Exploring the fate of emerging contaminants during hydrothermal regeneration of carbonaceous adsorbents

Sammanfattning: Wastewater from households and industries commonly contain emerging contaminants that are not easily removed by most wastewater treatment plants. These contaminants can be removed through adsorption onto adsorbents, such as activated carbon or biochars. Previously, attention has been given to waste residues from the agriculture and forestry industry as potential raw materials for activated biochars, which could replace coal and coconut, common feedstocks for activated carbon production. This thesis investigates the factors governing the adsorption efficiencies of these activated biochars and explores the potential of hydrothermal regeneration as a post-treatment. The adsorption experiments showed that iron-doped (i.e., magnetic) activated biochar had two times more adsorption capacity than non-doped activated biochar (i.e., non-magnetic). However, the adsorption capacity of magnetic activated biochar was still inferior to activated carbon for removing sulfamethoxazole (8 mg/g vs. 42 mg/g) and caffeine (40 vs. 56 mg/g). Of the three conditions tested (i.e., salts, humic acids, and pH), only pH had a significant influence on the adsorption of the three selected contaminants onto activated biochars, and the biochars preferentially adsorbed neutral species. This observation is most likely explained by the π-π bonds. Hydrothermal regeneration effectively degraded trimethoprim, sulfamethoxazole, and caffeine at temperatures above 240 °C in the absence of adsorbent. Only trimethoprim generated transformation products that could be identified and quantified from non-targeted analysis. In presence of adsorbent, caffeine was not completely degraded at 280 or even 320 °C, suggesting that the activated biochars adsorb and to some extent shelter the contaminants from degradation.After hydrothermal regeneration, the activated biochars had an enhanced adsorption capacity for sulfamethoxazole, whereas lower adsorption capacity was observed for trimethoprim and caffeine. These changes in performance are believed to be related to the alteration of surface characteristics of activated biochar induced by the adsorbed contaminants during the hydrothermal reaction. Overall, the regeneration efficiency for the activated biochars was found to exceed 50 %. After three regeneration cycles, the regeneration efficiency was as high as 320 %. The results of this thesis suggest that activated biochars could remove emerging contaminants in water and hydrothermal regeneration could degrade most of the emerging contaminants, allowing the spent adsorbent to be reused.