Louis J. Thibodeaux

Contaminant fluxes from sediment due to tubificid oligochaete bioturbation

The release of the hydrophobic organic compounds pyrene, dibenzofuran and phenanthrene from bioturbated freshwater sediments was studied in laboratory microcosms. Initial Tubificid oligochaete densities of 0, 6700 and 2.67 × 104 individuals · m−2 were employed. Under oxygen saturated conditions, the difference between the contaminant fluxes from the high-density bioturbated microcosms and controls remained essentially constant at 37 and 70 ng · cm2−2 · d−1 for pyrene and phenanthrene, respectively, corresponding to effective mass transfer coefficients of 0.16 and 0.37 cm/y. Under hypoxic conditions, worm defecation on the sediment surface increased and led to significantly increased fluxes to a maximum of 380, 490 and 940 ng · cm−2 · d−1, for pyrene, phenanthrene and dibenzofuran, respectively. Average bioturbation fluxes in the high-density microcosms of 246, 258 and 310 ng · cm−2 · d−1 for the respective compounds corresponded to effective mass transfer coefficients of 1.7, 3.2, and 7.5 cm/yr. Initial release rates from medium-density microcosms (25% of high density) were typically half the release rate of the high-density microcosms, indicating greater organism activity per individual at the lower density. The increased flux with the more soluble compounds likely reflects more rapid release at the sediment surface and the increased importance of porewater pumping over sediment particle reworking for migration of these compounds.

Relationships between micelle-water and octanol-water partition constants for hydrophobic organics of environmental interest

The partition constants between micelles of sodium dodecyl sulfate and water for eleven hydrophobic non-polar organics of environmental interest were correlated to their octanol-water partition constants, aqueous solubilities and total molecular surface areas. Octanol-water partition constant was found to be the best correlating parameter. Values of partition constants for micelles formed of dodecylbenzenesulfonate and hexadecyltrimethylammonium bromide surfactants were also correlated to the octanol-water partition constants. A possible practical application of the Km−Kow correlation to determine the micellar-mediated transport of hydrophobic organics in ground-water is illustrated.

ON THE ENRICHMENT OF HYDROPHOBIC ORGANIC COMPOUNDS IN FOG DROPLETS

Abstract The unusual degree of enrichment of hydrophobic organics in fogwater droplets reported by several investigators can be interpreted as a result of (a) the effects of temperature correction on the reported enrichment factors, (b) the effects of colloidal organic matter (both filterable and non-filterable) in fog water and (c) the effects of the large air-water interfacial adsorption of neutral hydrophobic organics on the tiny fog droplets. The enrichment factor was directly correlated to the hydrophobicity (or the activity coefficient in water) of the compounds, as indicated by their octanol-water partition constants. Compounds with large octanol-water partition coefficients (high activity coefficients in water) showed the largest enrichment. Available experimental data on the adsorption of hydrophobic compounds at the air-water interface and on colloidal organic carbon were used to show that the large specific air-water interfacial areas of fog droplets contribute significantly to the enrichment factor.

Partitioning of chloromethanes between aqueous and surfactant micellar phases

Abstract The partition constants for three volatile organic chemicals (VOCs)—methylene chloride, chloroform and carbon tetrachloride—between aqueous and surfactant micellar phases were determined using an equilibrium partitioning method. The surfactants used were sodium dodecylbenzene sulfonate (DDBS), sodium dodecylsulfate (DDS) and hexadecyltrimethyl ammonium bromide (HTAB). The vapor concentration of the VOCs remained unchanged until the critical micellar concentration (CMC) of the surfactant was reached. Above the CMC the vapor concentration decreased linearly with increasing surfactant concentration in the aqueous phase. The partition constants (Km) were observed to increase with increasing ionic strength of the solution. The partition constants also increased with increasing hydrophobicity of the VOCs. A pseudo-two-phase model was used to determine the Km values and was found to fit the experimental data well. Cationic micelles of HTAB gave larger Km values than anionic micelles of DDS and DDBS surfactants. Km values also increased with increasing temperature. From a thermodynamic treatment of the equilibrium process the values of the partition constants for transfer of VOCs from an ideal vapor phase to the micellar phase were calculated. These values were found to fall in between the values for aliphatic hydrocarbons and alcohols.

PCB transport into lake sediments. Conceptual model and laboratory simulation

A laboratory simulation study was performed to observe the transport of a polychlorinated biphenyl (PCB) in lake bottom sediment. A conceptual mathematical model, which describes PCB fate due to the combination of molecular diffusion in pore water and partitioning onto sediment solids, was assessed. An experimental apparatus was designed to simulate a lake bottom sediment/water interface condition, in which Aroclor 1242 was introduced. Sampling and analytical methods were devised, so that bed depth-PCB concentration profiles could be determined from the simulation. Very thin slices, 0.02-0.1 cm, allowed the construction of concentration profiles to quantify the time variation of penetration of {sup 14}C-labeled components into bottom sediment. The consistent behavior of effective diffusivities extracted from profiles suggests the model may adequately describe chemical transport in sediment.

Simulation of soil washing with surfactants

Abstract A mathematical model of soil washing that incorporates the surfactant enhanced mobilization and solubilization of organic compounds was implemented using a finite difference compositional reservoir simulator. The primary objective of the model was identification of the contributions of the various mechanisms—water displacement, surfactant mobilization and dissolution—on the removal of organic contaminants from soil. Mobilization of the organic phase was described by a reduction in the residual oil saturation caused by decreased interfacial tension. Increased aqueous solubility of organic compounds due to solubilization by surfactant micelles was modeled assuming local equilibrium. Parameters for the model were obtained from experimental measurements and literature sources. The model was implemented in a two-dimensional, two-phase system. Experimental data from surfactant flushing of columns contaminated with automatic transmission fluid and a mixture of chlorinated organics were used to evaluate the performance of the model. In most cases, the predicted organic recoveries were found to agree well with experimental results. For the surfactant sodium dodecyl sulfate, mobilization of organic contaminants was the main recovery mechanism for both waste liquids modeled. The results suggest that complete dissolution of a contaminant nonaqueous phase, rather than mobilization and subsequent vertical migration, may be difficult to achieve at the surfactant concentrations studied.

Gas-to-particle partitioning of polycyclic aromatic hydrocarbons in an urban atmosphere

Abstract Semi-volatile organic compounds (SOCs) are present in the atmosphere as vapors and are adsorbed to particulate matter. Knowledge of the gas-to-particle distribution of these compounds is important in understanding their atmospheric fate and transport. A baseline experimental study to estimate the vapor- and particle-phase-associated concentrations of the SOCs (specifically, polycyclic aromatic hydrocarbons, PAHS) in the Baton Rouge (Louisiana) air was carried out. A comparison of the levels of concentrations of PAHs obtained in Baton Rouge to those obtained in other parts of the world is made. The sampling artifacts associated with a traditional high-volume sampler introduced errors in the estimation of the partition coefficient. Different air sampling procedures (a modified. low-volume sampler and an annular denuder) were evaluated in order to obtain a more accurate measure of the gas-to-particle partition coefficient. The effects of temperature and precipitation on the partitioning of PAHs between the vapor and particulate phase were ascertained.

Equilibrium adsorption of chemical vapors on surface soils, landfills and landfarms — a review

Abstract The equilibrium adsorption of Volatile Organic Chemicals (VOCs) onto soils, such as occurring in surface soils, landfills and landfarms, is reviewed. On the basis of moisture content, soils can be classified into the “dryrd, “damp” and “wet” categories. Increasing moisture content in dry soils is known to lead to displacement of adsorbed nonpolar VOCs. This review addresses adsorption models for the three soil moisture regimes. An extended Brauner, Emmet, Teller (BET) adsorption theory is used and simplified isotherms are developed from it that enable the estimation of partial pressures of VOCs above soils. Available experimental data on VOC adsorption in the present context are also reviewed.

Transport of hydrophobic organics by colloids through porous media 1. Experimental results

Abstract The transport of two hydrophobic organic compounds (HOCs), namely, phenanthrene and pyrene, through model porous media (glass microsphere and quartz sand) in the presence of a homogenous colloid (polystyrene latex) was studied. The sorption and desorption of colloids were dependent on the pH and ionic strenght of the porewater. The sorption of colloids was only partially reversible even after modifying the aqueous solution chemistry, viz. the ionic strength and pH. The organic sorption on the porous media was directly related to the behavior of colloids. In the absence of an electrolyte, the HOCs showed an early brakthrough in the presence of the colloids compared to that without colloids. At higher ionic strength, the colloids were deposited on the porous medium. Owing to their high sorbed mass on the colloids, the HOCs remained on the porous material, thus decreasing their mobility in the column. Mobile colloids in porewater have been shown to impact the transport of HOCs through porous media.

Equilibrium vapor phase adsorption of volatile organic chemicals on dry soils

Abstract Equilibrium vapor phase adsorption of volatile organic chemicals (VOC) can be described adequately by the BET equation. The adsorption of five VOCs (benzene, dichloropropane, methylcyclohexane, ethyl ether and methanol) on four types of soils of moisture contents less than 2% at 30°C are reported. The adsorption of VOCs by dry soils is quite considerable and dominated by mineral adsorption. It also appears that most of the adsorption occurs on the external surface of the soils. Polar compounds show a larger degree of adsorption than non polar and slightly polar adsorbates. The high degree of adsorption observed will retard the movement of volatile organics from hazardous waste landfills and on surface soil for land application of hazardous waste when dry soil conditions exist.