Facet-dependent co-binding of Fe(II) and nalidixic acid on hematite

Abstract

Hematite, known for its varied crystal morphologies, often coexists with Fe(II) and significantly influences the environmental fate of organic contaminants. However, the interfacial binding reactivity of Fe(II)-hematite towards redox-inactive contaminants remains poorly understood. This study investigates the adsorptive interactions of nalidixic acid (NA), a prototypical redox-inactive quinolone antibiotic, with hematite nanostructures. Specifically, we compared the NA adsorption onto hematite nanoplates (HNPs) exposing primarily the {001} facets and nanorods (HNRs) with substantial exposure of both {001} and {110} facets, under various pH and Fe(II) concentrations. Through batch adsorption experiments, Fourier transform infrared spectroscopy, and surface complexation modeling, we demonstrated that NA predominantly interacts with surface Fe sites via bidentate coordination. HNRs exhibit higher NA adsorption capacity than HNPs, with adsorption densities of 0.47 and 0.77 NA/nm² for the {001} and {110} facets, respectively. HNRs also exhibit higher Fe(II) adsorption capacity than HNPs due to higher affinity of Fe(II) for {110} facets compared to {001} facets. Additionally, the presence of Fe(II) was found to enhance NA adsorption onto hematite in a facet-dependent manner, influenced by both Fe(II) concentration and pH levels. These findings are directly relevant to the environmental behavior of quinolones, particularly in oxic-anoxic interface, highlighting the importance of mineral facets in contaminant sequestration and mobility.