title: Influence of the pH on the accumulation of phosphate by red mud (a bauxite ore processing waste).
abstract: In the present work we investigated the interactions established between red mud (RM) and phosphate anions (P) at pH 4.0, 7.0 and 10.0. The amount of P sorbed by RM (P-RM) increased as the pH decreased being equal to 4.871 mmol g(-1) at pH 4.0, 0.924 mmol g(-1) at pH 7.0, and 0.266 mmol g(-1) at pH 10.0. Sequential extractions' data of P-RM equilibrated at pH 4.0 and 7.0, suggested that the phosphate sorption at these pH values was mainly regulated by two different mechanisms that gave rise to a chemical adsorption on RM phases, and to the formation of metal phosphate precipitates. By contrast, at pH 10.0 the P-sorption was regulated by a chemisorption mechanism on Fe-Al phases of RM. These findings were supported by FT-IR analysis, which showed a broad band at 1114 and 1105 cm(-1) in P-RM spectra at pH 4.0 and 7.0 respectively, attributable to P-O(H) stretching nu(3)-modes associated to inner-sphere complexes of phosphate on Fe-Al phases, or alternatively to stretching vibrations of PO(4)(3-) tetrahedra, arising from a precipitate of aluminium phosphate. Importantly, the FT-IR spectroscopy showed a phosphate-promoted dissolution of tectosilicates, notably cancrinite and sodalite, in RM exchanged with phosphate at pH 4.0 and 7.0.
authors.affiliation: Dipartimento di Scienze Ambientali Agrarie e Biotecnologie Agro-Alimentari, University of Sassari, Viale Italia 39, 07100 Sassari, Italy. castaldi@uniss.it
authors.forename: Paola
authors.initials: P
authors.lastname: Castaldi
authors.forename: Margherita
authors.initials: M
authors.lastname: Silvetti
authors.forename: Giovanni
authors.initials: G
authors.lastname: Garau
authors.forename: Salvatore
authors.initials: S
authors.lastname: Deiana
mesh_terms: D000327:Adsorption; D000537:Aluminum Oxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008906:Mining; D010710:Phosphates / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D013816:Thermodynamics
chemical_list: D010710:Phosphates; D000537:Aluminum Oxide