Evidence for the microbially mediated abiotic formation of reversible and non-reversible sulfamethoxazole transformation products during denitrification

Abstract

The antibiotic sulfonamide drug sulfamethoxazole (SMX) is extensively used in both human and veterinary medicine. Since it cannot be completely eliminated by the typical state-of-the-art wastewater treatment technology, it is frequently detected in the water cycle. SMX, as aromatic amine, can undergo abiotic transformations with the under denitrifying conditions produced nitrogen species nitric oxide (NO) and nitrite (NO(2)(-)). NO and aromatic amines are commonly known to form diazonium cations. Depending on the reaction conditions the diazonium cation disintegrates under cleavage of elementary nitrogen and substitutes its diazo-group by an NO(2)-group or by hydrogen. Following this approach, two transformation products (TPs) of the persistent SMX under denitrifying conditions were hypothesized and synthesized: 4-nitro-N-(5-methylisoxazol-3-yl)-benzenesulfonamide (4-nitro-SMX) and N-(5-methylisoxazol-3-yl)-benzenesulfonamide (desamino-SMX). The synthesized compounds were identified by Nuclear Magnetic Resonance (NMR) spectroscopy and used as reference standards for their confirmation and quantification in denitrifying water/sediment batch experiments and in environmental samples. During the denitrifying degradation experiment SMX was no longer detected after 10 days whereas increasing concentrations of the two TPs were observed. However, at day 87 the SMX concentration recovered to 53 ± 16% of the initial concentration after most of the nitrate was consumed. A retransformation of 4-nitro-SMX to SMX was postulated and confirmed by another anoxic water/sediment test in the absence of nitrate as electron acceptor. Both TPs were also detected in karst spring samples, highlighting the need and benefit of focusing on transformation products in environmental studies. Furthermore, the consideration of the retransformation potential of 4-nitro-SMX can substantially improve the understanding of SMX behavior during processes such as bank filtration and artificial recharge.