Degradation of oxytetracycline in the aquatic environment: a proposed steady state kinetic model that takes into account hydrolysis, photolysis, microbial degradation and adsorption by colloidal and sediment particles

Abstract

Environmental context. Pollution of the aquatic environment by oxytetracycline can lead to microbial resistance thereby compromising the efficacy of current medication regimes. Adsorption by colloidal and sediment particles reduces the rate at which oxytetracycline degrades, whereas the longer the antimicrobial remains in the aquatic environment, the greater the danger of microbial resistance. There is need therefore for a fuller understanding of the kinetics of degradation of oxytetracycline in aquatic ecosystems before measures for mitigating pollution by the antimicrobial can be designed.Abstract. The persistence of oxytetracycline in an aquatic microcosm and distilled water control experiments, was studied over a period of 90 days. An immediate 35 % loss as a result of adsorption by the sediment was observed in the microcosm experiment soon after charging. Subsequently triphasic linear rates of oxytetracycline degradation were observed for both the water phase (3.1 Â 10 À2 , 5.8 Â 10 À3 and 1 Â 10 À3 mg g À1 day À1 ) and sediment phase (4.8 Â 10 À2 , 6.5 Â 10 À3 and 2 Â 10 À4 mg g À1 day À1 ). Degradation is attributed to photolysis and microbial degradation of the free oxytetracycline in solution, and microbial degradation of the colloidal and sediment particle adsorbed speciation forms. The distilled water control exhibited biphasic zero order kinetics attributed to hydrolysis (2 Â 10 À6 mg g À1 day À1 ) and microbial degradation (2.7 Â 10 À3 mg g À1 day À1 ) under dark conditions, and monophasic zero order kinetics attributed to photolysis (6.9 Â 10 À3 mg g À1 day À1 ) under sunlight exposure. A kinetic model that takes into account hydrolysis, photolysis, microbial degradation and adsorption of the antibiotic by colloidal and sediment particles, is presented to account for the monophasic, biphasic and triphasic zero order kinetics observed in the control and microcosm experiments. Possible remediation strategies for mitigating aquatic environments polluted by the antimicrobial are discussed.

Speciation and persistence of doxycycline in the aquatic environment: Characterization in terms of steady state kinetics

“…Hydrolysis can be represented by the steps shown in Table 4. It can be shown that [17] dP dt Rates of degradation in distilled water exposed to natural light In the exposed distilled water experiment, the initial linear rate of degradation at 2.9 £ 10 ¡3 mgg ¡1 day ¡1 (Fig. 7(ii) and Table 2) is about five times slower than the final rate at 9.8 £ 10 ¡3 mgg ¡1 day ¡1 .…”

“…[17] Dispersive solid phase extraction has been previously used to extract tetracycline antibacterials Speciation and persistence of doxycycline in aquatic environment 909 from food samples such as milk and meat. [19,20] Coupling to ultrasonication was found to improve reproducibility and percentage recovery (>80% for most extractions).…”

“…8(i) and Table 2) in the water phase of the microcosm experiment is attributed to a combination of hydrolysis, photolysis, and microbial degradation of free doxycycline in the solution in the water phase of the experiment. [17,[24][25][26] From the distilled water experiments, the combined contribution of hydrolysis and photolysis is 2.9 £ 10 ¡3 mgg ¡1 day ¡1 (see Table 2) so that the contribution of microbial degradation is 2.01 £ 10 ¡2 mgg ¡ day ¡1 .…”

“…[17,[24][25][26] Distilled water experiments were set up as controls. Two basic assumptions were made in designing the distilled water control experiments.…”

“…Recent studies have shown that degradation of tetracyclines can be enhanced or decreased by the presence of organic carbon, [15,16] implying that adsorption of organic matter plays a significant role in the rate of degradation of tetracyclines. Similarly, Zaranyika et al [17] studied the degradation of oxytetracycline in the aquatic environment, and showed that degradation of oxytetracycline followed triphasic linear rates consistent with an enzymatic kinetic model that takes into account adsorption/desorption equilibria of the antimicrobial onto colloidal and sediment particles. The photochemical degradation of doxycycline in soil and its microbial degradation in animal manure were studied by Szatm ari et al [12] Varying half-lives ranging from 4.5 days to 76.3 days were reported.…”