Mass transfer effects on microbial uptake of naphthalene from complex NAPLs

The role of intra-NAPL diffusion on mass transfer from MGP residuals

An experimental and computational study was performed to investigate the role of multi-component intra-NAPL diffusion on NAPL-water mass transfer. Molecular weight and the NAPL component concentrations were determined to be the most important parameters affecting intra-NAPL diffusion coefficients. Four NAPLs with different viscosities but the same quantified mass were simulated. For a spherical NAPL body, a combination of NAPL properties and interphase mass transfer rate can result in internal diffusion limitations. When the main intra-NAPL diffusion coefficients are in the range of self-diffusion coefficients (10^-5 to 10^-6 cm²/s), dissolution is not limited by internal diffusion except for high mass transfer rate coefficients (>180 cm/day). For a complex and relatively high viscous NAPL (>50 g/(cm s)), smaller intra-NAPL diffusion coefficients (<10^-8) are expected and even low mass transfer rate coefficients (~6 cm/day) can result in diffusion-limited dissolution.

Kinetic modelling of a diesel-polluted clayey soil bioremediation process

A mathematical model is proposed to describe a diesel-polluted clayey soil bioremediation process. The reaction system under study was considered a completely mixed closed batch reactor, which initially contacted a soil matrix polluted with diesel hydrocarbons, an aqueous liquid-specific culture medium and a microbial inoculation. The model coupled the mass transfer phenomena and the distribution of hydrocarbons among four phases (solid, S; water, A; non-aqueous liquid, NAPL; and air, V) with Monod kinetics. In the first step, the model simulating abiotic conditions was used to estimate only the mass transfer coefficients. In the second step, the model including both mass transfer and biodegradation phenomena was used to estimate the biological kinetic and stoichiometric parameters. In both situations, the model predictions were validated with experimental data that corresponded to previous research by the same authors. A correct fit between the model predictions and the experimental data was observed because the modelling curves captured the major trends for the diesel distribution in each phase. The model parameters were compared to different previously reported values found in the literature. Pearson correlation coefficients were used to show the reproducibility level of the model.

Surface hydrophobicity of petroleum hydrocarbon degrading Burkholderia strains and their interactions with NAPLs and surfaces

Bacterial cell surface hydrophobicity (CSH) is an important factor governing the growth and adhesion behavior of microorganisms on non-aqueous phase liquids (NAPLs). In this work CSH and surface charge was quantified for three oil degrading Burkholderia cultures: aliphatic degrader Burkholderia cepacia (ES1) and two strains of aromatic degrading Burkholderia multivorans (NG1 and HN1) based on contact angle and zeta potential measurement. Model non-aqueous phase liquids (NAPLs) were formulated using n-hexadecane, naphthalene, phenanthrene and pyrene in varying concentration. Adhesion on to glass surfaces of varying hydrophobicity and adherence to n-hexadecane was quantified and correlated with hydrophobicity of the surface; variation in CSH of the culture in response to model NAPL used as growth substrate; and variation in zeta potential as a result of variation in growth substrate, ionic strength and pH of resuspension solution. B. cepacia (ES1) and B. multivorans (HN1) depicted comparable CSH which was higher than that of B. multivorans (NG1). For each culture, CSH was found to vary with the model NAPL used as growth substrate. Adhesion to glass increased with increase in CSH of the bacterial culture and with increase in hydrophobicity of the glass surface. B. cepacia (ES1) with lower negative zeta potential consistently depicted greater adhesion compared to B. multivorans (HN1). Adherence to n-hexadecane was significantly affected by various other factors, such as, growth substrate, pH, resuspension solution and their interactions as revealed through statistical analysis. These factors affected both the zeta potential and adherence to n-hexadecane to varying degree for the three Burkholderia cultures.

Biodegradation of oil in oily sludges from steel mills

Lab-scale batch studies were conducted to determine the biodegradability of oil associated with oily sludge from a steel mill using two microbial cultures enriched in the laboratory. After 60 days of biodegradation the residual oil content in mill sludge was reduced from 4.5-5% to 2.7-3.0%, corresponding to 40-45% loss with respect to initial. The rate of degradation was different for the two enrichment cultures studied. Significant loss of oil was observed in the un-inoculated controls while loss in the azide killed controls was negligible. Bioavailability limitations and the presence of structurally complex high molecular weight hydrocarbons in lubricating oil are responsible for the slow rate of degradation. Significant loss of oil in un-inoculated controls indicated the presence of indigenous microorganisms in oily mill sludge. The association of biomass with sludge solids and presence of a high level of residual oil may adversely affect the recyclability of iron-fines associated with the sludge.

Naphthalene and phenanthrene sorption to very low organic content diatomaceous earth: modeling implications for microbial bioavailability

Naphthalene and phenanthrene sorption was investigated on microporous/high surface area and low-microporous/low surface area particles with very low organic (f(oc)) content. Partitioning coefficients (K(p)) for naphthalene were similar to those predicted from the Karickhoff equation in both competitive and non-competitive sorption isotherms, even given the very low f(oc). In contrast, phenanthrene K(p) values in competitive isotherms were 10-fold higher than predicted by Karickhoff, suggesting phenanthrene out-competes naphthalene for sorption sites. Naphthalene exhibited greater non-competitive K(p) at higher concentrations on the microporous particles, as evidenced by a Freundlich n=0.74. Both compounds had 100-fold lower adsorption and desorption mass flux on the microporous particles. Adsorption followed first order kinetics, with phenanthrene adsorbing at 1.5 and 3 times the rate of naphthalene on the low surface area and high surface area particles, respectively. Naphthalene and phenanthrene desorption kinetics were well-described by a Fickian diffusion model with observed diffusivities (D(obs)) of 1.7-1.9 x 10(-8) and 0.93-1.9 x 10(-8) cm(2) s(-1) for naphthalene and phenanthrene, respectively. Phenanthrene D(obs) were 3-5 orders of magnitude faster than those reported in organic-rich sediments. Naphthalene D(obs) were 100-fold lower than fast-domain diffusivities, indicating access to micropores. Naphthalene sorption non-linearity was investigated via simulations with two coupled desorption-biodegradation models. Results indicate that non-linearity would not significantly affect bioavailability in low f(oc) geosorbents. In contrast, sorption non-linearity would result in greatly decreased bioavailability in organic-rich geosorbents, indicating that desorption non-linearity should be considered for surface soils and sediments but may not be critical for low f(oc) aquifer material.

Experimental evaluation and mathematical modeling of microbially enhanced tetrachloroethene (PCE) dissolution

Experiments to assess metabolic reductive dechlorination (chlororespiration) at high concentration levels consistent with the presence of free-phase tetrachloroethene (PCE) were performed using three PCE-to-cis-1,2-dichloroethene (cis-DCE) dechlorinating pure cultures (Sulfurospirillum multivorans, Desulfuromonas michiganensis strain BB1, and Geobacter lovleyi strain SZ) and Desulfitobacterium sp. strain Viet1, a PCE-to-trichloroethene (TCE) dechlorinating isolate. Despite recent evidence suggesting bacterial PCE-to-cis-DCE dechlorination occurs at or near PCE saturation (0.9-1.2 mM), all cultures tested ceased dechlorinating at approximately 0.54 mM PCE. In the presence of PCE dense nonaqueous phase liquid (DNAPL), strains BB1 and SZ initially dechlorinated, but TCE and cis-DCE production ceased when aqueous PCE concentrations reached inhibitory levels. For S. multivorans, dechlorination proceeded at a rate sufficient to maintain PCE concentrations below inhibitory levels, resulting in continuous cis-DCE production and complete dissolution of the PCE DNAPL. A novel mathematical model, which accounts for loss of dechlorinating activity at inhibitory PCE concentrations, was developed to simultaneously describe PCE-DNAPL dissolution and reductive dechlorination kinetics. The model predicted that conditions corresponding to a bioavailability number (Bn) less than 1.25 x 10(-2) will lead to dissolution enhancement with the tested cultures, while conditions corresponding to a Bn greater than this threshold value can result in accumulation of PCE to inhibitory dissolved-phase levels, limiting PCE transformation and dissolution enhancement. These results suggest that microorganisms incapable of dechlorinating at high PCE concentrations can enhance the dissolution and transformation of PCE from free-phase DNAPL.

Reduction of benzene and naphthalene mass transfer from crude oils by aging-induced interfacial films

Semi-rigid films or skins form at the interface of crude oil and water as a result of the accumulation of asphaltene and resin fractions when the water-immiscible crude oil is contacted with water for a period of time or "aged". The time varying patterns of area-independent mass transfer coefficients of two compounds, benzene and naphthalene, for dissolution from crude oil and gasoline were determined. Aqueous concentrations of the compounds were measured in the eluent from flow-through reactors, where a nondispersed oil phase and constant oil-water interfacial area were maintained. For Brent Blend crude oil and for gasoline amended with asphaltenes and resins, a rapid decrease in both benzene and naphthalene mass transfer coefficients over the first few days of aging was observed. The mass transfer coefficients of the two target solutes were reduced by up to 80% over 35 d although the equilibrium partition coefficients were unchanged. Aging of gasoline, which has negligible amounts of asphaltene and resin, did not result in a change in the solute mass transfer coefficients. The study demonstrates that formation of crude oil-water interfacial films comprised of asphaltenes and resins contribute to time-dependent decreases in rates of release of environmentally relevant solutes from crude oils and may contribute to the persistence of such solutes at crude oil-contaminated sites. It is estimated that the interfacial film has an extremely low film mass transfer coefficient in the range of 10(-6) cm/min.

Bacterial chemotaxis to naphthalene desorbing from a nonaqueous liquid

Bacterial chemotaxis has the potential to increase the rate of degradation of chemoattractants, but its influence on degradation of hydrophobic attractants initially dissolved in a non-aqueous-phase liquid (NAPL) has not been examined. We studied the effect of chemotaxis by Pseudomonas putida G7 on naphthalene mass transfer and degradation in a system in which the naphthalene was dissolved in a model NAPL. Chemotaxis by wild-type P. putida G7 increased the rates of naphthalene desorption and degradation relative to rates observed with nonchemotactic and nonmotile mutant strains. While biodegradation alone influenced the rate of substrate desorption by increasing the concentration gradient against which desorption occurred, chemotaxis created an even steeper gradient as the cells accumulated near the NAPL source. The extent to which chemotaxis affected naphthalene desorption and degradation depended on the initial bacterial and naphthalene concentrations, reflecting the influences of these variables on concentration gradients and on the relative rates of mass transfer and biodegradation. The results of this study suggest that chemotaxis can substantially increase the rates of mass transfer and degradation of NAPL-associated hydrophobic pollutants.