Heavy metals often occur as co-contaminants with polycyclic aromatic hydrocarbons (PAHs) and reportedly have adverse effects on biodegradation. In this study, the development of C-14-phenanthrene mineralisation in soil co-contaminated with aged or freshly added Al or Fe amendment was assessed.

C-14-phenanthrene mineralisation was assessed using respirometry; respirometers incorporated a Teflon-lined screw-capped CO2 trap containing 1-M NaOH within a glass scintillation vial. The production of (CO2)-C-14 was assessed by the addition of Ultima Gold liquid scintillation fluid to the CO2 traps and subsequent liquid scintillation counting. Enumeration of phenanthrene-degrading bacteria was achieved by counting the colony forming unit count using the spread plate method.

This investigation considered the effects of Al and Fe (50, 100, 250 and 500 mg/kg) on C-14-phenanthrene biodegradation in soil over 63-day contact time. Fresh Al amendments at lower concentrations (50 and 100 mg/kg) stimulated phenanthrene catabolism (p <0.05) at t = 21 and 42 days which may reflect an 'Arndt-Schulz' effect, but phenanthrene catabolism was significantly reduced (p <0.05) in 500 mg/kg aged Al this could be due to Al toxicity to phenanthrene degraders. Phenanthrene mineralisation was stimulated in the highest Fe concentration (500 mg/kg) in aged and fresh Fe amendments at t = 21 days. This could be because Fe is an essential requirement for microbial growth.

The impact of Al or Fe on the catabolism of C-14-phenanthrene was dependent on incubation time and Al was more toxic than Fe to soil PAH catabolic activity. This could be because Al is a non-essential microbial requirement. Bioremediation of soils co-contaminated with PAH and heavy metal is a complex problem; therefore, studies on the impact of metals on PAHs biodegradation highlight the risks and biodegradation potential in contaminated soil.