X-ray diffractometry investigation for ion-exchange properties on alpha-type manganese dioxides
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X-ray diffractometry investigation for ion-exchange properties on alpha-type manganese dioxides Yasuo Tanaka Seawater Science Research Laboratory, Japan Tobacco Inc., 4-13-20 Sakawa, Odawara, Kanagawa 256, Japan (Received 22 September 1994; accepted 10 January 1996)
Two materials of a-type manganese dioxide were synthesized and examined. They were prepared by the pyrolysis of mixtures of MnCO3 and (CH3 )3 COK. An ill-ordered material was obtained when prepared at large (CH3 )3 COK content. Both samples behave as acids, but their apparent capacities are obviously different: about 0.9 meqyg for a well-ordered sample and about 2.6 meqyg for an ill-ordered sample. Ion-exchange properties were examined on Kielland’s plot. Zero intersections of the two samples are almost the same M [logsKH dXM !0 ø 23.5 for Na1 exchange and 4–5 for K1 exchange], but slopes are different (about 250 for a well-ordered sample and about 210 for an ill-ordered sample). The difference in slope is likely caused by the flexibility. An evidence of the flexibility can be seen by x-ray diffractometry.
I. INTRODUCTION
Some early reports discussing the adsorption properties of manganese oxides were made by authors with geochemical interests.1–3 They commented on the influence of manganese oxides of suspension form on the compositions of the oceanic water. Experiments in those reports were only observations of pH change when metal-ion-containing solutions were spiked into suspensions of manganese oxides. Substantial ion-exchange studies of the manganese dioxides started by using the bulk form of the materials.4–6 Tsuji et al.7–11 investigated the ion-exchange properties of synthesized a-type manganese oxide systematically. Investigations were made on ion-exchange reactions at various conditions,7,8 column operations,9,10 and XRD investigations on the exchanged materials.11 The crystal structure of a-type manganese dioxide is a 2 3 2 tunnel-framed structure with exchangeable cations.12 As seen in Fig. 1, a double-chain of oxygenmanganese octahedra (two units of manganese dioxides in every step) is running in the c direction. One doublechain connects with other double-chains and forms a three-dimensional tunnel structure. Cations are situated in the tunnel structure at the total ratio of manganese atom and the tunnel cation is 4 : 1, in the perfect crystal of a –MnO2 . This manganese dioxide exhibits a selective ion-exchange property toward alkali metal ions and alkaline earth metal ions with an effective ionic radius ˚ e.g., K1 , Rb1 , and Ba21 . The tunnel of about 1.4 A, cations are probably able to exchange with ones in external solutions. Then, the selectivity stems from the steric effect in the tunnel structure. The author reported a new method to synthesize a –MnO2 , and confirmed that the material showed the 1030
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 4, Apr 1996
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potassium selective ion-exchange property.13 The synthesis method is a pyrolysis of mixtu
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