Exchange Behaviour of Coen33+ on Soil and Pest Fulvic Acid

The study of the exchange pattern on soil and peat fulvic acids (SFA and PFA) reveals that the uptake of Coen₃³⁺ increases with increase in pH but, after reaching a maximum at pH 6.5 it starts decreasing, and finally attains a steady value. The release isotherms indicate the following order of reactivity: Li ≈ Na < K < Rb < Cs < H for monovalent cations, (the reversal of exchangeability for Li and Na taking place at lower concentration) and Mg < Ca < Sr < Ba for the divalent cations. However, for organic cations the order of exchangeability is (C₂H₅)₄N > (CH₃)₄N at lower concentration but the reverse is observed at higher concentration. The release isotherms with monovalent cations distinctly display that the exchange processes occur in three stages. Kielland's equation has been harnessed to find out the thermodynamic equilibrium constants and Gibbs free energy change for exchange reactions involving bivalent cations. It is found that Log of corrected selectivity coefficients or thermodynamic equilibrium constants, when plotted separately against hydrated ionic radii and the reciprocal of Debye-Huckel parameter a⁰, the relative affinities of the counter ions for the exchange substrates are much better correlated with a⁰ rather than with hydrated ionic radii.