Sorption and exchange of potassium on montmorillonitic soil clay mineral

Naspendra, Zuldadan Sorption and exchange of potassium on montmorillonitic soil clay mineral. National Taiwan University.

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Abstract

One of the main problems of montmorillonitic soils is that of high nutrient retention, especially of potassium. Understanding sorption and exchange processes is essential to predicting the retention of potassium on montmorillonitic soils because it can affects on deficiency for plants. Our research was designed to observe: (i) the dynamics of sorption and exchange of potassium on various fraction size of montmorillonitic soil clay minerals, and (ii) the effect of fraction size, abundance of montmorillonite, and ionic strength on the sorption and exchange of potassium. Soil samples were collected from four sites in western Java, Indonesia, namely Lebak (MS1), Karawang (MS2), Cianjur (MS3), and Cirebon (MS4). The samples were fractionated to obtain coarse clay (2-0.2 µm), medium clay (0.2-0.08 µm), and fine clay (< 0.08 µm). Sorption and exchange experiments of K+ followed batch equilibrium method. Statistical analysis used pseudo 1st and 2nd order for kinetic sorption and used Freundlich and Langmuir equation for sorption isotherm. Kinetic sorption of K+ was accurately described by the pseudo-second order model, indicating the sorption of K+ was controlled by valency forces through sharing electron between adsorbent and K+. In term of sorption isotherm, the process were best adjusted to the Freudlich models which demonstrated that at a given maximum concentration of K+ (440 mg.L-1), soil fraction, total clay, and all clay fractions had more capacity to adsorb K+. Based on the particle size, the sorption of K+ increased on the finer clay fraction which was linear to the abundance of montmorillonite in the finer clay fraction and CEC value. Sorption of K+ was higher at ionic strength concentration 0.1 mol.L-1 NaCl than 0.01 mol.L-1. However, amount of exchangable K+ after adsorption expriments was only 42.38-37.75%, indicating that most K+ was strongly adsorbed in montmorillonite. These results showed that a high affinity of K+ to montmorillonitic soils impact on low availability of potassium for plants.

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