Kinetics of potassium chloride-induced Mn release from selected Mn-toxic soils of Indonesia and the United States of America

The Mn release by M KCl from two Oxisols from Indonesia and one Inceptisol from Oregon, USA was studied at different temperatures (25, 35 and 45 °C) to understand the kinetics and mechanism of Mn release from these soils, which were reported to have a Mn toxicity problem. The Mn release by M KCl continued to increase with time up to at least 772 h without reaching equilibrium. The kinetics of Mn release followed a parabolic diffusion model during the 0.25–772 h reaction period. The activation energy of Mn release, calculated from the overall diffusion coefficients, varied from 34 to 47 kJ mol−1. Even after removing the exchangeable Mn from the soils with M calcium nitrate, significant amounts of Mn were released by M KCl. Data presented indicate that ionic strength effect coupled with Cl complexation was the main mechanism governing the KCl-induced Mn release. The Mn release from the soils during the initial reaction period (0.25 h) was fast and accounted for 34–39% of the total amount of Mn released during the 772 h reaction period. The significantly high Mn release during the initial 0.25 h reaction time, the high overall diffusion coefficients of Mn release, and the high values of the pre-exponential factor (calculated from the Arrhenius equation) of Mn release from Mn-toxic soils apparently contributed to the KCl-induced Mn toxicity in these soils.