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

Hydrocarbon markers for assessing the influence of human activities in the tropical semi-arid region (Acaraú River, state of Ceará, Brazil)

Coastal ecosystems are facing increasing anthropogenic stressors, including rapid urbanization rates and extensive fossil fuel usage. Nevertheless, the distribution of hydrocarbons in the Brazilian semi-arid region remains relatively uncharacterized. In this study, we analyzed ten surface sediment samples (0-2 cm) along the banks of the Acaraú River to assess the chronic contributions of aliphatic and aromatic hydrocarbons. The Acaraú River is a crucial riverine-estuarine area in the semi-arid region of Northeast Brazil. Ultrasound-assisted extraction and gas chromatograph coupled to a mass spectrometer were used to identify target compounds: 45 PAHs, 27 n-alkanes (C10-C38), and two isoprenoids. At most stations, the predominant grain size was sand, and the organic carbon content was less than 1%. The total n-alkanes concentration ranged from 14.1 to 170.0 μg g-1, while individual pristane and phytane concentrations ranged from not detected (nd) to 0.4 μg g-1 and nd to 0.7 μg g-1, respectively. These concentrations resemble those found in unpolluted sediments and are lower compared to samples from urbanized coastal areas. The total USEPA PAHs concentration varied from 157.8 to 1364 ng g-1, leading to the characterization of sediment samples as moderately polluted. Based on diagnostic ratios calculated from both alkane and PAH concentrations, the sediment samples were predominantly deriving from pyrolytic sources, with some contribution from petrogenic sources. The most abundant group was 5-ring PAHs (mean: 47.3 ± 36.7%), followed by 3-ring PAHs (mean: 17.9 ± 13.7%). This predominance indicates a pyrolytic origin of hydrocarbons in the Acaraú River. The concentrations reported here were representative of the level of background hydrocarbons in the region. Regarding the sediment quality assessment, BaP TPE calculated for the Acaraú River ranged from 13.2 to 1258.4 ng g-1 (mean: 409.3 ± 409.4 ng g-1). When considering site-specific sediment quality values for the coast of the state of Ceará, half of the stations are classified as strongly contaminated, and toxic effects are expected to occur (SQGq >0.25) for the ∑16 PAHs measured in the samples, especially due to dibenz [a,h]anthracene concentrations.

Complex behavior of petroleum hydrocarbons in vadose zone: A holistic analysis using unsaturated soil columns

The migration of petroleum hydrocarbons in vadose zone involves complex coupled processes such as downward displacement and natural attenuation. Despite its significance in determining groundwater vulnerability to petroleum contamination and optimizing the remedial strategy, it has not been comprehensively studied in terms of overall processes under field-relevant conditions. In this study, a series of unsaturated soil column experiments were conducted by simulating subsurface diesel contamination within a vadose zone using different soil textures at different soil bulk densities and initial diesel concentrations, while partly exposing them to simulated precipitation. The results showed that the soil column with less fine fraction was favorable for the downward migration of diesel but unfavorable for its natural degradation. However, precipitation complicated the relative conductivities of multiple fluids (water, air, and diesel) through the pore network, therby decreasing diesel migration and degradation. For example, the downward migration of diesel in the SL column decreased by 8.4% under precipitation, while the overall attenuation rate dropped to almost 0.24% of its original state. Lowering bulk density (from 1.5 to 1.23 g/cm3), however, could enhance the attenuation rate presumably due to the secured void space for the incoming fluids. A high initial concentration of diesel (2%; w/w) inhibited its natural attenuation, while its influence on its vertical propagation after the precipitation was not significant. The present findings provide a mechanistic basis for approximating the behavior of petroleum hydrocarbons in a random vadose zone.

Coupled effect of porous network and water content on the natural attenuation of diesel in unsaturated soils

The natural attenuation potential of a vadose zone against diesel is critical for optimizing remedial actions and determining groundwater vulnerability to contamination. Here, diesel attenuation in unsaturated soils was systematically examined to develop a qualitative relationship between physical soil properties and the natural attenuation capacity of a vadose zone against diesel. The uniformity coefficient (C_u) and water saturation (S_w, %) were considered as the proxies reflecting the degree of effects by porous network and water content in different soils, respectively. These, in turn, are related to the primary diesel attenuation mechanisms of volatilization and biodegradation. The volatilization of diesel was inversely proportional to C_u and S_w, which could be attributed to effective pore channels facilitating gas transport. Conversely, biodegradation was highly proportional to C_u under unsaturated conditions (S_w = 35-71%), owing to nutrients typically associated with fine soil particles. The microbial community in unsaturated soils was affected by S_w rather than C_u. The overall diesel attenuation including volatilization and biodegradation was optimized at S_w = 35% for all tested soils.

Mineralization of pyrene (polycyclic aromatic hydrocarbon) in clay soil supplemented with animal organic carbon source

Biostimulation of polycyclic aromatic hydrocarbons (PAHs) is a major concern in the environment due to their toxic nature and ubiquitous occurrence. The study aimed to determine the best animal supplement of organic carbon source (cow dung, goat dung, pig dung and poultry (fowl droppings) and indigenous microbes capable of mineralizing pyrene in clay soil. Clay soil sample was collected from the top surface soil (0 - 15 cm) in Ikpoba River and upper Lawani river bank, both in Benin City with the aid of stainless steel trowel and gloved hands and stored in a polyethylene bag. The isolation and identification process were evaluated by staining method, microscopic examination and biochemical tests. The degradation experiment was conducted in bioreactors and supplemented cow dung, pig dung, goat dung and fowl droppings were added in varying amounts of 25 g/l, 50 g/l, 75 g/l and 100 g/l. The potential effects of animal organic carbon as nutrient supplements to enhance mineralization of pyrene were investigated. The rates of mineralization of pyrene were studied for a remediation period of 35 days at room temperature of about 25 °C. Results obtained from microbial isolation showed that B. substilis and A. niger were the most populated in the soil and were therefore selected. The degradation experiment showed that mixed culture of B. substilis and A. niger using 50 g/l supplement with the dungs and droppings gave the highest percentage degradation with fowl droppings 98.3%, pig dung 98.1%, goat dung 97.8%, and cow dung 97.7%. The least degradation was observed in supplement with 100 g/l in a single culture of A. niger with fowl droppings yielded 64.5% degradation, pig dung 63.8%, goat dung 63.7% and cow dung 62.7%. The overall results of the study showed that 50 g/l of animal wastes supplement in the ratio of 1:4 with pyrene contaminated soils are most effective in degradation processes. Also the mixed culture of bacteria and fungi enhanced pyrene mineralization and degradation more than single culture.

Influence of rhamnolipid biosurfactant and Brij-35 synthetic surfactant on 14C-Pyrene mineralization in soil

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants in soil and are considered priority pollutants due to their carcinogenicity. Bioremediation of PAH-contaminated soils is often limited by the low solubility and strong sorption of PAHs in soil. Synthetic surfactants and biosurfactants have been used to enhance the bioavailability of PAHs and to accelerate microbial degradation. However, few studies have compared synthetic and biosurfactants in their efficiency in promoting PAH biodegradation in either native or bioaugmented soils. In this study, we evaluated mineralization of ¹⁴C-pyrene in soils with or without the augmentation of Mycobacterium vanbaalenii PYR-1, and characterized the effect of Brij-35 (synthetic) and rhamnolipid biosurfactant at different amendment rates. Treatment of rhamnolipid biosurfactant at 140 or 1400 μg surfactant g-dry soil^-1 rates resulted in a significantly longer lag period in ¹⁴C-pyrene mineralization in both native and bioaugmented soils. In contrast, amendment of Brij-35 generally increased ¹⁴C-pyrene degradation, and the greatest enhancement occurred at 21.6 or 216 μg surfactant g-dry soil^-1 rates, which may be attributed to increased bioavailability. Brij-35 and rhamnolipid biosurfactant were found to be non-toxic to M. vanbaalenii PYR-1 at 10X CMC, thus indicating rhamnolipid biosurfactant likely served as a preferential carbon source to the degrading bacteria in place of ¹⁴C-pyrene, leading to delayed and inhibited ¹⁴C-pyrene degradation. Mineralization of ¹⁴C-pyrene by M. vanbaalenii PYR-1 was rapid in the unamended soils, and up to 60% of pyrene was mineralized to ¹⁴CO₂ after 10 d in the unamended or Brij-35 surfactant-amended soils. Findings of this study suggest that application of surfactants may not always lead to enhanced PAH biodegradation or removal. If the surfactant is preferentially used as an easier carbon substrate than PAHs for soil microorganisms, it may actually inhibit PAH biodegradation. Selection of surfactant types is therefore crucial for the effectiveness of surfactant-aided bioremediation of PAH-contaminated soils.

Removal mechanisms in aerobic slurry bioreactors for remediation of soils and sediments polluted with hydrophobic organic compounds: An overview

Hydrophobic organic compound (HOC)-contaminated soils are a great environmental and public health concern nowadays. Further research is necessary to develop environmental friendly biotechnologies that allows public and private sectors to implement efficient and adaptable treatment approaches. Aerobic soil-slurry bioreactor technology has emerged as an effective and feasible technique with a high remediation potential, especially for silt and clay soil fractions, which often contain the highest pollutant concentration levels and are usually difficult to remove by implementing conventional methods. However, the mechanisms involved in the HOC removal in bioslurry reactor are still not completely understood. Gas-liquid and solid-liquid mass transfer, mass transport and biodegradation phenomena are the main known processes taking place in slurry bioreactors. This review compiles the most up-to-date information available about these phenomena and tries to link them, enlightening the possible interactions between parameters. It gathers the basic information needed to understand the complex bioremediation technology and raises awareness of some considerations that should be made.

Bioremediation of PAH-contamined soils: Consequences on formation and degradation of polar-polycyclic aromatic compounds and microbial community abundance

A bioslurry batch experiment was carried out over five months on three polycyclic aromatic compound (PAC) contaminated soils to study the PAC (PAH and polar-PAC) behavior during soil incubation and to evaluate the impact of PAC contamination on the abundance of microbial communities and functional PAH-degrading populations. Organic matter characteristics and reactivity, assessed through solvent extractable organic matter and PAC contents, and soil organic matter mineralization were monitored during 5 months. Total bacteria and fungi, and PAH-ring hydroxylating dioxygenase genes were quantified. Results showed that PAHs and polar-PACs were degraded with different degradation dynamics. Differences in degradation rates were observed among the three soils depending on PAH distribution and availability. Overall, low molecular weight compounds were preferentially degraded. Degradation selectivity between isomers and structurally similar compounds was observed which could be used to check the efficiency of bioremediation processes. Bacterial communities were dominant over fungi and were most likely responsible for PAC degradation. Abundance of PAH-degrading bacteria increased during incubations, but their proportion in the bacterial communities tended to decrease. The accumulation of some oxygenated-PACs during the bioslurry experiment underlines the necessity to monitor these compounds during application of remediation treatment on PAH contaminated soils.

Fate Characterization of Benzene Kresoxim-Methyl (a Strobilurin Fungicide) in Different Aerobic Soils

Benzene kresoxim-methyl (BKM) is a promising broad-spectrum strobilurin fungicide widely used to control fungal pathogens in crops. However, information on its environmental fate is limited. To broaden our understanding of this fungicide's kinetic fate in aerobic soils, we labeled BKM with C on its benzoate ring and used ultralow-level liquid scintillation counting coupled with high-performance liquid chromatography analysis. Results show that degradation, mineralization, and bound residue (BR) formation of BKM was controlled by soil type and microbial community composition. Degradation of BKM followed first-order dynamics, and the half-lives () were 51.7, 30.8, and 26.8 d for clay, loamy, and saline soils, respectively. After 100 d, about 0.13, 4.35, and 5.94% of the initial C-BKM was mineralized, and 14.43, 19.90, and 28.81% was formed as BRs in the clay, loamy, and saline soils, respectively. About 60 to 85% of the C-BKM residue in soil was extractable; of this fraction, 30 to 50% was composed of incomplete degradation intermediates. Up to 40% of extractable C-BKM in soil was readily available. Our results suggest that BKM and its incomplete intermediates had a relatively long persistence in soil, which may lead to exposure for nontarget organisms. Soil microbes may play a dominant role in controlling the fate of BKM in soil as sterilization sharply decreased its mineralization rate from 4.35 to 0.03%, increased from 30.8 to 85.6 d, and decreased the BR fraction from 19.90 to 3.25%.

Removal of a mixture of pesticides by a Streptomyces consortium: Influence of different soil systems

Although the use of organochlorine pesticides (OPs) is restricted or banned in most countries, they continue posing environmental and health concerns, so it is imperative to develop methods for removing them from the environment. This work is aimed to investigate the simultaneous removal of three OPs (lindane, chlordane and methoxychlor) from diverse types of systems by employing a native Streptomyces consortium. In liquid systems, a satisfactory microbial growth was observed accompanied by removal of lindane (40.4%), methoxychlor (99.5%) and chlordane (99.8%). In sterile soil microcosms, the consortium was able to grow without significant differences in the different textured soils (clay silty loam, sandy and loam), both contaminated or not contaminated with the OPs-mixture. The Streptomyces consortium was able to remove all the OPs in sterile soil microcosm (removal order: clay silty loam > loam > sandy). So, clay silty loam soil (CSLS) was selected for next assays. In non-sterile CSLS microcosms, chlordane removal was only about 5%, nonetheless, higher rates was observed for lindane (11%) and methoxychlor (20%). In CSLS slurries, the consortium exhibited similar growth levels, in the presence of or in the absence of the OPs-mixture. Not all pesticides were removed in the same way; the order of pesticide dissipation was: methoxychlor (26%)>lindane (12.5%)>chlordane (10%). The outlines of microbial growth and pesticides removal provide information about using actinobacteria consortium as strategies for bioremediation of OPs-mixture in diverse soil systems. Texture of soils and assay conditions (sterility, slurry formulation) were determining factors influencing the removal of each pesticide of the mixture.

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.

Polycyclic aromatic hydrocarbon-contaminated soils: bioaugmentation of autochthonous bacteria and toxicological assessment of the bioremediation process by means of Vicia faba L

Two bacterial strains, Achromobacter sp. (ACH01) and Sphingomonas sp. (SPH01), were isolated from a heavily polycyclic aromatic hydrocarbon (PAH)-contaminated soil (5431.3 ± 102.3 ppm) for their capacity to use a mixture of anthracene, pyrene, phenanthrene and fluorene as sole carbon sources for growth and for the capacity to produce biosurfactants. The two strains were exploited for bioaugmentation in a biopile pilot plant to increase the bioavailability and the degradation of the residual PAH contamination (99.5 ± 7.1 ppm) reached after 9 months of treatment. The denaturing gel gradient electrophoresis (DGGE) profile of the microbial ecology of the soil during the experimentation showed that the bioaugmentation approach was successful in terms of permanence of the two strains in the soil in treatment. The bioaugmentation of the two bacterial isolates positively correlated with the PAH depletion that reached 7.9 ± 2 ppm value in 2 months of treatment. The PAH depletion was assessed by the loss of the phyto-genotoxicity of soil elutriates on the model plant Vicia faba L., toxicological assessment adopted also to determine the minimum length of the decontamination process for obtaining both the depletion of the PAH contamination and the detoxification of the soil at the end of the process. The intermediate phases of the bioremediation process were the most significant in terms of toxicity, inducing genotoxic effects and selective DNA fragmentation in the stem cell niche of the root tip. The selective DNA fragmentation can be related to the selective induction of cell death of mutant stem cells that can compromise offsprings.

Probing insulin bioactivity in oral nanoparticles produced by ultrasonication-assisted emulsification/internal gelation

Alginate-dextran sulfate-based particles obtained by emulsification/internal gelation technology can be considered suitable carriers for oral insulin delivery. A rational study focused on the emulsification and particle recovery steps was developed in order to reduce particles to the nanosize range while keeping insulin bioactivity. There was a decrease in size when ultrasonication was used during emulsification, which was more pronounced when a cosurfactant was added. Ultrasonication add-on after particle recovery decreased aggregation and led to a narrower nanoscale particle-size distribution. Insulin encapsulation efficiency was 99.3%±0.5%, attributed to the strong pH-stabilizing electrostatic effect between insulin and nanoparticle matrix polymers. Interactions between these polymers and insulin were predicted using molecular modeling studies through quantum mechanics calculations that allowed for prediction of the interaction model. In vitro release studies indicated well-preserved integrity of nanoparticles in simulated gastric fluid. Circular dichroism spectroscopy proved conformational stability of insulin and Fourier transform infrared spectroscopy technique showed rearrangements of insulin structure during processing. Moreover, in vivo biological activity in diabetic rats revealed no statistical difference when compared to nonencapsulated insulin, demonstrating retention of insulin activity. Our results demonstrate that alginate-dextran sulfate-based nanoparticles efficiently stabilize the loaded protein structure, presenting good physical properties for oral delivery of insulin.

Impact of clay mineral, wood sawdust or root organic matter on the bacterial and fungal community structures in two aged PAH-contaminated soils

The high organic pollutant concentration of aged polycyclic aromatic hydrocarbon (PAH)-contaminated wasteland soils is highly recalcitrant to biodegradation due to its very low bioavailability. In such soils, the microbial community is well adapted to the pollution, but the microbial activity is limited by nutrient availability. Management strategies could be applied to modify the soil microbial functioning as well as the PAH contamination through various amendment types. The impact of amendment with clay minerals (montmorillonite), wood sawdust and organic matter plant roots on microbial community structure was investigated on two aged PAH-contaminated soils both in laboratory and 1-year on-site pot experiments. Total PAH content (sum of 16 PAHs of the US-EPA list) and polar polycyclic aromatic compounds (pPAC) were monitored as well as the available PAH fraction using the Tenax method. The bacterial and fungal community structures were monitored using fingerprinting thermal gradient gel electrophoresis (TTGE) method. The abundance of bacteria (16S rRNA genes), fungi (18S rRNA genes) and PAH degraders (PAH-ring hydroxylating dioxygenase and catechol dioxygenase genes) was followed through qPCR assays. Although the treatments did not modify the total and available PAH content, the microbial community density, structure and the PAH degradation potential changed when fresh organic matter was provided as sawdust and under rhizosphere influence, while the clay mineral only increased the percentage of catechol-1,2-dioxygenase genes. The abundance of bacteria and fungi and the percentage of fungi relative to bacteria were enhanced in soil samples supplemented with wood sawdust and in the plant rhizospheric soils. Two distinct fungal populations developed in the two soils supplemented with sawdust, i.e. fungi related to Chaetomium and Neurospora genera and Brachyconidiellopsis and Pseudallescheria genera, in H and NM soils respectively. Wood sawdust amendment favoured the development of PAH-degrading bacteria holding Gram-negative PAH-ring hydroxylating dioxygenase, catechol-1,2-dioxygenase and catechol-2,3-dioxygenase genes. Regarding the total community structure, bacteria closely related to Thiobacillus (β-Proteobacteria) and Steroidobacter (γ-Proteobacteria) genera were favoured by wood sawdust amendment. In both soils, plant rhizospheres induced the development of fungi belonging to Ascomycota and related to Alternaria and Fusarium genera. Bacteria closely related to Luteolibacter (Verrucomicrobia) and Microbacterium (Actinobacteria) were favoured in alfalfa and ryegrass rhizosphere.

Relationship of bifenthrin sediment concentrations to grain size and total organic carbon in California waterbodies: implications for ecological risk

A summary analysis of data sets from six California waterbodies was conducted to determine the relationship of bifenthrin sediment concentrations to: % sand/gravel; % silt; % clay; % silt/clay and % total organic carbon (TOC). The relationship of TOC to % sand/gravel, % silt, % clay, and % silt/clay was also analyzed. Statistically significant and meaningful direct relationships were reported between bifenthrin and % TOC, % silt, % clay and % silt/clay while a significant and meaningful inverse relationship was reported between bifenthrin and % sand/gravel. A significant and meaningful inverse relationship was reported between % TOC and % sand/gravel, while a significant and meaningful direct relationship was reported between % TOC and % silt, % clay and % silt/clay. Significant bifenthrin sediment concentrations would not be expected in non-depositional (sand/gravel) areas which have been reported to be dominant in various streams in California's Central Valley and are also the preferred habitat for many benthic macroinvertebrate taxa.

PAHs biodegradation in intertidal surface sediment by indigenous microorganisms

In this study, the 30-day aerobic microorganism-mediated biodegradation of polycyclic aromatic hydrocarbons (PAHs) was investigated in four size fractions (i.e., <0.002, 0.002-0.031, 0.031-0.063 and >0.063 mm) of sand-dominated sediment S1 and mud-dominated S2 collected from intertidal zones in Bohai Bay (China). Prior to biodegradation, the total quantity of phenanthrene, fluoranthene and pyrene comprised more than 80% of the total quantity of 16 EPA-priority PAHs in each size fraction, with the exception of 70.33% found in the >0.063 mm fraction of sediment S1. Among the three dominant compounds, the intermediate size fraction (0.031-0.063 mm) showed higher levels of biodegradation than other size fractions in sediment S1 and S2. After pooling data from sediment S1 and S2 for joint analysis, it was observed that the biodegraded portion of the three dominant compounds showed negative correlations with both total organic carbon (TOC) and humic coverage index (HCI) in the size fractions. The observed negative correlation with TOC was in agreement with findings in many other studies, but the negative correlation with HCI had not been observed in early studies, which only investigated aged sediment/soil samples. The findings in this study indicated that the greatest bioavailability of PAHs in intertidal surface sediment may be present in sediment particles of intermediate size and mobility, and that intertidal sediment particles are less likely to experience sufficient ageing given periodical tidal disturbance. These findings have important implications for the assessment of the environmental fate of PAHs in intertidal regions.

Correlations between PAH bioavailability, degrading bacteria, and soil characteristics during PAH biodegradation in five diffusely contaminated dissimilar soils

The natural biodegradation of seven polycyclic aromatic hydrocarbons (PAHs) by native microorganisms was studied in five soils from Normandy (France) from diffusely polluted areas, which can also pose a problem in terms of surfaces and amounts of contaminated soils. Bioavailability tests using cyclodextrin-based extractions were performed. The natural degradation of low molecular weight (LMW) PAHs was not strongly correlated to their bioavailability due to their sorption to geosorbents. Conversely, the very low degradation of high molecular weight (HMW) PAHs was partly correlated to their poor availability, due to their sorption on complexes of organic matter and kaolinites or smectites. A principal component analysis allowed us to distinguish between the respective degradation behaviors of LMW and HMW PAHs. LMW PAHs were degraded in less than 2-3 months and were strongly influenced by the relative percentage of phenanthrene-degrading bacteria over total bacteria in soils. HMW PAHs were not significantly degraded, not only because they were less bioavailable but also because of a lack of degrading microorganisms. Benzo[a]pyrene stood apart since it was partly degraded in acidic soils, probably because of a catabolic cooperation between bacteria and fungi.

Concentrations and sources of polycyclic aromatic hydrocarbons in surface coastal sediments of the northern Gulf of Mexico

BACKGROUND

Coastal sediments in the northern Gulf of Mexico have a high potential of being contaminated by petroleum hydrocarbons, such as polycyclic aromatic hydrocarbons (PAHs), due to extensive petroleum exploration and transportation activities. In this study we evaluated the spatial distribution and contamination sources of PAHs, as well as the bioavailable fraction in the bulk PAH pool, in surface marsh and shelf sediments (top 5 cm) of the northern Gulf of Mexico.

RESULTS

PAH concentrations in this region ranged from 100 to 856 ng g-1, with the highest concentrations in Mississippi River mouth sediments followed by marsh sediments and then the lowest concentrations in shelf sediments. The PAH concentrations correlated positively with atomic C/N ratios of sedimentary organic matter (OM), suggesting that terrestrial OM preferentially sorbs PAHs relative to marine OM. PAHs with 2 rings were more abundant than those with 5-6 rings in continental shelf sediments, while the opposite was found in marsh sediments. This distribution pattern suggests different contamination sources between shelf and marsh sediments. Based on diagnostic ratios of PAH isomers and principal component analysis, shelf sediment PAHs were petrogenic and those from marsh sediments were pyrogenic. The proportions of bioavailable PAHs in total PAHs were low, ranging from 0.02% to 0.06%, with higher fractions found in marsh than shelf sediments.

CONCLUSION

PAH distribution and composition differences between marsh and shelf sediments were influenced by grain size, contamination sources, and the types of organic matter associated with PAHs. Concentrations of PAHs in the study area were below effects low-range, suggesting a low risk to organisms and limited transfer of PAHs into food web. From the source analysis, PAHs in shelf sediments mainly originated from direct petroleum contamination, while those in marsh sediments were from combustion of fossil fuels.

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.

BDE-47 sorption and desorption to soil matrix in single- and binary-solute systems

Three loamy-clay soil samples (LC1-3) with different properties were collected as the geosorbents to preliminarily investigate the sorption and desorption of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) in single system and binary system with the presence of decabromodiphenyl ether (BDE-209), which can provide information in order to further understand the sorption mechanisms and evaluate the adsorption sites. A concentration of 10 μg L(-1) BDE-209 suppressed the sorption of BDE-47, and the trend became more and more significant with the increase of BDE-47 equilibrium concentration, however, BDE-47 caused no competitive effect on BDE-209 sorption, which was related with the better accessibility of more hydrophobic molecules to adsorption sites. In the binary system, nonlinearity of the BDE-47 sorption isotherms for the three samples changed in different ways, which originated from the varied soil properties. Desorption hysteresis was observed in all cases, which was estimated due to irreversible surface adsorption between sorbent and sorbate. BDE-209 made desorption of BDE-47 more hysteretic from soil samples, which was estimated to be ascribed to the accelerated sorbent state transition and new sites creation caused by BDE-209 sorption.

Influence of single-walled carbon nanotubes on microbial availability of phenanthrene in sediment

Increasing production and use of single-walled carbon nanotubes (SWCNT) will inevitably lead to release of these nanoparticles to aquatic ecosystems. Similar to black carbon (BC) particles, SWCNT have a high affinity for hydrophobic organic contaminants (HOCs) and therefore the presence of SWCNT in sediment may lead to altered bioavailability of HOCs. We compared SWCNT with biochar and charcoal on their effect on the microbial degradability of 0.05 mg kg(-1) (14)C-phenanthrene (PHE) by Mycobacterium vanbaalenii PYR-1 in two sediments with different organic carbon (OC) contents. When the amendment rate of SWCNT or BC was 1 mg g(-1), PHE mineralization was inhibited much more significantly by SWCNT than by either biochar or charcoal. After 360 h of incubation, the mineralized fraction of PHE in the presence of SWCNT was 59.5% of the non-amended control in the sediment with low OC content, and only 42.4% in the other sediment with a higher OC content. Analysis of the freely dissolved concentration (C (free)) using disposable polydimethylsiloxane (PDMS) fibers showed that SWCNT decreased C (free) by 85-95%, apparently due to preferential sorption of PHE to SWCNT particles that had a much larger specific surface area and pore volume than biochar or charcoal. However, pre-interaction of SWCNT with dissolved organic matter (peptone, tannic acid, and humic acid) led to attachment of polar functional groups and reduced surface area on SWCNT, resulting in decreased PHE sorption and an alleviated effect on PHE biodegradation in the order of peptone > tannic acid > humic acid.