Abstract
Tetracycline drugs, extensively used as human and veterinary broad spectrum antibiotics, enter surface and drinking water sources through runoff and seepage following application of manure in agriculture, and direct discharge in pharmaceutical plant and hospital effluent. The aim of the present work was to study the bio-accessibility of tetracycline drugs in the aquatic environment to higher fauna. The study was achieved by first proposing an aquatic bio-accessibility coefficient towards higher fauna, defined as the fraction of the substance in solution in the water column over the mean concentration of the substance in a given aquatic system, and calculated assuming that non-settling adsorbed speciation forms are desorbed upon contact with the gastrointestinal fluid. The definition further assumes that organic substances residing within the sediment are not bioaccessible to higher fauna, but become bioaccessible following sediment-to-water-column equilibrium transfer. The bioaccessibility coefficients of tetracycline antibiotics in a typical tropical river ecosystem were studied by determining the distribution of the antibiotics between the water column and the sediment over a period of 90 days using microcosm experiments. Data are presented showing that, based on theoretical values calculated taking into account the different rates of degradation of different speciation forms of the antibiotic in both water and sediment phases of the aquatic ecosystem, oxytetracycline and chlortetracycline exhibit medium high aquatic bioaccessibility coefficients (1.0 – 1.5), while doxycycline exhibits a high aquatic bioaccessibility coefficient (1.5 – 2.0). The differences observed in the theoretical bio-accessibility coefficients of TCs are attributed to differences in the strengths of the adsorption bonds of different tetracyclines to colloidal particles. Data are further presented showing that experimental values of the aquatic bio-accessibility coefficient and water-clumn/sediment partition coefficient may also deviated considerably from their corresponding theoretical values, as a result of errors arising from re-suspension of antibiotic adsorbed to settling colloidal particles during sampling.
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