Reach-scale modeling of reaction cascades and spatially-dependent reactions in the hyporheic zone

2020 ◽  
Author(s):  
Kevin Roche ◽  
Jennifer Drummond ◽  
Nicole Sund ◽  
Rina Schumer ◽  
Marco Dentz
Keyword(s):  
Metabolic Activity ◽  
Hyporheic Zone ◽  
Reaction Rates ◽  
Breakthrough Curves ◽  
Reaction Pathways ◽  
Aerobic Respiration ◽  
Small Scale ◽  
Tracer Injection ◽  
Respiration Rates ◽  
Reaction Cascades

<p>Stream tracer injection experiments are widely used  to characterize reach-scale transport and reaction in rivers. Results from tracer injection experiments (i.e., concentration vs. time profiles, or breakthrough curves) are often used to estimate reach-averaged processes controlling solute fate. Advances in both tracer technology have greatly improved our ability to infer finer scale processes from the integrated, reach-scale result. However, to better meet the demands of improved tracer technology and the small-scale processes they elucidate, we need a model that incorporates process-based understanding of solute transport and reactivity. In brief, smarter tracers require smarter models. </p><p>A noteworthy example of the disconnect between measurement and modeling capabilities is the resazurin-resorufin (Raz-Rru) tracer system. Raz is a fluorescent chemical that transforms irreversibly to Rru at a rate proportional to the local rate of aerobic metabolic activity. Co-injections of Raz and a conservative tracer provide experimentalists with a “smart tracer” system that is commonly used to estimate aerobic metabolic activity in streams, particularly within the hyporheic zone. At present, aerobic respiration rates are challenging to estimate from breakthrough curves for two reasons. First, multiple reaction pathways are possible beyond the target parent-to-daughter transformation of Raz to Rru. This implies that aerobic respiration rates inferred from breakthrough curve concentrations may be confounded by additional, zone-specific reactions such as the abiotic degradation of Rru after it is created. Second, field campaigns using the Raz-Rru system have demonstrated that aerobic respiration rates vary strongly with depth in the hyporheic zone. Nevertheless, existing reach-scale models assume uniform reaction rates throughout the hyporheic zone for analytical tractability. This assumption biases the rates of metabolic activity inferred from tracer injection experiments in streams where metabolic rates are spatially variable. </p><p>Here, we present recent advances in reach-scale analytical modeling that address both challenges. We generalize a classic mobile-immobile model to account for multiple reaction pathways of Raz (e.g., via aerobic metabolic activity and abiotic decay) and Rru (e.g., via Raz transformation and abiotic decay). We then extend this framework to account for spatial variability in the hyporheic zone, and we validate semi-analytical model solutions against reach-scale simulations for reactive transport. Together, these advances provide a simple way to estimate reactivity of the benthic biolayer – a known hotspot of reach-scale ecosystem respiration – using established methods. The new framework also opens the door for modeling other chemical constituents transformed through reaction cascades in streams. </p>

Identifying flow transience in the hyporheic zone by Electric Resistivity Tomography

2020 ◽  
Author(s):  
Joakim Riml ◽  
Liwen Wu ◽  
Robert Earon ◽  
Stefan Krause ◽  
Theresa Blume
Keyword(s):  
Hyporheic Zone ◽  
Phenomenological Study ◽  
Stream Water ◽  
Fold Increase ◽  
Hyporheic Exchange ◽  
Small Scale ◽  
Tracer Injection ◽  
Resistivity Tomography ◽  
Research Attention ◽  
Flood Hydrograph

<p>The importance of hydrological interactions between groundwater and surface waters and the consequential transport of mass and energy across the streambed – water interface has gained significant research attention lately. In this phenomenological study we investigated the transient nature of hyporheic exchange as a response to flood events by performing a stream manipulation experiment in a small boreal stream within the Krycklan catchment, Sweden. The stream flow was manipulated in order to create a flood event and investigate the responding dynamically changing spatial extent of the hyporheic zone. The artificial flood caused an approximately 5-fold increase in stream discharge.</p><p>The experimental set-up consisted of both geophysical and hydrological methods, including time-lapse Electrical Resistivity Tomography (ERT) along the thalweg of a 6.3 m long stream section, with a 0.1 m longitudinal spacing of the electrodes. A constant stream water electric conductivity (EC) was obtained throughout the experiment by using a variable rate tracer injection of chloride. Additional measurements of background EC in the streambed sediments as well as streambed topography (from a total station) and subsurface structures (from Ground Penetrating Radar) were used to support the results from the ERT.</p><p>With combined experimental and numerical modeling approaches, the hyporheic response to transient hydrologic boundary conditions and small scale streambed heterogeneities were investigated. Results indicated that a quick response of the hyporheic zone to the changing pressure distribution on the streambed was strongly controlled by the shape of the flood hydrograph. Moreover, the response resulted in an alteration of the hyporheic flowpaths, which increased the hyporheic zone depth and contributed to a dynamically-changing residence time distribution within the hyporheic zone. This alteration was further complicated by the local streambed heterogeneities. The observed substantial variabilities in the hyporheic fluxes over the time span of a flood hydrograph and longitudinally over the measured stream section has direct consequences on the biogeochemical and hydro-ecological functioning of the hyporheic zone, which would be inadequately estimated using homogenous, steady-state approaches.</p>

Dissolved oxygen gradients in hyporheic zones depend on fine sediment and associated respiration rates

2021 ◽  
Author(s):  
David Piatka ◽  
Romy Wild ◽  
Jürgen Geist ◽  
Robin Kaule ◽  
Ben Gilfedder ◽  
...  
Keyword(s):  
Dissolved Oxygen ◽  
Hyporheic Zone ◽  
Stream Water ◽  
Stable Carbon Isotope ◽  
Open Water ◽  
Fine Sediment ◽  
Small Scale ◽  
Order Stream ◽  
Respiration Rates ◽  
Sediment Fraction

<p>Dissolved oxygen (DO) in the hyporheic zone (HZ) is a crucial parameter for the survival of many stream organisms and is involved in a multitude of aerobic chemical reactions. However, HZ DO budgets are easily perturbed by climate change and anthropogenic processes that have caused increased deposition of fine sediments (< 2 mm) in many stream beds. The fine sediment fraction hampers exchange of DO-rich stream water with the HZ. In this study we performed a raster sampling approach (0.90 cm length x 1.50 cm width; 30 cm distance between sampling points) at sediment depths of 10 and 25 cm with a focus on DO and its stable isotopes (δ<sup>18</sup>O<sub>DO</sub>). The aim was to analyze small-scale turnover patterns in a forested (site 1) and an anthropogenically influenced stream section (site 2) in a 3<sup>rd</sup> order stream in southern Germany. Grain size analyses showed similar average fine sediment fractions at site 1 (42.5 ±13.7 %) and site 2 (46.3 ±10.8 %). They increased with depth at both sites (38.5 ± 6.3 %, 0-15 cm; 46.5 ± 17.4 %, 15-30 cm at site 1 and 40.6 ±4.5 %, 0-15 cm; 52.0 ±12.2 %, 15-30 cm at site 2). DO concentrations in the HZ ranged from 1.4 to 4.5 mg L<sup>-1</sup> (2.0 ±0.7 mg L<sup>-1</sup>) and 1.5 to 1.8 mg L<sup>-1</sup> (1.7 ±0.1 mg L<sup>-1</sup>) at site 1 and from 1.2 to 2.9 mg L<sup>-1</sup> (1.6 ±0.5) and 1.0 to 2.4 mg L<sup>-1</sup> (1.6 ±0.4) at site 2 at 10 and 25 cm depth, respectively. The low DO concentrations in the HZ suggest high DO consumption rates and reduced exchange with stream water. This is possibly a result of increased fine sediment proportions. However, other factors such as organic carbon contents and increased respiration rates may also influence DO gradients. In contrast, the stream water had an average DO concentration of 9.8 ±0.2 mg L<sup>-1</sup>. Associated δ<sup>18</sup>O<sub>DO</sub> values of the open water (23.4 ±0.1 ‰) differed from those of sediment waters that showed averages of +22.5 ±0.5 ‰ and +22.4 ±0.3 ‰ at site 1 and +22.5 ±0.4 ‰ and +22.3 ±0.2 ‰ at site 2 at 10 and 25 cm depth, respectively. These sedimentary values indicated dominant photosynthesis, even though due to absence of light in the subsurface this process seems unlikely. Therefore, kinetically-driven processes such as diffusion, interactions with Fe or unknown DO sources within the HZ might have caused such <sup>16</sup>O-enriched values. Our findings suggest that the analyses of DO, δ<sup>18</sup>O<sub>DO</sub> and fine sediment gradients in the HZ should be combined with stable carbon isotope measurements to further our understanding of hyporheic processes relevant for stream biota.</p><p> </p>

Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method

2013 ◽  
Author(s):  
Scott Martin ◽  
Aleksandar Jemcov ◽  
Björn de Ruijter
Keyword(s):  
Turbulent Combustion ◽  
Large Scale ◽  
Reaction Rates ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Small Scale ◽  
Conditional Moment ◽  
Progress Variable ◽  
Conditional Moment Closure ◽  
Scale Turbulence

Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided. The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled. The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors.

Control of Chemical Reaction Pathways by Light–Matter Coupling

2021 ◽  
Vol 72 (1) ◽  
Author(s):  
Dinumol Devasia ◽  
Ankita Das ◽  
Varun Mohan ◽  
Prashant K. Jain
Keyword(s):  
Chemical Reactivity ◽  
Reaction Rates ◽  
Reaction Pathways ◽  
Metal Nanostructures ◽  
Annual Review ◽  
Publication Date ◽  
Strong Light ◽  
Radical Intermediates ◽  
Catalytically Active ◽  
Light Excitation

Because plasmonic metal nanostructures combine strong light absorption with catalytically active surfaces, they have become platforms for the light-assisted catalysis of chemical reactions. The enhancement of reaction rates by plasmonic excitation has been extensively discussed. This review focuses on a less discussed aspect: the induction of new reaction pathways by light excitation. Through commentary on seminal reports, we describe the principles behind the optical modulation of chemical reactivity and selectivity on plasmonic metal nanostructures. Central to these phenomena are excited charge carriers generated by plasmonic excitation, which modify the energy landscape available to surface reactive species and unlock pathways not conventionally available in thermal catalysis. Photogenerated carriers can trigger bond dissociation or desorption in an adsorbate-selective manner, drive charge transfer and multielectron redox reactions, and generate radical intermediates. Through one or more of these mechanisms, a specific pathway becomes favored under light. By improved control over these mechanisms, light-assisted catalysis can be transformational for chemical synthesis and energy conversion. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 72 is April 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Can Water Dilution Avoid Flashback on a Hydrogen Enriched Micro Gas Turbine Combustion? A Large Eddy Simulations Study

2020 ◽  
Author(s):  
Alessio Pappa ◽  
Laurent Bricteux ◽  
Pierre Bénard ◽  
Ward De Paepe
Keyword(s):  
Gas Turbines ◽  
Reaction Rates ◽  
Operating Conditions ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Reference Case ◽  
Large Eddy ◽  
Pure Methane ◽  
Water Dilution ◽  
Air Humidification

Abstract Considering the growing interest in Power-to-Fuel, i.e. production of H2 using electrolysis to store excess renewable electricity, combustion-based technologies still have a role to play in the future of power generation. Especially in a decentralized production with small-scale cogeneration, micro Gas Turbines (mGTs) offer great advantages related to their high adaptability and flexibility, in terms of operation and fuel. Hydrogen (or hydrogen enriched methane) combustion is well-known to lead to flame and combustion instabilities. The high temperatures and reaction rates reached in the combustor can potentially lead to flashback. In the past, combustion air humidification (i.e. water addition) has proven effective to reduce temperatures and reaction rates, leading to significant NOx emission reductions. Therefore, combustion air humidification can open a path to stabilize hydrogen combustion in a classical mGT combustor. However accurate data assessing the impact of humidification on the combustion is still missing for real mGT combustor geometries and operating conditions. In this framework, this paper presents a comparison between pure methane and hydrogen enriched methane/air combustions, with and without combustion air humidification, in a typical mGT combustion chamber (Turbec T100) using Large Eddy Simulations (LES) analysis. In a first step, the necessary minimal water dilution, to reach stable and low emissions combustion with hydrogen, was assessed using a 1D approach. The one-dimensional unstretched laminar flame is computed for both pure methane (reference case) and hydrogen enriched methane/air combustion cases. The results of this comparison show that, for the hydrogen enriched combustion, the same level of flame speed as in the reference case can be reached by adding 10% (in mass fraction) of water. In a second step, the feasibility and flexibility of humidified hydrogen enriched methane/air combustion in an industrial mGT combustor have been demonstrated by performing high fidelity LES on a 3D geometry. Results show that steam dilution helped to lower the reactivity of hydrogen, and thus prevents flashback, enabling the use of hydrogen blends in the mGT at similar CO levels, compared to the reference case. These results will help to design future combustor towards more stability.

scholarly journals Insight into the influence of local streambed heterogeneity on hyporheic-zone flow characteristics

2020 ◽  
Vol 28 (8) ◽  
pp. 2697-2712
Author(s):  
Robert Earon ◽  
Joakim Riml ◽  
Liwen Wu ◽  
Bo Olofsson
Keyword(s):  
Surface Water ◽  
Hydraulic Conductivity ◽  
Penetration Depth ◽  
Hyporheic Zone ◽  
Flow Characteristics ◽  
Time Lapse ◽  
Hyporheic Exchange ◽  
Tracer Injection ◽  
Hyporheic Flow ◽  
Exchange Processes

AbstractInteraction between surface water and groundwater plays a fundamental role in influencing aquatic chemistry, where hyporheic exchange processes, distribution of flow paths and residence times within the hyporheic zone will influence the transport of mass and energy in the surface-water/groundwater system. Geomorphological conditions greatly influence hyporheic exchange, and heterogeneities such as rocks and clay lenses will be a key factor for delineating the hyporheic zone. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to investigate the streambed along a 6.3-m-long reach in order to characterise geological layering and distinct features which may influence parameters such as hydraulic conductivity. Time-lapse ERT measurements taken during a tracer injection demonstrated that geological features at the meter-scale played a determining role for the hyporheic flow field. The penetration depth of the tracer into the streambed sediment displayed a variable spatial pattern in areas where the presence of highly resistive anomalies was detected. In areas with more homogeneous sediments, the penetration depth was much more uniformly distributed than observed in more heterogeneous sections, demonstrating that ERT can play a vital role in identifying critical hydraulic features that may influence hyporheic exchange processes. Reciprocal ERT measurements linked variability and thus uncertainty in the modelled resistivity to the spatial locations, which also demonstrated larger variability in the tracer penetration depth, likely due to local heterogeneity in the hydraulic conductivity field.

scholarly journals Modeling Emissions from Concentrated Sources into Large-Scale Models: Theory and apriori Testing

Atmosphere ◽  
2020 ◽  
Vol 11 (8) ◽  
pp. 863
Author(s):  
Roberto Paoli
Keyword(s):  
Large Scale ◽  
General Procedure ◽  
Chemical Transport ◽  
Reaction Rates ◽  
Small Scale ◽  
Background Concentrations ◽  
Transport Models ◽  
Chemical Transport Models ◽  
Scale Models ◽  
Effective Reaction

This paper presents a general procedure to incorporate the effects emissions from localized sources, such as aircraft or ship engines, into chemical transport models (CTM). In this procedure, the species concentrations in each grid box of a CTM are split into plume or small-scale concentrations and background concentrations, respectively, and the corresponding conservation equations are derived. The plume concentrations can be interpreted as subgrid contributions for the CTM grid-box averaged concentrations. The chemical reactions occurring inside the plume are parameterized by introducing suitable “effective” reaction rates rather than modifying the emission indices of the species inside the plume. Various methods for implementation into large-scale models are discussed that differ by the accuracy of the description of plume process. The mathematical consistency of the method is verified on simple idealized setting consisting of a reactive plume in homogeneous turbulence.

scholarly journals Green synthesis via electrolysis in microemulsions

2001 ◽  
Vol 73 (12) ◽  
pp. 1895-1905 ◽  
Author(s):  
James F. Rusling
Keyword(s):  
Turnover Rate ◽  
Recent Progress ◽  
Reaction Pathway ◽  
Reaction Rates ◽  
Synthetic Methods ◽  
Reaction Pathways ◽  
Lower Toxicity ◽  
Reduction Potentials ◽  
Covalently Linked ◽  
High Yields

Electrolysis in microemulsions is a promising approach for environmentally friendly chemical synthetic methods of the future. Employing microemulsions instead of organic solvents for electrosynthesis has the advantages of lower toxicity and cost, high dissolving power for reactants and mediators of unlike solubility, enhancement of reaction rates by controlling the reduction potentials of mediators, possible reaction pathway control, and recycling of microemulsion components. This paper reviews recent progress in using microemulsions for direct and mediated electrosynthesis, including formation of carbon­carbon bonds. Rates of mediated reactions can be controlled by manipulating microemulsion composition. Examples are presented, in which reaction pathways of direct and mediated electrolyses can be controlled with microemulsions to give desired products in high yields. Such control has been demonstrated with dissolved and surface-bound mediators. For a covalently linked scaffold of poly(l-lysine) and cobalt corrin vitamin B12 hexacarboxylate attached to graphite, catalytic turnover rate for reduction of 1,2-dibromocylcohexane was optimized by optimizing microemulsion composition.

scholarly journals Carbon limitation leads to thermodynamic regulation of aerobic metabolism

2020 ◽  
Author(s):  
Vanessa A. Garayburu-Caruso ◽  
James C. Stegen ◽  
Hyun-Seob Song ◽  
Lupita Renteria ◽  
Jaqueline Wells ◽  
...  
Keyword(s):  
Nitrogen Content ◽  
Biogeochemical Cycles ◽  
Freshwater Ecosystems ◽  
Aerobic Respiration ◽  
Carbon Limitation ◽  
New Paradigm ◽  
Ultrahigh Resolution ◽  
Respiration Rates ◽  
Biogeochemical Models ◽  
Global Biogeochemical Cycles

AbstractOrganic matter (OM) metabolism in freshwater ecosystems is a critical source of uncertainty in global biogeochemical cycles, yet aquatic OM cycling remains poorly understood. Here, we present the first work to explicitly test OM thermodynamics as a key regulator of aerobic respiration, challenging long-held beliefs that organic carbon and oxygen concentrations are the primary determinants of respiration rates. We pair controlled microcosm experiments with ultrahigh-resolution OM characterization to demonstrate a clear relationship between OM thermodynamic favorability and aerobic respiration under carbon limitation. We also demonstrate a shift in the regulation of aerobic respiration from OM thermodynamics to nitrogen content when carbon is in excess, highlighting a central role for OM thermodynamics in aquatic biogeochemical cycling particularly in carbon-limited ecosystems. Our work therefore illuminates a structural gap in aquatic biogeochemical models and presents a new paradigm in which OM thermodynamics and nitrogen content interactively govern aerobic respiration.

scholarly journals CO2-Enabled Cyanohydrin Synthesis and Facile Homologation Reactions

2020 ◽  
Author(s):  
Martin Juhl ◽  
Allan Petersen ◽  
JIWOONG LEE
Keyword(s):  
Atmospheric Pressure ◽  
Chemical Process ◽  
Reaction Rates ◽  
Protecting Groups ◽  
Reaction Pathways ◽  
Easy Access ◽  
Kinetic Control ◽  
Direct Reaction ◽  
Fischer Synthesis ◽  
Cyanohydrin Synthesis

Thermodynamic and kinetic control of a chemical process is the key to access desired products and states. Changes are made when desired product is not accessible; one may manipulate the reaction with additional reagents, catalysts and/or protecting groups. Here we report the use of carbon dioxide to direct reaction pathways in order to selectively afford desired products in high reaction rates while avoiding the formation of byproducts. The utility of CO<sub>2</sub>-mediated selective cyanohydrin synthesis was further showcased by broadening Kiliani-Fischer synthesis to offer an easy access to variety of polyols, cyanohydrins, linear alkylnitriles, by simply starting from alkyl- and arylaldehydes, KCN and atmospheric pressure of CO<sub>2</sub>.

Sign in / Sign up

Export Citation Format

Share Document