Bioremediation of oil-contaminated soil : a South African case study.
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Date
1996
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Abstract
In 1990, an oil recycling plant situated in Hammarsdale, South Africa, was
decommissioned and a decision was taken by management to rehabilitate the
site in preparation for resale. The heavily impacted area covered over two
hecatares and oil contamination penetrated soil to depths in excess of three
metres, making excavation and removal of the soil very expensive. The
options for remediation of the site were limited. No facility for incineration of
contaminated soil exists in South Africa, and landfilling was not permitted.
The emphasis in developing a remediation strategy, therefore, focussed upon
the possibility of in situ remediation with minimal excavation of soil. This
study, the first of its kind in South Africa, was subsequently initiated to
assess the feasibility of this approach, the results of which would underpin a
full-scale cleanup programme.
The development of such a strategy involved four key stages of work : (1) a
comprehensive site investigation to evaluate and fully understand the
particular problems at the site; (2) treatability studies to determine the
potential for biological treatment of the contaminated soil and the
optimisation of such treatments, particularly in terms of time and cost; (3) the
testing of some of the more effective treatments on a pilot-scale; and (4)
recommendations for full-scale bioremediation of the contaminated site.
various conditions unique to South Africa had to be considered at each stage
viz. the lack of funds and remediation experience, which created numerous
problems and emphasised the requirement for a simple, "low-tech"
approach.
Site investigations revealed that in situ remediation may be possible due to
the high permeability of the sandy soils and low concentrations of heavy
metals. Laboratory experiments also showed that a mixed association of
indigenous microorganisms was present which, once stimulated by nutrient supplementation at C:N:P, ratios of between 10:1:1 and 20:1:1, was capable of degrading total petroleum hydrocarbons at an average rate of 11% week -1. Further experimentation, aimed at reducing the cost of remediation and
improving the soil quality, focussed on the efficacy of oil solubilisers, a soil ameliorant (composted pine-bark), indigenous fungi and higher plants in the remedial process.
Three commercial surfactants (Arkopal N-050, N-060 and E2491) and one natural solubiliser (soybean lecithin) were tested for their biotoxicity, solubilisation and biodegradability at various concentrations (0.01 - 1.0%).
Formulation E2491 was able to support a microbial population and was
selected as the preferred commercial surfactant if soil washing was to be
recommended; however, lecithin was considered to be more useful in situ
because of its localised solubilising effect, biological origin and nutritional
contribution.
The use of fungi was of particular interest in addressing the persistent
organic compounds, such as the heavy fractions of oil, for which bacterial
remediation methods have been slow or ineffective. While it was not possible,
however, to demonstrate in the laboratory that the indigenous fungi
contributed significantly towards the degradation of the contaminating oil,
the basic trends revealed that the fungal component of the indigenous
microbial population was readily stimulated by the addition of nutrient
supplements. The bulking-up process was also a success and additional
exploratory work was proposed in the form of a larger scale composting
design.
Finally, the potential for using higher plants and 20% (v / v) composted pinebark
(in addition to nutrients) to increase the microbial degradation of the
contamination was investigated in both greenhouse and field plot studies.
Greenhouse investigations employed soybeans which were postulated to have soil quality and cost benefits. However, although the soybeans were
found to significantly enhance the remedial process, the complex soil-contaminant-
plant interactions gave rise to strange nutritional effects and, in
some cases, severe stunting. In contrast, the field studies employed grasses
that had previously established on the site and which ultimately
demonstrated a better tolerance for the contaminated conditions. Scanning
electron microscopy revealed that there were considerable differences
between the root tips of soybean plants which had been grown in
contaminated soil and those which had been grown in uncontaminated soil. It
was concluded that toxicity symptoms, which are readily observed in the
root, could be used as an early indicator for determining the suitability of
vegetation for remediation purposes. In both instances, despite the
differences, the addition of composted pine-bark and nutrients (nitrogen and
phosphorus) resulted in total petroleum hydrocarbon reductions of >85%,
illustrating the benefits of plant establishment and oxygen availability.
The need to link results from laboratory or pilot-scale experiments to achieve
reliable predictions of field-scale behaviour was an essential component of
this research. The results of the field study provided evidence, similar to that
found in the pot trial, of the accelerated disappearance of organic compounds
in the rhizosphere. All experiments incorporated parallel measurements of
hydrocarbon residues, microbial activity and pH changes in the contaminated
soil, the results of which strongly supported the argument that
biodegradation was the dominant component of the remediation process.
Thus, after consideration of the significant interactions which dominated the
study (time-contaminant-nutrient; time-contaminant-pine-bark; and time-contaminant-
pine-bark-plant), it was clear that, aside from these limiting
factors, little should preclude the in situ bioremediation of the impacted soil.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1996.
Keywords
Oil pollution of soils., Petroleum--Biodegradation., Bioremediation., Theses--Microbiology.