BIODEGRADATION - SELECTION OF SUITABLE MODEL

Biodegradation of five herbicides, two acetanilides (propanil and prop achlor), and three phenylureas (diuron, monuron, and fenuron) was anal yzed in samples of river water during a period of 6-8 weeks. Concentra tions of 0.2-5.0 mg/liter of the herbicides were used for biodegradati on. Two types of river water samples with different numbers of microor ganisms were collected from the Neris River: upstream and downstream o f the city of Vilnius. The initial concentration of microorganisms var ied from 4.7 x 10(5) to 2.7 x 10(6) cells/liter and from 1.4 x 10(8) t o 53 x 10(8) cells/liter in water samples from the Neris River upstrea m and downstream of Vilnius, respectively. Chemical analysis was perfo rmed by the HPLC technique, using standards of herbicides and likely d egradation products. Chemical parameters of different river water samp les used in biodegradation experiments were analyzed. A second-order r eaction rate model was used for the analysis of biodegradation data. V alues of the first-order rate constants (K-a) revealed the following d ecrease in the biodegradation rate of herbicides: propanil much greate r than diuron greater than or equal to monuron = propachlor>fenuron. T his sequence was constant for all water samples analyzed. The set of d ecreasing value of second-order biodegradation rate constants (K-b) di ffered from the set of first-order constants (K,) because the total nu mber of bacteria in the water samples varied by up to two orders of ma gnitude, and this variation influenced the calculated values of K-b. T hus, different sets of K-b values were obtained for the water samples from the river upstream and downstream of the city. Schemes of a varie ty of biodegradation models are presented, and the suitability of the second-order reaction rate model for the description of biodegradation of xenobiotics is discussed. (C) 1998 Academic Press.