Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure
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Sridhar Komarneni Materials Research Laboratory and Department of Agronomy, The Pennsylvania State University, University Park, Pennsylvania 16802-4801 (Received 26 June 1992; accepted 23 October 1992)
The ion-exchange selectivity of divalent transition metal ions on cryptomelane-type manganic acid (CMA) with tunnel structure has been studied using the distribution coefficients (Kd) at a small fractional exchange in nitrate media. All metal ions studied showed linear relationships with a slope of —2 on the log-log plot of Kd vs [HNO3] which clearly indicated that the adsorption process is an "ideal" ion-exchange. The selectivity increased in the following order: Pb > Mn > Co > Cu > Hg > Cd > Zn > Ni. The high selectivity of the manganic acid was successfully utilized in the removal of Co2+ from seawater and tap water.
I. INTRODUCTION
II. EXPERIMENTAL
Hydrous oxides of manganese (iv) have been extensively studied for possible use as scavengers, coprecipitating agents, and ion exchangers.1"3 The uptake of Mn2+ and other transition metal ions on colloidal manganese dioxide has attracted the special attention of many authors. 45 The sorption of divalent transition metal ions on manganous manganites and delta-type MnO 2 has been believed to occur through the following process6"9:
A. Preparation of CMA
= Mn - OH + M 2 + = Mn - O - M + + H + i.e., the 1:1 exchange of H+ from the solid with M2+ in the solution on a mole basis. Adsorption of alkali and alkaline earth metal ions has also been reported to follow a similar mechanism by the above authors. However, few details have been reported on the selectivity order of these materials in spite of the extensive work. Adsorption of alkali and alkaline earth metal ions on a cryptomelane-type manganic acid (CMA) has been discovered to show stoichiometric ion exchange on an equivalent basis.10'11 The ion-exchange selectivity occurred in the order of Li+ < Na+ < Cs+ «K + , Rb+ for alkali metal ions, and Mg2+ < Ca2+ < Sr2+ < Ba2+ for alkali metal ions in the fractional exchange. The selectivity is maximum for cations whose crystal ionic radius is approximately 1.4 A, e.g., K+, Rb + , and Ba 2+ . This selectivity can be understood based on the 2 X 2 type tunnel structure.11'12 In this paper, we report the fundamental cation exchange and selectivity of transition metal ions in cryptomelane-type manganic acid of tunnel structure. J. Mater. Res., Vol. 8, No. 3, Mar 1993 http://journals.cambridge.org
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The CMA phase in H+ form was synthesized as described previously.10'11 A precipitate was prepared by adding 0.5 M KMnO4 containing 1 M H 2 SO 4 to a mixed solution of 1 M MnSO4 and 1 M H 2 SO 4 at 60 °C. It was aged in the mother liquor overnight and washed with 6 M HNO 3 (0.5 L) and then water by using a centrifuge. The product was dried at ca. 70 °C for 3 days, and then ground and sieved. K+ and SOl" incorporated in the product were removed by the conventional column technique, as follows: the sieved material (100-200 mesh) was loaded into a glass column of
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