Separation of Photochemical and Non-Photochemical Diurnal In-Stream Attenuation of Micropollutants

For a better process
understanding of in-stream attenuation of
trace organic contaminants (TrOCs), quantitative comparisons between
field studies under different environmental conditions and controlled
laboratory experiments are important to separate different processes.
However, this is hampered by the challenge to transfer kinetics from
the laboratory to different field conditions due to the lack of good
quantitative measures to account for different boundary conditions.
For phototransformation, in situ light conditions in a river are difficult
to determine because light is reduced, for instance, by absorption,
scattering on suspended particles, and shading effects. In this study,
we present an approach to separate photochemical from non-photochemical
diurnal in-stream attenuation based on rate constants relative to
diclofenac, as a reference compound, to account for the difference
in the in situ light conditions combined with laboratory experiments.
12 out of 45 detected target TrOCs showed a diurnal attenuation at
a selected river stretch. A non-photochemical process, potentially
biotransformation, was responsible for the diurnal attenuation of
bisoprolol, metoprolol, O-desmethylvenlafaxine, tramadol, and venlafaxine.
Attenuation of amisulpride, flufenamic acid, hydrochlorothiazide,
naproxen, and xipamide can be quantitatively explained by phototransformation,
partially for sotalol. Attenuation rate constants of hydrochlorothiazide
at different field sites from this study and from published data range
over 2 orders of magnitude. Differences can be quantitatively explained
by different light exposures but not by water chemical parameters.