Development of rhizoremediation as a treatment technology in the removal of Polycyclic aromatic Hydrocarbons (PAHs) from the environment

Abstract

Numerous classes and types of chemicals, which contaminate soil, complicate the removal of many toxic compounds from the environment. For example, many soils are contaminated with one or more metals, radioactive and inorganic compounds. Large areas are polluted with recalcitrant organic substances that pose environmental problems due to their toxicity and tendency to disperse through wind and water errosion. Polycyclic aromatic Hydrocarbons (PAHs) are ubiquitous pollutants found in soil at wood preservation plants and gasworks. PAHs are chemical compounds that persist in the environment and thus cause pollution worldwide. Their persistence in the environment is due to their low water solubility. The major source of PAHs is from the combustion of organic material such as coal, tar, wood and rubber. PAHs have been detected in a wide variety of environmental samples including air, soil, sediments, water, oils, tars and foodstuff. Most people are exposed to PAHs when they breathe smoke, autoemissions or industrial fumes. Because PAHs are toxic, mutagenic and carcinogenic to humans and animals, their elimination from the environment is of paramount importance. The estimated costs for the clean up of PAHs contaminated sites with conventional techniques such as incineration and landfilling are enormous. The search for alternative methods to restore polluted sites in a less expensive, less labour intensive, safe and environmentally friendly way is required. Such an alternative method is rhizoremediation, which is defined as the use of plants in association with microorganisms to degrade environmental pollutants such as PAHs. Microbial communities exposed to hydrocarbons become adapted, exhibiting selective enrichment and genetic changes resulting in an increased proportion of hydrocarbon degrading bacteria and bacteria plasmid encoding hydrocarbon catabolic genes. Adapted microbial communities have higher proportions of hydrocarbon degraders that can respond to the presence of hydrocarbon pollutants. The aim of this study was to identify bacteria isolated from the rhizosphere of Elusine coracana, Biddens pilosa, Brantha serratia and Cyperus esculentus grown in polluted and unpolluted soil and to evaluate the potential of bacteria isolated from the rhizosphere of these plants grown in polluted soil for their ability to bioremediate Polycyclic aromatic Hydrocarbons (PAHs). Different concentrations (1%, 3% and 5%) of naphthalene and acenaphthene (PAHs) were made and added to 500ml Bacteriological agar.1000µl of bacterial suspensions were spread onto the surface of naphthalene and acenaphthene-based agar plates and incubated for 48h at 37OC. Results showed that Brevibacillus brevis, Brevindimonas versicularis, Vibrio vulnificus, Chryseo indologenes, Micrococcus spp, Bacillus stearothermophilus, Pseudomonas putida and Pseudomonas spinosa showed excellent growth in all concentrations in naphthalene based agar medium. However, Micrococcus spp and Pseudomonas spinosa showed limited growth in acenaphthene based agar medium. Amongst all these bacteria tested for their potential in utilizing PAHs, B. brevis, V. vulnificus, C. indologenes, B. stearothermophilus, and P. putida were the most promising for biodegradation of PAHs, since none of them were affected by any change in concentration either in naphthalene or acenaphthene.