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

Microbe cultivation guidelines to optimize rhamnolipid applications

In the growing landscape of interest in natural surfactants, selecting the appropriate one for specific applications remains challenging. The extensive, yet often unsystematized, knowledge of microbial surfactants, predominantly represented by rhamnolipids (RLs), typically does not translate beyond the conditions presented in scientific publications. This limitation stems from the numerous variables and their interdependencies that characterize microbial surfactant production. We hypothesized that a computational recipe for biosynthesizing RLs with targeted applicational properties could be developed from existing literature and experimental data. We amassed literature data on RL biosynthesis and micellar solubilization and augmented it with our experimental results on the solubilization of triglycerides (TGs), a topic underrepresented in current literature. Utilizing this data, we constructed mathematical models that can predict RL characteristics and solubilization efficiency, represented as logPRL = f(carbon and nitrogen source, parameters of biosynthesis) and logMSR = f(solubilizate, rhamnolipid (e.g. logPRL), parameters of solubilization), respectively. The models, characterized by robust R2 values of respectively 0.581-0.997 and 0.804, enabled the ranking of descriptors based on their significance and impact-positive or negative-on the predicted values. These models have been translated into ready-to-use calculators, tools designed to streamline the selection process for identifying a biosurfactant optimally suited for intended applications.

Critical evaluation of the performance of rhamnolipids as surfactants for (phyto)extraction of Cd, Cu, Fe, Pb and Zn from copper smelter-affected soil

Rhamnolipids are biosurfactants produced by bacteria belonging to the Pseudomonas genus. They are discussed to complex heavy metal cations stronger than cations of Fe, Ca, Mg. It is therefore suggested to employ rhamnolipids in phytoextraction where their addition to soil should result in preferential complexation of heavy metals that can be taken up by plants, thus enabling rapid and ecological clean-up of contaminated soil. In order to test this concept, we evaluated the rhamnolipid-mediated phytoextraction of heavy metal from soil collected from the vicinity of a copper smelter. The following aspects were investigated: i) selectivity of rhamnolipids towards Cu, Zn, Pb, Cd and Fe during soil washing; ii) phytoextraction efficiency of each ion with respect to the effective concentration of rhamnolipids; iii) possible phytotoxic effects; iv) effect of micro-sized polystyrene amendment. The experiments evaluated soil washing efficiency, BCR (Community Bureau of Reference) sequential extraction to determine the impact of rhamnolipids on the mobility of metal ions, phytoextraction with maize (Zea mays L.) and phytotoxic effects based on dry matter, chlorophyll fluorescence and content. The obtained results indicated that rhamnolipids lack desired selectivity towards heavy metal ions as Fe was complexed more efficiently by 80 % of the available rhamnolipids compared to priority pollutants like Zn, Cu, Pb, which were complexed by only 20 % of the tested rhamnolipids. With increased concentration of rhamnolipids, the soil washing efficiency increased and shifted in favour of Fe, reaching values of approx. 469 mg for Fe and only 118 mg in total of all tested heavy metals. Phytoextraction also favoured the accumulation of Fe, while Cd was not removed from the soil even at the highest applied rhamnolipid concentrations. Considering the selectivity of rhamnolipids and the costs associated with their production, our results suggest the need to search for other alternative (bio)surfactants with better selectivity and lower price.

A novel eco-friendly strategy for removing phenanthrene from groundwater: Synergism of nanobubbles and rhamnolipid

Nanobubbles (NBs), given their unique properties, could theoretically be paired with rhamnolipids (RL) to tackle polycyclic aromatic hydrocarbon contamination in groundwater. This approach may overcome the limitations of traditional surfactants, such as high toxicity and low efficiency. In this study, the remediation efficiency of RL, with or without NBs, was assessed through soil column experiments (soil contaminated with phenanthrene). Through the analysis of the two-site non-equilibrium diffusion model, there was a synergistic effect between NBs and RL. The introduction of NBs led to a reduction of up to 24.3 % in the total removal time of phenanthrene. The direct reason for this was that with NBs, the retardation factor of RL was reduced by 1.9 % to 15.4 %, which accelerated the solute replacement of RL. The reasons for this synergy were multifaceted. Detailed analysis reveals that NBs improve RL's colloidal stability, increase its absolute zeta potential, and reduce its soil adsorption capacity by 13.3 %-19.9 %. Furthermore, NBs and their interaction with RL substantially diminish the surface tension, contact angle, and dynamic viscosity of the leaching solution. These changes in surface thermodynamic and rheological properties significantly enhance the migration efficiency of the eluent. The research outcomes facilitate a thorough comprehension of NBs' attributes and their relevant applications, and propose an eco-friendly method to improve the efficiency of surfactant remediation.

Characterization and Production of a Biosurfactant Viscosin from Pseudomonas sp. HN11 and its Application on Enhanced oil Recovery During oily Sludge Cleaning

Biosurfactants are renewable resources with versatile applications on environmental bioremediation and industrial processes. Pseudomonas species are one of the promising biosurfactant producers. However, besides rhamnolipids, little is known about Pseudomonas-derived biosurfactants on solubilization of polycyclic aromatic hydrocarbons (PAHs) and oily sludge treatment. In this study, Pseudomonas sp. HN11-derived biosurfactant was purified by chromatographic methods and was characterized as viscosin via bioinformatic analysis, spectrometric and spectroscopic analyses, Marfey's method and (C-H)α NMR fingerprint matching approach. Viscosin is a potent biosurfactant with critical micelle concentration of 5.79 mg/L and is stable under various stresses. Moreover, viscosin was produced at 0.42 g/L at 48 h of liquid fermentation. Further data have shown that emulsifying agent viscosin is capable of promoting the solubilization of PAHs and displays enhanced oil recovery during oily sludge treatment. More specifically, viscosin has shown significantly enhanced solubilization on fluoranthene compared with control (0.04 mg/L), 2.21 mg/L and 1.27 mg/L fluoranthene was recovered from 100 mg/L and 200 mg/L viscosin treatment, respectively. However, only 200 mg/L viscosin has significantly enhanced the solubilization of phenanthrene (0.75 mg/L) and benzo[a]pyrene (0.51 mg/L) compared to each control (0.23 mg/L for phenanthrene and 0.09 mg/L for benzo[a]pyrene). Viscosin treatment of oily sludge (recovering of 0.58 g oil) has shown a significant oil recovery compared to that of control (recovering of 0.42 g oil). This study shows the great potential of viscosin-type biosurfactant on oily sludge treatment.

Simultaneous methanogenesis and denitrification in an anaerobic moving bed biofilm reactor for landfill leachate treatment: Ameliorative effect of rhamnolipids

In this study, an anaerobic moving bed biofilm reactor (AnMBBR) was developed for simultaneous methanogenesis and denitrification (SMD) to treat high-strength landfill leachate for the first time. A novel strategy using biosurfactant to ameliorate the inhibition of landfill leachate on the SMD performance was proposed and the underlying mechanisms were explored comprehensively. With the help of rhamnolipids, the chemical oxygen demand (COD) removal efficiency of landfill leachate was improved from 86.0% ± 2.9% to 97.5% ± 1.6%, while methane yields increased from 50.1 mL/g-COD to 69.6 mL/g-COD, and the removal efficiency of NO3--N was also slightly increased from 92.5% ± 1.9% to 95.6% ± 1.0%. The addition of rhamnolipids increased the number of live cells and enhanced the secretion of extracellular polymeric substances (EPS) and key enzyme activity, indicating that the inhibitory effect was significantly ameliorated. Methanogenic and denitrifying bacteria were enhanced by 1.6 and 1.1 times, respectively. Analysis of the microbial metabolic pathways demonstrated that landfill leachate inhibited the expression of genes involved in methanogenesis and denitrification, and that their relative abundance could be upregulated with the assistance of rhamnolipids addition. Moreover, extended Deraguin - Landau - Verwery - Oxerbeek (XDLVO) theory analysis indicated that rhamnolipids reduced the repulsive interaction between biofilms and pollutants with a 57.0% decrease in the energy barrier, and thus accelerated the adsorption and uptake of pollutants onto biofilm biomass. This finding provides a low-carbon biological treatment protocol for landfill leachate and a reliable and effective strategy for its sustainable application.

Valorization of oily sludge waste using biosurfactant-producing bacteria

Oily sludge generated by the petroleum industry is not only an environmental hazard, but since it contains crude oil too, it is a valuable resource as well. This study demonstrates a methodology for the valorization of the oily sludge that allows the recovery of oil fractions by the action of microbes producing surface-active metabolites. Two bacterial isolates were used in the study that were producing different biosurfactants, identified via FTIR analysis as well as through genomic mapping of the biosurfactant pathways using RAST, ANTISMASH 7.0, STRING databases. Serratia spp. AKBS12, produced a mono-rhamnolipid, while Acinetobacter spp. AKBS16, produced emulsan. Although recovery efficiency of both biosurfactants was similar, the recovery profile with respect to the class of hydrocarbons differed. The rhamnolipid produced by Serratia spp. AKBS12 extracted mono-chained paraffins and linear alkanes, while emulsan, produced by Acinetobacter spp. AKBS16 could extract heavier paraffins. The extraction procedure is simple and involves mixing the biosurfactant with oily sludge at a temperature of 30 °C with an incubation of 9 days. Sulphuric acid precipitation releases the oil trapped in the oily sludge. The study is the first step in developing user-friendly, innovative technologies that can be linked to the concept of a circular economy.

Polycyclic aromatic hydrocarbons remobilization from contaminated porous media by (bio)surfactants washing

Biosurfactants, surface-active agents produced by microorganisms, are increasingly studied for their potential use in soil remediation processes because they are more environmentally friendly than their chemically produced homologues. In this work, we report on the use of a crude biosurfactant produced by a bacterial consortium isolated from a PAHs-contaminated soil, compared with other (bio)surfactants (Tween80, Sodium dodecyl sulfate - SDS, rhamnolipids mix), to wash PAHs from a contaminated porous media. Assays were done using columns filled with sand or sand-clay mixtures (95:5) spiked with four model PAHs. The crude biosurfactant showed less adsorption to the [sand] and the [sand + clay] columns compared to Tween 80, SDS and the rhamnolipid mix. The biosurfactant showed the second best capacity to remove PAHs from the columns (as dissolved and particulate phases), both from [sand] and [sand + clay], after SDS when applied at lower concentrations than the other sufactants. The effluent concentrations of phenanthrene (PHE), pyrene (PYR) and benzo[a]pyrene (BAP) increased in the presence of the crude biosurfactant. Compared to the control experiment using only water, the global PAHs washed mass (amount of PAHs removed from the columns) increased between 9 and 1000 times for PHE and BAP in the [sand] column, and between 55 and 6000 times respectively for PHE and BAP in the [sand + clay] columns. Moreover, in the [sand + clay] columns, leaching of a part of the clays was observed in the SDS and the biosurfactant injections assays. This clay leaching resulted in higher PAHs removal, due not to desorption but rather to particulate transport. In the context of washing PAH-contaminated soils in biopiles or subsurface remediation, our results could help in sizing the remediation approach using an environmental friendly biosurfactant, before a pump-and-treat process.

Effect of rhamnolipids on the biodegradation of m-dichlorobenzene in biotrickling filters: Performance and mechanism

In this study, the effect of rhamnolipids (RL) on m-dichlorobenzene (m-DCB) removal and biofilm was investigated in two biotrickling filters (BTF) (BTF1: blank control; BTF2: RL addition). The critical micelle concentration (CMC) value of RL was 75.6 mg L^-1, and the RL could significantly improve the solubilization of m-DCB. The results showed that the optimal concentration of RL was 180 mg L^-1. The removal efficiency (RE) of m-DCB dropped by 42.4% for BTF1 no fed with RL and only 28.2% for BTF2 fed with RL when the inlet concentration increased from 200 to 1400 mg m^-3 at an empty bed time (EBRT) of 60 s. RL increased the secretion of extracellular polymers (EPS) and the ratio of Protein/Polysaccharide, which improved the mass transfer of m-DCB to the biofilm. RL also had a facilitating effect on catechol-1,2-dioxygenase (C₁₂O) enzyme activity. Furthermore, RL increased Zeta potential and facilitated microorganisms to form biofilm. The dominant microorganisms of microbial community were increased and the application of RL promoted the enrichment of them.

Rhamnolipid-Enhanced ZVI-Activated Sodium Persulfate Remediation of Pyrene-Contaminated Soil

In soil, polycyclic aromatic hydrocarbons (PAHs) are tightly bound to organic components, but surfactants can effectively transform them from a solid to a liquid phase. In this study, the biosurfactant rhamnolipid (RL) was selected as the eluent; shaking elution in a thermostatic oscillator improved the elution rate of pyrene, and the effects of RL concentration, temperature, and elution time on the elution effect were compared. After four repeated washings, the maximum elution rate was 75.6% at a rhamnolipid concentration of 20 g/L and a temperature of 45 °C. We found that 38 μm Zero-Valent Iron (ZVI) had a higher primary reaction rate (0.042 h^-1), with a degradation rate of 94.5% when 3 g/L ZVI was added to 21 mM Na₂S₂O₈ at 60 °C. Finally, electron paramagnetic resonance (EPR) detected DMPO-OH and DMPO-SO₄ signals, which played a major role in the degradation of pyrene. Overall, these results show that the combination of rhamnolipid elution and persulfate oxidation system effectively remediated pyrene-contaminated soil and provides some implications for the combined remediation with biosurfactants and chemical oxidation.

Study on the thermal washing of oily sludge used by rhamnolipid/sophorolipid binary mixed bio-surfactant systems

Demulsification and crude oil desorption are usually a necessary step for the treatment of oily sludge in the petroleum industry. In this study a binary mixed bio-surfactant (rhamnolipid / sophorolipid, RL/SL) was used to strengthen the removing oil efficiency for oily sludge by thermal washing method. Surface tension values of the single and the mixed surfactants were carried out to investigate the effect of mixing systems on reducing critical micelle concentrations (CMC) value. The models proposed by Clint, Rubingh and Gibbs et al. had been employed to interpret the formation of mixed micelles and synergism and found out in case of the mass ratios of 4:6 the synergism was the strongest in RL and SL mixed surfactant systems, which was selected as the washing agents to treat the oily sludge produced from Huabei oilfield. Through the optimization of oil washing process parameters, the oil removal rate reached the maximum value (95.66%, residual oil rate 1.98%) at the condition of heating temperature of 45 °C, detergents concentration of 500 mg/L, washing time of 3 h, liquid/solid mass ratio of 1:4, stirring speed of 300 r/min, and washing 4 times. The factors affecting the oil washing effect were analyzed from the composition and performance characteristics of oily sludge samples, washing oil system and washing process parameters. The results showed that low oil content of oily sludge, small specific surface area, strong wetting and solubilization of the oil-washing system all can increase the oil-washing effect and the washing time and temperature had a great influence on the oil-washing effect. Compared with the results of other researchers, the oil washing temperature and the concentration of oil washing agent were significantly lower and high oil removal rate and low residual oil rate were obtained in this study. It was confirmed that thermal oil washing method using RT/SL binary bio-surfactant mixing system was proved to a high-efficiency, low-consumption and wide range of applications technology.

Microbial biosurfactants: a review of recent environmental applications

Microbial biosurfactants are low-molecular-weight surface-active compounds of high industrial interest owing to their chemical properties and stability under several environmental conditions. The chemistry of a biosurfactant and its production cost are defined by the selection of the producer microorganism, type of substrate, and purification strategy. Recently, biosurfactants have been applied to solve or contribute to solving some environmental problems, with this being their main field of application. The most referenced studies are based on the bioremediation of contaminated soils with recalcitrant pollutants, such as hydrocarbons or heavy metals. In the case of heavy metals, biosurfactants function as chelating agents owing to their binding capacity. However, the mechanism by which biosurfactants typically act in an environmental field is focused on their ability to reduce the surface tension, thus facilitating the emulsification and solubilization of certain pollutants (in-situ biostimulation and/or bioaugmentation). Moreover, despite the low toxicity of biosurfactants, they can also act as biocidal agents at certain doses, mainly at higher concentrations than their critical micellar concentration. More recently, biosurfactant production using alternative substrates, such as several types of organic waste and solid-state fermentation, has increased its applicability and research interest in a circular economy context. In this review, the most recent research publications on the use of biosurfactants in environmental applications as an alternative to conventional chemical surfactants are summarized and analyzed. Novel strategies using biosurfactants as agricultural and biocidal agents are also presented in this paper.

Determining the bioavailability of benzo(a)pyrene through standardized desorption extraction in a certified reference 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.

Identification of pyrene degraders via DNA-SIP in oilfield soil during natural attenuation, bioaugmentation and biostimulation

Pyrene is a model contaminant of high molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs), which are compounds that have potential carcinogenic effects and pose a serious threat to human health. Finding effective pyrene-degrading bacteria is crucial for removing PAHs from soil. In this study, DNA-based stable isotope probing (DNA-SIP) technology was used to investigate pyrene degraders in PAH-contaminated oilfield soil during natural attenuation (NA), bioaugmentation (BA) and biostimulation (BS). The results show that BA played an important role in pyrene degradation with the highest pyrene removal rate (~95%) after 12 days incubation, followed by removal rates of ~90% for NA and ~30% for BS. In addition, 6 novel pyrene degraders were identified, while 12 well-known PAH degraders were demonstrated to participate in the biodegradation of pyrene. Additionally, the external homologous strains introduced during BA promoted the degradation of pyrene, but not by directly participating in the metabolism of the target compound. Rhamnolipid supplementation during BS promoted the involvement of more microorganisms in the degradation of pyrene, which was beneficial to identifying more pyrene degraders via DNA-SIP. These findings provide new insight into the effects of external homologous strains and supplementary rhamnolipids on pyrene degradation.

Biosurfactant matrix for the environmental clean-up of dichlorophenol from aqueous medium and soil

Chlorophenols are used in many industries for their importance in preservation and herbicide preparation even though they possess high-risk factors. The prolonged usage of these compounds makes it very complicated to remove them from water and soil by conventional treatment methods. Biosurfactant are the promising structures with the ability to remove contaminants effectively. In this work, an attempt has been made to eliminate 2,4-dichlorophenol from soil and water using amino acid-enhanced cationic biosurfactant obtained from Bacillus axarquiensis. The produced BS has the ability to reduce the surface tension to 30.0 mN m^-1. From RSM, the optimum conditions for the maximum production of BS were obtained at time 95 h; pH 7; temperature 35 °C, and concentration of substrate 5%. The BS was immobilized using a solid support matrix for the stability. The environmental factors such as temperature and pH have no effect on the matrix used and found to be viable even under extreme conditions. The removal efficiency was achieved in the range of 93-96% from water and 80-85% from soil. Additionally, the recyclability and reusability of the matrix were also analyzed, and it withstands up to 8 cycles. As a result, the significance of biosurfactant by enhancing the amino acid content was explored in remediation technology.

Remediation of soils and sediments polluted with polycyclic aromatic hydrocarbons: To immobilize, mobilize, or degrade?

Polycyclic aromatic hydrocarbons (PAHs) are generated due to incomplete burning of organic substances. Use of fossil fuels is the primary anthropogenic cause of PAHs emission in natural settings. Although several PAH compounds exist in the natural environmental setting, only 16 of these compounds are considered priority pollutants. PAHs imposes several health impacts on humans and other living organisms due to their carcinogenic, mutagenic, or teratogenic properties. The specific characteristics of PAHs, such as their high hydrophobicity and low water solubility, influence their active adsorption onto soils and sediments, affecting their bioavailability and subsequent degradation. Therefore, this review first discusses various sources of PAHs, including source identification techniques, bioavailability, and interactions of PAHs with soils and sediments. Then this review addresses the remediation technologies adopted so far of PAHs in soils and sediments using immobilization techniques (capping, stabilization, dredging, and excavation), mobilization techniques (thermal desorption, washing, electrokinetics, and surfactant assisted), and biological degradation techniques. The pros and cons of each technology are discussed. A detailed systematic compilation of eco-friendly approaches used to degrade PAHs, such as phytoremediation, microbial remediation, and emerging hybrid or integrated technologies are reviewed along with case studies and provided prospects for future research.

Combined application of rhamnolipid and agricultural wastes enhances PAHs degradation via increasing their bioavailability and changing microbial community in contaminated soil

Either biosurfactants or agricultural wastes were frequently used to enhance degradation of PAHs in soil, but there is still not clear whether combined application of biosurfactants and agricultural wastes is more efficient. Rhamnolipid and/or agricultural wastes (mushroom substrate or maize straw) were mixed with PAHs-contaminated soil to explore their performances in the removal of PAHs. The present study showed that rhamnolipid combined with mushroom substrate (MR, 30.36%) or maize straw (YR, 30.76%) significantly enhanced the degradation of soil PAHs compared with single application of mushroom substrate (M, 25.53%) or maize straw (Y, 25.77%) or no addition (19.38%). The addition of agricultural wastes significantly (p < 0.001) enhanced concentration of dissolved organic carbon (DOC) in soil. The combined application obviously improved the bioavailability of PAHs in soils and exhibited synergistic effects on concentration of organic acid-soluble HMW PAHs and the degradation rate of total HMW PAHs. Meanwhile, the combined application significantly (p < 0.01) enhanced the abundance of dominant bacterial and fungal genera being connected with PAHs degradation. The removal rate of PAHs was positively correlated with the dominant genera of bacteria (r = 0.539-0.886, p < 0.05) and fungi (r = 0.526-0.867, p < 0.05) related to PAHs degradation. Overall, the combined application exhibited a better performance in the removal of PAHs in contaminated soil via increasing their bioavailability and changing microbial communities in soil.

Salt selected for hydrocarbon-degrading bacteria and enhanced hydrocarbon biodegradation in slurry bioreactors

Hydrocarbon and salt contamination of surface and groundwater resources often co-occur from oil production activities. However, salt is often considered as a potential inhibitor of microbial activity. The feasibility of microbiome-based biotechnologies to treat the hydrocarbon contamination is contingent on the ability of the indigenous community to adapt to saline conditions. Here, we demonstrate enhanced hydrocarbon biodegradation in soil slurries under saline conditions of up to ~1 M (5%) compared to non-saline systems and the underlying causes. The mineralization extent of hexadecane was enhanced by salinity in the absence of nutrients. Salinity, similar to nutrients, enhanced the mineralization but through ecological selection. Microbial community analysis indicated a significant enrichment of Actinobacteria phylum and an increase in the absolute abundance of the hydrocarbon-degrading Dietzia genus, but a decrease in the total population size with salinity. Moreover, the in situ expression of alkane hydroxylases genes of Dietzia was generally increased with salinity. The data demonstrate that indigenous halotolerant hydrocarbon degraders were enriched, and their hydrocarbon degradation genes upregulated under saline conditions. These findings have positive implications for engineered biotreatment approaches for hydrocarbons in saline environments such as those affected with produced waters and oil sands tailing ponds.

Synergistic Effect of Nanofluids and Surfactants on Heavy Oil Recovery and Oil-Wet Calcite Wettability

In recent years, unconventional oils have shown a huge potential for exploitation. Abundant reserves of carbonate asphalt rocks with a high oil content have been found; however, heavy oil and carbonate minerals have a high interaction force, which makes oil-solid separation difficult when using traditional methods. Although previous studies have used nanofluids or surfactant alone to enhance oil recovery, the minerals were sandstones. For carbonate asphalt rocks, there is little research on the synergistic effect of nanofluids and surfactants on heavy oil recovery by hot-water-based extraction. In this study, we used nanofluids and surfactants to enhance oil recovery from carbonate asphalt rocks synergistically based on the HWBE process. In order to explore the synergistic mechanism, the alterations of wettability due to the use of nanofluids and surfactants were studied. Nanofluids alone could render the oil-wet calcite surface hydrophilic, and the resulting increase in hydrophilicity of calcite surfaces treated with different nanofluids followed the order of SiO2 > MgO > TiO2 > ZrO2 > γ-Al2O3. The concentration, salinity, and temperature of nanofluids influenced the oil-wet calcite wettability, and for SiO2 nanofluids, the optimal nanofluid concentration was 0.2 wt%; the optimal salinity was 3 wt%; and the contact angle decreased as the temperature increased. Furthermore, the use of surfactants alone made the oil-wet calcite surface more hydrophilic, according to the following order: sophorolipid (45.9°) > CTAB (49°) > rhamnolipid (53.4°) > TX-100 (58.4°) > SDS (67.5°). The elemental analysis along with AFM and SEM characterization showed that nanoparticles were adsorbed onto the mineral surface, resulting in greater hydrophilicity of the oil-wet calcite surface, and the roughness was related to the wettability. Surfactant molecules could aid in the release of heavy oil from the calcite surface, which exposes the uncovered calcite surface to its surroundings; additionally, some surfactants adsorbed onto the oil-wet calcite surface, and the combined role made the oil-wet calcite surface hydrophilic. In conclusion, the study showed that hybrid nanofluids showed a better effect on wettability alteration, and the use of nanofluids and surfactants together resulted in synergistic alteration of oil-wet calcite surface wettability.

A rhamnolipid biosurfactant increased bacterial population size but hindered hydrocarbon biodegradation in weathered contaminated soils

Surfactants are used to enhance the bioavailability of recalcitrant residual petroleum contamination during bioremediation. However, surfactants in some cases inhibit biodegradation, which is often attributed to their toxicity. Herein, we show that a rhamnolipid biosurfactant likely served as a carbon source and exhibited physiological inhibition on petroleum biodegradation. The addition of biosurfactants in mixed, batch, slurry bioreactors with soils from a petroleum-contaminated site led to a dose-dependent shift in the microbial community with a decrease in diversity and increase in population size and delayed biodegradation. Microbial community analysis indicated the enrichment of Alphaproteobacteria affiliated taxa such as Sphingomonadaceae in systems amended with biosurfactant. The diversity was significantly lower in systems with higher doses of biosurfactants compared to systems without biosurfactant. Droplet Digital PCR indicated a 30-90 fold increase in 16S rRNA copy numbers in systems with higher doses of biosurfactant than control systems without surfactant and nutrients, whereas the nutrient amendment alone led to a two-fold increase in population size. Total petroleum hydrocarbon analysis showed that the biodegradation extent was negatively impacted by rhamnolipid at the highest dose compared to lower doses (23% vs. 40%) or without the biosurfactant. Indigenous isolates cultivated from the oil-amended soil exhibited growth on rhamnolipid as a sole carbon source. A novel insight gained is how dose-dependent responses of microbial communities to biosurfactants alter the biodegradation time profile of hydrocarbons. The study highlights the significance of microbial assessment prior to surfactant-mediated bioremediation practices.

Enhancement of PAHs biodegradation in biosurfactant/phenol system by increasing the bioavailability of PAHs

The poor bioavailability of polycyclic aromatic hydrocarbons (PAHs) is the main limiting factor for their biodegradation in contaminated sites. The addition of biosurfactant is an effective method for enhancing the bioavailability of PAHs. Suitable low molecular weight (LMW) organic matters have been shown to increase the bioavailability of PAHs. Therefore, we investigated the effect of phenol, which often co-exists with PAHs, on the biodegradation of PAHs in biosurfactant solution. The results show that the critical micelle concentration (CMC) of the biosurfactant decreased after phenol was added. The formation of mixed micelles resulted in enhancement of PAHs dissolution. The weight solubilization ratio (WSR) values of biosurfactant for Phe, Pyr and BaP in phenol solution are approximately 1.34, 1.40 and 1.67 times that of the control group, respectively. Phenol, therefore, can assist biosurfactant to increase the availability of PAHs by microbes. The bioavailability of PAHs in sludge increased from 27.7% to 43.1% after the biosurfactant was added, and reached a maximum of 49.2%, following the simultaneous addition of phenol and biosurfactant. Phenol also improved the degradation of PAHs by Stenotrophomonas sp. N5 in biosurfactant solution.

Sequential solvent extraction as a tool for evaluating hydrocarbons speciation in soil after electrochemical treatment

Soluble and total extractable concentrations used for predicting contaminants' environmental fate may lead to uncertainties due to the lack of understanding of soil-contaminants interactions. The present study focuses on the influence of a controlled electric field on the distribution of polycyclic aromatic hydrocarbons in soil samples evaluated through a speciation scheme. Soil samples were spiked with 25,000 mg (hexadecane, phenanthrene, and pyrene 100:1:1 w/w) per kg of soil, and speciation of hydrocarbons was determined by employing a novel Sequential Solvent Extraction procedure, resulting in five fractions: soluble, pseudosoluble, desorbable, extractable, and sequestered. The distribution of hydrocarbons was then changed through the application of an electric field (72 h, 0.708 mA cm^-2, 2.95 ± 0.13 V cm^-1), which modified the interactions in the soil-water interface. The electrochemical treatment significantly increased the pyrene soluble, desorbable and sequestered fractions by 340, 1.3 and 19-fold (p < 0.05); the hexadecane soluble fraction increased in 6-fold (p < 0.05) and the phenanthrene desorbable fraction increased in 1.3-fold (p < 0.05). The use of the speciation scheme proposed in this study provides a wider view of hydrocarbons distribution in soils, rather than using water-soluble or total extractable concentrations. Finally, this speciation scheme is proposed as a tool to evaluate the environmental fate of organic contaminants in soils.

Biosurfactant production from newly isolated Rhodotorula sp.YBR and its great potential in enhanced removal of hydrocarbons from contaminated soils

One of the very promising methods in the field of bioremediation of hydrocarbons is the application of biosurfactant- producing microorganisms based on the use of wastewater as renewable substrates of culture media, contributing to the reduction of costs. With this aim, the production, characterization and properties of the yeast strain YBR producing a biosurfactant newly isolated from an oilfield in Algeria, using wastewater from olive oil mills (OOMW) as a substrate for a low-cost and effective production, have been investigated. Screening of biosurfactant production was carried out with different tests, including emulsification index test (E24), drop collapse test, oil spreading technique and measurement of surface tension (ST). The isolated yeast strain was found to be a potent biosurfactant producer with E24 = 69% and a significant reduction in ST from 72 to 35 mN m^-1. The study of the cultural, biochemical, physiological and genetic characteristics of the isolate allowed us to identify it as Rhodotorula sp. strain YBR. Fermentation was carried out in a 2.5 L Minifors Bioreactor using crude OOMW as culture medium, the E₂₄ value reached 90% and a reduction of 72 to 35 mN m^-1 in ST. A biosurfactant yield = 10.08 ± 0.38 g L^-1 was recorded. The characterization by semi-purification and thin layer chromatography (TLC) of the crude extract of biosurfactant showed the presence of peptides, carbohydrates and lipids in its structure. The crude biosurfactant exhibited interesting properties such as: low critical micellar concentration (CMC), significant reduction in ST and strong emulsifying activity. In addition, it has shown stability over a wide range of pH (2-12), temperature (4-100 °C) and salinity (1-10%). More interestingly, the produced biosurfactant has proven to be of great potential application in the remobilization of hydrocarbons from polluted soil with a removal rate of greater than 95%.

Enhanced treatment of the oil-contaminated soil using biosurfactant-assisted washing operation combined with H2O2-stimulated biotreatment of the effluent

A real crude oil-contaminated soil was treated using a two-step method: biosurfactant-assisted soil washing and the biostimulated biotreating of the effluent. The mixture of surfactin and rhamnolipid could enhance the TPH removal from an oil-contaminated soil (32 g/kg) in the soil washing operation. 86% of TPH was removed from the oil-contaminated soil in the soil washing operation under the mixed biosurfactant (surfactin + rhamnolipid) of 0.6 g/L, the soil/water ratio of 20 w/v%, the temperature of 30 °C, and the washing time of 24 h, leaving an effluent containing 5028 mg/L TPH. The effluent was efficiently biotreated in the bioprocess with 5 g/L acclimate biomass daily stimulated with 0.1 mM H₂O₂, and the concentrtion of TPH decreased to 26 mg/L within 17 d corresponding a TPH biodegradation over 99%. The biostimulation with H₂O₂ caused the production of a high amount of peroxidase that could accelerate the biodegradation of TPH. Accordingly, the findings suggest that the biosurfactant-assisted washing operation combined with the H₂O₂-stimulated biodegradation process could be an enhanced green method for efficient treatment of the heavy oil-contaminated soils.

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.

Application of alkyl polyglycosides for enhanced bioremediation of petroleum hydrocarbon-contaminated soil using Sphingomonas changbaiensis and Pseudomonas stutzeri

Bioremediation is considered a cost-effective and environmentally sound method for degradation of petroleum hydrocarbons in contaminated soils. This study investigated the effects of biosurfactant alkyl polyglycosides (APG) on enhanced biodegradation of petroleum hydrocarbon-contaminated soils using Sphingomonas changbaiensis and Pseudomonas stutzeri and explored the mechanism responsible for the enhanced petroleum hydrocarbon degradation. To accomplish this, the following treatments were evaluated: (1) bioaugmentation with Sphingomonas changbaiensis; (2) bioaugmentation with Pseudomonas stutzeri; (3) a combination of Sphingomonas changbaiensis and APG; and (4) a combination of Pseudomonas stutzeri and APG. The results showed that the degradation rates of total petroleum hydrocarbons (TPH) in contaminated soil samples bioaugmented with S. changbaiensis and P. stutzeri for 30 days were 39.2 ± 1.9% and 47.2 ± 1.2%, respectively. The addition of biosurfactant APG enhanced the bioremediation processes and improved the biodegradation rates. The biodegradation rate at 1.5 g/kg APG in soil samples bioaugmented with S. changbaiensis was 52.1 ± 2.0%, while the rate at 1.5 g/kg APG in soil samples bioaugmented with P. stutzeri was 59.0 ± 1.8%. The half-life decreased from 39.7 d to 24.5 d and from 29.6 to 20.1 d when the dosage of APG was 1.5 g/kg in contaminated soil samples bioaugmented with S. changbaiensis and P. stutzeri, respectively. Mechanism studies showed that the addition of APG can increase the TPH solubility, promote the sorption of TPH onto microbial cells and subsequent trans-membrane transport by APG-induced structural changes, stimulate microbial activities and participate in the co-metabolism. Therefore, the combination of bioaugmentation and APG is an effective method for remediation of petroleum hydrocarbon-contaminated soil.

Impact of bacterial motility on biosorption and cometabolism of pyrene in a porous medium

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.

Removal of polycyclic aromatic hydrocarbons during anaerobic biostimulation of marine sediments

This study proposes the supplementation of digestate, fresh organic fraction of municipal solid waste (OFMSW) and a nutrient solution during the anaerobic biostimulation of marine sediments contaminated by polycyclic aromatic hydrocarbons (PAHs). The experimental activity was conducted with four PAHs (i.e. phenanthrene, anthracene, fluoranthene and pyrene) under controlled mesophilic conditions (37 ± 1 °C) in 100 mL serum bottles maintained at 130 rpm. After 120 days of incubation, the highest total PAH degradation of 53 and 55% was observed in the experiments with digestate + nutrients and OFMSW + nutrients, respectively. Phenanthrene was the most degraded PAH and the highest removal of 69% was achieved with OFMSW + nutrients. The anaerobic PAH degradation proceeded through the accumulation of volatile fatty acids and the production of hydrogen and methane as biogas constituents. The highest cumulative biohydrogen production of 80 mL H₂·g VS^-1 was obtained when OFMSW was used as the sole amendment, whereas the highest biomethane yield of 140 mL CH₄·g VS^-1 was obtained with OFMSW + nutrients. The evolution of PAH removal during anaerobic digestion revealed a higher impact of the methanogenic phase rather than acidogenic phase on PAH degradation.

Effects of biochar and organic substrates on biodegradation of polycyclic aromatic hydrocarbons and microbial community structure in PAHs-contaminated soils

A incubation experiment was conducted to investigate whether combined amendment of biochar (B) and compost (CP), mushroom residue (M) and corn straw (Y) could enhance biodegradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils. After 77 days of incubation, both B + M and B + Y significantly (p < 0.01) increased removal rate of PAHs compared with amendment of biochar alone. However, B+CP resulted in a significant (p < 0.01) decreasing of PAHs removal. Compared with no biochar and no organic substrates addition (CK) and B, both B+M and B+Y significantly (p < 0.01) enhanced concentrations of dissolved organic carbon (DOC) and were favorable for the microbial growth reflected by microbial biomass carbons (MBC) and emission of carbon dioxide. Redundancy analysis (RDA) indicated that B + CP, B + M and B + Y separated the bacterial community compared with CK and B. However, the community composition structure in B + CP was different from that of B + M and B + Y. Moreover, the abundance of some PAHs degraders and PAH degradation genes predicted by PICRUSt software was promoted by B + M or B + Y, whereas that was inhibited under B + CP. The present study suggested that both B + M and B + Y could accelerate biodegradation of PAHs mainly through increasing the concentration of DOC and the abundances of microbial PAH degraders in soils.

Different bioavailability of phenanthrene to two bacterial species and effects of trehalose lipids on the bioavailability

Effects of trehalose lipids produced from Rhodococcus erythropolis ATCC 4277 on phenanthrene (PHE) mineralization by two soil microorganisms were investigated. Biodegradation experiments were conducted, with and without the biosurfactant, in three batch systems: water, soil, and soil-water slurry. PHE sorption to the soil did not limit the mineralization by the test microorganisms, Pseudomonas strain R (PR) and Sphingomonas sp. strain P5-2 (SP5-2). Both microorganisms, however, demonstrated significant difference in the PHE mineralization capability in the systems. While SP5-2 mineralized PHE faster than PR in liquid culture, PR having more hydrophobic surface greatly exceeded SP5-2 in ability to access soil-sorbed PHE. While the addition of the biosurfactant little affected the apparent cell hydrophobicity of SP5-2, it substantially improved PHE mineralization by this strain in all systems tested. Contrary to SP5-2, the apparent cell hydrophobicity was significantly stimulated with increasing concentration of the biosurfactant for PR. However, the biosurfactant had no significant effect on PHE mineralization by this microorganism. The results demonstrated that the addition of the biosurfactant may have great potential for remediation of sites contaminated with polycyclic aromatic hydrocarbons but its effects and benefits may be dependent on characteristics of microorganisms involved and environmental conditions.

Bacteria involved in biodegradation of creosote PAH - A case study of long-term contaminated industrial area

Polycyclic aromatic hydrocarbons (PAH) contained in creosote oil are particularly difficult to remove from the soil environment. Their hydrophobic character and low bioavailability to soil microorganisms affects their rate of biodegradation. This study was performed on samples of soil that were (for over forty years) subjected to contamination with creosote oil, and their metagenome and physicochemical properties were characterized. Moreover, the study was undertaken to evaluate the biodegradation of PAHs by autochthonous consortia as well as by selected bacteria strains isolated from long-term contaminated industrial soil. From among the isolated microorganisms, the most effective in biodegrading the contaminants were the strains Pseudomonas mendocina and Brevundimonas olei. They were able to degrade more than 60% of the total content of PAHs during a 28-day test. The biodegradation of these compounds using AT7 dispersant was enhanced only by Serratia marcescens strain. Moreover, the addition of AT7 improved the effectiveness of fluorene and acenaphthene biodegradation by Serratia marcescens 6-fold. Our results indicated that long-term contact with aromatic compounds induced the bacterial strains to use the PAHs as a source of carbon and energy. We observed that supplementation with surfactants does not increase the efficiency of hydrocarbon biodegradation.

Effect of Rhamnolipids on Microbial Biomass Content and Biochemical Parameters in Soil Contaminated with Coal Tar Creosote

The objective of the present study was to compare the effect of rhamnolipids on the microbial biomass content and the activity of dehydrogenases (DHA), acid phosphatase (ACP), alkaline phosphatase (ALP), and urease (URE) in soil contaminated with two types of coal tar creosote: type C and type GX-Plus. The experiment was carried out on samples of sandy clay loam under laboratory conditions. Coal tar creosote was added to soil samples at a dose of 0 and 10 g·kg⁻¹ DM, along with rhamnolipids at a dose of 0, 10, 100, and 1000 mg·kg⁻¹ DM. The humidity of the samples was brought to 60% maximum water holding capacity, and the samples were incubated at 20°C. Microbial and biochemical parameters were determined on days 1, 7, 21, and 63. The obtained results demonstrated that the addition of rhamnolipids did not result in any significant changes in the activity of the determined parameters in the uncontaminated soil. However, it was observed that the application of these biosurfactants, particularly at the dose of 1000 mg·kg⁻¹ DM, largely decreased the effect of coal tar creosote on the determined parameters. Moreover, the microbial biomass and the activity of ALP and URE were found to be the best indicator of bioremediation of soil contaminated with coal tar creosote.

Biosurfactant: A new frontier for greener technology and environmental sustainability

Petroleum hydrocarbons, oil, heavy metals pollution is becoming additional severe problem due to the growing call for crude oil and crude oil products related products in several fields of application. Such pollution have fascinated much considerations and attractions as it leads to ecological damages in both marines, aquatic and terrestrial ecosystems. Thus, different techniques including chemical surfactants and complex technologies have been proposed for their clean up from the environment, which in turn has detrimental effects on the environment. As of late, biosurfactant compounds have added much deliberation since they are considered as a reasonable option and eco-accommodating materials for remediation technology. The present society is confronting a few difficulties of usage, authorizing ecological protection and environmental change for the next generations. Biosurfactants hold the special property of minimizing and reducing the interfacial tension of liquids. Such features endure biosurfactants to afford a major part in emulsification, de-emulsification, biodegradability, foam formation, washing performance, surface activity, and detergent formulation, which have potential applications in the diverse industrial set-up. Conversations on cost-effective technologies, renewable materials, novel synthesis, downstream, upstream, emerging characterization techniques, molecular, and genetical engineering are substantial to produce biosurfactant of quality and quantity. Therefore, greater attention is being paid to biosurfactant production by identifying their environmental, and biotechnological applications. Be that as it may, the extravagant cost drew in with biosurfactants biotechnological synthesis and recovery can hamper their application in those areas. Notwithstanding these costs, biosurfactants can be used as these parts shows outstandingly high benefits that can at present beat the expenses incurred in the initial purification and downstream processes. Biosurfactant production by microorganisms is relatively considered one of the crucial know-how for improvement, growth, advancement, and environmental sustainability of the 21st century. There is a developing conversation around environmental safety and the significant role that biosurfactants will progressively play soon, for instance, the use of renewable by-products as substrates, potential reduction, re-use and recycling of waste and waste products. The review confers the usefulness of biosurfactants in the removal of environmental contaminants and, consequently, expanding environmental safety and drive towards greener technology.

Biodegradation of mixed polycyclic aromatic hydrocarbons by pure and mixed cultures of biosurfactant producing thermophilic and thermo-tolerant bacteria

Applicability of thermophilic and thermo-tolerant microorganisms for biodegradation of polycyclic aromatic hydrocarbons (PAHs) with low water solubility is an interesting strategy for improving the biodegradation efficiency. In this study, we evaluated utility of thermophilic and thermo-tolerant bacteria isolated from Unkeshwar hot spring (India) for biodegradation of four different PAHs. Water samples were enriched in mineral salt medium (MSM) containing a mixture of four PAHs compounds (anthracene: ANT, fluorene: FLU, phenanthrene: PHE and pyrene: PYR) at 37 °C and 50 °C. After growth based screening, four potent strains obtained which were identified as Aeribacillus pallidus (UCPS2), Bacillus axarquiensis (UCPD1), Bacillus siamensis (GHP76) and Bacillus subtilis subsp. inaquosorum (U277) based on the 16S rRNA gene sequence analysis. Degradation of mixed PAH compounds was evaluated by pure as well as mixed cultures under shake flask conditions using MSM supplemented with 200 mg/L concentration of PAHs (50 mg/L of each compound) for 15 days at 37 °C and 50 °C. A relatively higher degradation of ANT (92%- 96%), FLU (83% - 86%), PHE (16% - 54%) and PYR (51% - 71%) was achieved at 50 °C by Aeribacillus sp. (UCPS2) and mixed culture. Furthermore, crude oil was used as a substrate to study the degradation of same PAHs using these organisms which also revealed with similar results with the higher degradation at 50 °C. Interestingly, PAH-degrading strains were also positive for biosurfactant production. Biosurfactants were identified as the variants of surfactins (lipopeptide biosurfactants) based on analytical tools and phylogenetic analysis of the surfactin genes. Overall, this study has shown that hot spring microbes may have a potential for PAHs degradation and also biosurfactant production at a higher temperature, which could provide a novel perspective for removal of PAHs residues from oil contaminated sites.