Physicochemical and Thermodynamic Adsorption Studies of a Ferric Luvisol Soil in Western Nigeria

Abstract

An adequate knowledge of the physico-chemical properties and interaction of cations with soils is a critical research need for developing improved understanding of sustainable agricultural practices. This work therefore focuses on the study of the physico-chemical properties and equilibrium thermodynamics of adsorption of cations on Ferric Luvisols from Western Nigeria. All soil samples showed moderately similar characteristics in terms of the physico-chemical parameters investigated. The pH of the various soil samples in water varied from 6.70–7.40, typical of soils found in humid regions. The organic matter contents were low, in the range of 0.93–3.23%. The exchangeable bases and cation exchange capacity were also moderately low, with the latter having a range of 1.42–1.99 cmol_c/kg. This may be attributed to their low clay content (range 8.32–18.32%). The exchange reactions of calcium (Ca) and potassium (K) at 30°C and 60°C were studied. Thermodynamic parameters derived from experimental results were determined for K-Ca and Ca-K exchange reactions on the soil samples. The selectivity coefficients (K_c) of the Ca-K and K-Ca exchange were found to decrease with increase in K and Ca-loading. The decrease was more pronounced for K-Ca exchange. The low selectivity coefficients with an increase in K loading indicated a greater affinity of the soils for Ca than K. The high equilibrium constants and the negative values of the standard free energy (ΔGdeg;) for the exchange of K for Ca revealed a preference of all the soils for Ca relative to K ions. Also ΔGdeg; values for the reaction Ca-soil to K-soil were positive, which indicated the reluctance of Ca-soil to be converted to K-soil.

Keywords:

• cation exchange
• langmuir
• organic matter
• selectivity
• thermodynamics