blocking was the dominant mechanism for the enhanced breakthrough of REEs during co-transport, as compared to single transport. The co-transport of REEs and kaolinite demonstrated that kaolinite has a slight influence on the transport of REEs in QS under adsorption kinetics. However, REEs inhibited the transport and strongly enhanced the retention of kaolinite in QS due to a decreasing electrostatic repulsion between kaolinite and QS in the presence of REEs, even if the adsorption of REEs onto kaolinite was weak under adsorption kinetics. Therefore, this study increases our understanding of the transport mechanisms of REEs in the environment.

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH₄⁺ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH₄⁺ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH₄⁺, the co-transport mechanism became significantly different. During transport by the NH₄⁺-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH₄⁺, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH₄⁺. The co-transport of NH₄⁺ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH₄⁺ to be more likely to be transported at a higher ionic strength (IS = 0.05 m, CaCl₂) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH₄⁺ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20 mg/L then declines at 40 mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

Silica colloids as non-carriers facilitate Pb2+ transport in saturated porous media under a weak adsorption condition: effects of Pb2+ concentrations

Transport of environmental pollutants in groundwater systems can be greatly influenced by colloids. In this study, the cotransport of Pb²⁺ and silica (SiO₂) colloids at different Pb²⁺ concentrations was systematically investigated by batch adsorption and saturated sand column experiments. Results showed that SiO₂ colloids had low adsorption capacity for Pb²⁺ (less than 1% of the input) compared with sands. In saturated porous media, SiO₂ colloids showed a high mobility; however, with the increase of Pb²⁺ concentration in the sand column, the mobility of SiO₂ colloids gradually decreased. Notably, SiO₂ colloids could facilitate Pb²⁺ transport, although they did not serve as effective carriers of Pb²⁺. Under the condition of low Pb²⁺ concentration, SiO₂ colloids promoted the Pb²⁺ transport mainly through the way of "transport channel," while changing the porosity of the medium and masking medium adsorption sites were the main mechanisms of SiO₂ colloid-facilitated Pb²⁺ transport under the condition of high Pb²⁺ concentration. The discovery of this non-adsorption effect of colloids would improve our understanding of colloid-facilitated Pb²⁺ transport in saturated porous media, which provided new insights into the role of colloids, especially colloids with weak Pb²⁺ adsorption capacity, in Pb²⁺ occurrence and transport in soil-groundwater systems.

Biochar-amendment-reduced cotransport of graphene oxide nanoparticles and dimethyl phthalate in saturated porous media

Production and application of graphene oxide (GO) and biochar for water and soil treatment is steadily growing, driving the necessity to understand the cotransport behavior of contaminants and GO nanoparticles in porous media and the possible effect of biochar to reduce their cotransport. The cotransport of GO nanoparticles and dimethyl phthalate (DMP) as a model in a sand column and biochar-amended sand column (biochar column) was compared. The transport of DMP in the test columns was independent of the solution ionic strength (IS), while the transport of GO decreased with increased IS due to the enhanced aggregation of GO nanoparticles. The sand column had no retention capacity (less than 1%) for DMP, while the biochar column had significantly increased retention of DMP (100%). The retention of GO in the biochar column was significantly higher than that of the sand column because biochar can improve the roughness of the media and adsorb GO via π-π interactions. Under low-IS conditions, GO facilitated DMP transport by providing vehicles and adsorption sites (vehicle effect). Due to reversible adsorption-desorption, the adsorbed DMP on GO could be released, resulting in tailing during the flushing phase. The vehicle effect of GO on DMP transport was significantly weakened in the biochar columns, and DMP tailing during the flushing phase was not observed in the biochar columns, which was attributed to the strong retention/adsorption of the biochar columns for both GO and DMP, higher affinity of DMP on biochar than GO, and desorption hysteresis of DMP on biochar. These observations are important for evaluating the potential role of biochar in soil and water remediation, as well as mitigating the health risks of GO and organic contaminants in the environment.

Cotransport of Cu with Graphene Oxide in Saturated Porous Media with Varying Degrees of Geochemical Heterogeneity

Graphene oxide (GO) is likely to encounter heavy metals due to its widespread use and inevitable release into the subsurface environment, where the ubiquitous presence of iron oxides (e.g., hematite) would affect their interaction and transport. The present study aimed to investigate the cotransport of GO (20 mg L−1) and copper (0.05 mM CuCl2) in the presence of varying degrees of geochemical heterogeneity represented by iron oxide-coated sand fractions (ω = 0‒0.45) in water-saturated columns under environmentally relevant physicochemical conditions (1 mM KCl at pH 5.0‒9.0). The Langmuir-fitted maximum adsorption capacity of Cu2+ by GO reached 137.6 mg g−1, and the presence of 0.05 mM Cu2+ decreased the colloidal stability and subsequent transport of GO in porous media. The iron oxide coating was found to significantly inhibit the transport of GO and Cu-loaded GO in sand-packed columns, which can be explained by the favorable deposition of the negatively charged GO onto patches of the positively charged iron oxide coatings at pH 5.0. Increasing the solution pH from 5.0 to 9.0 promoted the mobility of GO, with the exception of pH 7.5, in which the lowest breakthrough of GO was observed. This is possibly due to the fact that the surface charge of iron oxide approaches zero at pH 7.5, suggesting that new “favorable” sites are available for GO retention. This study deciphered the complicated interactions among engineered nanomaterials, heavy metals, and geochemical heterogeneity under environmentally relevant physicochemical conditions. Our results highlight the significant role of geochemical heterogeneity, such as iron oxide patches, in determining the fate and transport of GO and GO-heavy metal association in the subsurface environment.

Key factors affecting graphene oxide transport in saturated porous media

This study focuses on the transport in porous media of graphene oxide nanoparticles (GONP) under conditions similar to those applied in the generation of in-situ reactive zones for groundwater remediation (i.e. GO concentration of few tens of mg/l, stable suspension in alkaline solution). The experimental tests evaluated the influence on GO transport of three key factors, namely particle size (300-1200 nm), concentration (10-50 mg/L), and sand size (coarse to fine). Three sources of GONP were considered (two commercial and one synthesized in the laboratory). Particles were stably dispersed in water at pH 8.5 and showed a good mobility in the porous medium under all experimental conditions: after injection of 5 pore volumes and flushing, the highest recovery was around 90%, the lowest around 30% (only for largest particles in fine sand). The particle size was by far the most impacting parameter, with increasing mobility with decreasing size, even if sand size and particle concentration were also relevant. The source of GONP showed a minor impact on the mobility. The transport test data were successfully modeled using the advection-dispersion-deposition equations typically applied for spherical colloids. Experimental and modeling results suggested that GONP, under the explored conditions, are retained due to both blocking and straining, the latter being relevant only for large particles and/or fine sand. The findings of this study play a key role in the development of an in-situ groundwater remediation technology based on the injection of GONP for contaminant degradation or sorption. Despite their peculiar shape, GONP behavior in porous media is comparable with spherical colloids, which have been more studied by far. In particular, the possibility of modeling GONP transport using existing models ensures that they can be applied also for the design of field-scale injections of GONP, similarly to other particles already used in nanoremediation.

Cotransport of Herbaspirillum chlorophenolicum FA1 and heavy metals in saturated porous media: Effect of ion type and concentration

Predicting the cotransport of functional microorganisms and heavy metals in porous media is essential to both bioremediation and pollutant risk assessment. In this study, batch and column experiments were conducted to explore the cotransport behaviors of functional bacteria (FA1) and heavy metals (Pb²⁺/Cd²⁺) in saturated sand media under different conditions. The sorption capacity of heavy metals on FA1 was much greater than that of the sand, while both FA1 and sand showed stronger affinity to Pb²⁺ than Cd²⁺. The surface properties, especially zeta potential, of the bacteria and sand were altered by metal adsorption. As a result, the co-existence of Pb²⁺ decreased the transport of FA1 more significantly than that of Cd²⁺, and the influence was more significant with higher heavy metal concentration. On the other hand, the co-existence of FA1 inhibited the mobility of Pb²⁺ and Cd²⁺ in most scenarios, except when the cotransport concentration of Pb²⁺ was 5 mg L^-1, and the inhibition was more pronounced for Pb²⁺ than Cd²⁺. Increase in metal concentrations decreased the FA1-associated Pb²⁺/Cd²⁺ in effluents due to the remarkable decrease in FA1 mobility, and free soluble Pb²⁺/Cd²⁺ became the major migration species. In addition, due to stronger attractive forces and affinity between Pb²⁺ and FA1, nearly all presorbed-Pb²⁺ by sand was remobilized by FA1 and transported mainly in FA1-associated form other than soluble Pb²⁺. Findings from this study indicated that the cotransport of biocolloids and heavy metals are highly sensitive to the ion type and concentration, and evaluation of their transport in the subsurface should be carefully carried out to avoid inaccurate estimations.