Adsorption of enrofloxacin in presence of Zn(II) on a calcareous soil

• The presence of Zn slightly increased ENR retention.
• Adsorption, speciation and XANES data were combined to study ENR/Zn co-adsorption.
• ENR was primarily adsorbed directly onto the soil surface.
• A small proportion of ENR was adsorbed as a ternary surface complex ENR-Zn-soil.
• ENR mobility in calcareous soils decreased in presence of zinc.

As a result of their consumption, excretion, disposal and persistence, antibiotics enter the soil environment and may be transported to surface and ground waters. During their transfer through soils, retention processes play a key role in their mobility. Antibiotics often coexist with heavy metals in soils due to agricultural practices and other sources of inputs. In this context, this study deals with the co-adsorption of Zn(II) and enrofloxacin (ENR), a widely-used veterinary antibiotic, on a calcareous soil using batch retention experiments and X-ray Absorption Near Edge Structure (XANES) spectroscopy. To improve our understanding of the interaction of this emerging organic contaminant with metal cations at the water–soil interface, the ternary system containing ENR, Zn(II) and a selected calcareous soil was investigated over a pH range between 7 and 10, at different solid–solution contact times and ENR concentrations. The presence of Zn(II) slightly influenced the retention of the antibiotic, leading to an increase of the adsorbed ENR amounts. The distribution coefficient Kd value increased from 0.66 L g−1 for single ENR adsorption to 1.04 L g−1 in presence of Zn(II) at a 1/2 ENR/Zn(II) ratio. The combination of adsorption isotherm data, solution speciation diagrams and XANES spectra evidenced a small proportion of Zn(II)–ENR complexes at soil pH leading to the slight increase of ENR adsorption in presence of zinc. These results suggest that it is necessary to consider the interaction between ENR and metal cations when assessing the mobility of ENR in soils.