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Overestimation of risk is one of the main problems in environmental risk assessments if only total concentration of organic pollutants is considered. In this study, we integrated bioavailability measurements into persistence testing of pollutants in soil to show that it is the key to have a more realistic environmental risk assessment (ERA). To this integration, two standardized methods were used: OECD 307, as persistence test, and ISO 16751: 2020, to bioavailability measurements based on 20 h extractions with a strong adsorbent (Tenax), using pyrene and carbamazepine as model test substances. Because the ISO method was initially designed for nonpolar compounds with log Kow > 3, a slight adaptation was necessary for carbamazepine (log Kow = 2.7), assuming this also as an extension of the applicability domain of the method. During the biodegradation of these compounds, the mineralization extents did not exceed 4 %, giving rise to transformation products. Therefore, the bioavailability measurements covered both the parent compound and the metabolites produced. In the case of pyrene, the partial transformation carried out by a specialized microbial inoculum accounted for, respectively, 32 % or 40 % of the initial concentration (4 mg kg−1) in unamended or compost-amended soil. Only 1 % was present as hydrophilic transformation products that were not trapped by Tenax, but partitioned into the water. The nonbioavailable residue increased in both soils after biodegradation. The distribution of chemicals in the different phases of the system was the key to assess more realistically the shifts in bioavailability during persistence testing. The same procedure was carried out for carbamazepine where an additional desorption study showed a slower desorption rate of the parent compound after incubation. In this case, 25 % of transformation products was mobilized to the aqueous phase. Our results show that bioavailability measurements provide valuable information when integrated

The productive application of motile microorganisms for degrading hydrophobic contaminants in soil is one of the most promising processes in modern remediation due to its sustainability and low cost. However, the incomplete biodegradation of the contaminants and the formation of the intermediary metabolites in the process may increase the toxicity in soil during bioremediation, and motile inoculants may mobilize the pollutants through biosorption. Therefore, controlling these factors should be a fundamental part of soil remediation approaches. The aim of this study was to evaluate the sources of risk associated with the cometabolism-based transformation of 14C-labeled pyrene by inoculated Pseudomonas putida G7 and identify ways to minimize risk. Our model scenario examined the increase in bioaccessibility to a distant source of contamination facilitated by sunflower (Helianthus annuus L.) roots. A biochar trap for mobilized pollutant metabolites and bacteria has also been employed. The experimental design consisted of pots filled with a layer of sand with 14C-labeled pyrene (88 mg kg−1) as a contamination focus located several centimeters from the inoculation point. Half of the pots included a biochar layer at the bottom. The pots were incubated in a greenhouse with sunflower plants and P. putida G7 bacteria. Pots with sunflower plants showed a higher biodegradation of pyrene, its mobilization as metabolites through the percolate and the roots, and bacterial mobilization toward the source of contamination, also resulting in increased pyrene transformation. In addition, the biochar layer efficiently reduced the concentrations of pyrene metabolites collected in the leachates. Therefore, the combination of plants, motile bacteria and biochar safely reduced the risk caused by the biological transformation of pyrene.

In response to declineing natural water sources, treated wastewater has been introduced into the water cycle as a new water source for irrigation. However, this practice exposes the agricultural environment to various contaminants of emerging concern. To better understand their fate in the soil and to effectively predict their bioavailability for plant uptake, there is a need to quantify their concentrations in soil solutions. In this study, we examined the concentrations of treated wastewater-derived ​contaminants of emerging concern in soil solutions under three scenarios: (1) shifting from irrigation with freshwater to treated wastewater (FW→TWW scenario), (2) long-term continuous irrigation with treated wastewater (TWW→TWW scenario), and (3) prolonged irrigation with treated wastewater followed by freshwater (TWW→FW scenario). Contaminants of emerging concern including carbamazepine, 1H-benzotriazole, lamotrigine, venlafaxine, and thiabendazole were ubiquitous in the treated wastewater (mean concentrations of 125, 945, 180, 3630, and 90 ​ng/L, respectively) and irrigated soils. Interestingly, their concentrations in the soil solutions were different (higher or lower) from the corresponding concentrations in the irrigation water. In both the freshwater to wastewater (FW→TWW) and treated wastewater to freshwater (TWW→FW) irrigation scenarios, lower contaminant concentrations were observed in soil solutions compared to the prolong treated wastewater irrigation scenario (TWW→TWW), indicating that a steady state condition was not achieved after a single irrigation season. For example, the concentrations of 1H-benzotriazole in Nir Oz soil solutions were 638, 310, and 1577 ​ng/L for the three irrigation scenarios, respectively. Moreover, the ​contaminants concentrations in soil solutions were slightly lower in the TWW→FW irrigation scenario compared to the TWW→TWW scenario. Our data suggest that rain-fed crops are also exposed to treated wastewater-derived contaminants

As is the case with many other industrial activities, the organic contaminants at military-impacted sites may pose significant hazards to the environment and human health. Given the expected increase in defense investments globally, there is a need to make society aware of the risks of emissions of organic contaminants generated by military activities and to advance risk minimization approaches. The most recent advances in environmental analytical chemistry, persistence, bioavailability and risk assessment of organic contaminants indicate that efficient risk reductions through biological means are possible. This review debates the organic contaminants of interest associated with military activities, the methodology used to extract and analyze these contaminants, and the nature-based remediation technologies available to recover these sites. In addition, we revise the military environmental regulatory frameworks designed to sustain such actions. Military activities that potentially release organic contaminants on land could be classified as infrastructure and base operations, training exercises and armed conflicts; additionally, chemicals may include potentially toxic compounds, energetic compounds, chemical warfare agents and military chemical compounds. Fuel components, PFASs, TNT, RDX and dyphenylcyanoarsine are examples of organic contaminants of environmental concern. Particularly in the case of potentially toxic and energetic compounds, bioremediation and phytoremediation are considered eco-friendly and low-cost technologies that can be used to remediate these contaminated sites. In addition, this article identifies implementing the bioavailability of organic contaminants as a justifiable approach to facilitate the application of these nature-based approaches and to reduce remediation costs. More realistic risk assessment in combination with new and economically feasible remediation methods that reduce risk by reducing bioavailability (instead of lowering the t

The evaluation of a chemical substance's persistence is key to understanding its environmental fate, exposure concentration, and, ultimately, environmental risk. Traditional biodegradation test methods were developed many years ago for soluble, nonvolatile, single-constituent test substances, which do not represent the wide range of manufactured chemical substances. In addition, the Organisation for Economic Co-operation and Development (OECD) screening and simulation test methods do not fully reflect the environmental conditions into which substances are released and, therefore, estimates of chemical degradation half-lives can be very uncertain and may misrepresent real environmental processes. In this paper, we address the challenges and limitations facing current test methods and the scientific advances that are helping to both understand and provide solutions to them. Some of these advancements include the following: (1) robust methods that provide a deeper understanding of microbial composition, diversity, and abundance to ensure consistency and/or interpret variability between tests; (2) benchmarking tools and reference substances that aid in persistence evaluations through comparison against substances with well-quantified degradation profiles; (3) analytical methods that allow quantification for parent and metabolites at environmentally relevant concentrations, and inform on test substance bioavailability, biochemical pathways, rates of primary versus overall degradation, and rates of metabolite formation and decay; (4) modeling tools that predict the likelihood of microbial biotransformation, as well as biochemical pathways; and (5) modeling approaches that allow for derivation of more generally applicable biotransformation rate constants, by accounting for physical and/or chemical processes and test system design when evaluating test data. We also identify that, while such advancements could improve the certainty and accuracy of persistence assessments, th

Due to the extensive oil extraction and transportation that occurs in oil-producing countries, many lands remain contam-inated because of accidental leakages. Despite its low cost and environmentally safe nature, bioremediation technology is not always successful, mainly because of the soil toxicity to the degrading microbial populations and plants. Here we re-port a three-year microfield experiment on the influence of natural sorbents of mineral (zeolite, kaolinite, vermiculite, di-atomite), organic (peat), carbonaceous (biochar) origin, and a mixed sorbent ACD (composed of granular activated carbon and diatomite) on the bioremediation of grey forest soil contaminated with weathered crude oil (40.1 g total petroleum hydrocarbons (TPH) kg-1). Optimal doses of the sorbents significantly accelerated bioremediation of petroleum -contaminated soil through bioaugmentation followed by phytoremediation. The main reason for the influence of the sor-bent amendments relied upon the creation of optimal conditions for the activation of hydrocarbon-utilizing bacteria and plant growth due to the reduction of soil toxicity, as well as maintaining an optimal pH and water-air regime in the soil. That happened because of reducing the soil hydrophobicity, increasing porosity and water holding capacity. The content of the TPH in the best samples (2 % biochar or ACD) reduced to their local permissible concentration accepted for remediated soils in the Russian Federation (<= 5 g kg-1) after two warm seasons compared to that after three warm sea-sons in the other samples. Although some sorbents decelerated biodegradation of highly condensed polycyclic aromatic hydrocarbons (PAHs, including benzo(a)pyrene) in the soil, the overall risk from the residual contaminants present in the remediated soil and plants was minimized. The final total content of the main PAHs in the sorbent-amended soils did not exceed the maximal permissible levels that are accepted in most EU countries (1000-40,000 mu

There is a strong need for certified reference materials in the quality assurance of nonionic soil contaminant bioavailability estimations through physicochemical methods. We applied desorption extraction, a method recently standardized as ISO16751, to determine the bioavailable concentration of the most commonly regulated polycyclic aromatic hydrocarbon (PAH), benzo(a)pyrene (BaP), in the reference industrial soil BCR-524 with a certified BaP total concentration of 8.60 mg kg−1. This concentration represented BaP levels found in many PAH-polluted soils. The method, based on single-point extraction of the analyte desorbed into the aqueous phase by a receiving phase (Tenax or cyclodextrin), was applied ten times. The data fulfilled highly demanding quality criteria based on recovery and repeatability. The bioavailable BaP concentration detected through Tenax extraction, 1.82 mg kg−1, was comparable to bioavailable concentrations determined in field-contaminated soils and to environmental quality standards based on previously observed total BaP concentrations. There was good agreement (Student's t-test, P ≤ 0.05) with the bioavailable BaP concentration determined by cyclodextrin extraction (1.53 mg kg−1). The methods were extended to four other certified 4- and 5-ringed PAHs for comparative purposes. We suggest ways of improving of the ISO16751 standard related to further systematic assessment of the Tenax-to-soil ratio and incorporation of mass balances. Furthermore, BCR-524 is suitable for quality-assurance protocols with these methods when used in site-specific risk assessments of PAH-polluted environments.

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 (KLW) and leaf-air (KLA) 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. © 2021 Elsevier B.V.

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. © 2020 Elsevier B.V.

In an article published recently, Waring et al. (2020) introduced a conceptual framework, named PROMISE, to explore controls on belowground carbon cycling. This “fundamentally novel approach to measuring and modeling critical carbon cycle processes” represents soil pore space as an array of possible particle locations belonging to one of three pore size classes. Several aspects of this conceptual framework raise questions about its adequacy to describe the fate of carbon in heterogeneous soils. More than a decade ago already, Kuka et al. (2007) advocated the significance of the size of pores when modeling carbon turnover in soils and proposed a model, called CIPS (“Carbon Turnover in Pore Space”), which is not mentioned by Waring et al. (2020) even though in several respects, CIPS is very similar to PROMISE, and shares the same aims and approach. In CIPS, pores are also divided into three classes according to their size. Like PROMISE, CIPS does not assume any spatial arrangement, distance, or differences in connectivity among the three classes of pores.

Biodegradable chemicals may become persistent due to reductions in their bioavailability thereby impacting on the rate and extent of biodegradation in soils and sediments. This chapter examines this – commonly neglected – contradictory face of persistence assessments from the light of the latest advancements in bioavailability science. They include the microbial influences on bioavailability, the different sorption capacities of carbonaceous components of soils and sediments, and the dissimilar bioavailability shown by chemicals when they are present as non-extractable residues. We also discuss possible pathways to improve the realism in persistence assessments from standardized biodegradation tests by incorporating new bioavailability-based approaches. Innovations of the standard tests are possible through the modified chemical application of enhanced dispersion and passive dosing. In addition, we offer a proposal for integrating bioavailability measurements into standard simulation tests with soils and sediments, by using desorption extraction and passive sampling methods to assess the removal of the bioavailable fractions, in addition to the total extractable concentration of the chemical.

The bioavailability of potentially hazardous organic chemicals (persistent organic pollutants, pesticides, biocides, pharmaceuticals, and others) in soil and sediment has a major impact on the environmental and human health risks of these chemicals and is an important area of scientific research. However, this area remains only partially recognized by regulators. Based on the positive experiences from the previous implementation for metals, regulatory frameworks have recently started to include bioavailability within retrospective risk assessment (rRA) and remediation for organic chemicals. In this regard, realistic decision-making in terms of hazard definition and priority setting will ensure the protection of environmental and public health, in contrast to the established approach of using total extractable concentrations, which has been shown to be inappropriate. Moreover, by addressing bioavailability reduction instead of only pollutant removal as a paradigm shift, new remediation strategies become possible. However, the implementation of bioavailability for rRA remains difficult because scientific developments on bioavailability do not always translate into practical approaches for regulators, thus requiring specific measures. For the same reason, bioavailability remains largely unexplored within prospective regulatory frameworks (e.g., REACH, pesticide RA) that address the approval and regulation of organic chemicals.

Urban trees provide important ecosystem services, including air quality improvement. Polycyclic aromatic hydrocarbons (PAHs) are among the most important pollutants in air, due to their elevated concentrations and toxicity. Plants can act as filters of PAHs and as "chemical reactors" for pollutant removal, therefore reducing air concentrations. Here, the first assessment of photo- vs. biodegradation of PAHs on leaves of urban trees is presented. A dynamic air-vegetation-soil model (SoilPlusVeg) was improved to simulate the fate of two representative PAHs with contrasting physico-chemical properties (phenanthrene and benzo[a]pyrene). Simulations were performed for two different environmental scenarios from Italy (Como and Naples), selected for their dissimilar meteorological parameters, plant species and emission levels. The effect of photo- and biodegradation on leaf concentrations and fluxes towards air and soil was investigated comparing deciduous (maple, cornet and hazelnut) and evergreen (holm oak) broadleaf woods. The results showed that biodegradation in the phyllosphere could not be neglected when evaluating the ecosystem services provided by urban trees, as this process contributed significantly to the reductions (up to 25% on average) in PAH leaf concentrations and fluxes to air and soil; however, the reductions revealed ample variations with time (up to more than two orders of magnitude) showing the dependence on meteorological parameters, air compartment structure, as well as type of woods. These findings permitted to improve the ecological realism of the simulations and obtain more accurate results when predicting organic contaminant uptake and release by plant leaves, including potential for food chain transfer and long-range transport. (C) 2020 Elsevier B.V. All rights reserved.

A key motivation for this handbook arose from the different perceptions of scientists and regulators of the scientific advances made during the last 30 years in the field of bioavailability of organic chemicals in soil and sediment. This last chapter provides a distillation of the general messages extracted from the individual chapters from this handbook, by answering the following questions: (1) Is bioavailability science ready for use in regulation? (2) How should bioavailability be measured? And (3) how should it be implemented? We conclude the chapter with the research needs covering the knowledge gaps that still remain after this effort, focusing on the methodologies for measuring bioavailability, the environmental risks of non-bioavailable compounds, and the innovative remediation of contaminated waters, sediments, and soils.

Due to the extended oil extraction and transportation in Russia and other oil-producing countries, many lands remain contaminated because of accidental spills. This situation requires the cost-effective and efficient remediation of petroleum-contaminated soils. Bioremediation of soils contaminated with high concentrations of crude oil is usually hampered by high toxicity thresholds for microbial degraders. We have performed a two-year microfield experiment on the influence of a mixed adsorbent (ACD) composed of granular activated carbon and diatomite on bioremediation of a grey forest soil contaminated with crude oil at concentrations (5–15 % w/w) that would theoretically not result in a successful pollutant removal due to toxicity. Remediation of these soils was evaluated after treating with the ACD adsorbent (from 4 to 12% w/w) and a biopreparation (BP) containing hydrocarbon-degrading bacteria, separately or in combination. Reduction of total petroleum hydrocarbons content was significantly greater in highly contaminated soils with the combined amendments than in the respective controls (through the activation of indigenous degrading microorganisms by fertilizing and mixing) by 9-10% and 5-8% at the end of the first and second years, respectively, depending on the contamination level. Significantly higher counts of petroleum-degrading microorganisms (as indigenous and introduced by the BP), as well as much less phytotoxicity was detected in the ACD-amended soils, as compared with the samples without adsorbent. In addition, the ACD mixture drastically reduced the wash-out of polar petroleum metabolites (evidently oxidized hydrocarbons) and the phytotoxicity of the lysimetric waters, especially in highly contaminated soils. The results indicate that the mixture of activated carbon and diatomite is a prospective adsorbent for the in situ bioremediation of soils highly contaminated with crude oil.

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.

The risks of pollution by polycyclic aromatic hydrocarbons (PAHs) may increase in bioremediated soils as a result of the formation of toxic byproducts and the mobilization of pollutants associated to suspended colloids. In this study, we used the motile and chemotactic bacterium Pseudomonas putida G7 as an experimental model for examining the potential role of bacterial motility in the cometabolism and biosorption of pyrene in a porous medium. For this purpose, we conducted batch and column transport experiments with 14C-labelled pyrene loaded on silicone O-rings, which acted as a passive dosing system. In the batch experiments, we observed concentrations of the 14C-pyrene equivalents well above the equilibrium concentration observed in abiotic controls. This mobilization was attributed to biosorption and cometabolism processes occurring in parallel. HPLC quantification revealed pyrene concentrations well below the 14C-based quantifications by liquid scintillation, indicating pyrene transformation into water-soluble polar metabolites. The results from transport experiments in sand columns revealed that cometabolic-active, motile cells were capable of accessing a distant source of sorbed pyrene. Using the same experimental system, we also determined that salicylate-mobilized cells, inhibited for pyrene cometabolism, but mobilized due to their tactic behavior, were able to sorb the compound and mobilize it by biosorption. Our results indicate that motile bacteria active in bioremediation may contribute, through cometabolism and biosorption, to the risk associated to pollutant mobilization in soils. This research could be the starting point for the development of more efficient, low-risk bioremediation strategies of poorly bioavailable contaminants in soils. © 2020 Elsevier B.V.

The interaction between root exudates and soil microbes has been hypothesised as the primary mechanism for the biodegradation of organic pollutants in the rhizosphere. However, the mechanisms governing this loss process are not completely understood. This study aimed to investigate the effect of two important compounds within root exudates (citric and malic acid) on 14C-phenanthrene desorption and bioaccessibility in soil. Overall results showed that the presence of both citric and malic acid (>100 mmol l−1) enhanced the desorption of 14C-phenanthrene; this appeared to be concentration dependant. Increases in extractability were not reflected in a higher bioaccessibility. Despite enhancing the desorption of 14C-phenanthrene in soil, there is no direct evidence indicating that citric or malic acid have the ability to promote the biodegradation of 14C-phenanthrene from soil. Results from this study provide a novel understanding of the role that substrates, typically found within the rhizosphere due to root exudation, play in the bioaccessibility and biodegradation of hydrocarbons in contaminated soil.

There is a strong need for certified reference materials in the quality assurance of nonionic soil contaminant bioavailability estimations through physicochemical methods. We applied desorption extraction, a method recently standardized as ISO16751, to determine the bioavailable concentration of the most commonly regulated polycyclic aromatic hydrocarbon (PAH), benzo(a)pyrene (BaP), in the reference industrial soil BCR-524 with a certified BaP total concentration of 8.60 mg kg−1. This concentration represented BaP levels found in many PAH-polluted soils. The method, based on single-point extraction of the analyte desorbed into the aqueous phase by a receiving phase (Tenax or cyclodextrin), was applied ten times. The data fulfilled highly demanding quality criteria based on recovery and repeatability. The bioavailable BaP concentration detected through Tenax extraction, 1.82 mg kg−1, was comparable to bioavailable concentrations determined in field-contaminated soils and to environmental quality standards based on previously observed total BaP concentrations. There was good agreement (Student's t-test, P ≤ 0.05) with the bioavailable BaP concentration determined by cyclodextrin extraction (1.53 mg kg−1). The methods were extended to four other certified 4- and 5-ringed PAHs for comparative purposes. We suggest ways of improving of the ISO16751 standard related to further systematic assessment of the Tenax-to-soil ratio and incorporation of mass balances. Furthermore, BCR-524 is suitable for quality-assurance protocols with these methods when used in site-specific risk assessments of PAH-polluted environments.

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

We applied a standardized desorption extraction method (Tenax extraction), to assess the bioavailability of native polycyclic aromatic hydrocarbons (PAHs) present in contaminated soils. Single-time point Tenax extraction at 20 h has been recently proposed by the International Organization for Standardization as one of the chemical methods to measure environmental bioavailability of nonionic pollutants (ISO/TS 16751). This work is one of the first ones that use this ISO method systematically in the field of bioremediation, and shows its advantages when used in combination with total concentrations determined with conventional, exhaustive solvent extraction. This method has been applied to different PAHs contaminated soils which had a different level of total PAHs (66-4370 mg kg(-1)) and which were from different contaminated sites and dissimilar bioremediation approaches. Inmost samples the study was focused on phenanthrene and benzo(a)pyrene as representative pollutants, although the profile of total PAHs was also studied in some samples. The results from this study show that the pollutant fractions extracted with Tenax during 20 h (D20) decreased after traditional bioremediation (biostimulation and phytoremediation), but they often increased in bioavailability-oriented treatments involving either biosurfactants or bioaugmentation with specialized microbial inocula. Therefore, D20-based assessments provided information on the bioremediation performance, not directly evident through the measurement of total PAH concentrations. (C) 2019 Elsevier B.V. All rights reserved.

The current project is a cooperation of 5 partner from 4 European countries under the PRIMA research programme. It started in August 2020 and we are going to present the project concept and first project results. The goal of the projec is to enhance food quality/security by an evaluation of the risks associated with agricultural techniques, using sewage sludge for soil fertilization and treated wastewater for irrigation. Associated contaminants may reach the soils and plants, representing a risk to the consumer. In the project, we develop an improved treatment of waste materials prior to spreading based on biochar techniques. We study the fate and distribution of selected relevant contaminants from treatment of the waste materials until crop harvest. 14C-radiolabelled contaminants allow an advanced evaluation to extend the risk assessment for transformation products and bound residues. As an integral part of the project, soil qualities and sustainable use of waste materials in agriculture will be an important result of the study. To ensure practical relevance of the project, advancement will be pursued by the involvement of stakeholders (farmers¿ cooperatives, trade associations) to evaluate the applicability of the proposed technologies. Further, a user manual will be prepared that can serve as guidance on how to implement the research results in agricultural practise. As countries like Israel have no choice to use waste materials in agriculture, the project will support e.g. the Israel export agricultural business. In Spain and Italy basically the same applies: the high rate of treated sewage sludge and wastewater used in agriculture raises the same concerns. This demonstrates the project relevance, because it will provide new insights into the risk associated to this agricultural practice, as related to chemical pollution and food-safety issues. For countries like Germany, where water limitations might get more relevance in the future as a result of climatic chan

Regulators are starting to consider bioavailability within retrospective risk assessment frameworks for organic chemicals, however, implementation is not straightforward because the developments of bioavailability science have not always been translated into ready-to-use approaches for regulators. Possible pathways for translating bioavailability science into regulation of organic chemicals have recently been identified (Environ. Sci. Technol. 49:10255-10264, 2015). A simplified approach was proposed in which the assessments of soil/sediment and the target chemicals should be based on two measurable values: the total extractable concentration and the bioavailable concentration as measured with robust and reproducible chemical and/or biological methods. One of the chemical methods which has been proposed to measure bioavailability of hydrophobic chemicals (HOCs) such as PAHs is the desorption extraction with Tenax during 20 h (ISO 17402) (Environ. Toxicol. Chem. 20:706–711, 2001; Integr Environ Assess Manag, 11:208–220, 2015). Understanding the role of bioavailability in the biodegradation of chemicals is relevant not only for retrospective contaminated land management but also prospective risk assessment applied in the approval and regulation of organic chemicals. With the aim of providing pathways for implementation into regulatory contexts, we carrying out desorption extraction measurements with Tenax in a greenhouse experiment in which different strategies (use of microorganisms, plants and (bio)surfactants) in a PAHs contaminated soil, oriented to decrease the fraction of bioavailable pollutants. The most relevant result in this study was that bioavailability increases in planted soils receiving rhamnolipids, as evidenced by Tenax extraction and it was accompanied by an increased biodegradation in soil slurries. In conclusion, tenax extraction during 20 h resulted a reliable and robust method to determine bioavailable concentrations in a wide set of operational

Polycyclic aromatic hydrocarbons (PAHs) are the best representatives of chemicals for which specific limitations in bioremediation exist due to low bioavailability. The residual concentrations of the PAHs after bioremediation are crucial because they may limit the use of the area after treatment, or land use might not even be possible if the residual concentrations do not meet the legal requirements. For this reason bioavailability should be incorporated into soil bioremediation and a very important question is how should it be measured. One of the chemical methods which has been proposed to measure bioavailability of PAHs is the desorption extraction with Tenax during 20 h (ISO 17402) (Environ. Sci. & Technol. 49, 10255-10264, 2015; Environ Toxicol Chem 20:706¿711, 2001; Integr Environ Assess Manag, 11:208¿220, 2015). We have used this method in a greenhouse experiment in which different strategies have been used (use of microorganisms, plants and (bio)surfactants) in a PAHs contaminated soil, oriented to decrease the fraction of bioavailable pollutants. These agents have been shown previously in our group to act as bioavailability-promoting agents (Environ. Technol. 21, 1099-1110, 2000; Soil Biology and Biochemistry, 57, 830-840, 2013; Environ. Sci. & Technol. 45, 3019-3026, 2011). The greenhouse experiment was carried out with a creosote polluted soil with a total concentration of PAHs of 500 mg/kg. For this study, we used sunflower plants and a rhamnolipid biosurfactant (R90). There were four different treatments: contaminated soil (used as control); contaminated soil with rhamnolipid; planted contaminated soil; and planted contaminated soil with rhamnolipid. After certain time intervals, the soil was sampled in the different treatments. The rhamnolipid was applied to the soil after 75 days of treatment. The total concentration of PAHs was determined by Soxhlet extraction and HPLC analysis; the bioavailable concentration was determined by Tenax extraction. In ad

Chemoeffector-mediated bacterial motility and tactic swimming are major drivers for contaminant accessibility and biodegradation at submillimeter scales. In sand-filled percolated columns we tested how and to what degree chemoeffectors influenced bacterial transport and thereby promoted accessibility and degradation of distantly located 14C-naphthalene (NAH) at the centimeter scale. Sunflower root exudates and silver nanoparticles (AgNPs) were used as chemoeffectors to stimulate opposing effects of motility and tactic swimming of NAH-degrading Pseudomonas putida G7. Sunflower exudates prompted smooth bacterial movement and positive taxis, while AgNPs induced tortuous movement and repellent responses. Compared to chemoeffector-free controls exudates reduced deposition and stimulated bacterial transport during percolation experiments. AgNPs, however, provoked bacterial deposition and concomitant saturation of the collector surfaces (filter blocking) that led to progressively increased percolation of cells. Despite mechanistic differences, both motility patterns supported bacterial transport and promoted mineralization rates of NAH desorbing from a source placed at the column outlet. Observed mineralization rates in the presence of the chemoeffectors were 5-fold higher than those in their absence and similar to NAH-mineralization in well-stirred batch assays. Our results indicate that chemically mediated, small-scale bacterial motility patterns may become relevant for long-distance bacterial transport and the biodegradation of patchy contaminants at higher scales, respectively

Bioavailability for contaminant degradation requires a deep understanding of the ecology of degrader microbial systems. It hence should be perceived as a microbial systems property. In this chapter we summarize recent research on microbial ecology of contaminant biodegradation in the mycosphere (i.e., the microhabitat surrounding and affected by mycelia). By forming unique transport networks, mycelial fungi are highly adapted to cope with complex heterogeneous habitats and to grow under conditions of uneven availability of their vital resources. Combining concepts from bioavailability, ecophysiology, and microbial ecology, our chapter discusses the impact of fungal networks on chemical and bacterial transport and their effects on contaminant bioavailability and degradation. It thereby provides generic information on key factors, processes, and ecological principles that drive contaminant biotransformation in the mycosphere.

The controlled dispersal of motile bacteria through contaminated soils is currently considered as one of the most promising aspects in bioremediation. The difficulty caused by the heterogeneous distribution of the pollutants and the degrading bacteria can be overcome by the use of different chemoefectors. These compounds provide the control over bacterial tactic motility, what results in an increase in the efficiency of the microorganisms dispersion and subsequently, enhanced biodegradation rates (Environ. Sci. Technol. 42: 1131-1137, 2008; Environ. Sci. Technol. 46: 6790-6797, 2012). The objetive of this study was to evaluate the dispersal capacity of the chemotactic bacterium Pseudomonas putida G7 (cell dimensions: 1 µm x 3 µm) through bioreactors assays. The innovative experimental design consisted in a bioreactor system with two chambers separated by a membrane with specific pore sizes that restrict the bacterial dispersal (Environ. Sci. Technol. 49: 14368-14375, 2015). Different chemoffectors (salicylate, glucose, zero-valent iron nanoparticles [Environ. Pollut. 213:438-445, 2016] and the root exudates components:¿-aminobutyric acid (GABA) and citrate) were used to modulate the motile bacterial behavior throught restricted porosities (1 µm to 12 µm). The initial results were promising. The transport of P. putida G7 through the membrane occurred at higher rates than the control for the majority of chemoeffectors. Progress in this field will undoubtedly open up new possibilities for bioremediation processes in contaminated soils, improving the existing techniques for the treatment of poorly bioaccessible contaminants.

Chemoeffector-mediated bacterial motility and tactic swimming are major drivers for contaminant accessibility and biodegradation at submillimeter scales. In sand-filled percolated columns we tested how and to what degree chemoeffectors influenced bacterial transport and thereby promoted accessibility and degradation of distantly located 14C-naphthalene (NAH) at the centimeter scale. Sunflower root exudates and silver nanoparticles (AgNPs) were used as chemoeffectors to stimulate opposing effects of motility and tactic swimming of NAH-degrading Pseudomonas putida G7. Sunflower exudates prompted smooth bacterial movement and positive taxis, while AgNPs induced tortuous movement and repellent responses. Compared to chemoeffector-free controls exudates reduced deposition and stimulated bacterial transport during percolation experiments. AgNPs, however, provoked bacterial deposition and concomitant saturation of the collector surfaces (filter blocking) that led to progressively increased percolation of cells. Despite mechanistic differences, both motility patterns supported bacterial transport and promoted mineralization rates of NAH desorbing from a source placed at the column outlet. Observed mineralization rates in the presence of the chemoeffectors were 5-fold higher than those in their absence and similar to NAH-mineralization in well-stirred batch assays. Our results indicate that chemically mediated, small-scale bacterial motility patterns may become relevant for long-distance bacterial transport and the biodegradation of patchy contaminants at higher scales, respectively.

The biodegradation of hydrocarbons in the environment is often slow due to restricted bioavailability. Research performed during the last 20 years has shown possible pathways to increase the bioavailability of hydrocarbons without necessarily increasing the risk to the environment. Pollutant solubilization through (bio)surfactants, microbial transport, and attachment to pollutant interfaces can increase bioavailability, which translates into an enhancement of biodegradation rates. These strategies can not only be integrated into optimized bioremediation protocols that lead to lower decontamination endpoints in soils and sediments but also help to improve biodegradation in other environmental contexts, such as wastewater treatment and natural attenuation.

The bioavailability of polycyclic aromatic hydrocarbons (PAHs) in soil can be enhanced through a variety of microbial and plant functions, that can be incorporated into optimized bioremediation technologies. In this review, we examine the potential of (bio)surfactants, the chemotactic mobilization of pollutant-degrading bacteria, and the role of bacterial attachment, to enhance biodegradation of PAHs. Plants can also play an active role in enhancing bioavailability of PAHs through rhizosphere-related mechanisms associated to specific exudate components that affect bacterial chemotaxis, pollutant mobilization, and intra-aggregate bacterial growth.