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

•The contribution of dissolution and cation-exchange reactions to NEK release was compared.•Cation-exchange reaction was prominent in NEK release when extracted by acid solution.•NEK release in NaTPB solution was an almost complete cation-exchange reaction.•NEK released through cation-exchange reaction may make up the main soil K pool available to plant.

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, 1 M boiling nitric acid (HNO3), 2 M hot hydrochloric acid (HCl) and 0.2 M 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.