Thomas C. Voice

Effect of solids concentration on the sorptive partitioning of hydrophobic pollutants in aquatic systems

Etude menee sur des sediments du Lac Michigan avec 4 polluants prioritaires hydrophobes: chlorobenzene, naphtolene, trichlorobiphenyle et hexachlorobiphenyle

Assessment of Bioavailability of Soil-Sorbed Atrazine

ABSTRACT Bioavailability of pesticides sorbed to soils is an important determinant of their environmental fate and impact. Mineralization of sorbed atrazine was studied in soil and clay slurries, and a desorption-biodegradation-mineralization (DBM) model was developed to quantitatively evaluate the bioavailability of sorbed atrazine. Three atrazine-degrading bacteria that utilized atrazine as a sole N source (Pseudomonas sp. strain ADP, Agrobacterium radiobacter strain J14a, and Ralstonia sp. strain M91-3) were used in the bioavailability assays. Assays involved establishing sorption equilibrium in sterile soil slurries, inoculating the system with organisms, and measuring the CO2 production over time. Sorption and desorption isotherm analyses were performed to evaluate distribution coefficients and desorption parameters, which consisted of three desorption site fractions and desorption rate coefficients. Atrazine sorption isotherms were linear for mineral and organic soils but displayed some nonlinearity for K-saturated montmorillonite. The desorption profiles were well described by the three-site desorption model. In many instances, the mineralization of atrazine was accurately predicted by the DBM model, which accounts for the extents and rates of sorption/desorption processes and assumes biodegradation of liquid-phase, but not sorbed, atrazine. However, for the Houghton muck soil, which manifested the highest sorbed atrazine concentrations, enhanced mineralization rates, i.e., greater than those expected on the basis of aqueous-phase atrazine concentration, were observed. Even the assumption of instantaneous desorption could not account for the elevated rates. A plausible explanation for enhanced bioavailability is that bacteria access the localized regions where atrazine is sorbed and that the concentrations found support higher mineralization rates than predicted on the basis of aqueous-phase concentrations. Characteristics of high sorbed-phase concentration, chemotaxis, and attachment of cells to soil particles seem to contribute to the bioavailability of soil-sorbed atrazine.

Biological activated carbon in fluidized bed reactors for the treatment of groundwater contaminated with volatile aromatic hydrocarbons

Abstract A comparison of fluidized bed reactor systems with (1) adsorptive removal capacity only using granular activated carbon (GAC) without microbial growth, (2) combined biological and adsorptive removal mechanisms using GAC with microbial growth and (3) biological removal only using nonactivated carbon with microbial growth was performed. These three systems were fed groundwater contaminated with benzene, toluene and xylene (BTX). The breakthrough profiles, steady-state removal of BTX and system responses to step increases in applied organic loading rates were investigated. During start-up, even though the same amount of inoculum was added to the two biological systems, the time required until effective biodegradation commenced in the system employing GAC as a biomass carrier was less than that observed for the system using non-activated carbon (200 vs 500 h). Significantly less BTX was released during this period by the system with combined removal mechanisms and the development of a contiguous biofilm was more rapid. Under constant, steady state, organic loading conditions (3 and 6 kg-COD/m 3 -day), BTX removals were similar for the two biological systems, although the system employing the GAC carrier had lower effluent concentrations at the lower loading rate. More than 90% of the BTX were removed in both systems. During step increases in organic loading, however, the combination of biological and adsorptive removal capacity resulted in enhanced BTX removal and more stable operation. Scanning electron microscopy was used to examine the extent of surface coverage of the GAC and non-activated carbon by the biofilm. Particles from both systems were observed to be completely covered by a contiguous, thick biofilm.

Effects of degree of water saturation on dispersivity and immobile water in sandy soil columns

Abstract Three natural nonaggregated soil samples, with similar grain-size distributions, have been used to determine the dispersive behavior of porous media under steady, saturated and unsaturated flow conditions. Tritium was used as a tracer and was found to have no sorption on the solid matrix. Generated breakthrough curves (BTCs) for the unsaturated experiments were symmetrical with no evidence of tailing. The unsaturated experiments for two of the soils were adequately described by considering all the water in the pore volume as mobile. However, about 10% of the pore water, independent of the degree of saturation, was found to be immobile in the case of the third soil during unsaturated flow. For this soil, there was no mass transfer between the two water regions, indicating that the immobile water is essentially isolated from the flowing water fraction. For all three soils, dispersivity under unsaturated conditions was found to be higher, independent of the degree of water saturation, than the value determined for the saturated experiments. This is inconsistent with what would be expected from the simple bundle-of-capillary-tubes model and does not agree well with a more sophisticated conceptualization of the porous medium. The data, however, clearly indicate a wider range in pore-water velocities when these soils are desaturated.

Effects of residence time and degree of water saturation on sorption nonequilibrium parameters

This study reports the impact of the degree of water saturation on sorption nonequilibrium parameters. Two nonionic organic compounds (benzene and dimethylphthalate) and three nonaggregated sandy soils were utilized. Local equilibrium assumptions were found to be invalid for describing the transport of these compounds even at pore-water velocities as low as 0.7 cm/h. Sorption nonequilibrium appeared to be of a diffusive nature rather than due to a slow chemical reaction. Sorption mass-transfer coefficients varied proportionally with pore-water velocity. A strong correlation between the mass-transfer coefficient and residence time was found utilizing present and previously reported laboratory data. A similar relationship was also found for the mass-transfer coefficient between mobile and immobile water regions. Field data indicate that the sorption mass-transfer coefficient may continue to decrease in a consistent way even at residence times as large as 5×103 h. Variations in the degree of water saturation had no impact on the value of the sorption mass-transfer coefficient other than what would be expected due to changes in the residence time. This suggested that movement into the solid grains of the large emptied pores through diffusion from the water-filled pores into stagnant water covering these grains was relatively fast compared to the sorption rate.

Long-term evaluation of adsorption capacity in a biological activated carbon fluidized bed reactor system

The adsorption capacity of biofilm coated activated carbon from a biological fluidized bed reactor which treated toluene contaminated water was determined systematically over an extended period of operation. The adsorption capacity of biocoated carbon remained at greater than 70% of initial levels during the first two months. After six months of operation, the remaining capacity was still approximately 40, 52 and 57% of the initial value for equilibrium toluene concentrations of 0.1, 3 and 10 mg/L, respectively. If the adsorption removal mechanism is used to provide a buffer during the transient conditions, this remaining capacity may be sufficient without carbon replacement. There was no direct relation between the amount of biomass or the thickness of the biofilm on the carbon and the remaining adsorptive capacity. The loss of adsorption capacity over an extended period of operation appears to result directly from bioactivity, rather than from toluene loading. Bioregeneration of this long-term capacity loss was not observed with continued operation for 100 days after the toluene feed was stopped and the nutrient and oxygen supplies continued.

Critical Evaluation of Environmental Exposure Agents Suspected in the Etiology of Balkan Endemic Nephropathy

Abstract Balkan endemic nephropathy (BEN), a kidney disease that occurs in rural villages in Bosnia, Bulgaria, Croatia, Romania, and Serbia, is thought to be linked to an environmental toxin. The authors review literature on proposed environmental exposure agents, report the results of field sampling and analysis studies to evaluate potentials for exposure to proposed agents, and propose criteria for future testing. They used these criteria to evaluate the evidence for suggested hypotheses, concluding that several proposed agents can be eliminated or considered unlikely based on apparent inconsistencies between clinical or epidemiologic evidence related to BEN and toxicologic or exposure evidence related to the agents. Mycotoxins and aristolochic acid are the primary targets of current toxicologic investigations, and while the evidence on exposures for both is potentially consistent, it is insufficient.

Stormwater dissolved organic matter: Influence of land cover and environmental factors

Dissolved organic matter (DOM) plays a major role in defining biological systems and it influences the fate and transport of many pollutants. Despite the importance of DOM, understanding of how environmental and anthropogenic factors influence its composition and characteristics is limited. This study focuses on DOM exported as stormwater from suburban and urban sources. Runoff was collected before entering surface waters and DOM was characterized using specific ultraviolet absorbance at 280 nm (a proxy for aromaticity), molecular weight, polydispersity and the fraction of DOM removed from solution via hydrophobic and H-bonding mechanisms. General linear models (GLMs) incorporating land cover, precipitation, solar radiation and selected aqueous chemical measurements explained variations in DOM properties. Results show (1) molecular characteristics of DOM differ as a function of land cover, (2) DOM produced by forested land is significantly different from other landscapes, particularly urban and suburban areas, and (3) DOM from land cover that contains paved surfaces and sewers is more hydrophobic than from other types of land cover. GLMs incorporating environmental factors and land cover accounted for up to 86% of the variability observed in DOM characteristics. Significant variables (p < 0.05) included solar radiation, water temperature and water conductivity.

Development of a kinetic basis for bioavailability of sorbed naphthalene in soil slurries

The degradation of naphthalene in soil-slurry systems was studied using four different organisms and two soils. Organisms with zero-order, first-order, and Michaelis-Menten rates were selected. The soils had substantially different sorption distribution coefficients. Sorption and desorption was evaluated in abiotic soil-slurry systems. The desorption process was described by a model that accounts for equilibrium, rate-limited and non-desorbing sites. Biodegradation parameters were measured in soil-extract solutions. Bioavailability assays, inoculated soil slurries, were conducted and both liquid- and sorbed-phase naphthalene concentrations were measured over time. For the less sorptive soil, the results could be explained by sequential desorption and degradation processes. For the other soil, enhanced degradation was clearly observed for the organisms with first-order and Michaelis-Menten rates. Several explanations are explored for these observations including direct sorbed-phase degradation and the development of elevated substrate concentrations at the organism/sorbent interface. No enhancement was found for the organism with zero-order kinetics.

Microscopic observation of a NAPL in a three-fluid-phase soil system

Abstract The focus of this study was to observe the location and form of a NAPL in a three-fluid-phase (air-NAPL-water) soil system utilizing an advanced microscopic technique, cryo-scanning electron microscopy (cryo-SEM) with X-ray analysis. A sandy aquifer material was brought to residual water (NaCl saturated) saturation (∼4% by pore volume) followed by NAPL (iodobenzene) saturation, ranging from ∼1% to 80%, using modified Tempe® pressure cells. A small intact soil core was obtained from the pressure cell and quickly frozen in liquid nitrogen. The core was fractured and chromium coated at a high vacuum and −130°C. Secondary electron images (SEI) and X-ray dot maps (Si, Cl and I) were made of samples with different NAPL saturations. Photomicrographs and X-ray dot maps confirmed the existence of continuous NAPL films on soil (containing a residual water saturation) at high NAPL saturations. Photomicrographs revealed v-shaped wedges, pendular rings and films on irregular shaped sand grains. At low NAPL saturations (∼1% by pore volume), iodine was virtually nondetectable in the overall X-ray analysis of the sample. This suggests that the small quantity of NAPL present in the sample probably existed as thin films or small isolated lenses or blobs.