Effects of water quality and ozone dosing on reaction pathways and ozonation products on the example of tramadol- and venlafaxine N-oxide

The input of pharmaceuticals into receiving waters via the effluents of the wastewater treatment plants is often abated by ozonation of these effluents. During ozonation, the target compounds are usually transformed into one or several products, which can arise either from the parent compound or from further oxidation of the first generation ozonation products. It is known that the removal is dependent on dissolved organic carbon (DOC) concentration in the water, it is however less known, whether DOC has an effect on the reaction pathways. To fill this gap, the N-oxides of tramadol and venlafaxine were chosen as model compounds to study the effects of DOC on reactions pathways and thus the formed transformation products (TPs). TPs resulting from the direct reaction with ozone were formed with higher abundance in pure water compared to effluent wastewater, while the abundance of TPs resulting from the reactions with hydroxyl radicals were higher in effluent wastewater as more ozone is transformed into OH radicals in the presence of abundant DOC. Some TPs were solely formed in reactions conducted in effluent wastewater. Consequently, identification of ozonation products should always be conducted in effluent water at least in comparison. Risk studies on ozonation products will otherwise focus on the wrong transformation products.