Surface characterization of natural apatites used to remove phosphates from wastewater in constructed wetlands: which techniques to use?

Phosphorus is an essential element for all life forms. However, an excessive concentration of phosphates is the most common cause of water bodies eutrophication as phosphorous is usually the limiting nutrient involved in plant growth. In constructed wetlands (CWs) treating domestic wastewaters, good treatment performances are obtained on main pollutants except with phosphates. In order to improve phosphate removal by CWs, different experiments were performed considering a series of reactive substrates. Among those, natural apatitic materials showed a high sorption capacity for phosphates. This paper presents the various analytical techniques that were used to characterize five natural apatitic materials which were tested in experiments performed at different scales (laboratory, pilot plant and full-scale experiments). These apatitic materials have been studied using X-ray powder diffraction, Raman spectroscopy and scanning electron microscopy. In addition, chemical compositions were obtained from X-ray fluorescence and ICP-MS analyses. The results suggest that the studied apatitic materials present a wide spectrum of physical, mineralogical and chemical characteristics. This variety is related to the presence of impurities in the natural materials (5–60% w/w). Therefore, the characterization of the natural apatitic materials available is necessary in order to select the ones that would lead to the best performances in terms of phosphate removal. Moreover, the observation of the surface of the grains before and after the percolation of phosphate containing solutions showed that the primary apatitic peloids were progressively covered with a carbonate hydroxyapatite precipitate layer formed with the phosphates removed from the treated solution