The role of root exuded low molecular weight organic anions in facilitating petroleum hydrocarbon degradation: current knowledge and future directions

Rhizoremediation is a bioremediation technique whereby enhanced microbial degradation of organic contaminants occurs within the plant root zone (rhizosphere). It is considered an effective and affordable ‘green technology’ for remediating soils contaminated with petroleum hydrocarbons (PHCs). This paper critically reviews the potential role of root exuded compounds in rhizoremediation, with emphasis on commonly exuded low molecular weight aliphatic organic acid anions (carboxylates). The extent to which remediation is achieved shows wide disparity among plant species. Therefore, plant selection is crucial for the advancement and widespread adoption of this technology. Root exudation is speculated to be one of the predominant factors leading to microbial changes in the rhizosphere and thus the potential driver behind enhanced petroleum biodegradation. Carboxylates can form a significant component of the root exudate mixture and are hypothesised to enhance petroleum biodegradation by: i) providing an easily degradable energy source; ii) increasing phosphorus supply; and/or iii) enhancing the contaminant bioavailability. These differing hypotheses, which are not mutually exclusive, require further investigation to progress our understanding of plant–microbe interactions with the aim to improve plant species selection and the efficacy of rhizoremediation.

Just add water and salt: the optimisation of petrogenic hydrocarbon biodegradation in soils from semi-arid Barrow Island, Western Australia

We investigated the potential of soil moisture and nutrient amendments to enhance the biodegradation of oil in the soils from an ecologically unique semi-arid island. This was achieved using a series of controlled laboratory incubations where moisture or nutrient levels were experimentally manipulated. Respired CO2 increased sharply with moisture amendment reflecting the severe moisture limitation of these porous and semi-arid soils. The greatest levels of CO2 respiration were generally obtained with a soil pore water saturation of 50–70%. Biodegradation in these nutrient poor soils was also promoted by the moderate addition of a nitrogen fertiliser. Increased biodegradation was greater at the lowest amendment rate (100 mg N kg−1 soil) than the higher levels (500 or 1,000 mg N kg−1 soil), suggesting the higher application rates may introduce N toxicity. Addition of phosphorous alone had little effect, but a combined 500 mg N and 200 mg P kg−1 soil amendment led to a synergistic increase in CO2 respiration (3.0×), suggesting P can limit the biodegradation of hydrocarbons following exogenous N amendment.

Hydrocarbon biodegradation and soil microbial community response to repeated oil exposure

Bioremediation strategies continue to be developed to mitigate the environmental impact of petroleum hydrocarbon contamination. This study investigated the ability of soil microbiota, adapted by prior exposure, to biodegrade petroleum. Soils from Barrow Is. (W. Australia), a class A nature reserve and home to Australia’s largest onshore oil field, were exposed to Barrow production oil (50 ml/kg soil) and incubated (25 °C) for successive phases of 61 and 100 days. Controls in which oil was not added at Phase I or II were concurrently studied and all treatments were amended with the same levels of additional nutrient and water to promote microbial activity. Prior exposure resulted in accelerated biodegradation of most, but not all, hydrocarbon constituents in the production oil. Molecular biodegradation parameters measured using gas chromatography–mass spectrometry (GC–MS) showed that several aromatic constituents were degraded more slowly with increased oil history. The unique structural response of the soil microbial community was reflected by the response of different phospholipid fatty acid (PLFA) sub-classes (e.g. branched saturated fatty acids of odd or even carbon number) measured using a ratio termed Barrow PLFA ratio (B-PLFAr). The corresponding values of a previously proposed hydrocarbon degrading alteration index showed a negative correlation with hydrocarbon exposure, highlighting the site specificity of PLFA-based ratios and microbial community dynamics. B-PLFAr values increased with each Phase I and II addition of production oil. The different hydrocarbon biodegradation rates and responses of PLFA subclasses to the Barrow production oil probably relate to the relative bioavailability of production oil hydrocarbons. These different effects suggest preferred structural and functional microbial responses to anticipated contaminants may potentially be engineered by controlled pre-exposure to the same or closely related substrates. The bioremediation of soils freshly contaminated with petroleum could benefit from the addition of exhaustively bioremediated soils rich in biota primed for the impacting hydrocarbons.

Sequential hydrocarbon biodegradation of a soil from arid coastal Australia oil-treated under laboratory controlled conditions

A general consistency in the sequential order of petroleum hydrocarbon reduction in previous biodegradation studies has led to the proposal of several molecularly based biodegradation scales. Few studies have investigated the biodegradation susceptibility of petroleum hydrocarbon products in soil media, however, and metabolic preferences can change with habitat type. A laboratory based study comprising gas chromatography–mass spectrometry (GC–MS) analysis of extracts of oil-treated soil samples incubated for up to 161 days was conducted to investigate the biodegradation of crude oil exposed to sandy soils of Barrow Island, home to both a Class ‘‘A” nature reserve and Australia’s largest on-shore oil field. Biodegradation trends of the hydrocarbon-treated soils were largely consistent with previous reports but some unusual behaviour was recognised both between and within hydrocarbon classes. For example, the n-alkanes persisted at trace levels from day 86 to 161 following the removal of typically more stable dimethyl naphthalenes and methyl phenanthrenes. The relative susceptibility to biodegradation of different di- tri- and tetramethylnaphthalene isomers also showed several features distinct from previous reports. The unique biodegradation behaviour of Barrow Is. soil likely reflects difference in microbial functioning with physiochemical variation in the environment. Correlation of molecular parameters, reduction rates of selected alkyl naphthalene isomers and CO2 respiration values with a delayed (61 d) oil-treated soil identified a slowing of biodegradation with microcosm incubation; a reduced function or population of incubated soil flora might also influence the biodegradation patterns observed.

Improved Prediction of Hydrocarbon Flash Points from Boiling Point Data

Flash points (T(FP)) of hydrocarbons are calculated from their flash point numbers, N(FP), with the relationship T(FP) (K) = 23.369N(FP)(2/3) + 20.010N(FP)(1/3) + 31.901 In turn, the N(FP) values can be predicted from experimental boiling point numbers (Y(BP)) and molecular structure with the equation N(FP) = 0.987 Y(BP) + 0.176D + 0.687T + 0.712B - 0.176 where D is the number of olefinic double bonds in the structure, T is the number of triple bonds, and B is the number of aromatic rings. For a data set consisting of 300 diverse hydrocarbons, the average absolute deviation between the literature and predicted flash points was 2.9 K.

Polycyclic aromatic hydrocarbon emission loads and profiles from the burning of locally grown wood species in Victoria, Australia - Some pilot project results

A testing facility for combustion of biomass and sampling of emissions has been established at Deakin University. In this pilot project using this facility, four kinds of locally grown wood species were burned and the particle emissions sampled and analysed for Polycyclic Aromatic Hydrocarbons (PAHs). The selected wood species covering pine, red gum, yellow box and sugar gum, are the most popular domestic fuel wood in Australia. Particulate matter emissions from burning of each load of wood were sampled from the flue using a standard stack emission sampling train. The particle-laden filters were extracted and the .extract analysed to determine PAH concentrations by Gas Chromatographyl Mass Spectrometer (Gc/MS). The sampling was conducted under two different burning conditions with the air inlet of the combustion chamber fully open and with it half open. A suite of 15 PAHs, ranging from naphthalene (C IOHB) to dibenzolahlanthracene (C12H14), were selected for analysis. PAH profiles for the four wood species, under the different burning regimes, have been generated. Some preliminary emission factors for the different wood species have been derived as microgram of summed PAHs (rPAHs) emittedlkilogram of wood burned. Total Particulate Matter (TPM) emission factors were also obtained from gravimetric measurement of the sample filter before and after the combustion. Based on these emission factors, pine displayed the highest level of rPAHs emitted from the combustion of the four wood species, with sugar gum showing the lowest level of rPAHs emission. Emission factors associated with the slow burning condition clearly showed higher l:PAH levels compared to the faster burning condition. During the faster burning condition, red gum and pine show a higher percentage of rPAH to TPM than sugar gum or yellow box. Under the slower burning. the l:PAHlTPM ratio in every case was greater.

Photocatalytic reduction of carbon dioxide into gaseous hydrocarbon using TiO2 pellets

It has been shown that CO₂ could be transformed into hydrocarbons when it is in contact with water vapour and catalysts under UV irradiation. This paper presents an experimental set-up to study the process employing a new approach of heterogeneous photocatalysis using pellet form of catalyst instead of immobilized catalysts on solid substrates. In the experiment, CO₂ mixed with water vapour in saturation state was discharged into a quartz reactor containing porous TiO₂ pellets and illuminated by various UV lamps of different wavelengths for 48 h continuously. The gaseous products extracted were identified using gas chromatography. The results confirmed that CO₂ could be reformed in the presence of water vapour and TiO₂ pellets into CH₄ under continuous UV irradiation at room conditions. It showed that when UVC (253.7 nm) light was used, total yield of methane was approximately 200 ppm which was a fairly good reduction yield as compared to those obtained from the processes using immobilized catalysts through thin-film technique and anchoring method. CO and H₂ were also detected. Switching from UVC to UVA (365 nm) resulted in significant decrease in the product yields. The pellet form of catalyst has been found to be attractive for use in further research on photocatalytic reduction of CO₂.

The influence of inert impurities on the catalyst lifetime and properties of nanofibrous carbon produced by utilization of diluted hydrocarbon gases

Experimental studies were focused on the feasibility of utilization of hydrocarbons diluted with inert gases (such as associated oil gases) during the synthesis of nanofibrous carbon. The carbon yield and catalyst lifetime were studied regarding the initial reaction mixture parameters. Varying the composition of the initial gas mixture, it is possible to control textural characteristics of the resulting carbon product.

Evolution of cuticular hydrocarbon diversity in ants

The cuticular hydrocarbons (CHCs) of ants provide important cues for nestmate and caste recognition. There is enormous diversity in the composition of these CHCs, but the manner in which this diversity has evolved is poorly understood. We gathered data on CHC profiles for 56 ant species, relating this information to their phylogeny. We deduced the mode of evolution of CHC profiles by reconstructing character evolution and then relating the number of changes in CHC components along each branch of the phylogeny to the length of the branch. There was a strong correlation between branch length and number of component changes, with fewer changes occurring on short branches. Our analysis thereby indicated a gradual mode of evolution. Different ant species tend to use specific CHC structural types that are exclusive of other structural types, indicating that species differences may be generated in part by switching particular biosynthetic pathways on or off in different lineages. We found limited, and contradictory, evidence for abiotic factors (temperature and rainfall) driving change in CHC profiles.

Measurement of forces due to structure in hydrocarbon liquids

Direct measurements of the force between two molecularly smooth mica sheets immersed in cyclohexane show not a monotonic van der Waals attraction, but an oscillatory function of distance, where the spacing between successive minima corresponds to the molecular diameter of cyclohexane. As surface separation increases the oscillations become less pronounced, and beyond 5 nm (typically seven or eight oscillations) they are no longer detected. These results accord with theoretical ideas on structural forces resulting from the inhomogeneous arrangement of molecules of the liquid near the solid surface. In n-octane the force law does not show the same pronounced oscillations, except at very small separations where repulsive barriers are found. These are attributed to the difficulty of removing the last layers of adsorbed molecules of the liquid from the mica surfaces, and they reduce the mice-mica adhesion significantly. Small amounts of water in the hydrocarbon liquids condense to form a bridge between the surfaces at small separations, causing a very strong adhesion between them. Some implications of these results for the stability of colloids in organic media are discussed.

In support of hydrocarbon exploration and discovery using clouds

Seismic data gathered from the Hydrocarbon Exploration and Discovery Operation is essential to identify possible hydrocarbon existence in a geologically surveyed area. However, the discovery operation takes a long time to be completed and computational processing of the acquired data is often delayed. Hydrocarbon exploration may end up needlessly covering an area without any hydrocarbon traces due to lack of immediate feedback from geophysical experts. This feedback can only be given when the acquired seismic data is computationally processed, analysed and interpreted. In response, we propose a comprehensive model to facilitate Hydrocarbon Exploration and Discovery Operation using encryption, decryption, satellite transmission and clouds. The model details the logical design of Seismic Data Processing (SDP) that exploits clouds and the ability for geophysical experts to provide on-line decisions on how to progress the hydrocarbon exploration operation at a remote location. Initial feasibility assessment was carried out to support our model. The SDP, data encryption and encryption for the assessment were carried out on a private cloud. The assessment shows that the overall process of hydrocarbon exploration from data acquisition, satellite data transmission through to SDP could be executed in a short time and at low costs.