Bioavailability of Tetracycline Sorbed By Carbonaceous Sorbents to Escherichia coli for Expression of Antibiotic Resistance.

Tetracycline antibiotics have been extensively used in livestock production. Large portions of tetracycline administered to animals are excreted in animal wastes and eventually disseminated to the environment with soils/sediments as the major reservoirs. Carbonaceous sorbents (e.g. activated carbons, biochars) are promising soil amendments with strong sorption affinity for tetracyclines due to their large specific surface areas, abundant pore volumes and functional groups. In this study, E. coli MC4100/pTGM whole-cell bioreporter was used as an effective tool to probe the bioavailability of tetracycline sorbed by carbonaceous sorbents including four activated carbons (ACs), three biochars (BCs). Tetracycline sorbed by ACs was less bioavailable to bacteria than that in BCs. The strong affinity of tetracycline for the porous structures in ACs limited its desorption into solution and bacterial access to the sorbed tetracycline in small pores. Among the four ACs studied, E. coli bioreporter might be able to access the sorbed tetracycline in larger mesopores (AC-G60 and AC-FM), and emitted stronger fluorescence than the bacteria in suspension. In contrast, for the ACs with dominant microporous structures (AC-WPC and AC-TOG), the sorbed tetracycline manifested minimal bioavailability to the bioreporter. Bioavailability of tetracycline in biochars was related to the production temperature as BG600 < BG500 < BG400, which were produced from bagasse at 600, 500 and 400 ⁰C. The biochars produced at higher temperature were characterized with larger pore volume and surface area, along with higher degrees of graphitization, which favors sorption of tetracycline and hence reducing its bioavailability. The results from this study provide useful information to evaluate the potential of using carbonaceous sorbents as soil amendments to mitigate the bioavailability of tetracycline in soils.