Effect of heavy metal co-contamination on the biodegradation of polycyclic aromatic hydrocarbons in an urban soil with high organic carbon content

Ekumankama, Chinedu (2015) Effect of heavy metal co-contamination on the biodegradation of polycyclic aromatic hydrocarbons in an urban soil with high organic carbon content. Doctoral thesis, Northumbria University.

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Abstract

Biodegradation is a commonly used approach for the removal of organic contaminants from soil, relying on naturally present microorganisms that utilise the pollutants as an energy source. Often these sites are co-contaminated with heavy metals and the aim of the current research was to investigate how this affects the biodegradation of 16 US EPA priority polycyclic aromatic hydrocarbons (PAHs), both in terms of removal rates and the overall functioning of the soil microbial community.

Soil samples were obtained from a Greenfield site in Newcastle upon Tyne. The soil had a high organic content (11.0 %) and also contained elevated lead concentrations as a result of past atmospheric deposition from adjacent industrial activities. PAHs were applied to the soil using a coal tar source dissolved in acetone, giving a total PAH concentration in the spiked soil of 2166 mg kg-1. Individual PAH concentrations ranged from 1.44 mg kg-1 (acenaphthylene) to 325 mg kg-1 (benzo[b]fluoranthene); the benzo[a]pyrene concentration was 255 mg kg-1. The effect of heavy metal co-contaminants on the biodegradation was investigated using separate amendments of cadmium and lead to give respective total concentrations ranging from 133 to 620 mg kg-1 and 340 to 817 mg kg-1. Mercury amendment was used to give an abiotic control. The study was carried out over 40 weeks.

For all treatments, the degradation of PAHs was observed to be biphasic. A novel kinetic model was developed to explain this dependence. In the absence of metal amendment, it was found that PAHs comprising two and three benzene rings generally degrade at a faster rate than four- five and six-membered rings. In the presence of metal amendments, overall % biodegradation after 40 weeks is relatively unaffected for two to four-ring PAHs but shows significant impairment for five and six-ring PAHs. Nevertheless, degradation rates generally decrease with increasing metal concentration, as do soil respiration rate, Shannon Diversity Index, and microbial biomass content. Lead appears to exert the greatest inhibitory effect. The novelty of this study arises from the integrated approach to investigating the effect of metal co-contaminants on the biodegradation of all 16 US EPA priority PAHs together with parameters relating to the functioning and diversity of the soil microbial community.

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