Optimization of the preparation method of K-selective manganese oxide for assessing the availability of soil K

Heating mixed solutions containing manganese sulfate, ammonium peroxodisulfate and ammonium sulfate at a molar ratio of 1:1:5 at 150°C gave NH₄-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 NH₄-cryptomelane was detected. Column washing with 6 mol/L nitric acid was found to be suitable for exchanging NH₄⁺ with H⁺. The H-exchanged cryptomelanes were highly selective for K and the K 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.