Kinetic Analysis Applied to Iron in a Natural Water Model Containing Ions,Organic Complexes,Colloids, and Particles

Abstract

Kinetic analysis using complex formation reactions is applied to “mixtures” containing (variously), Fe³⁺, Fe²⁺, iron complexed to a fulvic acid, hydrous oxide colloids, and non-settleable particulate iron. Such mixtures can be directly resolved if the kinetics of complex formation reactions are pseudo first order and differences among rate constants are large enough. At pH = 4, it is found that fulvic acid causes substantial reduction of Fe³⁺ to Fe²⁺ and that it causes complete dissolution and depolymerization of colloidal and particulate iron at 1:1 ratio. Addition of one equivalent of phosphate causes precipitation of ferric phosphate even in the presence of fulvic acid. This system is very useful for modeling natural water because the kinetic technique allows convenient analysis of components of varying particle size. The present results are strongly indicative of the role of fulvic acid in mediating metal ion chemistry in natural water.