Biodegradation of s-triazine xenobiotics

Abstract

Abstract Biodegradation of xenobiotic compounds is examined with s-triazines as an example and with biological treatment of wastewater containing s-triazines as an aim. s-Triazines have been termed recalcitrant, but examination of the literature indicates that a potential for biodegradation exists. Nitrogen-limited enrichment cultures yield organisms able to degrade by-products of the industrial synthesis of s-triazine herbicides as sources of nitrogen. The choice of inoculum for these enrichments is important and often allows for successful enrichment after simple batch culture, but organisms containing several degradative reactions could be obtained only after selection in extended culture. Routine, specific determinations of all s-triazines (by HPLC) were essential throughout the work. Molar growth yields show complete mass balances for the utilization of s-triazines. Kinetic experiments indicate that specific degradation rates of s-triazines in growing cells are about 0.4 mkat/kg of protein. Characterised biochemical pathways consist of a series of hydrolytic cleavages of chloro-, amino- and alkylamino-groups from the s-triazine ring. Pathways converge to cyanuric acid, which is subject to hydrolytic ring cleavage to CO2 and NH4+ via hydrolysis of biuret and urea. Our cultures degraded all significant s-triazines in real wastewater. But the system was not practicable because the specific activities of some enzymes were too low, because of inhibition by salt in the wastewater and because expensive carbon sources were necessary. Improved planning for enrichment cultures is seen to be necessary and this depends on adequate knowledge of the chemistry of the wastes.