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Download fileKinetics and Pathways of the Aqueous Photolysis of Pharmaceutical Pollutants: A Versatile Laboratory or Remote Learning Investigation
journal contribution
posted on 2021-06-30, 16:04 authored by Jeffrey M. Buth, Rachele Ossola, Sarah B. Partanen, Kristopher McNeill, William A. Arnold, Meghan O’Connor, Douglas E. LatchIn
this laboratory experiment, students explore the aquatic photochemical
fate of ranitidine and cimetidine, two common pharmaceutical pollutants
found in wastewater. It provides an engaging environmental context
for students to develop knowledge of reaction kinetics and photochemistry
as well as skill in using analytical instrumentation. This versatile
experiment consists of two basic modules, three optional advanced
modules, and additional add-ons that may be performed in various combinations
to meet the unique learning objectives of general, analytical, physical,
and environmental chemistry courses and science outreach activities.
It may be performed as a traditional lab experiment or as an entirely
remote exercise with an increased focus on data analysis and interpretation
using provided example data sets. All of the photolysis experiments
are carried out by preparing solutions of ranitidine or cimetidine
in various matrices, irradiating the samples, and periodically removing
subsamples for HPLC analysis of the compound of interest. Pseudo-first
order kinetic plots are then generated to determine rate constants
that are used to draw conclusions about photolysis pathways or to
calculate additional kinetic parameters. In the two basic modules,
cimetidine is found to degrade appreciably only when irradiated in
the presence natural organic matter (NOM), indicating an indirect,
photosensitized degradation pathway. In contrast, ranitidine degrades
in pure buffer and in the presence of NOM with comparable rate constants,
highlighting the predominant role of direct photolysis. In the advanced
modules, students calculate ranitidine direct photolysis quantum yields
and examine the significance of singlet oxygen as a photochemically
produced reactive intermediate. The two basic modules may be completed
in two 3 hour lab periods, while the advanced modules require additional
time. This experiment requires only an HPLC instrument, inexpensive
chemicals, and common glassware and lab equipment if performed in
person and a personal computer if performed remotely.