Phytoremediation and bioremediation of petroleum contaminated soils and wastes
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Bioremediation of petroleum contamination has been used by the petroleum industry for decades. Phytoremediation is an emerging technology that is too new to be widely accepted. There are many unknowns in petroleum phytoremediation. This research focuses on furthering our understanding of the potential for phytoremediation of petroleum-contaminated soils & sludges. A series of experiments were conducted to achieve the goal. In addition, critical reviews on petroleum biodegradation, kinetics, volatilization, seed germination, soil microbial-pollutants-root relationship, the rhizosphere, and the plant root systems as well as recent research relating to petroleum bioremediation and phytoremediation are presented. Screening tests were conducted in cups and pots using 200 g of soil for 12 weeks. TPH (Total Petroleum Hydrocarbons) levels, seed germination, and plant biomass were measured. The results show that petroleum contaminants in soil have adverse effects on plant seed germination as well as plant growth. Soil freshly contaminated with diesel at 2% (w/w) level could totally hinder ryegrass and bromus grass seed germination. Oil sludge is found to be less toxic to both plant species. Ryegrass is found to be more tolerant to diesel and oil sludge soil than bromus grass. The possible contribution of volatilization loss to landfarming of diesel and oil sludge soils was investigated under conditions similar to those of other experiments of this study. The results show that oil sludge is non-volatile; although intense diesel flux volatilization from fresh liquid diesel is found within a short period of time (≤30 days), the actual diesel flux volatilization from diesel soils is far less than from liquid diesel. Germination of plant seeds in petroleum contaminated soils have been found to be a common difficulty for many researchers, especially when lighter molecular hydrocarbons exist in the soil. Germination experiments were conducted inside a 20°C incubator with 5.5cm Petri dishes containing 15 g soil sown with ryegrass seeds. Results show that ryegrass seed treatment in a 20% PEG (polyethylene glycol) solution and incubated at 20°C for three days increases the ryegrass seed germination rate from 20% to 90% in 3% (w/w) diesel soil (freshly contaminated). Similar effects were found for seeds sowed in oil sludge soils. Soil microcosms with 200 g soil in a 1.65 1 glass jar were conducted to investigate the biotreatability of diesel and oil sludge. The results indicate that both diesel and oil sludge compounds are biodegradable by indigenous soil microorganisms with various degradation rates. For example, TPH reduction and CO₂ evolution for 2% diesel soil continued for the whole test duration (189 days). For 3% oil sludge soil, TPH reduction slowed down and CO₂ evolution almost stopped after 50 days. A series of experiments were conducted in 40cm deep columns with 4.0kg soil. The columns were monitored for TPH levels, root development and CO₂ concentration over 102 days. The experimental design of the columns allows one to monitor soil CO₂ concentration directly, and is a design that hasn't been used in other research. The soil gas (CO₂) analysis shows that diesel soil columns planted with ryegrass had higher soil CO₂ concentration than un-planted ones, which implies that microbial activities are stimulated by the growth of rye grass roots. (A comparison of the results with data from the screening test show that higher rooting intensity (mg root/kg soil) in diesel soil results in better diesel degradation. The results indicate that living plant root growth and distribution in diesel-contaminated soil play an important role in the effectiveness of phytoremediation. Experiments were conducted outdoors with 4kg soil in plastic trays over 331 days to evaluate the feasibility of combining land treatment and phytoremediation. The results indicate that land treatment with or without phytoremediation achieved similar TPH removal. Since phytoremediation is likely to reduce the operation cost of a land treatment project, it could be economically feasible to link landfarming and phytoremediation as a treatment strategy. A rough estimate of the contribution of several mechanisms to the TPH loss in land treatment was done by comparing the data obtained from various experiments in this study. The contribution of biodegradation, volatilization, and un-extractable TPH loss are estimated to be: 38 to 48% biodegradation, 18% volatilization, and 19% un-extractable TPH loss for 2.0% diesel soil, and 33 to 34% biodegradation, negligible volatilization, and 7% un-extractable TPH loss for 3.0% oil sludge soil. This dissertation represents a systematic approach to investigate and develop the information and knowledge that would be useful in the application of phytoremediation for these petroleum-contaminated soils. Discussion and recommendations on further research are provided.