Heating mixed solutions containing manganese sulfate, ammonium peroxodisulfate and ammonium sulfate at a molar ratio of 1:1:5 at 150°C gave NH 4 -cryptomelane. The higher the starting manganese concentration, the higher the purity of the products. At a concentration of 0.5 mol/L any products other than NH 4cryptomelane was detected. Column washing with 6 mol/L nitric acid was found to be suitable for exchanging NH 4 + with H + . The H-exchanged cryptomelanes were highly selective for K and the

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... adsorption capacity increased as the volume of nitric acid used for washing increased. The maximum adsorption capacity achieved in this study was about 800 mmol/kg.