Transport of volatile compounds in porous media in the presence of a trapped gas phase

The presence of an immobile gaseous phase in an otherwise-saturated porous medium affects the transport of volatile compounds. The linear theory of partitioning tracers suggests that a volatile tracer introduced into such a system should be retarded with a constant retardation factor. Using high concentrations, however, the saturation of the gaseous phase will change as an effect of the tracer test itself. Competitive gas transfer among all volatile compounds and the change of saturation may lead to tracer concentrations that are temporarily higher than those injected. We analyze the system in the framework of the coherence theory by Helfferich [Soc. Pet. Eng. J. 21 (1) (1981) 51]. The governing equations are formulated as functions of total concentration, i.e., the mass of solutes in all phases per pore volume. Neglecting dispersion and mass-transfer kinetics, we derive the characteristic form of the resulting system of hyperbolic equations. In a system with N volatile compounds, a variation of the concentrations splits up into N waves, each traveling with its own characteristic velocity. If the presence of a gaseous phase is sustained, one wave will be a standing one. We perform numerical model calculations for tracers with various Henry's law coefficients and show that the results agree with the semi-analytical solution obtained by coherence theory.     

The velocity of DNAPL fingering in water-saturated porous media: laboratory experiments and a mobile-immobile-zone model.

Dense nonaqueous phase liquids (DNAPLs) are immiscible with water and can give rise to highly fingered fluid distributions when infiltrating through water-saturated porous media. In this paper, a conceptual mobile-immobile-zone (MIZ) model is presented to describe the structure of a DNAPL finger in water-saturated porous media and the velocity of finger propagation. A finger is composed of a finger body and a tip. The finger body has a mobile core and an immobile sheath. All the DNAPL within the tip of a finger is mobile. Lab experiments utilizing image analyses of a DNAPL (PCE) penetrating into water-saturated homogeneous glass beads were carried out in a two-dimensional transparent chamber. The results show that the fingers elongated almost linearly with time. The fingers did not grow laterally after the tip of the finger had passed. The average finger diameters were between 3.9 and 5.4 mm for PCE propagation in water-saturated glass bead porous media with mean particle diameters from 0.32 to 1.36 mm. The estimated mobile core diameters were 51-60% of the average finger diameters.     

Management, disposal and recycling of waste industrial organic solvents in Hong Kong.

An attempt has been made to establish a mass balance of industrial organic solvents in Hong Kong. It is estimated that only a small portion, less than 4%, of all the organic solvents consumed in Hong Kong are collected as waste solvents and properly treated, while the remainder are used either in the formulation of solvent containing products, or are lost to the environment through vapour emissions, leaks and spills, or dumped illegally. It was found that solvent recycling has been a common practice in some industries but the existing level of solvent recycling in Hong Kong is difficult to estimate. About 87.4% of all the waste organic solvents disposed of at the licensed facilities are potentially recyclable although whether they can be recycled in practice depends on many factors. Examples of existing waste organic solvent management and recycling practices from selected industries in Hong Kong are presented. The economic feasibility of current and future potential recycling systems is evaluated for a few selected cases. An integrated waste organic solvent management strategy is proposed to minimize adverse impacts of organic solvents to the environment and human health.     

Effect of composting on characterization and leaching of copper, manganese, and zinc from swine manure.

This research was conducted to study the influence of composting on the concentrations, water solubility, and phase association of Cu, Mn, and Zn from swine (Sus domesticus) feces. Composting of separated swine manure was performed in two piles for 122 days. The metal concentrations increased rapidly during the first 49 days and leveled off thereafter. All metal concentrations increased approximately 2.7-fold in the final compost due to decomposition of organic matter. A sequential extraction protocol was used to evaluate the humification process and partition metals into water-soluble, exchangeable, organically complexed, organically bound, solid particulate, and residual fractions. Temporal changes in the water-soluble fractions of Cu, Mn, and Zn were reflected by water-soluble organic C concentrations, which rapidly increased to a maximum at Day 18 and declined thereafter. An increase in the humic acid/fulvic acid ratio in Na4P2O7 or NaOH extracts at various stages of composting represented the humification process. During composting, the major portions of Cu, Mn, and Zn were in the organically-bound, solid particulate, and organically complexed fractions, respectively. Metal distributions in different chemical fractions were generally independent of composting age and, thus, independent of respective total metal concentrations in the composts.     

The acute toxicity of agricultural surfactants to the tadpoles of four Australian and two exotic frogs

Nonionic surfactants are frequently incorporated into pesticide formulations, and are therefore a group of chemicals to which amphibians may be exposed in agricultural or urban landscapes. However, little is known about the effects of surfactant exposure in amphibians. Feeding stage tadpoles of Bufo marinus, Xenopus laevis and four species of Australian frogs (Crinia insignifera, Heleioporus eyrei, Limnodynastes dorsalis and Litoria moorei) were exposed to nonylphenol ethoxylate (NPE) and alcohol alkoxylate in static-renewal acute toxicity tests. All species exhibited nonspecific narcosis following exposure to both these surfactants. The 48-h EC50 values for NPE ranged between 1.1 mg/l (mild narcosis) and 12.1 mg/l (full narcosis). The 48-h EC50 values for alcohol alkoxylate ranged between 5.3 mg/l (mild narcosis) and 25.4 mg/l (full narcosis). Replicate acute toxicity tests with B. narinus exposed to NPE at 30 degrees C over 96 h indicated that the narcotic effects were not particularly time dependant. The mean 24, 48, 72, and 96-h EC50 (mild narcosis) values were 3.6, 3.7, 3.5 and 3.5 mg/l, respectively. The mean 24, 48, 72 and 96-h EC50 (full narcosis) were 4.0, 4.1, 4.2 and 4.0, respectively. Acute toxicity tests with B. marinus exposed to NPE at 30 degrees C under conditions of low dissolved oxygen (0.8-2.3 mg/l) produced a two to threefold increase in toxicity.     

Hydrolysis of chlorpyrifos in natural waters of the Chesapeake Bay

Chlorpyrifos is the most widely used insecticide in the Chesapeake Bay region. Recent studies show that this organophospate chemical is consistently present in the air, rain and surface waters of the Chesapeake Bay region, suggesting a long environmental half-life. Hydrolytic degradation of chlorpyrifos is likely a dominant removal process, but existing hydrolysis data do not reflect conditions in the Chesapeake Bay. In this project, hydrolysis rates of chlorpyrifos were measured in sterilized, ambient water from the mouth of four Chesapeake Bay tributaries ranging in salinity from 0 to 17 ppt. The measured hydrolysis half-lives varied from 24 d in the Patuxent River to 126 d in the Susquehanna River. These results indicate that pH alone cannot be used as a single parameter to predict hydrolysis under field conditions. The influence of copper concentration, and other water constituents, need to be further evaluated as they may emerge as independent predictors to assess the fate of pesticides in natural systems.     

Formation of PCDDs and PCDFs during the combustion of polyvinylidene chloride

In laboratory-scale combustion of polyvinylidene chloride (PVDC) with a quartz tubular furnace designed and fabricated to provide the desired combustion temperature and mixing state of combustion gas with air, it was found that at 800 degrees C or higher the level of polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans [corrected] (PCDDs/PCDFs) resulting from PVDC combustion was no higher than that from heating air alone, and thus far below the levels which resulted from PVDC combustion at 750 degrees C or lower. The results provide the first laboratory confirmation of the relation between PVDC incineration temperature and PCDD/PCDF formation, and of the primary importance of high temperature, turbulence for mixing between air and combustion gas, and sufficient residence time, as governing factors for the minimization of PCDD/PCDF formation in municipal solid waste incinerators.     

A reproducible approach to the reporting of organochlorine compounds in epidemiologic studies

A growing body of research indicates that the most biologically active PCB congeners and organochlorines are not the most abundant components in human and wildlife samples. As researchers attempt measurement on a wider pool of less abundant compounds, they inevitably face quantification problems. To address this problem and enhance comparability across studies, we propose a standardized approach to report organochlorines that is based on a reproducible method to determine the limit of quantification (LQ). Two statistical methods are incorporated into our approach, one by Gibbons termed the Alternative Minimum Level (AML), and one based on determining a region of stable relativestandard deviation in instrument response (RSD). We illustrate our approach using historical samples collected during the 1960s from a cohort of pregnant women enrolled in the Child Health and Development Study. The results are applicable to determining the LQ of any method, and are of utmost importance to environmental scientists conducting trace organic analyses of complex mixtures. Our results demonstrate that: (1) precision as measured by RSD is the most important criterion in determining LQ; (2) the AML routinely isolates a region of constant RSD; and (3) the precision of the instrument detector response as measured with pure standards locates the LQ applicable for real samples - that is, the true limits of quantification reside in the detector, not the matrix effects or analyte recoveries associated with real samples. A corollary of these findings is that bias due to matrix effects and analyte recoveries can be assessed separately from precision and LQdetermination. Previous approaches involved spiking matrix blanks to determine LQ, a problematic strategy for real world, complex matrices. We have now validated the use of pure standards in LQ determination, an approach that is practical and accessible to most analysts.     

Chemical kinetic modeling of de novo synthesis of PCDD/F in municipal waste incinerators

A kinetic model is developed for de novo synthesis of PCDD/F from carbon in incinerator fly ash. The main mechanistic steps considered in the model are carbon gasification, PCDD/F formation, desorption and degradation. Rate equations are derived which can relate PCDD/F formation with process variables including carbon concentration of fly ash, partial pressure of oxygen, reaction temperature and time. The kinetic model has been verified using laboratory de novo synthesis data reported in the literature. When the model is applied to industrial incinerator conditions, PCDD/F formation levels of 0.1-0.5 microg/N m3 in the gas phase and 0.1-1.2 microg/g in the solid phase are calculated, and both are in good agreement with incinerator measurements.