Effect of rhamnolipid solubilization on hexadecane bioavailability: enhancement or reduction?

Mechanisms for rhamnolipids-mediated biodegradation of hydrophobic organic compounds

The widespread existence of hydrophobic organic compounds (HOCs) in soil and water poses a potential health hazard to human, such as skin diseases, heart diseases, carcinogenesis, etc. Surfactant-enhanced bioremediation has been regarded as one of the most viable technologies to treat HOCs contaminated soil and groundwater. As a biosurfactant that has been intensively studied, rhamnolipids have shown to enhance biodegradation of HOCs in the environment, however, the underlying mechanisms are not fully disclosed. In this paper, properties and production of rhamnolipids are summarized. Then effects of rhamnolipids on the biodegradation of HOCs, including solubilization, altering cell affinity to HOCs, and facilitating microbial uptake are reviewed in detail. Special attention is paid to how rhamnolipids change the bioavailability of HOCs, which are crucial for understanding the mechanism of rhamnolipids-mediated biodegradation. The biodegradation and toxicity of rhamnolipids are also discussed. Finally, perspectives and future research directions are proposed. This review adds insight to rhamnolipids-enhanced biodegradation process, and helps in application of rhamnolipids in bioremediation.

Recovery of contaminated marine environments by biosurfactant-enhanced bioremediation

The need to remediate areas contaminated by petroleum products has led to the development of novel technologies for treating such contaminants in a non-conventional manner, that is, without the use of chemical or physical methods. Biosurfactants are amphipathic biomolecules produced by microorganisms that can be used in bioremediation processes in environments contaminated by petroleum products due to their excellent tensioactive properties. The aim of the present study was to produce a biosurfactant from Pseudomonas aeruginosa UCP 0992 cultivated in 0.5% corn steep liquor and 4.0% vegetable oil residue in a 1.2-L bioreactor employing a central composite rotatable design to optimize the cultivation conditions for maximum yield. The best results were achieved with aeration rate of 1.0 vvm and 3.0% inoculum at 225 rpm for 120 h, resulting in a surface tension of 26.5 mN/m and a biosurfactant yield of 26 g/L. Kinetic and static assays were then performed with the biosurfactant for the removal of motor oil adsorbed to sand, with removal rates around 90% and 80%, respectively, after 24 h. Oil degradation experiments with the bacterium and the combination of the bacterium and biosurfactant were also conducted to simulate the bioremediation process in sand and seawater samples (duration: 75 and 30 days, respectively). In both cases, oil degradation rates were higher than 90% in the presence of the biosurfactant and the producing species, indicating the potential of the biomolecule as an adjuvant in petroleum decontamination processes in the marine environment.

Efficient degradation of sulfamethazine in simulated and real wastewater at slightly basic pH values using Co-SAM-SCS /H2O2 Fenton-like system

The presence of antibiotics in aquatic environments has attracted global concern. Fenton process is an attractive yet challenging method for antibiotics degradation, especially when such a reaction can be conducted at neutral pH values. In this study, a novel composite Fe/Co catalyst was synthesized via the modification of steel converter slag (SCS) by salicylic acid-methanol (SAM) and cobalt nitrate (Co(NO₃)₂). The catalysts were characterized by N₂-Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results indicated that the Co-SAM-SCS/H₂O₂ Fenton-like system was very effective for sulfamethazine (SMZ) degradation at a wide pH range. At initial pH of 7.0, the degradation rate of SMZ in Co-SAM-SCS/H₂O₂ system was 2.48, 3.20, 6.18, and 16.21 times of that in Fe-SAM-SCS/H₂O₂, SAM-SCS/H₂O₂, Co(NO₃)₂/H₂O₂ and SCS/H₂O₂ system, respectively. The preliminary analysis suggested that high surface area of Co-SAM-SCS sample and synergistic effect between introduced Co and SAM-SCS are responsible for the efficient catalytic activity. During the degradation, three main intermediates were identified by high performance liquid chromatography-mass spectrometry (HPLC-MS) analysis. Based on this, a possible degradation pathway was proposed. The SEM images, XRD patterns and XPS spectra before and after the reactions demonstrate that the crystal and chemical structure of Co-SAM-SCS after five cycles are almost unchanged. Besides, the Co-SAM-SCS presented low iron and cobalt leaching (0.17 mg/L and 2.36 mg/L, respectively). The studied Fenton-like process also showed high degradation of SMZ in river water and municipal wastewater. The progress will bring valuable insights to develop high-performance heterogeneous Fenton-like catalysts for environmental remediation.

Degradation of di (2-ethylhexyl) phthalate in sediment by a surfactant-enhanced Fenton-like process

This is the premier study reporting the remediation of di (2-ethylhexyl) phthalate (DEHP) contaminated sediment by a surfactant-enhanced Fenton-like system. Three widely used non-ionic surfactants were tested, and the order for desorption and solubilization of DEHP was determined as Tween 80 > Triton X-100 > Brij 35. The degradation of DEHP was studied at a near natural pH of 6.0 by two Fenton treatments: (i) Fe³⁺/H₂O₂ and (ii) Fe³⁺/PCA/H₂O₂. Results show that the addition of PCA can significantly enhance DEHP removal from 48.9% to 92.5%. This is consistent with observation that PCA maintained at a relative high level of iron ions, which can catalyze H₂O₂ to generate the reactive hydroxyl radicals (OH). Most of the added Tween 80 and a portion of OM were co-oxidized together with DEHP due to the non-selective nature of OH, which leaded to an increase in DOC content and decreases in sediment pH and total N content. The results provide an efficient and eco-friendly technique for the remediation of DEHP contaminated sediment, and also give insight to its environmental implications.

Time-dependent bacterial community and electrochemical characterizations of cathodic biofilms in the surfactant-amended sediment-based bioelectrochemical reactor with enhanced 2,3,4,5-tetrachlorobiphenyl dechlorination

Applying an electric field to stimulate the microbial reductive dechlorination of polychlorinated biphenyls (PCBs) represents a promising approach for bioremediation of PCB-contaminated sites. This study aimed to demonstrate the biocathodic film-facilitated reduction of PCB 61 in a sediment-based bioelectrochemical reactor (BER) and, more importantly, the characterizations of electrode-microbe interaction from microbial and electrochemical perspectives particularly in a time-dependent manner. The application of a cathodic potential (-0.45 V vs. SHE) significantly improved the rate and extent of PCB 61 dechlorination compared to the open-circuit scenario (without electrical stimulation), and the addition of an external surfactant further increased the dechlorination, with Tween 80 exerting more pronounced effects than rhamnolipid. The bacterial composition of the biofilms and the bioelectrochemical kinetics of the BERs were found to be time-dependent and to vary considerably with the incubation time and slightly with the coexistence of an external surfactant. Excellent correlations were observed between the dechlorination rate and the relative abundance of Dehalogenimonas, Dechloromonas, and Geobacter, the dechlorination rate and the cathodic current density recorded from the chronoamperometry tests, and the dechlorination rate and the charge transfer resistance derived from the electrochemical impedance tests, with respect to the 120 day-operation. After day 120, PCB 61 was resistant to further appreciable reduction, but substantial hydrogen production was detected, and the bacterial community and electrochemical parameters observed on day 180 were not distinctly different from those on day 120.

Advances in applications of rhamnolipids biosurfactant in environmental remediation: A review

The objective of this review is to provide a comprehensive overview of the advances in the applications of rhamnolipids biosurfactants in soil and ground water remediation for removal of petroleum hydrocarbon and heavy metal contaminants. The properties of rhamnolipids associated with the contaminant removal, that is, solubilization, emulsification, dispersion, foaming, wetting, complexation, and the ability to modify bacterial cell surface properties, were reviewed in the first place. Then current remediation technologies with integration of rhamnolipid were summarized, and the effects and mechanisms for rhamnolipid to facilitate contaminant removal for these technologies were discussed. Finally rhamnolipid-based methods for remediation of the sites co-contaminated by petroleum hydrocarbons and heavy metals were presented and discussed. The review is expected to enhance our understanding on environmental aspects of rhamnolipid and provide some important information to guide the extending use of this fascinating chemical in remediation applications.

Evaluation methods for assessing effectiveness of in situ remediation of soil and sediment contaminated with organic pollutants and heavy metals

Soil and sediment contamination has become a critical issue worldwide due to its great harm to the ecological environment and public health. In recent years, many remediation technologies including physical, chemical, biological, and combined methods have been proposed and adopted for the purpose of solving the problems of soil and sediment contamination. However, current research on evaluation methods for assessing these remediation technologies is scattered and lacks valid and integrated evaluation methods for assessing the remediation effectiveness. This paper provides a comprehensive review with an environmental perspective on the evaluation methods for assessing the effectiveness of in situ remediation of soil and sediment contaminated with organic pollutants and heavy metals. The review systematically summarizes recent exploration and attempts of the remediation effectiveness assessment based on the content of pollutants, soil and sediment characteristics, and ecological risks. Moreover, limitations and future research needs of the practical assessment are discussed. These limitations are not conducive to the implementation of the abatement and control programs for soil and sediment contamination. Therefore, more attention should be paid to the evaluation methods for assessing the remediation effectiveness while developing new in situ remediation technologies in future research.

Kinetics of soil dehydrogenase in response to exogenous Cd toxicity

Soil dehydrogenase plays a role in the biological oxidation of soil organic matter and can be considered a good measure of the change of microbial oxidative activity under environmental pollutions. However, the kinetic characteristic of soil dehydrogenase under heavy metal stresses has not been investigated thoroughly. In this study, we characterized the kinetic characteristic of soil dehydrogenase in 14 soil types, and investigated how kinetic parameters changed under spiked with different concentrations of cadmium (Cd). The results showed that the K_m and V_max values of soil dehydrogenase was among 1.4-7.3mM and 15.9-235.2μMh^-1 in uncontaminated soils, respectively. In latosolic red soil and brown soil, the inhibitory kinetic mechanism of Cd to soil dehydrogenase was anticompetitive inhibition with inhibition constants (K_i) of 12 and 4.7mM, respectively; in other soils belonged to linear mixed inhibition, the values of K_i were between 0.7-4.2mM. Soil total organic carbon and K_i were the major factors affecting the toxicity of Cd to dehydrogenase activity. In addition, the velocity constant (k) was more sensitive to Cd contamination compared to V_max and K_m, which was established as an early indicator of gross changes in soil microbial oxidative activity caused by Cd contamination.

Integrated electrochemical treatment systems for facilitating the bioremediation of oil spill contaminated soil

Bioremediation plays an important role in oil spill management and bio-electrochemical treatment systems are supposed to represent a new technology for both effective remediation and energy recovery. Diesel removal rate increased by four times in microbial fuel cells (MFCs) since the electrode served as an electron acceptor, and high power density (29.05 W m^-3) at current density 72.38 A m^-3 was achieved using diesel (v/v 1%) as the sole substrate. As revealed by Scanning electron microscope images, carbon fibres in the anode electrode were covered with biofilm and the bacterial colloids which build the link between carbon fibres and enhance electron transmission. Trace metabolites produced during the anaerobic biodegradation were identified by gas chromatography-mass spectrometry. These metabolites may act as emulsifying agents that benefit oil dispersion and play a vital role in bioremediation of oil spills in field applications.

Effects of rhamnolipids on microorganism characteristics and applications in composting: A review

Biosurfactant rhmnolipids have been applied in many fields, especially in environmental bioremediation. According to previous researches, many research groups have studied the influence of rhamnolipids on microorganism characteristics and/or its application in composting. In this review, the effects of rhamnolipids on the cell surface properties of microorganisms was discussed firstly, such as cell surface hydrophobicity (CSH), electrical, surface compounds, etc. Moreover, the deeper mechanisms were also discussed, such as the effects of rhamnolipids on the structural characteristics and functional characteristics of the cell membrane, and the effects of rhamnolipids on the related enzymes and genes. Additionally, the application of rhamnolipids in composting was discussed, which is an important way for pollutant biodegradation and resource reutilization. It is believed that rhamnolipids will play more and more important role in composting.

Bioconversion of Welan Gum from Kitchen Waste by a Two-Step Enzymatic Hydrolysis Pretreatment

Kitchen waste (KW) is a worldwide issue, which can lead to environment pollution. Nevertheless, it is also a low-cost and sustainable resource for bio-production. Meanwhile, welan gum (WG) is one kind of the most important exopolysaccharide but with high material cost. The aim of this study was to adopt two-step enzymatic hydrolysis to improve the release and recovery of both sugar and protein in KW for subsequent WG production. As the results, the recovery rates of sugar and protein reached 81.07 and 77.38%, which were both satisfactory. After the conditions optimized in flasks, the welan fermentation was conducted in a 5-L fermentor, and the WG yield, utilization rates of reducing sugar and KDN, respectively, reached 5.57 g L^-1, 94.25% and 61.96%. Moreover, the kinetic analyses demonstrated that the WG fermentation in KWH was a partly growth-associated process. The KW was successfully treated by fermentation for the bioconversion to WG.

Land-use types and soil chemical properties influence soil microbial communities in the semiarid Loess Plateau region in China

Similar land-use types usually have similar soil properties, and, most likely, similar microbial communities. Here, we assessed whether land-use types or soil chemical properties are the primary drivers of soil microbial community composition, and how changes in one part of the ecosystem affect another. We applied Ion Torrent sequencing to the bacterial and fungal communities of five different land-use (vegetation) types in the Loess Plateau of China. We found that the overall trend of soil quality was natural forest > plantation > bare land. Dominant bacterial phyla consisted of Proteobacteria (42.35%), Actinobacteria (15.61%), Acidobacteria (13.32%), Bacteroidetes (8.43%), and Gemmatimonadetes (6.0%). The dominant fungi phyla were Ascomycota (40.39%), Basidiomycota (38.01%), and Zygomycota (16.86%). The results of Canonical Correspondence Analysis (CCA) and Redundancy Analysis (RDA) based on land-use types displayed groups according to the land-use types. Furthermore, the bacterial communities were mainly organized by soil organic carbon (SOC). The fungal communities were mainly related to available phosphorus (P). The results suggested that the changes of land use type generated changes in soil chemical properties, controlling the composition of microbial community in the semiarid Loess Plateau region. The microbial community could be an indicator for soil quality with respect to ecological restoration.