Fate and transport of bromide and mononuclear aromatic hydrocarbons in aqueous solutions through Berea Sandstone

A series of miscible displacement tests were performed on a 51 mm wide by 76 mm long well-laminated core of Berea Sandstone to determine the transport parameters of the anion bromide and a homologous series of seventeen mononuclear aromatic hydrocarbons (MAHs). In each test, a continuous input pulse of a single tracer was passed through the cylindrical core housed in a hydrostatic core holder at a confining pressure of 200 bar. The effluent concentration, as measured by in-line UV absorbance, versus time resulted in smooth high-resolution sinusoidal breakthrough curves (BTCs). In comparison to the near Gaussian BTCs of bromide, the transport of the MAHs was differentially retarded with minimal levels of delayed transport along the more rapid flow lines, but with progressively more along the slower flow paths. These results show that despite a lack of significant hydraulic heterogeneity, there is a high degree of heterogeneity among the sorption sites. The BTCs were aptly modeled with a one-dimensional flow model consisting of a mixture of instantaneous equilibrium and rate-limited reversible sorption sites. The relative fraction of instantaneous sites increased proportionately with the rate the subject MAH passed through the core. Potential quantitative structure-retention relationships (QSRR) between common chemical parameters of the MAHs and their overall retardation factors, sorption coefficients and the fraction of instantaneous equilibrium were evaluated. Among the compounds examined, relatively strong correlations were found with molecular weight, aqueous solubility, and octanol-water partitioning coefficient with which relative MAH transport retardation, the linear phase distribution coefficient, and the dimensionless partitioning coefficient between sorption sites.

Optimal experimental designs for estimating Henry's law constants via the method of phase ratio variation

When measuring Henry's law constants (k_H) using the phase ratio variation (PRV) method via headspace gas chromatography (G_C), the value of k_H of the compound under investigation is calculated from the ratio of the slope to the intercept of a linear regression of the inverse G_C response versus the ratio of gas to liquid volumes of a series of vials drawn from the same parent solution. Thus, an experimenter collects measurements consisting of the independent variable (the gas/liquid volume ratio) and dependent variable (the G_C^-1 peak area). A review of the literature found that the common design is a simple uniform spacing of liquid volumes. We present an optimal experimental design which estimates k_H with minimum error and provides multiple means for building confidence intervals for such estimates. We illustrate performance improvements of our design with an example measuring the k_H for Naphthalene in aqueous solution as well as simulations on previous studies. Our designs are most applicable after a trial run defines the linear G_C response and the linear phase ratio to the G_C^-1 region (where the PRV method is suitable) after which a practitioner can collect measurements in bulk. The designs can be easily computed using our open source software optDesignSlopeInt, an R package on CRAN.

Solubility enhancement effect of cyclodexin on groundwater pollutants

Cyclodextrin (CD) molecules are polycyclic glucose oligomers that have a hydrophilic exterior and a hydrophobic cavity. This structure provides CD the characteristic of enhancing the solubility of groundwater pollutants. The degree to which CD increases the apparent aqueous solubility of certain chemicals has been defined as the solubility enhancement factor (SEF). In this study, a novel and experimentally simple method has been developed to determine the SEF, which can be mathematically obtained by ratio of apparent and traditional Henry's law constants. The effects of temperature and CD concentration on the SEFs and CD cavity occupation have been investigated. Our results show that SEF values are inversely related to temperature for most examined chemicals, which is consistent with the assertion that the CD-chemical complexes are less stable at higher temperatures. The exception is toluene that shows the least SEF fluctuation within the temperature range studied (5 to 65 °C). This may indicate that the toluene-CD complex is particularly stable. As the definition of SEF predicted, linear relationships were found between the SEFs and CD concentrations for all the subject chemicals. The CD cavity occupation fraction at 5 °C were 3.14, 2.55, 2.04, 1.60, and 1.67 times greater than the values at 65 °C for of trichloroethylene, perchloroethylene, bezene, ethylbenze, and o-xylene, respectively. The fraction of CD cavities occupied was found to be inversely related to the CD concentration for all tested chemicals when pollutant mass are held constant. This study provides important information to accurately evaluate the performance of CD when used for aquifer remediation.

Dynamic interactions between cyclodextrin, an organic pollutant, and granular activated carbon in column studies

In this study, the dynamic interactions between cyclodextrin (CD), an organic chemical and granular activated carbon (GAC) were investigated using column studies. Breakthrough curves of a chlorinated solvent, trichloroethylene (TCE) were obtained over a range of concentrations of 2-hydroxypropyl-beta-cyclodextrin (HPCD) (0, 20, and 50 g L(-1)) and flow velocities (1.0, 4.0, and 10.2 mL min(-1)). Important transport parameters (i.e. residence time, dispersion coefficient, retardation factor) were estimated using truncated temporal moment analysis. Our result shows that increasing CD concentration resulted in earlier TCE breakthrough, demonstrated by decreasing residence times which are 306.23, 151.26, and 102.24 pore volumes for 0, 20, and 50 g L(-1) CD respectively. Comparison of the original breakthrough curves (BTCs) under different CD concentrations to the solubility-enhancement-rescaled BTCs showed (1) the presence of CD decreases the relative degree of TCE sorption to GAC and (2) all 3 curves exhibited similar rescaled times at which they reach 50% of the input concentration. The lowest flow rate, (1.0 mL min(-1)), resulted in a more symmetrical BTC, indicating more ideal conditions were achieved under the longer exposure time provided by this flow rate. As the flow rate increases the first appearance of TCE in the eluent occurs relatively earlier and exhibits comparatively greater delay in achieving full breakthrough, suggesting non-equilibrium processes are more significant at higher flow rates.

Transport and retention of Cryptosporidium parvum oocysts in sandy soils

A series of miscible-displacement experiments was conducted to examine the retention and transport behavior of oocysts in natural porous media. Three soils and a model sand were used that differed in physical and geochemical properties. Transport behavior was examined under various treatment conditions to help evaluate retention mechanisms. Significant retention of oocysts was observed for all media despite the fact that conditions were unfavorable for physicochemical interactions with respect to DLVO theory. The magnitude of retention was not influenced significantly by alterations in solution chemistry (reduction in ionic strength) or soil surface properties (removal of soil organic matter and metal oxides). On the basis of the observed results, it appears that retention by secondary energy minima or geochemical microdomains was minimal for these systems. The porous media used for the experiments exhibited large magnitudes of surface roughness, and it is suggested that this surface roughness contributed significantly to oocyst retention.

Carboxymethyl-beta-cyclodextrin mitigates toxicity of cadmium, cobalt, and copper during naphthalene biodegradation

Hazardous waste sites are commonly contaminated with both organic and metal pollutants. Many metal pollutants have been shown to inhibit organic pollutant biodegradation. We investigated the ability of a modified, polydentate cyclodextrin (carboxymethyl-beta-cyclodextrin, CMCD) to reduce the toxicity of 33.4 microM cadmium, cobalt or copper during naphthalene degradation by a Burkholderia sp. in 120 h aerobic, batch studies. The highest investigated concentration of CMCD, 3340 microM, reduced cadmium, cobalt, and copper toxicity. With each metal, the length of the lag phase was reduced (by as much as 108 h with cobalt or copper), the cell yield was increased (by as much as a factor of 16 with cobalt), and the growth rate was increased (by as much as a factor of 31 with cobalt). The degrader was unable to use CMCD as the sole source of carbon and energy. Our data suggest that the ability of CMCD to complex metals plays an important role in its ability to mitigate metal toxicity and that CMCD has the potential to enhance biodegradation in organic and metal co-contaminated environments.

The pseudophase approach to assessing chemical partitioning in air-water-cyclodextrin systems

Henry's Law constants (HLCs) of several common, subsurface hydrophobic organic pollutants (HOPs) including trichloroethylene (TCE), perchloroethylene (PCE) and benzene, toluene, ethylbenzene, and o-xylene (BTEX), were measured using a static headspace phase ratio (SHPR) method over a range of temperatures (35, 45, 55, and 65 degrees C) and experimentally and operationally relevant cyclodextrin (CD) concentrations (0, 10, 20, 50, and 100 g L(-1)). In aqueous CD solutions, HLC values decrease according to a power law relationship with increasing CD concentration due to an apparent solubility enhancement caused by HOP partitioning to the hydrophobic cavity of CD molecules. The temperature dependence of air-water partitioning under the influence of CD was well described by the van't Hoff equation for all HOPs tested. A three-phase equilibrium model was used to interpret air-water-CD partitioning data, treating CD as a pseudophase. Our results show that HOP CD-water partition coefficients decrease linearly with increasing temperature. CD-water partition coefficients were generally independent of CD concentration, with a few exceptions. Comparison of our results for hydroxypropyl-beta-CD and TCE to those from another study showed several major discrepancies, which were attributed to differences in the experimental methods employed. Our attempt to correlate CD-water partition coefficients with HOP chemical properties indicates that correlations based on individual chemical descriptors (e.g., aqueous solubility, octanol-water partition coefficient, molecular volume or ET (30) polarity index) will not be sufficient to obtain accurate estimates of HOP CD-water partition coefficients for other compounds with differing chemical structures.

Transport of microsporidium Encephalitozoon intestinales spores in sandy porous media

The retention and transport of microsporidium Encephalitozoon intestinales spores in two water-saturated sandy porous media was investigated in this study. The initial breakthrough of the spores in the column effluent occurred essentially simultaneously with that of a non-reactive tracer, indicating no significant velocity enhancement. A large fraction (45-73%) of the spores injected into the columns was not recovered in the effluent, indicating removal from solution through colloid retention processes of attachment and/or straining. The relative significance of attachment and straining to total retention was evaluated in additional experiments. An experiment was conducted with a sieved coarse fraction of porous media for which straining is unlikely to be of significance based on the relative diameters of the spores and porous-medium pores. The spore recovery for this experiment was similar to the recoveries obtained for microsporidia transport in the un-sieved parent porous medium. An additional experiment was conducted with a subsample of the coarse fraction that was acid-washed to reduce potential surface attachment sites. Spore recovery was complete for this experiment. These results suggest surface deposition was the primary removal mechanism in our system. This conclusion is supported by the results of an experiment wherein deionized water was flushed through a column that was previously flushed with electrolyte solution. The effluent spore concentrations were observed to increase upon injection of deionized water, indicating re-mobilization of spores upon a change in water chemistry. The measured data were successfully simulated using a mathematical model incorporating colloid filtration. The results of this study suggest that the transport of microspordia in sandy porous media is governed by established colloid-transport processes.

Influence of water chemistry and travel distance on bacteriophage PRD-1 transport in a sandy aquifer

Experiments were conducted to evaluate the impact of groundwater chemistry and travel distance on the transport and fate behavior of PRD-1, a bacteriophage employed as a surrogate tracer for pathogenic enteric viruses. The experiments were conducted in the unconfined aquifer at the United States Geological Survey Cape Cod Toxic-Substances Hydrology Research Site in Falmouth, Massachusetts. The transport behavior of bromide (Br(-)) and PRD-1 were evaluated in a sewage-effluent contaminated zone and a shallower uncontaminated zone at this site. Several multilevel sampling devices located along a 13-m transect were used to collect vertically discrete samples to examine longitudinal and vertical variability of PRD-1 retardation and attenuation. The concentration of viable bacteriophage in the aqueous phase decreased greatly during the first few meters of transport. This decrease is attributed to a combination of colloid filtration (attachment) and inactivation. The removal was greater (10(-12) relative recovery) and occurred within the first meter for the uncontaminated zone, whereas it was lesser (10(-9) relative recovery) and occurred over 4m in the contaminated zone. The lesser removal observed for the contaminated zone is attributed to the influence of sorbed and dissolved organic matter, phosphate, and other anions, which are present in higher concentrations in the contaminated zone, on PRD-1 attachment. After the initial decrease, the aqueous PRD-1 concentrations remained essentially constant in both zones for the remainder of the tests (total travel distances of 13 m), irrespective of variations in geochemical properties within and between the two zones. The viable, mobile PRD-1 particles traveled at nearly the rate of bromide, which was used as a non-reactive tracer. The results of this study indicate that a small fraction of viable virus particles may persist in the aqueous phase and travel significant distances in the subsurface environment.

Spatial variability of in situ microbial activity: biotracer tests

Biotracer tests have been proposed as a means by which to characterize the in situ biodegradation potential for field-scale systems. In this study, field experiments were conducted at two sites to evaluate the utility of the biotracer method for characterizing the spatial variability of microbial activity. The first site is a mixed waste-contaminated surficial aquifer in Utah, and the second site is a chlorinated solvent-contaminated regional aquifer in Tucson, Arizona. Mass recovery of the biotracer decreased approximately linearly with increasing residence time for the Tucson site. Similar behavior was observed at the Utah site, except in the region adjacent to the injection zone, where percent recoveries were much lower than those predicted using a correlation determined using data collected downgradient of the injection zone. First-order biodegradation rate coefficients obtained from model calibration of the tracer data varied between 0.2 and 0.5/day for the Tucson site. For the Utah site, the values varied between 0.1 and 0.6/day downgradient of the injection wells, and between 0.7 and 2.6/day near the injection wells. Considering the large range over which biodegradation rate coefficients can vary, the rate coefficient exhibited relatively minimal spatial variability (factor of 2.5) for the Tucson site. Conversely, the spatial variability of the rate coefficient was an order of magnitude greater for the Utah site. These differences in variability are consistent with conditions associated with the respective sites. For example, the greater microbial activity observed in the vicinity of the injection wells for the Utah site is consistent with the biomass distribution determined from analysis of core samples, which shows larger bacterial cell densities for the region near the injection wells. These results illustrate the utility of biotracer tests for in situ characterization of microbial activity (e.g., biodegradation potential), including evaluation of potential spatial variability.