Probing Oxytetracycline Sorption Mechanism on Kaolinite in a Single Ion and Binary Mixtures with Phosphate using In Situ ATR‐FTIR Spectroscopy

Abstract

Core Ideas Oxytetracycline retention on kaolinite in the presence of phosphate was evaluated. Advanced in situ ATR‐FTIR spectroscopic probe revealed detailed interaction mechanism. Both oxytetracycline and phosphate bonded on kaolinite via an inner‐sphere mechanism. Sorption increased for oxytetracycline and phosphate as pH and the initial concentration increased. Phosphate did not influence oxytetracycline adsorption. The presence of co‐adsorbing ions can influence antibiotic retention behavior on mineral surfaces. Thus, a thorough evaluation of antibiotic sorption mechanisms in single ion and in competitive sorption systems is essential to predict antibiotic fate and behavior in the environment. Molecular level understanding of these sorption mechanisms is possible by using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR). This tool can identify detailed surface interactions of the functional groups of antibiotics with soil minerals. This study investigates the sorption mechanisms of oxytetracycline (OTC) on kaolinite in a single ion and in binary mixtures with phosphate [P(V)] as a function of solution properties using in situ ATR‐FTIR spectroscopy. Three different systems were investigated: P(V) sorption, OTC sorption, and competitive sorption of OTC and P(V) on kaolinite. The results of P(V) alone and OTC alone systems indicated that sorption increased with decreasing pH and increasing initial adsorbate concentration. The major IR bands for P(V) indicated that the sorption occurred via an inner‐sphere mechanism. For OTC, involvement of amide (–CONH2), carbonyl (>C=O) and dimethyl amino groups [–N(CH3)2] in retention were noted by the IR bands. For sorption of OTC and P(V) in binary mixtures, the IR bands of OTC and P(V) sorption on kaolinite did not change relative to the single adsorbate systems, indicating a lack of competitive effects. These findings are important for assessing the potential mobilization of OTC in chemically‐complex soil and sediment environments where P(V) can co‐occur.