Influence of the pH on the accumulation of phosphate by red mud (a bauxite ore processing waste)

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 ν3-modes associated to inner-sphere complexes of phosphate on Fe-Al phases, or alternatively to stretching vibrations of PO43- 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.