To evaluate structural changes of TC, the UV–Vis spectra obtained before and after US/ S2O82− process in various time are shown in Fig. 10. The UV–Vis spectra obtained before process indicates two main absorption bands at 275 and 360 nm. The absorption of TC in 360 nm is due to aromatic rings B–D, such as the developed chromophores [68, 69]. With increase of reaction time, the absorption band slightly decreased because of the fragmentation of phenolic groups attached to aromatic ring B [69, 70]. The generation of acylamino and hydroxyl groups led to reduction of absorbance at 270 nm band [70]. The absorption decay at 360 nm band faster than 275 nm. This implies that the ring containing the N-groups (responsible for the absorbance at 276 nm) hardly opened than the other rings, or the created intermediate products absorbed at this wavelength [71]. The proposed degradation pathway for tetracycline based on loses of N-methyl, hydroxyl, and amino groups is shown in Fig. 11. This possible pathway corresponded with conducted studies by other researchers [72]. In addition, TC has a naphthol ring with high stability, which remains unchanged in the reaction and is not easily mineralized. Also, the absorption decay at 360 nm band was found with a relatively small absorption in the visible region. This could be due to the forming of 4a,12a-anhydro-4-oxo-4-dedimethylaminotetracycline according to Fig. 12 [73].
Fig 9 Removal of TC, COD and TOC by US/S2O8 2- process; [S2O8 2-] = 4 mM; US: 500 W, 35 KHz; pH=10; T=25 0C
Fig 10 Changes of UV–Vis spectra of 50 mg/L aqueous solution of TC during process of US/S2O8 2-
Fig 11 The proposed degradation pathway for tetracycline S2O8 2−
Fig 12 Photodegradation of TC into 4a, 12a-anhydro-4-oxo-4-dedimethylaminotetracycline