Behaviour of pharmaceuticals in rivers: flume experiments on the effect of different hydraulic conditions on elimination kinetics
Uwe Kunkel (01/2008)
Support: Michael Radke
Pharmaceutical residues are common micropollutants detected in sewage water, surface water, ground water and even in drinking water. In recent years, a few studies investigated their environmental fate in batch or column experiments. However, these experiments can not simulate interactions of surface water and pore water – which are decisive for the fate of pharmaceuticals in rivers. Therefore, experiments in an artificial streaming channel (flume) were conducted to investigate the influence of different flow conditions on the fate of six acidic pharmaceuticals (bezafibrate, clofibric acid, diclofenac, gemfibrozil, ibuprofen, naproxen). An annular flume was designed specifically for this study where experiments at different flow velocities which in turn induced different mechanisms of surface water/pore water interactions were conducted. Two different elimination kinetics were calculated. First, dissipation times (DT50) were deduced from the concentration trend of pharmaceuticals in the surface water. Furthermore, half life times (tH) of the microbial degradation of the pharmaceuticals were determined by the usage of clofibric acid as inert tracer and by inverse simulation. Finally, DT50 and tH were multiplied with the mean flow velocity of each experiment to obtain respective dissipation distances (DD50) and half life distances (dH). Pharmaceuticals behaved differently during the flume experiments. Ibuprofen was well biodegraded while bezafibrate and naproxen were moderately biodegraded. In contrast, clofibric acid behaved similar to bromide that was used as inert tracer. The behaviour of diclofenac and gemfibrozil strongly depended on the redox conditions in the sediment. Sorption was negligible for all pharmaceuticals. The calculated DT50 varied from 1.2 days for ibuprofen to 8.5 days for diclofenac. Higher flow velocities caused shorter DT50 due to enhanced exchange of surface and pore water. Calculated tH ranged from 2.4 days for ibuprofen to 18.6 days for diclofenac. Variations in degradation rates at different flow velocities referred to both transport limited degradation and different redox conditions in the sediment. Inverse simulation constituted no appropriate method for deriving half life times due to too few measuring data in the sediment. The dissipation distances (DD50) ranged from 24.1 kilometres (ibuprofen) to 146.9 kilometres (gemfibrozil), dH from 52.5 kilometres for ibuprofen to 278.3 kilometres for gemfibrozil. Despite the higher temporal elimination (shorter DT50 and tH) at higher flow velocities, respective elimination distances (DD50 and dH) were similar at both flow velocities. Comparative values for fields studies do hardly exist but DT50 are assumed to be shorter for some of the pharmaceuticals (diclofenac, naproxen) due to photodegradation.