Phytoremediation of petroleum hydrocarbons in tropical coastal soils I. selection of promising woody plants

GOAL, SCOPE AND BACKGROUND: This glasshouse study is aimed at evaluating tropical plants for phytoremediation of petroleum hydrocarbon-contaminated saline sandy subsurface soils. Tropical plants were selected for their ability to tolerate high salinity and remove No. 2 diesel fuel in coastal topsoil prior to further investigation of the phytoremediation feasibility in deep contaminated soils. The residual petroleum-hydrocarbon contaminant at the John Rogers Tank Farm site, a former petroleum storage facility, at Hickam Air Force Base, Honolulu, Hawaii, is located in a coastal area. It lies below a layer of silt in the subsurface, in loamy sand characterized by moderate salinity and high pH. Little is known regarding the ability of tropical plants to remediate petroleum hydrocarbon-contaminated subsurface soil in Hawaiian and other Pacific Island ecosystems although suitable plants have been identified and utilized for bioremediation in surface soil or marine sediments. METHODS: The experiments were conducted in long narrow pots under glasshouse conditions in two phases. A preliminary experiment was done with nine tropical plants: kiawe (Prosopis pallida), milo (Thespesia populnea), common ironwood (Casuarina equisetifolia), kou (Cordia subcordata), tropical coral tree (Erythrina variegata), false sandalwood (Myoporum sandwicense), beach naupaka (Scaevola sericea), oleander (Nerium oleander), and buffelgrass (Cenchrus ciliaris). These plants were screened for resistance to high salinity treatment (2% NaCl) and two diesel fuel levels (5 and 10 g No. 2 diesel fuel/kg soil) in separate treatments. Plants that showed good tolerance of both factors were further evaluated in a second phase for their efficacy in the phytoremediation of diesel-fuel petroleum hydrocarbons under moderate salinity treatment (1% NaCl). RESULTS: Tropical coral tree and buffelgrass were susceptible to either 2% NaCl or diesel fuel at 10 g/kg soil, but tolerant of diesel fuel at 5 g/kg soil. Kiawe, milo, kou, common ironwood, N. oleander, beach naupaka and false sandalwood were tolerant of high salinity (2% NaCl) or high diesel fuel level (10 g/kg soil). These seven plants were also tolerant of the combined adverse effects of a moderate salinity (1% NaCl) and 10 g diesel fuel/kg soil. Three trees, kiawe, milo and kou significantly accelerated the degradation of petroleum hydrocarbons in the soil spiked with 10 g diesel fuel/kg soil under a moderate salinity treatment (1% NaCl). CONCLUSION: Thus the tropical woody plants, kiawe, milo and kou showed potential for use in phytoremediation of petroleum hydrocarbons in coastal tropical soils. RECOMMENDATIONS AND OUTLOOK: Two fast growing trees, milo and kou, appeared promising for further phytoremediation evaluation in experiments that simulate the soil profile at the field site.     

Phytoremediation of petroleum hydrocarbons in tropical coastal soils II. microbial response to plant roots and contaminant

Goal, Scope and Background.  The goal of this study was to understand the interaction between plants and microorganisms during petroleum-hydrocarbon bioremediation in Pacific Islands coastal soils. Total bacteria and hydrocarbon-degrading microorganisms population dynamics were examined in the rhizospheres of tropical trees and shrubs, which were evaluated for their phytoremediation potential in a greenhouse experiment. The respective and combined effects of plant roots and diesel contaminant on the microbial populations were determined in relation to diesel fuel depletion. An increase in the size of the hydrocarbon-degrading populations of microbes, elicited by rhizodeposition, is generally regarded as conducive to an enhanced degradation of petroleum hydrocarbon polutants in veaetated soil. Conclusion  The results suggest the quality of the rhizodeposition is plant-dependent and governs the type of diesel-degrader populations that will be enhanced by a given plant. Recommendations and Outlook  In the proposed phytoremediation-benefit model plant roots maintain high levels of hydrocarbon de-graders in uncontaminated soil. When the root enters a contaminated zone of soil, those hydrocarbon degraders that prefer the contaminant would switch to the contaminant as a carbon source, effectively removing the hydrocarbons. If the root exudates and the contaminant are equally attractive to the hydrocarbon degraders, the contaminant degradation would be less effective.     

Comparison of the effects of variable site temperatures and constant incubation temperatures on the biodegradation of petroleum hydrocarbons in pilot-scale experiments with field-aged contaminated soils from a cold regions site

Temporal atmospheric temperature changes during summers at sub-Arctic sites often cause periodic fluctuations in shallow landfarm and surface soil temperatures. However, little information is available on the effect of site-relevant variations on biodegradation performance in cold climates. This study compares the rate and extents of biodegradation of petroleum hydrocarbons at variable site temperatures (1-10 °C) representative of summers at a sub-Arctic site reported previously with those obtained under a constant average temperature of 6 °C. The biodegradation was evaluated in pilot-scale landfarming experiments with field-aged petroleum-contaminated soils shipped from Resolution Island (61°30′N, 65°00′W), Nunavut, Canada. Under the variable site temperature conditions biodegradation rate constants of semi- (F2) and non-volatile (F3) hydrocarbon fractions were enhanced by over a factor of two during the 60-d experiment, compared to the constant temperature mode. The decrease in total petroleum hydrocarbons (TPH) under the variable site temperature mode was 55% compared to only 19% under the constant average temperature mode. The enhanced biodegradation is attributable to the non-linear acceleration of microbial activity between 4.7 and 10 °C and faster growth of indigenous hydrocarbon-degrading microbial populations. The first-order biodegradation rate constants of 0.018, 0.024 and 0.016 d⁻¹ for TPH, F2 and F3 fractions at the variable site temperature were in agreement with those determined by an on-site experiment at the same site.     

Utilising the Synergy between Plants and Rhizosphere Microorganisms to Enhance Breakdown of Organic Pollutants in the Environment (15 pp)

Background Phytoremediation is a promising technology for the cleanup of polluted environments. The technology has so far been used mainly to remove toxic heavy metals from contaminated soil, but there is a growing interest in broadening its applications to remove/degrade organic pollutants in the environment. Both plants and soil microorganisms have certain limitations with respect to their individual abilities to remove/breakdown organic compounds. A synergistic action by both rhizosphere microorganisms that leads to increased availability of hydrophobic compounds, and plants that leads to their removal and/or degradation, may overcome many of the limitations, and thus provide a useful basis for enhancing remediation of contaminated environments.Main Features The review of literature presented in this article provides an insight to the nature of plant-microbial interactions in the rhizosphere, with a focus on those processes that are relevant to the breakdown and/or removal of organic pollutants. Due consideration has been given to identify opportunities for utilising the plant-microbial synergy in the rhizosphere to enhance remediation of contaminated environments.Results and Discussion The literature review has highlighted the existence of a synergistic interaction between plants and microbial communities in the rhizosphere. This interaction benefits both microorganisms through provision of nutrients by root exudates, and plants through enhanced nutrient uptake and reduced toxicity of soil contaminants. The ability of the plant-microbial interaction to tackle some of the most recalcitrant organic chemicals is of particular interest with regard to enhancing and extending the scope of remediation technologies.Conclusions Plant-microbial interactions in the rhizosphere offer very useful means for remediating environments contaminated with recalcitrant organic compounds.Outlook A better knowledge of plant-microbial interactions will provide a basis for improving the efficacy of biological remediations. Further research is, however, needed to investigate different feedback mechanisms that select and regulate microbial activity in the rhizosphere.