The use of graphite-like carbon nitride (g-C3N4) combined with light-emitting diodes (LEDs) is a powerful tool for the degradation of organic contaminants in wastewaters and surface waters, since it provides high mineralisation rates under mild conditions, thus resulting in energy savings . Most photocatalytic studies employ ultrapure (UP) water spiked with individual target micropollutants (MPs), with concentrations higher than those found in real waters . However, the studies carried out with real waters, namely river and wastewaters, usually report removal rates lower than those in UP water due to the presence of organic and inorganic species that can trigger light attenuation, consumption or scavenging of oxidative free radicals and adsorption onto the catalyst surface .
In the present work, g-C3N4 was applied to study the photocatalytic degradation of two pharmaceuticals widely consumed worldwide and found in large quantities in wastewaters and surface waters . Specifically, we investigated the abatement of nonsteroidal anti-inflammatory diclofenac (DCF) and β-blocker metoprolol (MET), individually and as a mixture. The latter case to study the possible interfering effect of each pharmaceutical on the other. Also, we evaluated the impact of the water matrix type (river and wastewater), as well as the presence of inorganic ions, such as NO2-, NO3-, PO43- and SO42-.
g-C3N4 proved to be efficient for the degradation of both MPs under visible light irradiation, even when using real water matrices. When real waters were spiked with both pharmaceuticals, some photochemical instability was detected. The organic matter might be the main responsible for indirect photolysis for both MPs since for the inorganic species we observed a negligible effect. It was also possible to verify a competitive effect between these two pharmaceuticals, due to the decrease of the degradation kinetic constants of MET when co-occurring with DCF. In contrast, the presence of MET did not significantly affect the degradation rates of DCF. Lastly, the photocatalytic degradation mechanism of g-C3N4 was studied, and we concluded that both radicals and photogenerated holes play essential roles in the degradation of these MPs with different pKa. In sum, we concluded that the degradation rate of a particular MP might depend on other co-existing MPs.
Acknowledgements: This work was financially supported by projects POCI-01-0145-FEDER-030674 and POCI-01-0145-FEDER-029600 funded by European Regional Development Fund (ERDF) through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) - and by national funds (PIDDAC) through FCT/MCTES. We would also like to thank the scientific collaboration under Base Funding - UIDB/50020/2020 of the Associate Laboratory LSRE-LCM - funded by national funds through FCT/MCTES (PIDDAC). MAB acknowledges the research grant from FCT (Ref. SFRH/BD/145014/2019).
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