Removal of antibiotics, antibiotic-resistant bacteria and their associated genes by graphene-based TiO2 composite photocatalysts under solar radiation in urban wastewaters

The present work investigated: (i) the removal of the antibiotics sulfamethoxazole (SMX), erythromycin (ERY) and clarithromycin (CIA); (ii) the inactivation of the total and antibiotic-resistant E. coil along with their regrowth potential after treatment; (iii) the removal of the total genomic DNA content; and (iv) the removal of selected antibiotic resistance genes (ARGs), namely sell, ampC, ermB, mecA, as well as species-specific sequences, namely ecfX for Pseudomonas aeruginosa and enterococci-specific 23S rRNA, by graphene-based TiO2 composite photocatalysts under solar radiation, in real urban wastewaters. TiO2-reduced graphene oxide (TiO2-rGO) composite photocatalysts were synthesized by two ex-situ synthesis methods, namely hydrothermal (HD) treatment and photocatalytic (PH) treatment, starting from graphene oxide and Aeroxide P25 TiO2, and were characterized with various techniques, such as XRD, FT-IR, Raman, XPS, SEM and surface area (BET) analyses. The potential of the synthesized TiO2-rGO composites for the removal of the above mentioned antibiotic-related microcontaminants was compared to the efficiency shown by pristine Aeroxide P25 TiO2 under simulated solar radiation, in real urban wastewater effluents treated by a membrane bioreactor. The results showed that TiO2-rGO-PH was more efficient in the photocatalytic degradation of ERY (84 +/- 2%) and CIA (86 +/- 5%), while degradation of SMX (87 +/- 4%) was found to be slightly higher with Aeroxide P25 TiO2. It was also demonstrated that more than 180 min of treatment were satisfactory for the complete inactivation and complete absence of post-treatment regrowth of E. colt bacteria (< LOD) even 24 h after the end of the treatment, for all examined photocatalytic materials. The least amount of regrowth at all experimental times was observed in the presence of TiO2-rGO-HD. Moreover, the synthesized graphene-based photocatalysts successfully removed ampC and significantly reduced ecfX abundance of Pseudomonas aeruginosa, but sell, ermB and 23S rRNA for enterococci sequences were found to be persistent throughout treatment with all catalyst types. Finally, the total DNA concentration remained stable throughout the photocatalytic treatment (4.2-4.8 ng mu L-1), indicating the high total genomic DNA stability in treated wastewater and its resistance to photocatalytic treatment.