Rhamnolipid Concentration Research Articles

Enhanced low-cost optimization strategies for antimicrobial rhamnolipid production by Pseudomonas aeruginosa PAO1

The use of advanced biotechnological methods with agro-industrial waste is an outstanding approach for low-cost production. This study aimed to enhance rhamnolipid production by Pseudomonas aeruginosa PAO1 using agro-industrial waste and statistical modeling. The Box-Behnken model significantly increased rhamnolipid yield by 7.6-fold (661.7 ± 2.57 mg/L) at 40 g/L glycerol, 20 g/L glucose, 30 g/L peptone, pH 9, and 40 °C for 5 days. The replacement of glycerol and glucose with prickly pear peel (Opuntia ficus-indica) resulted in an increase of rhamnolipids to 847.3 ± 4.5 mg/L. The rhamnolipid concentration increased to 1521 mg/L after optimizing the air flow rate and agitation speed in the pilot-scale bioreactor. The rhamnolipids showed antimicrobial activity against Candida albicans, Escherichia coli, and Staphylococcus aureus strains. A concentration of 115 mg/L resulted in a 42.3 % reduction of the surface tension from 65 to 37.5 mN/m. The optimization processes using agricultural waste are promising approaches for future low-cost industrial production.

Adhesion and adsorption properties of water, ethanol, rhamnolipid and Triton X-165 mixture in quartz-mixture-air system

Wetting behaviour of the solutions including water, ethanol (ET), rhamnolipid (RL) and Triton X-165 (TX165) with regard to the quartz surface tension was investigated. The investigations were based on the measurements of the contact angle on the quartz surface for the solutions at the constant ET and RL concentrations and variable of TX165. The obtained results were considered with regard to the adhesion work of the solution to the quartz surface, adsorption of particular components of the solution at the quartz-air, quartz-solution and solution-air interfaces as well as to solution interactions across the quartz-solution interface. The considerations of the adhesion work and parameter of solution interactions across the quartz-solution interface as well as the adsorption of ET, RL and TX165 at different interfaces were based on the quartz surface tension and its components determined by different methods. These research results allow to establish the correlations between the adhesion work and the adsorption film pressure at the quartz-air interface and those between the parameter of interactions at the quartz-solution interface and its tension. The mechanism of the adsorption of particular components of the solution at the interfaces and their standard Gibbs free energy of adsorption were also discussed.

Antibacterial mechanism of Pseudomonas aeruginosa UKMP14T rhamnolipids against multidrug resistant Acinetobacter baumannii

Rhamnolipids, a major category of glycolipid biosurfactant, have recently gained enormous attention in medical field because of their relevance as effective antibacterial agents against a wide variety of pathogenic bacteria. Our previous studies have shown that rhamnolipids from an environmental isolate of Pseudomonas aeruginosa UKMP14T possess antibacterial, anti-adhesive and anti-biofilm activity against multidrug-resistant ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter sp.) pathogens. However, the mechanism of their antibacterial action remains unclear. Thus, this study aimed to elucidate the mechanism of the antibacterial action of P. aeruginosa UKMP14T rhamnolipids by studying the changes in cells of one of the ESKAPE pathogens, Acinetobacter baumannii, which is the most difficult strain to kill. Results revealed that rhamnolipid treatment rendered A. baumannii cells more hydrophobic as evaluated through contact angle measurements. It also induced the release of cellular proteins measuring 510 μg/mL at a rhamnolipid concentration of 1000 μg/mL. In addition, rhamnolipids were found to be bactericidal in their action as they could permeate the inner membranes, leading to a leak-out of nucleotides. More than 50 % of the cells were found to be killed upon 1000 μg/mL rhamnolipid treatment as observed through fluorescence microscopy. Other cellular changes such as irregular shape and size, membrane perturbations, clumping, shrinkage and physical damage were clearly visible in SEM, FESEM and laser micrographs. Furthermore, rhamnolipid treatment inhibited the levels of acyl-homoserine lactones (AHLs) in A. baumannii, which are vital for their biofilm formation and virulence. The obtained results indicate that P. aeruginosa UKMP14T rhamnolipids target outer and inner bacterial membranes through permeation, including physical damage to the cells, leading to cell leakage. Furthermore, AHL inhibition appears to be the mechanism behind their anti-biofilm action. All these observations can be correlated to rhamnolipids’ antibacterial effect against A. baumannii.

The efficiency of micellar solubilization of naphthalene from aqueous solutions using rhamnolipid as a biological surfactant according to NMR diffusometry.

The micellar solubilization of naphthalene from its saturated aqueous solutions using the biosurfactant rhamnolipid was studied. Using the NMR diffusion method, selective measurements of the self-diffusion coefficients of molecules of all components of the solution-naphthalene, rhamnolipid, and water-were carried out at various rhamnolipid concentrations from 0.06 to 100 g/L. Based on the results of diffusometry, the distribution of naphthalene molecules between the states free in solution and states bound by micelles was found. With an increase in the concentration of rhamnolipids, the proportion of bound naphthalene molecules increases from 50% at CRL = 2g/L to 100% at CRL ≥ 50 g/L. The micelle-water partition coefficient Km and the molar solubilization ratio MSR were calculated.

Promoting Seed Germination of Some Plant Species by Rhamnolipid Produced by Pseudomonas aeruginosa.

With growing environmental concerns and a growing world population, there is an interest in developing cheap, efficient, and environmentally friendly substances that benefit humanity. Microbial surfactants are nontoxic, biodegradable bioproducts that provide sustainable solutions in agricultural applications due to their many benefits over their synthetic counterparts. Hence the crucial importance of scientific research to understand the effect of microbial surfactants on plant development. The present study aimed to assess the effect of rhamnolipid produced by Pseudomonas aeruginosa on seed germination of wheat (Triticum aestivum), barley (Hordeum vulgare), okra (Abelmoschus esculentus), onion (Allium cepa), and lettuce (Lactuca sativa) under laboratory conditions. The results showed that P. aeruginosa was capable of producing 3.83 g/L of viscous, honey-colored rhamnolipid, which was capable of lowering the surface tension to 30 ± 0.33 mN/m. Different concentrations of rhamnolipid ranging from 0.25 to 1.00 g/L were assessed, with distilled water acting as a control. After treatment of seeds, results showed that applying 0.25 g/L of rhamnolipid can significantly increase seed germination to 100% on the fourth day of sowing okra and lettuce, and on the fifth day of sowing onion seeds, compared to control groups that recorded 60%, 50%, and 55%, respectively. In wheat and barley seeds, applying rhamnolipid can protect seeds from pathogenic fungi while delaying their germination to 60% and 70% on the third day of sowing, while 90% and 100% have been reported in the control groups. Thus, this biological molecule demonstrates promising results in enhancing seed germination of the studied species by protecting them from phytopathogens and then aiding plant growth.

Removal of chromium using rhamnolipid biosurfactant, ferrous sulfate and cationic tannin‐based flocculant in a dissolved air floating system using a central composite design (CCD)

AbstractThis research studied the wastewater chromium removal efficiency by dissolved air flotation (DAF) using a rhamnolipid (RL) biosurfactant as a collector. An experimental flotation DAF apparatus with 6 vessels of 2 L containing a coupled saturator injecting compressed air at 5.88 kPa in the vessels was used. The total RL concentration of the broth resulting from fermentation was 9 ± 1.0 g/L. This broth was used in nature in the DAF experiments. A central composite design (CCD) was used to optimize removal of Cr(VI) and total Cr with regards to two independent variables, pH (3.17–8.83) and iron concentration of the medium (0–225.0 mg/L), with a three assays performed at the conditions of the central point of the design. The experimental conditions for DAF were an initial hexavalent Cr concentration of 100 mg/L; RL broth volume of 500 mL; saturated with oxygen water volume of 200 mL; and a rapid mixing time of 6 min through stirring at 120 rpm. The results showed that under acidic pH conditions and with high iron concentrations, both the Cr(VI) and total Cr removal rates were highest. The optimal removal region determination was at a pH of 3.5 and iron concentration of 180 mg/L. Subsequently, cationic tannin‐based flocculant was also evaluated as a collector, and ferrous sulfate was used as a coagulant during Cr(III) removal. The best Cr(III) removal percentage was obtained at cationic polymer concentrations of 300 mg/L with Cr(III) removal of 50.8% and a pH of 5.5.

Selective removal of Gallium from mixed metal solutions with Arsenic by ion flotation using the biosurfactant rhamnolipid

Rhamnolipids have received great attention in various environmental applications in terms of metal complexation and recovery. However, the effect of metal ions on interfacial and foaming properties in relation to flotation are not much studied. To assist, this study investigated the effect of metal ions alone and in a mixed metal system containing Gallium (Ga) and Arsenic (As) on these properties of rhamnolipid, along with isothermal titration calorimetry to study rhamnolipid-metal interactions. Further, the potential of rhamnolipid to recover and separate Ga from As was assessed using bioionflotation. Ga alone and in mixed metal system had a remarkable effect on the interfacial and foaming properties of rhamnolipid as compared to As alone. The effect of operating parameters like pH, rhamnolipid concentration, and airflow rate had a significant influence on the separation performance. Nearly 74% Ga was removed at 0.85 mM rhamnolipid concentration, pH 6, and an airflow rate of 80 ml/min. The selectivity index of Ga over As was highest (17.2) at 0.85 mM rhamnolipid concentration, pH 6, and an airflow rate of 40 ml/min. Also, the selective separation of Ga was dependent on the recovery of water from the foam. Altogether, the results provide new insights on the interfacial and foaming properties of rhamnolipid and its efficiency as an ion collector for Ga and showed that the optimized process parameters could be expected to provide very efficient separation and recovery of target metal via ion flotation.

The desorption of n-hexadecane in calcareous soils from a karst area: Insight into endogenous Cd/Pb

To comprehensively reveal the effects of endogenous heavy metals on the desorption and release of n-hexadecane in calcareous soils from a karst area, the present work conducted desorption kinetic experiments and column experiments. This study also focused on the relationship between heavy metals and n-hexadecane desorption under various environmental factors including pH, ionic strength (IS), and rhamnolipid (RL). Results of desorption kinetic experiments demonstrated that the higher desorption content of Pb in the solution with a lower pH level created additional absorption sites for n-hexadecane, thereby inhibiting the desorption of n-hexadecane in soil samples. The co-desorption of Pb and n-hexadecane was observed in the presence of IS/RL. Particularly, a significant positive coefficient between n-hexadecane desorption and IS was observed (r = 0.904, p < 0.01), and Pb desorption positively correlated to RL (r = 0.810, p < 0.01). In contrast, results of column experiments showed that neither Cd nor Pb were released from the soil column. The limited interaction between soil and solution in the column led to the negligible influence of Cd and Pb on the release of n-hexadecane. The increased release efficiency of n-hexadecane with lower pH, reduced IS, or higher RL concentration can be attributed to the soil properties such as ion enrichment, higher soil pH, and weaker mineral compositions. The findings provide valuable insights into petroleum hydrocarbons mobility in karst areas.

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.

Insight into the inhibitory mechanism of rhamnolipid biosurfactant on the mobility of sulfonamide antibiotics in saturated soil porous media

The mobility and deposition of sulfonamide antibiotics (SAs) in soil porous media may be impacted by the presence of rhamnolipid (a typical biosurfactant), which is ubiquitous in the natural environment. Herein, the transport characteristics of various SAs, such as sulfamerazine (SMZ), sulfamethazine (SMT), and sulfanilamide (SNM) in soil columns with rhamnolipid, were explored. The mobility of SAs in the soil columns was in the sequence of SNM > SMT > SMZ. The observation was mainly due to the differences in the hydrophobic properties of various antibiotics. Interestingly, rhamnolipid inhibited the mobility of the three SAs over a broad solution pH range (5.0–9.0) primarily because of the interactions between the hydrophobic groups of the deposited rhamnolipid on the soil surfaces and parts of SAs molecules (i.e., rhamnolipid acted as a bridging agent between soil and antibiotic). Furthermore, at a given rhamnolipid concentration, the inhibitory effect of rhamnolipid on SMZ transport was the greatest among the three antibiotics due to SMZ’s greatest hydrophobicity. Additionally, the transport-inhibition influence of rhamnolipid declined as the solution pH values rose. This phenomenon was attributed to the decline of the bridge effects induced by the decreased adsorbed rhamnolipid on soil surfaces. Innovative findings from this study provide helpful information regarding the contribution of biosurfactants on the environmental behaviors of these ubiquitous antibiotic contaminants in soil-water systems.

Modeling downstream impact of a quorum sensing system of Pseudomonas aeruginosa in colony spreading

A single bacterial colony typically consists of two or more individual cells. To further growth, the cells within the colony need to coordinate through Quorum Sensing (QS), which serves as a communication mechanism between cells. The Quorum Sensing signalling system in Pseudomonas aeruginosa is known to involve multiple control components, notably the las and rhl systems. The las system controls the rhl system in a hierarchical signalling cascade, and the rhl system regulates certain gene expressions, including the rhlA and rhlB genes. Transcription of rhlA and rhlB results in the production of the enzyme rhamnosyltransferase, which leads to the biosurfactant production identified as rhamnolipids. In this paper, we investigate how the las system controls the rhl system to influence rhamnolipids production. We illustrate how the pulsed production of signal molecules in the las system affects the rhl system, leading to an increase in rhamnolipids concentration. Rhamnolipids play a crucial role in bacterial spreading. We employ a simple symmetric diffusion model to simulate colony spreading and demonstrate how bacteria spread from the centre of the colony. Our findings shed light on the interaction between the las and rhl systems, which may enable cells to trigger rhamnolipids production at the edge of the bacterial colony.

Rhamnolipids combined with drip irrigation as a novel strategy for promoting cotton growth by reducing salinity stress and optimizing rhizosphere microbial communities

Soil salinization is a serious issue that is being faced worldwide. This study investigated the effects of different concentrations of rhamnolipids (CK: 0 mg/L, RhaL: 100 mg/L, RhaM: 300 mg/L and RhaH: 500 mg/L) combined with drip irrigation on the rhizosphere environment of the saline soil used for cotton cultivation. Results showed that rhamnolipids reduced soil pH and soil water-soluble salts (e.g., Na+, K+, and SO42−), whereas Cl− and electrical conductivity increased in RhaM and RhaH. In addition, rhamnolipids increased the activity of alkaline phosphatase, sucrase, and CAT by 16.9–44.8%, 84.0–108.5%, and 29.6–41.9%, respectively, and decreased urease by 33.2–39.0%. In addition, the bacterial abundance in the rhamnolipid-treated group increased, whereas that of fungi decreased with increasing rhamnolipid concentrations. Moreover, co-occurrence network analysis showed that rhamnolipids improved the stability of the rhizosphere microbial communities. Furthermore, functional gene predictions indicated that rhamnolipids enhanced soil C, N, P, S, and energy cycling. Ultimately, the cotton growth and stem diameter in rhamnolipid-added treatments increased by 24.6–65.3% and 2.1–10.7%, respectively. This study provides valuable information for establishing effective strategies to meliorate saline soils and contributes to the sustainable agricultural development.

Antibacterial Mechanism of Rhamnolipids against Bacillus cereus and Its Application in Fresh Wet Noodles.

Bacillus cereus (B. cereus) is a common foodborne pathogen causing food poisoning incidents. This study aimed to evaluate the antibacterial activity and underlying mechanism of rhamnolipids (RLs) against B. cereus. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of RLs for B. cereus were determined to be 16.0 mg/L and 32.0 mg/L, respectively. Scanning electron microscopy and fluorescence microscope images, as well as data of membrane potential, relative electric conductivity, and leakage of intracellular components revealed that RLs disrupted the integrity of the cell membrane. Furthermore, the reactive oxygen species content, catalase (CAT) and superoxide dismutase (SOD) activity indicated that RLs activated the oxidative stress response of B. cereus in response to RLs. Fresh wet noodles (FWN) were used as a food model, and RLs showed a significant killing effect on B. cereus with a sustained inhibitory effect at the concentrations ranging from 128.0 to 1024.0 mg/kg. Additionally, RLs promoted the conversion of free water to bound water in FWN, which improved the storage of FWN and made the taste more resilient and chewy. These results suggest that RLs could be a potential alternative to antimicrobial agents and preservatives for applications in food processing.

Intervention of rhamnolipid improves the rhizosphere microenvironment of cotton in desert saline lands

Rhamnolipids have been extensively studied for the remediation of soils contaminated with petroleum hydrocarbons. However, the literature on the effects of rhamnolipids on the soil microenvironment is scarce. In this study, we adopted a drip irrigation technique to apply a rhamnolipid solution to soil planted with cotton in a saline desert area. The results showed that the addition of rhamnolipids increased soil organic matter by 5.0–31.6 % and reduced soil electrical conductivity by 0.3–42.4 %. Additionally, it improved soil nutrient conditions, reshaped the composition and function of the microbial community in the rhizosphere soil, and ultimately promoted cotton growth by 3.3 – 9.0 %. Simultaneously, the addition of rhamnolipids enriched the diversity of the bacterial community. Although it reduced the diversity of fungal communities, it optimized the community structure of fungi. It is worth noting that higher concentrations of rhamnolipids may have toxic effects on soil fungi, the mechanism of which is unknown. These findings shed new light on understanding the effects of rhamnolipids on rhizosphere soil microorganisms and provide a new direction for sustainable agricultural production.

The effects of rhamnolipid on the formation of CH4 hydrate and separation of CH4/N2 via hydrate formation

Rhamnolipid (Rha) is an efficient green surfactant that is able to promote hydrate generation, it can be applied to the hydrate method of separating. However, the effect of rhamnolipid on the separation of methane and nitrogen by the hydrate method is not clear. Therefore, in this paper, we investigated the effect of rhamnolipid on methane hydrate formation and the separation between methane and nitrogen via hydrate method. Firstly, the effects of five concentrations (0 ppm, 200 ppm, 400 ppm, 600 ppm and 800 ppm), temperatures and pressures on methane hydrate formation were studied separately. The rate of methane hydrate formation was positively correlated with experimental pressure. The gas consumption increased rapidly with the rhamnolipid concentration, and then increased slowly to a steady state. The final gas consumption above 200 ppm rhamnolipid concentrations could be increased more than five times compared with pure water conditions. The promotion of methane hydrate production is most pronounced at 274.15 K, mainly depending on the diffusion of gas molecules in solution and the ease of hydrate nucleation. Then, the interfacial tension at different concentrations was measured by the suspension drop method, and the relationship between the interfacial tension and the rate of hydrate formation was studied. Hydrate formation rate shows a multiplicative power function with the interfacial tension. What’s more, the kinetics of hydrate formation and gas separation effect of 3:1 CH4/N2 at 274.15 K and 7 MPa were studied with five concentrations. And 200 ppm was found to be the optimum concentration for separation, the separation factor is 8.25. Finally, the kinetics and thermodynamics of hydrate formation and gas separation effect of 1:1 CH4/N2 at 274.15 K and 9 MPa were also investigated, and the separation factor reached a maximum of 12.7 at 800 ppm, and the methane gas recovery rate also reached the maximum at 40.46%. The results of this study can provide some theoretical guidance for the optimal selection of green surfactants in separation by hydrate method.

Biosurfactant-affected mobility of oxytetracycline and its variations with surface chemical heterogeneity in saturated porous media

Herein, the influences of rhamnolipid (a typical biosurfactant) on oxytetracycline (OTC) transport in the porous media and their variations with the surface heterogeneities of the media (uncoated sand, goethite (Goe)-, and humic acid (HA)-coated sands) were explored. Compared to uncoated sand, goethite and HA coatings suppressed OTC mobility by increasing deposition sites. Interestingly, rhamnolipid-affected OTC transport strongly depended on the chemical heterogeneities of aquifers and biosurfactant concentrations. Concretely, adding rhamnolipid (1–3 mg/L) inhibited OTC mobility through sand columns because of the bridging effect of biosurfactant between sand and OTC. Unexpectedly, rhamnolipid of 10 mg/L did not further improve the inhibition of OTC transport owing to the fact that the deposition capacity of rhamnolipid reached its maximum. OTC mobility in Goe-coated sand columns was inhibited by 1 mg/L rhamnolipid. However, the inhibitory effect decreased with the increasing rhamnolipid concentration (3 mg/L) and exhibited a promoted effect at 10 mg/L rhamnolipid. This surprising observation was that the increased rhamnolipid molecules gradually occupied the favorable deposition sites (i.e., the positively charged sites). In comparison, rhamnolipid facilitated OTC transport in the HA-coated sand column. The promotion effects positively correlated with rhamnolipid concentrations because of the high electrostatic repulsion and deposition site competition induced by the deposited rhamnolipid. Another interesting phenomenon was that rhamnolipid's enhanced or inhibitory effects on OTC transport declined with the increasing solution pH because of the decreased rhamnolipid deposition on porous media surfaces. These findings benefit our understanding of the environmental behaviors of antibiotics in complex soil-water systems containing biosurfactants.

Physiological and Transcriptomic Analyses of Escherichia coli Serotype O157:H7 in Response to Rhamnolipid Treatment.

Rhamnolipid (RL) can inhibit biofilm formation of Escherichia coli O157:H7, but the associated mechanism remains unknown. We here conducted comparative physiological and transcriptomic analyses of cultures treated with RL and untreated cultures to elucidate a potential mechanism by which RL may inhibit biofilm formation in E. coli O157:H7. Anti-biofilm assays showed that over 70% of the E. coli O157:H7 biofilm formation capacity was inhibited by treatment with 0.25-1 mg/mL of RL. Cellular-level physiological analysis revealed that a high concentration of RL significantly reduced outer membrane hydrophobicity. E. coli cell membrane integrity and permeability were also significantly affected by RL due to an increase in the release of lipopolysaccharide (LPS) from the cell membrane. Furthermore, transcriptomic profiling showed 2601 differentially expressed genes (1344 up-regulated and 1257 down-regulated) in cells treated with RL compared to untreated cells. Functional enrichment analysis indicated that RL treatment up-regulated biosynthetic genes responsible for LPS synthesis, outer membrane protein synthesis, and flagellar assembly, and down-regulated genes required for poly-N-acetyl-glucosamine biosynthesis and genes present in the locus of enterocyte effacement pathogenicity island. In summary, RL treatment inhibited E. coli O157:H7 biofilm formation by modifying key outer membrane surface properties and expression levels of adhesion genes.

Phenanthrene removal from soil washing eluent by Bacillus subtilis embedded in alginate-carboxymethyl cellulose-diatomite beads

ABSTRACT The processes of surfactant-enhanced soil washing have been widely applied to the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil, accompanied by the production of soil washing eluent. In this study, novel composite materials of beads containing alginate, carboxymethyl cellulose and diatomite (SCD) were used to encapsulate Bacillus subtilis to remove phenanthrene (PHE) from simulated soil washing eluent with rhamnolipid. The effects of dosage, pH and temperature on the PHE removal performance were explored, and the optimal PHE removal conditions [SCD bead dose 16.2% (w:v), pH 7.1 and 30.6°C] were determined using response surface methodology. After incubation in simulated soil washing eluent for 7 d, SCD beads removed 84.92% of PHE, which was 49.18% higher than by free bacteria. In addition, SCD beads mainly removed PHE through biodegradation processes, and the degradation rate (1.38 mg L−1 d−1) was higher than that of free bacteria (0.64 mg L−1 d−1). Characterization results revealed that the immobilized substrate provided the micro-environment for bacteria and reduced the intense effect of high rhamnolipid concentration. Reusability results showed that SCD beads could be recycled four times to remove 80.05% of PHE. Collectively, SCD beads have great prospects for the decontamination of soil-washing eluent containing complex components.

Rhamnolipid Biosurfactant: Use for the Removal of Phenol from Aqueous Solutions by Micellar Solubilization.

Selective measurements of the self-diffusion coefficients of molecules of the biological surfactant rhamnolipid (RL) in individual aqueous solutions and in solutions with phenol as a solubilizate were carried out by nuclear magnetic resonance (NMR) diffusometry. Based on the obtained results, the solubilization characteristics of RLs were calculated. They are the fraction of solubilized phenol molecules, the phenol micelle-water distribution coefficient, the molar solubilization coefficient, the hydrodynamic radii of RL monomers and micelles, the aggregation numbers of micelles, and the solubilization capacity of micelles. Fraction of the solubilized phenol molecules increases and approaches 80-90% with increasing RL concentration. The solubilization capacity of the micelles increases from several units to 102 with an increase in both the concentration of RLs and the concentration of phenol in solution.

Towards green flotation: Investigating the effect of rhamnolipid biosurfactant on single bubble adhesion dynamics

In this study we performed an experimental analysis of the influence of rhamnolipid (RL) biosurfactant produced by Pseudomonas aeruginosa on the dynamics of bubble adhesion onto model surfaces of various hydrophobicity. The attachment time of a single bubble to a solid surface with contact angles of 6, 34, 49 and 80° was evaluated at different concentrations of RL in the range of 0 to 500 mg·dm−3, both at pH 5 (non-ionic form) and pH 10 (anionic form). The dependence of the three-phase contact (TPC) formation time on biosurfactant concentration was determined by monitoring the bubble-solid surface interactions using a high-speed camera. It was found that as the concentration of RL rises, the formation time of the TPC extends as a result of an increase in the film drainage time. A significant effect of RL on the process of TPC expansion (lower rate) and the size of the contact area between the bubble and the surface (smaller surface area) was also noted. This study provides a better understanding of the effect of one of the most popular biosurfactants on the fundamental act of the flotation process: attachment of the particle to the bubble.