Denis M. O'Carroll

Enhanced stability and dechlorination activity of pre-synthesis stabilized nanoscale FePd particles

Nanoscale zero-valent iron (NZVI) particles are promising materials for the in-situ remediation of a wide variety of source zone contaminants. This study presents the results of a systematic investigation of the stability of bimetallic FePd nanoparticle suspensions in water and their capability to degrade trichloroethylene (TCE) synthesized in the presence of various stabilizers (i.e., carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP), and guar gum). Results indicate a dramatic improvement in FePd suspension stability when the stabilizer is present in the matrix during the nanoparticle synthesis step. Stability enhancement is controlled by iron nanoparticle/stabilizer electrostatic and steric interactions, which are a function of the molecular structure of the stabilizer. Stabilization mechanisms differed for each stabilizer with CMC and guar gum exhibiting the best nanoparticle suspension stability improvement. Results suggest that the complexation of iron precursors with the stabilizer, during synthesis, plays a key role in nZVI stability improvement. In case of guar gum, gelation during synthesis significantly increased suspension viscosity, enhancing suspension stability. The capability of these materials to degrade TCE was also investigated. Results demonstrated that when stabilizers were present in the matrix dechlorination rates increased significantly. FePd nanoparticles in CMC had the highest observed rate constant; however the highest surface area-normalized rate constant was obtained from FePd stabilized in PVP360K. Results from this study can be used to aid in the selection of appropriate iron nanoparticle stabilizers. Stabilizer selection should be assessed on a case by case basis as no stabilizer will meet the needs of all in-situ remediation applications.

Impact of nZVI stability on mobility in porous media

Nano-scale zero valent iron (nZVI) has received significant attention because of its potential to rapidly reduce a number of priority source zone contaminants. In order to effectively deliver nZVI to the source zone the nZVI particles must be stable. Previous laboratory studies have demonstrated the mobility of polymer modified suspensions of low concentration nZVI. More recently studies have shown potential for higher concentration nZVI suspensions to be transmitted through porous media. However, with increasing nZVI concentration aggregation is accelerated, reducing the available time for injection before nZVI settles. In this study the colloidal stability and mobility of nZVI concurrently synthesized and stabilized in the presence of carboxy-methyl-cellulose (CMC) are evaluated in one-dimensional column experiments. Low pore water velocity nZVI injections (4, 2, and 0.25 m/day) conducted over periods as long as 80 h with no mixing of the influent reservoir were used to investigate the effects of prolonged aggregation and settling of colloids on transport. A numerical simulator, based on colloid filtration theory, but accounting for particle aggregation and settling was used to evaluate the contributions of aggregation and settling on nZVI mobility. Results suggest that the prediction of nZVI sticking efficiency in column experiments becomes increasingly influenced by aggregation and settling in the influent reservoir as the period of injection increases. Consideration of nZVI stability is required for the prediction of nZVI mobility at the field scale and for the design of successful nZVI remediation plans.

Experimental investigation of dynamic effects in capillary pressure: Grain size dependency and upscaling

[1] The macroscopic flow equations used to predict two-phase flow typically utilizes a capillary pressure-saturation relationship determined under equilibrium conditions. Theoretical reasoning, experimental evidence, and numerical modeling results have indicated that when one fluid phase replaces another fluid, this relationship may not be unique but may depend on the rate at which the phase saturations change in response to changes in phase pressures. This nonuniqueness likely depends on a variety of factors including soil-fluid properties and possibly physical scale. To quantify this dependency experimentally, direct measurements of equilibrium and dynamic capillary pressure-saturation relationships were developed for two Ottawa sands with different grain sizes using a 20 cm long column. A number of replicate air-water experiments were conducted to facilitate statistical comparison of capillary pressure-saturation relationships. Water and air pressures and phase saturations were measured at three different vertical locations in the sand column under different desaturation rates (1) to measure local capillary pressure-saturation relationships (static and dynamic); (2) to quantify the dynamic coefficient T , a measure of the magnitude of observed dynamic effects, as a function of water saturation for different grain sizes and desaturation rates; (3) to investigate the importance of grain size on measured dynamic effects; and (4) to assess the importance of sample scale on the magnitude of dynamic effects in capillary pressure. A comparison of the static and dynamic P c -S w relationships showed that at a given water saturation, capillary pressure measured under transient water drainage conditions is statistically larger than capillary pressure measured under equilibrium or static conditions, consistent with thermodynamic theory. The dynamic coefficient T , used in the expression relating the static and dynamic capillary pressures to the desaturation rate was dependant on porous media mean grain size but not on the desaturation rate. Results also suggest that the magnitude of the dynamic coefficient did not increase with the increased averaging volume considered in this study, as has been reported in the literature. This work suggests that dynamic effects in capillary pressure should be included in numerical models used to predict multiphase flow in systems when saturations change rapidly, particularly in fine-grained soil systems (e.g., CO 2 sequestration, enhanced oil recovery, air sparging for remediation).

The influence of humic acid and clay content on the transport of polymer-coated iron nanoparticles through sand

The introduction of nanoscale zero valent iron (nZVI) into the subsurface has recently received significant attention as a potentially effective method for remediation of source zones of chlorinated solvents present as dense nonaqueous phase liquids (DNAPL). One of the challenges in the deployment of nZVI is to achieve good subsurface nZVI mobility to permit delivery of the nZVI to the target treatment zone. Stabilization of nZVI with various polymers has shown promise for enhancing nZVI subsurface mobility, but the impact of subsurface conditions on nZVI mobility has not been fully explored. In this study, the effect of humic acid and kaolinite on the transport of polymer-stabilized nZVI (carboxylmethyl cellulose-surface modified nZVI, CMC90K-RNIP) in sand was investigated using column experiments. In addition, effects of electrolytes on the stability of CMC90K-RNIP in the presence of humic acid, and the stability of humic acid-coated reactive nanoscale iron particles (HA-RNIP) at various humic acid concentrations were investigated. Humic acid enhanced the mobility of bare RNIP, whereas the transport of CMC90K-RNIP was not significantly affected by humic acid injected as a background solution, except at the highest concentration of 500mg/L. At lower pore water velocity, the effect of humic acid on the transport of CMC90K-RNIP was greater than that at high water velocity. Adding kaolinite up to 2% by weight to the sand column reduced the retention of CMC90K-RNIP, but further increases in kaolinite content (to 5%) did not significantly affect nZVI retention. The impact of kaolinite on nZVI retention was more pronounced at lower pore water velocities.

An evaluation of Sherwood-Gilland models for NAPL dissolution and their relationship to soil properties.

Predicting the longevity of non-aqueous phase liquid (NAPL) source zones has proven to be a difficult modeling problem that has yet to be resolved. Research efforts towards understanding NAPL depletion have focused on developing empirical models that relate lumped mass transfer rates to velocities and organic saturations. These empirical models are often unable to predict NAPL dissolution for systems different from those used to calibrate them, indicating that system-specific factors important for dissolution are not considered. This introduces the need for a calibration step before these models can be reliably used to predict NAPL dissolution for systems of arbitrary characteristics. In this paper, five published Sherwood-Gilland models are evaluated using experimental observations from the dissolution of two laboratory-scale complex three-dimensional NAPL source zones. It is shown that the relative behavior of the five models depends on the system and source zone characteristics. Through a theoretical analysis, comparing Sherwood-Gilland type models to a process-based, thermodynamic dissolution model, it is shown that the coefficients of the Sherwood-Gilland models can be related to measurable soil properties. The derived dissolution model with soil-dependent coefficients predicts concentrations identical to those predicted by the thermodynamic dissolution model for cases with negligible hysteresis. This correspondence breaks down when hysteresis has a significant impact on interfacial areas. In such cases, the derived dissolution model will slightly underestimate dissolved concentrations at later times, but is more likely to capture system-specific dissolution rates than Sherwood-Gilland models.

Association of Arsenic and Phosphorus with Iron Nanoparticles between Streams and Aquifers: Implications for Arsenic Mobility.

The microbial oxidation of organic matter coupled to reductive iron oxide dissolution is widely recognized as the dominant mechanism driving elevated arsenic (As) concentrations in aquifers. This paper considers the potential of nanoparticles to increase the mobility of As in aquifers, thereby accounting for discrepancies between predicted and observed As transport reported elsewhere. Arsenic, phosphorus, and iron size distributions and natural organic matter association were examined along a flow path from surface water via the hyporheic zone to shallow groundwater. Our analysis demonstrates that the colloidal Fe concentration (>1 kDa) correlates with both colloidal P and colloidal As concentrations. Importantly, increases in the concentration of colloidal P (>1 kDa) were positively correlated with increases in the concentration of nominally dissolved As (<1 kDa), but no correlation was observed between colloidal As and nominally dissolved P. This suggests that P actively competes for adsorption sites on Fe nanoparticles, displacing adsorbed As, thus mirroring their interaction with Fe oxides in the aquifer matrix. Dynamic redox fronts at the interface between streams and aquifers may therefore provide globally widespread conditions for the generation of Fe nanoparticles, a mobile phase for As adsorption currently not a part of reactive transport models.

The impact of immobile zones on the transport and retention of nanoparticles in porous media: IMMOBILE ZONES AND NANOPARTICLE TRANSPORT AND RETENTION

Nanoparticle transport and retention within porous media is treated by conceptualizing the porous media as a series of independent collectors (e.g., Colloid Filtration Theory). This conceptualization assumes that flow phenomena near grain-grain contacts, such as immobile zones (areas of low flow), exert a negligible influence on nanoparticle transport and assumes that retention and release of particles depends only on surface chemistry. This study investigated the impact of immobile zones on nanoparticle transport and retention by employing Synchrotron X-ray Computed Microtomography (SXCMT) to examine pore-scale silver nanoparticle distributions during transport through three sand columns: uniform iron oxide, uniform quartz and well graded quartz. Extended tailing was observed during the elution phase of all experiments suggesting that hydraulic retention in immobile zones, not detachment from grains, was the source of tailing. A numerical simulation of fluid flow through an SXCMT dataset predicted the presence of immobile zones near grain-grain contacts. SXCMT-determined silver nanoparticle concentrations observed that significantly lower nanoparticle concentrations existed near grain-grain contacts throughout the duration of all experiments. In addition, the SXCMT-determined pore-scale concentration gradients were found to be independent of surface chemistry and grain size distribution, suggesting that immobile zones limit the diffusive transport of nanoparticles towards the collectors. These results suggest that the well-known overprediction of nanoparticle retention by traditional CFT may be due to ignoring the influences of grain-grain contacts and immobile zones. As such, accurate prediction of nanoparticle transport requires consideration of immobile zones and their influence on both hydraulic and surface retention. This article is protected by copyright. All rights reserved.

nZVI injection into variably saturated soils: Field and modeling study

Nano-scale zero valent iron (nZVI) has been used at a number of contaminated sites over the last decade. At most of these sites, significant decreases in contaminant concentrations have resulted from the application of nZVI. However, limited work has been completed investigating nZVI field-scale mobility. In this study, a field test was combined with numerical modeling to examine nZVI reactivity along with transport properties in variably saturated soils. The field test consisted of 142L of carboxymethyle cellulose (CMC) stabilized monometallic nZVI synthesized onsite and injected into a variably saturated zone. Periodic groundwater samples were collected from the injection well, as well as, from two monitoring wells to analyze for chlorinated solvents and other geochemistry indicators. This study showed that CMC stabilized monometallic nZVI was able to decrease tricholorethene (TCE) concentrations in groundwater by more than 99% from the historical TCE concentrations. A three dimensional, three phase, finite difference numerical simulator, (CompSim) was used to further investigate nZVI and polymer transport at the variably saturated site. The model was able to accurately predict the field observed head data without parameter fitting. In addition, the numerical simulator estimated the mass of nZVI delivered to the saturated and unsaturated zones and distinguished the nZVI phase (i.e. aqueous or attached). The simulation results showed that the injected slurry migrated radially outward from the injection well, and therefore nZVI transport was governed by injection velocity and viscosity of the injected solution. A suite of sensitivity analyses was performed to investigate the impact of different injection scenarios (e.g. different volume and injection rate) on nZVI migration. Simulation results showed that injection of a higher nZVI volume delivered more iron particles at a given distance; however, the travel distance was not proportional to the increase in volume. Moreover, simulation results showed that using a 1D transport equation to simulate nZVI migration in the subsurface may overestimate the travel distance. This is because the 1D transport equation assumes a constant velocity while pore water velocity radially decreases from the well during injection. This study suggests that on-site synthesized nZVI particles are mobile in the subsurface and that a numerical simulator can be a valuable tool for optimal design of nZVI field applications.

Effect of gold nanoparticles and ciprofloxacin on microbial catabolism: a community‐based approach

The effect of gold nanoparticles (AuNPs) and ciprofloxacin on the catabolism of microbial communities was assessed. This was accomplished through an ex situ methodology designed to give a priori knowledge on the potential for nanoparticles, or other emerging contaminants, to affect the catabolic capabilities of microbial communities in the environment. Microbial communities from a variety of sources were incubated with 31 prespecified carbon sources and either National Institute of Standards and Technology reference material 10-nm AuNPs or ciprofloxacin on 96-well microtiter plates. From the ciprofloxacin study, dose-response curves were generated and exemplified how this method can be used to assess the effect of a toxicant on overall catabolic capabilities of microbial communities. With 10-nm AuNPs at concentrations ranging from 0.01 µg/mL to 0.5 µg/mL, rhizosphere communities from Typha roots were only slightly catabolically inhibited at a single concentration (0.05 µg/mL); no effects were seen on wetland water communities, and a minor positive (i.e., enhanced catabolic capabilities) effect was observed for loamy soil communities. This positive effect might have been because of a thin layer of citrate found on these AuNPs that initiated cometabolism with some of the carbon sources studied. Under the conditions considered, the possible adverse effects of AuNPs on the catabolic capabilities of microbial communities appears to be minimal.

Effect of Water Chemistry and Aging on Iron—Mica Interaction Forces: Implications for Iron Particle Transport

The transport of particles through groundwater systems is governed by a complex interplay of mechanical and chemical forces that are ultimately responsible for binding to geological substrates. To understand these forces in the context of zero valent iron particles used in the remediation of groundwater, atomic force microscopy (AFM)-based force spectroscopy was employed to characterize the interactions between AFM tips modified with either carbonyl iron particles (CIP) or electrodeposited Fe as a function of counterion valency, temperature, particle morphology, and age. The measured interaction forces were always attractive for both fresh and aged CIP and electrodeposited iron, except in 100 mM NaCl, as a consequence of electrostatic attraction between the negatively charged mica and positively charged iron. In 100 mM NaCl, repulsive hydration forces appeared to dominate. Good agreement was found between the experimental data and predictions based on the extended DLVO (XDLVO) theory. The effect of aging on iron particle composition and morphology was assessed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) revealing that the aged particles comprising a zero valent iron core passivated by a mixture of iron oxides and hydroxides. Force spectroscopy showed that aging caused variations in the adhesive force due to the changes in particle morphology and contact area.