K.T. Valsaraj

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.

Reusable adsorbents for dilute solution separation. 6. Batch and continuous reactors for the adsorption and degradation of 1,2-dichlorobenzene from dilute wastewater streams using titania as a photocatalyst

Two types of external lamp reactors were investigated for the titania catalyzed photodegradation of 1,2-dichlorobenzene (DCB) from a dilute water stream. The first one was a batch mixed slurry reactor and the second one was a semi-batch reactor with continuous feed recycle with titania immobilized on inert supports (quartz and low density polyethylene, LDPE). The batch reactor was used to study the intermediates and reaction kinetics of DCB degradation. Four intermediate products were observed in the degradation of DCB; these being 2-chlorophenol (CP), 2,3-dichlorophenol (DCP), 1,2-dihydroxybenzene (catechol) and, o-benzoquinone. A mechanism based on these observations is proposed. Increasing the pH of the solution increased the intial rate of photodegradation of DCB. Added oxidant (hydrogen peroxide) did not have any appreciable effect on the degradation of DCB. The Langmuir–Hinshelwood kinetic parameters for DCB, DCP and CP were obtained. The steady state removals and apparent rate constants were obtained for the plug-flow reactor with different supports and compared under similar conditions. The titania supported on LDPE showed a better rate of photocatalysis than titania supported on quartz, although the titania film thickness on LDPE was five times lower than on quartz. The modification of titania surface by adsorption of a non-photodegradable polyfluorinated surfactant vastly improved the rate of DCB degradation on both LDPE and quartz. The rate of photodegradation in the immobilized tubular reactor was mass transfer controlled for the flow regimes investigated, viz., Reynolds numbers, Re<550. The steady state removal was directly proportional to the radiant flux within the range 4–16 mW cm−2.

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.

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.

Reusable adsorbents for dilute solution separation. 5. Photodegradation of organic compounds on surfactant-modified titania

A semiconductor titania (TiO2) surface was modified by surfactant adsorption to make it more hydrophobic and to increase the adsorption of hydrophobic organic compounds (HOCs) and their photodegradation rates under UV irradiation. Photocatalytic experiments using TiO2 suspensions were conducted in the batch mode in a 10 ml reactor made of quartz. Two anionic surfactants, a hydrocarbon-based surfactant, sodium dodecylsulfate (SDS), and a fluorocarbon-based surfactant, potassium perfluorooctylsulfonate (PFOS), were used for titania surface modification. A hydrophobic organic compound namely, 1,2-dichlorobenzene (DCB), a chlorinated aromatic compound, was used as the test HOC. PFOS, which is resistant to UV irradiation, increased both the adsorption and photodegradation of DCB on titania. SDS, however, competed for photodegradation and inhibited the degradation of PCB.

Transport of hydrophobic organics by colloids through porous media 2. Commercial humic acid macromolecules and polyaromatic hydrocarbons

Abstract In Part 1 of this series [Colloids Surfaces A: Physicochem. Eng. Aspects, 94 (1995) 197] we reported the influence of a model colloid (polystyrene latex) on the transport of pollutants through a model porous medium (glass beads). In this paper the influence of commercial Aldrich humic acid macromolecules on the partitioning and transport of polyaromatic hydrocarbons (PAHs) through two different porous media under carefully controlled pH and ionic strength conditions are reported. The PAHs studied were pyrene, phenanthrene and dibenzofuran, and the porous media used were glass beads and quartz sand subjected to extensive cleaning procedures to obtain reproducible surfaces. The influence of ionic strength on the transport of PAHs through the porous media in the presence and absence of humic acid macromolecules were explored. Under low ionic strength conditions, the humic acid macromolecules enhanced the transport of PAHs through the porous media; however, at high ionic strength the deposition of humic acid on the porous media retarded the transport of PAHs. The effects of different types of inorganic salts were studied. CaCl 2 had a greater effect than KCl in retarding the movement of humic acid macromolecules and PAHs through the column.

The effect of moisture on volatile organic chemical gas-to-particle partitioning with atmospheric aerosols—competitive adsorption theory predictions

Abstract Junges (Junge C.E., 1977, in Fate of Pollutants in the Air and Water Environments , pp7–25, John Wiley, New York) aerosol/vapor adsorption algorithm for organic air pollutants is extended to account for the water vapor content in air. A model, based on classical competitive adsorption theory and supported by ancillary data, is proposed to quantify the role of relative humidity (r.h.) on the equilibrium distribution of semivolatile organic chemicals (SOCs) between the aerosol (solid) and vapor phases. In general the model shows that aerosols contain significantly less SOCs for r.h. in the range of 10–90%. Quantitative predictions are made and one calculation shows that as air r.h. increases to 60% the aerosol associated SOC fraction is reduced by 80%. The model also suggest that dry aerosols and those with complete water films have the highest SOC content.

Emulsions in porous media. I. Transport and stability of polyaphrons in sand packs

Abstract Polyaphrons are ‘biliquid foams’ where micrometer size oil droplets are encapsulated within a water film. They have large ratios of dispersed phase to continuous phase volume. Polyaphrons require the presence of both an oil phase surfactant and an aqueous phase surfactant. The aqueous surfactant forms a soapy shell surrounding the inner oil phase core. The oil phase surfactant (non-ionic) influences the size of the oil droplets, while the aqueous phase surfactant (anionic or cationic) influences the surface charge of the polyaphrons. Distinct sizes of oppositely charged polyaphrons were created by altering the type and concentration of the oil phase and aqueous phase surfactants. The polyaphrons were stable when diluted in deionized water, showing negligible change in diameter after four months of storage. The polyaphrons were pumped through sand packs to study their stability and flow characteristics in a typical porous medium. The surface charge and size of polyaphrons were found to affect the capture of polyaphrons in the sand and the effective permeability of the sand. The effect of ionic strength was studied at pH 7.0 with anionic polyaphrons. Increasing ionic strength did not affect the average size of polyaphrons in the effluent, but resulted in slightly greater permeability reduction than low ionic strength. Sand packs consisting of larger, more uniform size distributions captured less polyaphrons and had less permeability reduction than smaller, more varied grain-sized sand.

Transport of hydrophobic organic compounds by colloids through porous media 3. Diffusion from sediment porewater to overlying water in laboratory microcosms

Abstract The diffusion of colloidal organic carbon from the porewater of a local sediment University Lake, Baton Rouge, LA) was studied in laboratory microcosms. The effects of porewater ionic strength on the flux of dissolved organic carbon (DOC) was studied using two different electrolytes, namely NaCl and CaCl 2 . The sediment porewater DOC concentration frofiles were used to obtain the effective diffusivities ( D e ) in the sediment. The values of D e (0.019 and 0.021 cm 2 d −1 ) obtained for the sediment sample used in this work indicated low colloid mobility in porewaters. The D e values increased slightly in the presence of electrolytes in the porewaters (0.030 cm 2 d −1 for 1 M CaCl 2 and 0.023 cm 2 d −1 for 0.5 M NaCl). The porewater concentration of colloidal DOC was consistently low in the presence of electrolytes, indicating larger adsorption to the sediment solids. The D e values were found to range from 0.019 to 0.103 cm 2 d −1 for sediments collected from different regions and during different seasons. The effect of ionic strength on the depletion of a hydrophobic organic contaminant (namely pyrene) from the sediment by diffusion was found to be minimal under the conditions of these experiments.

Soil-Water Partitioning and Desorption Hysteresis of Volatile Organic Compounds from a Louisiana Superfund Site Soil

The adsorption and desorption of three volatile organic compounds (1,2- dichloroethane, 1,1,2- trichloroethane and 1,1,2,2-tetrachloroethane) from a previously uncontaminated clayey soil sample from a Superfund site in North Baton Rouge,Louisiana was studied. In the linear range of the adsorption isotherm, the partition constants were not affected by the presence of the co-solutes. The adsorption isotherms over a wide concentration range on the soil followed the nonlinearFreundlich isotherm. The desorption of the compounds showedsignificant hysteresis at all concentrations studied. Approximately 20 to 70% of the adsorbed mass of organic compounds resisted the desorption even after five months ofsuccessive desorption steps. The desorption of four compounds(1,2-dichloroethane, 1,1,2-trichloroethane, 1,4-dichlorobenzeneand hexachlorobutadiene) from a contaminated soil sample fromthe same site was also studied. The aqueous concentration declined as the successive desorption steps progressed. For hexachlorobutediene the desorption can be visualized as occurring in two stages. The first stage involved a ‘loosely bound’ or ‘reversible’ fraction and the second stage involveda ‘tightly bound’ or ‘resistant’ fraction.