Bioremediation of spent lubricating oil-contaminated sediments in mangrove microcosm

Abstract

Oil pollution has been recognized as one of the most serious anthropogenic threats to marine and coastal environments. Coastal wetlands distributed along oil transporting routes are vulnerable to oil pollution, and these habitats are often contaminated with oil residues and petroleum hydrocarbons (PH). Bioremediation, the use of biological processes to remove, destroy or sequester hazardous substances from the environment, has received increasing attention in recent years for clean-up purposes. The potential of mangrove wetlands in removing heavy metals, inorganic and organic pollutants from contaminated sediment have been reported. The present study aims to explore the feasibility of using mangrove wetlands to remedy sediment contaminated by spent lubricating oil. A series of microcosm studies were conducted in a greenhouse to determine the potential of mangrove seedlings of different ages, and the importance of oil-degrading microorganisms in the bioremediation process. The one-year old seedlings of two mangrove species, namely Bruguiera gymnorrhiza (Bg) and Acanthus ilicifolius (Ai) were planted in sediment contaminated by spent lubricating oil at a dose of 9.04 ± 1.03 mg oil g-1 sediment fresh weight. The growth and physiological responses of plants during the four-month experiment were also investigated. The performance of these two seedlings was compared with the three-month old A. ilicifolius (3MAi). The results demonstrated that the microcosm planted with one-year old Ai had the highest removal percentage (average of 44%), followed by 3MAi (41%), Bg (36%) and the unplanted microcosm (just natural attenuation) had the lowest removal (only 22%). Not only did the three-month old seedling have a poorer removal efficiency than the one-year old seedling of the same species, oxidative stress was found in the roots of the oil treated 3MAi, suggesting that 3MAi was more susceptible to oil pollution and was not suitable for bioremediation. Growth, measured in terms of leaf number and root biomass, also supported that one-year old Ai was more resistant to oil toxicity than its younger seedlings, and Bg and was more advantageous for remedying oil-contaminated sediments. The sediment properties, including redox potential and microbial count, showed that more oxygen was consumed in the sediment contaminated by spent lubricating oil. Further, the mangrove plants helped increase its oxygen status, leading to more oil degradation by microorganisms. The effects of mangrove plants and the importance of oil-degrading microorganisms on bioremediation of oil-contaminated sediment were further assessed. A greenhouse microcosm study was conducted to study the four commonly used bioremediation methods, namely natural attenuation (without plants and without inoculation of oil-degrading microorganisms), phytoremediation (with one-year old Ai), biostimulation (with the addition of slow-release-fertilizers as extra nutrients) and bioaugmentation (with the inoculation of an oil-degrading microbial consortium enriched from mangrove sediment). A total of nine treatments were prepared to compare the efficiency of each of the four methods and their various combinations in removing spent lubricating oil from the contaminated sandy mangrove sediment. At the end of the four-month treatment, the growth and physiological responses of Ai in the oil-contaminated sediment was comparable to that in the oil-free control, indicating that Ai could tolerate the toxicity of spent lubricating oil. With the addition of nutrients to the contaminated sediments, the root of Ai had the lowest content of malondialdehyde, an indicator of membrane lipid peroxidation and damage due to free radicals, suggesting that biostimulation enhanced the plant’s vigor, as well as its resistance to the reactive oxygen stress caused by oil pollution. This may, in turn, improve the remediation potential of mangrove plants. The residual concentrations of total petroleum hydrocarbon (TPH) in the aliphatic (TPH-F1) and aromatic (TPH-F2) fractions in sediment were measured at the end of the experiment. The mass balance of TPH-F1 and TPH-F2 showed that the TPH taken up by the mangrove plant (one-year old Ai) could only account for a very small amount of its total loss, about 0.4 - 8.4 %, even though Ai could tolerate the oil toxicity. This indicated that the loss of TPH from the mangrove microcosm was mostly due to biodegradation by microorganisms in the sediment. The overall bioremediation process was significantly faster in the microcosm with the inoculation of oil-degrading consortium (bioaugmentation) than that with biostimulation (with nutrient amendment) or phytoremediation (with Ai), and >50% of TPH-F1 was removed with bioaugmentation treatment. The microcosm with the combination of bioaugmentation and biostimulation achieved more than 80% loss of TPH-F2, irrespective of whether it was planted or unplanted. These findings further demonstrated that the oil-degrading inoculants played a more important role in the degradation of oil, especially the aromatic PH, than the mangrove plant. The potential of employing the oil-degrading microbial consortium enriched from mangrove sediment to remedy oil-contaminated wetland habitats should be further explored.