The antibiotic sulfamethoxypyridazine (SMP) undergoes phototransformation without complete mineralization. New phototransformation products (PTPs) are formed [1]. The photocatalytic degradation of SMP in ultrapure water (UP) and sewage treatment plant effluent (STP) was examined during the irradiation with a medium pressure Hg-lamp in the presence of TiO2, FeCl3 and TiO2/FeCl3 catalysts at natural pH (without using buffer or pH adjustment) in UP and STP effluent samples and pH3 in STP effluent samples. The primary elimination of SMP was monitored and structures of PTPs were assessed by LC-UV-MS/MS and total organic carbon (TOC) analyzer. The biodegradation of SMP samples after photolysis and photocatalysis was performed using Closed Bottle Test (CBT; OECD 301 D). Additional data for an environmental risk assessment of SMP and its PTPs were collected using in silico QSAR models.
It was found that SMP underwent photocatalytic degradation and mineralization in the presence of TiO2, FeCl3 and TiO2/ FeCl3. The total degradation time decreased from 128 min in direct photolysis in UP to 16, 4, and 4 min in photocatalysis with TiO2, FeCl3 and TiO2/FeCl3, respectively and complete mineralization took place in case of TiO2 catalyst. In STP effluent, the degradation time decreased from about 64 min in direct photolysis to 32, 8, and 8 min in photocatalysis with TiO2, FeCl3 (pH3) and TiO2/FeCl3 (pH3), respectively. Based on the identified PTPs, a degradation pathway was proposed and a comparison between the three catalysts concerning the formed PTPs intensities was performed. On the one hand it was found that SMP and its PTPs were not readily biodegradable. On the other hand in HPLC-UV analysis, it was found that one of the PTPs showed full elimination at day 28 in CBT. QSAR results revealed that the formed PTPs might be more toxic than SMP to the environment with a higher risk of these PTPs on bacteria. Therefore, photocatalysis will be more preferable than photolysis as it offers complete mineralization. However the appropriate time period of irradiation has to be chosen.
Acknowledgements: Waleed M. M. Mahmoud Ahmed thanks the Egyptian-German Research Long Term Scholarship (DAAD and MHESR Egypt) program for the scholarship. The authors wish to thank MultiCASE Inc. for providing CASE Ultra.

[1] N. D. H. Khaleel, W. M. M. Mahmoud, G. M. Hadad, R. A. A. Salam, K. Kümmerer, Photolysis of sulfamethoxypyridazine in various aqueous media: aerobic biodegradation and identification of photoproducts by LC-UV-MS/MS. Journal of Hazardous Materials 2013 (244-245) 654-661.