Cephalexin interaction with biosolids-derived dissolved organic matter: Binding mechanism and implications for adsorption by biochar and clay

Authors: Schmidt, Michael - Mike; Ashworth, Daniel; Ibekwe, Abasiofiok - Mark

Submitted to: Environmental Science: Water Research & Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2024
Publication Date: 2/16/2024
Citation: Schmidt, M.P., Ashworth, D.J., Ibekwe, A.M. 2024. Cephalexin interaction with biosolids-derived dissolved organic matter: Binding mechanism and implications for adsorption by biochar and clay. Environmental Science: Water Research & Technology. 10:949-959. https://doi.org/10.1039/D3EW00590A.
DOI: https://doi.org/10.1039/D3EW00590A

Interpretive Summary: The antibiotic cephalexin is commonly found at trace levels (parts per billion or µg per liter) in wastewater globally. As such, it represents a possible factor for development of antibiotic resistance in the environment. Cephalexin removal from wastewater is therefore an important process for protecting environmental health. Removal from wastewater may be impacted by cephalexin interaction with dissolved organic matter (DOM) species in wastewater. This process has not been studied previously, however, leaving the interaction between cephalexin and DOM as well as subsequent impact on cephalexin removal unclear. The objective of this study was to probe the interaction between cephalexin and DOM and then determine how this interaction might affect the adsorption of cephalexin by materials used in water treatment (biochar and clay). We showed that cephalexin binds to protein-like components of DOM and that it binds more strongly with decreasing pH. Binding was not impacted by changes in solution ionic strength or cation charge. These results suggest cephalexin binds to DOM through aromatic functional groups on both species. Adsorption of cephalexin onto biochar and clay was reduced in the presence of DOM under conditions where cephalexin-DOM binding occurs, suggesting solution interactions may play a role in treatment. These results will strengthen future efforts to treat wastewater by enhancing our understanding of the environmental fate of this prominent antibiotic in wastewater and wastewater irrigated systems.

Technical Abstract: Trace levels of the ß-lactam antibiotic cephalexin are known to persist through wastewater treatment processes, representing a possible route for promoting antibiotic resistance in environments that receive treated wastewater. Processes that impact antibiotic treatment in wastewater, such as binding to dissolved organic matter (DOM), regulate the environmental fate of antibiotics. Here we studied cephalexin binding onto sludge-derived DOM using fluorescence quenching. Interaction was studied under varying solution pH (4 to 8), ionic strength (deionized water and I = 0.1 M) and background cations (Na+ and Ca2+) to determine binding mechanism(s). We observed no binding at pH = 8, while cephalexin binding to protein-like fluorophores in DOM was increasingly strong with decreasing pH from 7 to 4. Conditional equilibrium constants varied considerably with pH, with log Kc = 8.48, 7.15 and 5.33 for pH = 4, 5 and 7, respectively. Cephalexin-DOM interaction was insensitive to changes in solution ionic strength and cation charge. These results suggest p-p bonding likely mechanism drives cephalexin-DOM binding, with the aromatic group of cephalexin interacting with substituted aromatic groups on DOM. Lastly, we showed reduced adsorption of cephalexin from synthetic wastewater onto biochar and clay by DOM under conditions where cephalexin-DOM interaction occurs. This work demonstrates that cephalexin-DOM interaction likely occurs in aquatic environments, with interaction most strongly controlled by pH. We also show that this binding may influence adsorptive treatment and fate in wastewater streams and, therefore, may impact cephalexin fate in the environment.