Biodegradation: selection of suitable model.

Biodegradation of five herbicides, two acetanilides (propanil and propachlor), and three phenylureas (diuron, monuron, and fenuron) was analyzed in samples of river water during a period of 6-8 weeks. Concentrations of 0.2-5.0 mg/liter of the herbicides were used for biodegradation. Two types of river water samples with different numbers of microorganisms were collected from the Neris River: upstream and downstream of the city of Vilnius. The initial concentration of microorganisms varied from 4.7 x 10(5) to 2.7 x 10(6) cells/liter and from 1.4 x 10(8) to 5.3 x 10(8) cells/liter in water samples from the Neris River upstream and downstream of Vilnius, respectively. Chemical analysis was performed by the HPLC technique, using standards of herbicides and likely degradation products. Chemical parameters of different river water samples used in biodegradation experiments were analyzed. A second-order reaction rate model was used for the analysis of biodegradation data. Values of the first-order rate constants (Ka) revealed the following decrease in the biodegradation rate of herbicides: propanil >> diuron > or = monuron = propachlor > fenuron. This sequence was constant for all water samples analyzed. The set of decreasing value of second-order biodegradation rate constants (Kb) differed from the set of first-order constants (Ka) because the total number of bacteria in the water samples varied by up to two orders of magnitude, and this variation influenced the calculated values of Kb. Thus, different sets of Kb values were obtained for the water samples from the river upstream and downstream of the city. Schemes of a variety of biodegradation models are presented, and the suitability of the second-order reaction rate model for the description of biodegradation of xenobiotics is discussed.