Biodegradation of non-desorbable naphthalene in soils

Microbial Involvement in the Bioremediation of Total Petroleum Hydrocarbon Polluted Soils: Challenges and Perspectives

Nowadays, soil contamination by total petroleum hydrocarbons is still one of the most widespread forms of contamination. Intervention technologies are consolidated; however, full-scale interventions turn out to be not sustainable. Sustainability is essential not only in terms of costs, but also in terms of restoration of the soil resilience. Bioremediation has the possibility to fill the gap of sustainability with proper knowledge. Bioremediation should be optimized by the exploitation of the recent “omic” approaches to the study of hydrocarburoclastic microbiomes. To reach the goal, an extensive and deep knowledge in the study of bacterial and fungal degradative pathways, their interactions within microbiomes and of microbiomes with the soil matrix has to be gained. “Omic” approaches permits to study both the culturable and the unculturable soil microbial communities active in degradation processes, offering the instruments to identify the key organisms responsible for soil contaminant depletion and restoration of soil resilience. Tools for the investigation of both microbial communities, their degradation pathways and their interaction, will be discussed, describing the dedicated genomic and metagenomic approaches, as well as the interpretative tools of the deriving data, that are exploitable for both optimizing bio-based approaches for the treatment of total petroleum hydrocarbon contaminated soils and for the correct scaling up of the technologies at the industrial scale.

Contribution of phenanthrene in different binding sites to its biodegradation in biochar-amended soils

Biochars can strongly sorb hydrophobic organic contaminants in soils. However, contribution of contaminants in different binding sites to their biodegradation in biochar-amended soils is not clear. In this work, wheat straw biochars were prepared at pyrolysis temperatures of 400 °C (BC400) and 700 °C (BC700). During a 42-day experiment, degradation rate constant of phenanthrene in soils was in the order of treatment without biochar (1.64 × 10-2 d-1) > treatment with BC700 (0.96 × 10-2 d-1) > treatment with BC400 (0.30 × 10-2 d-1). At the beginning, amendment of BC400 and BC700 reduced the rapidly desorbing fraction of phenanthrene in soils by 44.8% and 92.5%, respectively. At the end, both phenanthrene and microbial biomass highly concentrated on the biochar separated from soils. The results of a coupled model of desorption and biodegradation revealed that only phenanthrene in rapidly desorbing sites was degraded in BC400-amended soils, whereas degradation of phenanthrene in both rapidly and slowly desorbing sites occurred in BC700-amended soils, contributing 24.4% and 75.6% of the degradation, respectively. High fraction (>95%) of biodegradable phenanthrene in slowly desorbing sites was the key reason for higher biodegradation rate of phenanthrene in soils with BC700 than in soils with BC400.

Effect of Potamogeton crispus L. on bioavailability and biodegradation activity of pyrene in aged and unaged sediments

In order to clarify the effect of Potamogeton crispus L. (P. crispus) on bioavailability and biodegradation activity of pyrene in aged and unaged sediments, model calculation based on experimental results was carried out. During a 36-day experiment, the dissipation ratio of pyrene was increased by planting but decreased by aging. P. crispus improved the dissipation more significantly in aged sediments (45.9%) than in unaged sediments (17.6%). Results derived from a two-compartment desorption model showed that the decrease of rapidly desorbing fraction of pyrene was in the order of aged sediments without plant (A)>unaged sediments without plant (U)>unaged sediments with plant (UP)>aged sediments with plant (AP). Moreover, the results of biodegradation kinetic model showed that the first-order biodegradation coefficient was in the order of AP>UP>U and A, which was consistent with that of sediment redox potential. These modeling results indicated that planting could enhance the bioavailability (73.9%) and biodegradation activity (277%) of pyrene more significantly in aged sediments as compared to unaged sediments (13.1% and 150%, respectively), which should be the key reasons leading to more significant dissipation increment of pyrene in aged sediments by P. crispus.

Degradation of cyflumetofen and formation of its main metabolites in soils and water/sediment systems

Cyflumetofen is a novel benzoyl acetonitrile acaricide without cross-resistance to existing acaricides. In the present study, for the first time, the environmental behaviors of cyflumetofen and the formation of its main metabolites, 2-(trifluoromethyl) benzoic acid (B-1) and 2-(trifluoromethyl) benzamide (B-3), in the four types of soil (black soil, sierozem, krasnozem, and fluvo-aquic soil) and three types of water/sediment systems (Northeast Lake, Hunan paddy field, and Beijng Shangzhuang reservoir) under aerobic and anaerobic conditions were investigated. The degradation dynamics of cyflumetofen followed first-order kinetics. Under aerobic environment, the half-lives of cyflumetofen in black soil, sierozem, krasnozem and fluvo-aquic soil were 11.2, 10.3, 12.4, and 11.4 days. Under water anaerobic conditions, the half-lives were 13.1, 10.8, 13.9, and 12.8 days. The effects of different conditions and soil types on the half-lives of cyflumetofen were studied using a one-way ANOVA test with post hoc comparison (Tukey's test). It was shown that the differences in black soil, krasnozem, and fluvo-aquic soil were extremely significant difference (p < 0.05) under aerobic and water anaerobic conditions. And there is a strong correlation between half-life and pH. Under aerobic environment, the half-lives of cyflumetofen in Northeast Lake, Hunan paddy field, and Beijng Shangzhuang reservoir were 15.4, 16.9, and 15.1 days. Under anaerobic conditions, they were 16.5, 17.3, and 16.1 days. Analyzing the differences of the half-lives under aerobic and anaerobic conditions, the difference only in Shangzhuang reservoir was extremely significant difference (p < 0.05). In soils, cyflumetofen degraded metabolites B-1 and B-3, from the first day 0.24 % B-1 was generated, while, only very low levels of B-3 generated at the same time. As time increased, B-3 gradually increased, cyflumetofen reduced gradually. Until 100 days, there were about 3.5 % B-1 and B-3 in the soils. In the water/sediment systems, from the first day, it degraded into B-1 in the sediment, and in the water mainly degraded into B-3.

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.

Degradation of Fluxapyroxad in Soils and Water/Sediment Systems Under Aerobic or Anaerobic Conditions

The persistence and fate of fluxapyroxad were investigated including studies with four soils from Hunan (HN), Shanxi (SX), Jiangsu (JS), and Heilongjiang (HLJ) and two water/sediment systems (water/sediment systems 1 and 2) from Beijing, China. The results demonstrated that the biodegradation efficiency of fluxapyroxad in soils under aerobic conditions was higher than that observed under anaerobic conditions. The order of degradation capability was HLJ soil > JS soil > SX soil > HN soil, and fluxapyroxad dissipated faster in water/sediment system 2 than in system 1. The tested systems (four soils and two water/sediments systems) with rich organic matter content, high oxygen level and neutral pH had a high potential to degrade fluxapyroxad, possibly because rich organic matter and oxygen level stimulated microbial activity and the neutral pH was suitable for microbial growth. These results showed that fluxapyroxad exhibited high persistence in tested systems, with half-lives ≥157.6 day.

Biosorption and biodegradation of phenanthrene and pyrene in sterilized and unsterilized soil slurry systems stimulated by Phanerochaete chrysosporium

To assess the "bioaccessible" pool of mycelia-bound polycyclic aromatic hydrocarbons (PAHs) and to quantify its biodegradation kinetics in soil, a soil-slurry system containing mycelial pellets of Phanerochaete chrysosporium as a separable biophase was set up. In sterilized and unsterilized soil-slurry, the distribution and dissipation of phenanthrene and pyrene in soil, fungal body of P. chrysosporium and water were independently quantified over the incubation periods. Biosorption and biodegradation contributions to bio-dissipation of dissolved- and sorbed-PAHs were identified. The biodegradation kinetics of PAHs by allochthonous P. chrysosporium and soil wild microorganisms was higher than those predicted by a coupled desorption-biodegradation model, suggesting both allochthonous and wild microorganisms could access sorbed-PAHs. The obvious hysteresis of PAHs in soil reduced their biodegradation, while the biosorbed-PAHs in P. chrysosporium body as an interim pool exhibited reversibly desorption and were almost exhausted via biodegradation. Both biosorption and direct biodegradation of PAHs in soil slurry were stimulated by allochthonous P. chrysosporium. After 90-day incubation, the respective biodegradation percentages for phenanthrene and pyrene were 63.8% and 51.9% in the unsterilized soil without allochthonous microorganisms, and then increased to 94.9% and 90.6% when amended with live P. chrysosporium. These indicate that allochthonous and wild microorganisms may synergistically attack sorbed-PAHs.

Effects of pH and organic co-solvents on the oxidation of naphthalene with peroxosulfate catalyzed by iron(III) tetrakis(p-sulfonatophenyl)porphyrin

The oxidation of naphthalene was investigated in a biomimetic catalytic system using KHSO 5 and iron(III) tetrakis(p-sulfonatophenyl)porphyrin ( FeTPPS ) in order to elucidate the influence of solution forms, such as buffer pH and co-solvent types, on the reaction. 1,4-naphthoquinone was the main byproduct and the efficiency of oxidation, particularly 1,4-naphthoquinone formation, was influenced by pH and the type of co-solvent used. The most efficient conversion of 1,4-naphthoquinone was observed at an acidic pH (= 3). At higher pH , the formation of μ-oxo species ( OFe 2( TPPS )2) leads to a decrease of the percentage of naphthalene oxidized. In addition, the organic co-solvents (methanol, ethanol, 2-propanol and acetonitrile) influenced the amounts of naphthalene conversion. The amounts of 1,4-naphthoquinone formed were related to the electron donating character of the organic co-solvents.

Biodegradation of pyrene in sand, silt and clay fractions of sediment

Microbial degradation is the dominant pathway for natural attenuation of PAHs in environmental compartments such as sediments, which in turn depends on the bioavailability of PAHs. The bioavailability of PAHs has seldom been studied at the sediment particle size scale. We evaluated biodegradation of pyrene by Mycobacterium vanbaalenii PYR-1 as a function of sediment particle sizes, and investigated the relationship between the rate of degradation on sand, silt and clay particles with their individual desorption kinetics measured with the Tenax extraction method. Regression analysis showed that the total organic carbon (TOC), black carbon (BC), and specific surface area (SSA) of the specific particle size fractions, instead of the particle size scale itself, were closely related (P < 0.01) with the mineralization rate. While the fraction in the rapid desorption pool (F_rapid) ranged from 0.11 to 0.38 for the whole sediments and different size groups, the fractions mineralized after 336-h incubation (0.52 to 0.72) greatly surpassed the F_rapid values, suggesting utilization of pyrene in the slow desorption pool (F_slow). A biodegradation model was modified by imbedding a two-phase desorption relationship describing sequential Tenax extractions. Model analysis showed that pyrene sorbed on silt and clay aggregates was directly utilized by the degrading bacteria. The enhanced bioavailability may be attributed to the higher chemical concentration, higher TOC or larger SSA in the silt and clay fractions, which appeared to overcome the reduced bioavailability of pyrene due to sorption, making pyrene on the silt and clay particles readily available to degrading microbes. This conjecture merits further investigation.

Desorption kinetics of benzene in a sandy soil in the presence of powdered activated carbon

Desorption kinetics of benzene was investigated with a modified biphasic desorption model in a sandy soil with five different powdered activated carbon (PAC) contents (0, 1, 2, 5, 10% w/w) as sorbents. Sorption experiments followed by series dilution desorption were conducted for each sorbent. Desorption of benzene was successively performed at two stages using deionized water and hexane. Modeling was performed on both desorption isotherm and desorption rate for water-induced desorption to elucidate the presence of sorption-desorption hysteresis and biphasic desorption and if present to quantify the desorption-resistant fraction (q (irr)) and labile fraction (F) of desorption site responsible for rapid process. Desorption isotherms revealed that sorption-desorption exhibited a severe hysteresis with a significant fraction of benzene being irreversibly adsorbed onto both pure sand and PAC, and that desorption-resistant fraction (q (irr)) increased with PAC content. Desorption kinetic modeling showed that desorption of benzene was biphasic with much higher (4-40 times) rate constant for rapid process (k (1)) than that for slow process (k (2)), and that the difference in the rate constant increased with PAC content. The labile fraction (F) of desorption site showed a decreasing tendency with PAC. The experimental results would provide valuable information on remediation methods for soils and groundwater contaminated with BTEX.

An integrated numerical and physical modeling system for an enhanced in situ bioremediation process

Groundwater contamination due to releases of petroleum products is a major environmental concern in many urban districts and industrial zones. Over the past years, a few studies were undertaken to address in situ bioremediation processes coupled with contaminant transport in two- or three-dimensional domains. However, they were concentrated on natural attenuation processes for petroleum contaminants or enhanced in situ bioremediation processes in laboratory columns. In this study, an integrated numerical and physical modeling system is developed for simulating an enhanced in situ biodegradation (EISB) process coupled with three-dimensional multiphase multicomponent flow and transport simulation in a multi-dimensional pilot-scale physical model. The designed pilot-scale physical model is effective in tackling natural attenuation and EISB processes for site remediation. The simulation results demonstrate that the developed system is effective in modeling the EISB process, and can thus be used for investigating the effects of various uncertainties.

Predominance of char sorption over substrate concentration and soil pH in influencing biodegradation of benzonitrile

Incomplete combustion of field crop residues results in the production of char, a material rich in charcoal-type substances. Consequently, char is an effective adsorbent of organic compounds and when incorporated into soil may adsorb soil-applied pesticides, thereby altering their susceptibility to biodegradation. We investigated the relative importance of char, soil pH and initial substrate concentration in biodegradation of pesticides in soils by measuring the biodegradation of benzonitrile in soil as a function of soil char content (0% and 1% by weight), initial benzonitrile concentration (0.1, 1.06, and 10.2 mg l(-1)) and soil pH (5.2, 6.9 and 8.5). Preliminary experiments revealed that wheat straw char had a much greater benzonitrile sorption capacity than did soil to which the char was added. The extent of benzonitrile degradation decreased as initial benzonitrile concentration increased in both buffer solution and soil slurry. In contrast, the degradation increased as initial benzonitrile concentration increased in char-amended slurry. In un-amended soil slurry, the benzonitrile degradation was lower at pH 5.2 than at pH 6.9 or 8.5, but in char-amended soil slurry the degradation was not affected by pH, again presumably due to adsorption of benzonitrile by the char. Adsorption by soil char appears to be more important than either initial substrate concentration or soil pH in controlling benzonitrile degradation in char-amended soil slurry. The presence of crop residue-derived chars may alter pesticide degradation patterns normally observed in soils and thus significantly affect their environmental fate.

Sustainable oil and grease removal from synthetic stormwater runoff using bench-scale bioretention studies

One of the principal components of the contaminant load in urban stormwater runoff is oil and grease (O&G) pollution, resulting from vehicle emissions. A mulch layer was used as a contaminant trap to remove O&G (dissolved and particulate-associated naphthalene, dissolved toluene, and dissolved motor oil hydrocarbons) from a synthetic runoff during a bench-scale infiltration study. Approximately 80 to 95% removal of all contaminants from synthetic runoff was found via sorption and filtration. Subsequently, approximately 90% of the sorbed naphthalene, toluene, oil, and particulate-associated naphthalene was biodegraded within approximately 3, 4, 8, and 2 days after the event, respectively, based on decreases in contaminant concentrations coupled with increases of microbial populations. These results indicate the effectiveness and sustainability of placing a thin layer of mulch on the surface of a bioretention facility for reducing O&G pollution from urban stormwater runoff.

Bioavailability of diuron in soil containing wheat-straw-derived char

This study evaluated the bioavailability of diuron in soil as influenced by char arising from the burning of wheat straw. The wheat char was a highly effective sorbent for diuron. The presence of 1% wheat char in soil resulted in a 7-80 times higher diuron sorption. A 10-week incubation resulted in <40% of 0.5 mg/kg diuron in 0.5% char-amended soil microbially degraded, as compared to 50% in char-free soil under the same conditions. Over the experimental range of diuron application rates from 0 to 12 mg/kg and of char contents from 0% to 1.0%, a 4-week bioassay indicated that both the barnyardgrass survival rating and the fresh weight of aboveground biomass decreased with increasing diuron application at given char contents but increased with increasing char content at potentially damaging diuron application rates. Residual analyses of bioassayed soils showed that the soils with char contents of 0.5% and higher and diuron application rates of 3.0 mg/kg and higher, as compared to those with no or low (0.05%) char and a diuron application rate of 1.5 mg/kg, had higher residual diuron levels but higher barnyardgrass survival ratings and fresh weights. These results suggest that enhanced sorption of diuron in soil in the presence of wheat char reduced the bioavailability of diuron, as manifested by reduced microbial degradation of diuron and its herbicidal efficacy to barnyardgrass. This study may have greater implication than for burning of wheat straw that field burning of vegetations may reduce bioavailability of pesticides.

Isolation of soil bacteria adapted to degrade humic acid-sorbed phenanthrene

The goal of these studies was to determine how sorption by humic acids affected the bioavailability of polynuclear aromatic hydrocarbons (PAHs) to PAH-degrading microbes. Micellar solutions of humic acid were used as sorbents, and phenanthrene was used as a model PAH. Enrichments from PAH-contaminated soils established with nonsorbed phenanthrene yielded a total of 25 different isolates representing a diversity of bacterial phylotypes. In contrast, only three strains of Burkholderia spp. and one strain each of Delftia sp. and Sphingomonas sp. were isolated from enrichments with humic acid-sorbed phenanthrene (HASP). Using [14C]phenanthrene as a radiotracer, we verified that only HASP isolates were capable of mineralizing HASP, a phenotype hence termed "competence." Competence was an all-or-nothing phenotype: noncompetent strains showed no detectable phenanthrene mineralization in HASP cultures, but levels of phenanthrene mineralization effected by competent strains in HASP and NSP cultures were not significantly different. Levels and rates of phenanthrene mineralization exceeded those predicted to be supported solely by the metabolism of phenanthrene in the aqueous phase of HASP cultures. Thus, competent strains were able to directly access phenanthrene sorbed by the humic acids and did not rely on desorption for substrate uptake. To the best of our knowledge, this is the first report of (i) a selective interaction between aerobic bacteria and humic acid molecules and (ii) differential bioavailability to bacteria of PAHs sorbed to a natural biogeopolymer.

Assessment of contaminant lability during phytoremediation of polycyclic aromatic hydrocarbon impacted soil

Polycyclic aromatic hydrocarbons (PAHs) are recalcitrant compounds, some of which are known carcinogens, often found in high residual soil concentrations at industrial sites. Recent research has confirmed that phytoremediation holds promise as a low-cost treatment method for PAH contaminated soil. In this study, the lability of soil bound PAHs in the rhizosphere was estimated using solid phase extraction resin. An extraction time of 14 days was determined to be appropriate for this study. Resin-extractable PAHs, which are assumed to be more bioavailable, decreased during plant treatments. Significant reductions in the labile concentrations of several PAH compounds occurred over 12 months of plant growth. The differences in concentration between the unplanted and the planted soil indicate that the presence of plant roots, in addition to the passage of time, contributes to reduction in the bioavailability of target PAHs.

Naphthalene biodegradation kinetics in an aerobic slurry-phase bioreactor

The research was focused on the slurry-phase biodegradation of naphthalene in soil. Among ex situ techniques, the slurry phase offers the advantage of increased availability of contaminants to bacteria. From naphthalene contaminated soil, a Pseudomonas putida M8 strain capable to degrade naphthalene was selected. Experiments were performed in a stirred and oxygenated reactor. In this study, the influence of air flow rate and agitation rate on volatilisation and biodegradation of naphthalene was investigated. The hydrocarbon disappearance, the carbon dioxide production, and the ratio of total heterotrophic and naphthalene-degrading bacteria was monitored. The results obtained confirm that the selected bioremediation technology is successful in the treatment of contaminated soils.

Combustive approach for measuring total volatile phosphorus content in landfill gas

A technique was developed to measure the total gaseous phosphorus content in biogas. The amount of air needed for a neutral to oxidising flame was mixed with the biogas. The gas mixture was burnt in a closed quartz burner and the combustion gasses were bubbled through a nitric acid solution. The phosphate content in the bubbling liquid was determined with sector field ICP-MS. The technique was validated in the lab with phosphine. Afterwards the set-up was installed on a landfill. The total gaseous phosphorus content in the landfill gas, measured with the combustive technique, ranged from 1.65 to 4.44 microg P/m3. At the same time the phosphine concentration in the landfill gas was determined gas chromatographically (GC). The phosphine (PH3) content measured with GC ranged from 7.6 to 16.7 microg PH3-P/m3. Since the phosphine-P content (GC) was consistently higher than the total gaseous phosphorus content (burner/ICP-MS), the hypothesised presence of highly toxic gaseous phosphorus compounds other than phosphine could not be demonstrated.

Sorbed atrazine shifts into non-desorbable sites of soil organic matter during aging

Soil-chemical contact time (aging) is an important determinant of the sorption and desorption characteristics of the organic contaminants and pesticides in the environment. The effects of aging on mechanism-specific sorption and desorption of atrazine were studied in soil and clay slurries. Sorption isotherm and desorption kinetic experiments were performed, and soil-water distribution coefficients and desorption rate parameters were evaluated using linear and non-linear sorption equations and a three-site desorption model, respectively. Aging time for sorption of atrazine in sterilized soil and clay slurries ranged from 2 days to 8 months. Atrazine sorption isotherms were nearly linear (r(2)>0.97) and sorption coefficients were strongly correlated to soil organic carbon content. Sorption distribution coefficients (K(d)) increased with increase in age in all five soils studied, but not for K-montmorillonite. Sorption non-linearity did not increase with increase in age except for the Houghton muck soil. Desorption profiles were well described by the three-site desorption model. The equilibrium site fraction (f(eq)) decreased and the non-desorbable site fraction (f(nd)) increased as a function of aging time in all soils. For K-montmorillonite, f(nd) approximately 0 regardless of aging, showing that aging phenomena are sorbent/mechanism specific. In all soils, it was found that when normalized to soil organic matter content, the concentration of atrazine in desorbable sites was relatively constant, whereas that in non-desorbable site increased. This, and the lack of aging effects on desorption from montmorillonite, suggests that sorption into non-desorbable sites of soil organic matter is primary source of increased atrazine sorption in soils during aging.

Interactions of organic contaminants with mineral-adsorbed surfactants

Sorption of organic contaminants (phenol, p-nitrophenol, and naphthalene) to natural solids (soils and bentonite) with and without myristylpyridinium bromide (MPB) cationic surfactant was studied to provide novel insightto interactions of contaminants with the mineral-adsorbed surfactant. Contaminant sorption coefficients with mineral-adsorbed surfactants, Kss, show a strong dependence on surfactant loading in the solid. At low surfactant levels, the Kss values increased with increasing sorbed surfactant mass, reached a maximum, and then decreased with increasing surfactant loading. The Kss values for contaminants were always higher than respective partition coefficients with surfactant micelles (Kmc) and natural organic matter (Koc). At examined MPB concentrations in water the three organic contaminants showed little solubility enhancement by MPB. At low sorbed-surfactant levels, the resulting mineral-adsorbed surfactant via the cation-exchange process appears to form a thin organic film, which effectively "adsorbs" the contaminants, resulting in very high Kss values. At high surfactant levels, the sorbed surfactant on minerals appears to form a bulklike medium that behaves essentially as a partition phase (rather than an adsorptive surface), with the resulting Kss being significantly decreased and less dependent on the MPB loading. The results provide a reference to the use of surfactants for remediation of contaminated soils/sediments or groundwater in engineered surfactant-enhanced washing.

Assessment of bioavailability of soil-sorbed atrazine

Bioavailability of pesticides sorbed to soils is an important determinant of their environmental fate and impact. Mineralization of sorbed atrazine was studied in soil and clay slurries, and a desorption-biodegradation-mineralization (DBM) model was developed to quantitatively evaluate the bioavailability of sorbed atrazine. Three atrazine-degrading bacteria that utilized atrazine as a sole N source (Pseudomonas sp. strain ADP, Agrobacterium radiobacter strain J14a, and Ralstonia sp. strain M91-3) were used in the bioavailability assays. Assays involved establishing sorption equilibrium in sterile soil slurries, inoculating the system with organisms, and measuring the CO(2) production over time. Sorption and desorption isotherm analyses were performed to evaluate distribution coefficients and desorption parameters, which consisted of three desorption site fractions and desorption rate coefficients. Atrazine sorption isotherms were linear for mineral and organic soils but displayed some nonlinearity for K-saturated montmorillonite. The desorption profiles were well described by the three-site desorption model. In many instances, the mineralization of atrazine was accurately predicted by the DBM model, which accounts for the extents and rates of sorption/desorption processes and assumes biodegradation of liquid-phase, but not sorbed, atrazine. However, for the Houghton muck soil, which manifested the highest sorbed atrazine concentrations, enhanced mineralization rates, i.e., greater than those expected on the basis of aqueous-phase atrazine concentration, were observed. Even the assumption of instantaneous desorption could not account for the elevated rates. A plausible explanation for enhanced bioavailability is that bacteria access the localized regions where atrazine is sorbed and that the concentrations found support higher mineralization rates than predicted on the basis of aqueous-phase concentrations. Characteristics of high sorbed-phase concentration, chemotaxis, and attachment of cells to soil particles seem to contribute to the bioavailability of soil-sorbed atrazine.

Development of a kinetic basis for bioavailability of sorbed naphthalene in soil slurries

The degradation of naphthalene in soil-slurry systems was studied using four different organisms and two soils. Organisms with zero-order, first-order, and Michaelis-Menten rates were selected. The soils had substantially different sorption distribution coefficients. Sorption and desorption was evaluated in abiotic soil-slurry systems. The desorption process was described by a model that accounts for equilibrium, rate-limited and non-desorbing sites. Biodegradation parameters were measured in soil-extract solutions. Bioavailability assays, inoculated soil slurries, were conducted and both liquid- and sorbed-phase naphthalene concentrations were measured over time. For the less sorptive soil, the results could be explained by sequential desorption and degradation processes. For the other soil, enhanced degradation was clearly observed for the organisms with first-order and Michaelis-Menten rates. Several explanations are explored for these observations including direct sorbed-phase degradation and the development of elevated substrate concentrations at the organism/sorbent interface. No enhancement was found for the organism with zero-order kinetics.