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The antiviral drug ribavirin is widely used and continuously released into the environment through various pathways, which will cause serious water pollution and virus resistance. However, only few works have examined ribavirin retention and transport groundwater environment. This study investigates the transport behavior of ribavirin in saturated quartz sand and limestone porous media, considering ionic strength (IS), ionic type and humic acid (HA), through column experiments and mathematical modeling. The findings reveal that, ribavirin exhibits strong mobility within saturated quartz sand media is less affected by different hydrochemical factors, posing a high risk of migration. In limestone porous media, the migration of ribavirin decreased. In comparison to Na+, the presence of Ca2+ causes the effluent recovery rate of ribavirin decreases. And as the IS of Ca2+ background solution increases, the effluent recovery rate of ribavirin also decreases. The presence of HA causes more ribavirin to flow out of the column, and its effect becomes more pronounced with increasing concentration. Furthermore, simulated precipitation scenarios, the ribavirin adsorbed on the of media surface is released again and transport with the water flow. These findings enhance our understanding about the environmental fate of ribavirin and facilitating ecological restoration efforts. [ABSTRACT FROM AUTHOR]

A method is established to generate pore networks within domains of arbitrary shape, as long as the domain can be mathematically described by a set of inequalities. In this method, a stochastic network algorithm is adopted to construct pore network skeletons, which are then cut into the desired shapes using a new pore network cutting algorithm. The latter can be embedded into other methods to transplant its ‘pore network cutting’ function. Using this method, pore networks with four archetypical two-dimensional shapes (namely, cross-sections of one-holed rings, trilobes, four-holed rings, and wheels) and four three-dimensional shapes (namely, spheres, cylinders, trilobes, and hollow cylinders) are constructed as examples. Then, some of these pore networks are applied to simulate diffusion and reaction in Pd/γ-alumina catalyst particles for hydrogenation of benzene to cyclohexane. It is shown that the randomness of the pore network and the external particle shape significantly affect the performance of catalysts, because of their impact on effective diffusivity and diffusion length, respectively, indicating that this structural information must be accounted for to achieve a model with high accuracy. The versatile method proposed in this article is ideal to study the effect of particle shape and pore network structure on the performance of porous materials for catalysis and other applications. [ABSTRACT FROM AUTHOR]

Dispersal of functional microorganisms is a rate-limiting process during in situ bioremediation of contaminated soil and groundwater. In this work, series of column experiments were conducted to investigate the retention and transport behaviors of Herbaspirillum chlorophenolicum FA1, a promising bacterial agent for bioremediation, in saturated porous media under conditions of different combinations of grain size, solution pH, solution ionic strength (IS), and humic acid (HA) concentration. Experimental data showed that the mobility of FA1 in saturated porous media was strongly dependent on the physicochemical conditions. The breakthrough curves (BTCs) indicated that the amounts of FA1 in the effluent increased with increasing in sand size, solution pH, and HA concentration, but decreased with increase of solution IS. The shape of retention profiles (RPs) was hyper-exponential. The amounts of retained bacteria in the media also varied with the experimental conditions with opposite trends to that of effluent. Both experimental BTCs and RPs were simulated by a mathematical model that accounted for deposition kinetics to better interpret the effects of physicochemical conditions on FA1 deposition dynamics. Findings from this study showed that fate and transport of the functional bacterium FA1 in porous media strongly relied on the environmental conditions. Both experimental and modeling results can provide guidelines for field application of functional bacteria for soil and groundwater remediation. [ABSTRACT FROM AUTHOR]

Accurately predicting the fate and transport of graphene oxide (GO) in porous media is critical to assess its environmental impact. In this work, sand column experiments were conducted to determine the effect of input concentration and grain size on transport, retention, and size perturbation of GO in saturated porous media. The mobility of GO in the sand columns reduced with decreasing grain size and almost all GO were retained in fine sand columns for all of the tested conditions. This result can be explained with colloid filtration and XDLVO theories. Input concentration also influenced the retention and transport of GO in the sand columns because of the ‘blocking’ mechanism that reduces the particle retention rate. After passing through the column, average GO sizes increased dramatically. In addition, the sizes of GO retained in the sand also increased with travel distance. These results suggested that transport through the porous media induced GO aggregation. A mathematical model based on the advection–dispersion equation coupled with the second-order kinetics to reflect the blocking effect simulated the experimental data well. [ABSTRACT FROM AUTHOR]

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 types 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. Image 1 • Lower moisture content result in higher air-water interfacial adsorption of PFOA. • Larger surface roughness lead to greater retardation of PFOA in unsaturated media. • Rougher surface cause larger air-water interfacial areas and more PFOA adsorption. [ABSTRACT FROM AUTHOR]

Predicting the cotransport of functional microorganisms and heavy metals in porous media is essential to both bioremediation and pollutant risk assessment. In this study, batch and column experiments were conducted to explore the cotransport behaviors of functional bacteria (FA1) and heavy metals (Pb2+/Cd2+) in saturated sand media under different conditions. The sorption capacity of heavy metals on FA1 was much greater than that of the sand, while both FA1 and sand showed stronger affinity to Pb2+ than Cd2+. The surface properties, especially zeta potential, of the bacteria and sand were altered by metal adsorption. As a result, the co-existence of Pb2+ decreased the transport of FA1 more significantly than that of Cd2+, and the influence was more significant with higher heavy metal concentration. On the other hand, the co-existence of FA1 inhibited the mobility of Pb2+ and Cd2+ in most scenarios, except when the cotransport concentration of Pb2+ was 5 mg L−1, and the inhibition was more pronounced for Pb2+ than Cd2+. Increase in metal concentrations decreased the FA1-associated Pb2+/Cd2+ in effluents due to the remarkable decrease in FA1 mobility, and free soluble Pb2+/Cd2+ became the major migration species. In addition, due to stronger attractive forces and affinity between Pb2+ and FA1, nearly all presorbed-Pb2+ by sand was remobilized by FA1 and transported mainly in FA1-associated form other than soluble Pb2+. Findings from this study indicated that the cotransport of biocolloids and heavy metals are highly sensitive to the ion type and concentration, and evaluation of their transport in the subsurface should be carefully carried out to avoid inaccurate estimations. Image 1 • The mobility of FA1 decreased with increasing Pb2+/Cd2+ concentration. • Co-existence of Pb2+ decreased bacteria FA1 mobility more significantly than Cd2+. • The migration of Pb2+ and Cd2+ was mostly impeded by bacteria FA1 in sand columns. • FA1 can remobilize pre-adsorbed Pb2+ from the quartz sand surface. [ABSTRACT FROM AUTHOR]

Endocrine disrupter, like bisphenol A (BPA) and bisphenol S (BPS), is frequently detected in the subsurface environment, imposing threats to the groundwater quality and public health. However, current understanding of environmental fate and transport of BPA/BPS is still not clear, especially with respect to those in the subsurface media with trapped non-aqueous phase liquids (NAPLs). In this study, the effect of residual NAPLs on the retention and transport of BPA/BPS in the saturated sand and soil media was investigated using column experiments. The results showed that residual NAPLs (i.e., xylene and perchloroethylene (PCE)) inhibited the transport of BPA in the sand columns, and the inhibit effect of xylene was greater than that of PCE. While the presence of NAPLs showed little influence on the transport of BPS in the sand columns, in soil A (sandy soil), the residual NAPLs had similar effect on the retention and transport of BPA and BPS. Both BPA and BPS showed higher retention in the soil B columns than in the sand and soil A columns due to soil B's higher organic matter contents, which can strongly sorb BPA/BPS. The presence of residual NAPLs reduced the retention of BPA/BPS in the soil B columns because the NAPLs blocked the sorption sites of the soil organic matter for BPA/BPS. This study demonstrates the importance of residual NAPLs, types of media, properties of contaminants, and their interactions to the fate and transport of BPA/BPS in saturated porous media. [ABSTRACT FROM AUTHOR]

Pentadiamond (PD), a recently proposed carbon allotrope, has garnered considerable attention owing to its unique covalent framework comprising both sp2 and sp3 carbon atoms. Despite its intriguing structure with potentially useful thermal, electronic and optical properties, PD is supposed to be thermodynamically very unstable, due to the strain imposed by adjacent pentagonal rings in the carbon framework, which poses a major concern for its practical production. Here, we employ first-principles simulations to demonstrate that inserting alkali or alkaline-earth metals into the bigger cavities of PD significantly enhances its thermodynamic stability. Moreover, the resulting doped materials exhibit high kinetic stability at high temperatures typical for synthesis of fullerenes and metallofullerenes. We systematically investigate the structures and the modulation of mechanical properties and electronic structures of metal-doped PDs. Most remarkably, beryllium is revealed to be the energetically most favorable dopant among the metals considered. Be-doped PD exhibits an increased shear and Young's moduli (by about 55.6 % and 44.4 %, respectively), along with a substantially reduced Poisson's ratio (from 0.23 to 0.14). Furthermore, doping with Be transforms PD from an indirect-gap material to a direct-gap semiconductor, narrowing the band gap from 3.58 eV to 2.67 eV. We hope these findings may stimulate the experimental endeavors to synthesize metal-doped PD materials and unlock their potential applications. [Display omitted] • Doping with alkali or alkaline-earth metals improves the stability of pentadiamond and makes its synthesis more viable • Beryllium-doped pentadiamond shows remarkably enhanced mechanical properties with a Poisson's ratio of only 0.14 • Doping with most of the alkali or alkaline-earth metals converts pentadiamond from a semiconductor to a conductor • Doping pentadiamond with Be not only changes the band gap from indirect to direct but reduces significantly the gap width [ABSTRACT FROM AUTHOR]

The release of bisphenols such as bisphenol A (BPA) and its alternative bisphenol S (BPS) into the subsurface environment may cause serious pollutions to soil and groundwater. However, only few works have examined their fate and transport in porous media. In this work, batch and column experiments and mathematical modeling were conducted to study the transport behaviors of BPA and BPS in water-saturated limestone porous media. The effects of contaminant input concentration, solution ion type, and solution ionic strength on the retention and transport of BPA and BPS in the columns were investigated. BPS had higher mobility in limestone porous media than that of BPA. With its input concentration decreased, BPA showed lower mobility, while the transport of BPS in the media was not affected by the input concentration perturbations. The retention of both BPA and BPS was higher in divalent calcium ion solution than that in monovalent sodium solution in limestone porous media. Ionic strength showed little effect on the retention and transport of BPA and BPS except that high concentration of Ca2+ inhibited the migration of BPS in the media. Because of its relatively high mobility and toxicity, BPS may present a great risk to groundwater quality and thus may not be an environmentally friendly bisphenol alternative. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • LTV technology was effective in visualizing GO transport in the 2-D porous media. • Preferential flow affected GO transport in 2-D heterogeneous porous media. • GO spread vertically through dispersion in 2-D porous media. • Double Monod model described GO transport and retention very well. Abstract Graphene oxide (GO) has been indicated to be biotoxic and risky in environment, its environmental behavior thus has received increasing attention in recent. In this study, homogeneous and heterogeneous sand tanks were used to examine the transport behaviors of GO nanoparticles in two-dimensional (2-D) porous media under various conditions. Light transmission visualization (LTV) technology was applied to visualize the real-time transport, retention, and release of GO. GO transport in 2-D porous media was simulated with a simplified Double Monod model. GO mobility decreased with the increasing solution ionic strength (IS) and decreasing media grain size. Preferential flow played an important role in GO transport in 2-D heterogeneous porous media. Even without vertical flow in the sand tanks, GO still spread vertically through dispersion, suggesting the importance of the dispersion process to nanoparticle fate and transport in 2-D porous media. LTV images and breakthrough curves showed that some of the previous retained GO particles were instantaneously remobilized with IS decreasing. With the consideration of the vertical dispersion, simulations of the Double Monod model matched the experimental data well. Findings from this work contribute to expand current knowledge of environmental fate and transport of GO, leading to better assessment and prediction of its environmental risks. [ABSTRACT FROM AUTHOR]

• A model for PFAS transport in unsaturated porous media is developed. • A second-order accurate implicit finite volume method is proposed. • The effects of the numerical methods on the simulation results are revealed. • The effects of parameters relevant to the properties of porous media as well as the relation between the surface tension are explored. Per- and polyfluoroalkyl substances (PFASs) have become emerging contaminants of critical concern. Comprehensive understanding of the transport and fate of PFAS in the vadose-zone, a type of water-unsaturated porous media, is key to determination of the risks of the PFAS contamination in the subsurface and to the development of the effective remediation strategies. Accurate modeling of the PFAS transport in the unsaturated porous media is still a challenge due to the variable surface tension induced by the adsorption of PFAS to the air-water interfaces. In an effort to address this challenge, we propose a multidimensional modeling framework for the transient PFAS transport in the unsaturated porous media based on the second order accuracy finite volume method. In the modeling, the adsorption of PFAS to the solid surfaces and to the air-water interfaces is described by the two-domain sorption kinetics model, i.e., both the instantaneous and the rate limited PFAS adsorptions are taken into account. The diffusive and convective terms in the governing equations for the PFAS transport and the water flow are discretized by the central difference and the quadratic upstream interpolation for convective kinetics schemes, respectively. We investigate the effects of the convergent criteria, coupling method, and variation of the surface tension on the average and the local PFAS concentration and water content in the computational domain. We find that the convergent criteria should be chosen carefully so as to get the accurate results. The differences between the different coupling methods are affected by the boundary conditions. The variation in the surface tension due to the variation of the PFAS concentration cannot be neglected. The dimensionless parameters relevant to the properties of porous media as well as the relation between the surface tension and the PFAS concentration play an important role in transport of PFAS in the unsaturated porous media. The effects of the Péclet number, Damköhler values, and fraction of instantaneous sorption are not significant in the ranges studied in this work. These findings provide a better understanding of transport of PFAS in the vadose zone. [ABSTRACT FROM AUTHOR]

In this work, effects of graphene oxide (GO) on the co-transport of the two typical Fluoroquinolones (FQs) - levofloxacin (LEV) and ciprofloxacin (CIP) in saturated and unsaturated quartz sand media were studied. The adsorption isotherms showed that GO had much larger sorption capacities to LEV and CIP than sand with the largest Langmuir adsorption capacity of 409 mg g −1 (CIP-GO); while the sorption affinity of the two FQs onto the two adsorbents might follow the order of CIP-sand > LEV-sand > LEV-GO > CIP-GO. GO promoted the mobility of the two FQs in both saturated and unsaturated porous media due to its strong mobility and sorption capacity. The GO-bound LEV/CIP was responsible for the LEV/CIP transport in the porous media, and transport of GO-bound FQs increased with the increasing of initial GO concentration. Under unsaturated conditions, moisture showed little effect on the transport of GO-bound CIP; however, the mobility of GO-bound LEV reduced with the decreasing of moisture content, suggesting the transport of adsorbed LEV from GO to air-water interface. GO sorption reduced the antibacterial ability of the two FQs, but they were still effective in inhibiting E. coli growth. [ABSTRACT FROM AUTHOR]

Understanding the fate and transport of antibiotics in porous media can help reduce their contamination risks to soil and groundwater systems. In this work, batch and column experiments were conducted to determine the interactions between two representative antibiotics, sulfacetamide (SA) and levofloxacin (LEV), and sand porous media under various solution pH, humic acid (HA) concentration, grain size, and moisture content conditions. Batch sorption experimental results indicated that the sand had relatively strong bonding affinity to LEV, but little sorption of SA under different pH, HA concentration, grain size conditions. Results from the packed sand column experiments showed that SA had extremely high mobility in the porous media for all combinations of pH, HA concentration, grain size, and moisture content. The mass recovery of SA was higher than 98.5% in all the columns with the exception of the one packed with fine sand (97.2%). The retention of LEV in the columns was much higher and the recovery rates ranged from 0% to 71.1%. Decreases in solution pH, HA concentration, grain size, or moisture content reduced the mobility of LEV in the columns under the tested conditions. These results indicated that type of antibiotics and environmental conditions also played an important role in controlling their fate and transport in porous media. Mathematical models were applied to simulate and interpret experimental data, and model simulations described the interactions between the two antibiotics and sand porous media very well. Findings from this study elucidated the key factors and processes controlling the fate of SA and LEV in porous media, which can inform the prediction and assessment of the environmental risks of antibiotics. [ABSTRACT FROM AUTHOR]

Accurately predicting the fate and transport of nano-TiO 2 in porous media is critical to assess its environmental impact. This study was designed to understand the effects of gain size and structural heterogeneity under different ionic strength (IS) on the fate and transport of nano-TiO 2 in saturated porous media. In the columns packed homogenously with sand of different grain sizes (920, 550, 390, and 275 μm), the transport of nano-TiO 2 decreased when the IS increased from 0.1 to 1 or 10 mM. For all the three IS conditions, the retention of the nano-TiO 2 particles in the columns increased when the gain size decreased, and the mobility of the nano-TiO 2 was the lowest in the sand at size of 275 μm with recovery rates of 0.30% to 1.72%. The mass recovery rates of TiO 2 in other homogeneous columns were higher and ranged from 0.37% to 59.9%. Structural heterogeneity created two flow domains for the retention and transport of nano-TiO 2 particles in the saturated porous media. The fast-flow domain dominated the flow and transport processes of the nano-TiO 2 in the heterogeneous columns under the tested conditions. As a result, the transport of nano-TiO 2 in the heterogeneous porous media was faster and higher than that in the homogeneous columns under similar experimental conditions. Because of the dominance of the fast-flow domain, the recovery rates of the nano-TiO 2 in the heterogeneous columns were similar and ranged from 59.8% to 66.9%. These results reflected the importance of preferential flow to the fate and transport of nano-TiO 2 particle in porous media. Simulations from a two-domain model matched the experimental breakthrough curves very well. [ABSTRACT FROM AUTHOR]

High efficiency and cost-effective remediation technologies are needed to remove tetrachloroethylene (PCE) from the source zones in soil and groundwater systems. This work evaluated the ability of a biodegradable surfactant (Tween 80) coupled with an effective oxidant (Fe 2+ (activation agent)–citrate (chelating agent) activated persulfate (PS, oxidant)) to remove PCE from porous media. Batch experiments were used to optimize the oxidant by testing the effects of PS, citrate, and Fe 2+ concentrations on the PCE degradation. The optimal molar ratio of PS/citrate/Fe 2+ /PCE was 15:15:5:1 for PCE degradation in presence of the surfactant. Four sandbox experiments were performed to compare the effectiveness of the surfactant and the combination of the surfactant with the optimized oxidant in the removal of PCE from homogeneous and heterogeneous porous media. PCE saturation distributions in the sandboxes were quantified using a light transmission system. In addition, the PCE, PS, and Cl − concentrations in the effluents were also monitored during the experiments. While the Tween 80 solution removed about 52% and 67% of PCE from the homogeneous and heterogeneous porous media; the combination solution was more effective with removal rates of 80% and 84%, respectively. The enhanced PCE removal was due to the combined effects of the surfactant solubilization and PS oxidation: (1) Tween 80-enhanced solubilization resulted in an enlarged effective contact and reaction between PCE and PS; and (2) PS degraded the PCE from the soluble phase and thus facilitated its dissolution from the source zone, resulting in more efficient use of Tween 80. Findings from this work indicate that the combination of surfactant and oxidant can provide an effective way for the remediation of PCE in soils and groundwater. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stable CdS and Pt NPs were highly dispersed in mesopore channel of mesoporous Beta. [•] The novel catalyst was obtained by a two-step pore modification process. [•] The catalyst shows an excellent catalytic activity and stability for H2 evolution. [•] The mesoporous Beta as a rigid framework can effectively protect CdS from photocorrosion. [Copyright &y& Elsevier]

Dissolved surface active agents such as surfactant and natural organic matter can affect the distribution and fate of dense nonaqueous liquids (DNAPLs) in soil and groundwater systems. This work investigated how two common groundwater surface active agents, humic acid (HA) and Tween 80, affected tetrachloroethylene (PCE) migration and source zone architecture in saturated porous media under environmentally relevant conditions. Batch experiments were first conducted to measure the contact angles and interfacial tensions (IFT) between PCE and quartz surface in water containing different amount of surface active agents. Results showed that the contact angle increased and IFT decreased with concentration of surface active agent increasing, and Tween 80 was much more effective than HA. Five 2-D flow cell experiments were then conducted. Correspondingly, Tween 80 showed strong effects on the migration and distribution of PCE in the porous media due to its ability to change the medium wettability from water-wet into intermediate/NAPL-wet. The downward migration velocities of the PCE in three Tween 80 cells were slower than those in the other two cells. In addition, the final saturation of the PCE in the cells containing surface active agents was higher than that in the water-only cell. Results from this work indicate that the presence of surface active agents in groundwater may strongly affect the fate and distribution of DNAPL through altering porous medium wettability. [ABSTRACT FROM AUTHOR]

Knowledge of the fate and transport of graphene (GR) nanosheets in porous media is essential to understand their environmental impacts. In this work, sand column experiments were conducted to investigate the retention and transport of surfactant-dispersed GR nanoparticles under various conditions. An anionic surfactant, sodium dodecylbenzene sulfonate (SDBS) and a cationic surfactant, cetyltrimethylammonium bromide (CTAB) were used to disperse and stabilize GR in aqueous solutions. Both surfactants were effective in stabilizing the GR particles, even at low concentration (0.004% w:v) because the surfactant coating introduced negative (SDBS) or positive (CTAB) charges on the GR surfaces. As a result, the SDBS- and CTAB-GR showed different retention and transport behaviors in the saturated porous media. At low surfactant concentration, the transport of SDBS-GR was much higher than that of the CTAB-GR, which was almost immobile in the sand columns with mass recovery rate only about 4%. When the surfactant concentration increased from 0.004% to 0.4%, it reduced the transport of SDBS-GR and dramatically enhanced the mobility of the CTAB-GR (with mass recovery rate of 91%). It is suggested that the presence of ‘free’ SDBS ions may reduce the electrosteric repulsions between SDBS-GR and sand surfaces by compressing the electrical double layer. The ‘free’ CTAB ions, however, may compete with the CTAB-GR for adsorption sites on the sand surfaces. Findings from this study indicated that the dispersion method plays an important role in affecting the environmental fate and transport of GR particles. [ABSTRACT FROM AUTHOR]

[Display omitted] • SDBS inhibited PFOA retardation during transport in unsaturated sand. • PFOA had a marginal effect on SDBS transport in unsaturated sand. • SDBS with greater surface activity than PFOA dominated mixture surface activity. • SDBS outcompeted PFOA for the limited air–water interfacial areas. Per– and polyfluoroalkyl substances (PFAS) contamination is ubiquitous and likely co–exist with hydrocarbon surfactants in the subsurface environment. The transport of PFAS in unsaturated porous media (quartz sand–packed columns) with an anionic hydrocarbon surfactant was investigated using perfluorooctanoic acid (PFOA) and sodium dodecyl benzene sulfonate (SDBS) as representative PFAS and hydrocarbon surfactant at 1.5 and 30 mM NaCl, respectively. Our results show that SDBS inhibited the retardation of PFOA during transport. The degree of the inhibited retardation was more significant at higher concentration of SDBS (C SDBS) for a given concentration of PFOA (C PFOA) and higher ionic strength. In contrast, PFOA only marginally affected the transport of SDBS under all tested conditions. This is mainly due to the greater surface activity of SDBS with respect to PFOA, favoring the competitive preferential adsorption of SDBS at the air–water interface and then the transport of PFOA in the unsaturated sand. This assumption is further supported by the greater air–water adsorption coefficient (K ai) and surface excess (Γ) for SDBS than those for PFOA, as well as the surface tension isotherms that fairly weighted toward to SDBS in the binary PFOA–SDBS system. Our findings indicate the critical role of anionic hydrocarbon surfactant in retarding PFOA transport in unsaturated zone. This should be considered when assessing the transport potential and environmental risk, and developing remedial strategies of PFAS in the subsurface environment. [ABSTRACT FROM AUTHOR]

Highlights: [•] Enteromorpha prolifera-based activated carbon was activated by KOH and H4P2O7. [•] The properties of carbons prepared by H4P2O7 and KOH were compared. [•] The activated carbon with high surface area of 3500m2/g was prepared by KOH. [•] The prepared carbons exhibited significant adsorption capacity for GR. [ABSTRACT FROM AUTHOR]

Root exudates are a major source of dissolved organic matters that strongly affect the stability and transport behaviors of nanomaterials in porous media. This study investigated the effect of citric acid (CA) and oxalic acid (OA), two common low molecular weight root exudates, on the stability and transport of graphene oxide (GO) in saturated sand columns under different combinations of pH (4.5, 7.0), ionic strength (IS: 10, 50 mM), and organic acid concentrations (10, 25 mM). Both OA and CA accelerated GO aggregation, especially under high IS and acid concentration conditions. With the presence of OA/CA (≥ 10 mM), the transport of GO was higher at pH of 7.0 than 4.5, and the GO mobility decreased with increasing IS and OA/CA concentrations, whereas, enhanced GO transport was observed at a low concentration of OA/CA (0.1 mM), indicating that the influence of organic acid was concentration-dependent. All the results suggest that perturbations of surface potential of GO and sand, as well as the chemical structure of organic acids under different solution chemistry conditions are crucial in controlling GO stability and transport behaviors. Mathematical models based on the advection-dispersion equation with one-site kinetics simulated the experimental breakthrough curves of GO very well. [Display omitted] • Organic acid type and concentration strongly affected GO stability and mobility. • Enhancement or inhibition of OA/CA on GO mobility was concentration-dependent. • GO aggregation and retention increased with increasing OA/CA concentration (≥ 10 mM). • Inhibition on GO mobility by OA/CA was more profound at lower pH, higher IS. • Mathematical models simulated the experimental breakthrough curves of GO very well. [ABSTRACT FROM AUTHOR]

• Increasing ionic strength promoted PFOA retardation in unsaturated porous media. • Ca2+ caused greater PFOA retardation in unsaturated porous media than Na+. • Increasing ionic strength promoted PFOA adsorption on the air-water interface. • Ca2+ promoted more PFOA adsorption on the air-water interface than Na+. Current understanding of perfluorooctanoic acid (PFOA) transport in unsaturated porous media is still limited with significant variability in solution chemistry. Column experiments were conducted to systematically evaluate the impacts of ionic strength (1.5−30 mM) and cation type (Na+ and Ca2+) on PFOA transport in unsaturated quartz sand. The results showed that an increase in ionic strength (1.5−30 mM) led to greater PFOA retardation in unsaturated columns. Meanwhile, Ca2+ caused more PFOA retardation than Na+ at the same unsaturated conditions. These findings were supported by bubble column experiments, which indicated greater PFOA adsorption at the air-water interface with increasing ionic strength or in the presence of Ca2+ in comparison to Na+. Furthermore, the air-water interfacial (AWI) adsorption coefficients calculated from surface tension isotherms also increased with increasing ionic strength or in the presence of Ca2+ in comparison to Na+. These results clearly confirm that higher ionic strength or cation valence significantly promoted PFOA adsorption at the air-water interface, and thus caused greater PFOA retardation during transport in unsaturated porous media. This work points out the importance of considering solution ionic strength and cation type in assessing the transport behavior of PFOA in unsaturated porous media. [ABSTRACT FROM AUTHOR]

• A novel model of nanoparticle transport and release in layered media was proposed. • Heterogeneity results in remarkable concentration gradient along transverse direction. • Mass interaction of two layers was determined by dispersion across interface. The rapid increase in production and application of nanoparticles (NPs) across a wide range of commercial and industrial processes has resulted in the extensive invasion of NPs into subsurface environments. Accurately predicting the transport and release of NPs in subsurface environments is important for assessing a wide range of environmental and human health risks. In this study, a two-dimensional NP transport and release model was proposed to investigate the behavior of NPs in a layered heterogeneous medium with transient ionic strength (IS). The layered heterogeneous porous medium comprised a slow flow domain (SFD) and a fast flow domain (FFD). The transport of NPs in the two layers was described by two coupled advection–dispersion equations (ADEs). The NP deposition and release were related to the IS of the electrolyte solution (ES) in the medium and were represented by reversible and irreversible deposition terms in the ADEs. The results indicated that heterogeneity has a significant impact on the transport and release of NPs. When NPs pass through the two‐layer medium, initially part of the NPs in the FFD will migrate into the SFD. A larger amount of NPs are deposited in the SFD than the FFD owing to the higher filtration capacity of the SFD. The retained NPs in both the SFD and FFD will be released when the IS declines. The release of the retained NPs in the FFD is faster than that in the SFD because the FFD has a higher intrinsic release rate and lower IS owing to the more rapid transport of ES in the FFD. The proposed model was applied to laboratory experiments and compared to the double-region model. The proposed model fitted the experimental breakthrough curves better than the double-region model, and it is capable of fitting the release of NPs, which cannot be accomplished with the double-region model. [ABSTRACT FROM AUTHOR]

Understanding subsurface transport of per- and polyfluoroalkyl substances (PFASs) is of critical importance for the benign use and risk management of PFASs. As one of the most commonly found PFASs, perfluorooctanoic acid (PFOA) is used as a representative PFAS and water-saturated column experiments were conducted to investigate the effect of Al/Fe oxyhydroxide coating and ionic strength on its transport at an environmentally relevant PFOA concentration (6.8 μg L−1). Our results showed a clear increase in PFOA retardation in Al/Fe oxyhydroxide coated sand (retardation factor: Al: 1.87–5.58, Fe: 1.28–4.05) than those in uncoated sand (1.00–1.05), due to the stronger electrostatic attraction between anionic PFOA and Al/Fe oxyhydroxide coated sand surface. Notably, Al oxyhydroxide have a more profound effect on PFOA retention and retardation than Fe oxyhydroxide. Besides, higher ionic strength significantly inhabited PFOA retention and retardation in positively charged sand, and the considerable retention of PFOA (∼90%) in deionized water than those in 1.5 mM and 30.0 mM NaCl (<10%) clearly proves the role of competitive adsorption of Cl− on PFOA transport in positively charged sand. In contrast, higher ionic strength (0 mM–30 mM NaCl) slightly increased PFOA retardation in negatively charged sand, illustrating the dominance of electrostatic interaction. Our findings advance current knowledge to understand PFOA transport in natural media with different surface charge property under environmental PFOA concentrations. Image 1 • Al/Fe oxyhydroxide coating on sand strongly increased PFOA retardation. • Al oxyhydroxide affected PFOA retardation more than Fe oxyhydroxide. • Competitive adsorption of Cl− affected PFOA transport in positively charged sand. • Electrostatic interaction controlled PFOA transport in negatively charged sand. [ABSTRACT FROM AUTHOR]

• Retention and release of bacteria in saturated limestone porous media were studied. • Common and unexpected behaviors were observed through various column experiments. • Characteristics of limestone affected bacteria retention and release greatly. • Perturbations of flow rate caused no release of previously retained bacteria. • DLVO theory and non-traditional mechanisms were involved. Laboratory saturated columns packed with granular limestone grains were used to explore the retention and remobilization of functional bacteria FA1 under various physicochemical conditions. The unique surface properties of limestone and FA1 caused some unexpected phenomena. Solution IS, cation type, temperature and surface biological property all affected FA1 retention in the columns. The IS effect was temperature dependent and initial solution pH showed little influence due to the strong buffering ability of limestone. Perturbations of solution IS caused slight release of previously retained bacteria in some columns with NaCl as the background electrolyte, while increase in flow rate caused no release at all. When CaCl 2 was the background, bacterial remobilization only occurred following both cation exchange and IS reduction. DLVO forces incorporating with surface roughness calculation were determined to assist with interpretation of interaction mechanisms. All the experimental evidences suggest the importance of cation bridging, cation exchange, surface roughness, and hydrophobic interaction in controlling bacterium transport in saturated limestone porous media. [ABSTRACT FROM AUTHOR]