Modeling the effect of adsorption on the degradation rate of propiconazole in profiles of Polish Luvisols

Laboratory adsorption and degradation studies were carried out to determine the effect of time-dependent adsorption on propiconazole degradation rates in samples from three Polish Luvisols. Strong propiconazole adsorption (organic carbon normalized adsorption coefficients Koc in the range of 1217-7777 mL/g) was observed in batch experiments, with a typical biphasic mechanism with a fast initial step followed by the time-dependent step, which finished within 48 h in the majority of soils. The time-dependent step observed in incubation experiments was longer (duration from 5 to 23 d), and its contribution to total adsorption was from 20% to 34%. The half-lives obtained at 25 °C and 40% maximum water holding capacity of soil, were in the range of 34.7-112.9 d in the Ap horizon and in the range of 42.3-448.8 d for subsoils. The very strong correlations, between degradation rates in pore water and soil organic carbon and soil microbial activity, indicated that microbial degradation of propiconazole was most likely the only significant process responsible for the decay of this compound under aerobic conditions for the whole of the examined soil profiles. Modeling of the processes showed that only models coupling adsorption and degradation were able to correctly describe the experimental data. The analysis of the bioavailability factor values showed that degradation was not limited by the rate of propiconazole desorption from soil, but sorption affected the degradation rate by decreasing its availability for microorganisms.