Effect of Leonardite on Adsorption Behavior and Distribution of Chemical Forms of Zinc in a Sandy Loam Soil

In order to study the effect of Leonardite on adsorption behavior and chemical forms of Zn a soil, an experiment was conducted in a completely randomized design (CRD) with three levels of Leonardite (0, 2 and 5%) in three replications. The soil was mixed with 2 and 5% of Leonardite and incubated at 25±1 °C for 2 weeks. After 2 weeks, adsorption experiments carried out with different initial concentration of Zn (0, 30, 90, 120, 150, 200, 250 and 300 mg L-1 Zn) with 0.01 M CaCl2 as a background solution, and Zn-extractable by DTPA method and Zn distribution by Tessier sequential extraction method were determined. Among the adsorption equations, the Froundlich equation (with higher R2 and lower SE) was better fitted to experimental data compared to Langmuier, Temkin and Dubinin-Radushkevich equations, and sorption capacity factors (qmax, B, KF, qD) and sorption energy factors (KT, KL,1/n) decreased with application of leonardite. The sorption energy parameter (E) of Dubinin-Radushkevich isotherm indicated that the Zn adsorption process was physical. The application of Leonardite increased the exchangeable, carbonateand organic matter forms and decreased oxide and residual forms. The MF Values increased with increasing Leonardite levels, demonstrating an increase in the mobility of Zn in the studied soil. It was concluded that addition of the Leonardite in soil lead to transformation of the zinc from insoluble forms into more soluble forms and increased zinc bioavailability.