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Unlocking the sorption mechanism of perfluoroalkyl acids (PFAAs) on geosynthetics: Case of the geotextile components of geosynthetic clay liners

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... Some OMPs may form chemical complexes or undergo interactions with functional groups on the adsorbent surface [127]. The kinetics of these chemical interactions influence the entire adsorption process [128]. Faster response rates contribute to faster OMP uptake, while slower reaction kinetics may lengthen the time required to attain equilibrium [129]. ...
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This paper gives an overview of the current state of the practice on the interaction between geosynthetic liners and per- and polyfluoroalkyl substances (PFASs). The importance of PFAS chemistry and the need to make a clear distinction between on-site repositories and landfill applications is highlighted, as the liners will be subjected to widely different concentration loadings and types of dominating PFASs. Consequently, the selection of geosynthetic liner components needs to account for these differences and distinguish between the expected exposure conditions. In particular, geomembranes need to be selected carefully as their resin type will dictate their performance. Preliminary hydraulic conductivity test results using unamended and activated carbon amended geosynthetic clay liners indicate they can play a role in minimising the migration of PFAS, although the tests are still ongoing.
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The current study addresses the cracking and self-healing capacity of Geosynthetic clay liners (GCLs) subjected to drying and wetting in a divalent salt solution. Commercially available GCLs, initially saturated under a load of 3.92 kPa, were stepwise dried for different durations in an oven at 30 °C and rewetted afterwards in deionised water and divalent salt solutions (CaCl2) of different molarities (0.05 mol/l, 0.5 mol/l and 0.05 mol/l prehydrated). The evolution of cracks and their patterns were studied by analysing X-ray images. In parallel, the water retention behavior was tested on the raw bentonite using micro-cells and a chilled-mirror hygrometer. The morphology of the crack patterns in the GCLs was highly affected by the pore fluid, which was reasoned by reduced tensile strength caused by the salt induced aggregation. The ability to retain water at a given suction was found to be higher for the samples subjected to CaCl2-solutions in comparision to the sample saturated with deionised water. However, a calculation of the osmotic suction caused by the additional CaCl2 in the porewater shows that the matric suction of the samples subjeted to CaCl2 decreases. The crack intensitiy factor (CIF) followed a similar trend and three different drying regimes were identified.
Article
Diffusion of perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) through 0.1 mm and 0.75 mm LLDPE and 0.1 mm and 0.75 mm LLDPE coextruded with ethyl vinyl alcohol (denoted as CoEx) at room temperature (23 °C), 35 °C, and 50 °C is examined. These tests had negligible source depletion throughout the monitoring period, indicating limited contaminant partitioning and diffusion through the LLDPE. At 483 days, 23 °C receptor PFOA and PFOS concentrations, cr, were <8 μg/L (cr/co < 3.2 × 10⁻⁴) for all tests, and at 399 days elevated temperature receptor concentrations were < 0.4 μg/L (cr/co < 1.6 × 10⁻⁵) at 35 °C and <0.5 μg/L (cr/co < 2.0 × 10⁻⁵) at 50 °C for both PFOA and PFOS. LLDPE partitioning coefficient, Sgf was 0.9–1.4 (PFOA) and 2.8–5.3 (PFOS) based on sorption tests at 23 °C. Based on the best estimates of permeation coefficient, PgCoEx, for CoEx was consistently lower than PgLLDPE. For PFOA, CoEx had PgCoEx < 0.26 × 10⁻¹⁶ m²/s at 23 °C, <11 × 10⁻¹⁶ m²/s (35 °C), and < 10 × 10⁻¹⁶ m²/s (50 °C) while LLDPE had PgLLDPE < 3.1 × 10⁻¹⁶ m²/s (23 °C), <13 × 10⁻¹⁶ m²/s (35 °C), and <19 × 10⁻¹⁶ m²/s (50 °C). For PFOS, CoEx and LLDPE had PgCoEx < 0.55 × 10⁻¹⁶ m²/s and PgLLDPE < 3.2 × 10⁻¹⁶ m²/s (23 °C), PgCoEx < 8.3 × 10⁻¹⁶ m²/s and PgLLDPE < 40 × 10⁻¹⁶ m²/s (35 °C), and PgCoEx < 8.2 × 10⁻¹⁶ m²/s and PgLLDPE < 52 × 10⁻¹⁶ m²/s (50 °C). These values are preliminary and may change (e.g., decrease) as more data comes available over time. The Pg values deduced for PFOA and PFOS are remarkably lower than those reported for other contaminants of concern, excepting BPA, which exhibits similar behaviour.
Article
Understanding the subsurface transport of perfluorooctanoic acid (PFOA) is of considerable interest for evaluating its potential risks to humans and ecosystems. In this study, packed-column experiments were conducted to examine the influence of surface roughness on PFOA transport in unsaturated glass beads, quartz sand and limestone porous media. Results showed decreasing moisture content significantly increased the air-water interfacial adsorption of PFOA and led to greater retardation in all three typed of porous media. Particularly, rougher surface (limestone > quartz sand > glass beads) and smaller grain size (i.e. a larger solid specific surface area, SSSA) significantly enhanced PFOA retardation under unsaturated conditions. These results were further supported by bubble column experiments and SSSA analysis of porous media, which demonstrate that except for the factors affecting PFOA transport in solid-water interface (e.g. surface charge and chemical heterogeneity), the greater retardation of PFOA during transport is attributed to the larger air-water interfacial areas associated with rougher surface and smaller grain size and hence greater interfacial adsorption of PFOA. Our results indicated the importance of surface roughness on the retention and transport of PFOA in the unsaturated zone.
Article
Microplastics (MPs) have been recognized as transport vectors for micropollutants in the natural water environment and the food web; therefore, the sorption behaviour of contaminant on MPs has recently gained an increased attention. However, a consensus has not yet been reached and information about the adsorption of water contaminants on real MPs remains elusive. Herein, we raise the question of “Should we continue using pure polymers as surrogates for real MPs?” This first systematic study compared the adsorption of multiple micropollutants (i.e. a pesticide, a pharmaceutical, and perfluoroalkyl substances (PFAS)) on a large set of MPs (i.e. 20 well-characterized MPs) and kaolin. Material characterizations results showed various physicochemical and compositional differences between real and pure MPs. Pure polymers had lower normalized uptake values than real MPs in most cases. This was attributed to the surface roughness and/or the presence of fillers (e.g. talc and glass fiber) in real samples. Further, preloaded MPs with natural organic matter (NOM) showed an increased uptake of micropollutants due to forming a complex with NOM and/or co-sorption. These findings indicate that employing real MPs in research studies is critical for obtaining environmentally meaningful results, and the evaluation of MPs sorption behavior without NOM preloading can result in a significant underestimation for their actual values. We also provided an outlook the key areas for further investigations.
Article
Poly- and perfluoroalkyl substances (PFAS) are a wide group of environmentally persistent organic compounds of industrial origin, which are of great concern due to their harmful impact on human health and ecosystems. Amongst long-chain PFAS, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) are the most detected in the aquatic environment, even though their use has been limited by recent regulations. Recently, more attention has been posed on the short-chain compounds, due to their use as an alternative to long-chain ones, and to their high mobility in the water bodies. Therefore, short-chain PFAS have been increasingly detected in the environmental compartments. The main process investigated and implemented for PFAS removal is adsorption. However, to date, most adsorption studies have focused on synthetic water. The main objective of this article is to provide a critical review of the recent peer-reviewed studies on the removal of long- and short-chain PFAS by adsorption. Specific objectives are to review 1) the performance of different adsorbents for both long- and short-chain PFAS, 2) the effect of organic matter, and 3) the adsorbent regeneration techniques. Strong anion-exchange resins seem to better remove both long- and short-chain PFAS. However, the adsorption capacity of short-chain PFAS is lower than that observed for long-chain PFAS. Therefore, short-chain PFAS removal is more challenging. Furthermore, the effect of organic matter on PFAS adsorption in water or wastewater under real environmental conditions is overlooked. In most studies high PFAS levels have been often investigated without organic matter presence. The rapid breakthrough of PFAS is also a limiting factor and the regeneration of PFAS exhausted adsorbents is very challenging and needs more research.
Article
Bench-scale experiments were performed to assess uptake of poly- and perfluoroalkyl acids (PFAS), both single compounds and mixtures, at the air-water interface. The focus was on evaluating uptake at field-relevant PFAS concentrations (< 2 x 10-4 mol m-3 or 0.1 mg L-1), assessing the impacts of various PFAS mixtures, and quantifying the impacts of background NaCl concentrations. Both interfacial tension measurements and direct quantification of PFAS mass sorbed at the air-water interface in water films were used to evaluate PFAS interfacial partitioning. Results showed that a Freundlich-based model, rather than a Langmuir-based model, described perfluorooctanoic acid (PFOA) and perfluorosulfonic acid (PFOS) interfacial uptake. At lower and field-relevant PFOS and PFOA concentrations, the Langmuir-based model underpredicted interfacial uptake by up to several orders of magnitude. The interfacial partition coefficient, kaw, increased as PFAS concentrations decreased. Results also showed that the interfacial tension and interfacial uptake of PFAS mixtures were (within a factor of 2) predicted based on the single solute systems assuming ideal dilute behavior. Furthermore, the addition of NaCl at concentrations of up to 0.01 M increased PFOS uptake by less than a factor of 2 at field-relevant PFOS concentrations. The results presented herein have important implications for PFAS migration in unsaturated soils, as well as for remedial technologies that rely on PFAS interfacial sorption, particularly at field-relevant PFAS concentrations.
Article
The interface transmissivity (θ) and hydraulic conductivity (k) are measured for two geosynthetic clay liners (GCLs), one with polymer-enhanced bentonite, when hydrated and permeated with saline (brine) solutions at three different concentrations and Reverse Osmosis (RO) water. Two interface transmissivity values are reported, the 2-week (θ 2-week ) and the steady-state (θ steady-state ) interface transmissivity. For saline solution (brine) permeation, the 2-week interface transmissivity (θ 2-week ) is one to two orders of magnitude higher than the steady-state values. In addition, the steady-state interface transmissivity (θ steady-state ) with respect to brine is almost an order of magnitude higher than that for RO water permeation. Geomembrane (GMB) stiffness has a limited effect on interface transmissivity at 150 kPa, whereas at 10 kPa the interface transmissivity decreases as the GMB stiffness decreases. GMB texture has only a small effect on interface transmissivity at different stress levels. Water prehydration reduces the effect of brine permeation on interface transmissivity and hydraulic conductivity, especially at 150 kPa. Transmissivity tends to increase as the salt concentration increases but the effect was significant at all concentrations considered (440 to 4400 mmol/l). While the effect of bentonite enhancement on interface transmissivity is unclear, the hydraulic conductivity (k) is generally lower for enhanced bentonite.
Article
In this perspective, we evaluate key and emerging epidemiological and toxicological data concerning immunotoxicity of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) and seek to reconcile conflicting conclusions from two reviews published in 2016. We summarize ways that immunosuppression and immunoenhancement are defined and explain how specific outcomes are used to evaluate immunotoxicity in humans and experimental animals. We observe that different approaches to defining immunotoxicological outcomes, particularly those that do not produce clinical disease, may lead to different conclusions from epidemiological and toxicological studies. The fundamental point that we make is that aspects of epidemiological studies considered as limitations can be minimized when data from toxicological studies support epidemiological findings. Taken together, we find that results of epidemiological studies, supported by findings from toxicological studies, provide strong evidence that humans exposed to PFOA and PFOS are at risk for immunosuppression.
Article
The adsorption of perfluorinated acids (PFAs) onto soils with different physicochemical properties was investigated in this study. The adsorption kinetics for all PFAs onto the soil with the highest contents of total organic carbon (TOC) and iron oxide were well described by a biexponential adsorption model, indicating that two types of binding sites characterized by a fast and a slow sorption rates were involved in the adsorption, and the time required for achieving adsorption equilibrium was <48 h for all PFAs. The adsorption isotherms were well represented by both of Freundlich equation (R2 = 0.9547-0.9977) and/or Virial equation (R2 = 0.8720-0.9995). The interfacial capacitances derived from the Virial isotherm were substantially low (in the range of 33.7 to 851 μF/m2) for all soils, but were not analyte-independent for all PFAs onto the same soil. The linear regression between distribution coefficient (Kd) and individual soil property as well as principle component analysis were conducted for determining the dominant soil physicochemical properties affecting the adsorption of PFAs onto soil in the present study. The results indicated that the content of protein rather than of total organic carbon (TOC) was the dominant property, and then followed by anion exchange capacity (AEC) and the content of iron oxides. For the other properties, the influences of fulvic acid (FA) and aluminum oxides were PFA-dependent, while there were no effects of saccharide, humic acid (HA), specific surface area (SSA) and cation exchange capacities (CEC) on the adsorption.
Article
High-resolution X-ray tomography was used to observe a partially hydrated geosynthetic clay liner (GCL) specimen to gain a better understanding of the interaction of its compnents (i.e., geotextiles, fibres and bentonite) on partial hydration when deployed as part of a composite liner system. Detailed in-situ studies of hydration processes in GCLs has proven difficult despite more than two decades of effort. X-ray tomographs were collected at spatial resolutions of 12 and 7 μm to identify the different components within a GCL, as well as to examine in finer detail their interaction within the GCL after initial partial hydration. Tomograph projections provided an excellent aspect of the interaction of these components and some concepts, such as the presence of shearing features within the bentonite component, may require re-consideration based on evidence from X-ray tomography.
Article
The effect of water salinity on the water retention curve of geosynthetic clay liners (GCLs), under constant volume condition is examined. The results indicate that at a constant gravimetric moisture content the total suction increases as the salinity of the wetting liquid increases. Furthermore, the difference in total suction between the GCL hydrated by saline water and distilled water is greater than the difference in the osmotic potential of the wetting water. This behaviour is possibly caused by the matric suction being affected by the expected chemically induced pore size change of the bentonite component of the GCL.
Article
The permeability of coal is the key parameter both in primary and enhanced coalbed methane recovery. The natural cleat system in coal serves as the primary pathway for gas flow in coal seams though mineralisation in cleats and is known to significantly reduce coal permeability. This paper reports on a numerical simulation of the pore network evolution of coal subject to cleat demineralisation. A high-resolution micro-computed tomography scanner was used to characterize the micro-structures of three anthracite coal samples. The mineral phases available in the coal samples were selectively removed to different extents (20%, 40%, 60%, 80% and 100%) and merged into the pore space using image processing techniques. In this way, the coal demineralisation process could be simulated and its impact on porosity and permeability studied. Comprehensive pore structure characterizations, including porosity, connectivity and tortuosity, were then conducted on the reconstructed pore network using Avizo software. Pore network models were also extracted to investigate changes in the pore and throat attributes. The lattice Boltzmann method was adopted to identify the absolute permeability changes with cleat demineralisation. The results reveal that demineralisation can increase coal porosity and permeability up to a percolation threshold. Although porosity was enhanced prior to the percolation threshold, the coal permeability was not enhanced due to poor pore connectivity. The permeability changed rapidly close to the percolation threshold, depending on the degree of demineralisation, and an exponential relation was observed between permeability and the amount of demineralisation. According to the observations, complete removal of the mineral phase can significantly increase the connected porosity while reducing the pore tortuosity, resulting in several orders of magnitude increase in coal permeability. This study shows that cleat demineralisation is an effective permeability enhancement technique for coalbed methane recovery, if very high demineralisation can be achieved.
Article
Gas diffusion and gas permeability tests were performed sequentially on powder and granular partially hydrated needle-punched geosynthetic clay liners (GCLs) over a range of gravimetric water content using a gas flow unified measurement system under 2 kPa and 20 kPa vertical stresses. Most of the changes in diffusion and advection occurred at intermediary levels of saturation or gravimetric water contents where diffusive and advective gas migration in the granular GCL tended to be higher than in the powder GCL. When the GCLs were relatively dry, their gas diffusion and gas permeability remained constant due to the large interconnected air voids present in the bentonites. For relatively wet conditions, the difference in their gas diffusion and gas permeability was minimal as the bentonites developed a relatively uniform gel structure. The results suggest that at a nominal overburden pressure of 20 kPa, GCLs such as the ones studied need to be hydrated to more than 160% gravimetric water content or >80% apparent degree of saturation before gas diffusion and permeability drop to 1.0×10⁻¹¹ m²/s and 2×10⁻¹³ m/s, respectively.
Article
A comprehensive understanding of the transport and fate of per- and poly-fluoroalkyl substances (PFAS) in the subsurface is critical for accurate risk assessments and design of effective remedial actions. A multi-process retention model is proposed to account for potential additional sources of retardation for PFAS transport in source zones. These include partitioning to the soil atmosphere, adsorption at air-water interfaces, partitioning to trapped organic liquids (NAPL), and adsorption at NAPL-water interfaces. An initial assessment of the relative magnitudes and significance of these retention processes was conducted for two PFAS of primary concern, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), and an example precursor (fluorotelomer alcohol, FTOH). The illustrative evaluation was conducted using measured porous-medium properties representative of a sandy vadose-zone soil. Data collected from the literature were used to determine measured or estimated values for the relevant distribution coefficients, which were in turn used to calculate retardation factors for the model system. The results showed that adsorption at the air-water interface was a primary source of retention for both PFOA and PFOS, contributing approximately 50% of total retention for the conditions employed. Adsorption to NAPL-water interfaces and partitioning to bulk NAPL were also shown to be significant sources of retention. NAPL partitioning was the predominant source of retention for FTOH, contributing ~98% of total retention. These results indicate that these additional processes may be, in some cases, significant sources of retention for subsurface transport of PFAS. The specific magnitudes and significance of the individual retention processes will depend upon the properties and conditions of the specific system of interest (e.g., PFAS constituent and concentration, porous medium, aqueous chemistry, fluid saturations, co-contaminants). In cases wherein these additional retention processes are significant, retardation of PFAS in source areas would likely be greater than what is typically estimated based on the standard assumption of solid-phase adsorption as the sole retention mechanism. This has significant ramifications for accurate determination of the migration potential and magnitude of mass flux to groundwater, as well as for calculations of contaminant mass residing in source zones. Both of which have critical implications for human-health risk assessments.
Article
Leachate from 27 landfills was analysed for nine perfluoroalkyl substances (PFASs). Five PFASs were detected ubiquitously, with perfluorohexanoate (PFHxA) the predominant PFAS (mean 1700 ng/L; range 73 − 25000 ng/L). Despite the complexity of landfill-specific factors, some general trends in PFAS concentrations were observed. Mean concentrations of eight PFASs were higher in operating landfills (or landfill cells) accepting primarily municipal waste, compared to closed municipal landfills. Landfills accepting primarily construction and demolition wastes produced leachate that had higher mean PFAS concentrations than municipal landfills. Younger landfills appeared to have a higher burden of waste containing PFASs (or their precursors), as significant relationships (p < 0.05) were observed between selected PFAS concentrations and landfill age. Increasing pH and total organic carbon (TOC) in leachate were associated with increased concentrations of several PFASs. Eight landfills discharged leachate to wastewater treatment plants (WWTPs). Estimated masses of PFASs discharged reached a maximum of 62 g annually (PFHxA), with a national estimate reaching 31 kg (PFHxA) annually. The practise of treating leachate at WWTPs allows redistribution of PFASs between the solid and liquid waste streams, although the contribution of leachate to the total load of PFASs entering WWTPs is minor compared to domestic waste water sources.
Article
To create more active sites on biochar surface, biochar-graphene (BG) nanosheet composites were prepared by one step facile dip coating following thermal route at three different temperatures, in addition to biochars (300, 500, 700°C). The morphology and structural composition of biochars and BG composites were examined by SEM, TEM, BET–N2 and CO2, Raman, FTIR, XPS, TGA and CHN elemental analysis. It was found that graphene nanosheets (GNS, ∼1µm, 0.1% mass) could ensure relatively higher surface area (N2 and CO2), porous structure and thermal stability within BG composites. BG composites portrayed the existence of GNS bearing cavities and evidently increased the graphitic structure. The adsorption capabilities of biochars and BG composites towards dimethyl phthalate (DMP), diethyl phthalate (DEP), and dibutyl phthalate (DBP) as model phthalic acid esters (PAEs) were examined by batch sorption technique. The BG composites exhibited the increased adsorption capacity comparatively to biochars. The pseudo-second-order kinetic rate (k2) illustrated that adsorption of PAEs was efficient and smaller molecule (DMP) was quicker to pore-diffusion mechanism. The aromatic sheets of biochars and GNS on biochars dominated the π–π EDA (electron donor–acceptor) interaction for ring structure of DMP molecule, whereas adsorption of DBP was attributed to hydrophobicity. The surface morphology and composition of biochars can be regulated with GNS to promote the adsorption capacity and kinetics for effective pollutant remediation and could be considered as promising adsorbent for various organic contaminants.
Article
In this study, a series of membrane/diffusion tests were conducted on specimens of a dense prehydrated geosynthetic clay liner (DPH GCL) subjected to KCl solutions (source concentration, Co = 8.7–160 mM) in rigid-wall cells. The source KCl solutions and de-ionized water (DIW) were circulated across the top and bottom specimen boundaries, respectively, and membrane efficiency coefficients, ω, were measured based on the differential pressures induced across the specimens due to prevention of liquid flux. Also, effective salt-diffusion coefficients, , and apparent tortuosity factors, τa, were determined for each specimen based on the steady-state diffusive Cl− flux measured at the exit (bottom) boundary. The DPH GCL specimens exhibited higher ω and lower (and, likewise, lower τa) relative to conventional (granular, non-prehydrated) GCL specimens tested under similar conditions. The results were consistent with the lower hydraulic conductivities, k, measured for the DPH GCL specimens and are attributed primarily to the higher bentonite dry densities in the DPH GCL specimens (1.1 Mg/m3) relative to the conventional GCL specimens (∼0.4 Mg/m3), although differences in bentonite texture (powdered versus granular), bentonite type (treated versus untreated), and testing apparatus (rigid wall versus flexible wall) may have been contributing factors. Both the DPH GCL and the conventional GCL exhibited similar trends of decreasing τa with increasing ω. The τa values are considered to be a function of both a matrix tortuosity factor, τm, that accounts for the geometry of the interconnected pores, and a restrictive tortuosity factor, τr, that accounts for solute exclusion due to membrane behavior. Whereas the DPH GCL exhibited a lower τm relative to the conventional GCL, both GCLs exhibited similar trends of decreasing τr with increasing ω. The relationship between τr and ω for both GCLs is reasonably well represented by τr = 1 – ω, an expression that has been proposed for clay membranes in previous theoretical and experimental studies.
Chapter
An approach is proposed for the treatment of equilibrium swelling of networks obtained by crosslinking polypropylene glycol (PPG) chains with a triisocyanate crosslinker, ‘Desmodur RF’ (DRF). Three models are used for the analysis of swelling of these polyurethane networks in toluene and methanol. It is shown that the χ parameter of the copolymer network depends on the interactions between the solvent and the individual components of the network and between these components themselves. Swelling data in methanol and toluene appear to yield the same value for the interaction parameter of the network components provided a) the differences in molecular surface area between the various building blocks of the systems and b) the molecular association of methanol are taken into account. Two networks, identical in number average chain length between crosslinks but differing in chain length distribution, have Identical enthalpic contributions to the effective interaction parameter but different entropic terms. The conclusions are affected neither by the choice of the model used for the front factors nor by possibly necessary corrections for the number of elastically active network chains. A simple solubility parameter treatment provides a fair prediction of swelling in mixtures of methanol and toluene.
Article
The sorption of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), and perfluoroheptanoic acid (PFHpA) on granular activated carbon (GAC) was characterized and compared to explore the underlying mechanisms. Sorption of the three perfluoroalkyl acids (PFAAs) on GAC appeared to be a rapid intra-particle diffusion process, which were well represented by the pseudo-second-order rate model with the sorption rate following the order PFOS > PFOA > PFHpA. Sorption isotherm data were well fitted by the Freundlich model with the sorption capacity (Kf) of PFOS, PFOA and PFHpA being 4.45, 2.42 and 1.66 respectively. This suggests that the hydrophilic head group on PFAAs, i.e. sulfonate vs carboxylic, has a strong influence on their sorption. Comparison between PFOA and PFHpA revealed that hydrophobicity could also play a role in the sorption of PFAAs on GAC when the fluorocarbon chain length is different. Analyses using Attenuated Total Reflection (ATR)-Fourier Transform Infrared (FTIR) spectroscopy suggested possible formation of a negative charge-assisted H-bond between PFAAs and the functionalities on GAC surfaces, including non-aromatic ketones, sulfides, and halogenated hydrocarbons.
Article
The ability of geosynthetic clay liners (GCLs) to contain acidic mining leachates is examined. The results of saturated hydraulic conductivity (k) of two GCLs permeated with sulfuric acid solutions (H2SO4) at 0.015 M, 0.125 M and 0.5 M concentrations are reported. Also, the saturated k values of consolidated (35 kPa) bentonite cakes made from sodium bentonite extracted from both GCLs were compared to a commonly used magnesium-sodium form bentonite. Chemical compatibility and effects of pre-hydration and effective stress were assessed as part of this study. Results indicated that an increased acid concentration (ionic strength) increased the k of all tested specimens. The ratio of the k0.5 values for non-prehydrated specimens permeated with 0.5 M H2SO4 to the kw values for specimens permeated with deionized (DI) water (k0.5/kw) ranged from 10 to 110. Pre-hydration (50-140% water content) and increased effective stress (35-200 kPa) improved the performance of GCLs (lower k). Strong correlations were observed between k and liquid limit and swell index parameters independent of pre-hydration and effective stress in this study. However, care should still be taken when using these correlations to evaluate hydraulic performance because the intrinsic micro-structure properties of bentonite, such as porosity, should also be considered. This work showed that, for example, high SI of bentonite does not translate necessarily to a better hydraulic performance of GCLs.
Article
Artifacts are commonly encountered in clinical CT and may obscure or simulate pathology. There are many different types of CT artifacts, including noise, beam hardening, scatter, pseudoenhancement, motion, cone-beam, helical, ring and metal artifacts. We review the cause and appearance of each type of artifact, correct some popular misconceptions and describe modern techniques for artifact reduction. Noise can be reduced using iterative reconstruction or by combining data from multiple scans. This enables lower radiation dose and higher resolution scans. Metal artifacts can also be reduced using iterative reconstruction, resulting in a more accurate diagnosis. Dual-and multi-energy (photon counting) CT can reduce beam hardening and provide better tissue contrast. Methods for reducing noise and out-of-field artifacts may enable ultra-high resolution limited field of view imaging of tumors and other structures.