Simultaneous incorporation of Mn and Al in the goethite structure

Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural c...

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Autor principal:	Sileo, Elsa Ester
Publicado:	2007
Materias:	aluminum crystal structure goethite manganese Rietveld analysis X-ray diffraction
Acceso en línea:	https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167037_v71_n4_p1009_Alvarez http://hdl.handle.net/20.500.12110/paper_00167037_v71_n4_p1009_Alvarez
Aporte de:	Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA) de Universidad de Buenos Aires

id	paper:paper_00167037_v71_n4_p1009_Alvarez
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spelling	paper:paper_00167037_v71_n4_p1009_Alvarez2023-06-08T14:38:46Z Simultaneous incorporation of Mn and Al in the goethite structure Sileo, Elsa Ester aluminum crystal structure goethite manganese Rietveld analysis X-ray diffraction Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the simultaneous substitution in goethite. XRD patterns revealed that all the obtained solids remain in a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the increasing Mn substitution and consequent decrease of Al substitution causes an increase in the unit cell volume. This change is accompanied by the increment of the various Me-Me distances. XANES spectra at the Al and Mn K-edge confirm the octahedral coordination of Al and the trivalent oxidation state of the Mn ion in all the synthesized samples. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom remains practically constant upon (Mn,Al) substitution. Measurements in the Mn K-edge show that distances Mn-Me suffer different changes with the increase in Mn substitution: a marked decrease in E and a slight decrease in E′, while DC remains constant. E and E′ values correspond to the distance between one Mn and one neighboring Me (Fe, Mn, Al) atom, both situated in two polyhedra linked by an edge. These polyhedra belong to the same double row of the goethite structure. DC value corresponds to the distance between one Mn and one Me (Fe, Mn, Al) atom, situated in two octahedral linked by one corner and belonging to two adjacent double chains. All the intermetallic distances are minor than the corresponding singly substituted goethites, this fact is attributed to the structure contraction due to the presence of Al(III) which restrains the axial distortion of Mn. Dissolution-time curves, resulting from exposure to 6 M HCl at 318 K, show that the dissolution rate slows with increasing Al substitution and consequent decrease of Mn substitution, and the shape of the curve becomes increasingly sigmoidal for mixed goethite with large Al content and Al-goethite. Dissolution kinetics of most samples are well described by the Kabai equation. Al dissolves almost congruently with respect to Fe, implying that it is homogeneously distributed in the structure. However, the convex χMn:χFe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles. © 2006 Elsevier Inc. All rights reserved. Fil:Sileo, E.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. 2007 https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167037_v71_n4_p1009_Alvarez http://hdl.handle.net/20.500.12110/paper_00167037_v71_n4_p1009_Alvarez
institution	Universidad de Buenos Aires
institution_str	I-28
repository_str	R-134
collection	Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA)
topic	aluminum crystal structure goethite manganese Rietveld analysis X-ray diffraction
spellingShingle	aluminum crystal structure goethite manganese Rietveld analysis X-ray diffraction Sileo, Elsa Ester Simultaneous incorporation of Mn and Al in the goethite structure
topic_facet	aluminum crystal structure goethite manganese Rietveld analysis X-ray diffraction
description	Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the simultaneous substitution in goethite. XRD patterns revealed that all the obtained solids remain in a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the increasing Mn substitution and consequent decrease of Al substitution causes an increase in the unit cell volume. This change is accompanied by the increment of the various Me-Me distances. XANES spectra at the Al and Mn K-edge confirm the octahedral coordination of Al and the trivalent oxidation state of the Mn ion in all the synthesized samples. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom remains practically constant upon (Mn,Al) substitution. Measurements in the Mn K-edge show that distances Mn-Me suffer different changes with the increase in Mn substitution: a marked decrease in E and a slight decrease in E′, while DC remains constant. E and E′ values correspond to the distance between one Mn and one neighboring Me (Fe, Mn, Al) atom, both situated in two polyhedra linked by an edge. These polyhedra belong to the same double row of the goethite structure. DC value corresponds to the distance between one Mn and one Me (Fe, Mn, Al) atom, situated in two octahedral linked by one corner and belonging to two adjacent double chains. All the intermetallic distances are minor than the corresponding singly substituted goethites, this fact is attributed to the structure contraction due to the presence of Al(III) which restrains the axial distortion of Mn. Dissolution-time curves, resulting from exposure to 6 M HCl at 318 K, show that the dissolution rate slows with increasing Al substitution and consequent decrease of Mn substitution, and the shape of the curve becomes increasingly sigmoidal for mixed goethite with large Al content and Al-goethite. Dissolution kinetics of most samples are well described by the Kabai equation. Al dissolves almost congruently with respect to Fe, implying that it is homogeneously distributed in the structure. However, the convex χMn:χFe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles. © 2006 Elsevier Inc. All rights reserved.
author	Sileo, Elsa Ester
author_facet	Sileo, Elsa Ester
author_sort	Sileo, Elsa Ester
title	Simultaneous incorporation of Mn and Al in the goethite structure
title_short	Simultaneous incorporation of Mn and Al in the goethite structure
title_full	Simultaneous incorporation of Mn and Al in the goethite structure
title_fullStr	Simultaneous incorporation of Mn and Al in the goethite structure
title_full_unstemmed	Simultaneous incorporation of Mn and Al in the goethite structure
title_sort	simultaneous incorporation of mn and al in the goethite structure
publishDate	2007
url	https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167037_v71_n4_p1009_Alvarez http://hdl.handle.net/20.500.12110/paper_00167037_v71_n4_p1009_Alvarez
work_keys_str_mv	AT sileoelsaester simultaneousincorporationofmnandalinthegoethitestructure
_version_	1768543307930009600

Simultaneous incorporation of Mn and Al in the goethite structure

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