Recalcitrance of polycyclic aromatic hydrocarbons in soil contributes to background pollution

Microbial degradation of pyrene in holm oak (Quercus ilex) phyllosphere: Role of particulate matter in regulating bioaccessibility

In this study we first measured the mineralization of pyrene on leaves of urban holm oak (Quercus ilex) by autochthonous microorganisms and an inoculated PAH degrading bacterium (i.e., Mycobacterium gilvum), selected as a model phyllosphere species, as well as the leaf-water (K_LW) and leaf-air (K_LA) partition coefficients for this chemical. Mineralization was investigated in two different experimental systems in terms of leaf and microorganism environment. Additionally, the influence on pyrene partitioning and mineralization when particulate matter (PM) was present on the leaf surface or removed was studied. Mineralization of ¹⁴C-labeled pyrene by autochthonous microorganisms was lower than 1% after approximately two weeks, while M. gilvum mineralized 5% to 17% of pyrene. These extents corresponded to mineralization half-lives that ranged between ~30 to ~200 days. We proposed that PM present at the leaf surface reduced the accumulation of pyrene by inner compartments (cuticle) distantly located from microbial cells and enhanced the bioaccessibility of pyrene, speeding up microbial activity and therefore mineralization. These results highlight that plant-phyllosphere microorganism interaction is more complex than currently established and deserves additional studies to further comprehend the air purification ecosystem service of phyllosphere microorganisms.

Microplastics in terrestrial ecosystems: Moving beyond the state of the art to minimize the risk of ecological surprise

Microplastic (plastic particles measuring <5mm) pollution is ubiquitous. Unlike in other well-studied ecosystems, for example, marine and freshwater environments, microplastics in terrestrial systems are relatively understudied. Their potential impacts on terrestrial environments, in particular the risk of causing ecological surprise, must be better understood and quantified. Ecological surprise occurs when ecosystem behavior deviates radically from expectations and generally has negative consequences for ecosystem services. The properties and behavior of microplastics within terrestrial environments may increase their likelihood of causing ecological surprises as they (a) are highly persistent global pollutants that will last for centuries, (b) can interact with the abiotic environment in a complex manner, (c) can impact terrestrial organisms directly or indirectly and (d) interact with other contaminants and can facilitate their transport. Here, we compiled findings of previous research on microplastics in terrestrial environments. We systematically focused on studies addressing different facets of microplastics related to their distribution, dispersion, impact on soil characteristics and functions, levels of biological organization of tested terrestrial biota (single species vs. assemblages), scale of experimental study and corresponding ecotoxicological effects. Our systematic assessment of previous microplastic research revealed that most studies have been conducted on single species under laboratory conditions with short-term exposures; few studies were conducted under more realistic long-term field conditions and/or with multi-species assemblages. Studies targeting multi-species assemblages primarily considered soil bacterial communities and showed that microplastics can alter essential nutrient cycling functions. More ecologically meaningful studies of terrestrial microplastics encompassing multi-species assemblages, critical ecological processes (e.g., biogeochemical cycles and pollination) and interactions with other anthropogenic stressors must be conducted. Addressing these knowledge gaps will provide a better understanding of microplastics as emerging global stressors and should lower the risk of ecological surprise in terrestrial ecosystems.

Root-mediated bacterial accessibility and cometabolism of pyrene in soil

Partial transformation of pollutants and mobilization of the produced metabolites may contribute significantly to the risks resulting from biological treatment of soils polluted by hydrophobic chemicals such as polycyclic aromatic hydrocarbons (PAHs). Pyrene, a four-ringed PAH, was selected here as a model pollutant to study the effects of sunflower plants on the bacterial accessibility and cometabolism of this pollutant when located at a spatially distant source within soil. We compared the transformation of passively dosed ¹⁴C-labeled pyrene in soil slurries and planted pots that were inoculated with the bacterium Pseudomonas putida G7. This bacterium combines flagellar cell motility with the ability to cometabolically transform pyrene. Cometabolism of this PAH occurred immediately in the inoculated and shaken soil slurries, where the bacteria had full access to the passive dosing devices (silicone O-rings). Root exudates did not enhance the survival of P. putida G7 cells in soil slurries, but doubled their transport in column tests. In greenhouse-incubated soil pots with the same pyrene sources instead located centimeters from the soil surface, the inoculated bacteria transformed ¹⁴C-labeled pyrene only when the pots were planted with sunflowers. Bacterial inoculation caused mobilization of ¹⁴C-labeled pyrene metabolites into the leachates of the planted pots at concentrations of approximately 1 mg L^-1, ten times greater than the water solubility of the parent compound. This mobilization resulted in a doubled specific root uptake rate of ¹⁴C-labeled pyrene equivalents and a significantly decreased root-to-fruit transfer rate. Our results show that the plants facilitated bacterial access to the distant pollutant source, possibly by increasing bacterial dispersal in the soil; this increased bacterial access was associated with cometabolism, which contributed to the risks of biodegradation.

Rhizosphere-enhanced biosurfactant action on slowly desorbing PAHs in contaminated soil

We studied how sunflower plants affect rhamnolipid biosurfactant mobilization of slowly desorbing fractions of polycyclic aromatic hydrocarbons (PAHs) in soil from a creosote-contaminated site. Desorption kinetics of 13 individual PAHs revealed that the soil contained initially up to 50% slowly desorbing fractions. A rhamnolipid biosurfactant was applied to the soil at the completion of the sunflower cycle (75 days in greenhouse conditions). After this period, the PAHs that remained in the soil were mainly present in a slowly desorbing form as a result of the efficient biodegradation of fast-desorbing PAHs by native microbial populations. The rhamnolipid enhanced the bioavailable fraction of the remaining PAHs by up to 30%, as evidenced by a standardized desorption extraction with Tenax, but the enhancement occurred with only planted soils. The enhanced bioavailability did not decrease residual PAH concentrations under greenhouse conditions, possibly due to ecophysiological limitations in the biodegradation process that were independent of the bioavailability. However, biodegradation was enhanced during slurry treatment of greenhouse planted soils that received the biosurfactant. The addition of rhamnolipids caused a dramatic shift in the soil bacterial community structure, which was magnified in the presence of sunflower plants. The stimulated groups were identified as fast-growing and catabolically versatile bacteria. This new rhizosphere microbial biomass possibly interacted with the biosurfactant to facilitate intra-aggregate diffusion of PAHs, thus enhancing the kinetics of slow desorption. Our results show that the usually limited biosurfactant efficiency with contaminated field soils can be significantly enhanced by integrating the sunflower ontogenetic cycle into the bioremediation design.

Rhamnolipid-enhanced solubilization and biodegradation of PAHs in soils after conventional bioremediation

The application of a rhamnolipid biosurfactant for enhanced solubilization and biodegradation of slowly desorbing polycyclic aromatic hydrocarbons (PAHs) in contaminated soils was determined in this study. The soil samples exhibited different levels of pollution and different bioremediation stages: the first soil originated from a creosote-polluted site, contained 4370 mg kg ^-1 of PAHs and had not been bioremediated; the second soil was the same as the first but had received bioremediation treatment with nutrient amendment in biopiles for a period of 5 months and contained 580 mg kg ^-1 of PAHs after this treatment; the third soil was treated by bioremediation for several years to reduce the concentration of PAHs to 275 mg kg ^-1. The kinetics of PAH desorption were determined to assess the magnitude of the slowly desorbing fractions present in the polluted soil and to optimize the biosurfactant effectiveness in terms of biodegradation. The soils that had been treated by bioremediation were enriched in slowly desorbing PAHs. The rhamnolipid at a concentration above its critical micelle concentration enhanced biodegradation in the soils that had been bioremediated previously. The measurement of residual concentrations of native PAHs showed the promoting effect of the biosurfactant on the biodegradation of the slowly desorbing fractions. Interestingly, benzo(a)pyrene was biodegraded in the soil that had been bioremediated for a long time. Rhamnolipid can constitute a valid alternative to chemical surfactants in promoting the biodegradation of slow-desorption PAHs, which is one of the most important problems in bioremediation, but the efficiency depends strongly on the bioremediation stage in which the biosurfactant is applied.

Bioremediation of soil long-term contaminated with PAHs by algal-bacterial synergy of Chlorella sp. MM3 and Rhodococcus wratislaviensis strain 9 in slurry phase

Remediation of soil contaminated with pollutants using biological agents is more a sustainable and greener approach as compared to physico-chemical technologies. We recently confirmed that a microalga, Chlorella sp. MM3, and a bacterium, Rhodococcus wratislaviensis strain 9, can degrade high-molecular weight PAHs. In this study, an algal-bacterial system of these two strains was developed by long-term growth on a mixture of phenanthrene, pyrene, and benzo[a]pyrene (BaP). In a soil spiked with 50 mg L^-1 phenanthrene, 10 mg L^-1 of pyrene and 10 mg L^-1 of BaP, the algal-bacterial system degraded these PAHs almost completely in slurry phase within 30 days. Also, the algal-bacterial system was able to successfully remediate these three PAHs in a soil long-term contaminated with 245.1 mg kg^-1 of 16 PAHs and several heavy metals under slurry phase in 21 days. Use of such appropriate assays as chlorophyll estimation for the microalga and semi-quantitative PCR for the bacterium confirmed survival of both the strains during soil bioremediation. Moreover, the residual toxicity test involving Escherichia coli DH5α that expresses green fluorescent protein indicated the successful bioremediation of PAHs-contaminated soil in slurry phase. For the first time, here we demonstrate the great potential of an algal-bacterial synergy in bioremediation of soil long-term contaminated with PAHs even in the presence of toxic heavy metals.

Occurrence and levels of micropollutants across environmental and engineered compartments in Austria

Occurrence and concentration of a broad spectrum of micropollutants are investigated in Austrian river catchments, namely polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), organotin compounds, perfluoroalkyl acids (PFAAs) and metals. The parallel analysis across multiple environmental and engineered compartments sheds light on the ratio of dissolved and particulate transport and on differences in concentration levels between point and diffuse emission pathways. It is found that some PAHs and organotins are present in rivers, groundwater and bulk deposition at higher concentrations than in municipal wastewater effluents. Among PFAAs and metals, highest concentrations were recorded either in atmospheric deposition or in discharges from wastewater treatment plants. The relevance of the analysis across compartments is best shown by the case of perfluorooctanesulfonic acid (PFOS). Despite municipal wastewater effluents being the emission pathway with highest concentrations, this study reveals that not only rivers, but also atmospheric deposition and groundwater sometimes exceed the environmental quality standard for surface waters. Moreover, this work reveals partially counterintuitive patterns. In rivers with treated wastewater discharges, increasing levels of dissolved compounds were measured at rising flow conditions, whereas the opposite would be expected owing to the dilution effect. This might derive from the mobilisation from soil or suspended particulate matter or rather find its explanation in high concentrations in atmospheric deposition. These hypotheses require however being tested through targeted studies. Additional future research includes the analysis of how regional or catchment specific characteristics might alter the relative importance of different emission pathways, and the modelling of emission and river loads to assess their relative contribution to river pollution.

Bioremediation of soils contaminated by PAHs: Mutagenicity as a tool to validate environmental quality

Bioremediation can be used as one of the decontamination techniques for areas contamined by polycyclic aromatic hydrocarbons (PAHs). However the effective biodegradation of these compounds must take into account the possible toxic and mutagenic effects that might persist. In this study the mutagenic potential of soil samples from an area contaminated by wood preservatives was evaluated. The area had already been submitted to a simulated bioremediation process in a microcosm, using two different inoculums (1 and 2), and comparing them to the decay of PAHs. Organic extracts were prepared before and after bioremediation, where the 16 PAHs considered a priority by USEPA were analyzed and tested using the Salmonella/microsome assay. The extracts were analyzed in strains TA98, TA97a and TA100 (+S9mix/-S9mix), YG1041 and YG1042. Considering Inoculum 1 only as bioaugmented and Inoculum 2 also stimulated and enriched, the concentrations of PAHs and mutagenic effect were different. The former identified a greater reduction of mutagenesis and a smaller decrease of PAHs while the latter showed greater mutagenic power even associated with the greatest reduction of PAHS. The possible generation of degradation byproducts with high mutagenic power after a partial biodegradation process can be considered. In strains YG1041 and YG 1042 the mutagenesis values before bioremediation were 747 and 567 rev/g soil, respectively. Although the efficiency of bioremediation was observed, the associated damage indicates that the analysis of contaminants and their relationship with mutagenic effects are a fundamental stage for the effective evaluation of the risks and efficiency of bioremediation processes.

Opportunistic Bacteria Dominate the Soil Microbiome Response to Phenanthrene in a Microcosm-Based Study

Bioremediation offers a sustainable approach for removal of polycyclic aromatic hydrocarbons (PAHs) from the environment; however, information regarding the microbial communities involved remains limited. In this study, microbial community dynamics and the abundance of the key gene (PAH-RHDα) encoding a ring hydroxylating dioxygenase involved in PAH degradation were examined during degradation of phenanthrene in a podzolic soil from the site of a former timber treatment facility. The 10,000-fold greater abundance of this gene associated with Gram-positive bacteria found in phenanthrene-amended soil compared to unamended soil indicated the likely role of Gram-positive bacteria in PAH degradation. In contrast, the abundance of the Gram-negative PAHs-RHDα gene was very low throughout the experiment. While phenanthrene induced increases in the abundance of a small number of OTUs from the Actinomycetales and Sphingomonadale, most of the remainder of the community remained stable. A single unclassified OTU from the Micrococcaceae family increased ~20-fold in relative abundance, reaching 32% of the total sequences in amended microcosms on day 7 of the experiment. The relative abundance of this same OTU increased 4.5-fold in unamended soils, and a similar pattern was observed for the second most abundant PAH-responsive OTU, classified into the Sphingomonas genus. Furthermore, the relative abundance of both of these OTUs decreased substantially between days 7 and 17 in the phenanthrene-amended and control microcosms. This suggests that their opportunistic phenotype, in addition to likely PAH-degrading ability, was determinant in the vigorous growth of dominant PAH-responsive OTUs following phenanthrene amendment. This study provides new information on the temporal response of soil microbial communities to the presence and degradation of a significant environmental pollutant, and as such has the potential to inform the design of PAH bioremediation protocols.

Humic substances, their microbial interactions and effects on biological transformations of organic pollutants in water and soil: A review

Depicted as large polymers by the traditional model, humic substances (HS) tend to be considered resistant to biodegradation. However, HS should be regarded as supramolecular associations of rather small molecules. There is evidence that they can be degraded not only by aerobic but also by anaerobic bacteria. HS presence alters biological transformations of organic pollutants in water and soil. HS, including humin, have a great potential for an application in aerobic and anaerobic wastewater treatment as well as in bioremediation. Black carbon materials, including char (biochar) and activated carbon (AC), long recognized effective sorbents, have been recently discovered to act as effective redox mediators (RM), which may significantly accelerate degradation of organic pollutants in a way similar to HS. Humic-like coating on the biochar surface has been identified. Explanation of mechanisms and possibility of applications of black carbon materials have only started to be explored. Results of many original and review papers, presented and discussed in this article, show an enormous potential for an interesting, multidisciplinary research as well as for a development of new, green technologies for biological wastewater treatment and bioremediation. Future research areas have been suggested.

Strategies for oxidation of PAHs in aged contaminated soil by batch reactors

Polycyclic aromatic hydrocarbons (PAH) are neutral, nonpolar and hydrophobic molecules that tend to sorb onto soil organic matter. Chemical oxidation is a good choice to avoid the limitations of bioremediation. To evaluate the efficiency of different types of oxidation (permanganate, hydrogen peroxide, and persulfate) and activation (heat, alkaline, and iron), batch reactors were prepared. The soil was contaminated with phenanthrene and pyrene (1200 ± 200 and 2800 ± 100mg per kg of dry soil, respectively) and aged for fifteen months. Treatments were prepared with 10g of contaminated dry soil and 20ml of water and incubated at room temperature for 7 days. Analyses of phenanthrene and pyrene concentrations, soil pH and electric conductivity were performed. Counts of heterotrophic cultivable bacteria on R2A medium and PAH-degraders were carried out after 7 days of treatment. The persulfate treatment at room temperature, without the addition of activators, achieved better results than treatments with the same doses of permanganate or hydrogen peroxide. All the strategies to improve persulfate treatments yielded higher degradation of pyrene than the biological control, as expected from the structural description of this compound by Clar's model. The thermal activation of persulfate (65°C for 6h) led to the degradation of more than 90% of both PAHs after 7 days of treatment.

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.

Role of desorption kinetics in the rhamnolipid-enhanced biodegradation of polycyclic aromatic hydrocarbons

The main aim of this study was to investigate the effect of a rhamnolipid biosurfactant on biodegradation of (14)C-labeled phenanthrene and pyrene under desorption-limiting conditions. The rhamnolipid caused a significant solubilization and enhanced biodegradation of PAHs sorbed to soils. The enhancement was, however, negatively influenced by experimental conditions that caused an enrichment of slow desorption fractions. These conditions included aging, a higher organic matter content in soil, and previous extraction with Tenax to remove the labile-desorbing chemical. The decline in bioavailability caused by aging on sorbed (14)C-pyrene was partially reversed by rhamnolipids, which enhanced mineralization of the aged compound, although not so efficiently like with the unaged chemical. This loss in biosurfactant efficiency in promoting biodegradation can be explained by intra-aggregate diffusion of the pollutant during aging. We suggest that rhamnolipid can enhance biodegradation of soil-sorbed PAHs by micellar solubilization, which increase the cell exposure to the chemicals in the aqueous phase, and partitioning into soil organic matter, thus enhancing the kinetics of slow desorption. Our study show that rhamnolipid can constitute a valid alternative to chemical surfactants in promoting the biodegradation of slow desorption PAHs, which constitutes a major bottleneck in bioremediation.

The effect of humic acids on biodegradation of polycyclic aromatic hydrocarbons depends on the exposure regime

Binding of polycyclic aromatic hydrocarbons (PAHs) to dissolved organic matter (DOM) can reduce the freely dissolved concentration, increase apparent solubility or enhance diffusive mass transfer. To study the effects of DOM on biodegradation, we used phenanthrene and pyrene as model PAHs, soil humic acids as model DOM and a soil Mycobacterium strain as a representative degrader organism. Humic acids enhanced the biodegradation of pyrene when present as solid crystals but not when initially dissolved or provided by partitioning from a polymer. Synchronous fluorescence spectrophotometry, scintillation counting and a microscale diffusion technique were applied in order to determine the kinetics of dissolution and diffusive mass transfer of pyrene. We suggest that humic acids can enhance or inhibit biodegradation as a result of the balance of two opposite effects, namely, solubilization of the chemicals on the one hand and inhibition of cell adhesion to the pollutant source on the other.

Levels and spatial distribution of persistent organic pollutants in the environment: a case study of German forest soils

The Of/Oh-horizons of 447 forest stands in Germany were evaluated for concentrations and spatial distribution of selected polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and organochlorine pesticides (OCPs). While concentrations of dichlorodiphenyltrichloroethane (DDT) and PCBs show similar spatial distribution patterns for all measured compounds within each compound class, significantly different distributions were identified for concentrations of low-molecular-weight PAHs [2- and 3-ring PAHs plus fluoranthene (FLA) and pyrene (PYR)] in contrast to high-molecular-weight PAHs (4-6-ring PAHs without FLA and PYR). Maxima of persistent organic pollutant (POP) concentrations could be mostly explained by specific locatable sources. Because of the slow degradation rates of these target substances, this is especially relevant for historic contamination sources, such as extensive 1980s DDT usage in the former German Democratic Republic and industrial facilities that produced hexachlorobenzene (HCB) or PCBs. A contribution of ubiquitous background pollution derived from long-range atmospheric transport is likely for some compounds in the studied area, e.g., DDT in the western part of Germany and dieldrin. However, most target compounds appear to be mainly sourced from local or regional emissions. This is supported by the absence of clear dependencies between POP concentrations and most evaluated environmental and local parameters. We suggest that these results generally reflect the distribution of POPs in densely populated and industrialized countries located in temperate regions.

Concentrations and geographic distribution of selected organic pollutants in Scottish surface soils

Concentrations of selected persistent organic pollutants (POPs) representing three chemical classes (polycyclic aromatic hydrocarbons (PAH), polybrominated diphenyl ethers (PBDE) and polychlorinated biphenyls (PCB) and the organic pollutant diethylhexyl phthalate (DEHP), were determined in surface soil samples (0-5 cm) collected at 20 km grid intersects throughout Scotland over a three-year period. Detectable amounts of all chemical classes and most individual congeners were present in all samples. There were no consistent effects of soil or vegetation type, soil carbon content, pH, altitude or distance from centres of population on concentrations which exhibited extreme variation, even in adjacent samples. It is concluded that soil POPs and DEHP concentrations and associated rates of animal and human exposure were highly variable, influenced by multiple, interacting factors, and not clearly related to local sources but possibly related to wet atmospheric deposition and the organic carbon content of the soil.

Is it possible to increase bioavailability but not environmental risk of PAHs in bioremediation?

The current poor predictability of end points associated with the bioremediation of polycyclic aromatic hydrocarbons (PAHs) is a large limitation when evaluating its viability for treating contaminated soils and sediments. However, we have seen a wide range of innovations in recent years, such as an the improved use of surfactants, the chemotactic mobilization of bacterial inoculants, the selective biostimulation at pollutant interfaces, rhizoremediation and electrobioremediation, which increase the bioavailability of PAHs but do not necessarily increase the risk to the environment. The integration of these strategies into practical remediation protocols would be beneficial to the bioremediation industry, as well as improve the quality of the environment.

Assessing microbial carbon sources and potential PAH degradation using natural abundance 14C analysis

Natural abundance (14)C analysis was applied to PLFAs collected from an industrial site in southern Ontario in order to assess microbial carbon sources and potential PAH biodegradation in soils. Δ(14)C of microbial phospholipid fatty acids (PLFA) at the site ranged from +54‰ to -697‰. Comparison of these values to surrounding carbon sources found that microbial carbon sources were derived primarily from vegetation and/or natural organic matter present in the soils rather than PAHs. This study highlights that microbes are able to utilize almost all available pools of organic matter including older pools which are thought to contain recalcitrant compounds. Furthermore, it shows that even with the presence of an active microbial community, there may be little biodegradation of PAHs. This study illustrates challenges in assessing microbial activity in the environment and the advantage of using natural abundance (14)C analysis as a tool to elucidate microbial carbon sources.

Organo/LDH nanocomposite as an adsorbent of polycyclic aromatic hydrocarbons in water and soil-water systems

Polycyclic aromatic hydrocarbons (PAHs) are considered as priority pollutants because of their high risk to human health. In this paper, we addressed the issue of using hydrotalcite-based nanocomposites as adsorbents of six low molecular weight PAHs (acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene and pyrene) to reduce their negative effects on the environment. A nanocomposite (HT-DDS) was prepared by intercalating the organic anion dodecylsulfate (DDS) in a Mg-Al hydrotalcite (HT), and then characterized using several analytical techniques. A Mediterranean soil was selected for being a high-risk scenario of groundwater contamination by leaching of pollutants. The nanocomposite displayed enhanced affinity for the PAHs in water as compared to carbonate-hydrotalcite (HTCO(3)) and its calcined product (HT500), and showed a high irreversibility of the adsorption process (hysteresis coefficient, H<0.15). The results revealed an increase of the pollutants retention in the soil by the addition of the nanocomposite that depended on the nanocomposite application rate and also on the hydrophobicity of each PAH. Accordingly, the use of HT-DDS as an amendment or barrier in contaminated soil is proposed for reducing the mobility of PAHs and, consequently, the adverse effect derived from rapid transport losses of the pollutants to the adjoining environmental compartments.