Influence of hydroxypropyl-beta-cyclodextrin on the extraction and biodegradation of phenanthrene in soil

A study was conducted to investigate the effect of hydroxypropyl-beta-cyclodextrin (HPCD) on the aging and biodegradation of phenanthrene (PHE) in soil. Soil was spiked with PHE at 25 mgPHE/kgSOIL and HPCD at a range of concentrations from 0 to 3.5 gHPCD/kgSOIL and aged for 1, 84, and 322 d. At each time point, a variety of analyses were performed to assess the loss and aging of the PHE in the soil. Methods included determination of total PHE remaining, dichloromethane (DCM) and butan-1-ol (BuOH) extractions, and determination of PHE extractable by an aqueous HPCD shake extraction. Mineralization assays were also carried out to assess the availability of the PHE to a PHE-degrading bacterial inoculum. It was found that the presence of HPCD in the soils increased PHE loss from the aged soil systems, particularly at the higher application rates. Dichloromethane and BuOH extractabilities were reduced with aging and increasing HPCD concentration, as was the amount of PHE that was extractable using an aqueous HPCD shake extraction or that was available for mineralization. The DCM and BuOH extraction yielded similar results, and both greatly overestimated the availability of the PHE to the degraders, whereas the HPCD extraction results were very similar to that of PHE biodegradation. This study indicates that cyclodextrins have potential for use as alternatives to surfactants in enhancing the desorption/solubilization and degradation of recalcitrant organic contaminants in soil.     

Mechanisms regulating bioavailability of phenanthrene sorbed on a peat soil-origin humic substance

The organic matter-mineral complex plays an important role in regulating the fate of hydrophobic organic compounds (HOCs) in the environment. In the present study, the authors investigated the microbial bioavailability of phenanthrene (PHE) sorbed on the original and demineralized humic acids (HAs) and humin (HM) that were sequentially extracted from a peat soil. Demineralization treatment dramatically decreased the 720-h mineralized percentage of HM-sorbed PHE from 42.5 ± 2.6% to 3.4 ± 1.3%, whereas the influence of this treatment on the biodegradability of HA-associated PHE was much lower. Degradation kinetics of HA- and HM-sorbed PHE showed that its initial degradation rate was negatively correlated with the aromatic carbon content of humic substances (p<0.05). This was attributed to the strong interactions between PHE and the aromatic components of humic substances, which hampered its release and subsequent biodegradation. The 720-h mineralized percentage of PHE was inversely correlated with the estimated thickness of the organic matter layer at the surfaces of HAs and HMs. Therefore, in a relatively long term, diffusion of PHE within the organic matter layer could be an important factor that may limit the bioavailability of PHE to bacteria. Results of the present study highlight the molecular-scaled mechanisms governing bioavailability of PHE sorbed on humic substances.     

Assessment of bioavailability limitations during slurry biodegradation of petroleum hydrocarbons in aged soils

In an effort to determine whether bioavailability limitations are responsible for the slow or incomplete hydrocarbon biodegradation in aged soils, both the rate of desorption (rdes) and biodegradation (rbio) was measured for n-alkanes and polynuclear aromatic hydrocarbons (PAHs) at different times during the slurry biotreatment of six different soils. While all n-alkanes were biodegraded to various degrees depending on their respective carbon number and the soil organic matter content, none of them were desorbed to a significant extent, indicating that these saturated hydrocarbons do not need to be transferred from the soil particles into the aqueous phase in order to be metabolized by microorganisms. Most two- and three-ring PAHs biodegraded as fast as they were desorbed (rbio = rdes); that is, desorption rates controlled biodegradation rates. By contrast, the biodegradation kinetics of four-, five-, and six-ring PAHs was limited by microbial factors during the initial phase (rbio < rdes) while becoming mass-transfer rate limited during the final phase of bioremediation treatment (rbio = rdes). Whenever PAH biodegradation stalled or did not occur at all (rbio = 0), it was never due to bioavailability limitations (rdes > 0) but was more likely caused by microbial factors. such as the absence of specific PAH degraders or cometabolic substrates. Consequently, PAHs that are found to be microbially recalcitrant in aged soils may not be so because of limited bioavailability and thus could pose a greater risk to the environment than previously thought.     

Liver microsomal levels of cytochrome P450IA1 as biomarker for exposure and bioavailability of soil-bound polycyclic aromatic hydrocarbons

The bioavailability of soil-bound polycyclic aromatic hydrocarbons (PAHs) for mammalian species was studied with rats fed with a diet containing contaminated soil preparations. The extent of cytochrome P450IA1 (CYP1A1) induction in the liver correlated with the amount of 5- and 6-ring PAHs in the soil samples but not with the total PAH content. Other cytochromes P450 were much less affected by the soil-contaminants. The highest induction of CYP1A1 was obtained with a sample containing 274 mg 5- and 6-ring PAH/kg soil, resulting in a nearly 360-fold increase in the ethoxyresorufin deethylase (EROD) activity. In a semilogarithmic plot, a linear correlation was found between the 5- and 6-ring PAH concentration in the soil and the microsomal CYP1A1 content. As a model for the action of intestinal fluids, soil samples were extracted by bile acid solution. In these experiments, the selectivity in the solubilization of individual PAHs parallels that of toluene extraction, although the yield is lower than the latter and varies with the soil sample. The bioavailability of PAHs for microorganisms, but not for mammals, was shown to be considerably reduced in the presence of high total organic carbon (TOC) values of the soil samples. This may have implications for decontamination strategies, diminishing the effectiveness of biological decontamination in cases with high TOC values. The data suggest that CYP1A1 induction in rats is a parameter that may be useful in risk assessments of contaminated soils for mammalian species.Part of this work was presented at the International Congress on Cytochrome P450: Biochemistry, Biophysics and Molecular Biology, Lisbon, 1993 (Roos et al. 1994a) and as a lecture on ECOINFORMA, Wien, 1994 (Roos et al. 1994b, 1994c)     

Aging of triazine amine in soils demonstrated through sorption, desorption, and bioavailability measurements

The aging of triazine amine in soil was studied during a time course of 119 d by measuring bioavailability in terms of mineralization after inoculation of the triazine amine-degrading bacterium Rhodococcus erythropolis TA57. The bioavailability was measured in four soil samples: A-, B-, and C-horizons from an agricultural soil profile and in a peat soil. The sorption of triazine amine in the soil samples was quantified during the period of aging in terms of sorption distribution coefficients (Kd) and desorption distribution coefficients (Kd,des). Measures of bioavailability and triazine amine concentration in the nonavailable fraction showed effects of aging in the soils that were rich in organic matter. The triazine amine bioavailability declined significantly during the aging period in soils containing greater than 2% organic carbon, whereas the B- and C-horizons showed no signs of aging, in agreement with their low content of organic material. Corresponding to this, desorption decreased significantly in the A-horizon but, surprisingly, not in the peat soil. Analyses by thin-layer chromatography indicated an association of aqueous triazine amine and dissolved organic matter in the peat soil. This gives an explanation for both the significant decrease in bioavailability and the noncorresponding stability of the nonavailable (i.e., nondesorbed) fraction.     

Impact of metals on the biodegradation of organic pollutants

Forty percent of hazardous waste sites in the United States are co-contaminated with organic and metal pollutants. Data from both aerobic and anaerobic systems demonstrate that biodegradation of the organic component can be reduced by metal toxicity. Metal bioavailability, determined primarily by medium composition/soil type and pH, governs the extent to which metals affect biodegradation. Failure to consider bioavailability rather than total metal likely accounts for much of the enormous variability among reports of inhibitory concentrations of metals. Metals appear to affect organic biodegradation through impacting both the physiology and ecology of organic degrading microorganisms. Recent approaches to increasing organic biodegradation in the presence of metals involve reduction of metal bioavailability and include the use of metal-resistant bacteria, treatment additives, and clay minerals. The addition of divalent cations and adjustment of pH are additional strategies currently under investigation.     

Biodegradation during contaminant transport in porous media: 6. Impact of sorption on coupled degradation-transport behavior

Bioavailability is one of the critical factors influencing the biodegradation and bioremediation of organic compounds. The bioavailability of many organic contaminants is controlled in part by the nature, magnitude, and rate of sorption/desorption processes. This study investigates the impact of sorption and associated retardation on the bioavailability and biodegradation of aromatic hydrocarbons during transport in porous media. Miscible-displacement experiments were conducted using naphthalene and 2-naphthol as the model sorbing compounds and salicylate, a degradation product of naphthalene, as a nonsorbing reference compound. Two porous media were used, one (Eustis soil, FL, USA) with moderate sorption capacity and one (quartz sand) with no measurable sorption of the compounds. The porous media were sterilized and inoculated with Pseudomonas putida RB1353, an organism that degrades naphthalene and its derivatives. The biodegradation and transport of all three substrates in quartz sand were significantly influenced by microbial lag, the effects of which were observed within two to three pore volumes (3-4.5 h). This was also true for salicylate transport in the Eustis soil system. Conversely, biodegradation lag effects were not observed for naphthalene or 2-naphthol in the Eustis soil system. In addition, the masses of naphthalene and 2-naphthol degraded were significantly greater for the Eustis soil system compared to the quartz sand system. As noted previously, naphthalene and 2-naphthol were sorbed by the Eustis soil but not the quartz sand, while salicylate was not sorbed by either media. These results indicate that the increased residence time associated with sorption of naphthalene and 2-naphthol by Eustis soil enhanced overall biodegradation and obviated the impact of lag on observed transport behavior.     

Enhancing the attenuation of explosives in surface soils at military facilities: combined sorption and biodegradation

This research evaluated soil amendments designed to enhance the adsorption and biodegradation of explosives at military training facilities, thus minimizing their potential for transport to subsurface environments. Several carbon cosubstrates were tested in soil slurries for their ability to stimulate the biodegradation of 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (royal demolition exposive [RDX]), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (high-melting explosive [HMX]) by indigenous soil microorganisms. Crude soybean oil and molasses stimulated mineralization of RDX (30-40%) and HMX (approximately 10%). The TNT was not significantly mineralized in any of the treatments, but high-performance liquid chromatography (HPLC) analysis indicated extensive transformation of TNT to amino-containing compounds. The biodegradation of explosives was then examined in unsaturated soil microcosms amended with crude soybean oil and molasses combined with sphagnum peat moss and sawdust. Minimal TNT mineralization was observed, and HMX mineralization was only observed with molasses addition. In contrast, RDX mineralization was extensive in microcosms amended with soybean oil or molasses. The presence of peat moss decreased soybean oil-stimulated RDX mineralization by approximately 5%, but resulted in about 5% greater RDX mineralization compared with molasses only. Sawdust markedly decreased mineralization regardless of cosubstrate type. Mass balance results indicated that the formation of bound residues likely was occurring, especially for TNT. These results indicate that the application of inexpensive adsorbents and cosubstrates to soils may significantly improve the protection of groundwater resources underlying live fire ranges.     

Investigation of an onsite wastewater treatment system in sandy soil: modeling the fate of surfactants

Field monitoring data for three common laundry detergent surfactants were used to test the applicability of a mathematical model that was developed as a screening-level tool for predicting the fate and transport of consumer product ingredients in septic systems. This model takes into account the simultaneous effects of sorption and biodegradation on the transport of chemicals through a septic system. Predicted groundwater concentrations of alcohol ethoxylate (AE) and alcohol ethoxy sulfate (AES) surfactants were in excellent agreement with measured values. This good agreement was to some extent due to the fact that the biodegradation rates of AE and AES do not vary significantly as a function of the degree of oxygenation of the soil. However, using laboratory-measured soil biodegradation rates for linear alkylbenzene sulfonate (LAS), the model underpredicted measured LAS concentrations in groundwater downgradient from the drainage field. This underprediction was due to the fact that the groundwater beneath the drainage field was anoxic during certain parts of the year and LAS is not degradable under this condition. Measured LAS concentrations were consistent with an assumed in situ soil biodegradation rate that was lower than the rate measured under fully oxygenated laboratory conditions. A limitation of the model is that only one soil biodegradation rate can be input for the saturated zone, even though biodegradation rates may vary seasonally or with distance from the drainage field. However, the model was appropriate and useful as a screening tool for the sorbable organic compounds studied. The applicability of the model to other classes of compounds should be assessed before broader application.     

Indices for bioavailability and biotransformation potential of contaminants in soils

Bioavailability is an important consideration in risk assessment of soil contaminants and in the selection of appropriate remediation technologies for polluted sites. The present study examined the bioavailability and biodegradation potential of phenanthrene with respect to a pseudomonad in 15 different soils through separate measurements of mineralization, transformation, and desorption to a polymeric infinite sink (Tenax) after 180-d sterile pre-equilibration with phenanthrene. Fractions strongly resistant to desorption and mineralization at long times were evident in all cases. After correcting for bioconversion (moles mineralized per mole transformed) determined in aqueous particle-free soil extracts, a correlation was found between the biotransformation-resistant fraction and the Tenax desorption-resistant fraction. Indices are proposed to assess bioavailability (BAt) and biotransformation potential (BTPt) of a compound in a soil based on parallel desorption and degradation studies over a selected period t. The BAt is the ratio of moles biotransformed to moles desorbed to an infinite sink, and it reflects the biotransformation rate relative to the maximal desorption rate. Values of BA30 (30-d values) ranged from 0.64 (for dark gray silt loam) to 1.12 (Wurtsmith Air Force Base [AFB] 2B, Oscoda, MI, USA). The BTPt is the ratio between moles biotransformed and moles of contaminant remaining sorbed after maximal desorption. The BTPt provides an indication of the maximum extent of biotransformation that may be expected in a system, assuming desorption is a prerequisite for biodegradation. Values of BTP30 ranged between 0.3 (Wurtsmith AFB 1B) and 13 (Mount Pleasant silt loam, NY, USA). The combination of BAt and BTPt provides insights regarding the relationship between physical availability (desorption) and biological processes (biotransformation kinetics, toxicity, other soil factors) that occur during biodegradation and are suggested to represent the remediation potential of the chemical. The BA30 values less than 0.9 and BTP30 values less than five indicate poor potential for site remediation.     

A potential link between the turnover of soil organic matter and the release of aged organic contaminants

Soil column experiments were conducted to investigate the release of aged organic contaminants (adsorbable organohalogens, [AOX]) and the potential effects of microbial turnover of soil organic matter onto the contaminant release from two different soils of former wastewater infiltration sites. Under reference conditions, the total carbon flux ranged from 6 to 9 mg/kg/d from the two soils. More than 92% of it was due to mineralization (CO2) and below 8% to the dissolved organic carbon (DOC) in the column effluent. Despite very different organohalogen levels in the two soils, the fluxes of AOX were similar (1 microg/kg/d). Microbial activity was altered by changing temperature and the solution pH and by drying. In all experiments where increased microbial respiration (CO2 production) was observed, the release of organic contaminants in the column effluent also increased. Fluxes of nickel, zinc, and phosphate showed a completely different pattern, suggesting that their retention in soil is determined by chemical parameters. A linear correlation analysis provided clear evidence that AOX and DOC fluxes are closely linked to each other. However, the superimposition of abiotic and biotic processes in the soils did not allow a causal relation to be established between the mineralization of soil organic matter and the release of organic contaminants sequestered in the soil. Our findings suggest that microbial activity may have a significant impact on the extent to which contaminated soils may act as a source of persistent organic pollutants.     

Enhancement of aerobic microbial degradation of polychlorinated biphenyl in soil microcosms

This article reports the results of various biodegradation experiments on polychlorinated biphenyl (PCB)-contaminated sandy soil employing a mixed culture of acclimatized bacteria. Following the optimization of different variables without chemical pretreatment, the elimination rate achieved of Aroclor 1242 in slurry-phase reactors was 61% after four months of treatment, with the presence of biphenyl as cosubstrate being the most important factor affecting PCB biodegradation. The biodegradation occurred as a first-order process, and it proved most effective in respect to dichlorinated biphenyls (100% removal), followed by trichlorinated (92%) and tetrachlorinated biphenyls (24%). The results also showed that the degradability of PCBs in soil may be enhanced by an advanced oxidation pretreatment (Fenton reaction), producing almost 100% elimination of PCBs at the end of the integrated chemical-biological process and 72% mineralization of the intermediates generated during the chemical pretreatment.     

Pyrene and chrysene fate in surface soil and sand microcosms

Polycyclic aromatic hydrocarbons (PAHs) are major components of wastes from municipal gas plants and many wood preservatives. Soil contaminated with these wastes is a potential threat to human health because of the carcinogenicity of many PAHs. This study follows the fate of two four-ring PAHs, pyrene and chrysene, in three matrices: an adapted soil (obtained from a site contaminated with PAHs for more than 75 years), an uncontaminated soil (with and without an inoculum of adapted soil), and sand mixed with an inoculum of adapted soil. Radiolabeled pyrene, chrysene, and salicylic acid (a metabolite of PAH biodegradation) were used to trace the mineralization, transformation, extractability, and formation of an unextractable residual over time. Linear approximations of the rates of these processes were made. High-performance liquid chromatography (HPLC) analysis of extracts from inoculated soil showed the transient formation of two known metabolites: 1-hydroxypyrene (from pyrene) and 1-hydroxy-2-naphthoic acid (from chrysene). The amount of extractable label diminished steadily over the course of the study in systems that were not inhibited with sodium azide, whereas the amount of extractable label remained relatively constant in inhibited systems. Correspondingly, the amount of nonextractable residual label generally increased during each incubation in uninhibited systems, whereas the amount of this residual label remained relatively constant in inhibited systems. In contrast, the rate and extent of mineralization varied widely across matrix types. This suggests that alterations of the PAH that impact extractability and residual formation are common, in contrast to mineralization, which was apparently limited to adapted communities.     

Impact of Phenanthrene on Organic Acids Secretion and Accumulation by Perennial Ryegrass, <Emphasis Type="Italic">Lolium perenne</Emphasis> L., Root

A solution culture experiment was performed to investigate the impact of phenanthrene (PHE) on organic acids secretion and accumulation by Lolium perenne L. root. Data showed that, oxalic acid was the dominant composition of organic acids in root and root exudates. In root exudates, increased levels of PHE resulted in higher oxalic acid and its secrete proportion; oxalic acid arranged from 3.00 to 4.72 mg/g FW under spiked PHE treatments, in control, it was 2.33 mg/g FW. In root, oxalic acid rose to 25.61 mg/g FW at 1 mg/L PHE treatment, while the PHE concentration was continuously increasing, organic acids in root decreased.     

Biodegradation kinetics of linear alkylbenzene sulfonate in sludge-amended agricultural soils

The kinetics of ultimate biodegradation (mineralization to CO2) of linear alkylbenzene sulfonate (LAS) were studied in sludge-amended agricultural soils for a series of pure chain length LAS homologs containing 10 to 14 carbon atoms in the alkyl chain. Degradation rates were measured by following the production of 14CO2 from uniformly 14C-ring-labeled material. In general, degradation of LAS was rapid in soil over a broad concentration range (0.1 to 10 times the expected environmental concentration) and demonstrated little variation among different homologs. Half-lives for mineralization of the benzene ring ranged from 18 to 26 days and were not significantly different for any homolog over the range of alkyl chain lengths tested. Half-lives measured for LAS degradation in these studies were comparable to values reported in the literature and also to values obtained for naturally occurring materials (stearic acid, cellulose) typically present in soil environments. On the basis of the results of the present studies and those of other investigators, it is concluded that soil environments exposed to LAS in sewage sludges contain microbial communities which can actively metabolize this material. Rates of biodegradation of the benzene ring, the final step in the LAS biodegradation pathway prior to complete mineralization, are also sufficient to prevent LAS from accumulating in soil environments.     

Statistical Implications of Pyrene and Phenanthrene Sorptive Phenomena: Effects of Sorbent and Solute Properties

A 3 × 2 factorial experiment in a generalized randomly complete block was conducted to assess the effects of soil type, soil preparation, and solute concentration on the sorptive behavior of pyrene (PYR) and phenanthrene (PHE). Three bulk soils were treated to remove the soil organic matter (SOM) or clay fractions, then spiked with an initial PYR/PHE concentration of either 3 or 15 mg/L. On average, 98.3% PYR and 91.3% PHE were sorbed to the bulk soils in 24 h, with 4.96 mg PYR kg⁻¹ soil and 22.48 mg PHE kg⁻¹ soil desorbed after three successive 24-h desorption steps. Both clay minerals and SOM greatly contributed to the sorptive behavior. For example, an average 95.1% and 96.1% of the initial PYR sorbed to the clay-removed and SOM-removed subsoils, respectively. Conversely, 16.5 mg/kg and 12.9 mg/kg of the sorbed PYR was desorbed from the clay-removed and SOM-removed subsoils, respectively. Received: 18 January 2002/Accepted: 13 May 2002     

Prediction of polycyclic aromatic hydrocarbon biodegradation in contaminated soils using an aqueous hydroxypropyl-beta-cyclodextrin extraction technique

This study investigated the use of an aqueous hydroxypropyl-beta-cyclodextrin (HPCD) shake extraction to predict the degree of microbial degradation of polycyclic aromatic hydrocarbons (PAHs) in soils. Three different aged PAH-contaminated soils were studied: A soil from a former coke works (CW) and two artificially contaminated soils (AC1 and AC2). First, the catabolic activity of the indigenous soil microflora was assessed with 14C-respirometry, using a range of 14C-labeled aromatic compounds. Extensive mineralization of several compounds occurred in the CW and the AC2 soils, suggesting that both soils contained catabolically active microorganisms. No significant mineralization occurred in the AC1 soil, implying that either it did not contain an indigenous PAH-degrading microbial population or that degradation, but not mineralization, occurred. The soils then were subjected to three sets of analyses: dichloromethane (DCM) soxhlet extraction, six-week biodegradation assay followed by DCM extraction, and extraction with HPCD followed by DCM extraction. A general decrease in PAHs present in the soils occurred after the biodegradation assay. In the CW and the AC1 soils, strong correlations were observed between the amount of PAHs biodegraded and the fraction of PAHs removed from the soils using the HPCD extraction. However, the AC2 soil showed a more modest correlation between the biodegradable fraction and the HPCD extractable fraction, with the HPCD extraction slightly underestimating the extent of PAH biodegradation. The results of this study indicated that an aqueous HPCD extraction may be a useful tool in assessing the microbial availability of aged contaminant mixtures in soils, although further validation is required.     

Effects of organic fertilizer treatments and old humus on thiofanox and aldicarb soil metabolisms in sugar beets

Cow manure, pig slurry, or a mixture of both were applied on field plots of a sugar beet trial, 1 month before sowing and an insecticide thiofanox soil treatment. During the first 2 months crop period, the S-+SO-+SO₂-thiofanox soil half-lives were about 43 days in the organic fertilizer treated plots and 24 days in the organic fertilizer untreated control plots. In another sugar beet trial, soil was treated at sowing with the insecticide aldicarb, and the field was divided into two parts. The first part had been treated with cow manure in the autumn preceding sugar beet sowing; its organic matter concentration was 2.4%; that cow manure treatment had been repeated every 3 years for 18 years. In the second part, the soil contained a high concentration of old soil organic matter, humus (4.3%); this corresponded to a meadow ploughed 18 years ago; since then, no organic fertilizer had been applied. During the first 2 months crop period, the S-+SO-+SO₂-aldicarb soil half-lives in the first and second part of the field were 63 and 29 days, respectively. The results show that the recent soil organic matter slows down insecticide soil metabolisms, and increases their protection efficiencies. However, the old soil organic matter, humus, had no significant effect on biodegradation, in spite of its high soil concentration.     

Comparison of synthetic surfactants and biosurfactants in enhancing biodegradation of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbon (PAH) contamination of the environment represents a serious threat to the health of humans and ecosystems. Given the human health effects of PAHs, effective and cost-competitive remediation technologies are required. Bioremediation has shown promise as a potentially effective and low-cost treatment option, but concerns about the slow process rate and bioavailability limitations have hampered more widespread use of this technology. An option to enhance the bioavailability of PAHs is to add surfactants directly to soil in situ or ex situ in bioreactors. Surfactants increase the apparent solubility and desorption rate of the PAH to the aqueous phase. However, the results with some synthetic surfactants have shown that surfactant addition can actually inhibit PAH biodegradation via toxic interactions, stimulation of surfactant degraders, or sequestration of PAHs into surfactant micelles. Biosurfactants have been shown to have many of the positive effects of synthetic surfactants but without the drawbacks. They are biodegradable and nontoxic, and many biosurfactants do not produce true micelles, thus facilitating direct transfer of the surfactant-associated PAH to bacteria. The results with biosurfactants to date are promising, but further research to elucidate surfactant-PAH interactions in aqueous environments is needed to lead to predictive, mechanistic models of biosurfactant-enhanced PAH bioavailability and thus better bioremediation design.