Kinetics of Reaction of Fulvic Acid-Iron Complexes in Soils

The kinetics of reaction of fulvic acid-iron complexes in five soils collected from different soil associations of Bihar was studied at 15°C and 26°C. The loss of iron applied to soil through Fe-FA complex was observed to be through complex process involving at least two simultaneous reactions. The experimental data, adequately described by Elovich equation, enabled calculation of the rate constants of two simultaneous reactions operating at different rates by different mechanisms. The instantaneous fast reaction not obeying inverse exponential first order kinetics was thought to be surface adsorption of Fe-FA molecules on the solid matrix of the soils. The reaction was endothermic but higher clay content of soils required lesser activation energy for reaction. The loss of Fe from soil solution by another process obeying inverse exponential first order kinetics of reaction was mainly through displacement of iron from the fulvic acid ligand by competing cations of soil solution like Ca²⁺, Mg²⁺, Zn²⁺ and Cu²⁺ and subsequent precipitation of iron into various solid phases. Formation of metal fulvates in the proportion more than 1: 1 and their coagulation as mechanism of loss of iron from soil solution in some highly alkaline soils was also evident. The mechanisms of reaction of metal-FA complexes are discussed.