The study reports on the biodegradation of phenanthrene in soil by an indigenous soil microbial biomass in the presence of zinc. An uncontaminated soil was spiked with zinc and phenenathrene as both single and co-contaminants, and microbial metabolic activity was measured using an intracellular dehydrogenase enzyme bioassay. The extent of phenanthrene biodegradation in the co-contaminated soil was measured over a period of 37 days. Contaminants were spiked to soil in accordance to values specified in ‘The New Dutchlist’ (2000) which states an optimum zinc concentration for soil of 140 mg/kg, and advises corrective action at 720 mg/kg. Double this zinc concentration I.e. 1440 mg/kg was also spiked to soil samples to represent serious soil zinc contamination. The amount of phenanthrene spiked to soil was also according to ‘The New Dutchlist’ which stipulates an action value for total PAH of 40 mg/kg, and an optimum value of 1mg/kg. There is no value stipulated for individual PAHs. Phenanthrene was spiked to soil to the following concentrations: 1, 40 and 80mg/kg of soil, representing optimum, action and serious contamination levels respectively. Measurements of microbial activity in soils contaminated with zinc or phenanthrene alone indicated the presence of tolerant, albeit inhibited soil microorganisms, relative to an biotic control, where a lower soil zinc concentration of 140 mg/kg in the cocontaminated soil resulted in a marginal stimulation in the rate of phenanthrene biodegradation. However, elevated concentrations of zinc, above the optimum value stipulated in ‘The New Dutchlist’ were inhibitory to phenanthrene degradation, thereby highlighting the potential constraints of bioremediation in co-contaminated soils.
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