ABSTRACT
The availability of molecular oxygen has a profound effect on the biodegradation of various compounds. Oxygen limitation often is the most troublesome problem facing in situ bioremediation for hydrocarbons and other pollutants that are biodegraded aerobically. It is observed that the rates of HC biodegradation are limited by abiotic environmental factors and molecular oxygen is required for rapid HC biodegradation (Atlas and Bartha, 1992). The conditions for oxygen limitation do not exist in the upper levels of the water column in marine and freshwater environments (Cooney, 1985) but its limitation and the demand for oxygen can be overcome at contaminated aquifers by the addition of hydrogen peroxide (Berwanger and Barker, 1988), which can support microbial utilization of hydrocarbons. However, hydrogen peroxide being toxic to microorganisms at higher concentrations may lower the actual rates of HC biodegradation. Berwanger and Barker (1988) investigated in situ biorestoration stimulating aerobic biodegradation in a contaminated anaerobic, methane saturated groundwater using hydrogen peroxide as an oxygen source. Batch biodegradation experiments conducted with groundwater and Canadian landfill samples have indicated that hydrogen peroxide at non-toxic levels promoted the rapid biodegradation of benzene, toluene, ethyl benzene, and o, m, and p-xylene. There is a growing interest in anaerobic hydrocarbon metabolism and in recent years, this is recognized as an important process in contaminated anoxic environments. Numerous reports have appeared on the anaerobic biodegradation of some low-molecular weight aromatic hydrocarbons (Zeyer, 1993). Denitrifying, iron, manganese and sulphate reducing bacteria have been isolated that have the ability to degrade simple aromatic or aliphatic hydrocarbons under anoxic conditions.
