Use of X-ray diffractometry and analysis of the atomic pair distribution function to study the interactions between smectite and emerging organic pollutants
Aristilde L.; Lanson B.; Charlet L. (2013) Langmuir, 29,International audienceMany emerging pollutants from pharmaceuticals, pesticides or products for domestic or industrial use are found in the environment and pose the problem of achieving good chemical or ecological status of this system. In the environment, these pollutants can interact with different phases that make up soils or sediments, such as organic matter, natural oxyhydroxides, or clay minerals. However, to date there is very little data describing the reaction mechanisms and the nature of the bonds involved in these interactions (Aristilde et al., 2013). In this context, and in order to improve the state of knowledge on the behaviour of emerging pollutants in the environment, we propose to study the adsorption of two organic molecules, sulfamethoxazole (SMX) and metoprolol tartrate (MPT), considered as emerging pollutants onto a Ca-montmorillonite. SMX and MPT are an antibiotic and a cardioselective beta blocker, respectively, and are widely found in soils and aquatic ecosystems, with concentrations up to several micrograms per liter. In this study, SMX and MPT have been in contact with Ca-montmorillonite for 24 hours at pH 6. Different concentrations, varying from0.1 mg/L to 1 mg/L, were studied. The resulting solids were characterized by X-ray powder diffractometry at constant temperature (25°C) and different humidities (from 10 to 90%). Initial results showed differences in the position (interlayer distance) of the 001 reflection of montmorillonite. In the presence of organic molecules and at low humidity, the interlayer distance is greater than that observed in the case of Ca-montmorillonite. This result indicates that the interlayer space has been modified, in particular by the insertion of organic molecules. For relative humidities above 50%, there are no more differences between the samples with and without organic molecules. Due to their small size, organic molecules do not induce significant swelling when the interlayer space contains many water molecules. To complete this study, analyses of the atomic pair distribution function (PDF), were carried out on the starting materials (Ca-montmorillonite, SMX and MPT) as well as on the montmorillonite after having been in contact with the organic molecules. The comparison between the PDFs confirms the adsorption of organic molecules since characteristic peaks of C-H bonds belonging to the CH 3 groups of SMX and MPT are visible on the montmorillonite PDFs after contact with the organic molecules. However, the interatomic distances between C and H seem to be slightly modified indicating a potential reaction mechanism involving these groups. Aristilde L.; Lanson B.; Charlet L. (2013) Langmuir, 29, 4492-4501
