Two-dimensional (2D) pore-scale models have successfully simulated microfluidic experiments of aqueous-phase flow with mixing-controlled reactions in devices with small aperture. A standard 2D model is not generally appropriate when the presence of mineral precipitate or biomass creates complex and irregular three-dimensional (3D) pore geometries. We modify the 2D lattice Boltzmann method (LBM) to incorporate viscous drag from the top and bottom microfluidic device (micromodel) surfaces, typically excluded in a 2D model. Viscous drag from these surfaces can be approximated by uniformly scaling a steady-state 2D velocity field at low Reynolds number. We demonstrate increased accuracy by approximating the viscous drag with an analytically-derived body force which assumes a local parabolic velocity profile across the micromodel depth. Accuracy of the generated 2D velocity field and simulation permeability have not been evaluated in geometries with variable aperture. We obtain permeabilities within approximately 10% error and accurate streamlines from the proposed 2D method relative to results obtained from 3D simulations. In addition, the proposed method requires a CPU run time approximately 40 times less than a standard 3D method, representing a significant computational benefit for permeability calculations.
Salt used to make brines for regeneration of ion exchange (IX) resins is the dominant economic and environmental liability of IX treatment systems for nitrate-contaminated drinking water sources. To reduce salt usage, the applicability and environmental benefits of using a catalytic reduction technology to treat nitrate in spent IX brines and enable their reuse for IX resin regeneration were evaluated. Hybrid IX/catalyst systems were designed and life cycle assessment of process consumables are used to set performance targets for the catalyst reactor. Nitrate reduction was measured in a typical spent brine (i.e., 5000 mg/L NO3−">NO3− and 70,000 mg/L NaCl) using bimetallic Pd–In hydrogenation catalysts with variable Pd (0.2–2.5 wt%) and In (0.0125–0.25 wt%) loadings on pelletized activated carbon support (Pd–In/C). The highest activity of 50 mgNO3−">NO3−/(min − gPd) was obtained with a 0.5 wt%Pd–0.1 wt%In/C catalyst. Catalyst longevity was demonstrated by observing no decrease in catalyst activity over more than 60 days in a packed-bed reactor. Based on catalyst activity measured in batch and packed-bed reactors, environmental impacts of hybrid IX/catalyst systems were evaluated for both sequencing-batch and continuous-flow packed-bed reactor designs and environmental impacts of the sequencing-batch hybrid system were found to be 38–81% of those of conventional IX. Major environmental impact contributors other than salt consumption include Pd metal, hydrogen (electron donor), and carbon dioxide (pH buffer). Sensitivity of environmental impacts of the sequencing-batch hybrid reactor system to sulfate and bicarbonate anions indicate the hybrid system is more sustainable than conventional IX when influent water contains <80 mg/L sulfate (at any bicarbonate level up to 100 mg/L) or <20 mg/L bicarbonate (at any sulfate level up to 100 mg/L) assuming 15 brine reuse cycles. The study showed that hybrid IX/catalyst reactor systems have potential to reduce resource consumption and improve environmental impacts associated with treating nitrate-contaminated water sources.
We used in vitro selection to identify new DNA aptamers for two endocrine-disrupting compounds often found in treated and natural waters, 17β-estradiol (E2) and 17α-ethynylestradiol (EE). We used equilibrium filtration to determine aptamer sensitivity/selectivity and dimethyl sulfate (DMS) probing to explore aptamer binding sites. The new E2 aptamers are at least 74-fold more sensitive for E2 than is a previously reported DNA aptamer, with dissociation constants (Kd values) of 0.6 μM. Similarly, the EE aptamers are highly sensitive for EE, with Kd of 0.5–1.0 μM. Selectivity values indicate that the E2 aptamers bind E2 and a structural analogue, estrone (E1), equally well and are up to 74-fold selective over EE. One EE aptamer is 53-fold more selective for EE over E2 or E1, but the other binds EE, E2, and E1 with similar affinity. The new aptamers do not lose sensitivity or selectivity in natural water from a local lake, despite the presence of natural organic matter (∼4 mg/L TOC). DMS probing suggests that E2 binding occurs in relatively flexible single-stranded DNA regions, an important finding for rational redesign of aptamers and their incorporation into sensing platforms. This is the first report of aptamers with strong selectivity for E2 and E1 over EE, or with strong selectivity for EE over E2 and E1. Such selectivity is important for achieving the goal of creating practically useful DNA-based sensors that can distinguish structurally similar estrogenic compounds in natural waters.
Noble metal nanoparticles have been applied to mediate catalytic removal of toxic oxyanions and halogenated hydrocarbons in contaminated water using H2 as a clean and sustainable reductant. However, activity loss by nanoparticle aggregation and difficulty of nanoparticle recovery are two major challenges to widespread technology adoption. Herein, we report the synthesis of a core–shell-structured catalyst with encapsulated Pd nanoparticles and its enhanced catalytic activity in reduction of bromate (BrO3–), a regulated carcinogenic oxyanion produced during drinking water disinfection process, using 1 atm H2 at room temperature. The catalyst material consists of a nonporous silica core decorated with preformed octahedral Pd nanoparticles that were further encapsulated within an ordered mesoporous silica shell (i.e., SiO2@Pd@mSiO2). Well-defined mesopores (2.3 nm) provide a physical barrier to prevent Pd nanoparticle (∼6 nm) movement, aggregation, and detachment from the support into water. Compared to freely suspended Pd nanoparticles and SiO2@Pd, encapsulation in the mesoporous silica shell significantly enhanced Pd catalytic activity (by a factor of 10) under circumneutral pH conditions that are most relevant to water purification applications. Mechanistic investigation of material surface properties combined with Langmuir–Hinshelwood modeling of kinetic data suggest that mesoporous silica shell enhances activity by promoting BrO3– adsorption near the Pd active sites. The dual function of the mesoporous shell, enhancing Pd catalyst activity and preventing aggregation of active nanoparticles, suggests a promising general strategy of using metal nanoparticle catalysts for water purification and related aqueous-phase applications.
M. Oostrom, Mehmani, Y., Romero-Gomez, P., Tang, Y., Liu, H., Yoon, H., Kang, Q., Joekar-Niasar, V., Balhoff, M. T., Dewers, T., Tartakovsky, G. D., Leist, E. A., Hess, N. J., Perkins, W. A., Rakowski, C. L., Richmond, M. C., and A., J., “
A lattice Boltzmann color-fluid model, which was recently proposed by Liu et al. (2012) based on a concept of continuum surface force, is improved to simulate immiscible two-phase flows in porous media. The new improvements allow the model to account for different kinematic viscosities of both fluids and to model fluid–solid interactions. The capability and accuracy of this model is first validated by two benchmark tests: a layered two-phase flow with a variable viscosity ratio, and a dynamic capillary intrusion. This model is then used to simulate liquid CO2 (LCO2) displacing water in a dual-permeability pore network. The extent and behavior of LCO2 preferential flow (i.e., fingering) is found to depend on the capillary number (Ca), and three different displacement patterns observed in previous micromodel experiments are reproduced. The predicted variation of LCO2 saturation with Ca, as well as variation of specific interfacial length with LCO2 saturation, are both in reasonable agreement with the experimental observations. To understand the effect of heterogeneity on pore-scale displacement, we also simulate LCO2 displacing water in a randomly heterogeneous pore network, which has the same size and porosity as the simulated dual-permeability pore network. In comparison to the dual-permeability case, the transition from capillary fingering to viscous fingering occurs at a higher Ca, and LCO2 saturation is higher at low Ca but lower at high Ca. In either pore network, the LCO2–water specific interfacial length is found to obey a power-law dependence on LCO2 saturation.
T. Boyd, Yoon, H., Zhang, C., Oostrom, M., Hess, N., Fouke, B., Valocchi, A. J., and Werth, C. J., “
Environmental impacts of conventional and emerging perchlorate drinking water treatment technologies were assessed using life cycle assessment (LCA). Comparison of two ion exchange (IX) technologies (i.e., nonselective IX with periodic regeneration using brines and perchlorate-selective IX without regeneration) at an existing plant shows that brine is the dominant contributor for nonselective IX, which shows higher impact than perchlorate-selective IX. Resource consumption during the operational phase comprises >80% of the total impacts. Having identified consumables as the driving force behind environmental impacts, the relative environmental sustainability of IX, biological treatment, and catalytic reduction technologies are compared more generally using consumable inputs. The analysis indicates that the environmental impacts of heterotrophic biological treatment are 2–5 times more sensitive to influent conditions (i.e., nitrate/oxygen concentration) and are 3–14 times higher compared to IX. However, autotrophic biological treatment is most environmentally beneficial among all. Catalytic treatment using carbon-supported Re–Pd has a higher (ca. 4600 times) impact than others, but is within 0.9–30 times the impact of IX with a newly developed ligand-complexed Re–Pd catalyst formulation. This suggests catalytic reduction can be competitive with increased activity. Our assessment shows that while IX is an environmentally competitive, emerging technologies also show great promise from an environmental sustainability perspective.
A three-dimensional multiphase numerical model was used to simulate the infiltration and dissolution of a dense nonaqueous phase liquid (DNAPL) release in two experimental flow cells containing different heterogeneous and well-characterized permeability fields. DNAPL infiltration was modeled using Brooks-Corey-Burdine hysteretic constitutive relationships. DNAPL dissolution was simulated using a rate-limited mass transfer expression with a velocity-dependent mass transfer coefficient and a thermodynamically based calculation of DNAPL-water interfacial area. The model did not require calibration of any parameters. The model predictions were compared to experimental measurements of high-resolution DNAPL saturations and effluent concentrations. The predicted concentrations were in close agreement with measurements for both domains, indicating that important processes were effectively captured by the model. DNAPL saturations greatly influenced mass transfer rates through their effect on relative permeability and velocity. Areas with low DNAPL saturation were associated with low interfacial areas, which resulted in reduced mass transfer rates and nonequilibrium dissolution. This was captured by the thermodynamic interfacial area model, while a geometric model overestimated the interfacial areas and the overall mass transfer. This study presents the first validation of the thermodynamic dissolution model in three dimensions and for high aqueous phase velocities; such conditions are typical for remediation operations, especially in heterogeneous aquifers. The demonstrated ability to predict DNAPL dissolution, only requiring prior characterization of soil properties and DNAPL release conditions, represents a significant improvement compared to empirical dissolution models and provides an opportunity to delineate the relationship between source zone architecture and the remediation potential for complex DNAPL source zones.
Predicting the longevity of non-aqueous phase liquid (NAPL) source zones has proven to be a difficult modeling problem that has yet to be resolved. Research efforts towards understanding NAPL depletion have focused on developing empirical models that relate lumped mass transfer rates to velocities and organic saturations. These empirical models are often unable to predict NAPL dissolution for systems different from those used to calibrate them, indicating that system-specific factors important for dissolution are not considered. This introduces the need for a calibration step before these models can be reliably used to predict NAPL dissolution for systems of arbitrary characteristics.
In this paper, five published Sherwood–Gilland models are evaluated using experimental observations from the dissolution of two laboratory-scale complex three-dimensional NAPL source zones. It is shown that the relative behavior of the five models depends on the system and source zone characteristics. Through a theoretical analysis, comparing Sherwood–Gilland type models to a process-based, thermodynamic dissolution model, it is shown that the coefficients of the Sherwood–Gilland models can be related to measurable soil properties. The derived dissolution model with soil-dependent coefficients predicts concentrations identical to those predicted by the thermodynamic dissolution model for cases with negligible hysteresis. This correspondence breaks down when hysteresis has a significant impact on interfacial areas. In such cases, the derived dissolution model will slightly underestimate dissolved concentrations at later times, but is more likely to capture system-specific dissolution rates than Sherwood–Gilland models.
This work presents a pore-scale biofilm model that solves the flow field using the lattice Boltzmann method, the concentration field of chemical species using the finite difference method, and biofilm development using the cellular automaton method. We adapt the model from a previous work and expand it by implementing biofilm shrinkage in the cellular automaton method. The new pore-scale biofilm model is then evaluated against a previously published pore-scale biofilm experiment, in which two microfluidic flow cells, one with a homogeneous pore network and the other with an aggregate pore network, were tested for aerobic degradation of a herbicide. The simulated biofilm distribution and morphology, biomass accumulation, and contaminant removal are generally consistent with the experimental data. Biofilm detachment in this model occurs when the local shear stress is above a critical value. We use the critical value from our previously published modeling study and find it works well in this case, even though we now have a different pore network and a different microbial species. We also use the model to show that the interaction between flow and biofilm growth is important to predict contaminant removal. The computational time of the new model is reduced 90% compared to our prior work due to implementation of biofilm shrinkage in the cellular automaton method. To the best of our knowledge, this is the first time that biofilm shrinkage has been incorporated into a pore-scale model for simulation of pollutant biodegradation in porous media.
H. Liu, Valocchi, A. J., Werth, C. J., and Kang, Q., “