Dynamics of Potassium Release from Potassium-Bearing Minerals as Influenced by Oxalic and Citric Acids

The effect of selected low molecular weight organic acids on release of K from K-bearing minerals was investigated by reacting biotite, muscovite, microcline and orthoclase (20–50 µm) with 0.01 mol L⁻¹ oxalic and citric acids or water for 1 h to 30 d. The rates of the K release varied with the nature of organic acids and chemical composition, atomic bonding, crystal structure, and formation sequence of K-bearing minerals. The rate constants of K release from biotite in citric and oxalic acid solutions were 63 and 123 times, respectively, greater than those from muscovite. The K release from biotite in these two organic acid solutions was 14 to 18 times faster than that from K-feldspars. The rates of K release from these two K-feldspars were almost identical in the respective organic acid solution but were 4 to 7 times faster than those of muscovite. The renewal of organic acid solutions replenished the H ions and organic ligands as well as depleted the level of the structural cations released and thus enhanced the rate of dissolution of the minerals. The sequence of K release from these K-bearing minerals through the attack of the organic acids was biotite > microcline ≃ orthoclase > muscovite, while that through the cation exchange process in the sodium tetraphenylboron systems was biotite > muscovite > microcline ≃ orthoclase. The rates of release of structural cations from minerals by organic acids followed the order: Al(Fe, Mg) > K > Si. The nature of soluble products and surface features of selected minerals after reactions with oxalic acid was revealed by transmission and scanning electron microscopy. The present data indicate that the sequence of relative rates of K release from K-bearing minerals varies with the mechanisms of the release. The role of organic acids in affecting the K-supplying power of soils thus merits close attention.