Key factors affecting graphene oxide transport in saturated porous media

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

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

Statuses, shortcomings, and outlooks in studying the fate of nanoplastics and engineered nanoparticles in porous media respectively and borrowable sections from engineered nanoparticles for nanoplastics

This review discussed the research statuses, shortcomings, and outlooks for the fate of nanoplastics (NPs) and engineered nanoparticles (ENPs) in porous media and borrowable sections from ENPs for NPs. Firstly, the most important section was that we reviewed the research statuses on the fate of NPs in porous media and the main influencing factors, and explained the influencing mechanisms. Secondly, in order to give NPs a reference of research ideas and influence mechanisms, we also reviewed the research statuses on the fate of ENPs in porous media and the factors and mechanisms influencing the fate. The main mechanisms affecting the transport of ENPs were summarized (Retention or transport modes: advection, diffusion, dispersion, deposition, adsorption, blocking, ripening, and straining; Main forces and actions: Brownian motion, gravity, electrostatic forces, van der Waals forces, hydration, filtration, bridging; Affecting elements of the forces and actions: the ENP and media grain surface functional groups, size, shape, zeta potential, density, hydrophobicity, and roughness). Instead of using the findings of ENPs, thorough study on NPs was required because NPs and ENPs differed greatly. Based on the limited existing studies on the NP transport in porous media, we found that although the conclusions of ENPs could not be applied to NPs, most of the influencing mechanisms summarized from ENPs were applicable to NPs. Combining the research thoughts of ENPs, the research statuses of NPs, and some of our experiences and reflections, we reviewed the shortcomings of the current studies on the NP fate in porous media as well as the outlooks of future research. This review is very meaningful for clarifying the research statuses and influence mechanisms for the NP fate in porous media, as well as providing a great deal of inspiration for future research directions about the NP fate in porous media.

Interplay of compound pollutants with microplastics transported in saturated porous media: Effect of co-existing graphene oxide and tetracycline

Co-existence of microplastics, nanomaterials, and antibiotics may lead to intensified multifaceted pollution, which may influence their fate in soils. This study investigated the co-transport behavior of polystyrene microplastics (PS) and compound pollutants of graphene oxide (GO) and tetracycline (TC). Packed column experiments for microplastic with or without combined pollutants were performed in KCl (10 and 30 mM) and CaCl2 solutions (0.3 and 1 mM). The results showed transport of PS was facilitated at low ionic strengths and inhibited at high ionic strengths by GO with or without TC under examined conditions. Carrier effect of GO as well as the aggregation of PS in the presence of co-exiting GO or GO-TC could be the contributor. Although the existence of TC relieved the ripening phenomenon of PS and GO deposition due to enhanced electronegativity of sand media, the effect of GO on the PS transport has not been significantly impacted, indicating the dominant role of GO during cotransport process. Furthermore, the transport of PS was increased by TC owing to competition for deposition sites on sand surfaces. In turn, the transport of TC was mainly affected by PS whether graphene was present or not. The increase in electrostatic repulsive force (transport-promoting) and addition adsorption sites (transport-inhibiting) may be responsible for the observations. Our findings could improve understandings of complex environmental behaviors of microplastics and provide insight into investigation on cotransport of emerging contaminants under various conditions relevant to the subsurface environment.

Size effect of graphene oxide from quantum dot to nanoflake on the mobility of nanoplastics in seawater-saturated sand

Marine sedimentary environment serves as an important sink of terrigenous nanoplastics (NP) and graphene oxides (GO). In this study, we discovered that GO of varying sizes exhibited distinct binding modes with 200 nm NP in 35 practical salinity unit (PSU) seawater, resulting in varying impacts on the mobility of NP in porous media. GO-8, with a size of 8±2 nm, firmly adhered to the surface of NP and formed stable primary heterogeneous aggregates, which promoted NP mobility and increased the mass recovery of effluent (Meff) from 24.74% to 31.08%. GO-250 (246±10 nm) partly enveloped NP and only slightly increased the volume of heteroaggregates, which had minimal effect on NP transport. Conversely, GO-850 (855±55 nm) wrapped numerous NP particles to form large secondary heteroaggregates that clung to sand surfaces, providing additional attachment sites for NP, resulting in complete inhibition of NP mobility in porous media (Meff = 0%). In brackish water with 3.5 PSU, all GO-8, GO-250 and GO-850 achieved enhanced mobility of NP, with Meff increasing from 50.35% to 85.62%, 69.45% and 75.41%, respectively. The results indicate that GO size effects on NP mobility are also salinity-dependent.

Transport of biochar colloids under unsaturated flow condition: Roles of chemical aging and cation type

Biochar colloids released from biochar materials are ubiquitous in the environment and undergo environmental transformation processes that may alter their properties. Natural subsurface environments are usually under unsaturated conditions, which could affect the transport of biochar colloids. This study investigated the transport of pristine and aged biochar colloids under unsaturated conditions by aggregation test, bubble column experiment, and sand column experiment. After aging, the biochar showed a more negative, hydrophilic, and rougher surface. Compared with pristine biochar colloids, aged biochar colloids in NaCl solution were not retained at the air-water interface (AWI) due to their more hydrophilic and rougher surface. In CaCl₂ solution, more pristine and aged biochar colloids were retained at the AWI because Ca²⁺ weakened the electrostatic repulsion between biochar colloids and the AWI. With the decrease in saturation, the transport of pristine and aged biochar colloids decreased by 17 %‑67 % through the retention at AWI and air-water-solid (AWS) interface. The transport of biochar colloids in NaCl solution was increased by 10 %‑20 % after aging as the aged biochar was not retained at the AWI. The difference of transport between pristine and aged biochar colloids in CaCl₂ solution (<8 %) was lower than that in NaCl solution due to the enhanced retention of aggregated biochar colloids at the AWI and AWS interfaces. These results highlight the importance of the surface structure of biochar on its behavior in the environment, which is essential for assessing the potential of biochar application for carbon sequestration and environmental protection.

A pore-scale investigation of the effect of nanoparticle injection on properties of sandy porous media

Nanoremediation is a new groundwater remediation technology in which nanoparticles (NPs) are injected into the sub-surface to promote in-situ degradation of aquifer contaminants. Although nanoremediation is an effective process to eliminate contaminants in-situ, its success relies on sufficiently mobile NPs that can reach the contaminated zones and remain there to facilitate chemical degradation of contaminants. Therefore, understanding the main parameters that control the mobility and retention of NPs in saturated porous media is a key component of designing a successful nanoremediation process. This work presents the outcome of a pore-scale study of nZVI NP (zero-valent iron) transport in sandy porous media using the non-destructive 3D imaging technique, X-ray computed micro-tomography (X-ray micro-CT). We investigate the effect of grain size (fine, coarse, carbonate and mixed sand) and composition (carbonate vs sand grains) on the mobility and retention of NPs in sand columns. To achieve this, we used four columns packed with grains of different sizes and compositions. The main contribution of this work is, therefore, to understand the effect of NP injection on the structural and geometric properties of sandy porous media and to identify the main pore-scale mechanisms controlling NP transport and entrapment. Our experiment shows that the pore geometries change because of NP injection. Pore clogging is evidenced through pore size and throat size distribution displaying a shift to the left with a noticeable reduction in pore connectivity in all the columns. The porosity and permeability of the columns studied display significant reduction as result of the NP injection.

The roles of Fe oxyhydroxide coating and chemical aging in pyrogenic carbon nanoparticle transport in unsaturated porous media

Pyrogenic carbon (PyC) nanoparticles are widespread in the environment, which is important to global carbon cycle. PyC can exist for millions of years and undergo various environmental aging processes. To better understand the roles of Fe oxyhydroxides and water content on the pristine and aged PyC transport, adsorption and column experiments were conducted under three saturations (100%, 70%, and 40%) and three pH (5, 7, and 9) in both clean and Fe oxyhydroxide-coated sand. At high water saturations (100% and 70%), the mobility of both the pristine and aged PyC was enhanced at high pH due to strong electrostatic repulsion, and the aged PyC showed higher mobility than the pristine PyC because of its more negative charge and hydrophilic surface. The coating of Fe oxyhydroxides on sand decreased the mobility of both the pristine and aged PyC due to weak electrostatic repulsion, large specific surface area, and high roughness. At low saturation (40%), solution pH showed little effect on both the pristine and aged PyC mobility, and water saturation became the main factor affecting PyC mobility. Almost no pristine or aged PyC transported out from the Fe oxyhydroxide-coated sand column because Fe oxide increased the roughness of the sand surface, which led to a sharp increase in the air-water-solid interface and retention sites. This study demonstrates that water content, environmental aging, and Fe oxyhydroxides are significant in the fate and transport of PyC nanoparticles in environments, which provides a good fundamental understanding for the assessment of pyrogenic carbon application in environmental protection and carbon sequestration.

Effects of polyamide microplastic on the transport of graphene oxide in porous media

With the rapid development of the nano-material and chemical industry, more and more microplastic (MP) and nano-material were discharged into the environment. In this study, a two-dimensional (2D) surface of Extended Darjaguin-Landau-Verwe-Overbeek (XDLVO) is proposed to quantitatively investigate the effect of polyamide (PA) on the transport of graphene oxide (GO) in porous media. The influences of mass fraction of PA, flow rate, GO concentration, ionic type and strength on the migration of GO in saturated porous media are investigated by column experiments and numerical models. The two-dimensional (2D) surfaces of XDLVO interaction energy between GO and GO, GO and QS, GO and PA, are firstly calculated to analyze the transport of GO in saturated porous media. Experimental results suggest the mobility of GO is enhanced when flow velocity and initial concentration of GO are increased. However, the mobility of GO is inhibited when the mass fraction of PA and ionic strength are increased. More important, the inhibitory effect of divalent cations on GO migration is stronger than that of monovalent cations. Simultaneously, XDLVO results suggest that ionic types and strengths are important factors affecting the mobility of GO in porous media, and the critical ionic strength is observed from the continuous variation of the secondary minimum trap of XDLVO interaction energy. Model results show that there is a linear relationship between the logarithm of the secondary minimum trap of XDLVO interaction energy and the parameters related to GO mobility, which suggests XDLVO energy surface has an important application significance in the accurate quantification of GO mobility in porous media. These findings contribute to GO transport affected by microplastic in porous media, thus laying a significant foundation for the environmental risk and contamination remediation.

Nanoplastics removal during drinking water treatment: Laboratory- and pilot-scale experiments and modeling

Microplastics detected in potable water sources and tap water have led to concerns about the efficacy of current drinking water treatment processes to remove these contaminants. It is hypothesized that drinking water resources contain nanoplastics (NPs), but the detection of NPs is challenging. We, therefore, used palladium (Pd)-labeled NPs to investigate the behavior and removal of NPs during conventional drinking water treatment processes including ozonation, sand and activated carbon filtration. Ozone doses typically applied in drinking water treatment plants (DWTPs) hardly affect the NPs transport in the subsequent filtration systems. Amongst the different filtration media, NPs particles were most efficiently retained when aged (i.e. biofilm coated) sand was used with good agreements between laboratory and pilot scale systems. The removal of NPs through multiple filtration steps in a municipal full-scale DWTP was simulated using the MNMs software code. Removal efficiencies exceeding 3-log units were modeled for a combination of three consecutive filtration steps (rapid sand filtration, activated carbon filtration and slow sand filtration with 0.4-, 0.2- and 3.0-log-removal, respectively). According to the results from the model, the removal of NPs during slow sand filtration dominated the overall NPs removal which is also supported by the laboratory-scale and pilot-scale data. The results from this study can be used to estimate the NPs removal efficiency of typical DWTPs with similar water treatment chains.

Vertical transport and retention behavior of polystyrene nanoplastics in simulated hyporheic zone

The ecological risk of microplastics (MPs) usually depends on their environmental behavior, however, few studies focused on the impact of hydrodynamic perturbations on the fate of MPs in hyporheic zone. This study chose quartz sand (250-425 μm) as simulated porous medium to investigate the transport of 100 nm polystyrene nanoplastics (PSNPs) under hydrodynamic factors, including flow rates (0.5, 1.0, and 2.0 mL/min), flow orientations (up-flow, down-flow, and horizontal-flow), and water saturations (50%, 80%, and 100%), as well as different salinities and temperatures. The breakthrough curves (BTCs) and retained profiles (RPs) of PSNPs were compared and analyzed by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Due to the small size and moderate density of PSNPs, as well as high flow rates, the flow orientation exhibited little effect on the PSNP transport. However, high flow rate, low salinity, high water saturation, and low temperature would facilitate the mobility of PSNPs. The increase in salinity from zero to 35 PSU (practical salinity units) caused the compression of electrical double layer and weakened the electrostatic repulsion between PSNPs and sands, which dramatically decreased the penetration rate from 100% to zero. Especially, the lower energy barrier of PSNPs-PSNPs at 3.5 and 35 PSU (16.45 k_BT and zero, respectively) facilitated the adsorption of PSNPs on sand via ripening mechanism. Due to the strong adsorption of PSNPs by sand at high salinity, the effect of flow rate on PSNP transport was more pronounced at low salinity. The mobility of PSNPs at 0.035 PSU was enhanced by 41.4%-75.3% as the flow rate increased from 0.5 to 2.0 mL/min, which was contributed from the reversible deposition in lower secondary energy minimum depth at low salinity and the stronger hydrodynamic drag force generated by the high flow rate. However, the sufficient molecular diffusion at low flow rate promoted the occupation of PSNPs on adsorption sites. In addition, the penetration rate of PSNPs decreased by 25.0% as the water saturation decreased from 100% to 50%, indicating that the film straining at the air-water interface would hinder the transport of PSNPs. Finally, temperature increase impeded the penetration of PSNPs by 6.26%-23.1% via blocking mechanism. Our results suggest that low-salinity, high-flow river systems may be at greater risk of MPs contamination due to enhanced vertical transport capability.

Modeling graphene oxide transport and retention in biochar

Experimental data from fixed-bed column studies and a numerical model based on convection-dispersion equations were used to describe transport and retention of Graphene Oxide (GO) in sand, biochar (BC), and BC modified with nanoscale zero-valent iron (BC-nZVI). Three blocking functions, namely no blocking, site-blocking, and depth-dependent blocking, were used to analyze GO transport and retention behavior in each media as a function of Ionic Strength (IS). An inverse modeling approach was implemented to determine the attachment coefficient (K_a) and maximum solid-phase retention capacity (S_max). The Langmuirian attachment model with site-blocking function effectively described experimental GO breakthrough curves (R² ~ 0.70-0.99) compared to other models, indicating the importance of introducing a limit on the attachment capacity of the media. The K_a values for BC and BC-nZVI were significantly higher than sand, attributable to high porosity, roughness, and surface chemical properties. The models predicted an increasing trend in K_a (0.065 to 0.615 min^-1) in BC with increasing IS (0.1 to 10 mM), while K_a values decreased (2.26 to 0.349 min^-1) for BC-nZVI. A consistent increase in S_max was observed for both BC and BC-nZVI with increasing IS. Scenario analysis was conducted to further understand the effect of influent IS, GO concentration, and treatment depth. BC-nZVI exhibited a higher K_a and S_max and as a result, higher GO retention than BC at lower IS (0.1 and 1.0 mM). BC-nZVI had a relatively lower K_a (0.349 min^-1) at 10 mM IS, however, it outperformed BC when GO retention capacities are compared over a longer period attributable to a higher S_max (6.47). Complete GO breakthrough occurred in a 5 cm media after 350 and 465 days for BC and BC-nZVI, respectively at 10 mM IS and influent concentration of 0.1 mg·L^-1. GO breakthrough time increased with increasing treatment depth, however, the relation was non-linear.

The detection and characterization techniques for the interaction between graphene oxide and natural colloids: A review

The high dispersibility of graphene oxide (GO) and the universality of natural colloids (clay minerals, (hydr)oxides of Al, Fe, silica, etc.) make them interact easily. Many kinds of analytical methods have been used to study the interaction between GO and natural colloids. This review provides a comprehensive overview of analytical methods for the detection and quantification of interaction process. We highlighted the influence of the most relevant environmental factors (ionic strength, pH, etc.) on batch experiment, quartz crystal microbalance with dissipation monitoring measurements, and column experiments. Besides, the benefits and drawbacks of spectroscopic, microscopic techniques, theoretical models, calculation and time-resolved dynamic light scattering methods also have discussed in this work. This review can give some guidance to researchers in their selection and combination of the technique for the research of the interaction between GO and natural colloids.

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 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.

Effects of filtration-induced size change on the subsequent transport and fate of graphene oxide in saturated porous media

A particle size change occurs ubiquitously during transport of nanoparticles in the subsurface and is likely to influence nanoparticle fate and transport behaviours. The effects of this size change on the subsequent transport of eluted graphene oxide (GO) in saturated media were therefore investigated under various ionic strength (IS) and filtration degree conditions. Aggregation kinetics revealed that size change after filtration only occurred at relatively high IS conditions. As the filtration column length increased from 15 cm to 30 cm, sizes of aggregates in filtrates for large-sized and small-sized GO populations decreased and increased, respectively, and both approached to their steady aggregate sizes. Aggregation, straining, sedimentation, bridging, DLVO interactions, or a combination of these mechanisms were involved in the size change process during filtration. After passing through the 30 cm filtration column, filtered GO, in comparison with original GO, exhibited stronger mobility than expected, suggesting neglecting size change will result in underestimation of the nanoparticle mobility in porous media.

Graphene oxide transport and retention in biochar media

This study explores the use of biochar (BC), an inexpensive filtration media, for removing graphene oxide (GO) contaminants from the aquatic subsurface environments. Mass balance approaches and column dissection tests were used to analyze the retention behavior of GO in a series of model fixed-bed columns as a function of ionic strength (IS) and flowrate. The column based on the biochar media (BC) displayed 3.6-fold higher retention compared to the quartz sand (control). To overcome the challenges of unfavorable electrostatic interactions between GO and BC, we used a facile functionalization strategy to modify the BC surfaces with nanoscale zero-valent iron (BC-nZVI). The BC-nZVI (5:1, w/w) retained 2.6-fold higher amounts of GO compared with bare biochar. Furthermore, the performance of BC-nZVI increased with decreasing values of IS, attributed to the attachment of GO to nZVI where nZVI was partially dissolved by the presence of higher chloride ion at high IS. A better GO retention (86%) at higher IS was observed in BC where the GO was primarily retained due to the higher aggregation via straining.

Combined effect of physicochemical factors on the retention and transport of g-C3N4 in porous media

The environmental behaviors of graphitic carbon nitride (g-C₃N₄) have drawn increasing attention in recent. Understanding the fate and transport of g-C₃N₄ in porous media is necessary for evaluating its environmental risks. Column experiments were used in this study to investigate the combined effect of ionic strength (IS) and other common physicochemical factors (i.e. sand grain size, solution pH, and humic acid concentration) on g-C₃N₄ transport. The one-site kinetic models were applied to simulate the retention and transport of g-C₃N₄ in porous media, which fitted the breakthrough curves very well. Experimental and model results showed that g-C₃N₄ had a weak mobility with the transport mass recovery (TMR) less than 39.6% at pH 6.0 in absence of humic acid (HA). The mobility of g-C₃N₄ was inhibited with decreasing sand grain size, and the inhibited efficiency was enhanced with IS. However, g-C₃N₄ transport was significantly enhanced with increasing pH and HA concentration, and the enhanced efficiency was more obviously at high IS. The maximum TMR (78.3%) of g-C₃N₄ was observed with the presence of 5 mg L^-1 HA. These results indicated that physicochemical factors played an important and combined role in controlling g-C₃N₄ transport in porous media, which would lead to the more complex evaluation on the environmental behaviors of g-C₃N₄.

Pore-scale investigation of the use of reactive nanoparticles for in situ remediation of contaminated groundwater source

Chlorinated solvents are among the most recalcitrant aquifer contaminants, which can cause serious health problems such as kidney and liver damage, and some are considered carcinogenic. They are a global problem due to their wide industrial use since the beginning of the 20th century (e.g., in metal-processing plants). Conventionally, pump and treat technology (ex situ method) has been used to treat such contaminated groundwater resources. Recently, in situ techniques have been applied to lower the remediation costs (e.g., energy/water consumption) while also limiting the disruption. Nanoremediation is a new in situ technology that has shown promising results at laboratory, pilot, and field scales. This study uses 4D (time-resolved 3D) imaging to capture the dynamics of nanoremediation at the pore scale.

Nanoscale zero-valent iron (nZVI) particles have excellent capacity for in situ remediation of groundwater resources contaminated by a range of organic and inorganic contaminants. Chlorinated solvents are by far the most treated compounds. Studies at column, pilot, and field scales have reported successful decrease in contaminant concentration upon injection of nZVI suspensions in the contaminated zones. However, the field application is far from optimized, particularly for treatments at—or close to—the source, in the presence of residual nonaqueous liquid (NAPL). The knowledge gaps surrounding the processes that occur within the pores of the sediments hosting those contaminants at microscale limit our ability to design nanoremediation processes that are optimized at larger scales. This contribution provides a pore-scale picture of the nanoremediation process. Our results reveal how the distribution of the trapped contaminant evolves as a result of contaminant degradation and generation of gaseous products. We have used state-of-the-art four-dimensional (4D) imaging (time-resolved three-dimensional [3D]) experiments to understand the details of this degradation reaction at the micrometer scale. This contribution shows that the gas released (from the reduction reaction) remobilizes the trapped contaminant by overcoming the capillary forces. Our results show that the secondary sources of NAPL contaminations can be effectively treated by nZVI, not only by in situ degradation, but also through pore-scale remobilization (induced by the evolved gas phase). The produced gas reduces the water relative permeability to less than 1% and, therefore, significantly limits the extent of plume migration in the short term.

A Large-Scale 3D Study on Transport of Humic Acid-Coated Goethite Nanoparticles for Aquifer Remediation

Humic acid-coated goethite nanoparticles (HA-GoeNPs) have been recently proposed as an effective reagent for the in situ nanoremediation of contaminated aquifers. However, the effective dosage of these particles has been studied only at laboratory scale to date. This study investigates the possibility of using HA-GoeNPs in remediation of real field sites by mimicking the injection and transport of HA-GoeNPs under realistic conditions. To this purpose, a three-dimensional (3D) transport experiment was conducted in a large-scale container representing a heterogeneous unconfined aquifer. Monitoring data, including particle size distribution, total iron (Fetot) content and turbidity measurements, revealed a good subsurface mobility of the HA-GoeNP suspension, especially within the higher permeability zones. A radius of influence of 2 m was achieved, proving that HA-GoeNPs delivery is feasible for aquifer restoration. A flow and transport model of the container was built using the numerical code Micro and Nanoparticle transport Model in 3D geometries (MNM3D) to predict the particle behavior during the experiment. The agreement between modeling and experimental results validated the capability of the model to reproduce the HA-GoeNP transport in a 3D heterogeneous aquifer. Such result confirms MNM3D as a valuable tool to support the design of field-scale applications of goethite-based nanoremediation.

Injection of Zerovalent Iron Gels for Aquifer Nanoremediation: Lab Experiments and Modeling

One of the main technical problems faced during field-scale injections of iron microparticles (mZVI) for groundwater nanoremediation is related to their poor colloidal stability and mobility in porous media. In this study, a shear-thinning gel, composed of a mixture of two environmentally friendly biopolymers, i.e., guar gum and xanthan gum, was employed to overcome these limitations. The slurry rheology and particle mobility were characterized by column transport tests. Then, a radial transport experiment was performed to mimic the particle delivery in more realistic conditions. The gel, even at a low polymeric content (1.75 g/L), proved effective in enhancing the mobility of high concentrated mZVI suspensions (20 g/L) in field-like conditions. The high radius of influence (73 cm) and homogeneous iron distribution were achieved by maintaining a low injection overpressure (<0.4 bar). Based only on the information derived from column tests, the MNMs 2018 software (Micro- and Nanoparticle transport, filtration, and clogging Model-Suite) was able to predict the particle distribution and pressure build-up measured in the radial domain. Experimental and simulated results showed good agreement, thus proving that a simplified experimental-modeling procedure based on 1D column tests could be used to effectively upscale the slurry behavior to more representative scales, e.g., radial domains.