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X-ray diffraction and thermal analysis of bauxite ore-processing waste (red mud) exchanged with arsenate and phosphate

Castaldi, Paola and Silvetti, Margherita and Enzo, Stefano and Deiana, Salvatore Andrea (2011) X-ray diffraction and thermal analysis of bauxite ore-processing waste (red mud) exchanged with arsenate and phosphate. Clays and Clay Minerals, Vol. 59 (2), p. 189-199. eISSN 1552-8367. Article.

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The use of waste materials from mineral ore processing has much potential for immobilizing pollutants such as arsenic (As) in natural soils and waters. The purpose of the present study was to investigate red mud (RM, a finely textured bauxite-ore residue) as a sequestering agent for arsenate and phosphate, including characterization of the types of surface complexes formed. The mineralogical and structural changes occurring in RM were investigated after exchange with arsenate [As(V)-RM] and phosphate [P(V)-RM] anions at pH 4.0, 7.0, and 10.0. Eight different phases were present in the untreated red mud (RMnt), though 80 wt.% of the crystalline phase consisted of sodalite, hematite, gibbsite, and boehmite. The X-ray diffraction (XRD) data for As(V)-RM revealed an anion-promoted dissolution of the gibbsite, suggesting that this phase was the most active for As(V) sequestration. In addition, the lattice parameters of cancrinite were different in As(V)-RM at pH 7.0 and 10.0 from those in RMnt. The changes may be related to the incorporation of arsenate in the cancrinite cages. X-ray diffraction patterns of P(V)-RM at pH 4.0 and 7.0 revealed the dissolution of sodalite, hematite, and gibbsite, and the formation of a novel phase, berlinite [(α,β)AlPO4]. The new phases detected through XRD and thermal (TG/DTG) analysis in P(V)-RM probably originated through an initial phosphate-promoted dissolution of some RM phases, followed by a precipitation reaction between the phosphate and Al/Fe ions. The results obtained suggest that phosphate and arsenate, though with different reactivities, were strongly bound to some RM phases, such as gibbsite, cancrinite, sodalite, and hematite through mechanisms such as chemical sorption and coprecipitation reactions. The knowledge acquired will be helpful in selecting alternative materials such as red muds, which currently pose critical economic and environmental challenges related to their disposal, for the decontamination of soils and waters polluted with As.

Item Type:Article
ID Code:6694
Uncontrolled Keywords:X-ray diffraction, Red Mud (RM), arsenate, phosphate
Subjects:Area 03 - Scienze chimiche > CHIM/02 Chimica fisica
Area 07 - Scienze agrarie e veterinarie > AGR/13 Chimica agraria
Divisions:001 Università di Sassari > 01 Dipartimenti > Chimica
001 Università di Sassari > 01 Dipartimenti > Scienze ambientali agrarie e biotecnologie agro-alimentari
Publisher:Clay Minerals Society
Deposited On:25 Nov 2011 13:19

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