Sorption Isotherm Analysis of Trace Metals on a Sandy Textured Coconut Growing Soil

The nature of adsorption of trace metallic plant nutrients, Cu, Mn and Zn, on a coconut growing sandy textured soil (Madampe series) was investigated in 0.050 M KNO₃ medium. Different adsorption parameters, viz contact time, soil mass, initial concentration and pH were optimized for the study. Variation of the mass of each metal ion sorbed on soil and the initial concentration at constant pH was investigated to understand the nature of adsorption. Further, the data at adsorption equilibrium were fitted to two commonly used isotherm models, namely, Langmuir and Freundlich. The optimum parameters were determined as, shaking time: 3 hours; settling time: 30 minutes; soil mass: 1 g; pH: 5-7; and initial concentrations for Mn(II) and Zn(II): 20 mg/L and 15 mg/L, respectively. The experimental data of Mn(II) and Zn(II) lead to regression coefficients (R2) of 0.957 to 0.970, respectively, for the Langmuir model and 0.812 to 0.925, respectively, for the Freundlich model. Although the Langmuir model fitted better with respect to R2 values, both Langmuir and Freundlich models could be used to illustrate the trace metal sorption on the soil. Initially, trace metal ions probably interacted with specific sites where soil surface is homogeneous; this was followed by iteration at nonspecific sites leading to multilayers or through the different modes of mass transfer fulfilling the conditions of the Freundlich model. The adsorption capacity of soil for the three trace metal ions varied in the order of Cu (II) > Zn(II) > Mn(II).