First-order Mass Research Articles

Characterization of oxygenated derivatives of isoprene related to 2‐methyltetrols in Amazonian aerosols using trimethylsilylation and gas chromatography/ion trap mass spectrometry

In the present study, we have tentatively identified the structures of three oxygenated derivatives of isoprene in Amazonian rain forest aerosols as the C(5) alkene triols, 2-methyl-1,3,4-trihydroxy-1-butene (cis and trans) and 3-methyl-2,3,4-trihydroxy-1-butene. The formation of these oxygenated derivatives of isoprene can be explained by acid-catalyzed ring opening of epoxydiol derivatives of isoprene, namely, 1,2-epoxy-2-methyl-3,4-dihydroxybutane and 1,2-dihydroxy-2-methyl-3,4-epoxybutane. The structural proposals of the C(5) alkene triols were based on chemical derivatization reactions and detailed interpretation of electron and chemical ionization mass spectral data, including data obtained from first-order mass spectra, deuterium labeling of the trimethylsilyl methyl groups, and MS(2) ion trap experiments. The characterization of 2-methyl-1,3,4-trihydroxy-1-butene (cis and trans) and 3-methyl-2,3,4-trihydroxy-1-butene in forest aerosols is important from an atmospheric chemistry viewpoint in that these compounds hint at the formation of intermediate isomeric epoxydiol derivatives of isoprene and as such provide mechanistic insights into the formation of the previously reported 2-methyltetrols through photooxidation of isoprene.

Reactive Solute Transport in a Nonstationary, Fractured Porous Medium: A Dual-Porosity Approach

A numerical method of moment is developed for solute flux through a nonstationary, fractured porous medium. Solute flux is described as a space-time process where time refers to the solute flux breakthrough and space refers to the transverse displacement distribution at a control plane. A first-order mass diffusion model is applied to describe interregional mass diffusion between fracture (advection) and matrix (nonadvection) regions. The chemical is under linear equilibrium sorption in both fracture and matrix regions. Hydraulic conductivity in the fracture region is assumed to be a spatial random variable. In this study, the general framework of Zhang et al.(2000) is adopted for solute flux in a nonstationary flow field. A time retention function related to physical and chemical sorption in the dual-porosity medium is developed and coupled with solute advection along random trajectories. The mean and variance of total solute flux are expressed in terms of the probability density function of the parcel travel time and transverse displacement. The influences of various factors on solute transport are investigated. These factors include the interregional mass diffusion rate between fracture and matrix regions, chemical sorption coefficients in both regions, water contents in both regions, and location of the solute source. In comparison with solute transport in a one-region medium, breakthrough curves of the mean and variance of the total solute flux in a two-region medium have lower peaks and longer tails. As compared with the classical stochastic studies on solute transport in fractured media, the numerical method of moment provides an approach for applying the stochastic method to study solute transport in more complicated fractured media.

Experimental Determination of Effective Diffusion Parameters in the Matrix of Fractured Till

Diffusion is often the dominant mode of solute transport in soils when advection is minimal. This paper describes the application of a radial diffusion cell method to estimate the effective diffusion coefficient ( D e ) and effective diffusive porosity (θDe ) for use in solute transport models for fractured-porous media. Twenty-four experiments were conducted for 28 d using three conservative solutes (Br, PFBA, and PIPES) on eight late Wisconsinan and Pre-Illinoian till samples from Iowa. The mean value of the total porosity (θT) of the till samples was 30.0%. Concentrations of the three tracers in the reservoir decreased with time and eventually approached equilibrium concentrations. A model simulated the observed concentration data and the modified goodness-of-fit ( d 1) values ranged from 0.878 to 0.950. Mean values of θDe from the model were 28.3 (Br−), 26.5 (PFBA), and 21.6% (PIPES) and there were significant differences in θDe among the three tracers ( p = 0.05). Mean values of D e were 5.6 × 10−10 m2 s−1 (Br−), 2.9 × 10−10 m2 s−1 (PFBA), and 1.3 × 10−10 m2 s−1 (PIPES). Values of D e differed significantly by compound and were significantly different ( p = 0.05) from the aqueous diffusion coefficient ( D ). Calculated mean values of the first-order mass exchange coefficient (α) were 8.4 × 10−7 (Br−), 4.1 × 10−7 (PFBA), and 1.6 × 10−7 s−1 (PIPES); they differed by compound ( p = 0.05) and generally decreased with increasing molecular weight of the tracer. This study confirmed that the radial diffusion cell method is an efficient method to estimate effective diffusion parameters necessary to accurately model solute transport in fractured till and soil.

Sediment accumulation and organic material flux in a managed mangrove ecosystem: estimates of land–ocean–atmosphere exchange in peninsular Malaysia

Rates of sediment accumulation and organic matter decomposition in mangrove forests of different age were examined at the Matang Mangrove Forest Reserve in peninsular Malaysia. These data were used with previous findings to construct a first-order mass balance of carbon to determine whether the ecosystem is net heterotrophic or autotrophic, and to estimate land–ocean–atmosphere exchange. Measurements of various carbon and nitrogen processes in sediments suggest that organic matter is rapidly and efficiently mineralized to a depth of 1 m. Rates of total carbon oxidation in sediments ( T COX) ranged from 77 to 118 mmol C m −2 day −1 with no significant differences with forest age. Sulfate reduction (range: 19–53 mmol S m −2 day −1) appeared to be the dominant decomposition pathway, accounting for an average of 51–75% of sediment T COX. Aerobic respiration (range: 5–20 mmol C m −2 day −1) accounted for ≈5–20% of mean T COX. Methanogenesis was not measurable. Denitrification (range: 0.4–11.0 mmol N 2 m −2 day −1) was rapid but very variable, and may have accounted for ≈25% of T COX at the old-growth forest. Rates of sediment mass accumulation (range: 2.2–11.4 kg m −2 year −1) were rapid compared with previous measurements in mangroves. Efficiency of carbon burial (range: 23–29 mmol C m −2 day −1) in sediments increased from 16% to 27% from the youngest to the oldest forest. The ratio of sediment T COX to mangrove net primary production decreased from 28% to 7% with increasing forest age. The efficiency of N mineralization (range: 67–81%) and burial (range: 10–29%) in sediments showed the opposite pattern. Tree N demand required from 8 to 22 mmol N m −2 day −1. Surface N 2 fixation was measurable only in two of six trials, but was rapid (range: 2.3–3.0 mmol N 2 m −2 day −1). A mass balance of carbon for the entire reserve suggests that the ecosystem is currently net autotrophic, with a net ecosystem production of ≈21×10 9 mol C year −1 and an ecosystem P/ R of 1.4. Despite considerable uncertainty, forest production and respiration clearly dominates carbon flow, accounting for ≈95% of total inputs and ≈79% of total outputs, respectively. River inputs, phytoplankton primary production and trash fish inputs to fish cage aquaculture in the waterways were comparatively minor. Export of mangrove detritus equated to 25% of net canopy production; only 2% of total carbon input to the ecosystem was buried in mangrove sediments. Clear felling and thinning of forests accounted for <1% of total outputs; 73% of total carbon inputs to the ecosystem were respired. Our budget indicates that the major exchange of carbon in this managed ecosystem is between forest and atmosphere.

The abundance and source of mercury-binding organic ligands in Long Island Sound

The abundance and strength of mercury (Hg)-complexing organic matter was measured in samples collected from Long Island Sound (LIS) and related locations. A range in ligand-equivalent concentrations was found in LIS (0.3–6 nN). Rivers, lakes, sewage effluent, and marine porewaters were also sampled. When the ligand concentrations were normalized to dissolved organic carbon (DOC) concentrations, the samples fell into broad groups. Offshore and sewage effluent samples were relatively ligand-poor, while river, lake, and porewater samples were ligand-rich. Normalized ligand abundance in LIS water was intermediate. A first-order mass balance for ligand and DOC suggests that terrestrial organic matter and phytoplankton exudates are the dominant sources of ligand to LIS, while tidal exchange and an unknown term are the dominant sinks. Concentration distributions through the Connecticut River (CTR) estuary suggest pseudo-conservative mixing of the ligand derived from the Connecticut River watershed. The identity of the material is unknown, but ligand-to-DOC ratios are well below the ratio of bulk reduced sulfur to DOC.

Structural analysis of 2,6-[bis(alkyloxy)methyl]-phenyltin derivatives using electrospray ionization mass spectrometry.

Electrospray ionization (ESI) mass spectrometry was applied to the structural analysis of 23 2,6-[bis(alkyloxy)methyl]phenyltin(IV) derivatives. The mass spectra were measured in both polarity modes and multistage tandem mass spectrometric (MS(n)) measurements were performed on the ion trap analyser for positively charged tin-containing ions. The sum of complementary ions observed in the positive-ion mode (i.e. [M-R(3)](+) ion) and in the negative-ion mode (i.e. [R(3)](-) ion) permits molecular mass determination in spite of the fact that the molecular adducts were often missing even in the first-order mass spectra. The subsequent fragmentation of [M-R(3)](+) ions studied by MS(n) and the correlation of observed fragment ions with the expected structures of synthesized organotin(IV) compounds allowed us to understand the fragmentation behaviour and the mechanism of the ion formation for studied compounds. The typical neutral losses are alkenes, alcohols and aldehydes. The fragmentation pattern of one selected compound was supported by MS(n) measurements of an isotopically labelled analogue to confirm unusual ion-molecule reactions of some fragment ions with water in the ion trap.

Natural Attenuation of Fuel Hydrocarbon Contaminants:Correlation of Biodegradation with Hydraulic Conductivity in a Field Case Study

Two biodegradation models are developed to represent natural attenuation of fuel-hydrocarbon contaminants as observed in a comprehensive natural-gradient tracer test in a heterogeneous aquifer on the Columbus Air Force Base in Mississippi, USA. The first, a first-order mass loss model, describes the irreversible losses of BTEX and its individual components, i.e., benzene (B), toluene (T), ethyl benzene (E), and xylene (X). The second, a reactive pathway model, describes sequential degradation pathways for BTEX utilizing multiple electron acceptors, including oxygen, nitrate, iron and sulfate, and via methanogenesis. The heterogeneous aquifer is represented by multiple hydraulic conductivity (K) zones delineated on the basis of numerous flowmeter K measurements. A direct propagation artificial neural network (DPN) is used as an inverse modeling tool to estimate the biodegradation rate constants associated with each of the K zones. In both the mass loss model and the reactive pathway model, the biodegradation rate constants show an increasing trend with the hydraulic conductivity. The finding of correlation between biodegradation kinetics and hydraulic conductivity distributions is of general interest and relevance to characterization and modeling of natural attenuation of hydrocarbons in other petroleum-product contaminated sites.

Reactive solute transport in macroscopically homogeneous porous media: analytical solutions for the temporal moments

In this work, we investigate one-dimensional solute transport affected by rate-limited sorption, first-order mass transfer, and first-order transformation. Analytical expressions are obtained for the temporal moments of the solute in the solution phase. The effect of various rate coefficients on the temporal moments is examined. It was found that, in the presence of transformation reactions, the mean arrival time, and the spread and skewness of the breakthrough curves, are not monotonic functions of the rate coefficients. These solutions will be useful as a preliminary analysis tool for ascertaining the relative importance of various processes under given conditions. They may also be used to analyze the accuracy of various numerical techniques used for simulation of reactive transport.

Mass transfer from theophylline hydrogels of a‐PVA/H2O and a‐PVA/NaCl/H2O system on heating

AbstractTheophylline hydrogels of atactic‐poly(vinyl alcohol) (a‐PVA)/H2O and a‐PVA/NaCl/H2O systems were prepared followed by cyclic freezing (−30°C for 16 hr)–thawing (at room temperature for 8 hr) and one cycle gelation (at −20°C for 24 hr) processes, respectively. In order to prepare xerogels (dried hydrogels) of these hydogel systems, an apparently first‐order mass transfer phenomenon of water as evaporation was observed for a‐PVA/H2O hydrogel system, while heating at 60°C. The rate of evaporation decreased with increasing time in hyperbolic fashion. The total surface area (both lateral and two end surfaces of hydrogel matrix disc) decreased linearly for the first 90 min and thereafter had a tendency towards the steady‐state. The total mass flux showed time dependent linear reduction phenomenon, which is a characteristic physical behavior for these hydrogel systems on heat treatment. When NaCl was included in a‐PVA/H2O system mass transfer of water followed fourth‐order polynomial. But in consideration of a comparative study, sustained mass transfer was found from the hydrogel matrices of a‐PVA/H2O/NaCl system (gelation at −20°C). Copyright © 2003 John Wiley &amp; Sons, Ltd.

Nonequilibrium Nonaqueous Phase Liquid Mass Transfer Model for Soil Vapor Extraction Systems

Soil vapor extraction is a popular soil remediation technology that is hampered by less than optimal performance in the field due to mass transfer limitations. Therefore, laboratory column venting experiments were completed to quantify mass transfer limitations for the removal of multicomponent nonaqueous phase liquid (NAPL) contaminants from a silt loam soil at three water contents. The observed mass transfer limitations were quantified using a four phase multicomponent, nonequilibrium contaminant transport model based on first-order mass transfer kinetics. The overall mass transfer coefficient Kga was treated as a variable and modeled as a linear function of the NAPL volumetric fraction using two adjustable parameters (m, the slope parameter and Kgamin, the intercept). Both were back calculated from column venting data. The agreement between the calibrated model and experimental results were favorable for the removal of single and binary contaminants under conditions ranging from near equilibrium to severe mass transfer limitations and extended tailing. A strong dependency of Kga on water content was evident by the differences in Kgamin and to a lesser extent, m, at the three water contents investigated. A single expression Kga captured the performance of both components in the binary mixture. For the quaternary venting experiments a single expression for Kga captured the performance of all four components well under air dry conditions. However, the agreement between the hexane model versus the experimental result deteriorated significantly as the water content increased. This difference is attributed to hexane’s lower affinity for the water phase relative to the other three components in the mixture.

Procedure for the measurement of soil inputs of plant-protection agents washed off through vineyard canopy by rainfall.

Soil inputs produced by rainfall waters washed off through a Ribeiro vineyard (Galicia, northwestern Spain) confirmed that levels recovered are <5% of the added fungicides. Laboratory studies showed that some fungicides were degraded in the ODS cartridge used for the collection of fungicides washed off by rainfall. Procymidone wash-off data obtained in the vineyard can be explained by a first-order rate mass transfer model. A procedure for the collection of wash-off waters based on octadecylsilane solid phase extraction, followed by an analytical method of reverse-phase liquid chromatography with diode array detection is presented for the evaluation of fungicide inputs into soils after being washed off vineyards by rainfalls. Quality parameters of the analytical method yielded good precision (RSD < 10%) and low detection limits (ranging between 1 and 21 microg/L).

A transfer function technique to describe odor causing VOCs transport in a ventilated airspace with mixing/adsorption heterogeneity

A model describing odor causing volatile organic compounds (VOC-odor) transport in a ventilated airspace influenced by heterogeneity of adsorption surface of ambient aerosol and air mixing pattern is proposed and analyzed based on a transfer function modeling technique. In this study an advection–reaction impulse/step response function for VOC-odor is assumed. The system process presented by an ensemble transfer function is solved analytically in the Laplace domain. The analytical results are then numerically inverted using a modified fast Fourier transform algorithm. The model requires the specification of probability density function for residence time of airflow and for both equilibrium linear partitioning and first-order mass transfer rate parameters to quantify the specific air mixing pattern and transport processes. The model predicts the ensemble mean VOC-odor concentrations for a variety of adsorption kinetics and mixing pattern combinations as a function of the boundary impulse/step response inputs as well as residence time and adsorption rate statistics. The general behavior of output VOC-odor profiles is analyzed through the effects of mean adsorption rate coefficient, mean linear partitioning constant, mixing efficiency, mean residence time and coefficient of variations of both linear partitioning and rate coefficients. It indicates that when mixing/adsorption heterogeneity exists, simple complete mixing assumption and simple distribution of rate constant is inherently not sufficient to represent a more generally distributed mixing/adsorption process of VOC-odor transport in a ventilated airspace.

Optimization of a liquid chromatography method based on simultaneous electrospray ionization mass spectrometric and ultraviolet photodiode array detection for analysis of flavonoid glycosides.

Different reversed-phase liquid chromatography (LC) columns of conventional dimensions were coupled to an ultraviolet photodiode array detector (UV-DAD) and a magnetic sector-type spectrometer, equipped with an electrospray ionization (ESI) source, by a laboratory-made flow splitter. A mixture of three flavonoid-O-glycosides was employed to examine the effects of the solvent composition, the flow rate, the stationary phase, the pH and the organic acid added, on the chromatographic separation, the UV-DAD detection, the ESI process and the entire LC system with ESI-MS and UV-DAD detection. In the positive ion mode, methanol containing 1% acetic acid was by far the most sensitive in ESI-MS analysis, whereas an acetonitrile/water mobile phase containing 0.5% formic acid was proved to give the best sensitivity in LC/ESI-MS/UV-DAD analysis. In the negative ion mode, the highest sensitivity was obtained with a mobile phase containing 0.1% formic acid, while addition of bases decreased the sensititvity. The optimal flow rate was higher in negative ESI (20-50 micro L/min) than in positive ESI (5 micro L/min), and the percentage of organic phase had an influence on the sensitivity of ESI-MS detection. With regard to the selection of a suitable C(18) reversed-phase LC column, a column which is well end-capped is to be preferred, because residual silanol groups appear to impair the separation of flavonoid glycosides. The optimized LC/ESI-MS/UV-DAD method was applied to a commercial Crataegus extract, which is used in phytomedicine to treat cardiovascular problems and is known to be rich in flavonoids. It is demonstrated how UV spectra and first-order ESI mass spectra allow a fast characterization of flavonoids, even if reference compounds are not available or at hand.

Uptake and release of zinc by aquatic bryophytes ( Fontinalis antipyretica L. ex. Hedw.)

The zinc uptake and posterior release by an aquatic bryophyte— Fontinalis antipyretica L. Ex Hedw .—was experimentally studied in laboratory exposing the plants to different zinc concentrations in the range, 1.0–5.0 mg l −1, for a 144 h contamination period, and then exposed to metal-free water for a 120 h decontamination period. The experiments were carried out in perfectly mixed contactors at controlled illumination, using mosses picked out in February 1997, with a background initial zinc concentration of 263 mg g −1 (dry wt.). A first-order mass transfer kinetic model was fitted to the experimental data to determine the uptake and release constants, k 1 and k 2, the zinc concentration in mosses at the end of the uptake period, C mu, and at the equilibrium, for the contamination and decontamination stages, C me and C mr, respectively. A bioconcentration factor, BCF= k 1/ k 2 (zinc concentration in the plant, dry wt./zinc concentration in the water) was determined. A biological elimination factor defined as BEF=1− C mr/ C mu was also calculated. BCF decreases from about 4500 to 2950 as Zn concentration in water increases from 1.05 to 3.80 mg l −1. BEF is approximately constant and equal to 0.80. Comparing Zn and Cu accumulation by Fontinalis antipyretica, it was concluded that the uptake rate for Zn (145 h −1) is much lower than for Cu (628 h −1) and the amount retained by the plant decreased by a factor of about seven.

Spatial Moments for Reactive Transport in Heterogeneous Porous Media

The transport of reactive chemicals through porous media is generally affected by multiple reactions with different rate constants. In this work, we investigate one-dimensional transport affected by rate-limited sorption, first-order mass transfer, and first-order transformation and derive analytical solutions for the spatial moments of the solute in the solution phase using an exponentially increasing dispersivity function to account for the spatial heterogeneity of hydraulic conductivity. Illustrative examples are used to discuss the effect of various rate constants on the travel distance, variance, and skewness. It is shown that the spatial heterogeneity of material properties can be satisfactorily accounted for by using an increasing macrodispersivity function. These solutions will be useful as a preliminary analysis tool for ascertaining the relative importance of various processes under given conditions. They may also be used to analyze the accuracy of various numerical techniques used for simulatio...

A simple special relativistic perturbation scheme for yielding the general relativistic behavior of point particles and photons in the gravitational field of stars

Using the principle of the equivalence of mass and energy of the special theory of relativity, it is shown that a Lagrangian yielding the same equations of motion that Einstein originally derived for the precession of the perihelion of Mercury and the bending of light by the sun may be derived by using a new perturbation scheme applied to the Lagrangian for an orbiting point particle. The magnitude of the Newtonian potential energy term appearing in the Lagrangian of an orbiting point particle is divided by c2 to yield a first-order mass correction to the potential energy. The corrected first-order potential energy term is in turn divided by c2 to yield a second-order mass correction to the total potential energy, and so forth. Successive correction terms are shown to yield successively smaller mass corrections to the total potential energy. This scheme, up to the first-order term in perturbation, leads to the astronomically observed corrections to the advance of the perihelion of Mercury and the bending of light by the sun attributable to general relativistic effects.

Modeling of polychlorinated biphenyl removal from contaminated soil using steam.

Microwave-generated steam technology shows promise as an effective remediation alternative for removal of polychlorinated biphenyls (PCBs) from contaminated soils, based on our laboratory-scale experiments. The overall process can be described by a nonisothermal, unsteady, coupled heat and multicomponent PCB mass-transport model in a multiphase, variably saturated, porous soil medium. In this paper, a multicomponent PCB mass-transport model is presented that assumes evaporation is an important removal mechanism and that is based on first-order mass transfer between the interface of PCB films and the bulk steam. The model was calibrated using the experimental data, and the calibrated model was verified by computational mass-balance checks and comparisons with laboratory-scale column experimental results. From a qualitative point of view, the calibrated model successfully simulated the transport of PCBs in variably saturated soil media. The calculated increase/decrease factors of physicochemical properties of PCBs as a function of temperature in the soil, water, and free phases were consistent with the model hypothesis of an evaporation process. The effects of mass-transfer coefficients and initial PCB concentrations in the soils on PCB removal rates were investigated using the numerical code. It was determined that the PCB removal rates were sensitive to mass-transfer coefficients and initial PCB concentrations. Although the steam:soil mass ratios required to achieve a given percentage removal were lower for lower initial PCB soil concentrations, steam: soil mass ratios required to achieve a given unit mass removal were higherfor lower initial PCB soil concentrations.

Tandem mass spectra of tryptic peptides at signal-to-background ratios approaching unity using electrospray ionization high-field asymmetric waveform ion mobility spectrometry/hybrid quadrupole time-of-flight mass spectrometry.

High-field asymmetric waveform ion mobility spectrometry (FAIMS) has been coupled to a quadrupole time-of-flight mass spectrometer for the tandem mass spectrometric analysis of tryptic peptides of pig hemoglobin. Using FAIMS, low levels (fmol/microL) of multiply charged tryptic peptides were separated from relatively intense chemical background such that their tandem mass spectra (MS/MS) lacked many background-related fragment ions observed using a conventional ESI-QqTOFMS instrument. Substantial improvements in both first-order and tandem mass spectra were realized while maintaining approximately the same absolute intensities.

A transfer function model to describe odor causing VOCs transport in a ventilated airspace with mixing/adsorption heterogeneity

The ability of a transfer function modeling technique is evaluated to explain the odor causing VOCs (VOC-odor) transport processes influenced by heterogeneity of adsorption surface of ambient aerosol and air mixing pattern in a ventilated airspace. An advection–reaction impulse/step response function is used to generalize the dynamic transport of VOC-odor in heterogeneous mixing/adsorption ventilated airspace. The system process presented by an ensemble transfer function is solved analytically in the Laplace domain. The model requires the specification of probability density function (pdf) for residence time of airflow and for both equilibrium linear partitioning and first-order mass transfer rate parameters of gas/solid phase to quantify the specific air mixing pattern and transport processes. The model predicts the ensemble mean VOC-odor concentrations for a variety of adsorption kinetics and mixing pattern combinations as a function of the boundary impulse/step response inputs as well as residence time and adsorption rate statistics. The general behavior of output VOC-odor profiles is analyzed through the effects of mean adsorption rate coefficient, mean linear partitioning constant, mixing efficiency, mean residence time and coefficient of variations of both linear partitioning and rate coefficients. This study indicates that when mixing/adsorption heterogeneity exists, simple complete mixing assumption and simple distribution of rate constant are inherently not sufficient to represent a more generally distributed mixing/adsorption process of VOC-odor transport in a ventilated airspace.

Mass transport effects on the kinetics of nitrobenzene reduction by iron metal.

To evaluate the importance of external mass transport on the overall rates of contaminant reduction by iron metal (Fe0), we have compared measured rates of surface reaction for nitrobenzene (ArNO2) to estimated rates of external mass transport in a permeable reactive barrier (PRB). The rate of surface reaction was measured at a polished Fe0 rotating disk electrode (RDE) in an electrochemical cell, and the rate of mass transport was estimated from a correlation for mass transport in packed-bed reactors. The kinetics of ArNO2 reduction were studied in pH 8.4 borate buffer at a potential below which an oxide film would form. The cathodic current measured in this system was dependent on the electrode rotation rate, and the measured first-order heterogeneous rate coefficient for surface reaction [krxn = (1.7 +/- 0.2) x 10(-3) cm s-1] was about 10 times faster than the first-order mass transport rate coefficient (kmt approximately 2 x 10(-4) cm s-1) estimated for PRBs. The similarity between rates of surface reaction and mass transport suggest that it may be important to consider mass transport processes in the design of PRBs for contaminants such as nitroaromatics that are highly reactive with Fe0.