Isotherms and kinetics of Si adsorption in soils

Many scientists consider Si to be a ‘quasi-essential’ element for plants. Farmers in a number of countries now apply Si-containing fertilizers to the soil in order to improve crop yield. The adsorption of Si onto soil particles is an important process affecting the availability of Si to plants, yet little is known about the fate of Si after it is added to the soil. The objective of this paper was to study the adsorption of Si in three soils at different initial Si concentrations, temperatures (293 and 303 K), and reaction times. Si-adsorption behavior varied significantly between soil types. Si adsorption and adsorption rate were highest in the Yellow Drab soil and lowest in the Lou soil. In all the three soils, the slopes of the adsorption isotherms remained constant as initial Si concentrations increased from 0 to 50 mg Si/L, but declined rapidly as initial Si concentrations increased from 50 to 200 mg Si/L. This suggests that these soils contained multiple Si-adsorption sites. Langmuir, Freundlich, and Temkin equations all described the adsorption of Si as a function of Si concentration reasonably well, however their goodness of fit varied according to soil type. The soils also showed significant differences in buffering capacities, supply parameters, and percent saturation. The reaction kinetics for Si adsorption in the Yellow Drab and Purple Paddy soils could be divided into two stages, an initial fast reaction (0~2 h) followed by a slow reaction (2~12 h). Si adsorption in the Lou soil was slow but steady throughout the reaction period. The correlation coefficients (r 2 ) for all the equations used in this study were significant at p<0.05 significant levels. Among the kinetic equations used in this study, the parabolic diffusion, bi-constants function, Langmuir, and Elovich equations were the best for describing the relationship between reaction time and the amount of Si adsorbed onto soil particles. An analysis of activation energy (E a ) suggested that the rate-limiting step for Si adsorption in the Yellow Drab soil and the Purple Paddy soil was a diffusion-controlled process, while Si adsorption in the Lou soil seemed to be a chemically controlled process. This study shows that there are significant differences in the adsorption of Si between soil types and highlights the importance of future studies to investigate the mechanisms for Si adsorption in soil.