Kinetics of decomposition of 2,4-dichlorophenoxyacetic acid by Alcaligenes eutrophus JMP134 and in soil

Data on the decomposition of 2,4-dichlorophenoxyacetic acid (2,4-D) by pure cultures of Alcaligenes eutrophus JMP134 and in soil were obtained to investigate the validity of the Eq. (1), c = c0 - k1t1/2 for decomposition at a decreasing rate by a constant amount of enzymes, and Eq. (2), c = c0 - k1t1/2, for decomposition at an increasing rate by an exponentially increasing amount of cells. In the equations, c is the concentration of 2,4-D at time t, c0 is the initial concentration of 2,4-D, q is the maximum metabolic rate, N0 is the initial amount of 2,4-D-degrading microorganisms, and k1 and k2 are rate constants. Equation 2 satisfactorily described the data on decomposition of 2,4-D by A. eutrophus at c0 of 100–400 μg mL−1, with N0 of 0.33-11.8 × 107 cells mL−1, and at initial pH values between 7.1 and 8.4. At an initial pH of 6.1 there was accumulation of 2,4-dichlorophenol (DCP), and the pattern of decomposition of 2,4-D was sigmoidal. When approximately 20 μg of DCP mL−1 had accumulated, the kinetics of decomposition switched from Eq. (2) to Eq. (1). Growth and DCP accumulation then also became linear with t1/2. Equation (1) was also valid for decomposition of 2,4-D under conditions of nitrogen starvation. In soil, Eq. (1) was valid when the number of 2,4-D degrading microorganisms was constant, and Eq. (2) was valid when the number increased exponentially. It is concluded that several occurring patterns of decomposition are described mathematically by Eq. (1), by Eq. (2), or by the sum of these equations. Sigmoidal curves are described by combining the equations in a sequence, thus providing an alternative to models where decomposition curves are treated by one continuous function.