A nano-scale study of the mechanisms of non-exchangeable potassium release from micas

Abstract

Non-exchangeable potassium (NEK) is released from minerals by chemical extraction methods through dissolution and cation-exchange reactions. Assessing the contribution of these two processes to release budgets is important to predicting plant/mineral interactions and soil sustainability. Three NEK extraction methods, 1M boiling nitric acid (HNO3), 2M hot hydrochloric acid (HCl) and 0.2M sodium tetraphenylboron (NaTPB), were applied to investigate the mechanisms of NEK release from trioctahedral biotite and dioctahedral muscovite. Atomic force microscopy and X-ray diffraction were used to investigate the surface morphology and transformation of mica, respectively. NEK released from biotite and muscovite through cation-exchange reaction accounted for 87.88%, 85.93% and 83.23%, 78.31% of the total extracted K when extracted by boiling HNO3 and hot HCl, respectively, with channel and elliptical dissolution pits on mica surface. NEK extracted by NaTPB was an almost complete cation-exchange reaction, and the extracted micas showed an obvious vermiculitic character and no dissolution features. Results showed the release of NEK from micas is essentially a cation-exchange reactions in both acid and salt solutions, indicating that NEK released through cation-exchange reaction but not via dissolution may make up the main soil K pool available to plant.