Mechanisms of Cephalosporin Antibiotic Adsorption to Quartz and Feldspar

Pharmaceutical contamination of ground water and surface water is an emerging environmental concern, and has stimulated recent research on antibiotic fate and transport, and related sorption processes. Cephapirin sodium salt (CHP) is a first generation cephalosporin antibiotic used widely in the dairy cattle industry. Raman spectroscopy was used to investigate the mechanisms of CHP adsorption onto quartz (SiO₂) and feldspars at different solution pH values. Depending on the charge and surface properties of the mineral, different reaction mechanisms including electrostatic attraction, monodentate and bidentate complexation were found to be responsible for CHP sorption. The zwitterion (CHP^o) adsorbs to a quartz⁽⁺⁾ surface by electrostatic attraction of the carboxylate anion group (–COO^-) at a low pH, but adsorbs to a quartz^(-) surface through electrostatic attraction of the pyridinium cation and possibly COO^- bridge complexes at higher pH conditions. CHP^- bonds to a quartz^(-) surface by bidentate complexation between one oxygen of –COO^- and oxygen from the carbonyl (C=O) of the acetoxymethyl group. On an orthoclase feldspar (KAlSi₃O₈) surface of mixed charge, CHP^o forms monodentate complexes between C=O as well as possible –COO^- bridges and/or electrostatic attachments to localized edge (hydr)oxy-Al surfaces. CHP^- adsorbs to orthoclase^(-) through monodentate C=O complexation. Additional preliminary data suggest that related mechanisms are responsible for CHP adsorption to albite (NaAlSi₃O₈) and other Na-Ca plagioclase feldspars. Mechanisms observed in the current study may operate for the sorption of other cephalosporin antibiotics with similar functional group chemistry. This research demonstrates that Raman spectroscopic techniques can be effective for evaluating sorption processes and mechanisms of a hydrophilic antibiotic at relatively low sorbed concentrations (~100 µmol/kg range).