Micromodel Investigation of Transport Effect on the Kinetics of Reductive Dissolution of Hematite

Reductive dissolution of hematite in porous media was investigated using a micromodel (8.1 × 4.5 × 0.028 mm) with realistic pore network structures that include distinctive advection domain, macropores and micropores created in silicon substrate. The micromodel pore surface was sputter deposited with a thin layer (230 nm) of hematite. The hematite in the micromodel was reduced by injecting pH-varying solutions (pH 5.0, 6.0, 7.0) containing a reduced form of flavin mononucleotide (FMNH₂, 100 μM), a biogenic soluble electron transfer mediator produced by <i>Shewanella</i> species. The reduction kinetics was determined by measuring effluent Fe­(II) (aq) concentration and by spectroscopically monitoring the hematite dissolution front in the micromodel. Batch experiment was also performed to estimate the hematite reduction rate under the well-mixed condition. Results showed significant spatial variation in local redox reaction rate that was controlled by the coupled transport and reaction. The overall rate of the redox reaction in the micromodel required a three-domain numerical model to effectively describe reaction kinetics either with distinctive apparent rate parameters or mass transfer coefficients in different pore domains. Results from this study demonstrated the feasibility of a domain-based modeling approach for scaling reaction rates from batch to porous media systems where reactions may be significantly limited by transport

Capacitive Membrane Stripping for Ammonia Recovery (CapAmm) from Dilute Wastewaters

A novel cost-effective flow-electrode capacitive deionization unit combined with a hydrophobic gas-permeable hollow fiber membrane contactor (designated “CapAmm”) is described here and used for efficient recovery of ammonia from dilute synthetic wastewaters. During operation, ammonia migrates across a cation exchange membrane and selectively accumulates in the cathode chamber of a flow electrode followed by transformation to dissolved NH₃ with subsequent stripping via a membrane contactor and recovery as ammonium sulfate. Our results demonstrate that the CapAmm process can achieve an ammonia removal efficiency of ∼90% and a recovery efficiency of ∼60%. At current densities of 5.8 and 11.5 A m^–2 (normalized by the effective cation exchange membrane area) and a hydraulic retention time of 1.48 min, the energies required for ammonia recovery were 9.9 and 21.1 kWh (kg of N)⁻¹, respectively, with these values being comparable with those of other similar electrochemical ammonia recovery systems. These findings suggest that the CapAmm technology described here has the potential for the dual purposes of cost-effective salt removal and the recovery of ammonia from wastewaters, with greater stability, better flexibility, and greater energy efficiency compared to those of other methods

Kinetic Modeling of the Anodic Degradation of Ni-EDTA Complexes: Insights into the Reaction Mechanism and Products

In this study, an electrochemical advanced oxidation process (EAOP) was employed to effectively degrade complexes of nickel and ethylenediaminetetraacetic acid (EDTA) present in electroless nickel plating wastewaters. Our results show that Ni-EDTA complexes can be effectively degraded by an EAOP with degradation of the complexes occurring at/near the anode surface via interaction with hydroxyl radicals generated on water splitting. Our results further show that the rate of Ni-EDTA degradation is not a function of the rate of any particular chemical reaction but, rather, is controlled by the rate of transport of Ni-EDTA to the anode surface. The oxidation of EDTA to smaller noncomplexing entities releases Ni2+, which is subsequently deposited onto the cathode as Ni0. While complete Ni-EDTA removal and Ni recovery are achieved within 2 h, the overall TOC removal by EAOP is limited, with only 50% TOC removal achieved after 2 h of treatment. The low affinity of small molecular weight EDTA degradation products (such as formic acid, glycine, oxamic acid, and acetic acid) for the anode surface limits oxidation of these compounds and overall TOC removal by the anodic oxidation process. We have developed a mathematical kinetic model that satisfactorily describes Ni-EDTA removal, Ni recovery, and TOC removal over a range of Ni and EDTA concentrations and provides a good description of the oxidation of various EDTA degradation intermediates. The mathematical model developed here, when coupled with the hydrodynamics of the electrochemical cell using a computational fluid dynamics tool, can assist in both cell design and the selection of operating parameters such that the performance of the EAOP process for Ni-EDTA degradation and TOC removal is optimized

A Ten Liter Stacked Microbial Desalination Cell Packed With Mixed Ion-Exchange Resins for Secondary Effluent Desalination

The architecture and performance of microbial desalination cell (MDC) have been significantly improved in the past few years. However, the application of MDC is still limited in a scope of small-scale (milliliter) reactors and high-salinity-water desalination. In this study, a large-scale (>10 L) stacked MDC packed with mixed ion-exchange resins was fabricated and operated in the batch mode with a salt concentration of 0.5 g/L NaCl, a typical level of domestic wastewater. With circulation flow rate of 80 mL/min, the stacked resin-packed MDC (SR-MDC) achieved a desalination efficiency of 95.8% and a final effluent concentration of 0.02 g/L in 12 h, which is comparable with the effluent quality of reverse osmosis in terms of salinity. Moreover, the SR-MDC kept a stable desalination performance (>93%) when concentrate volume decreased from 2.4 to 0.1 L (diluate/concentrate volume ratio increased from 1:1 to 1:0.04), where only 0.875 L of nonfresh water was consumed to desalinate 1 L of saline water. In addition, the SR-MDC achieved a considerable desalination rate (95.4 mg/h), suggesting a promising application for secondary effluent desalination through deriving biochemical electricity from wastewater

Development of Redox-Active Flow Electrodes for High-Performance Capacitive Deionization

An innovative flow electrode comprising redox-active quinones to enhance the effectiveness of water desalination using flow-electrode capacitive deionization (FCDI) is described in this study. The results show that, in addition to carbon particle contact, the presence of the aqueous hydroquinone (H₂Q)/benzoquinone (Q) couple in a flowing suspension of carbon particles enhances charge transfer significantly as a result of reversible redox reactions of H₂Q/Q. Ion migration through the micropores of the flow electrodes was facilitated in particular with the desalination rate significantly enhanced. The cycling behavior of the quinoid mediators in the anode flow electrode demonstrated a relatively high stability at the low pH induced, suggesting that the mediator would be suitable for long-term operation

In Situ Infrared Spectroscopic Study of Brucite Carbonation in Dry to Water-Saturated Supercritical Carbon Dioxide

In geologic carbon sequestration, whereas part of the injected carbon dioxide will dissolve into host brine, some will remain as neat to water saturated supercritical CO₂ (scCO₂) near the well bore and at the caprock, especially in the short term life cycle of the sequestration site. Little is known about the reactivity of minerals with scCO₂ containing variable concentrations of water. In this study, we used high-pressure infrared spectroscopy to examine the carbonation of brucite (Mg­(OH)₂) in situ over a 24 h reaction period with scCO₂ containing water concentrations between 0% and 100% saturation, at temperatures of 35, 50, and 70 °C, and at a pressure of 100 bar. Little or no detectable carbonation was observed when brucite was reacted with neat scCO₂. Higher water concentrations and higher temperatures led to greater brucite carbonation rates and larger extents of conversion to magnesium carbonate products. The only observed carbonation product at 35 °C was nesquehonite (MgCO₃·3H₂O). Mixtures of nesquehonite and magnesite (MgCO₃) were detected at 50 °C, but magnesite was more prevalent with increasing water concentration. Both an amorphous hydrated magnesium carbonate solid and magnesite were detected at 70 °C, but magnesite predominated with increasing water concentration. The identity of the magnesium carbonate products appears strongly linked to magnesium water exchange kinetics through temperature and water availability effects

Experimental Study of Crossover from Capillary to Viscous Fingering for Supercritical CO₂–Water Displacement in a Homogeneous Pore Network

Carbon sequestration in saline aquifers involves displacing brine from the pore space by supercritical CO₂ (scCO₂). The displacement process is considered unstable due to the unfavorable viscosity ratio between the invading scCO₂ and the resident brine. The mechanisms that affect scCO₂–water displacement under reservoir conditions (41 °C, 9 MPa) were investigated in a homogeneous micromodel. A large range of injection rates, expressed as the dimensionless capillary number (<i>Ca</i>), was studied in two sets of experiments: discontinuous-rate injection, where the micromodel was saturated with water before each injection rate was imposed, and continuous-rate injection, where the rate was increased after quasi-steady conditions were reached for a certain rate. For the discontinuous-rate experiments, capillary fingering and viscous fingering are the dominant mechanisms for low (log<i>Ca</i> ≤ −6.61) and high injection rates (log<i>Ca</i> ≥ −5.21), respectively. Crossover from capillary to viscous fingering was observed for log<i>Ca</i> = −5.91 to −5.21, resulting in a large decrease in scCO₂ saturation. The discontinuous-rate experimental results confirmed the decrease in nonwetting fluid saturation during crossover from capillary to viscous fingering predicted by numerical simulations by Lenormand et al. (<i>J. Fluid Mech.</i> <b>1988</b>, <i>189</i>, 165–187). Capillary fingering was the dominant mechanism for all injection rates in the continuous-rate experiment, resulting in monotonic increase in scCO₂ saturation

Pore-Scale Characterization of Biogeochemical Controls on Iron and Uranium Speciation under Flow Conditions

Etched silicon microfluidic pore network models (micromodels) with controlled chemical and redox gradients, mineralogy, and microbiology under continuous flow conditions are used for the incremental development of complex microenvironments that simulate subsurface conditions. We demonstrate the colonization of micromodel pore spaces by an anaerobic Fe­(III)-reducing bacterial species (<i>Geobacter sulfurreducens</i>) and the enzymatic reduction of a bioavailable Fe­(III) phase within this environment. Using both X-ray microprobe and X-ray absorption spectroscopy, we investigate the combined effects of the precipitated Fe­(III) phases and the microbial population on uranium biogeochemistry under flow conditions. Precipitated Fe­(III) phases within the micromodel were most effectively reduced in the presence of an electron shuttle (AQDS), and Fe­(II) ions adsorbed onto the precipitated mineral surface without inducing any structural change. In the absence of Fe­(III), U­(VI) was effectively reduced by the microbial population to insoluble U­(IV), which was precipitated in discrete regions associated with biomass. In the presence of Fe­(III) phases, however, both U­(IV) and U­(VI) could be detected associated with biomass, suggesting reoxidation of U­(IV) by localized Fe­(III) phases. These results demonstrate the importance of the spatial localization of biomass and redox active metals, and illustrate the key effects of pore-scale processes on contaminant fate and reactive transport

Flagella-Mediated Differences in Deposition Dynamics for <i>Azotobacter vinelandii</i> in Porous Media

A multiscale approach was designed to study the effects of flagella on deposition dynamics of <i>Azotobacter vinelandii</i> in porous media, independent of motility. In a radial stagnation point flow cell (RSPF), the deposition rate of a flagellated strain with limited motility, DJ77, was higher than that of a nonflagellated (Fla^–) strain on quartz. In contrast, Fla^– strain deposition exceeded that of DJ77 in two-dimensional silicon microfluidic models (micromodels) and in columns packed with glass beads. Both micromodel and column experiments showed decreasing deposition over time, suggesting that approaching cells were blocked from deposition by previously deposited cells. Modeling results showed that blocking became effective for DJ77 strain at lower ionic strengths (1 mM and 10 mM), while for the Fla^– strain, blocking was similar at all ionic strengths. In late stages of micromodel experiments, ripening effects were also observed, and these appeared earlier for the Fla^– strain. In RSPF and column experiments, deposition of the flagellated strain was influenced by ionic strength, while ionic strength dependence was not observed for the Fla^– strain. The observations in all three setups suggested flagella affect deposition dynamics and, in particular, result in greater sensitivity to ionic strength

Role of Collector Alternating Charged Patches on Transport of <i>Cryptosporidium parvum</i> Oocysts in a Patchwise Charged Heterogeneous Micromodel

The role of collector surface charge heterogeneity on transport of <i>Cryptosporidium parvum</i> oocyst and carboxylate microsphere in 2-dimensional micromodels was studied. The cylindrical silica collectors within the micromodels were coated with 0, 10, 20, 50, and 100% Fe₂O₃ patches. The experimental values of average removal efficiencies (η) of the Fe₂O₃ patches and on the entire collectors were determined. In the presence of significant (>3500 kT) Derjaguin–Landau–Verwey–Overbeek (DLVO) energy barrier between the microspheres and the silica collectors at pH 5.8 and 8.1, η determined for Fe₂O₃ patches on the heterogeneous collectors were significantly less (<i>p</i> < 0.05, <i>t</i> test) than those obtained for collectors coated entirely with Fe₂O₃. However, η calculated for Fe₂O₃ patches for microspheres at pH 4.4 and for oocysts at pH 5.8 and 8.1, where the DLVO energy barrier was relatively small (ca. 200–360 kT), were significantly greater (<i>p</i> < 0.05, <i>t</i> test) than those for the collectors coated entirely with Fe₂O₃. The dependence of η for Fe₂O₃ patches on the DLVO energy barrier indicated the importance of periodic favorable and unfavorable electrostatic interactions between colloids and collectors with alternating Fe₂O₃ and silica patches. Differences between experimentally determined overall η for charged heterogeneous collectors and those predicted by a patchwise geochemical heterogeneous model were observed. These differences can be explained by the model’s lack of consideration for the spatial distribution of charge heterogeneity on the collector surface