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Interpretation of the Variability of Selectivity Coefficients for Exchange Between Ions of Unequal Charge on Smectites

Published online by Cambridge University Press:  01 July 2024

M. B. McBride
M. B. McBride*
Affiliation:
Department of Agronomy, Cornell University, Ithaca, New York 14853
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Abstract

Ion-exchange experiments in expanding clay minerals conducted over a wide range of surface ionic compositions and ionic strength produce variable mass-action selectivity coefficients. When the exchanging ions are of unequal charge, tactoid structure appears to influence selectivity, although configurational entropy of adsorbed ions may also generate variable selectivity. The degree of deviation from ideal mass-action exchange is related to the dissimilarity of the ions undergoing exchange. Data involving trivalent ion adsorption on smectites suggest that mass-action is a poor approximation when the adsorbing and desorbing ions have different hydration energies and charge. No form of exchange equation is successful in describing ion exchange for a wide range of experimental conditions, although the fluctuation of the selectivity coefficient follows consistent trends with changing experimental conditions. The strong adsorption of high-charge ions on clays is not exothermic, but must be driven by the increasing disorder of ions and/or water.

Резюме

Резюме

Показано, что ионно-обменные эксперименты, проведенные на расширяющихся глинистых минералах в пределах поверхностных ионных составов и ионных сил дают переменные коэффициенты масс-действенной селективности. Когда обменные ионы имеют неравный заряд, тактоидная структура, по видимому, влияет на селективность, хотя конфигурационная энтропия адсорбированных ионов тоже может вызывать переменную селективность. Степень отклонения от идеального масс-действенного обмена связана с различием ионов, вовлеченных в обмен. Данные, включающие трехвалентную адсорбцию ионов смектитами, указывают на то, что масс-действие является плохим приближением, когда адсорбирующие и десорбирующие ионы имеют разные энергии гидратации и заряд. Ни одна форма уравнения обмена не является подходящей для описания ионного обмена для широкого диапазона экспериментальных условий, хотя колебание коэффициента селективности следует последоветельным тенденциям с изменением экспериментальных условий. Сильная адсорбция глинами ионов с высокими зарядами не является экзотермической, а должна поддерживаться увеличивающимся разупорядочиванием ионов и/или воды. [N.R.]

Resümee

Resümee

Experimente zum Ionenaustausch in quellfähigen Tonmineralen, wobei deren Oberflächen eine unterschiedliche Ionenzusammensetzung und Ionenstärke aufwiesen, fähren zu unterschiedlichen Selektivitätskoeffizienten. Wenn die austauschenden Ionen von ungleicher Ladung sind, scheint eine taktoide Struktur die Selektivität zu beeinflussen, obwohl auch die Konfigurationsentropie des adsorbierten Ions eine unterschiedliche Selektivität hervorrufen kann. Der abweichungsgrad vom idealen Austausch nach dem Massenwirkungsgesetz steht in Zusammenhang mit der Unterschiedlichkeit der Ionen, die am Austausch teilnehmen. Ergebnisse bei der Adsorption dreiwertiger Ionen an Smektit deuten darauf hin, daß das Massenwirkungsgesetz eine ungenaue Annäherung ist, wenn das zu adsorbierende und das zu desorbierende Ion unterschiedliche Hydratationsenergien und Ladungen haben. Keine Art von Austauschgleichung kann den Ionenaustausch für einen großen Bereich von experimentellen Bedingungen genau beschreiben, obwohl die Änderung des Selektivitätskoeffizienten bei sich ändernden experimentellen Bedingungen bestimmten Trends folgt. Die starke Adsorption von hochwertigen Ionen an Tonen ist nicht exotherm, sondern muß durch die zunehmende Unordnung der Ionen und/oder des Wassers bewirkt werden. [U.W.]

Résumé

Résumé

Des expériences d’échange d'ions dans des minéraux argileux en expansion faits sur une large gamme de compositions ioniques et de force ionique de surface produisent des coéficients de sélectivité d'action en masse variables. Lorsque les ions échangeants sont de charge inégale, une structure tactoide semble influencer la sélectivité, quoique l'entropie de configuration des ions adsorbés peut aussi générer une sélectivité variable. Le degré de déviation de l’échange d'action en masse idéal est apparenté à la dissimilar ité des ions subissants l’échange. Des donneés impliquant l'adsorption d'ions trivalents sur des smectites suggère que l'action en masse est une pauvre approximation lorsque les ions adsorbants et désorbants ont des énergies d'hydration et une charge différentes. Aucune forme d’équation d’échange ne réeussit à décrire l’échange d'ions pour une large gamme de conditions expérimentales, quoique la fluctuation du coéficient de sélectivité suit des directions régulières suivant des changements de conditions expérimentales. L'adsorption forte d'ions à haute charge sur les argiles n'est pas exothermique mais doit être pousseé par le désordre croissant d'ions et/ou d'eau. [D.J.]

Keywords

Cation exchangeCation selectivityEntropyMass actionSelectivity coefficientSmectite
Type
Research Article
Information
Clays and Clay Minerals , Volume 28 , Issue 4 , August 1980 , pp. 255 - 261
Copyright
Copyright © Clay Minerals Society 1980

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Footnotes

1

Agronomy Paper No 1335.

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