Complex soil-microorganism-pollutant interactions underpinning bioremediation of hydrocarbon/heavy metal contaminated soil.
This study evaluated the efficacy of bioremediation as a treatment option for a hydrocarbon and heavy metal contaminated soil. Microbial degradation of hydrocarbons under aerobic, nitrate-reducing and sulphate-reducing conditions was examined. Nutrient supplementation with nitrogen and phosphate as well as aeration seemed to be the most important factors for enhancing biodegradation. From initial batch studies, a carbon: nitrogen ratio of 50: 1 was found to be optimal for biodegradation. However, very low carbon to nitrogen ratios were undesirable since these inhibited microbial activity. Manipulation of the pH did not seem to be beneficial with regard to hydrocarbon biodegradation. However, low pH values induced elevated concentrations of leachate heavy metals. Aerobic conditions provided optimal conditions for hydrocarbon catabolism with up to 54% of the original contaminant degraded after 2 months of treatment. Further treatment for up to 20 months did not significantly increase hydrocarbon biodegradation. Under nitrate- and sulphatereducing conditions, 6% and 31 % respectively of the initial contaminant was degraded after 2 months while after a further 20 months, 50% and 42%, respectively were degraded. The addition of soil bulking agents and the use of sparging did not significantly increase biodegradation. Similarly, the addition of inoculum did not influence biodegradation rates to any great degree. The presence of heavy metals up to concentrations of 400 mgt1 Mn, 176 mgt1 Zn and 94 mgt1 Ni did not reduce microbial activity within the soil. During the treatment phase, heavy metal and hydrocarbon migration were limited even under water saturation and low pH conditions. A Biodegradation Index was developed and evaluated and may, potentially, find use as an in situ assessment technique for microbial hydrocarbon catabolism. The iodonitrophenyltetrazolium salt assay was also found to be an effective and rapid alternative assay for monitoring bioremediation progress.