Cation exchange during subsurface iron removal.

Subsurface iron removal (SIR), or in-situ iron removal, is an established treatment technology to remove soluble iron (Fe(2+)) from groundwater. Besides the adsorptive-catalytic oxidation theory, it has also been proposed that the injection of O(2)-rich water onsets the exchange of adsorbed Fe(2+) with other cations, such as Ca(2+) and Na(+). In sand column experiments with synthetic and natural groundwater it was found that cation exchange (Na(+)-Fe(2+)) occurs during the injection-abstraction cycles of subsurface iron removal. The Fe(2+) exchange increased at higher Na(+) concentration in the injection water, but decreased in the presence of other cations in the groundwater. Field results with injection of elevated O(2) concentrations (0.55 mM) showed increased Fe removal efficacy; the operational parameter V/Vi (abstraction volume with [Fe]<2 μM divided by the injection volume) increased from an average 7 to 16, indicating that not the exchangeable Fe(2+) on the soil material is the limiting factor during injection, but it is the supply of O(2) to the available Fe(2+).