Fate of trimethoprim, sulfamethoxazole and caffeine after hydrothermal regeneration of activated carbon

Emerging contaminants are found in all parts of our environment. Adsorption of these contaminants by activated carbon in water treatment plants is well-known; however, a problem resides in the handling of the spent adsorbents. As current regenerative technologies are expensive, the adsorbents are often destructed or landfilled. Here, we examine a novel regeneration method for the used adsorbents with subcritical water - i.e., hydrothermal treatment. The degradation of three well-known emerging contaminants - caffeine, trimethoprim and sulfamethoxazole - was studied with regard to processing temperature (160-280 degrees C), concentration (2 and 20 mg/L), and the impact of adsorbents. In addition to trimethoprim in the mix at 20 mg/L, the other contaminants were entirely degraded at 280 degrees C. To obtain insight into transformation products formed during hydrothermal regeneration, we performed non-target and targeted analyses with LC-MS-QTOF using two types of columns, C18 and ZIC-HILIC. This approach ensured a wide range of hydrophilicities. Results showed more transformation products for trimethoprim (20) compared to sulfamethoxazole and caffeine (4). To assess the regeneration efficiencies of the activated carbons, we conducted three cycles of regeneration at 280 degrees C and between 61 and 120 % degradation was achieved. Moreover, only two transformation products were detected and readsorbed on the adsorbent after regeneration. Hydrothermal regeneration efficiently degraded the target emerging contaminants, suggesting a potential approach for enabling alternative, sequential uses for regenerated activated carbon.

Automated summary

A novel regeneration method for used adsorbents, hydrothermal treatment, efficiently degrades emerging contaminants at 280 degrees C. LC-MS-QTOF analysis reveals different transformation products generated during hydrothermal regeneration depending on the contaminants. Three regeneration cycles achieve a degradation rate of 61-120%.