Biodegradation kinetics of two phenoxy acid herbicides, MCPP (±)-2-(4-chloro-2-methylphenoxy)propanoic acid; mecoprop and 2,4-D 2,4-dichlorophenoxyacetic acid were studied in laboratory batch microcosms at low concentrations (0.025-100μg/L) using 14{sup left}C technique with sediments and groundwater from a shallow aerobic sandy aquifer. Below a certain threshold concentration of approximately 1μg/L for 2,4-D and 10 μg/L for MCPP, the biodegradation followed first-order nongrowth kinetics, and no adaptation was observed within the experimental period of 341 d. Half-lifes for ultimate degradation were 500 d for 2,4-D and 1100 d for MCPP at 10℃ in unpolluted aquifer sediment in this environmentally relevant concentration regime. Above the threshold concentrations, the biodegradation rate accelerated gradually due to selective growth of specific biomass, which was ascertained from 14{sup left}C most probable number enumerations of specific phenoxy acid degraders. At the highest concentration tested (100μg/ L), specific degraders increased from 10{sup}(-1) to 10{sup}5 cells/g during the experiment, and half-lifes after adaptation decreased to approximately 5 d. The enhanced rate of degradation by adapted systems was maintained during degradation of the last residuals measured to less than 0.1μg/L. In situ long-term preexposure of the aquifer sediment also resulted in significant higher degradation rates of the phenoxy acids.
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