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Cation and Anion Retention by Natural and Synthetic Allophane and Imogolite

Published online by Cambridge University Press:  02 April 2024

C. J. Clark  and
M. B. McBride
C. J. Clark*
Affiliation:
Department of Agronomy, Cornell University, Ithaca, New York 14853
M. B. McBride
Affiliation:
Department of Agronomy, Cornell University, Ithaca, New York 14853
*
1Present address: Ruakura Soil and Plant Research Station, Private Bag, Hamilton, New Zealand.
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Abstract

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The negative surface charge of synthetic allophanes with a range of Si/Al ratios decreased and positive charge increased with increasing alumina content at a given pH. The phosphate adsorption capacity also increased with increasing Al content. That this relationship between composition and chemical reactivity was not found for the soil allophanes is attributed to the presence of specifically adsorbed organic or inorganic anions on the natural material. Both synthetic and natural imogolites had a much lower capacity to adsorb phosphate than the allophanes and adsorbed anomalously high amounts of Cl− and ClO4− at high pH. It is proposed that intercalation of salt occurs in imogolite, although electron spin resonance studies using spin probes failed to reveal the trapping of small organic molecules in imogolite tubes. These spin probes in the carboxylated form did, however, suggest an electrostatic retention of carboxylate by imogolite and a more specific adsorption by allophane involving ligand exchange of surface hydroxyl. The results illustrate the inherent differences in charge and surface properties of allophane and imogolite despite the common structural unit which the two minerals incorporate.

Резюме

Резюме

При данным pH и увеличивающимся содержании окиси алюминия отрицательный поверхностный заряд синтетических аллофанов с различными отношениями Si/Al уменьшался, а положительный заряд—увеличивался. Способность адсорбции фосфата также увеличивалась. Факт, что эта зависимость между составом и химической активностью не находилась для почвенных аллофанав, объясняется присутствием специфически адсорбированных органических и неорганических анионов в натуральном материале. Оба, синтетический и натуральный, имоголиты имели значительно меньшие способности адсоробции фосфата, чем аллофаны, и адсорбировали аномально большие количества Cl− и СlO4− при больших величинах pH. Предполагается, что прослойка соли назодится в имоголите, хотя исследования при помощи электронного спинового резонанса с использованием спиновых зондов не показали захватов малых органических молекул в трубках имоголита. Эти спиновые зонды в карбоксилированной форме показали, однако, электростатическое сохранение карбоксильных групп имоголитом и более специфическую адсорбцию аллофанами, включая лигандовый обмен поверхностных гидроксильных групп. Эти результаты указывают на врожденные различия в величинах заряда и свойствах поверхности аллофана и имоголита, несмотря на общую структурную единицу, котурую оба минерала включают. [E.G.]

Resümee

Resümee

Die negative Oberflächenladung von synthetischen Allophanen mit unterschiedlichen Si/Al-Verhältnissen nahm mit zunehmendem Al2O3-Gehalt bei einem gegebenen pH ab, während die positive Ladung zunahm. Die Phosphatadsorptionskapazität nahm ebenfalls mit zunehmendem Al-Gehalt zu. Daß diese Beziehung zwischen Zusammensetzung und chemischer Reaktionsfähigkeit bei Boden-Allo-phanen nicht gefunden wurde, wird auf die Anwesenheit von spezifisch adsorbierten organischen oder anorganischen Anionen auf dem natürlichen Material zurückgeführt. Sowohl synthetische als auch natürliche Imogolite hatten eine viel niedrigere Phosphatadsorptionskapazität als die Allophane und adsorbierten ungewöhnlich große Mengen von Cl− und ClO4− bei hohen pH-Werten. Es wird vorgeschlagen, daß in den Imogoliten ein Einbau von Salz auftritt, obwohl Elektronenspinresonanz-Untersuchungen mit Spinproben keine Einschlüsse von kleinen organischen Molekülen in den Imogolit-Röhren ergaben. Diese Spinproben in der karboxylierten Form deuteten jedoch auf eine elektrostatische Retention von Karboxylat durch Imogolit hin und auf eine mehr spezifische Adsorption durch Allophan, die einen Ligandenaustausch der Oberflächenhydroxyle mit einschließt. Diese Ergebnisse verdeutlichen die spezifischen Unterschiede bei der Ladung und den Oberflächeneigenschaften von Allophan und Imogolit trotz der gemeinsamen Struktureinheiten, die beide Minerale beinhalten. [U.W.]

Résumé

Résumé

La charge de surface négative d'allophanes synthétiques avec une gamme de proportions Si/Al a diminué et la charge positive a augmenté lorsqu'on a augmenté le contenu en alumine à un pH donné. La capacité d'adsorption de phosphate a aussi augmenté lorsqu'on a augmenté le contenu en Al. On attribue à la présence d'anions organiques et inorganiques spécifiquement adsorbés sur le matériau naturel le fait que la même relation entre la composition et la réactivité chimique n'a pas été trouvée dans les allophanes de sol. Les imogolites synthétiques et naturels avaient tous deux une capacité d'adsorption de phosphate beaucoup plus basse que les allophanes, et avaient adsorbé des quantité abnormalement élevées de Cl− et de ClO4− à un pH élevé. On propose que l'intercalation de sel se produit dans l'imogolite, quoique le spin à résonnance d’électrons utilisant des probes de spin a manqué de révéler l'entrappement de petites molécules organiques dans les tubes d'imogolite. Ces probes de spin dans la forme carboxylate ont cependant suggéré une rétention électrostatique de carboxylate par l'imogolite, et une adsorption plus spécifique par l'allophane impliquant l’échange ligand d'hydroxyles de surface. Les résultats illustrent les différences inhérentes dans les propriétés de charge et de surface de l'allophane et de l'imogolite malgré l'unité structurale commune que les deux minéraux incorprent. [D.J.]

Keywords

AdsorptionAllophaneAnion retentionCation retentionImogoliteSiO2/Al2O3 ratioSurface charge
Type
Research Article
Information
Clays and Clay Minerals , Volume 32 , Issue 4 , August 1984 , pp. 291 - 299
Copyright
Copyright © 1984, The Clay Minerals Society

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