Natural and Engineered Phenomena Impacting the Fate, Transport and Treatment of Environmental Contaminants

The effective implementation of in situ thermal treatment (ISTT) technologies requires understanding of gas production and migration in heterogenous media. However, investigations of the effects of high permeability contrast on gas formation, accumulation, and migration, as well as its potential effect on the redistribution of dense non-aqueous phase liquid (DNAPL), are relatively rare. In this study, electrical resistance heating (ERH) experiments were conducted in a thin sand-packed cell to simulate common yet not well-studied scenarios encountered during ISTT applications, such as coarse lenses surrounded by finer material. Two packing configurations were employed: 2 mm glass beads surrounded by 20/30 silica sand and 20/30 silica sand overlaying 40/50 silica sand. Each experiment contained an emplaced pool of trichloroethene (TCE) within the coarse material. If permeable material or pathways were present between the coarse lens and the upper cell boundary, the gas migrated along these pathways, and local DNAPL redistribution was limited to near the top of the pool before it vaporized. In contrast, if the coarse material was surrounded by finer material and contained a sufficient volume of DNAPL, the gas accumulated inside the coarse lens leading to DNAPL displacement from the lens. For five selected DNAPLs, this volume was estimated to be 0.1% to 0.5% of the total pore volume of the coarse material. The conceptual model developed in this study improves our understanding of this common geological scenario, demonstrating the importance of considering both lower- and higher-permeability material and their effects on multiphase flow during co-boiling, as well as the design of gas extraction systems during ISTT applications. Full article

Selenium is an essential micro-nutrient for living organisms, but elevated concentrations of it in water can adversely affect health. Nitrate is often found in selenium-contaminated water and negatively correlates with selenium removal. In this study, we investigate the effect of nitrate co-existence on selenium bioremediation in chemically modified zeolite columns. Dynamic sorption-reduction experiments were conducted using natural and iron-coated zeolite columns to remove selenite and selenate oxyanions separately, with and without nitrate anions. Anaerobic sludge was included as microbial inoculum, while lactate was the sole electron donor. The initial selenium concentration (Se^IV or Se^VI) was 790 µg/L, the nitrate concentration was 620 mg/L, the pH was 7.5, and the flow rate was 3 mL/min. Before introducing nitrate ions, selenium reduction in all four columns reached approximately 99%. However, after introducing nitrate ions, selenate and selenite reduction efficiencies were reduced to approximately 93% and 60%, respectively. Biofilm microbial community composition, assessed by 16S rRNA sequencing, was distinct between the communities with and without nitrate anions. Specifically, in the absence of nitrate, biofilm communities are mainly composed of selenium-reducing bacteria (Veillonella, Bacteroides and Escherichia). In contrast, the presence of nitrate led to mostly denitrifying bacteria (Anaeromusa-Anaeroarcus, Lentimicrobium, Azospirillum and Endomicrobium). Further, comparison of diversity indices (Shannon index, Faith PD and Pielou’s) shows alteration in all indices in the presence of nitrate. Full article

Selenium (Se) is an essential micro-nutrient for living organisms, but elevated concentrations in water can adversely affect health. In this research, we investigate the removal of selenium oxyanions (selenate and selenite) in aqueous systems by integration of adsorption on modified zeolites and microbial reduction. Dynamic sorption-reduction experiments were conducted using two sets of zeolite columns for the removal of selenite and selenate oxyanions, respectively. In each case, one column was fully packed with natural, unmodified zeolites, while the other column was composed of 80% natural and 20% iron-coated zeolites, by mass. The initial selenium concentration, selenite (Se^IV) or selenate (Se^VI), was 790 μg/L, the pH was 7.5, and the flow rate was 3 mL/min. Initially, as expected, the higher selenate removal (34%) was observed with coated zeolite, twice as high compared to the results with unmodified zeolite. Maximum selenite removal was 89% in the column with modified zeolite. Within approximately 14 days, as the biofilm developed inside the columns, selenium reduction in all four columns reached approximately 99%. Biofilm microbial community composition, assessed by 16S rRNA sequencing, is consistent with the presence of mainly selenium-reducing bacteria (Veillonella, Bacteroides, and Escherichia). Selenium oxyanions were reduced to elemental selenium, visible within the bioreactors as red-color aggregates. Full article

Among the most common amendments added to groundwater during site remediation are compounds used to adjust or maintain the pH. This research describes an approach to encapsulate mineral particles (MgO and CaCO₃) within oil droplets suspended within an aqueous phase for the purpose of delivery to the subsurface environment. A series of batch experiments was combined with mathematical modeling to illustrate the encapsulation and understand the influence of particle encapsulation on rates and extents of alkalinity release. The encapsulation of the alkalinity-releasing particles results in slower rates of amendment release as compared to rates obtained using suspensions of bare mineral particles, allowing for the possibility of control as a function of the pH. The results indicate that the alkalinity release from particle suspensions followed a mineral dissolution mechanism that could not explain the rate of the alkalinity release of the encapsulated particles. The reduction in mineral dissolution rates observed with the encapsulated particles was found to result from a mass transfer limitation. This limitation was well described using a linear driving force expression to account for the resistance to mass transfer at the oil–water interface. Full article

The “excess salt replication process” is a new simple method of fabrication of open-cell metal foam based on the commonly known salt replication method. Porous materials with porosity between 46% and 66% result when the employed alloy is 25% antimonial lead alloy and when it is 58% to 65% zamak 5. These foams are proposed as structured catalysts instead of packed beds in the treatment of wastewater. The local regimes influencing macroscopic air flow behaviour through these foams are delimited and boundaries are analysed in terms of sample length. Most of the experimental tests in this work exhibited a general trend of air flow in ESR foams dominated by the “strong inertia regime”. It was established that the law governing the unidirectional air flow through these foams was the full cubic law. The permeability and inertia coefficient of five samples with a cell diameter between 2.5 and 4.5 mm were calculated, and an empirical correlation was fitted. The irregular cuboid shape of salt grains used in the ESR foam was the origin of the special cell form of ESR foams leading to an anisotropic ordered porous media. This can explain the macroscopic turbulence of air flow because there were many dead zones present in the corner of each cubic cell, thus causing kinetic energy loss starting at earlier regimes. Full article

This study investigates the effect on varying flow rates and bubble sizes on gas–liquid flow through porous media in a horizontal microchannel. A simple bubble generation system was set up to generate bubbles with controllable sizes and frequencies, which directly flowed into microfluidic channels packed with different sizes of glass beads. Bubble flow was visualized using a high-speed camera and analyzed to obtain the change in liquid holdup. Pressure data were measured for estimation of hydraulic conductivity. The bubble displacement pattern in the porous media was viscous fingering based on capillary numbers and visual observation. Larger bubbles resulted in lower normalized frequency of the bubble breakthrough by 20 to 60 percent. Increasing the flow rate increased the change in apparent liquid holdup during bubble breakthrough. Larger bubbles and lower flow rate reduced the relative permeability of each channel by 50 to 57 percent and 30 to 64 percent, respectively. Full article

Treatment of nitrate rich groundwater using permeable reactive barriers (PRBs) established with injection of emulsified vegetable oil is receiving attention in areas where groundwater discharges contribute to eutrophication (e.g., Cape Cod, MA). To better understand the biogeochemical process kinetics when emulsified vegetable oil (EVO) is used to stimulate denitrification within the subsurface, microcosm experiments and process-based modeling were conducted for pH conditions ranging from 4 to 8. Biomass variability in soil and pH variations were found to affect denitrification, with limited nitrate reduction observed below pH 5.0. Different rates for denitrification associated with biomass variability suggest that a greater characterization of the indigenous biological community may improve PRB design and operation. Process-based modeling employed the activated sludge model No 3 (AMS3) framework that assumes denitrification as a two-step anoxic process dependent primarily on heterotrophic bacteria, soluble substrate, nitrate, and nitrite. Experimental data were used to calibrate the model under neutral to low pH, resulting in a robust set of equations that can be coupled with transport in future research to improve PRB effectiveness. Full article

The unsaturated zone of the subsurface plays a critical role in the environmental fate and transport of a wide range of environmental contaminants, and is also important for plant growth and agriculture. Quantitative prediction of processes in the unsaturated zone requires knowledge of how water content varies with elevation above the water table, a relationship known as the capillary pressure (P_c)–saturation (S) relationship. While the P_c–S relationship is conventionally thought of as being primarily a property of the porous medium and fluids, previous work found evidence suggesting that it actually results from a dynamic equilibrium between capillary forces and phenomena driven by evaporation. The focus of this work was on gaining a further understanding of the role of evaporation on the P_c–S relationship. The work made use of a tall instrumented laboratory column connected to an external reservoir for maintaining water table height. Following an initial imbibition experiment, the column was saturated and allowed to drain, and then water content was monitored in the column as a function of height over 1207 days (3.31 years). While initial imbibition and drainage were rapid, on the order of hours, redistribution and evaporation effects became apparent over longer time scales (hundreds of hours). A drying front moving downward in the column was apparent from changes in the slopes of saturation vs. time curves as it passed individual sensors; unlike previous experiments with a more fully-vented column, the evaporation front appeared to stall, balanced by the capillary-driven upward flow of water. Over the full multi-year duration of the experiment, seasonal trends in water saturation were apparent, with significant, reversible variations observed that closely followed atmospheric conditions. Specifically, saturation above the water table appeared to increase during the spring and summer months and decrease during the fall and winter months, despite the constant water table location, consistent with a changing driving force for evaporation. This result may suggest the likelihood of seasonal effects in the long-term transport and fate of contaminants in the unsaturated zone. Full article