Simultaneous sorption of phosphate and arsenate by red muds (a bauxite ore processing waste)

In the present work we investigated the interactions established between red mud (RM, a bauxite ore residue), arsenate and phosphate anions at pH 4.0, 7.0 and 10.0, through adsorption isotherms, FT-IR spectroscopy and thermal analysis. RM samples were artificially enriched with solutions containing increasing concentrations of As(V) and P(V) [As,P-RM]. The sorption of anions was higher at lower pH values and followed the order: RM-pH 4.0 [As(V) 0.669 mmol•g-1; P(V) 1.056 mmol•g-1]> RMpH 7.0 [As(V) 0.537 mmol•g-1; P(V) 0.857 mmol•g-1]> RM-pH 10.0 [As(V) 0.154 mmol•g-1; P(V) 0.336]. The lower sorption of arsenate compared to phosphate could be due to a higher affinity of phosphate towards the RM sorption sites, despite the similar chemistry of the two anions. The sorption of As(V) and P(V) was highlighted in the FTIR spectra by a band at 865 and 1114 cm−1 respectively. These peaks were likely due to As-O(H) and P-O(H) stretching- modes associated to inner-sphere complexes of arsenate and phosphate on Fe-Al phases. Thermal analysis showed that the sorption of anions increased the stability of RM. The results obtained highlighted that phosphate and arsenate, although with different reactivity, were strongly bounded to some RM phases, such as gibbsite and hematite, through a chemical sorption.