Modeling iron binding to organic matter

The aim of the present work is to model iron speciation during its interaction with natural organic matter. Experimental data for iron speciation were achieved with an insolubilised humic acid used as an organic matter analogue for 30 M to 1.8 mM total iron concentrations and 2 pH 5.5. IHA was found to be able to impose its redox potential to the solution and therefore the Fe(II)/Fe(III) ratio. Model VI and the NICA-Donnan model have been adjusted to experimental results of acid-base titrations, total iron measurements, and redox speciation in solution. They both describe well pH and concentration dependence of iron adsorption. For high iron concentration, Fe(III) solution activity is limited by precipitation of a poorly ordered Fe oxyhydroxide with a higher solubility (log Ks = 5.6-5.7) than ferryhydrite described in the litterature.