Transformation of benzothiazole in estuarine sediments

Benzothiazole (BT) is a natural and synthetic compound occurring in aquatic sediments and wastewater. The purpose of this work was to investigate BT biogeochemistry in controlled Eh/pH microcosms (CEPMs) containing estuarine sediments of different particle sizes (coarse, intermediate, fine) under oxidized and reduced conditions vs. killed controls, and tide simulation mesocosms (TSMs) containing plants and meiofauna under well-drained (oxidized), consistently saturated/flooded (reduced), and tidal (alternating oxidized/reduced) conditions. Benzothiazole was transformed into complex product mixtures under all conditions. Benzothiazole transformation rates in CEPMs were slower under reduced conditions vs. oxidized conditions in the fine- and intermediate-grain sediments, but the same in the coarse sediment. Quiescent (unstirred) CEPMs showed greatly reduced BT transformation rates in all sediments, with half-lives on the order of 2200 to >4000 h (unstirred) vs. 640 to 1000 h in the continuously stirred systems. The TSM data showed that tidal and drained systems processed BT at identical rates, far exceeding those observed in statically flooded (reduced) TSMs. Mixing was found to be a more significant variable in BT transformation rate than either Eh or sediment particle size breakdown, with constant stirring increasing observed degradation appreciably. Otherwise, BT was transformed more readily on sediments of high surface area under oxidized conditions vs. coarser sediments and those under reducing electrochemical conditions.