Etude macroscopique et spectroscopique de la fixation des Ions phosphate sur les oxyhydroxydes de Fer -Application aux sédiments aquatiques

Abstract

The present study is a contribution to the evaluation of the adsorbing properties of iron oxyhydroxides (Ferrihydrite, Goethite and Hematite) towards phosphate ions. As application, the evaluation of the retention of phosphate by mining sediments was evaluated. Ferrihydrite, goethite and hematite were prepared in laboratory. The used sediments came from a mining site located in the North-eastern of Algeria in the region of Oum Toub. The prepared iron oxyhydroxides and sediment samples were characterized by DRX, DRIFT, SEM, BET, PZC, TGA, DSC and particles size analysis,. The obtained results show that ferrihydrite has the highest surface area and particles size with the predominance of micropores. Goethite and hematite are characterized by the presence of mesopores. The sampled sediments consist essentially of quartz, jarosite, kaolinite, schwertmannite and goethite. Phosphate sorption was studied through macroscopics and spectroscopics analysis. The macroscopic experiments were carried out in batch in order to determine the effects of various parameters. Equilibrium and thermodynamic studies were performed. The macroscopic results show that the phosphate ions fixation on the prepared iron oxyhydroxides is strongly related to the pH; it is significant at acidic pH (pH <5). The phosphate adsorption capacities of the iron oxyhydroxides follow the order: Ferrihydrite> Goethite> Hematite. The calculated thermodynamic parameters show that the adsorption process is endothermic in the case of ferrihydrite and exothermic in the cases of goethite and hematite. The effects of the presence of foreign ions depend on the nature of the ion and the iron oxyhydroxide. At pH <7, a decrease in the binding of phosphate ions is recorded in the presence of Cd (II) ions in the case of goethite and hematite. In the presence of Cu (II), Pb (II) and Cd (II), a clear improvement in the binding is recorded in the case of ferrihydrite at pH> 7. In aquatic environment, the phosphate uptake by the mining sediments is related to their composition. The predominance of amorphous iron oxyhydroxides implies better adsorption. However, that of quartz causes a decrease in the fixing rate. The spectroscopic analysis (DRIFT) show that in the absence of competitive ions, phosphates form inner sphere complexes with the symmetry C3v in the case of ferrihydrite and the symmetry C2v in the case of goethite and hematite. The presence of chromate and oxalate ions involves a modification of the mechanism of phosphate ions retention. In the presence of cations, the formation of ternary complexes is shown in the case of ferrihydrite. In mining sediments, phosphate replaces sulphate in shwertmannite.