Transport of nZVI/copper synthesized by green tea extract in Cr(IV)-contaminated soil: modeling study and reduced toxicity

In this study, nano-zero-valent iron/copper was synthesized by green tea extracts (GT-nZVI/Cu) and produced a stable suspension than nano-zero-valent iron synthesized by green tea extracts (GT-nZVI) injected into Cr(VI)-containing soil column. The equilibrium 1D-CDE model was successfully used to fit the penetration curves of Fe(tot), Fe(aq), and Fe(0) in order to determine the relevant parameters. The hydrodynamic dispersion coefficient of chromium-contaminated soil was 0.401 cm2·h-1, and the pore flow rate was 0.144 cm·h-1. The stable C/C0 of Fe(tot), Fe(aq), and Fe(0) in the effluent were retarded to 0.39, 0.79, and 0.11, respectively, compared to a ratio of 1 for the concentration of the tracer Cl- in the effluent to the concentration in the influent. Additionally, the 1D-CDE model describes the migration behavior of Cr(VI) with a high R2 (> 0.97). The obtained blocking coefficients declined gradually with increasing concentration of GT-nZVI/Cu suspension and decreasing concentration of Cr(VI). The content of reduced chromium in the soil decreased from 2.986 to 1.121 after remediation, while the content of more stable oxidizable chromium and residual chromium increased from 2.975 and 20.021 to 16.471 and 27.612. The phytotoxicity test showed that mung bean seeds still had a germination rate of 90% (control of 100%), root length of 29.63 mm (control of 35.25 mm), and stem length of 17.9 cm (control of 18.96 cm) after remediation with GT-nZVI/Cu. These indicated that GT-nZVI/Cu was effective in immobilizing Cr(VI) in the soil column and reduced the ecological threat. This study provides an analytical basis and theoretical model for the migration of chromium-contaminated soil in practical application.

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.

Transport and retention of bacteria through a filtration system consisting of sands and geotextiles

Water-saturated column experiments were conducted to study the effect of nonwoven geotextiles on bacteria transport and deposition through two sandy porous media with grain sizes 1.05 and 3.25 mm. The breakthrough curves (BTCs) of tracer for the all porous media exhibited an asymmetrical shape with a substantial tailing, indicating that non-equilibrium and dispersive flow patterns in these porous media. The mass recovery of the bacteria from the effluent (M_eff) increased with grain size. The retention profiles (RPs) exhibited hyper-exponential behavior, especially in the finer sand. The presence of the geotextiles increased bacteria retention rate. For a given geotextile, greater retention was observed in the surrounding region close to the geotextile. Moreover, the retention of bacteria became more significant in the geotextile with a lower porosity. Results demonstrated that model simulations of bacteria transport and fate need to accurately account for both observed BTC and RP behaviors and also the geotextile placement can impact mechanisms of retention.

Modeling the risk of U(VI) migration through an engineered barrier system at a proposed Chinese high-level radioactive waste repository

The migration of U(VI) through the engineered barrier system (EBS) and into the natural environment in a geological repository for high-level radioactive waste depends on the chemical and physical environment of the repository. Modeling is widely used to understand the risk associated with migration of U(VI) for different barrier designs for repository sites. In this study, coupled thermal, hydrological, and chemical (THC) models were used to evaluate the risk of U(VI) migration at a proposed deep geological repository in northwestern China. The models incorporated two-site protolysis nonelectrostatic surface complexation, dissolution/precipitation of minerals and cation exchange as the major reactions controlling U(VI) migration. Modeling results showed that the main factors influencing U(VI) migration were pH, and the smectite content in the bentonite, as dissolution of the hydrous uranium oxide mineral schoepite is suppressed at higher pH values, and smectite is the most important adsorbent of dissolved U(VI). Therefore, an alkaline bentonite with a smectite volume fraction of >0.6 is suggested as the backfill material for this EBS. The THC model results also showed that in 100,000 years, U(VI) migration is constrained within EBS if the suggested bentonite is used as backfill in a repository that is hosted within Beishan granite. This study provides a feasible method for selecting a bentonite backfill and predicting the effect of environmental conditions on U(VI) migration.

Composition and emission of VOC from biogas produced by illegally managed waste landfills in Giugliano (Campania, Italy) and potential impact on the local population

The composition in Volatile Organic Compounds (VOC) of the biogas produced by seven landfills of Giugliano (Naples, Campania, Italy) was determined and VOC emission rates assessed to verify if these compounds represent a potential threat to the population living nearby. VOC composition in the biogas could not be predicted, as heterogeneous waste was dumped from the late 1980s to the early 2000s and then underwent biological degradation. No data are available on the amount and composition of VOC in the biogas before the landfills closure as no operational biogas collection system was present. In this study, VOC composition was determined by gas chromatography-mass spectrometry (GC-MS), after collecting samples from collection pipes and from soil fractures in cover soil or capping. Individual VOC were quantified and data compared with those collected at two landfills in Latium, when they were still in operation. Relevant differences were observed, mainly due to waste aging, but no specific VOC revealing toxic waste dumping was found, although the concurrent presence of certain compounds suggested that dumping of industrial wastes might have occurred. The average VOC emission was assessed and a dispersion model was run to find out if the emitted plume could affect the health of population. The results suggested that fugitive emissions did not represent a serious danger, since the concentrations simulated at the neighboring cities were below the threshold limits for acute and chronic diseases. However, VOC plume could cause annoyance at night when the steady state conditions of the atmosphere enhance pollutants accumulation in the lower layers. In addition, some of the emitted VOC, such as alkylbenzenes and monoterpenes, can contribute to tropospheric ozone formation.

Uncertainty and variability in laboratory derived sorption parameters of sediments from a uranium in situ recovery site

This research assesses the ability of a GC SCM to simulate uranium transport under variable geochemical conditions typically encountered at uranium in-situ recovery (ISR) sites. Sediment was taken from a monitoring well at the SRH site at depths 192 and 193 m below ground and characterized by XRD, XRF, TOC, and BET. Duplicate column studies on the different sediment depths, were flushed with synthesized restoration waters at two different alkalinities (160 mg/l CaCO₃ and 360 mg/l CaCO₃) to study the effect of alkalinity on uranium mobility. Uranium breakthrough occurred 25% - 30% earlier in columns with 360 mg/l CaCO₃ over columns fed with 160 mg/l CaCO₃ influent water. A parameter estimation program (PEST) was coupled to PHREEQC to derive site densities from experimental data. Significant parameter fittings were produced for all models, demonstrating that the GC SCM approach can model the impact of carbonate on uranium in flow systems. Derived site densities for the two sediment depths were between 141 and 178 μmol-sites/kg-soil, demonstrating similar sorption capacities despite heterogeneity in sediment mineralogy. Model sensitivity to alkalinity and pH was shown to be moderate compared to fitted site densities, when calcite saturation was allowed to equilibrate. Calcite kinetics emerged as a potential source of error when fitting parameters in flow conditions. Fitted results were compared to data from previous batch and column studies completed on sediments from the Smith-Ranch Highland (SRH) site, to assess variability in derived parameters. Parameters from batch experiments were lower by a factor of 1.1 to 3.4 compared to column studies completed on the same sediments. The difference was attributed to errors in solid-solution ratios and the impact of calcite dissolution in batch experiments. Column studies conducted at two different laboratories showed almost an order of magnitude difference in fitted site densities suggesting that experimental methodology may play a bigger role in column sorption behavior than actual sediment heterogeneity. Our results demonstrate the necessity for ISR sites to remove residual pCO2 and equilibrate restoration water with background geochemistry to reduce uranium mobility. In addition, the observed variability between fitted parameters on the same sediments highlights the need to provide standardized guidelines and methodology for regulators and industry when the GC SCM approach is used for ISR risk assessments.

Transport and long-term release behavior of polymer-coated silver nanoparticles in saturated quartz sand: The impacts of input concentration, grain size and flow rate

This study investigated the transport and long-term release of stabilized poly vinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) in a quartz sand column with various sand grain sizes (0.3-0.5 μm, 0.5-1.0 μm, 1.0-2.0 μm), input concentrations of PVP-AgNP solution (1, 5, 15, 25 mg/L), and flow rates corresponding to a filter velocities (1.0, 1.5, 2.0, 2.5 mL/min-0.14, 0.21, 0.28, 0.35 cm/min) by determining breakthrough curves, retention profiles, and long-term release curves. Breakthrough curves and retention profiles were simulated by a mathematical model based on the advection dispersion equation coupled with second-order kinetics. The increased transport of PVP-AgNPs in quartz sand occurred with increased grain sizes and reduced input concentrations, and the transport can be predicted by the colloid filtration theory and DLVO theory. The long-term (one week) release amounts of retained PVP-AgNPs were 42.78%, 31.45%, and 10.95% in the fine, medium, and coarse sand columns, respectively, and were 34.70%, 40.79%, 47.24%, and 57.32% at flow rates of 0.0363, 0.0436, 0.0545, and 0.0726 mL/min, respectively. The released quantity of retained PVP-AgNPs decreased as the sand grain size increased. This phenomenon is opposite with the trend of increased transport of PVP-AgNPs with increased grain size in the transport test, which most likely because colloidal filtration regulates the transport process and adsorption (and desorption) dominates the release process. Increasing the flow rate increased the shear force on the particles, which improved the release of PVP-AgNPs. The results of the release tests further verified our previous published studies showing that the long-term release of retained PVP-AgNPs in the quartz sand was mostly in the form of released nanoparticles rather than ions. The results of this study indicated that sand grain size, input concentration, and flow rate have a prominent influence on the transport and long-term release behavior of PVP-AgNPs in saturated quartz sand.

Physics-based modeling approaches of resistive switching devices for memory and in-memory computing applications

The semiconductor industry is currently challenged by the emergence of Internet of Things, Big data, and deep-learning techniques to enable object recognition and inference in portable computers. These revolutions demand new technologies for memory and computation going beyond the standard CMOS-based platform. In this scenario, resistive switching memory (RRAM) is extremely promising in the frame of storage technology, memory devices, and in-memory computing circuits, such as memristive logic or neuromorphic machines. To serve as enabling technology for these new fields, however, there is still a lack of industrial tools to predict the device behavior under certain operation schemes and to allow for optimization of the device properties based on materials and stack engineering. This work provides an overview of modeling approaches for RRAM simulation, at the level of technology computer aided design and high-level compact models for circuit simulations. Finite element method modeling, kinetic Monte Carlo models, and physics-based analytical models will be reviewed. The adaptation of modeling schemes to various RRAM concepts, such as filamentary switching and interface switching, will be discussed. Finally, application cases of compact modeling to simulate simple RRAM circuits for computing will be shown.

Effects of temperature on graphene oxide deposition and transport in saturated porous media

Laboratory batch sorption and sand column experiments were conducted to examine the effects of temperature (6 and 24°C) on the retention and transport of GO in water-saturated porous media with different combination of solution ionic strength (IS, 1 and 10mM), sand type (natural and acid-cleaned), and grain size (coarse and fine). Although results from batch sorption experiment showed that temperature affected the sorption of GO onto the sand grains at the low IS, the interactions between GO and the sand were relatively weak, which did make the temperature effect prominent. When the IS was 1mM, experimental temperature showed little effect on GO retention and transport regardless of the medium properties. GO was highly mobile in the sand columns with mass recovery rates ranged from 77.3% to 92.4%. When the IS increased to 10mM, temperature showed notable effects on GO retention and transport in saturated porous media. For all the combinations of sand type and grain size, the higher the temperature was, the less mobile GO particles were. The effects of temperature on GO retention and transport in saturated porous media were further verified though simulations from an advection-dispersion-reaction model.

Transport of stabilized iron nanoparticles in porous media: Effects of surface and solution chemistry and role of adsorption

Carboxymethyl cellulose (CMC) stabilized zero-valent iron (ZVI) (CMC-ZVI) nanoparticles have been extensively tested for remediation of soil and groundwater. This study investigated effects of iron oxide and aluminum oxide on retention and transport of CMC-ZVI nanoparticles, which have a mean hydrodynamic diameter of 155nm. Column breakthrough experiments showed that the metal oxides coatings on quartz sand greatly enhanced particle retention. A mechanistically sounder transport model was proposed by incorporating a Langmuir-type adsorption rate law into the classic convection-dispersion equation with the adsorption parameters derived from independent experiments. The model allows for a quantitative evaluation of the role of adsorption. While filtration is the primary mechanism for particle retention at lower pore velocities, adsorption becomes more significant at elevated velocities. The presence of 40-80mg-CL^-1 of natural organic matter and high ionic strength (up to 200mM CaCl₂) had negligible effect on the breakthrough profiles of the nanoparticles. Starch, a neutral polysugar stabilizer, was also tested as a stabilizer. Starch-stabilized ZVI nanoparticles, with a mean hydrodynamic diameter of 303nm, displayed a higher particle retention than CMC-ZVI. The information and modeling approach can facilitate our understanding of fate and transport of stabilized ZVI nanoparticles under various geochemical conditions.

Transport and deposition of stabilized engineered silver nanoparticles in water saturated loamy sand and silty loam

It is considered inevitable that the increasing production and application of engineered nanoparticles will lead to their release into the environment. However, the behavior of these materials under environmentally relevant conditions is still only poorly understood. In this study the transport and deposition behavior of engineered surfactant stabilized silver nanoparticles (AgNPs) in water saturated porous media was investigated in transport experiments with glass beads as reference porous medium and in two natural soils under various hydrodynamic and hydrochemical conditions. The transport and retention processes of AgNPs in the porous media were elucidated by inverse modeling and possible particle size changes occurring during the transport through the soil matrix were analyzed with flow field-flow fractionation (FlFFF). A high mobility of AgNPs was observed in loamy sand under low ionic strength (IS) conditions and at high flow rates. The transport was inhibited at low flow rates, at higher IS, in the presence of divalent cations and in a more complex, fine-grained silty loam. The slight decrease of the mean particle size of the AgNPs in almost all experiments indicates size selective filtration processes and enables the exclusion of homoaggregation processes.

Characterizing and modeling of extensive atrazine elution tailing for stable manure-amended agricultural soil

Non-ideal sorption and extensive elution tailing behavior of atrazine was evaluated for an agricultural soil with and without stable manure amendment (10% by weight). A series of laboratory experiments showed that the sorption of atrazine was described by rate-limited, nonlinear reversible processes (Freundlich isotherm) for both non-amended and amended soil. Non-ideal transport of atrazine exhibited extensive low concentration elution tailing due to the most likely organic carbon fraction in the soil. This tailing behavior was more pronounced and extensive for soil with 10% stable-manure amendment. Two-site transport modeling analyses including non-linear sorption and rate-limited sorption-desorption provided a reasonably good match to the atrazine breakthrough curves but were unable to match the long-term concentration tailing, even for non-amended soil. A mathematical model incorporating nonlinear, rate-limited sorption/desorption described by a continuous-distribution function was used to successfully simulate atrazine transport early-time breakthrough and long-term concentration tailing for both non-amended and amended soil conditions.

Modeling and sensitivity analysis on the transport of aluminum oxide nanoparticles in saturated sand: effects of ionic strength, flow rate, and nanoparticle concentration

Aluminum oxide nanoparticles have been widely used in various consumer products and there are growing concerns regarding their exposure in the environment. This study deals with the modeling, sensitivity analysis and uncertainty quantification of one-dimensional transport of nano-sized (~82 nm) aluminum oxide particles in saturated sand. The transport of aluminum oxide nanoparticles was modeled using a two-kinetic-site model with a blocking function. The modeling was done at different ionic strengths, flow rates, and nanoparticle concentrations. The two sites representing fast and slow attachments along with a blocking term yielded good agreement with the experimental results from the column studies of aluminum oxide nanoparticles. The same model was used to simulate breakthrough curves under different conditions using experimental data and calculated 95% confidence bounds of the generated breakthroughs. The sensitivity analysis results showed that slow attachment was the most sensitive parameter for high influent concentrations (e.g. 150 mg/L Al2O3) and the maximum solid phase retention capacity (related to blocking function) was the most sensitive parameter for low concentrations (e.g. 50 mg/L Al2O3).

Transport of nanoparticles with dispersant through biofilm coated drinking water sand filters

This article characterizes, experimentally and theoretically, the transport and retention of engineered nanoparticles (NP) through sand filters at drinking water treatment plants (DWTPs) under realistic conditions. The transport of four commonly used NPs (ZnO, CeO2, TiO2, and Ag, with bare surfaces and coating agents) through filter beds filled with sands from either acid washed and calcined, freshly acquired filter media, and used filter media from active filter media, were investigated. The study was conducted using water obtained upstream of the sand filter at DWTP. The results have shown that capping agents have a determinant importance in the colloidal stability and transport of NPs through the different filter media. The presence of the biofilm in used filter media increased adsorption of NPs but its effects in retaining capped NPs was less significant. The data was used to build a mathematical model based on the advection-dispersion equation. The model was used to simulate the performance of a scale-up sand filter and the effects on filtration cycle of traditional sand filtration system used in DWTPs.

Limited transport of functionalized multi-walled carbon nanotubes in two natural soils

Column experiments were conducted in undisturbed and in repacked soil columns at water contents close to saturation (85-96%) to investigate the transport and retention of functionalized (14)C-labeled multi-walled carbon nanotubes (MWCNT) in two natural soils. Additionally, a field lysimeter experiment was performed to provide long-term information at a larger scale. In all experiments, no breakthrough of MWCNTs was detectable and more than 85% of the applied radioactivity was recovered in the soil profiles. The retention profiles exhibited a hyper-exponential shape with greater retention near the column or lysimeter inlet and were successfully simulated using a numerical model that accounted for depth-dependent retention. In conclusion, results indicated that the soils acted as a strong sink for MWCNTs. Little transport of MWCNTs is therefore likely to occur in the vadose zone, and this implies limited potential for groundwater contamination in the investigated soils.