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A microfluidic gradient chamber (MGC) and a homogeneous batch culturing system were used to evaluate whether spatial concentration gradients of the antibiotic ciprofloxacin allow development of greater antibiotic resistance in Escherichia coli strain 307 (E. coli 307) compared to exclusively temporal concentration gradients, as indicated in an earlier study. A linear spatial gradient of ciprofloxacin and Luria-Bertani broth (LB) medium was established and maintained by diffusion over 5 days across a well array in the MGC, with relative concentrations along the gradient of 1.7-7.7× the original minimum inhibitory concentration (MICoriginal). The E. coli biomass increased in wells with lower ciprofloxacin concentrations, and only a low level of resistance to ciprofloxacin was detected in the recovered cells (∼2× MICoriginal). Homogeneous batch culture experiments were performed with the same temporal exposure history to ciprofloxacin concentration, the same and higher initial cell densities, and the same and higher nutrient (i.e., LB) concentrations as in the MGC. In all batch experiments, E. coli 307 developed higher ciprofloxacin resistance after exposure, ranging from 4 to 24× MICoriginal in all replicates. Hence, these results suggest that the presence of spatial gradients appears to reduce the driving force for E. coli 307 adaptation to ciprofloxacin, which suggests that results from batch experiments may over predict the development of antibiotic resistance in natural environments.

Subsurface environments often contain mixtures of contaminants in which the microbial degradation of one pollutant may be inhibited by the toxicity of another. Agricultural settings exemplify these complex environments, where antimicrobial leachates may inhibit nitrate bioreduction, and are the motivation to address this fundamental ecological response. In this study, a microfluidic reactor was fabricated to create diffusion-controlled concentration gradients of nitrate and ciprofloxacin under anoxic conditions in order to evaluate the ability of Shewanella oneidenisis MR-1 to reduce the former in the presence of the latter. Results show a surprising ecological response, where swimming motility allow S. oneidensis MR-1 to accumulate and maintain metabolic activity for nitrate reduction in regions with toxic ciprofloxacin concentrations (i.e., 50× minimum inhibitory concentration, MIC), despite the lack of observed antibiotic resistance. Controls with limited nutrient flux and a nonmotile mutant (Δ flag) show that cells cannot colonize antibiotic rich microenvironments, and this results in minimal metabolic activity for nitrate reduction. These results demonstrate that under anoxic, nitrate-reducing conditions, motility can control microbial habitability and metabolic activity in spatially heterogeneous toxic environments.

Rapid reduction of aqueous ClO4(-) to Cl(-) by H2 has been realized by a heterogeneous Re(hoz)2-Pd/C catalyst integrating Re(O)(hoz)2Cl complex (hoz = oxazolinyl-phenolato bidentate ligand) and Pd nanoparticles on carbon support, but ClOx(-) intermediates formed during reactions with concentrated ClO4(-) promote irreversible Re complex decomposition and catalyst deactivation. The original catalyst design mimics the microbial ClO4(-) reductase, which integrates Mo(MGD)2 complex (MGD = molybdopterin guanine dinucleotide) for oxygen atom transfer (OAT). Perchlorate-reducing microorganisms employ a separate enzyme, chlorite dismutase, to prevent accumulation of the destructive ClO2(-) intermediate. The structural intricacy of MGD ligand and the two-enzyme mechanism for microbial ClO4(-) reduction inspired us to improve catalyst stability by rationally tuning Re ligand structure and adding a ClOx(-) scavenger. Two new Re complexes, Re(O)(htz)2Cl and Re(O)(hoz)(htz)Cl (htz = thiazolinyl-phenolato bidentate ligand), significantly mitigate Re complex decomposition by slightly lowering the OAT activity when immobilized in Pd/C. Further stability enhancement is then obtained by switching the nanoparticles from Pd to Rh, which exhibits high reactivity with ClOx(-) intermediates and thus prevents their deactivating reaction with the Re complex. Compared to Re(hoz)2-Pd/C, the new Re(hoz)(htz)-Rh/C catalyst exhibits similar ClO4(-) reduction activity but superior stability, evidenced by a decrease of Re leaching from 37% to 0.25% and stability of surface Re speciation following the treatment of a concentrated "challenge" solution containing 1000 ppm of ClO4(-). This work demonstrates the pivotal roles of coordination chemistry control and tuning of individual catalyst components for achieving both high activity and stability in environmental catalyst applications.

The ability of Pseudomonas stutzeri strain DCP-Ps1 to drive CaCO3 biomineralization has been investigated in a microfluidic flowcell (i.e., micromodel) that simulates subsurface porous media. Results indicate that CaCO3 precipitation occurs during NO3(-) reduction with a maximum saturation index (SIcalcite) of ∼1.56, but not when NO3(-) was removed, inactive biomass remained, and pH and alkalinity were adjusted to SIcalcite ∼ 1.56. CaCO3 precipitation was promoted by metabolically active cultures of strain DCP-Ps1, which at similar values of SIcalcite, have a more negative surface charge than inactive strain DCP-Ps1. A two-stage NO3(-) reduction (NO3(-) → NO2(-) → N2) pore-scale reactive transport model was used to evaluate denitrification kinetics, which was observed in the micromodel as upper (NO3(-) reduction) and lower (NO2(-) reduction) horizontal zones of biomass growth with CaCO3 precipitation exclusively in the lower zone. Model results are consistent with two biomass growth regions and indicate that precipitation occurred in the lower zone because the largest increase in pH and alkalinity is associated with NO2(-) reduction. CaCO3 precipitates typically occupied the entire vertical depth of pores and impacted porosity, permeability, and flow. This study provides a framework for incorporating microbial activity in biogeochemistry models, which often base biomineralization only on SI (caused by biotic or abiotic reactions) and, thereby, underpredict the extent of this complex process. These results have wide-ranging implications for understanding reactive transport in relevance to groundwater remediation, CO2 sequestration, and enhanced oil recovery.

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 comprises80% 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 microfluidic pore structure etched into a silicon wafer was used as a two-dimensional model subsurface sedimentary system (i.e., micromodel) to study mineral precipitation and permeability reduction relevant to groundwater remediation and geological carbon sequestration. Solutions containing CaCl2 and Na2CO3 at four different saturation states (Ω = [Ca2+][CO32−]/KspCaCO3) were introduced through two separate inlets, and they mixed by diffusion transverse to the main flow direction along the center of the micromodel resulting in CaCO3 precipitation. Precipitation rates increased and the total amount of precipitates decreased with increasing saturation state, and only vaterite and calcite crystals were formed (no aragonite). The relative amount of vaterite increased from 80% at the lowest saturation state (Ωv = 2.8 for vaterite) to 95% at the highest saturation state (Ωv = 4.5). Fluorescent tracer tests conducted before and after CaCO3 precipitation indicate that pore spaces were occluded by CaCO3 precipi...

Recent work demonstrates reduction of aqueous perchlorate by hydrogen at ambient temperatures and pressures using a novel rhenium-palladium bimetal catalyst immobilized on activated carbon (Re/Pd-AC). This study examines the influence of Re speciation on catalyst activity and stability. Rates of perchlorate reduction are linearly dependent on Re content from 0-6 wt %, but no further increases are observed at higher Re contents. Surface-immobilized Re shows varying stability and speciation both in oxic versus H(2)-reducing environments and as a function of Re content. In oxic solutions, Re immobilization is dictated by sorption of the Re(VII) precursor, perrhenate (ReO(4)(-)), to activated carbon via electrostatic interactions. Under H(2)-reducing conditions, Re immobilization is significantly improved and leaching is minimized by ReO(4)(-) reduction to more reduced species on the catalyst surface. X-ray photoelectron spectroscopy shows two different Re binding energy states under H(2)-reducing conditions that correspond most closely to Re(V)/Re(IV) and Re(I) reference standards, respectively. The distribution of the two redox states varies with Re content, with the latter predominating at lower Re contents where catalyst activity is more strongly dependent on Re content. Results demonstrate that both lower Re contents and the maintenance of H(2)-reducing conditions are key elements in stabilizing the active Re surface species that are needed for sustained catalytic perchlorate treatment.

Knowledge of IFT values for chemical mixtures helps guide the design and analysis of various processes, including NAPL remediation with surfactants or alcohol flushing, enhanced oil recovery, and chemical separation technologies, yet available literature values are sparse. A comprehensive comparison of thermodynamic and empirical models for estimating interfacial tension (IFT) of organic chemical mixtures with water is conducted, mainly focusing on chlorinated organic compounds for 14 ternary, three quaternary, and one quinary systems. Emphasis is placed on novel results for systems with three and four organic chemical compounds, and for systems with composite organic compounds like lard oil and mineral oil. Seven models are evaluated: the ideal and nonideal monolayer models (MLID and MLNID), the ideal and nonideal mutual solubility models (MSID and MSNID), an empirical model for ternary systems (EM), a linear mixing model based on mole fractions (LMMM), and a newly developed linear mixing model based on volume fractions of organic mixtures (LMMV) for higher order systems. The two ideal models (MLID and MSID) fit ternary systems of chlorinated organic compounds without surface active compounds relatively well. However, both ideal models did not perform well for the mixtures containing a surface active compound. However, for these systems, both the MLNID and MSNID models matched the IFT data well. It is shown that the MLNID model with a surface coverage value (0.00341 mmol/m2) obtained in this study can practically be used for chlorinated organic compounds. The LMMM results in poorer estimates of the IFT as the difference in IFT values of individual organic compounds in a mixture increases. The EM, with two fitting parameters, provided accurate results for all 14 ternarysystems including composite organic compounds. The new LMMV method for quaternary and higher component systems was successfully tested. This study shows that the LMMV may be able to be used for higher component systems and it can be easily incorporated into compositional multiphase flow models using only parameters from ternary systems.

To better understand sorption, separation methods are needed to enrich soils and sediments in one or more types of carbonaceous materials (CM), especially in fine grain materials where physical separation is not possible. We evaluated a series of chemical and thermal treatment methods by applying them to four different CMs prepared in our laboratory: a humic acid (HA), a char, a soot, and a heat-treated soot (HN-soot). Before and after each treatment step, CM properties were evaluated including aqueous phase sorption with trichloroethene (TCE). Results indicate that treatment with hydrofluoric (HF) and hydrochloric acid (HCI) to remove silicate minerals, and with trifluoroacetic acid (TFA) to remove easily hydrolyzable organic matter, has relatively little effect on the humic acid mass (19% change) and TCE sorption to this material. Subsequent treatment with NaOH to extract fulvic and humic acids results in almost complete removal of the humic acid mass (92%) and has little to no effect on the masses of the char and two soots (8% change) and TCE sorption to these materials. Treatment with acid dichromate to remove kerogen and humin also has little effect on masses of the char and soots (16% change), but TCE sorption to these materials is significantly altered (by10x in some cases), and there is strong evidence of surface oxidation based on X-ray photoelectron and diffuse reflectance Fourier transform infrared spectroscopy results. The last step, thermal treatment, which targets char removal, also destroys96% of the soots pretreated with acid dichromate. However, when thermal treatment is applied to the original soots,32% of these materials are destroyed. Thermal oxidation also affects sorption to one of the soots (by approximately 2x at low concentration), and surface oxidation is evident. These results suggest that treatment with HCl, HCl/HF, TFA, and NaOH can be applied to soils and sediments to obtain CM enrichment fractions for sorption evaluation, but that acid dichromate and heat treatment may not be appropriate for these purposes.

Desorption profiles of trichloroethylene (TCE), tetrachloroethylene (PCE), and a TCE-PCE mixture were measured for three natural solids and four zeolites. Initial sorbed mass (M i ) in slow desorbing sites of natural solids and in micropores of zeolites were obtained from desorption profiles. In natural solids, M i increases with recalcitrant organic matter content. In zeolites, M i generally increases with decreasing micropore width and increasing micropore hydrophobicity, but the effect of hydrophobicity is stronger. In both natural solids and zeolites, competition between TCE and PCE causes M i for each sorbate in the mixture to be less than or similar to that for each sorbate alone. Zeolite results indicate that micropore width affects this competition more than micropore hydrophobicity for the solids examined. Desorption in all solids was simulated with the radial diffusion model, either alone or coupled with the advection-dispersion equation when necessary; diffusion rate constants (D/R 2 ) were obtained. In natural solids, mean values of D/R 2 increase with decreasing recalcitrant organic matter content. In zeolites, values of D/R 2 generally increase with increasing micropore width, while they are a weak function of hydrophobicity. In both natural solids and zeolites, competition between TCE and PCE causes D/R 2 for each sorbate in the mixture to generally be larger than that for each sorbate alone. Zeolite results indicate that the effects of competition on D/R 2 generally decrease with decreasing micropore width for the solids examined; a trend with micropore hydrophobicity is not apparent. For the three natural solids and four zeolites examined in this study, the similar effects of competition between TCE and PCE on values of M i and D/R 2 and the overlapping range of D/R 2 values support the hypothesis that diffusion through hydrophobic micropores affects and may control slow mass transfer processes in the recalcitrant organic matter of natural solids. These results contribute to the fundamental understanding of slow mass transfer processes in natural solids, and they indicate that characterization of micropore width and polarity may be necessary to predict organic chemical transport and fate.

The anaerobic halorespiring microorganism, Sulfurospirillum multivorans, was observed in the pore structure of an etched silicon wafer to determine how flow hydrodynamics and mass transfer limitations along a transverse mixing zone affect biomass growth. Tetrachloroethene (PCE, an electron acceptor, 0.2 mM) and lactate (an electron donor, 2 mM) were introduced as two separate and parallel streams that mixed along a reaction line in the pore structure. The first visible biomass occupied a single line of pores in the direction of flow, a few pore bodies from the micromodel centerline. This growth was initially present as small aggregates; over time, these grew and fused to form finger-like structures with one end attached to downgradient ends of the silicon posts and the other end extending into pore bodies in the direction of flow. Biomass did not grow in pore throats as expected, presumably because shear forces were not favorable. Over the next few weeks, the line of growth migrated upward into the PCE zone and extended over a width of up to five pore spaces. When the PCE concentration was increased to 0.5 mM, the microbial biomass increased and growth migrated down toward the lactate side of the micromodel. A new analytical model was developed and used to demonstrate that transverse hydrodynamic dispersion likely caused the biomass to move in the direction observed when the PCE concentration was changed. The model was unable, however, to explain why growth migrated upward when the PCE concentration was initially constant. We postulate that this occurred because PCE, not lactate, sorbed to biofilm components and that biomass on the lactate side of the micromodel was limited in PCE. A fluorescent tracer experiment showed that biomass growth changed the water flow paths, creating a higher velocity zone in the PCE half of the micromodel. These results contribute to our understanding of biofilm growth and will help in the development of new models to describe this complex process.

Magnetic resonance imaging (MRI) was used to determine the effects of pore-scale heterogeneity on the dissolution of a nonaqueous phase liquid (NAPL) in water-saturated flow-through columns (1.2 cm in diameter) packed with either approximately 500 or approximately 1,000 micron diameter angular silica gel (referred to as SG500 and SG1000, respectively). Columns were contaminated with 1,3,5-trifluorobenzene at residual saturation and then purged with water at a constant Darcy velocity of 1.83 m/day. Three-dimensional 19F images were acquired every 2-5 h at an imaging resolution of 59 x 234 x 234 microm3. Imaging results show that the specific NAPL surface area (a(t)) is linearly related to the NAPL volumetric fraction (theta(n)) and that the constant of proportionality between these parameters is determined by the blob size and geometry distribution. Overall (expressed as the modified Sherwood number, Sh') and intrinsic (expressed as the apparent Sherwood number, Sh(apt)) mass transfer rate coefficients were calculated. Values of Sh' and Sh(apt) for SG500 were approximately three times less than those for SG1000. For both solids, Sh' first increased or stayed the same and then decreased with decreasing theta(n), while Shapt generally increased with decreasing theta(n). These results suggest that during dissolution new flow paths were created (i.e., bypass zones were eliminated) as NAPL dissolved, decreasing the fraction of NAPL-water interfaces adjacent to pores filled with stagnant water and the average diffusion length scale. Since at for SG500 was dominated by less spherical multipore blobs (as opposed to more spherical singlets for SG1000), these results also suggest that the extent of flow bypassing (and the average diffusion length scale) increases in systems with more irregular blobs. These results are important because Sh' correlations and a "sphere" dissolution model do not account for transient changes in the fraction of NAPL surface area that contributes to dissolution or for the effect of initial blob size and geometry distribution on this fraction.

In the first of this two-paper series, a new model is presented that simulates the effects of a temperature perturbation on the rate of slow desorption as a function of mass remaining. The model assumes slow desorption is controlled by one-dimensional diffusion from a single or many hydrophobic micropores and that the micropores of a geosorbent are defined by a γ distribution of diffusion rate constants. Simulation results indicate that during slow desorption the relative increase in flux upon heating increases with decreasing micropore width. Simulation results also indicate that the relative increase in flux upon heating increases with desorption time when diffusion occurs from successively smaller width micropores with decreasing mass remaining. In paper 2, the model is tested and used to examine micropore geometry in natural and model solids by simulating results from temperature stepped desorption (TSD) experiments.

Results from temperature stepped desorption (TSD) experiments are presented and compared with simulations from the TSD model presented in the first of this two-paper series. TSD columns were filled with a sand, a sediment, a soil, or a silica gel, all at 100% relative humidity. Next, TSD columns were equilibrated with trichloroethene (TCE), initially purged at 30 °C, and then heated to 60 °C after 100, 1000, or 10 000 min of slow desorption. One γ distribution of diffusion rate constants at 30 °C and one γ distribution of diffusion rate constants at 60 °C were used to simulate column results at all three heating times for a single solid. At each heating time, diffusion rate constants of the γ distributions at 30 °C and 60 °C were used to calculated an effective activation energy, Eact,eff. Values of Eact,eff for all solids were between 47 and 94 kJ/mol, on the order of activation energy values found for diffusion in microporous solids. Between 100 and 10 000 min heating times, the value of Eact,eff increa...

A new model was developed to determine if reduced uptake rates observed during isotope exchange experiments could plausibly be attributed to sterically hindered counter-diffusion in one-dimensional micropores. During exchange experiments, hydrogenated trichloroethylene ({sup 1}HTCE) was displaced with deuterated TCE (DTCE) in the slow-desorbing sites of a silica gel, a groundwater sediment, and a clay and silt fraction. To describe this process, the model accounts for co- and counter-diffusion of TCE isotopes in one-dimensional micropores, where each micropore type is defined by a single codiffusion rate constant and a single counter-diffusion rate constant. For silica gel, isotope exchange was simulated in a single micropore type. For geosorbents, isotope exchange was simulated in a distribution of micropore types characterized by a {gamma} distribution of diffusion rate constants. Simulation results indicate that (1) the proposed model accounts for the mechanisms controlling isotope exchange in the silica gel and the groundwater sediment and (2) the rate of counter-diffusion is up to 6 times slower than the rate of codiffusion. This suggests that steric hindrance between counter-diffusing sorbates can significantly affect mass transfer and, consequently, the transport of chemical mixtures in the subsurface.

To investigate counter-diffusion in microporous sorbents, the rate of exchange between deuterated trichloroethylene (DTCE) in fast desorbing sites and nondeuterated TCE ({sup 1}HTCE) in slow desorbing sites was measured. Exchange rates were measured for a silica gel, a Santa Clara sediment, and a Livermore clay/silt fraction, all at 100% relative humidity and 30 C. Initially, solids were packed into stainless steel columns and incubated with {sup 1}HTCE for 1--3 weeks. After incubation, {sup 1}HTCE was replaced with DTCE in fast desorbing sites. Next, columns were capped, and DTCE was allowed to exchange with {sup 1}HTCE in slow desorbing sites for 1, 3, or 30 days. Elution profiles were then measured to determine the extent of exchange that occurred while the columns were capped. Results from experiments conducted with different exchange times support the hypothesis that slow sorption kinetics is controlled by diffusion in micropores. For the silica gel and the Santa Clara sediment, {sup 1}HTCE was incompletely exchanged with DTCE after 30 days. This indicates that the counter-diffusion rate of DTCE into {sup 1}HTCE-filled micropores is less than the diffusion rate of {sup 1}HTCE into micropores not filled with TCE.

Isothermal desorption rates were measured at 15, 30, and 60 °C for trichloroethylene (TCE) on a silica gel, an aquifer sediment, a soil, a sand fraction, and a clay and silt fraction, all at 100% relative humidity. Temperature-stepped desorption (TSD) rates were measured for these solids in columns prepared and equilibrated at 30 °C, but heated instantaneously to 60 °C after ∼1000 min of slow desorption. Fast and slow elution rates are observed for all solids. Modeling results for the fast eluting fraction of TCE show that fast desorption is controlled by diffusion through aqueous filled mesopores. Rates predicted from diffusion and surface-barrier models are compared to slow isothermal and TSD rates. Diffusion model fits are superior to surface-barrier model fits in all cases. Slow diffusion coef ficients and a high activation energy calculated from silica gel data (∼34 kJ/mol) indicate that slow desorption is controlled by activated diffusion in micropores. Initial amounts of slow desorbing TCE do not a...

A review of Restoration of Contaminated Aquifiers: Petroleum Hydrocarbons and Organic Compounds (2nd Edition)