High-Resolution Depth-Discrete Analysis of PFAS Distribution and Leaching for a Vadose-Zone Source at an AFFF-Impacted Site.

The long-term leaching of polyfluoroalkyl substances (PFAS) within the vadose zone of an AFFF application site for which the depth to groundwater is approximately 100 m was investigated by characterizing the vertical distribution of PFAS in a high spatial resolution. The great majority (99%) of PFAS mass resides in the upper 3 m of the vadose zone. The depths to which each PFAS migrated, quantified by moment analysis, is an inverse function of molar volume, demonstrating chromatographic separation. The PFAS were operationally categorized into three chain-length groups based on the three general patterns of retention observed. The longest-chain (>∼335 cm3/mol molar volume) PFAS remained within the uppermost section of the core, exhibiting minimal leaching. Conversely, the shortest-chain (<∼220 cm3/mol) PFAS accumulated at the bottom of the interval, which coincides with the onset of a calcic horizon. PFAS with intermediate-chain lengths were distributed along the length of the core, exhibiting differential magnitudes of leaching. The minimal or differential leaching observed for the longest- and intermediate-chain-length PFAS, respectively, demonstrates that retention processes significantly impacted migration. The accumulation of shorter-chain PFAS at the bottom of the core is hypothesized to result from limited deep infiltration and potential-enhanced retention associated with the calcic horizon.

Predicting Interfacial Tension and Adsorption at Fluid-Fluid Interfaces for Mixtures of PFAS and/or Hydrocarbon Surfactants.

Many per- and polyfluoroalkyl substances (PFAS) are surface-active and adsorb at fluid-fluid interfaces. The interfacial adsorption controls PFAS transport in multiple environmental systems, including leaching through soils, accumulation in aerosols, and treatment methods such as foam fractionation. Most PFAS contamination sites comprise mixtures of PFAS as well as hydrocarbon surfactants, which complicates their adsorption behaviors. We present a mathematical model for predicting interfacial tension and adsorption at fluid-fluid interfaces for multicomponent PFAS and hydrocarbon surfactants. The model is derived from simplifying a prior advanced thermodynamic-based model and applies to nonionic and ionic mixtures of the same charge sign with swamping electrolytes. The only required model inputs are the single-component Szyszkowski parameters obtained for the individual components. We validate the model using literature interfacial tension data of air-water and NAPL (non-aqueous phase liquid)-water interfaces covering a wide range of multicomponent PFAS and hydrocarbon surfactants. Application of the model to representative porewater PFAS concentrations in the vadose zone suggests competitive adsorption can significantly reduce PFAS retention (up to 7 times) at some highly contaminated sites. The multicomponent model can be readily incorporated into transport models to simulate the migration of mixtures of PFAS and/or hydrocarbon surfactants in the environment.

Human Health and Ecological Risks Associated with Total and Bioaccessible Concentrations of Cadmium and Lead in Urban Park Soils.

This study investigated the total and bioaccessible concentrations of cadmium (Cd) and lead (Pb) in urban soils and their associated human health and ecological risk. Total and bioaccessible metal concentrations were found within the safe limits except for Cd, surpassing the State Environmental Protection Administration (SEPA) China limit in 9.5% of parks. Bioaccessible concentrations were higher in the gastric (G) phase than the intestinal (I) phase, while Cd showed more bioaccessibility compared to Pb. Bioaccessible concentrations reduced Hazard Quotient (HQing) values by 2-22 times and 0-2 times for children and adults, respectively, while hazard index (HI) declined by 1.7 times, and the mean total bioaccessible risk of Pb decreased by 20.8 times. Further, the study revealed a low level of contamination factor (CF < 1) and a low degree of contamination (CD < 6), and Potential Ecological Risk Index (PERI) values for all the cities were less than 150, indicating low ecological risk.

General Backward Model to Identify the Source for Contaminants Undergoing Non-Fickian Diffusion in Water.

Pollutant source identification (PSI) has been conducted for four decades for tracking Fickian diffusive pollutants, while PSI for non-Fickian diffusion, well-documented in aquifers and rivers, requires novel, predictive models. To enable PSI for non-Fickian diffusive pollutants, this study derived a general backward model using the fractional-adjoint approach in sensitivity analysis for dissolved contaminants with transport governed by the spatiotemporal fractional advection-dispersion equation (fADE). The backward fADE contains a self-adjoint time-fractional term for subdiffusion and direction-dependent, non-self-adjoint space-fractional terms for superdiffusion. Field applications showed that the resultant backward location probability density function identified the point source location in all three test cases, one alluvial aquifer and two rivers. The backward model and boundary conditions derived in this study made it possible to reliably and efficiently backtrack pollutants (and may include other constituents, such as bedload) undergoing mixed sub- and superdiffusion in natural aquatic systems. The classical PSI model, however, underestimated the source location since it did not account for solute retention and preferential flow. In addition, the measured tracer snapshots (if available before PSI) can enhance the parameter predictability and improve the applicability of backward fADE PSI. Most importantly, a spatially variable dispersion coefficient is needed in the backward fADE since PSI is most likely scale dependent in natural hydrologic systems.

PFOS Mass Flux Reduction/Mass Removal: Impacts of a Lower-Permeability Sand Lens within Otherwise Homogeneous Systems.

Perfluorooctane sulfonic acid (PFOS) is one of the most common per- and polyfluoroalkyl substances (PFAS) and is a significant risk driver for these emerging contaminants of concern. A series of two-dimensional flow cell experiments was conducted to investigate the impact of flow field heterogeneity on the transport, attenuation, and mass removal of PFOS. A simplified model heterogeneous system was employed consisting of a lower-permeability fine sand lens placed within a higher-permeability coarse sand matrix. Three nonreactive tracers with different aqueous diffusion coefficients, sodium chloride, pentafluorobenzoic acid, and ß-cyclodextrin, were used to characterize the influence of diffusive mass transfer on transport and for comparison to PFOS results. The results confirm that the attenuation and subsequent mass removal of the nonreactive tracers and PFOS were influenced by mass transfer between the hydraulically less accessible zone and the coarser matrix (i.e., back diffusion). A mathematical model was used to simulate flow and transport, with the values for all input parameters determined independently. The model predictions provided good matches to the measured breakthrough curves, as well as to plots of reductions in mass flux as a function of mass removed. These results reveal the importance of molecular diffusion and pore water velocity variability even for systems with relatively minor hydraulic conductivity heterogeneity. The impacts of the diffusive mass transfer limitation were quantified using an empirical function relating reductions in contaminant mass flux (MFR) to mass removal (MR). Multi-step regression was used to quantify the nonlinear, multi-stage MFR/MR behavior observed for the heterogeneous experiments. The MFR/MR function adequately reproduced the measured data, which suggests that the MFR/MR approach can be used to evaluate PFOS removal from heterogeneous media.

Contribution of Nonaqueous-Phase Liquids to the Retention and Transport of Per and Polyfluoroalkyl Substances (PFAS) in Porous Media.

Per and polyfluoroalkyl substances (PFAS) cocontamination with nonaqueous-phase organic liquids (NAPLs) has been observed or suspected at various sites, particularly at fire-training areas at which aqueous film-forming foams (AFFFs) were applied. The objectives of this study are to (1) delineate the relative significance of specific PFAS-NAPL processes on PFAS retention, including partitioning into the bulk NAPL phase and adsorption to the NAPL-water interface; (2) investigate the influence of NAPL properties, saturation, and mass-transfer constraints on PFAS retention; and (3) determine whether PFAS may impact NAPL distribution through mobilization or dissolution. Perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are used as representative PFAS, and trichloroethene (TCE) and decane are used as representative NAPLs. NAPL-water interfacial adsorption was quantified with NAPL-water interfacial-tension measurements; partitioning into NAPL was quantified with batch experiments, and retardation factors (R) in the absence and presence of residual NAPL were determined with miscible-displacement transport experiments. R values increased in the presence of residual NAPL, with adsorption to the NAPL-water interface accounting for as much as ∼77% of retention and solid-phase adsorption also significantly contributing to retention. Additionally, this study provides the first QSPR analysis focused on NAPL-water interfacial adsorption coefficients, with results consistent with those from previous air-water studies. Lastly, this initial investigation into PFAS impacts on NAPL behavior determined that PFOS/PFOA are unlikely to enhance solubilization or mobilization of NAPL under the conditions present at many AFFF legacy sites.

Impact of a Hydrocarbon Surfactant on the Retention and Transport of Perfluorooctanoic Acid in Saturated and Unsaturated Porous Media.

The transport and retention behavior of perfluorooctanoic acid (PFOA) in the presence of a hydrocarbon surfactant under saturated and unsaturated conditions was investigated. Miscible-displacement transport experiments were conducted at different PFOA and sodium dodecyl sulfate (SDS) input ratios to determine the impact of SDS on PFOA adsorption at solid-water and air-water interfaces. A numerical flow and transport model was employed to simulate the experiments. The PFOA breakthrough curves for unsaturated conditions exhibited greater retardation compared to those for saturated conditions in all cases, owing to air-water interfacial adsorption. The retardation factor for PFOA with a low concentration of SDS (PFOA-SDS ratio of 10:1) was similar to that for PFOA without SDS under unsaturated conditions. Conversely, retardation was greater in the presence of higher levels of SDS (1:1 and 1:10) with retardation factors increasing from 2.4 to 2.9 and 3.6 under unsaturated conditions due to enhanced adsorption at the solid-water and air-water interfaces. The low concentration of SDS had no measurable impact on PFOA air-water interfacial adsorption coefficients (Kia) determined from the transport experiments. The presence of SDS at the higher PFOA-SDS concentration ratios increased the surface activity of PFOA, with transport-determined Kia values increased by 27 and 139%, respectively. The model provided very good independently predicted simulations of the measured breakthrough curves and showed that PFOA and SDS experienced various degrees of differential transport during the experiments. These results have implications for the characterization and modeling of poly-fluoroalkyl substances (PFAS) migration potential at sites wherein PFAS and hydrocarbon surfactants co-occur.

Transport of GenX in Saturated and Unsaturated Porous Media.

The objective of this research was to investigate the retention and transport behavior of GenX in five natural porous media with similar median grain diameters but different geochemical properties. Surface tensions were measured to characterize surface activity. Miscible-displacement experiments were conducted under saturated conditions to characterize the magnitude of solid-phase adsorption, while unsaturated-flow experiments were conducted to examine the impact of air-water interfacial adsorption on retention and transport. The results from surface-tension measurements showed that the impact of solution composition is greater for the ammonium form of GenX than for the acid form, due to the presence of the NH4 counterion. The breakthrough curves for the experiments conducted under saturated conditions were asymmetrical, and a solute-transport model employing a two-domain representation of nonlinear, rate-limited sorption provided reasonable simulations of the measured data. The magnitudes of solid-phase adsorption were relatively small, with the highest adsorption associated with the medium containing the greatest amount of metal oxides. The breakthrough curves for the experiments conducted under unsaturated conditions exhibited greater retardation due to the impact of adsorption at the air-water interface. The contributions of air-water interfacial adsorption to GenX retention ranged from ∼24% to ∼100%. The overall magnitudes of retardation were relatively low, with retardation factors < ∼3, indicating that GenX has significant migration potential in soil and the vadose zone. To our knowledge, the results presented herein represent the first reported data for solid-water and air-water interfacial adsorption of GenX by soil. These data should prove useful for assessing the transport and fate behavior of GenX in soil and groundwater.

Ferrous metal-organic frameworks with strong electron-donating properties for persulfate activation to effectively degrade aqueous sulfamethoxazole.

Three novel persulfate activators, Fe(II)-based metal-organic frameworks (MOFs) were synthesized for the degradation of sulfamethoxazole (SMX). The degradation experiment results showed that all the Fe(II)MOFs could effectively activate persulfate and degrade more than 97% SMX within 180 min, with higher than 77% persulfate decomposition efficiencies. It was found by Mössbauer spectra that the variation of organic ligands for synthesis have an influence on the content of Fe(II) of these MOFs, thus resulted in the order of activation capacities: Fe(Nic) > Fe(PyBDC) > Fe(PIP). It was demonstrated that the activation of persulfate was mainly ascribed to the heterogeneous process that accomplished by surface-bounded Fe(II) acted as the main active site to provided electrons for persulfate or dissolved oxygen. EPR and molecular probe studies confirmed the coexistence of SO4·-, ·OH, and O2·-, and differentiated their contributions in SMX degradation. Possible degradation pathways of SMX were proposed based on the detection results of intermediates by UPLC-MS/MS. This work provides a new prospect into the synthesis of high-performance MOFs with strong electron-donating properties as efficient persulfate activators, which may encourage the employ of MOFs in the wastewater treatment process.

A Mathematical Model for the Release, Transport, and Retention of Per- and Polyfluoroalkyl Substances (PFAS) in the Vadose Zone.

Per- and Polyfluoroalkyl Substances (PFAS) are emerging contaminants of critical concern. As surfactants, PFAS tend to accumulate at air-water interfaces and may stay in the vadose zone for long times before contaminating groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. We present the first mathematical model that accounts for surfactant-induced flow and solid-phase and air-water interfacial adsorption. We apply the model to simulate PFOS (a PFAS compound of primary concern) transport in the vadose zone at a model fire-training area site impacted by Aqueous Film-Forming Foam (AFFF). Air-water interfacial adsorption is shown to have a significant impact-amplified by the low water content due to gravity drainage-total retardation factors range from 233 to 1355 for the sand and 146 to 792 for the soil used in the study. The simulations illustrate it can take several decades or longer for PFOS to reach groundwater. Counterintuitively, the lower water content in the sand-due to stronger drainage and weaker capillary retention-leads to retardation factors greater than for the soil. Also, most PFOS is adsorbed at air-water interfaces with only 1-2% in the aqueous phase. The implications include 1) fine-texture materials could have lower retardation factors than sand due to higher retained water content, 2) soil PFAS concentrations are likely to be orders of magnitude higher than those in groundwater at source zones. Both implications are consistent with recent field observations at hundreds of AFFF-impacted sites.

Assessing XMT-Measurement Variability of Air-Water Interfacial Areas in Natural Porous Media.

This study investigates the accuracy and reproducibility of air-water interfacial areas measured with high-resolution synchrotron x-ray microtomography (XMT). Columns packed with one of two relatively coarse-grained monodisperse granular media, glass beads or a well-sorted quartz sand, were imaged over several years, encompassing changes in acquisition equipment, improved image quality, and enhancements to image acquisition and processing software. For the glass beads, the specific solid surface area (SSSA-XMT) of 31.6 ±1 cm-1 determined from direct analysis of the segmented solid-phase image data is statistically identical to the independently calculated geometric specific solid surface area (GSSA, 32 ±1 cm-1) and to the measured SSSA (28 ±3 cm-1) obtained with the N2BET method (NBET). The maximum specific air-water interfacial area (Amax) is 27.4 (±2) cm-1, which compares very well to the SSSA-XMT, GSSA, and SSSA-NBET values. For the sand, the SSSA-XMT (111 ±2 cm-1) and GSSA (113 ±1 cm-1) are similar. The mean Amax is 96 ±5 cm-1, which compares well to both the SSSA and the GSSA values. The XMT-SSSA values deviated from the GSSA values by 7-16% for the first four experiments, but were essentially identical for the later experiments. This indicates that enhancements in image acquisition and processing improved data accuracy. The Amax values ranged from 74 cm-1 to 101 cm-1, with a coefficient of variation (COV) of 9%. The maximum capillary interfacial area ranged from 12 cm-1 to 19 cm-1, for a COV of 10%. The COVs for both decreased to 5-6% for the latter five experiments. These results demonstrate that XMT imaging provides accurate and reproducible measurements of total and capillary interfacial areas.

Pore-Scale Modeling of Fluid-Fluid Interfacial Area in Variably Saturated Porous Media Containing Microscale Surface Roughness.

A pore-scale model is developed to simulate fluid-fluid interfacial area in variably saturated porous media, with a specific focus on incorporating the effects of solid-surface roughness. The model is designed to quantify total (film and meniscus) fluid-fluid interfacial area (Anw ) over the full range of wetting-phase fluid saturation (Sw ) based on the inherent properties of the porous medium. The model employs a triangular pore space bundle of cylindrical capillaries (BCC) framework, modified with three surface roughness-related parameters. The first parameter (surface roughness factor) represents the overall magnitude of surface roughness, whereas the other two parameters (interface growth factor and critical adsorptive film thickness) reflect the micro-scale structure of surface roughness. A series of sensitivity analyses was conducted for the controlling variables, and the efficacy of the model was tested using air-water interfacial area data measured for three natural porous media. The model produced good simulations of the measured Anw data over the full range of saturation. The results demonstrate that total interfacial areas for natural media are typically much larger than those for ideal media comprising smooth surfaces due to the substantial contribution of surface roughness to wetting-film interfacial area. The degree to which fluid-fluid interfacial area is influenced by roughness is a function of fluid-retention characteristics and the nature of the rough surfaces. The full impact of roughness may be masked to some degree due to the formation of thick wetting films, which is explicitly quantified by the model. Application of the model provides insight into the importance of the interplay between pore-scale distribution and configuration of wetting fluid and the surface properties of solids.

Characterization of the Micro-scale Surface Roughness Effect on Immiscible Fluids and Interfacial Areas in Porous Media Using the Measurements of Interfacial Partitioning Tracer Tests.

This study presents a model-based methodology to characterize the surface roughness effect on immiscible fluids in porous media using the measurements obtained with the gas-phase interfacial partitioning tracer test (IPTT). The characterization approach captures how adsorbed wetting film configuration on grain surfaces influences fluid-fluid interfaces in unsaturated porous media. The method establishes a novel representation of surface and interface roughness that delineates the micro-scale fractal nature of grain surfaces and the fluid-surface interactions at these scales. The method was tested using reported experimental data for several soils. The results showed that the methodology was effective for natural porous media comprising a range of physical and geochemical properties. Comparisons between characterized parameters of different media revealed that micro-scale surface roughness was only partially correlated to soil texture properties. Images of the test media obtained with scanning electron microscopy (SEM) illustrates the complexity of micro-scale surface roughness, and its variability among different media. Tests with an organic liquid-water system validated the generalness of surface roughness properties generated by the model. The proposed methodology is anticipated to provide a means to characterize and quantify the effects of surface roughness on fluid-solid interaction and fluid-fluid interfacial area, which are critical to various environmental disciplines.

Nonideal Transport and Extended Elution Tailing of PFOS in Soil.

The objective of this research was to examine the influence of nonideal sorption/desorption on the transport of polyfluorinated alkyl substances (PFASs) in soil, with a specific focus on characterizing and quantifying potential extended, mass-transfer-limited elution behavior. Perfluorooctane sulfonic acid (PFOS) was used as a representative PFAS, and miscible-displacement experiments were conducted with two soils comprising contrasting geochemical properties. The influence of nonlinear, rate-limited, hysteretic, and irreversible sorption/desorption on transport was investigated through experiments and model simulations. The breakthrough curves measured for PFOS transport in the two soils were asymmetrical and exhibited extensive elution tailing, indicating that sorption/desorption was significantly nonideal. The widely used two-domain sorption kinetics model could not fully simulate the observed transport behavior, whereas a multirate model employing a continuous distribution of sorption domains was successful. The overall results indicated that sorption/desorption was significantly rate-limited and that nonlinear, hysteretic, and irreversible sorption/desorption had minimal impact on PFOS transport. Comparison of PFOS transport data to data reported for two hydrophobic organic contaminants (HOCs) showed that the HOCs exhibited much more extensive elution tailing, likely reflecting differences in sorption/desorption mechanisms. The projected influence of rate-limited sorption/desorption on PFOS transport at the field scale was investigated through simulation. The results of the study suggest that rate-limited sorption/desorption may affect the field-scale transport of PFOS and other PFAS for systems influenced by transient or short-residence-time conditions and in some cases could possibly increase the amount of flushing required to reduce PFOS concentrations to levels below those associated with human-health concerns.

Mechanism of carbon tetrachloride reduction in ferrous ion activated calcium peroxide system in the presence of methanol.

This study investigated the reductive initiation for the depletion of highly oxidized/perhalogenated pollutants, specifically the degradation of carbon tetrachloride (CT) was induced by adding methanol (MeOH) into a ferrous ion (Fe(II)) activated calcium peroxide (CaO2) system. The results indicated that CT could be completely degraded within 20 min at CaO2/Fe(II)/MeOH/CT molar ratio of 30/40/10/1 in this system. Scavenging tests suggested that both superoxide radical anion (O2 •-) and carbon dioxide radical anion (CO2 •-) were predominant reactive species responsible for CT destruction. Hydroxymethyl radicals (•CH2OH), an intermediate in the transformation of MeOH, could also initiate CT degradation by reducing C-Cl bond. GC/MS analysis identified CHCl3, C2Cl4, and C2Cl6 as the intermediates accompanied by CT destruction, and a reduction mechanism for CT degradation was proposed accordingly. In addition, the impact of solution matrix and initial solution pH were evaluated, and the results showed that Cl-, NO3 -, and HCO3 - had adverse effects on CT degradation. Moreover, the alkaline condition was unfavorable to CT depletion. In conclusion, the results obtained in the actual groundwater tests encouragingly demonstrated that the CaO2/Fe(II)/MeOH process is a highly promising technique for the remediation of CT-contaminated groundwater.

Insight into CaO2-based Fenton and Fenton-like systems: strategy for CaO2-based oxidation of organic contaminants.

This study conducted a comparison of the CaO2-based Fenton (CaO2/Fe(II)) and Fenton-like (CaO2/Fe(III)) systems on their benzene degradation performance. The H2O2, Fe(II), Fe(III), and HO● variations were investigated during the benzene degradation. Although benzene has been totally removed in the two systems, the variation patterns of the investigated parameters were different, leading to the different benzene degradation patterns. In terms of the Fe(II)/Fe(III) conversion, the CaO2/Fe(II) and CaO2/Fe(III) systems were actually inseparable and had the inherent mechanism relationships. For the CaO2/Fe(III) system, the initial Fe(III) must be converted to Fe(II), and then the consequent Fenton reaction could be later developed with the regenerated Fe(II). Moreover, some benzene degradation intermediates could have the ability to facilitate the transformation of the Fe(III) to Fe(II) without the classic H2O2-associated propagation reactions. By varying the Fe(II) dosing method, an effective degradation strategy has been developed to take advantage of the two CaO2-based oxidation systems. The proposed strategy was further successfully tested in TCE degradation, therefore extending the potential for the application of this technique.

Degradation of trichloroethylene in aqueous solution by nanoscale calcium peroxide in the Fe(II)-based catalytic environments.

In this study, nCaO2 was synthesized successfully and applied in the Fe(II)-based catalytic environments in investigating trichloroethylene (TCE) removal performance. nCaO2 with the particle sizes in the range of 50-200 nm was prepared, and it performed better for TCE removal when compared to the conventional CaO2. Further experimental results showed that 70.4% of TCE could be removed in 180 min at the nCaO2/Fe(II)/TCE molar ratio of 1/2/1, while this data was elevated to 86.1% in the presence of citric acid (CA) at the nCaO2/Fe(II)/CA/TCE molar ratio of 1/2/2/1 in the same test period. Probe compound tests, specifically designed for free radicals confirmation, demonstrated the presence of HO• and O2 -•. Moreover, scavenging tests indicated that HO• was the major radical responsible for TCE degradation but O2 -• promoted the removal of TCE in both nCaO2/Fe(II) and nCaO2/Fe(II)-CA system. In addition, the effects of initial solution pH and anions (Cl-, HCO3 -) were also evaluated. The performance of TCE degradation in actual groundwater demonstrated that both nCaO2/Fe(II) and nCaO2/Fe(II)-CA systems can be applicable for TCE removal in ISCO practice and the nCaO2/Fe(II)-CA system is much promising technique. These fundamental data strongly confirmed the feasibility and potential of nCaO2 based technique in the remediation of TCE contaminated groundwater.

Modeling groundwater contaminant transport in the presence of large heterogeneity: A case study comparing MT3D and RWhet.

A case study is presented that implements two numerical models for simulating a 30-year PAT operation conducted at a large contaminated site for which high-resolution data sets are available. A Markov chain based stochastic method is used to conditionally generate the realizations with random distribution of heterogeneity for the Tucson International Airport Area (TIAA) federal Superfund site. The fields were conditioned to data collected for 245 boreholes drilled at the site. Both MT3DMS and the advanced random walk particle method (RWhet) were used to simulate the PAT-based mass removal process. The results show that both MT3DMS and RWhet represent the measured data reasonably, with Root Mean Square Error (RMSE) less than 0.03. The use of fine grids and the total-variation-diminishing method (TVD) limited the effects of numerical dispersion for MT3DMS. However, the effects of numerical dispersion were observed when compared to the simulations produced with RWhet using a larger number of particles, which provided more accurate results with RMSE diminishing from 0.027 to 0.024 to 0.020 for simulations with 1, 20, and 50 particles. The computational time increased with more particles used in the model, but was still much less than the time required for MT3DMS, which is an advantage of RWhet. By showing the results using both methods, this study provides guidance for simulating long-term PAT systems. This work will lead to improve understanding of contaminant transport and plume persistence, and in turn will enhance site characterization and site management for contaminated sites with large plumes.

Adsorption of PFOA at the Air-Water Interface during Transport in Unsaturated Porous Media.

Miscible-displacement experiments are conducted with perfluorooctanoic acid (PFOA) to determine the contribution of adsorption at the air-water interface to retention during transport in water-unsaturated porous media. Column experiments were conducted with two sands of different diameter at different PFOA input concentrations, water saturations, and pore-water velocities to evaluate the impact of system variables on retardation. The breakthrough curves for unsaturated conditions exhibited greater retardation than those obtained for saturated conditions, demonstrating the significant impact of air-water interfacial adsorption on PFOA retention. Retardation was greater for lower water saturations and smaller grain diameter, consistent with the impact of system conditions on the magnitude of air-water interfacial area in porous media. Retardation was greater for lower input concentrations of PFOA for a given water saturation, consistent with the nonlinear nature of surfactant fluid-fluid interfacial adsorption. Retardation factors predicted using independently determined parameter values compared very well to the measured values. The results showed that adsorption at the air-water interface is a significant source of retention for PFOA, contributing approximately 50-75% of total retention, for the test systems. The significant magnitude of air-water interfacial adsorption measured in this work has ramifications for accurate determination of PFAS migration potential in vadose zones.

Insight on the generation of reactive oxygen species in the CaO2/Fe(II) Fenton system and the hydroxyl radical advancing strategy.

Calcium peroxide (CaO2) is a stable hydrogen peroxide (H2O2) carrier, and the CaO2/Fe(II) system has been applied for treatment of various pollutants. It is commonly reported in the literature that hydroxyl radical (HO●) and superoxide radical anions (O2 ●-) are the two main reactive oxygen species (ROSs) generated in the CaO2/Fe(II) system. However, many of the reported results were deduced from degradation performance rather than specific testing of radical generation. Thus, the specific generation of ROSs and the influence of system conditions on ROSs yield is still unclear. To our knowledge, this is the first study specifically focusing on the generation of HO● and O2 ●- in the CaO2/Fe(II) system. Experimental conditions were optimized to investigate the production of HO● and O2 ●-. The results showed the influences of CaO2, Fe(II), and solution pH on HO● and O2 ●- generation, and the HO● generation efficiency was reported for the first time. In addition, the ROSs generation pathways in the CaO2/Fe(II) system were elucidated. A strategy for enhancing HO● yield is developed, based on the continuously dosing Fe(II). This proposed strategy has implications for the effective application of in situ chemical oxidation employing CaO2/Fe(II) for groundwater remediation.