Biodegradation of complex aromatic compounds in nuclear process water

Abstract

Nuclear energy generation results in the production of effluents and radioactive waste that are very difficult to treat and dispose. A considerable fraction of nuclear waste is discharged in the form of complex mixtures of hazardous organic compounds and metallic radionuclides. The most serious pollution is caused by polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls that are very difficult to remove from the environment. The nuclear industry faces certain challenges related to treatment and safe disposal of these mixed radioactive organic wastes due to the toxicity and recalcitrant nature of the organics. Techniques currently used in treating the waste include physical-chemical processes that have resulted in the generation of the secondary waste requiring further treatment before disposal to the environment. These conventional processes also require the use of strong oxidising agents and higher than natural pH and temperature. Therefore, it is of great importance to develop new environmentally friendly technologies. One suggested method employs specialised cultures of bacteria to completely mineralize the organic compounds without leaving traces of harmful byproducts. The efficiency of bacteria to remove these types of compounds may be improved by in situ application. During in situ application, the bacteria apply a variety of pathways to break down the compounds and use them as their energy and carbon sources. These processes may be carried out within the natural pH and temperature range capable of supporting life forms. In the current study, a more detailed analysis of the biodegradation capability of the organic compounds was conducted and the following were the major findings of the study: <ul><li> Wastewater from an actual radionuclide processing facility was characterised and was found to contain all the 16 priority PAHs in the range 0.001-25 mg/L. Acenaphthene (detected at 25.1 mg/L) was the most abundant. Most of the PAHs in the wastewater samples exceeded the WHO limit of 0.05ìg/L indicating the need for further treatment before final disposal to the environment.</li> <li> After purifying and sequencing the rRNA genes from the soil and mine water bacteria, a total of 5 and 3 bacterial isolates were found, respectively. The rRNA sequences were isolated from bacteria with some tolerance to PAH toxicity and were thus candidate species for naphthalene degradation. The bacteria from soil were predominated by aromatic compound degraders Pseudomonas aeruginosa, Microbacterium esteraromaticum and Alcaligenes sp. In mine water, only Pseudomonas putida was identified as a known aromatic ring cleaving species.</li> <li> The biodegradation of naphthalene by the purified cultures was determined to be limited by its solubility (30mg/L) and toxic effects of the aromatic compounds. A kinetic model was derived based on the metabolism and microbial growth kinetics. The model predicted the concentration remaining in solution under different initial (added) PAH concentrations. A simplified coupled dissolution-degradation model was used to model the kinetics of degradation. With help of the model, parameters were estimated and the sensitivity of parameter value was also evaluated. The aim of model was to help gain a better understanding of biological degradation. This could be used for optimisation of the process and scale up of the process to pilot and full-scale application.</li></ul>